fix: scope MCP replay to the requested stream - #1529
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RhysSullivan merged 2 commits intoAug 28, 2026
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CI note: the only red check is the 16-minute self-host E2E suite, and it is the same deterministic pre-existing failure on this PR's exact base (
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The agents@0.17.3 patch was rewritten on main by UsefulSoftwareCo#1754 (standalone listeners attach alongside each other instead of superseding, plus the _replayInFlight guard). Re-rolled this branch's change with bun patch on top of main's patched dist rather than merging the patch file textually: the Last-Event-ID branch no longer drains other streams, and the helper is now replayUndeliveredResponsesOnFreshGet, called only from the cursorless GET path. The fresh-GET fallback and its in-flight guard are unchanged.
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Summary
Keep a cursor-bearing MCP recovery GET scoped to the stream identified by its
Last-Event-ID, while preserving the existing cursorless fresh-GET fallback for completed POST responses that otherwise have no recovery cursor.Problem
The Streamable HTTP spec says a server MUST NOT replay messages that would have been delivered on a different stream. The current patched transport first replays the cursor's stream, then appends responses from every other undelivered POST stream to the same recovery GET.
That can lose the response the client actually needs: the TypeScript SDK stops reconnecting after the first JSON-RPC result/error, so an unrelated completed response can make it close before the cursor's own result arrives.
Change
Last-Event-IDbranchreplayUndeliveredResponsesOnFreshGetand call it only when GET has no cursorRegression proof
With the old cross-stream call deliberately restored, the new test fails immediately:
With this fix restored, the full replay E2E file passes all three scenarios.
Validation
bun run check:patchesbun run --cwd packages/hosts/cloudflare test— 61 passedbun run --cwd packages/hosts/cloudflare typecheckbun run --cwd e2e typecheckbunx vitest run --project cloud cloud/mcp-sse-replay.test.ts— 3 passedRelated context: #1345 and #1489.