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Add a `connectGeneration` counter to prevent stale `stateUpdateHandler` closures from resuming an already-completed `CheckedContinuation`. The crash occurred when: 1. connect() modelcontextprotocol#1 succeeds — continuation A is resumed via .ready 2. Connection breaks — receiveLoop calls connection.cancel() 3. cancel() triggers the old stateUpdateHandler, which queues a Task capturing continuation A 4. receiveLoop calls connect() modelcontextprotocol#2, resetting `connectionContinuationResumed` to false and creating continuation B 5. The queued Task from step 3 checks the flag (now false), passes the guard, and tries to resume continuation A — which was already resumed in step 1 → SIGTRAP (CheckedContinuation double-resume assertion) The fix increments a generation counter on each connect() call. Each stateUpdateHandler closure captures its generation, and handler methods verify the generation matches before touching the continuation. Also fixes Swift 6 strict concurrency warnings in ClientTests (mutable local variables captured in Sendable closures). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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This was referenced Sep 10, 2026
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Issue titles are now bare 'Upstream PR#<n>'; the merge/investigate/decline label carries the decision, so the title does not duplicate it. Two places named the old convention and are updated. The investigate count was also stale: it said twelve, which was true at triage on 2026-09-10. Four were declined since (modelcontextprotocol#280, modelcontextprotocol#226, modelcontextprotocol#118, modelcontextprotocol#204) and two were promoted and merged (modelcontextprotocol#266 as entries 9/9a, modelcontextprotocol#275 as 10/10a/10b), leaving six — modelcontextprotocol#178, modelcontextprotocol#213, modelcontextprotocol#216, modelcontextprotocol#257, modelcontextprotocol#258, modelcontextprotocol#259 — now listed by number rather than by count, so the sentence cannot drift again. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Eb9yGSXH1TVkg5Afsu9phk
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Summary
EXC_BREAKPOINT/SIGTRAPcrash caused byCheckedContinuationbeing resumed twice during NetworkTransport reconnectionconnectGenerationcounter that invalidates stalestateUpdateHandlerclosures from previousconnect()cyclesClientTests.swiftRoot cause
When a connection breaks and
receiveLoopinitiates reconnection, it callsconnection.cancel()followed byconnect(). Thecancel()triggers thestateUpdateHandlerfrom the firstconnect()cycle, which queues a Task capturing the old continuation. Meanwhile, the newconnect()resetsconnectionContinuationResumedtofalseand creates a new continuation.When the queued Task from the old handler finally runs, it checks the flag (now
falsedue to the reset), passes the guard, and attempts to resume the old continuation — which was already resumed successfully via.readyin the first cycle. This triggers Swift'sCheckedContinuationdouble-resume assertion (SIGTRAP).Fix
A
connectGenerationcounter (UInt64) is incremented on eachconnect()call and captured in thestateUpdateHandlerclosure. All handler methods (handleConnectionReady,handleConnectionFailed,handleConnectionCancelled) verify the generation matches before proceeding. Stale handlers from previous cycles are logged and safely ignored.Test plan
NetworkTransportTestspass (including the new regression test)testStaleHandlerDoesNotDoubleResumeverifies the specific crash scenario~/Library/Logs/DiagnosticReports/iMCP-2026-03-28-*.ips) showing identical stack traces🤖 Generated with Claude Code