Executor as MCP server: serve spec 2026-07-28 and consolidate onto SDK v2

[RhysSullivan]

Executor as an MCP server on spec 2026-07-28 (part 2 of 2, stacked on the client PR): every host serves modern clients, and all serving — both eras — consolidates onto SDK v2.

Serving the new spec:

• Tool server: one SDK-agnostic core on the v2 McpServer, serving both eras. Native elicitation = input_required + HMAC-signed requestState (principal+method bound, TTL'd) over the existing pause/resume engine; legacy clients get real elicitation/create via the SDK's shim. MCP Apps helpers vendored wire-identical pending upstream ext-apps v2 ([codex] Put typed base URL first in combobox #702).
• Neutral hosts (selfhost, local): era-routed after auth — modern requests to cached stateless handlers, legacy sessions on the v2 sessionful transport. stdio serves both eras via serveStdio. CORS preflights echo requested headers for dynamic Mcp-Param-<name>.
• Cloudflare hosts: modern reads worker-side (no DO touch); modern tools/call routes into DOs for execution affinity via the executionId-keyed owner directory (continuations verify signed state in the worker; same cross-bearer 403 contract as legacy).

Consolidation (deletes the transitional machinery):

• Cloud legacy sessions move off McpAgent onto our own Durable Object hosting the v2 stack: DO-storage EventStore (trim/oversize/best-effort), cross-stream replay of stranded POST responses onto the next standalone GET, compatibility priming for pre-2025-11-25 clients, SSE max-age rotation, transactional idle-dispose, parse guard so unknown/foreign session ids 404 cleanly.
• Deleted: patches/agents@0.17.3.patch (911 lines), the agents dependency from every MCP path, the v1 tool-server assembly, the ext-apps runtime dependency, all transitional V2 naming. @modelcontextprotocol/sdk v1 survives only as a devDependency for legacy-client interop tests and the deliberately-legacy plugins/mcp test fixture.

Evidence: full CI green at this tip (19 checks incl. all cloud e2e shards). The four patch-behavior suites — priming reconnect, SSE replay, oversize response, transport-brick regression — plus session survival are the acceptance gate for the consolidation, all green. Modern serving proven live with a 2026-07-28-pinned client; legacy proven by real v1-SDK clients in tests.

Deployment prerequisite (before merge): both workers need the shared continuation-signing secret (≥32 bytes): wrangler secret put MCP_REQUEST_STATE_KEY. No generated fallback.

One-time cutover: cloud sessions created before this deploy get a clean 404 and clients re-initialize transparently. Deploy in a quiet window and watch reconnects. (If a staged rollback is ever needed, the additive-serving/consolidation seam survives in the commit history at mcp-server-v2-cloud.)

---

Closes #1449.
Supersedes #1562 — with thanks to @Schreezer, whose independent implementation validated the architecture; this PR adopts its stronger requestState binding (account/org/resource/tool/code-digest), the MCP_2026_07_28_ENABLED inbound rollback switch, and the CLI stdio-bridge era handling.

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	executor-cloud	ce87083	Aug 16 2026, 09:07 PM

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✅ Deployment successful! View logs	executor-marketing	ce87083	Commit Preview URL Branch Preview URL	Aug 16 2026, 09:06 PM

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