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feat(sdk): add opt-in stdin for detached commands - #365
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runCommand accepts `stdin: true` (detached only), and Command gains writeStdin() and closeStdin(). Writes are serialized per command, split into 512 KiB chunks, and never retried so bytes can't be delivered twice.
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Workflow DevKit retries failed steps by default, which could resend stdin bytes, so writeStdin and closeStdin now run with maxRetries 0. Rate limited writes are retried again since they were never applied. A write that fails after possibly delivering data now blocks later writes instead of appending to a partial message, an abort while waiting in the queue takes effect, and CommandFinished rejects stdin calls with a clear error.
Each workflow step gets a freshly deserialized Command, so the write queue and the rejection after a failed write don't carry across steps.
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Each stdin request now carries the position it starts at, so a request that fails on a dropped connection or a restarting server is resent without delivering the same bytes twice. Timeouts are only resent while the process keeps reading.
Passing a Readable such as process.stdin as `stdin` pipes it to the command and closes stdin when it ends, with or without `detached`.
QuiiBz
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Oct 9, 2026
QuiiBz
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Oct 9, 2026
QuiiBz
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October 9, 2026 18:56
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This PR was opened by the [Changesets release](https://github.com/changesets/action) GitHub action. When you're ready to do a release, you can merge this and the packages will be published to npm automatically. If you're not ready to do a release yet, that's fine, whenever you add more changesets to main, this PR will be updated. # Releases ## sandbox@4.8.0 ### Minor Changes - Add `sandbox create claude` to open Claude Code in a new Vercel Sandbox. ([#371](#371)) - Add `sandbox create codex` to open Codex in a new Vercel Sandbox. ([#373](#373)) - Add `sandbox create opencode` to create a sandbox with the universal image and open OpenCode directly in the terminal. ([#370](#370)) - Add `sandbox create pi` to open Pi in a Sandbox with scoped reconnect and stop commands. ([#372](#372)) ### Patch Changes - Show a coding agent launch example first in the top-level CLI help. ([#378](#378)) - Update `@vercel/oidc` to v4. ([#375](#375)) - Updated dependencies [[`515f60b261fd7fa2ae9e5ae58fb8d950e4378089`](515f60b), [`31244aa3280dc3fdca0680933f85971570d73d2d`](31244aa)]: - @vercel/sandbox@3.7.0 ## @vercel/sandbox@3.7.0 ### Minor Changes - Add opt-in stdin for commands. Pass `stdin: true` to a detached `runCommand`, then use `command.writeStdin(data)` and `command.closeStdin()` while it runs, or pass a `Readable` such as `process.stdin` to have it piped to the command. ([#365](#365)) ### Patch Changes - Update `@vercel/oidc` to v4. ([#375](#375)) ## @vercel/sandbox-mock@3.7.0 ### Patch Changes - Updated dependencies [[`515f60b261fd7fa2ae9e5ae58fb8d950e4378089`](515f60b), [`31244aa3280dc3fdca0680933f85971570d73d2d`](31244aa)]: - @vercel/sandbox@3.7.0 Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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Every sandbox now supports stdin write offsets, so the SDK no longer
needs the fallbacks for ones that don't:
- `Command` always sends an offset, instead of switching to unsafe
writes when a response lacks `bytesWritten`. If it can't learn the
current offset, it fails before writing rather than assuming 0.
- A 504 no longer triggers an extra empty request to find the bytes
written; the timeout response already includes them.
<!-- project-ships:v1
{
"user_intent": "Finish the opt-in stdin rollout for Sandbox commands by
removing SDK compatibility code for sandboxes that predate stdin write
offsets, now that every sandbox supports them.",
"intended_outcome": "The @vercel/sandbox SDK always sends stdin offsets,
no longer probes for bytes written after a timeout, and fails loudly
instead of assuming offset 0 when the current offset is unknown.",
"related_prs": ["#365"],
"change_type": "chore"
}
-->
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Adds stdin for commands, mainly to run stdio MCP servers (like
agent-browser mcp) in a sandbox.stdin: trueneedsdetached: true. AReadableis written in order with backpressure, and stdin closes when it ends. The caller's stream is never destroyed.runCommandrejects and kills the command. When detached, the nextwait()rejects instead.closeStdinstill works.@vercel/sandbox-mocksupport is a follow-up.Needs backend support that ships separately. The integration tests fail until it's deployed.