From b80b8ab80f00c753cca9caa5a4252fad0f639dba Mon Sep 17 00:00:00 2001
From: "Reichenbach, Michael" <755327+Silthus@users.noreply.github.com>
Date: Sat, 31 Oct 2026 08:25:16 +0000
Subject: [PATCH 1/5] feat: send email content from code with emails.send

A send with template_key carries subject and html, text or both, flat at
the top of the body with distinct_id. With template_data, the content is a
local template sent by key and content hash, uploaded when PostHog answers
template_outdated. A code send also takes from_, reply_to, cc, bcc,
headers, tags, attachments and max_retries, and captures engagement unless
it passes capture_engagement=False. A template_id send doesn't take
capture_engagement. migrated_from on the client or on Emails goes out as
X-PostHog-Migrated-From on every email request.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .github/scripts/check_strict_types.sh |  33 +-
 .sampo/changesets/emails-code-form.md |   9 +
 posthog/async_client.py               |  11 +-
 posthog/client.py                     |  13 +-
 posthog/emails.py                     | 619 +++++++++++++++++++++++---
 posthog/test/test_emails.py           | 478 +++++++++++++++++++-
 references/public_api_snapshot.txt    |  39 +-
 7 files changed, 1111 insertions(+), 91 deletions(-)
 create mode 100644 .sampo/changesets/emails-code-form.md

diff --git a/.github/scripts/check_strict_types.sh b/.github/scripts/check_strict_types.sh
index 7b46c0af..67da012e 100755
--- a/.github/scripts/check_strict_types.sh
+++ b/.github/scripts/check_strict_types.sh
@@ -15,6 +15,7 @@ import atexit
 
 import posthog
 from posthog import FeatureFlagEvaluations, FlagValue, Posthog
+from posthog.emails import AsyncEmails, Emails, SendEmailResult
 from posthog.ai.evaluations import (
     AsyncOfflineEvaluations,
     BooleanScorerConfig,
@@ -40,6 +41,18 @@ span.end()
 
 _ = (flag_value, all_flags, enabled, payload, evaluations, active)
 
+emails = Emails("phs_example")
+template_send: SendEmailResult = emails.send(to="user@example.com", template_id="0190f6d2-7c1e-7b3a-9f5e-1c2d3e4f5a6b", template_data={"name": "Ada"})
+rendered_send: SendEmailResult = emails.send(to="user@example.com", template_key="password-reset", subject="Reset", html="<p>Reset</p>", from_="Acme <security@example.com>", reply_to=["help@example.com"], capture_engagement=False, max_retries=0)
+local_template_send: SendEmailResult = emails.send(to="user@example.com", template_key="welcome", subject="Hi {{ variables.name }}", text="Hi", template_data={"name": "Ada"})
+invite_send: SendEmailResult = emails.send(to="user@example.com", template_key="invite", subject="Invite", html="<p>Invite</p>", cc="team@example.com", bcc=["audit@example.com"], headers={"X-Entity-Ref-ID": "order-123"}, tags=[{"name": "category", "value": "invite"}], attachments=[{"filename": "invite.ics", "content": b"BEGIN:VCALENDAR", "content_type": "text/calendar"}, {"filename": "logo.png", "content": "iVBORw0KGgo=", "content_id": "logo"}])
+_ = (template_send, rendered_send, local_template_send, invite_send)
+
+
+async def async_email_types() -> None:
+    sent: SendEmailResult = await AsyncEmails("phs_example").send(to="user@example.com", template_key="welcome", subject="Hi", html="<p>Hi</p>")
+    _ = sent
+
 
 async def offline_evaluation_types(
     sync_client: OfflineEvaluations, async_client: AsyncOfflineEvaluations
@@ -64,15 +77,25 @@ async def offline_evaluation_types(
     _ = config
 PY
 
-cat > "$tmp/invalid_offline_types.py" <<'PY'
+cat > "$tmp/invalid_types.py" <<'PY'
 # pyright: strict
 from posthog.ai.evaluations import BooleanScorerConfig, CategoricalScorerConfig, NumericPassingRule, NumericScorerConfig, OfflineEvaluations
+from posthog.emails import AsyncEmails, Emails
 
 client = OfflineEvaluations(project_id=123, secret_key="phx_example")
 NumericPassingRule(operator="gt", threshold=0.8)  # expected-type-error
 NumericScorerConfig(unknown_setting=True)  # expected-type-error
 CategoricalScorerConfig(selection_mode="single")  # expected-type-error
 client.scorers.create(name="Bad config", kind="numeric", config=BooleanScorerConfig(true_is_failure=True))  # expected-type-error
+Emails("phs_example").send(to="user@example.com", template_id="0190f6d2-7c1e-7b3a-9f5e-1c2d3e4f5a6b", template_key="welcome", subject="Hi", html="<p>Hi</p>")  # expected-type-error
+Emails("phs_example").send(to="user@example.com", template_key="welcome", html="<p>Hi</p>")  # expected-type-error
+Emails("phs_example").send(to="user@example.com", template_id="0190f6d2-7c1e-7b3a-9f5e-1c2d3e4f5a6b", cc="team@example.com")  # expected-type-error
+Emails("phs_example").send(to="user@example.com", template_id="0190f6d2-7c1e-7b3a-9f5e-1c2d3e4f5a6b", capture_engagement=False)  # expected-type-error
+Emails("phs_example").send(to="user@example.com", template_key="welcome", subject="Hi", html="<p>Hi</p>", attachments=[{"filename": "a.txt", "content": [104, 105]}])  # expected-type-error
+
+
+async def invalid_async_email() -> None:
+    await AsyncEmails("phs_example").send(to="user@example.com", template_id="0190f6d2-7c1e-7b3a-9f5e-1c2d3e4f5a6b", subject="Hi")  # expected-type-error
 PY
 
 "$tmp/.venv/bin/python" - <<'PY' > "$tmp/public_api_access.py"
@@ -116,8 +139,8 @@ JSON
 cd "$tmp"
 "$tmp/.venv/bin/python" -m pyright strict_posthog_types.py public_api_access.py
 
-if "$tmp/.venv/bin/python" -m pyright --outputjson invalid_offline_types.py > invalid_types.json; then
-    echo "Invalid offline evaluation configurations unexpectedly passed type checking."
+if "$tmp/.venv/bin/python" -m pyright --outputjson invalid_types.py > invalid_types.json; then
+    echo "Invalid offline evaluation configurations and email sends unexpectedly passed type checking."
     exit 1
 fi
 "$tmp/.venv/bin/python" - <<'PY'
@@ -125,11 +148,11 @@ import json
 from pathlib import Path
 
 expected = {
-    index for index, line in enumerate(Path("invalid_offline_types.py").read_text().splitlines())
+    index for index, line in enumerate(Path("invalid_types.py").read_text().splitlines())
     if "expected-type-error" in line
 }
 diagnostics = json.loads(Path("invalid_types.json").read_text())["generalDiagnostics"]
 actual = {item["range"]["start"]["line"] for item in diagnostics if item["severity"] == "error"}
 assert actual == expected, diagnostics
-print("Offline evaluation configuration types reject invalid inputs.")
+print("Offline evaluation configuration and email send types reject invalid inputs.")
 PY
diff --git a/.sampo/changesets/emails-code-form.md b/.sampo/changesets/emails-code-form.md
new file mode 100644
index 00000000..71b12603
--- /dev/null
+++ b/.sampo/changesets/emails-code-form.md
@@ -0,0 +1,9 @@
+---
+pypi/posthog: minor
+---
+
+Send transactional email from content in your code with `posthog.emails.send(template_key=..., subject=..., html=...)`. Without `template_data`, the content is sent as is. With `template_data`, the content is a template: PostHog fills in its variables, and the client uploads it by key whenever PostHog doesn't hold its content yet, then sends again with the same idempotency key. A send that mixes `template_id` and `template_key` fails type checks and raises `TypeError`.
+
+A send with `template_key` captures the email's opens and clicks unless it passes `capture_engagement=False`. A template made in PostHog follows the project's setting, so a `template_id` send doesn't take `capture_engagement`. A code send takes `max_retries` like a template send. Set `migrated_from`, such as `"resend"`, on the client or on `Emails` to send it with every email request.
+
+A code send also takes `cc`, `bcc`, `headers`, `tags` and `attachments`. An attachment's `content` can be `bytes`, `bytearray` or `memoryview`, which the client encodes as Base64, or a Base64 string.
diff --git a/posthog/async_client.py b/posthog/async_client.py
index 57ed13e2..787705f9 100644
--- a/posthog/async_client.py
+++ b/posthog/async_client.py
@@ -133,6 +133,7 @@ def __init__(
         app_host: Optional[str] = None,
         email_request_timeout_seconds: float = 15,
         email_request_max_retries: int = 2,
+        migrated_from: Optional[str] = None,
     ) -> None:
         if flush_at <= 0:
             raise ValueError("flush_at must be greater than zero")
@@ -177,10 +178,13 @@ def __init__(
             else resolved_secret_key
         ) or None
         self.personal_api_key = self.secret_key
-        _validate_options(email_request_timeout_seconds, email_request_max_retries)
+        _validate_options(
+            email_request_timeout_seconds, email_request_max_retries, migrated_from
+        )
         self.app_host = app_host
         self.email_request_timeout_seconds = email_request_timeout_seconds
         self.email_request_max_retries = email_request_max_retries
+        self.migrated_from = migrated_from
         self.feature_flags_request_timeout_seconds = (
             feature_flags_request_timeout_seconds
         )
@@ -260,8 +264,8 @@ def __init__(
     @property
     def emails(self) -> AsyncEmails:
         """
-        Send transactional email from PostHog email templates, and read the
-        status of each email sent with ``get``.
+        Send transactional email from PostHog email templates or from content
+        in your code, and read the status of each email sent with ``get``.
 
         Needs ``secret_key``, a project secret API key with the ``hog_flow:write``
         scope, and ``posthog[async]``. Sends go to ``app_host``, or to the PostHog
@@ -288,6 +292,7 @@ def emails(self) -> AsyncEmails:
                 app_host=self.app_host,
                 timeout=self.email_request_timeout_seconds,
                 max_retries=self.email_request_max_retries,
+                migrated_from=self.migrated_from,
             )
         return self._emails
 
diff --git a/posthog/client.py b/posthog/client.py
index 388e20b2..fda66cf2 100644
--- a/posthog/client.py
+++ b/posthog/client.py
@@ -733,6 +733,7 @@ def __init__(
         app_host: Optional[str] = None,
         email_request_timeout_seconds: float = 15,
         email_request_max_retries: int = 2,
+        migrated_from: Optional[str] = None,
     ):
         """
         Initialize a new PostHog client instance.
@@ -793,6 +794,8 @@ def __init__(
             email_request_max_retries: Number of retries for an email send after a
                 network error, a rate limit, or an unavailable service. Defaults to 2.
                 Set to 0 when a job runner already retries the send.
+            migrated_from: The email provider you moved from, such as
+                ``"resend"``. ``emails`` sends it with every request.
             super_properties: Properties merged into every captured event.
             enable_exception_autocapture: Automatically capture uncaught
                 exceptions.
@@ -1068,10 +1071,13 @@ def __init__(
             else resolved_secret_key
         ) or None
         self.personal_api_key = self.secret_key
-        _validate_options(email_request_timeout_seconds, email_request_max_retries)
+        _validate_options(
+            email_request_timeout_seconds, email_request_max_retries, migrated_from
+        )
         self.app_host = app_host
         self.email_request_timeout_seconds = email_request_timeout_seconds
         self.email_request_max_retries = email_request_max_retries
+        self.migrated_from = migrated_from
         if debug:
             # Ensures that debug level messages are logged when debug mode is on.
             # Otherwise, defaults to WARNING level. See https://docs.python.org/3/howto/logging.html#what-happens-if-no-configuration-is-provided
@@ -1250,8 +1256,8 @@ def _request_identity_kwargs(self) -> Dict[str, str]:
     @property
     def emails(self) -> Emails:
         """
-        Send transactional email from PostHog email templates, and read the
-        status of each email sent with ``get``.
+        Send transactional email from PostHog email templates or from content
+        in your code, and read the status of each email sent with ``get``.
 
         Needs ``secret_key``, a project secret API key with the ``hog_flow:write``
         scope. Sends go to ``app_host``, or to the PostHog Cloud app host of
@@ -1279,6 +1285,7 @@ def emails(self) -> Emails:
                 app_host=self.app_host,
                 timeout=self.email_request_timeout_seconds,
                 max_retries=self.email_request_max_retries,
+                migrated_from=self.migrated_from,
             )
         return self._emails
 
diff --git a/posthog/emails.py b/posthog/emails.py
index 72b1ca3e..bde51056 100644
--- a/posthog/emails.py
+++ b/posthog/emails.py
@@ -1,4 +1,4 @@
-"""Transactional email, sent from an email template in PostHog.
+"""Transactional email, sent from an email template in PostHog or from content in your code.
 
 Setup:
 
@@ -22,6 +22,26 @@
     idempotency_key="welcome:user_123",
 )
 print(result.id)  # the queued email
+
+# Content you rendered for this recipient, named by a key
+posthog.emails.send(
+    to="user@example.com",
+    template_key="password-reset",
+    subject="Reset your password",
+    html=rendered_html,
+    distinct_id="user_123",
+)
+
+# A template in your code. PostHog fills in the variables, and the client
+# uploads the template whenever its content changes.
+posthog.emails.send(
+    to="user@example.com",
+    template_key="welcome",
+    subject="Welcome, {{ variables.name }}",
+    html=welcome_html,
+    template_data={"name": "Ada"},
+    distinct_id="user_123",
+)
 ```
 
 ``posthog.emails`` above is the client's property. For async code, use
@@ -44,6 +64,10 @@
 :class:`Emails`, when a job runner already retries the send, and pass an
 ``idempotency_key`` that names the email, such as ``welcome:user_123``.
 
+Moving from another email provider: set ``migrated_from``, such as
+``"resend"``, on the client or on :class:`Emails`, so PostHog knows where your
+sends came from.
+
 Errors: a send that is not queued raises :class:`EmailError`. Branch on
 ``code``. The ones a setup usually hits:
 
@@ -74,6 +98,8 @@
 from __future__ import annotations
 
 import asyncio
+import base64
+import hashlib
 import importlib
 import json
 import logging
@@ -85,10 +111,21 @@
 from dataclasses import dataclass
 from datetime import datetime
 from types import TracebackType
-from typing import Any, Callable, Mapping, Optional, TypeVar, Union
+from typing import (
+    Any,
+    Callable,
+    Mapping,
+    Optional,
+    Sequence,
+    TypedDict,
+    TypeVar,
+    Union,
+    overload,
+)
 from uuid import UUID, uuid4
 
 import requests
+from typing_extensions import NotRequired, Unpack
 
 from .capture_v1 import _parse_retry_after
 from .request import (
@@ -102,19 +139,27 @@
 
 __all__ = [
     "AsyncEmails",
+    "CodeEmailArgs",
+    "EmailAttachment",
     "EmailError",
     "EmailStatus",
+    "EmailTag",
     "EmailWarning",
     "Emails",
     "SendEmailResult",
+    "TemplateEmailArgs",
 ]
 
 log = logging.getLogger("posthog")
 
 _SEND_PATH = "/api/projects/@current/messaging/send/email/"
 _STATUS_PATH = "/api/projects/@current/messaging/emails/{message_id}/"
+_UPLOAD_PATH = "/api/projects/@current/messaging_templates/by_key/{template_key}/"
+_TEMPLATE_KEY = re.compile(r"[a-zA-Z0-9_-]{1,400}")
+_CONTENT_HASH_HEADER = b"posthog-email-template-v1\n"
 _MAX_RETRY_DELAY = 30.0
 _IDEMPOTENCY_KEY = re.compile(r"^[\x21-\x7e]{1,255}$")
+_HEADER_TOKEN = re.compile(r"[\x21-\x7e]+")
 _IN_FLIGHT_CODES = frozenset({"idempotency_key_in_flight"})
 _QUOTA_CODES = frozenset({"quota_limited"})
 
@@ -168,6 +213,122 @@ class SendEmailResult:
     warnings: tuple[EmailWarning, ...] = ()
 
 
+class EmailTag(TypedDict):
+    """A name and value that PostHog adds to the email's engagement events as ``$email_tags``."""
+
+    name: str
+    value: str
+
+
+class EmailAttachment(TypedDict):
+    """A file sent with the email.
+
+    Args:
+        filename: The file name the recipient sees, such as ``invite.ics``.
+        content: The file as ``bytes``, ``bytearray`` or ``memoryview``, or
+            already Base64 encoded as a string.
+        content_type: The MIME type. Without it, the type follows the file name.
+        content_id: Shows the file inline: reference it in ``html`` as
+            ``<img src="cid:logo">`` with ``content_id`` ``logo``.
+    """
+
+    filename: str
+    content: Union[bytes, bytearray, memoryview, str]
+    content_type: NotRequired[str]
+    content_id: NotRequired[str]
+
+
+class _RecipientArgs(TypedDict):
+    to: str
+    distinct_id: NotRequired[Optional[str]]
+    idempotency_key: NotRequired[Optional[str]]
+    max_retries: NotRequired[Optional[int]]
+
+
+class TemplateEmailArgs(_RecipientArgs):
+    """A send of an email template made in PostHog, named by its ID.
+
+    Args:
+        to: One recipient email address.
+        template_id: The UUID of a template made transactional in PostHog.
+        template_data: Values for the template's ``{{ variables.* }}``
+            placeholders.
+        distinct_id: The recipient's distinct ID in your product.
+        idempotency_key: 1 to 255 visible ASCII characters.
+        max_retries: Overrides the client's maximum number of retries for this
+            send. ``0`` turns retries off.
+    """
+
+    template_id: Union[str, UUID]
+    template_data: NotRequired[Optional[Mapping[str, Any]]]
+
+
+class CodeEmailArgs(_RecipientArgs):
+    """A send of email content that lives in your code, named by a key.
+
+    Without ``template_data``, the content is rendered for this one recipient
+    and sent as is. With it, the content is a template: PostHog fills in its
+    ``{{ variables.* }}`` placeholders, and the client uploads the template
+    whenever PostHog doesn't hold its content yet.
+
+    A PostHog version that doesn't take a field yet, such as ``cc``, refuses
+    the send with ``invalid_request`` and names the field in
+    ``EmailError.field``. It never drops the field and sends the rest.
+
+    Args:
+        to: One recipient email address.
+        template_key: Names the email, so its metrics, logs and status group
+            under it. Letters, digits, ``-`` and ``_``, at most 400.
+        subject: The subject line.
+        html: The HTML body. Give ``html``, ``text`` or both.
+        text: The plain-text body. Without it, PostHog derives it from
+            ``html``. HTML with no text in it, such as a single image, goes out
+            without a plain-text part.
+        from_: A verified sender, such as ``"Acme <hello@example.com>"``.
+        reply_to: One address or a list of addresses.
+        cc: One address or a list of addresses that get a visible copy.
+        bcc: One address or a list of addresses that get a hidden copy.
+        headers: Extra email headers by name, such as ``X-Entity-Ref-ID``.
+        tags: Names and values that PostHog adds to the email's engagement
+            events.
+        attachments: Files sent with the email.
+        capture_engagement: Capture the email's opens and clicks as events of
+            the recipient. Defaults to ``True``. A project whose admin turned
+            engagement capture off captures nothing, whatever the send asks. A
+            template made in PostHog follows the project's setting, so the
+            template form doesn't take it.
+        template_data: Values for the ``{{ variables.* }}`` placeholders.
+        distinct_id: The recipient's distinct ID in your product.
+        idempotency_key: 1 to 255 visible ASCII characters. Template sends
+            and code sends share one set of keys: a key already used for
+            another email, or for a ``template_id`` send, raises
+            ``idempotency_key_reused``. A retry with the same key returns the
+            first send's result, also when its workflow was paused since.
+        max_retries: Overrides the client's maximum number of retries for this
+            send. ``0`` turns retries off.
+    """
+
+    template_key: str
+    subject: str
+    html: NotRequired[str]
+    text: NotRequired[str]
+    from_: NotRequired[str]
+    reply_to: NotRequired[Union[str, Sequence[str]]]
+    cc: NotRequired[Union[str, Sequence[str]]]
+    bcc: NotRequired[Union[str, Sequence[str]]]
+    headers: NotRequired[Mapping[str, str]]
+    tags: NotRequired[Sequence[EmailTag]]
+    attachments: NotRequired[Sequence[EmailAttachment]]
+    capture_engagement: NotRequired[bool]
+    template_data: NotRequired[Mapping[str, Any]]
+
+
+@dataclass(frozen=True)
+class _PreparedUpload:
+    path: str
+    body: bytes
+
+
 @dataclass(frozen=True)
 class EmailStatus:
     """What happened to an email that ``send()`` queued.
@@ -200,6 +361,210 @@ class _PreparedSend:
     body: bytes
     idempotency_key: str
     max_retries: Optional[int] = None
+    upload: Optional[_PreparedUpload] = None
+
+
+def _prepare(args: Mapping[str, Any]) -> _PreparedSend:
+    is_template_form = "template_id" in args
+    if is_template_form == ("template_key" in args):
+        raise TypeError("Pass either template_id or template_key.")
+    form = TemplateEmailArgs if is_template_form else CodeEmailArgs
+    unexpected = sorted(set(args) - set(form.__annotations__))
+    if unexpected:
+        raise TypeError(
+            f"send() got arguments that the {'template' if is_template_form else 'code'} "
+            f"form doesn't take: {', '.join(unexpected)}."
+        )
+    if "to" not in args:
+        raise TypeError("send() is missing the argument to.")
+    if is_template_form:
+        return _prepare_send(
+            args["to"],
+            args["template_id"],
+            args.get("template_data"),
+            args.get("distinct_id"),
+            args.get("idempotency_key"),
+            args.get("max_retries"),
+        )
+    return _prepare_code_send(args)
+
+
+def _prepare_code_send(args: Mapping[str, Any]) -> _PreparedSend:
+    if "subject" not in args:
+        raise TypeError("send() is missing the argument subject.")
+    to, distinct_id, idempotency_key = _recipient(
+        args["to"], args.get("distinct_id"), args.get("idempotency_key")
+    )
+    max_retries = _retry_limit(args.get("max_retries"))
+    template_key = args["template_key"]
+    if not isinstance(template_key, str) or not _TEMPLATE_KEY.fullmatch(template_key):
+        raise ValueError(
+            "template_key can only have letters, digits, hyphens (-) and underscores (_), at most 400."
+        )
+    content = {"subject": _nonempty_text(args, "subject")}
+    content.update(
+        {field: _string(args, field) for field in ("html", "text") if field in args}
+    )
+    if "html" not in content and "text" not in content:
+        raise TypeError("send() needs html, text or both.")
+    if not (content.get("html") or content.get("text")):
+        raise ValueError("html or text must be a nonempty string.")
+
+    payload: dict[str, Any] = {"to": to}
+    upload = None
+    if "template_data" in args:
+        template_data = args["template_data"]
+        if not isinstance(template_data, Mapping):
+            raise TypeError("template_data must be a mapping.")
+        payload["template"] = {"key": template_key, "hash": _content_hash(content)}
+        payload["message_data"] = clean(dict(template_data))
+        if distinct_id is not None:
+            payload["identifiers"] = {"distinct_id": distinct_id}
+        upload = _PreparedUpload(
+            path=_UPLOAD_PATH.format(template_key=template_key),
+            body=_encode(
+                {
+                    "name": template_key,
+                    **content,
+                    "if_not_superseded": True,
+                    "if_code_managed": True,
+                }
+            ),
+        )
+    else:
+        payload.update({"template_key": template_key, **content})
+        if distinct_id is not None:
+            payload["distinct_id"] = distinct_id
+    payload.update(_sender(args))
+    return _PreparedSend(
+        path=_SEND_PATH,
+        body=_encode(payload),
+        idempotency_key=idempotency_key,
+        max_retries=max_retries,
+        upload=upload,
+    )
+
+
+def _content_hash(content: Mapping[str, str]) -> str:
+    """PostHog's content hash v1 of a template, so a send can say which content it holds."""
+    encoded = bytearray(_CONTENT_HASH_HEADER)
+    for field in ("subject", "preheader", "html", "text"):
+        value = content.get(field, "").replace("\r\n", "\n").replace("\r", "\n")
+        data = value.encode("utf-8")
+        encoded += f"{len(data)}:".encode() + data + b"\n"
+    return "v1:" + hashlib.sha256(encoded).hexdigest()
+
+
+def _sender(args: Mapping[str, Any]) -> dict[str, Any]:
+    sender: dict[str, Any] = {}
+    if "from_" in args:
+        sender["from"] = _nonempty_text(args, "from_")
+    for field in ("reply_to", "cc", "bcc"):
+        if field in args:
+            sender[field] = _addresses(field, args[field])
+    if "headers" in args:
+        sender["headers"] = _headers_field(args["headers"])
+    if "tags" in args:
+        sender["tags"] = _tags(args["tags"])
+    if "attachments" in args:
+        sender["attachments"] = _attachments(args["attachments"])
+    capture_engagement = args.get("capture_engagement", True)
+    if not isinstance(capture_engagement, bool):
+        raise TypeError("capture_engagement must be True or False.")
+    sender["capture_engagement"] = capture_engagement
+    return sender
+
+
+def _addresses(field: str, value: Any) -> list[str]:
+    addresses = [value] if isinstance(value, str) else value
+    if not isinstance(addresses, Sequence) or not all(
+        isinstance(address, str) for address in addresses
+    ):
+        raise TypeError(f"{field} must be one address or a list of addresses.")
+    if not all(address.strip() for address in addresses):
+        raise ValueError(f"{field} must not hold a blank address.")
+    return list(addresses)
+
+
+def _headers_field(headers: Any) -> dict[str, str]:
+    if not isinstance(headers, Mapping) or not all(
+        isinstance(name, str) and isinstance(value, str)
+        for name, value in headers.items()
+    ):
+        raise TypeError("headers must map header names to string values.")
+    return dict(headers)
+
+
+def _tags(tags: Any) -> list[dict[str, str]]:
+    if (
+        isinstance(tags, str)
+        or not isinstance(tags, Sequence)
+        or not all(_is_tag(tag) for tag in tags)
+    ):
+        raise TypeError("tags must be a list of {'name': str, 'value': str}.")
+    return [{"name": tag["name"], "value": tag["value"]} for tag in tags]
+
+
+def _attachments(attachments: Any) -> list[dict[str, str]]:
+    if isinstance(attachments, (str, bytes)) or not isinstance(attachments, Sequence):
+        raise TypeError("attachments must be a list of files.")
+    return [_attachment(index, file) for index, file in enumerate(attachments)]
+
+
+def _attachment(index: int, file: Any) -> dict[str, str]:
+    name = f"attachments[{index}]"
+    if not isinstance(file, Mapping) or not {"filename", "content"} <= set(file):
+        raise TypeError(f"{name} needs a filename and its content.")
+    unexpected = set(file) - set(EmailAttachment.__annotations__)
+    if unexpected:
+        raise TypeError(
+            f"{name} has fields an attachment doesn't take: {', '.join(sorted(unexpected))}."
+        )
+    if not all(isinstance(file[field], str) for field in file if field != "content"):
+        raise TypeError(
+            f"{name} needs its filename, content_type and content_id as strings."
+        )
+    return {**file, "content": _base64_content(name, file["content"])}
+
+
+def _base64_content(name: str, content: Any) -> str:
+    if isinstance(content, (bytes, bytearray, memoryview)):
+        return base64.b64encode(bytes(content)).decode("ascii")
+    if isinstance(content, str):
+        return content
+    raise TypeError(
+        f"{name}.content must be bytes or a Base64 string. "
+        "Convert a list of byte values with bytes(content)."
+    )
+
+
+def _is_tag(tag: Any) -> bool:
+    return (
+        isinstance(tag, Mapping)
+        and set(tag) == {"name", "value"}
+        and isinstance(tag["name"], str)
+        and isinstance(tag["value"], str)
+    )
+
+
+def _string(args: Mapping[str, Any], name: str) -> str:
+    value = args[name]
+    if not isinstance(value, str):
+        raise TypeError(f"{name} must be a string.")
+    return value
+
+
+def _nonempty_text(args: Mapping[str, Any], name: str) -> str:
+    value = _string(args, name)
+    if not value:
+        raise ValueError(f"{name} must not be empty.")
+    return value
+
+
+def _encode(payload: Mapping[str, Any]) -> bytes:
+    return json.dumps(
+        payload, cls=DatetimeSerializer, separators=(",", ":"), ensure_ascii=False
+    ).encode("utf-8")
 
 
 def _prepare_send(
@@ -210,10 +575,8 @@ def _prepare_send(
     idempotency_key: Optional[str],
     max_retries: Optional[int] = None,
 ) -> _PreparedSend:
-    if not isinstance(to, str):
-        raise TypeError("to must be one email address as a string.")
-    if not to.strip():
-        raise ValueError("to must not be empty.")
+    to, distinct_id, idempotency_key = _recipient(to, distinct_id, idempotency_key)
+    max_retries = _retry_limit(max_retries)
     try:
         template_uuid = (
             template_id if isinstance(template_id, UUID) else UUID(template_id)
@@ -222,6 +585,29 @@ def _prepare_send(
         raise ValueError("template_id must be the template's UUID.") from None
     if template_data is not None and not isinstance(template_data, Mapping):
         raise TypeError("template_data must be a mapping.")
+
+    payload: dict[str, Any] = {
+        "template": {"id": str(template_uuid)},
+        "to": to,
+        "message_data": clean(dict(template_data or {})),
+    }
+    if distinct_id is not None:
+        payload["identifiers"] = {"distinct_id": distinct_id}
+    return _PreparedSend(
+        path=_SEND_PATH,
+        body=json.dumps(payload, cls=DatetimeSerializer).encode("utf-8"),
+        idempotency_key=idempotency_key,
+        max_retries=max_retries,
+    )
+
+
+def _recipient(
+    to: Any, distinct_id: Any, idempotency_key: Any
+) -> tuple[str, Optional[str], str]:
+    if not isinstance(to, str):
+        raise TypeError("to must be one email address as a string.")
+    if not to.strip():
+        raise ValueError("to must not be empty.")
     if distinct_id is not None and (
         not isinstance(distinct_id, str) or not distinct_id
     ):
@@ -234,26 +620,17 @@ def _prepare_send(
         raise ValueError(
             "idempotency_key must be 1 to 255 visible ASCII characters without spaces."
         )
+    return to, distinct_id, idempotency_key
+
+
+def _retry_limit(max_retries: Any) -> Optional[int]:
     if max_retries is not None and (
         isinstance(max_retries, bool)
         or not isinstance(max_retries, int)
         or max_retries < 0
     ):
         raise ValueError("max_retries must be a nonnegative integer.")
-
-    payload: dict[str, Any] = {
-        "template": {"id": str(template_uuid)},
-        "to": to,
-        "message_data": clean(dict(template_data or {})),
-    }
-    if distinct_id is not None:
-        payload["identifiers"] = {"distinct_id": distinct_id}
-    return _PreparedSend(
-        path=_SEND_PATH,
-        body=json.dumps(payload, cls=DatetimeSerializer).encode("utf-8"),
-        idempotency_key=idempotency_key,
-        max_retries=max_retries,
-    )
+    return max_retries
 
 
 def _status_path(message_id: Union[str, UUID]) -> str:
@@ -266,7 +643,9 @@ def _status_path(message_id: Union[str, UUID]) -> str:
     return _STATUS_PATH.format(message_id=message_uuid)
 
 
-def _validate_options(timeout: float, max_retries: int) -> None:
+def _validate_options(
+    timeout: float, max_retries: int, migrated_from: Optional[str]
+) -> None:
     if (
         isinstance(timeout, bool)
         or not isinstance(timeout, (int, float))
@@ -280,6 +659,12 @@ def _validate_options(timeout: float, max_retries: int) -> None:
         or max_retries < 0
     ):
         raise ValueError("max_retries must be a nonnegative integer.")
+    if migrated_from is not None and not (
+        isinstance(migrated_from, str) and _HEADER_TOKEN.fullmatch(migrated_from)
+    ):
+        raise ValueError(
+            "migrated_from must be visible ASCII characters, such as 'resend'."
+        )
 
 
 def _text(value: Any) -> Optional[str]:
@@ -402,6 +787,12 @@ def _parse_status_response(
     )
 
 
+def _parse_upload(status: int, payload: Any, retry_after_header: Optional[str]) -> None:
+    if not 200 <= status < 300:
+        fields = payload if isinstance(payload, dict) else {}
+        raise _response_error(fields, status, retry_after_header)
+
+
 def _response_error(
     fields: Mapping[str, Any], status: int, retry_after_header: Optional[str]
 ) -> EmailError:
@@ -444,20 +835,33 @@ def _no_op() -> None:
     pass
 
 
+def _is_outdated(error: EmailError) -> bool:
+    return error.status_code == 409 and error.code == "template_outdated"
+
+
 def _log_warnings(result: SendEmailResult) -> None:
     for warning in result.warnings:
         log.warning("PostHog email warning %s: %s", warning.code, warning.message)
 
 
-def _headers(secret_key: str, idempotency_key: Optional[str]) -> dict[str, str]:
+def _headers(
+    secret_key: str,
+    migrated_from: Optional[str],
+    *,
+    has_body: bool = False,
+    idempotency_key: Optional[str] = None,
+) -> dict[str, str]:
     headers = {
         "Authorization": f"Bearer {secret_key}",
         "Accept": "application/json",
         "User-Agent": f"posthog-python/{VERSION}",
     }
-    if idempotency_key is not None:
+    if has_body:
         headers["Content-Type"] = "application/json"
+    if idempotency_key is not None:
         headers["Idempotency-Key"] = idempotency_key
+    if migrated_from is not None:
+        headers["X-PostHog-Migrated-From"] = migrated_from
     return headers
 
 
@@ -491,13 +895,15 @@ def __init__(
         timeout: float,
         max_retries: int,
         disabled: bool,
+        migrated_from: Optional[str],
     ) -> None:
-        _validate_options(timeout, max_retries)
+        _validate_options(timeout, max_retries, migrated_from)
         self.secret_key = secret_key.strip() if isinstance(secret_key, str) else None
         self.host = determine_app_host(host)
         self.timeout = timeout
         self.max_retries = max_retries
         self.disabled = disabled
+        self.migrated_from = migrated_from
         self._parent_disabled: Callable[[], bool] = lambda: False
         self._admit: Callable[[], Optional[Callable[[], None]]] = lambda: _no_op
         self._host_error: Optional[str] = None
@@ -526,7 +932,7 @@ def _secret_key(self, key_needed: str) -> str:
 
 
 class Emails(_EmailsBase):
-    """Sends transactional email from PostHog email templates.
+    """Sends transactional email from PostHog email templates or from content in your code.
 
     ``host`` is your PostHog app host. PostHog Cloud ingestion hosts map to the
     app host, so ``us.i.posthog.com`` sends to ``us.posthog.com``. Any other
@@ -536,7 +942,8 @@ class Emails(_EmailsBase):
     to ``max_retries`` times, with the same idempotency key. Set
     ``max_retries=0`` when a job runner already retries the send. ``timeout``
     is in seconds, per request. Pass ``session`` to send through your own
-    ``requests.Session``, for example a test double.
+    ``requests.Session``, for example a test double. ``migrated_from``, such
+    as ``"resend"``, tells PostHog which provider your sends came from.
     """
 
     def __init__(
@@ -548,6 +955,7 @@ def __init__(
         max_retries: int = 2,
         disabled: bool = False,
         session: Optional[requests.Session] = None,
+        migrated_from: Optional[str] = None,
     ) -> None:
         super().__init__(
             secret_key,
@@ -555,22 +963,25 @@ def __init__(
             timeout=timeout,
             max_retries=max_retries,
             disabled=disabled,
+            migrated_from=migrated_from,
         )
         self._session: Optional[requests.Session] = session
         self._owns_session = session is None
         self._session_lock = threading.Lock()
 
-    def send(
-        self,
-        *,
-        to: str,
-        template_id: Union[str, UUID],
-        template_data: Optional[Mapping[str, Any]] = None,
-        distinct_id: Optional[str] = None,
-        idempotency_key: Optional[str] = None,
-        max_retries: Optional[int] = None,
-    ) -> SendEmailResult:
-        """Queue one email from a transactional email template.
+    @overload
+    def send(self, **kwargs: Unpack[TemplateEmailArgs]) -> SendEmailResult: ...
+
+    @overload
+    def send(self, **kwargs: Unpack[CodeEmailArgs]) -> SendEmailResult: ...
+
+    def send(self, **kwargs: Any) -> SendEmailResult:
+        """Queue one email, from a template in PostHog or from content in your code.
+
+        Pass exactly one form. The template form names a template made in
+        PostHog with ``template_id``. The code form names the email with
+        ``template_key`` and passes ``subject`` and ``html``, ``text`` or both:
+        see :class:`CodeEmailArgs`.
 
         Args:
             to: One recipient email address.
@@ -608,17 +1019,33 @@ def send(
                 distinct_id="user_123",
                 idempotency_key="welcome:user_123",
             )
+            emails.send(
+                to="user@example.com",
+                template_key="password-reset",
+                subject="Reset your password",
+                html=rendered_html,
+                distinct_id="user_123",
+            )
             ```
         """
-        prepared = _prepare_send(
-            to, template_id, template_data, distinct_id, idempotency_key, max_retries
-        )
+        prepared = _prepare(kwargs)
         finish = self._start(_SEND_NOT_DONE)
         try:
-            headers = _headers(
-                self._secret_key(_SEND_KEY_NEEDED), prepared.idempotency_key
-            )
-            result = self._call(
+            result = self._deliver(prepared, self._secret_key(_SEND_KEY_NEEDED))
+        finally:
+            finish()
+        _log_warnings(result)
+        return result
+
+    def _deliver(self, prepared: _PreparedSend, secret_key: str) -> SendEmailResult:
+        headers = _headers(
+            secret_key,
+            self.migrated_from,
+            has_body=True,
+            idempotency_key=prepared.idempotency_key,
+        )
+        try:
+            return self._call(
                 "POST",
                 prepared.path,
                 prepared.body,
@@ -626,10 +1053,25 @@ def send(
                 _parse_response,
                 prepared.max_retries,
             )
-        finally:
-            finish()
-        _log_warnings(result)
-        return result
+        except EmailError as error:
+            if prepared.upload is None or not _is_outdated(error):
+                raise
+            self._call(
+                "PUT",
+                prepared.upload.path,
+                prepared.upload.body,
+                _headers(secret_key, self.migrated_from, has_body=True),
+                _parse_upload,
+                prepared.max_retries,
+            )
+        return self._call(
+            "POST",
+            prepared.path,
+            prepared.body,
+            headers,
+            _parse_response,
+            prepared.max_retries,
+        )
 
     def get(self, message_id: Union[str, UUID]) -> EmailStatus:
         """Read what happened to an email that :meth:`send` queued.
@@ -658,7 +1100,7 @@ def get(self, message_id: Union[str, UUID]) -> EmailStatus:
         path = _status_path(message_id)
         finish = self._start(_GET_NOT_DONE)
         try:
-            headers = _headers(self._secret_key(_GET_KEY_NEEDED), None)
+            headers = _headers(self._secret_key(_GET_KEY_NEEDED), self.migrated_from)
             return self._call("GET", path, None, headers, _parse_status_response)
         finally:
             finish()
@@ -753,6 +1195,7 @@ def __init__(
         max_retries: int = 2,
         disabled: bool = False,
         http_client: Any = None,
+        migrated_from: Optional[str] = None,
     ) -> None:
         super().__init__(
             secret_key,
@@ -760,21 +1203,19 @@ def __init__(
             timeout=timeout,
             max_retries=max_retries,
             disabled=disabled,
+            migrated_from=migrated_from,
         )
         self._client: Any = http_client
         self._owns_client = http_client is None
 
-    async def send(
-        self,
-        *,
-        to: str,
-        template_id: Union[str, UUID],
-        template_data: Optional[Mapping[str, Any]] = None,
-        distinct_id: Optional[str] = None,
-        idempotency_key: Optional[str] = None,
-        max_retries: Optional[int] = None,
-    ) -> SendEmailResult:
-        """Queue one email from a transactional email template.
+    @overload
+    async def send(self, **kwargs: Unpack[TemplateEmailArgs]) -> SendEmailResult: ...
+
+    @overload
+    async def send(self, **kwargs: Unpack[CodeEmailArgs]) -> SendEmailResult: ...
+
+    async def send(self, **kwargs: Any) -> SendEmailResult:
+        """Queue one email, from a template in PostHog or from content in your code.
 
         Takes the same arguments and raises the same errors as
         :meth:`Emails.send`.
@@ -789,15 +1230,26 @@ async def send(
             )
             ```
         """
-        prepared = _prepare_send(
-            to, template_id, template_data, distinct_id, idempotency_key, max_retries
-        )
+        prepared = _prepare(kwargs)
         finish = self._start(_SEND_NOT_DONE)
         try:
-            headers = _headers(
-                self._secret_key(_SEND_KEY_NEEDED), prepared.idempotency_key
-            )
-            result = await self._call(
+            result = await self._deliver(prepared, self._secret_key(_SEND_KEY_NEEDED))
+        finally:
+            finish()
+        _log_warnings(result)
+        return result
+
+    async def _deliver(
+        self, prepared: _PreparedSend, secret_key: str
+    ) -> SendEmailResult:
+        headers = _headers(
+            secret_key,
+            self.migrated_from,
+            has_body=True,
+            idempotency_key=prepared.idempotency_key,
+        )
+        try:
+            return await self._call(
                 "POST",
                 prepared.path,
                 prepared.body,
@@ -805,10 +1257,25 @@ async def send(
                 _parse_response,
                 prepared.max_retries,
             )
-        finally:
-            finish()
-        _log_warnings(result)
-        return result
+        except EmailError as error:
+            if prepared.upload is None or not _is_outdated(error):
+                raise
+            await self._call(
+                "PUT",
+                prepared.upload.path,
+                prepared.upload.body,
+                _headers(secret_key, self.migrated_from, has_body=True),
+                _parse_upload,
+                prepared.max_retries,
+            )
+        return await self._call(
+            "POST",
+            prepared.path,
+            prepared.body,
+            headers,
+            _parse_response,
+            prepared.max_retries,
+        )
 
     async def get(self, message_id: Union[str, UUID]) -> EmailStatus:
         """Read what happened to an email that :meth:`send` queued.
@@ -819,7 +1286,7 @@ async def get(self, message_id: Union[str, UUID]) -> EmailStatus:
         path = _status_path(message_id)
         finish = self._start(_GET_NOT_DONE)
         try:
-            headers = _headers(self._secret_key(_GET_KEY_NEEDED), None)
+            headers = _headers(self._secret_key(_GET_KEY_NEEDED), self.migrated_from)
             return await self._call("GET", path, None, headers, _parse_status_response)
         finally:
             finish()
@@ -915,10 +1382,15 @@ def _client_emails(
     app_host: Optional[str],
     timeout: float,
     max_retries: int,
+    migrated_from: Optional[str],
 ) -> _EmailsT:
     if app_host:
         emails = factory(
-            secret_key, host=app_host, timeout=timeout, max_retries=max_retries
+            secret_key,
+            host=app_host,
+            timeout=timeout,
+            max_retries=max_retries,
+            migrated_from=migrated_from,
         )
         emails._parent_disabled = is_disabled
         emails._admit = admit
@@ -930,6 +1402,7 @@ def _client_emails(
         host=inferred_host if is_cloud else None,
         timeout=timeout,
         max_retries=max_retries,
+        migrated_from=migrated_from,
     )
     if not is_cloud:
         # A custom host is often an ingestion proxy, which must not receive the secret key.
diff --git a/posthog/test/test_emails.py b/posthog/test/test_emails.py
index 0d2099a8..feb68680 100644
--- a/posthog/test/test_emails.py
+++ b/posthog/test/test_emails.py
@@ -176,6 +176,7 @@ def test_client_email_options_set_the_host_timeout_and_retries():
         app_host="https://posthog.example.com/",
         email_request_timeout_seconds=3,
         email_request_max_retries=0,
+        migrated_from="resend",
     )
     unavailable = response(503, {"code": "delivery_unavailable"})
 
@@ -188,6 +189,35 @@ def test_client_email_options_set_the_host_timeout_and_retries():
     request.assert_called_once()
     assert request.call_args.args[1].startswith("https://posthog.example.com/api/")
     assert request.call_args.kwargs["timeout"] == 3
+    sent = request.call_args.kwargs["auth"](
+        requests.Request("POST", "https://posthog.example.com/").prepare()
+    )
+    assert sent.headers["X-PostHog-Migrated-From"] == "resend"
+
+
+def test_names_the_provider_the_client_moved_from_on_every_request():
+    emails, adapter = emails_with([accepted(), status_reply()], migrated_from="resend")
+
+    send(emails)
+    emails.get(MESSAGE_ID)
+
+    assert [
+        request.headers["X-PostHog-Migrated-From"] for request in adapter.requests
+    ] == [
+        "resend",
+        "resend",
+    ]
+
+
+@parameterized.expand([("space", "re send"), ("not_text", 1), ("empty", "")])
+def test_rejects_a_provider_name_that_is_not_a_header_value(_name, migrated_from):
+    with pytest.raises(ValueError):
+        Posthog(
+            "phc_test",
+            secret_key=SECRET_KEY,
+            sync_mode=True,
+            migrated_from=migrated_from,
+        )
 
 
 @parameterized.expand(
@@ -282,6 +312,18 @@ def test_waits_for_retry_after_and_gives_up_on_a_longer_one(no_sleep):
         ("client_option", {"max_retries": 0}, {}, 1),
         ("per_send_override", {}, {"max_retries": 0}, 1),
         ("per_send_override_above_client", {"max_retries": 0}, {"max_retries": 1}, 2),
+        (
+            "code_send_override",
+            {},
+            {
+                "template_id": None,
+                "template_key": "password-reset",
+                "subject": "Reset",
+                "html": "<p>Reset</p>",
+                "max_retries": 0,
+            },
+            1,
+        ),
     ]
 )
 def test_max_retries_zero_leaves_retries_to_the_caller(
@@ -290,9 +332,12 @@ def test_max_retries_zero_leaves_retries_to_the_caller(
     emails, adapter = emails_with(
         [rejected(503, "delivery_unavailable")] * 3, **client_options
     )
+    args = {"to": "user@example.com", "template_id": TEMPLATE_ID, **send_options}
 
     with pytest.raises(EmailError):
-        send(emails, **send_options)
+        emails.send(
+            **{name: value for name, value in args.items() if value is not None}
+        )
 
     assert len(adapter.requests) == requests_made
 
@@ -572,3 +617,434 @@ def handler(request: httpx.Request) -> httpx.Response:
     assert [(request.method, str(request.url)) for request in seen] == [
         ("GET", STATUS_URL)
     ] * 2
+
+
+HTML = "<p>Reset it here.</p>"
+
+
+def send_code(emails: Emails, **overrides: Any):
+    return emails.send(
+        **{
+            "to": "user@example.com",
+            "template_key": "password-reset",
+            "subject": "Reset your password",
+            **overrides,
+        }
+    )
+
+
+@parameterized.expand(
+    [
+        ("html_only", {"html": HTML}, {"html": HTML}),
+        ("text_only", {"text": "Reset it here."}, {"text": "Reset it here."}),
+        (
+            "every_field",
+            {
+                "html": HTML,
+                "text": "Reset it here.",
+                "from_": "Acme <security@example.com>",
+                "reply_to": "help@example.com",
+                "capture_engagement": False,
+                "distinct_id": "user_123",
+            },
+            {
+                "html": HTML,
+                "text": "Reset it here.",
+                "from": "Acme <security@example.com>",
+                "reply_to": ["help@example.com"],
+                "capture_engagement": False,
+                "distinct_id": "user_123",
+            },
+        ),
+        (
+            "several_reply_to",
+            {"html": HTML, "reply_to": ["a@example.com", "b@example.com"]},
+            {"html": HTML, "reply_to": ["a@example.com", "b@example.com"]},
+        ),
+        (
+            "empty_address_lists",
+            {"html": HTML, "reply_to": [], "cc": [], "bcc": []},
+            {"html": HTML, "reply_to": [], "cc": [], "bcc": []},
+        ),
+    ]
+)
+def test_rendered_content_is_posted_as_is(_name, overrides, extra_body):
+    emails, adapter = emails_with([accepted("msg_1")])
+
+    result = send_code(emails, idempotency_key="reset:user_123", **overrides)
+
+    assert result.id == "msg_1"
+    [request] = adapter.requests
+    assert (request.method, request.url) == ("POST", SEND_URL)
+    assert request.headers["Idempotency-Key"] == "reset:user_123"
+    assert json.loads(request.body) == {
+        "to": "user@example.com",
+        "template_key": "password-reset",
+        "subject": "Reset your password",
+        "capture_engagement": True,
+        **extra_body,
+    }
+
+
+WELCOME = {
+    "template_key": "welcome",
+    "subject": "Welcome, {{ variables.first_name }}",
+    "html": "<p>Hi {{ variables.first_name }}</p>",
+}
+# The server's content_hash v1 test vectors, shared with posthog-node.
+WELCOME_HASH = "v1:f74cd913f3aa2e5667a588f2865634e56693df77793ca594d18e9b5f89e3d2e6"
+WELCOME_CRLF_TEXT_HASH = (
+    "v1:8112876ee1b2dd7327f571b04b546ed057bfff5a904d9f0ae507b6a449ab6453"
+)
+UPLOAD_URL = (
+    "https://us.posthog.com/api/projects/@current/messaging_templates/by_key/welcome/"
+)
+
+
+def send_local_template(emails: Emails, **overrides: Any):
+    return emails.send(
+        **{
+            "to": "user@example.com",
+            **WELCOME,
+            "template_data": {"first_name": "Jane"},
+            **overrides,
+        }
+    )
+
+
+@parameterized.expand(
+    [
+        ("html_only", {}, WELCOME_HASH),
+        ("crlf_text", {"text": "\r\n\r"}, WELCOME_CRLF_TEXT_HASH),
+        ("empty_text", {"text": ""}, WELCOME_HASH),
+    ]
+)
+def test_a_local_template_is_sent_by_key_and_content_hash(_name, overrides, hash):
+    emails, adapter = emails_with([accepted("msg_1")])
+
+    result = send_local_template(
+        emails, from_="Acme <hello@example.com>", distinct_id="user_123", **overrides
+    )
+
+    assert result.id == "msg_1"
+    [request] = adapter.requests
+    assert (request.method, request.url) == ("POST", SEND_URL)
+    assert json.loads(request.body) == {
+        "to": "user@example.com",
+        "template": {"key": "welcome", "hash": hash},
+        "message_data": {"first_name": "Jane"},
+        "from": "Acme <hello@example.com>",
+        "capture_engagement": True,
+        "identifiers": {"distinct_id": "user_123"},
+    }
+
+
+def uploaded() -> Reply:
+    return (201, {"id": "tpl_1", "key": "welcome", "result": "created"}, {})
+
+
+def test_an_outdated_local_template_is_uploaded_then_sent_again_with_the_same_key():
+    emails, adapter = emails_with(
+        [rejected(409, "template_outdated"), uploaded(), accepted("msg_1")]
+    )
+
+    result = send_local_template(emails, text="Hi {{ variables.first_name }}")
+
+    assert result.id == "msg_1"
+    first_send, upload, second_send = adapter.requests
+    assert (upload.method, upload.url) == ("PUT", UPLOAD_URL)
+    assert json.loads(upload.body) == {
+        "name": "welcome",
+        "subject": "Welcome, {{ variables.first_name }}",
+        "html": "<p>Hi {{ variables.first_name }}</p>",
+        "text": "Hi {{ variables.first_name }}",
+        "if_not_superseded": True,
+        "if_code_managed": True,
+    }
+    assert "Idempotency-Key" not in upload.headers
+    assert upload.headers["Authorization"] == f"Bearer {SECRET_KEY}"
+    assert second_send.body == first_send.body
+    assert (
+        second_send.headers["Idempotency-Key"] == first_send.headers["Idempotency-Key"]
+    )
+
+
+@parameterized.expand(
+    [
+        (
+            "refused_upload",
+            rejected(409, "template_edited_in_posthog"),
+            "template_edited_in_posthog",
+            2,
+        ),
+        ("outdated_again", None, "template_outdated", 3),
+    ]
+)
+def test_a_local_template_uploads_at_most_once_per_send(
+    _name, upload_reply, code, requests_made
+):
+    replies = (
+        [rejected(409, "template_outdated"), upload_reply]
+        if upload_reply
+        else [
+            rejected(409, "template_outdated"),
+            uploaded(),
+            rejected(409, "template_outdated"),
+        ]
+    )
+    emails, adapter = emails_with(replies)
+
+    with pytest.raises(EmailError) as raised:
+        send_local_template(emails)
+
+    assert (raised.value.status_code, raised.value.code) == (409, code)
+    assert len(adapter.requests) == requests_made
+
+
+def test_rendered_content_is_never_uploaded():
+    emails, adapter = emails_with([rejected(409, "template_outdated")])
+
+    with pytest.raises(EmailError):
+        send_code(emails, html=HTML)
+
+    assert [request.method for request in adapter.requests] == ["POST"]
+
+
+@pytest.mark.asyncio
+async def test_async_uploads_an_outdated_local_template_then_sends_again():
+    seen: list[httpx.Request] = []
+    replies = [
+        httpx.Response(409, json={"code": "template_outdated"}),
+        httpx.Response(201, json={"id": "tpl_1", "result": "created"}),
+        httpx.Response(202, json={"message_id": "msg_1"}),
+    ]
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen.append(request)
+        return replies.pop(0)
+
+    async with httpx.AsyncClient(transport=httpx.MockTransport(handler)) as client:
+        result = await AsyncEmails(SECRET_KEY, http_client=client).send(
+            to="user@example.com", **WELCOME, template_data={"first_name": "Jane"}
+        )
+
+    assert result.id == "msg_1"
+    assert [(request.method, str(request.url)) for request in seen] == [
+        ("POST", SEND_URL),
+        ("PUT", UPLOAD_URL),
+        ("POST", SEND_URL),
+    ]
+    first_send, upload, second_send = seen
+    assert json.loads(first_send.content)["template"] == {
+        "key": "welcome",
+        "hash": WELCOME_HASH,
+    }
+    assert "Idempotency-Key" not in upload.headers
+    assert second_send.content == first_send.content
+    assert (
+        second_send.headers["Idempotency-Key"] == first_send.headers["Idempotency-Key"]
+    )
+
+
+@parameterized.expand(
+    [
+        ("both_forms", {"template_id": TEMPLATE_ID, "html": HTML}, TypeError),
+        ("no_form", {"template_key": None}, TypeError),
+        ("no_content", {}, TypeError),
+        ("no_subject", {"subject": None, "html": HTML}, TypeError),
+        ("subject_not_text", {"subject": 1, "html": HTML}, TypeError),
+        ("from_not_text", {"html": HTML, "from_": 1}, TypeError),
+        ("from_keyword", {"html": HTML, "from": "a@example.com"}, TypeError),
+        (
+            "key_with_space",
+            {"template_key": "reset password", "html": HTML},
+            ValueError,
+        ),
+        (
+            "key_with_trailing_newline",
+            {"template_key": "welcome\n", "html": HTML},
+            ValueError,
+        ),
+        ("key_too_long", {"template_key": "k" * 401, "html": HTML}, ValueError),
+        ("empty_html", {"html": ""}, ValueError),
+        ("empty_html_and_text", {"html": "", "text": ""}, ValueError),
+        ("blank_cc", {"html": HTML, "cc": " "}, ValueError),
+        ("cc_not_text", {"html": HTML, "cc": 123}, TypeError),
+        ("template_data_list", {"html": HTML, "template_data": [1]}, TypeError),
+        (
+            "capture_engagement_on_template_form",
+            {
+                "template_id": TEMPLATE_ID,
+                "template_key": None,
+                "subject": None,
+                "capture_engagement": False,
+            },
+            TypeError,
+        ),
+        (
+            "cc_on_template_form",
+            {
+                "template_id": TEMPLATE_ID,
+                "template_key": None,
+                "subject": None,
+                "cc": "a@example.com",
+            },
+            TypeError,
+        ),
+    ]
+)
+def test_rejects_a_code_send_that_mixes_or_misses_fields(_name, overrides, error):
+    emails, adapter = emails_with([])
+    args = {
+        "to": "user@example.com",
+        "template_key": "password-reset",
+        "subject": "Reset your password",
+        **overrides,
+    }
+
+    with pytest.raises(error):
+        emails.send(
+            **{name: value for name, value in args.items() if value is not None}
+        )
+
+    assert adapter.requests == []
+
+
+COPIES_HEADERS_AND_TAGS = {
+    "cc": "manager@example.com",
+    "bcc": ["audit@example.com", "archive@example.com"],
+    "headers": {"X-Entity-Ref-ID": "order-123"},
+    "tags": [{"name": "category", "value": "password_reset"}],
+}
+COPIES_HEADERS_AND_TAGS_BODY = {
+    "cc": ["manager@example.com"],
+    "bcc": ["audit@example.com", "archive@example.com"],
+    "headers": {"X-Entity-Ref-ID": "order-123"},
+    "tags": [{"name": "category", "value": "password_reset"}],
+}
+
+
+def test_rendered_content_carries_copies_headers_and_tags():
+    emails, adapter = emails_with([accepted()])
+
+    send_code(emails, html=HTML, **COPIES_HEADERS_AND_TAGS)
+
+    [request] = adapter.requests
+    assert json.loads(request.body) == {
+        "to": "user@example.com",
+        "template_key": "password-reset",
+        "subject": "Reset your password",
+        "html": HTML,
+        "capture_engagement": True,
+        **COPIES_HEADERS_AND_TAGS_BODY,
+    }
+
+
+def test_a_server_that_does_not_take_a_field_refuses_the_send_and_names_it():
+    refused = {
+        "type": "invalid_request",
+        "code": "invalid_request",
+        "detail": "cc: PostHog doesn't take this field on this route.",
+        "attr": "cc",
+    }
+    emails, adapter = emails_with([(400, refused, {}), accepted()])
+
+    with pytest.raises(EmailError) as raised:
+        send_code(emails, html=HTML, cc="manager@example.com")
+
+    error = raised.value
+    assert (error.name, error.code, error.field) == (
+        "validation_error",
+        "invalid_request",
+        "cc",
+    )
+    assert len(adapter.requests) == 1
+
+
+def test_a_local_template_sends_copies_headers_and_tags_but_never_uploads_them():
+    emails, adapter = emails_with(
+        [rejected(409, "template_outdated"), uploaded(), accepted()]
+    )
+
+    send_local_template(emails, **COPIES_HEADERS_AND_TAGS)
+
+    first_send, upload, _ = adapter.requests
+    sent = json.loads(first_send.body)
+    assert {field: sent[field] for field in COPIES_HEADERS_AND_TAGS_BODY} == (
+        COPIES_HEADERS_AND_TAGS_BODY
+    )
+    assert not set(COPIES_HEADERS_AND_TAGS_BODY) & set(json.loads(upload.body))
+
+
+INVITE = b"BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n"
+
+
+def test_attachments_are_sent_base64_encoded_once_across_retries():
+    emails, adapter = emails_with([rejected(503, "delivery_unavailable"), accepted()])
+
+    send_code(
+        emails,
+        html=HTML,
+        attachments=[
+            {
+                "filename": "invite.ics",
+                "content": INVITE,
+                "content_type": "text/calendar; method=REQUEST",
+            },
+            {"filename": "logo.png", "content": "iVBORw0KGgo=", "content_id": "logo"},
+        ],
+    )
+
+    first, retry = adapter.requests
+    assert json.loads(first.body)["attachments"] == [
+        {
+            "filename": "invite.ics",
+            "content": "QkVHSU46VkNBTEVOREFSDQpFTkQ6VkNBTEVOREFSDQo=",
+            "content_type": "text/calendar; method=REQUEST",
+        },
+        {"filename": "logo.png", "content": "iVBORw0KGgo=", "content_id": "logo"},
+    ]
+    assert retry.body == first.body
+
+
+@parameterized.expand(
+    [
+        ("bytearray", bytearray(INVITE)),
+        ("memoryview", memoryview(INVITE)),
+        (
+            "strided_memoryview",
+            memoryview(bytes(b for byte in INVITE for b in (byte, 0)))[::2],
+        ),
+    ]
+)
+def test_attachment_content_from_any_bytes_buffer_is_base64_encoded(_name, content):
+    emails, adapter = emails_with([accepted()])
+
+    send_code(
+        emails, html=HTML, attachments=[{"filename": "invite.ics", "content": content}]
+    )
+
+    [request] = adapter.requests
+    assert json.loads(request.body)["attachments"] == [
+        {
+            "filename": "invite.ics",
+            "content": "QkVHSU46VkNBTEVOREFSDQpFTkQ6VkNBTEVOREFSDQo=",
+        }
+    ]
+
+
+@parameterized.expand(
+    [
+        ("list_of_ints", {"filename": "a.txt", "content": [104, 105]}),
+        ("no_filename", {"content": b"104"}),
+        ("unknown_field", {"filename": "a.txt", "content": b"104", "path": "/tmp/a"}),
+    ]
+)
+def test_rejects_an_attachment_that_is_not_a_file(_name, attachment):
+    emails, adapter = emails_with([])
+
+    with pytest.raises(TypeError) as raised:
+        send_code(emails, html=HTML, attachments=[attachment])
+
+    assert "104" not in str(raised.value)
+    assert adapter.requests == []
diff --git a/references/public_api_snapshot.txt b/references/public_api_snapshot.txt
index 3eadb81b..8a60e086 100644
--- a/references/public_api_snapshot.txt
+++ b/references/public_api_snapshot.txt
@@ -758,6 +758,7 @@ attribute posthog.async_client.AsyncClient.is_server = is_server
 attribute posthog.async_client.AsyncClient.log = logging.getLogger('posthog')
 attribute posthog.async_client.AsyncClient.log_captured_exceptions = log_captured_exceptions
 attribute posthog.async_client.AsyncClient.max_retries = max(0, max_retries)
+attribute posthog.async_client.AsyncClient.migrated_from = migrated_from
 attribute posthog.async_client.AsyncClient.on_error = on_error
 attribute posthog.async_client.AsyncClient.personal_api_key = self.secret_key
 attribute posthog.async_client.AsyncClient.project_root = os.getcwd()
@@ -828,6 +829,7 @@ attribute posthog.client.Client.log = logging.getLogger('posthog')
 attribute posthog.client.Client.log_captured_exceptions = log_captured_exceptions
 attribute posthog.client.Client.max_retries = max(0, max_retries)
 attribute posthog.client.Client.metrics: PostHogMetrics
+attribute posthog.client.Client.migrated_from = migrated_from
 attribute posthog.client.Client.on_error = on_error
 attribute posthog.client.Client.personal_api_key = self.secret_key
 attribute posthog.client.Client.poll_interval = poll_interval
@@ -884,6 +886,23 @@ attribute posthog.debug = False
 attribute posthog.default_client = None
 attribute posthog.disable_geoip = True
 attribute posthog.disabled = False
+attribute posthog.emails.CodeEmailArgs.attachments: NotRequired[Sequence[EmailAttachment]]
+attribute posthog.emails.CodeEmailArgs.bcc: NotRequired[Union[str, Sequence[str]]]
+attribute posthog.emails.CodeEmailArgs.capture_engagement: NotRequired[bool]
+attribute posthog.emails.CodeEmailArgs.cc: NotRequired[Union[str, Sequence[str]]]
+attribute posthog.emails.CodeEmailArgs.from_: NotRequired[str]
+attribute posthog.emails.CodeEmailArgs.headers: NotRequired[Mapping[str, str]]
+attribute posthog.emails.CodeEmailArgs.html: NotRequired[str]
+attribute posthog.emails.CodeEmailArgs.reply_to: NotRequired[Union[str, Sequence[str]]]
+attribute posthog.emails.CodeEmailArgs.subject: str
+attribute posthog.emails.CodeEmailArgs.tags: NotRequired[Sequence[EmailTag]]
+attribute posthog.emails.CodeEmailArgs.template_data: NotRequired[Mapping[str, Any]]
+attribute posthog.emails.CodeEmailArgs.template_key: str
+attribute posthog.emails.CodeEmailArgs.text: NotRequired[str]
+attribute posthog.emails.EmailAttachment.content: Union[bytes, bytearray, memoryview, str]
+attribute posthog.emails.EmailAttachment.content_id: NotRequired[str]
+attribute posthog.emails.EmailAttachment.content_type: NotRequired[str]
+attribute posthog.emails.EmailAttachment.filename: str
 attribute posthog.emails.EmailError.code = code
 attribute posthog.emails.EmailError.docs_url = docs_url
 attribute posthog.emails.EmailError.field = field
@@ -898,12 +917,16 @@ attribute posthog.emails.EmailStatus.id: str
 attribute posthog.emails.EmailStatus.reason: Optional[str] = None
 attribute posthog.emails.EmailStatus.status: str
 attribute posthog.emails.EmailStatus.status_at: datetime
+attribute posthog.emails.EmailTag.name: str
+attribute posthog.emails.EmailTag.value: str
 attribute posthog.emails.EmailWarning.code: str
 attribute posthog.emails.EmailWarning.fix: Optional[str] = None
 attribute posthog.emails.EmailWarning.message: str
 attribute posthog.emails.SendEmailResult.id: str
 attribute posthog.emails.SendEmailResult.replayed: bool = False
 attribute posthog.emails.SendEmailResult.warnings: tuple[EmailWarning, ...] = ()
+attribute posthog.emails.TemplateEmailArgs.template_data: NotRequired[Optional[Mapping[str, Any]]]
+attribute posthog.emails.TemplateEmailArgs.template_id: Union[str, UUID]
 attribute posthog.enable_exception_autocapture = False
 attribute posthog.enable_exception_autocapture_rate_limiting = False
 attribute posthog.enable_full_ai_capture = False
@@ -1320,21 +1343,25 @@ class posthog.ai.types.TokenUsage
 class posthog.ai.types.ToolInProgress 
 class posthog.args.OptionalCaptureArgs 
 class posthog.args.OptionalSetArgs 
-class posthog.async_client.AsyncClient(project_api_key: str, host: Optional[str] = None, debug: bool = False, max_queue_size: int = 10000, send: bool = True, on_error=None, flush_at: int = 100, flush_interval: float = 5.0, gzip: bool = False, max_retries: int = 3, timeout: float = 15, thread: int = 1, disabled: bool = False, disable_geoip: bool = True, is_server: bool = True, historical_migration: bool = False, super_properties: Optional[dict[str, Any]] = None, before_send=None, log_captured_exceptions: bool = False, project_root: Optional[str] = None, capture_exception_code_variables: bool = False, code_variables_mask_patterns=None, code_variables_ignore_patterns=None, code_variables_mask_url_credentials=None, code_variables_detect_secrets=None, in_app_modules: Optional[list[str]] = None, capture_mode: Optional[Union[CaptureMode, str]] = None, capture_compression: Optional[Union[CaptureCompression, str]] = None, capture_trace_context: bool = False, secret_key: Optional[str] = None, personal_api_key: Optional[str] = None, feature_flags_request_timeout_seconds: float = 3, feature_flags_request_max_retries: int = 1, app_host: Optional[str] = None, email_request_timeout_seconds: float = 15, email_request_max_retries: int = 2)
+class posthog.async_client.AsyncClient(project_api_key: str, host: Optional[str] = None, debug: bool = False, max_queue_size: int = 10000, send: bool = True, on_error=None, flush_at: int = 100, flush_interval: float = 5.0, gzip: bool = False, max_retries: int = 3, timeout: float = 15, thread: int = 1, disabled: bool = False, disable_geoip: bool = True, is_server: bool = True, historical_migration: bool = False, super_properties: Optional[dict[str, Any]] = None, before_send=None, log_captured_exceptions: bool = False, project_root: Optional[str] = None, capture_exception_code_variables: bool = False, code_variables_mask_patterns=None, code_variables_ignore_patterns=None, code_variables_mask_url_credentials=None, code_variables_detect_secrets=None, in_app_modules: Optional[list[str]] = None, capture_mode: Optional[Union[CaptureMode, str]] = None, capture_compression: Optional[Union[CaptureCompression, str]] = None, capture_trace_context: bool = False, secret_key: Optional[str] = None, personal_api_key: Optional[str] = None, feature_flags_request_timeout_seconds: float = 3, feature_flags_request_max_retries: int = 1, app_host: Optional[str] = None, email_request_timeout_seconds: float = 15, email_request_max_retries: int = 2, migrated_from: Optional[str] = None)
 class posthog.async_client.AsyncPosthog 
 class posthog.bucketed_rate_limiter.BucketedRateLimiter(bucket_size: Number, refill_rate: Number, refill_interval_seconds: Number, on_bucket_rate_limited: Optional[Callable[[Hashable], None]] = None, clock: Callable[[], float] = time.monotonic)
 class posthog.capture_compression.CaptureCompression 
 class posthog.capture_mode.CaptureMode 
 class posthog.capture_v1.CaptureV1Error(status: int | str, message: str, *, retry_after: Optional[float] = None, request_id: Optional[str] = None, attempts: Optional[int] = None, retry_exhausted: Optional[list[str]] = None, drops: Optional[list[tuple[str, Optional[str]]]] = None)
-class posthog.client.Client(project_api_key: str, host=None, debug=False, max_queue_size=10000, send=True, on_error=None, flush_at=100, flush_interval=5.0, gzip=False, max_retries=3, sync_mode=False, timeout: float = 15, thread=1, poll_interval=30, personal_api_key=None, disabled=False, disable_geoip=True, is_server=True, historical_migration=False, feature_flags_request_timeout_seconds: float = 3, feature_flags_request_max_retries=1, super_properties=None, enable_exception_autocapture=False, log_captured_exceptions=False, project_root=None, privacy_mode=False, before_send=None, flag_fallback_cache_url=None, enable_local_evaluation=True, flag_definition_cache_provider: Optional[FlagDefinitionCacheProvider] = None, capture_exception_code_variables=False, code_variables_mask_patterns=None, code_variables_ignore_patterns=None, code_variables_mask_url_credentials=None, code_variables_detect_secrets=None, in_app_modules: list[str] | None = None, enable_exception_autocapture_rate_limiting=False, exception_autocapture_bucket_size=ExceptionCapture.DEFAULT_BUCKET_SIZE, exception_autocapture_refill_rate=ExceptionCapture.DEFAULT_REFILL_RATE, exception_autocapture_refill_interval_seconds=ExceptionCapture.DEFAULT_REFILL_INTERVAL_SECONDS, capture_mode: Optional[Union[CaptureMode, str]] = None, capture_compression: Optional[Union[CaptureCompression, str]] = None, secret_key=None, metrics: Optional[dict] = None, enable_full_ai_capture=False, capture_trace_context=False, _use_ai_lane=False, _enable_multimodal_capture=False, traces: Optional[dict] = None, app_host: Optional[str] = None, email_request_timeout_seconds: float = 15, email_request_max_retries: int = 2)
+class posthog.client.Client(project_api_key: str, host=None, debug=False, max_queue_size=10000, send=True, on_error=None, flush_at=100, flush_interval=5.0, gzip=False, max_retries=3, sync_mode=False, timeout: float = 15, thread=1, poll_interval=30, personal_api_key=None, disabled=False, disable_geoip=True, is_server=True, historical_migration=False, feature_flags_request_timeout_seconds: float = 3, feature_flags_request_max_retries=1, super_properties=None, enable_exception_autocapture=False, log_captured_exceptions=False, project_root=None, privacy_mode=False, before_send=None, flag_fallback_cache_url=None, enable_local_evaluation=True, flag_definition_cache_provider: Optional[FlagDefinitionCacheProvider] = None, capture_exception_code_variables=False, code_variables_mask_patterns=None, code_variables_ignore_patterns=None, code_variables_mask_url_credentials=None, code_variables_detect_secrets=None, in_app_modules: list[str] | None = None, enable_exception_autocapture_rate_limiting=False, exception_autocapture_bucket_size=ExceptionCapture.DEFAULT_BUCKET_SIZE, exception_autocapture_refill_rate=ExceptionCapture.DEFAULT_REFILL_RATE, exception_autocapture_refill_interval_seconds=ExceptionCapture.DEFAULT_REFILL_INTERVAL_SECONDS, capture_mode: Optional[Union[CaptureMode, str]] = None, capture_compression: Optional[Union[CaptureCompression, str]] = None, secret_key=None, metrics: Optional[dict] = None, enable_full_ai_capture=False, capture_trace_context=False, _use_ai_lane=False, _enable_multimodal_capture=False, traces: Optional[dict] = None, app_host: Optional[str] = None, email_request_timeout_seconds: float = 15, email_request_max_retries: int = 2, migrated_from: Optional[str] = None)
 class posthog.consumer.Consumer(queue, api_key, flush_at=100, host=None, on_error=None, flush_interval=5.0, gzip=False, retries=10, timeout: float = 15, historical_migration=False, endpoint=EVENTS_ENDPOINT, max_msg_size=MAX_MSG_SIZE, capture_mode=CaptureMode.V0, capture_compression=CaptureCompression.NONE)
 class posthog.contexts.ContextScope(parent=None, fresh: bool = False, capture_exceptions: bool = True, client: Optional[Client] = None)
-class posthog.emails.AsyncEmails(secret_key: Optional[str], *, host: Optional[str] = None, timeout: float = 15, max_retries: int = 2, disabled: bool = False, http_client: Any = None)
+class posthog.emails.AsyncEmails(secret_key: Optional[str], *, host: Optional[str] = None, timeout: float = 15, max_retries: int = 2, disabled: bool = False, http_client: Any = None, migrated_from: Optional[str] = None)
+class posthog.emails.CodeEmailArgs 
+class posthog.emails.EmailAttachment 
 class posthog.emails.EmailError(*, name: str, code: str, message: str, status_code: Optional[int] = None, why: Optional[str] = None, fix: Optional[str] = None, docs_url: Optional[str] = None, field: Optional[str] = None, retry_after: Optional[float] = None)
 class posthog.emails.EmailStatus(id: str, status: str, created_at: datetime, status_at: datetime, reason: Optional[str] = None)
+class posthog.emails.EmailTag 
 class posthog.emails.EmailWarning(code: str, message: str, fix: Optional[str] = None)
-class posthog.emails.Emails(secret_key: Optional[str], *, host: Optional[str] = None, timeout: float = 15, max_retries: int = 2, disabled: bool = False, session: Optional[requests.Session] = None)
+class posthog.emails.Emails(secret_key: Optional[str], *, host: Optional[str] = None, timeout: float = 15, max_retries: int = 2, disabled: bool = False, session: Optional[requests.Session] = None, migrated_from: Optional[str] = None)
 class posthog.emails.SendEmailResult(id: str, replayed: bool = False, warnings: tuple[EmailWarning, ...] = ())
+class posthog.emails.TemplateEmailArgs 
 class posthog.exception_capture.ExceptionCapture(client: Client, rate_limiting_enabled=False, bucket_size=DEFAULT_BUCKET_SIZE, refill_rate=DEFAULT_REFILL_RATE, refill_interval_seconds=DEFAULT_REFILL_INTERVAL_SECONDS)
 class posthog.exception_utils.AnnotatedValue(value, metadata)
 class posthog.exception_utils.VariableSizeLimiter(max_size=DEFAULT_TOTAL_VARIABLES_SIZE_LIMIT)
@@ -1833,10 +1860,10 @@ method posthog.contexts.ContextScope.set_distinct_id(distinct_id: str)
 method posthog.contexts.ContextScope.set_session_id(session_id: str)
 method posthog.emails.AsyncEmails.aclose() -> None
 method posthog.emails.AsyncEmails.get(message_id: Union[str, UUID]) -> EmailStatus
-method posthog.emails.AsyncEmails.send(*, to: str, template_id: Union[str, UUID], template_data: Optional[Mapping[str, Any]] = None, distinct_id: Optional[str] = None, idempotency_key: Optional[str] = None, max_retries: Optional[int] = None) -> SendEmailResult
+method posthog.emails.AsyncEmails.send(**kwargs: Any) -> SendEmailResult
 method posthog.emails.Emails.close() -> None
 method posthog.emails.Emails.get(message_id: Union[str, UUID]) -> EmailStatus
-method posthog.emails.Emails.send(*, to: str, template_id: Union[str, UUID], template_data: Optional[Mapping[str, Any]] = None, distinct_id: Optional[str] = None, idempotency_key: Optional[str] = None, max_retries: Optional[int] = None) -> SendEmailResult
+method posthog.emails.Emails.send(**kwargs: Any) -> SendEmailResult
 method posthog.exception_capture.ExceptionCapture.capture_exception(exception, metadata=None)
 method posthog.exception_capture.ExceptionCapture.close()
 method posthog.exception_capture.ExceptionCapture.exception_handler(exc_type, exc_value, exc_traceback)

From 3f2428df858b6c40fa0a857f85ea08a97097e5b3 Mon Sep 17 00:00:00 2001
From: "Reichenbach, Michael" <755327+Silthus@users.noreply.github.com>
Date: Sat, 31 Oct 2026 08:37:05 +0000
Subject: [PATCH 2/5] fix: name a refused template upload validation_error

A 409 template_edited_in_posthog from a local-template upload is a
permanent refusal, not a concurrent request, so it carries the name
validation_error like template_outdated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 posthog/emails.py           | 3 ++-
 posthog/test/test_emails.py | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/posthog/emails.py b/posthog/emails.py
index bde51056..cb397bf6 100644
--- a/posthog/emails.py
+++ b/posthog/emails.py
@@ -162,6 +162,7 @@
 _HEADER_TOKEN = re.compile(r"[\x21-\x7e]+")
 _IN_FLIGHT_CODES = frozenset({"idempotency_key_in_flight"})
 _QUOTA_CODES = frozenset({"quota_limited"})
+_TEMPLATE_CONFLICTS = frozenset({"template_outdated", "template_edited_in_posthog"})
 
 
 class EmailError(Exception):
@@ -685,7 +686,7 @@ def _error_name(status: Optional[int], code: str) -> str:
     if status == 409:
         return (
             "validation_error"
-            if code == "template_outdated"
+            if code in _TEMPLATE_CONFLICTS
             else "concurrent_idempotent_requests"
         )
     if status == 422:
diff --git a/posthog/test/test_emails.py b/posthog/test/test_emails.py
index feb68680..b5f5aa08 100644
--- a/posthog/test/test_emails.py
+++ b/posthog/test/test_emails.py
@@ -797,7 +797,11 @@ def test_a_local_template_uploads_at_most_once_per_send(
     with pytest.raises(EmailError) as raised:
         send_local_template(emails)
 
-    assert (raised.value.status_code, raised.value.code) == (409, code)
+    assert (raised.value.status_code, raised.value.code, raised.value.name) == (
+        409,
+        code,
+        "validation_error",
+    )
     assert len(adapter.requests) == requests_made
 
 

From bcde99339846db54d50700c6ff25f16a637a8ba1 Mon Sep 17 00:00:00 2001
From: "Reichenbach, Michael" <755327+Silthus@users.noreply.github.com>
Date: Sun, 1 Nov 2026 16:55:09 +0000
Subject: [PATCH 3/5] fix: refuse a local template without html

PostHog stores a local template on upload, and its upload route needs an
HTML body. A text-only local template got 409 template_outdated, then a
400 on the upload, on every send. The client now raises ValueError before
any request, as posthog-node refuses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 posthog/emails.py           |  5 +++++
 posthog/test/test_emails.py | 10 ++++++++++
 2 files changed, 15 insertions(+)

diff --git a/posthog/emails.py b/posthog/emails.py
index cb397bf6..6b5bb06e 100644
--- a/posthog/emails.py
+++ b/posthog/emails.py
@@ -417,6 +417,11 @@ def _prepare_code_send(args: Mapping[str, Any]) -> _PreparedSend:
         template_data = args["template_data"]
         if not isinstance(template_data, Mapping):
             raise TypeError("template_data must be a mapping.")
+        # PostHog stores a local template, and a stored template needs HTML.
+        if not content.get("html"):
+            raise ValueError(
+                "A local template needs html. Pass the HTML body with template_data."
+            )
         payload["template"] = {"key": template_key, "hash": _content_hash(content)}
         payload["message_data"] = clean(dict(template_data))
         if distinct_id is not None:
diff --git a/posthog/test/test_emails.py b/posthog/test/test_emails.py
index b5f5aa08..27bcdf9e 100644
--- a/posthog/test/test_emails.py
+++ b/posthog/test/test_emails.py
@@ -875,6 +875,16 @@ def handler(request: httpx.Request) -> httpx.Response:
         ("blank_cc", {"html": HTML, "cc": " "}, ValueError),
         ("cc_not_text", {"html": HTML, "cc": 123}, TypeError),
         ("template_data_list", {"html": HTML, "template_data": [1]}, TypeError),
+        (
+            "local_template_without_html",
+            {"text": "Hi", "template_data": {"first_name": "Jane"}},
+            ValueError,
+        ),
+        (
+            "local_template_with_empty_html",
+            {"html": "", "text": "Hi", "template_data": {"first_name": "Jane"}},
+            ValueError,
+        ),
         (
             "capture_engagement_on_template_form",
             {

From 1fef822e48134142bb8808aa79effbef876dda8e Mon Sep 17 00:00:00 2001
From: "Reichenbach, Michael" <755327+Silthus@users.noreply.github.com>
Date: Sun, 1 Nov 2026 16:55:09 +0000
Subject: [PATCH 4/5] fix: take None as a left-out field of a code send

A wrapper that forwards its own optional values passed text=None or
template_data=None, and the code form raised TypeError, while
distinct_id=None and the template form's template_data=None already
counted as left out. The code form's optional fields now do too, and
their types say so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .sampo/changesets/emails-code-form.md |  4 +--
 posthog/emails.py                     | 51 ++++++++++++++++++++-------
 posthog/test/test_emails.py           | 18 ++++++++++
 references/public_api_snapshot.txt    | 22 ++++++------
 4 files changed, 70 insertions(+), 25 deletions(-)

diff --git a/.sampo/changesets/emails-code-form.md b/.sampo/changesets/emails-code-form.md
index 71b12603..df27efc6 100644
--- a/.sampo/changesets/emails-code-form.md
+++ b/.sampo/changesets/emails-code-form.md
@@ -2,8 +2,8 @@
 pypi/posthog: minor
 ---
 
-Send transactional email from content in your code with `posthog.emails.send(template_key=..., subject=..., html=...)`. Without `template_data`, the content is sent as is. With `template_data`, the content is a template: PostHog fills in its variables, and the client uploads it by key whenever PostHog doesn't hold its content yet, then sends again with the same idempotency key. A send that mixes `template_id` and `template_key` fails type checks and raises `TypeError`.
+Send transactional email from content in your code with `posthog.emails.send(template_key=..., subject=..., html=...)`. Without `template_data`, the content is sent as is. With `template_data`, the content is a template: PostHog fills in its variables, and the client uploads it by key whenever PostHog doesn't hold its content yet, then sends again with the same idempotency key. A local template needs `html`. A send that mixes `template_id` and `template_key` fails type checks and raises `TypeError`.
 
 A send with `template_key` captures the email's opens and clicks unless it passes `capture_engagement=False`. A template made in PostHog follows the project's setting, so a `template_id` send doesn't take `capture_engagement`. A code send takes `max_retries` like a template send. Set `migrated_from`, such as `"resend"`, on the client or on `Emails` to send it with every email request.
 
-A code send also takes `cc`, `bcc`, `headers`, `tags` and `attachments`. An attachment's `content` can be `bytes`, `bytearray` or `memoryview`, which the client encodes as Base64, or a Base64 string.
+A code send also takes `cc`, `bcc`, `headers`, `tags` and `attachments`. An attachment's `content` can be `bytes`, `bytearray` or `memoryview`, which the client encodes as Base64, or a Base64 string. An optional field of a code send passed as `None` counts as left out.
diff --git a/posthog/emails.py b/posthog/emails.py
index 6b5bb06e..97b2b6dc 100644
--- a/posthog/emails.py
+++ b/posthog/emails.py
@@ -272,6 +272,8 @@ class CodeEmailArgs(_RecipientArgs):
     ``{{ variables.* }}`` placeholders, and the client uploads the template
     whenever PostHog doesn't hold its content yet.
 
+    An optional field passed as ``None`` counts as left out.
+
     A PostHog version that doesn't take a field yet, such as ``cc``, refuses
     the send with ``invalid_request`` and names the field in
     ``EmailError.field``. It never drops the field and sends the rest.
@@ -281,7 +283,8 @@ class CodeEmailArgs(_RecipientArgs):
         template_key: Names the email, so its metrics, logs and status group
             under it. Letters, digits, ``-`` and ``_``, at most 400.
         subject: The subject line.
-        html: The HTML body. Give ``html``, ``text`` or both.
+        html: The HTML body. Give ``html``, ``text`` or both. A local template
+            needs ``html``.
         text: The plain-text body. Without it, PostHog derives it from
             ``html``. HTML with no text in it, such as a single image, goes out
             without a plain-text part.
@@ -311,17 +314,36 @@ class CodeEmailArgs(_RecipientArgs):
 
     template_key: str
     subject: str
-    html: NotRequired[str]
-    text: NotRequired[str]
-    from_: NotRequired[str]
-    reply_to: NotRequired[Union[str, Sequence[str]]]
-    cc: NotRequired[Union[str, Sequence[str]]]
-    bcc: NotRequired[Union[str, Sequence[str]]]
-    headers: NotRequired[Mapping[str, str]]
-    tags: NotRequired[Sequence[EmailTag]]
-    attachments: NotRequired[Sequence[EmailAttachment]]
-    capture_engagement: NotRequired[bool]
-    template_data: NotRequired[Mapping[str, Any]]
+    html: NotRequired[Optional[str]]
+    text: NotRequired[Optional[str]]
+    from_: NotRequired[Optional[str]]
+    reply_to: NotRequired[Optional[Union[str, Sequence[str]]]]
+    cc: NotRequired[Optional[Union[str, Sequence[str]]]]
+    bcc: NotRequired[Optional[Union[str, Sequence[str]]]]
+    headers: NotRequired[Optional[Mapping[str, str]]]
+    tags: NotRequired[Optional[Sequence[EmailTag]]]
+    attachments: NotRequired[Optional[Sequence[EmailAttachment]]]
+    capture_engagement: NotRequired[Optional[bool]]
+    template_data: NotRequired[Optional[Mapping[str, Any]]]
+
+
+# None stands for a field left out, as it does for distinct_id, so a wrapper
+# can pass its own optional values through.
+_OPTIONAL_CODE_FIELDS = frozenset(
+    {
+        "html",
+        "text",
+        "from_",
+        "reply_to",
+        "cc",
+        "bcc",
+        "headers",
+        "tags",
+        "attachments",
+        "capture_engagement",
+        "template_data",
+    }
+)
 
 
 @dataclass(frozen=True)
@@ -391,6 +413,11 @@ def _prepare(args: Mapping[str, Any]) -> _PreparedSend:
 
 
 def _prepare_code_send(args: Mapping[str, Any]) -> _PreparedSend:
+    args = {
+        name: value
+        for name, value in args.items()
+        if value is not None or name not in _OPTIONAL_CODE_FIELDS
+    }
     if "subject" not in args:
         raise TypeError("send() is missing the argument subject.")
     to, distinct_id, idempotency_key = _recipient(
diff --git a/posthog/test/test_emails.py b/posthog/test/test_emails.py
index 27bcdf9e..c7fc3cd8 100644
--- a/posthog/test/test_emails.py
+++ b/posthog/test/test_emails.py
@@ -666,6 +666,24 @@ def send_code(emails: Emails, **overrides: Any):
             {"html": HTML, "reply_to": [], "cc": [], "bcc": []},
             {"html": HTML, "reply_to": [], "cc": [], "bcc": []},
         ),
+        (
+            "none_as_left_out",
+            {
+                "html": HTML,
+                "text": None,
+                "from_": None,
+                "reply_to": None,
+                "cc": None,
+                "bcc": None,
+                "headers": None,
+                "tags": None,
+                "attachments": None,
+                "capture_engagement": None,
+                "template_data": None,
+                "distinct_id": None,
+            },
+            {"html": HTML},
+        ),
     ]
 )
 def test_rendered_content_is_posted_as_is(_name, overrides, extra_body):
diff --git a/references/public_api_snapshot.txt b/references/public_api_snapshot.txt
index 8a60e086..2eb20a58 100644
--- a/references/public_api_snapshot.txt
+++ b/references/public_api_snapshot.txt
@@ -886,19 +886,19 @@ attribute posthog.debug = False
 attribute posthog.default_client = None
 attribute posthog.disable_geoip = True
 attribute posthog.disabled = False
-attribute posthog.emails.CodeEmailArgs.attachments: NotRequired[Sequence[EmailAttachment]]
-attribute posthog.emails.CodeEmailArgs.bcc: NotRequired[Union[str, Sequence[str]]]
-attribute posthog.emails.CodeEmailArgs.capture_engagement: NotRequired[bool]
-attribute posthog.emails.CodeEmailArgs.cc: NotRequired[Union[str, Sequence[str]]]
-attribute posthog.emails.CodeEmailArgs.from_: NotRequired[str]
-attribute posthog.emails.CodeEmailArgs.headers: NotRequired[Mapping[str, str]]
-attribute posthog.emails.CodeEmailArgs.html: NotRequired[str]
-attribute posthog.emails.CodeEmailArgs.reply_to: NotRequired[Union[str, Sequence[str]]]
+attribute posthog.emails.CodeEmailArgs.attachments: NotRequired[Optional[Sequence[EmailAttachment]]]
+attribute posthog.emails.CodeEmailArgs.bcc: NotRequired[Optional[Union[str, Sequence[str]]]]
+attribute posthog.emails.CodeEmailArgs.capture_engagement: NotRequired[Optional[bool]]
+attribute posthog.emails.CodeEmailArgs.cc: NotRequired[Optional[Union[str, Sequence[str]]]]
+attribute posthog.emails.CodeEmailArgs.from_: NotRequired[Optional[str]]
+attribute posthog.emails.CodeEmailArgs.headers: NotRequired[Optional[Mapping[str, str]]]
+attribute posthog.emails.CodeEmailArgs.html: NotRequired[Optional[str]]
+attribute posthog.emails.CodeEmailArgs.reply_to: NotRequired[Optional[Union[str, Sequence[str]]]]
 attribute posthog.emails.CodeEmailArgs.subject: str
-attribute posthog.emails.CodeEmailArgs.tags: NotRequired[Sequence[EmailTag]]
-attribute posthog.emails.CodeEmailArgs.template_data: NotRequired[Mapping[str, Any]]
+attribute posthog.emails.CodeEmailArgs.tags: NotRequired[Optional[Sequence[EmailTag]]]
+attribute posthog.emails.CodeEmailArgs.template_data: NotRequired[Optional[Mapping[str, Any]]]
 attribute posthog.emails.CodeEmailArgs.template_key: str
-attribute posthog.emails.CodeEmailArgs.text: NotRequired[str]
+attribute posthog.emails.CodeEmailArgs.text: NotRequired[Optional[str]]
 attribute posthog.emails.EmailAttachment.content: Union[bytes, bytearray, memoryview, str]
 attribute posthog.emails.EmailAttachment.content_id: NotRequired[str]
 attribute posthog.emails.EmailAttachment.content_type: NotRequired[str]

From e5fdedf1a4d3d26596212e52f3a69b46ab30aa5f Mon Sep 17 00:00:00 2001
From: "Reichenbach, Michael" <755327+Silthus@users.noreply.github.com>
Date: Sun, 1 Nov 2026 17:09:06 +0000
Subject: [PATCH 5/5] chore: drop two comments the code already says

The docstring names None as a left-out field, and the error names the html rule.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 posthog/emails.py | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/posthog/emails.py b/posthog/emails.py
index 97b2b6dc..c65af278 100644
--- a/posthog/emails.py
+++ b/posthog/emails.py
@@ -327,8 +327,6 @@ class CodeEmailArgs(_RecipientArgs):
     template_data: NotRequired[Optional[Mapping[str, Any]]]
 
 
-# None stands for a field left out, as it does for distinct_id, so a wrapper
-# can pass its own optional values through.
 _OPTIONAL_CODE_FIELDS = frozenset(
     {
         "html",
@@ -444,7 +442,6 @@ def _prepare_code_send(args: Mapping[str, Any]) -> _PreparedSend:
         template_data = args["template_data"]
         if not isinstance(template_data, Mapping):
             raise TypeError("template_data must be a mapping.")
-        # PostHog stores a local template, and a stored template needs HTML.
         if not content.get("html"):
             raise ValueError(
                 "A local template needs html. Pass the HTML body with template_data."