fix(mcp): guard against duplicate serve() on streamable-http MCP servers - #8435
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thinhlpg merged 2 commits intoJul 15, 2026
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Remote streamable-http MCP servers (type "http") intermittently failed to connect with `400 Bad Request ... when send initialize request`, or connected then dropped a few seconds later. Root cause: `start_mcp_server` had no idempotency guard, so a server could be serve()'d twice (e.g. a rename fires activate + syncServersAndRestart, and `restart_active_mcp_servers` restarts active servers without clearing them first). Each serve() spins up an independent rmcp client that sends its own `initialize` (both with request id 0); the second is rejected by the server as a duplicate on an already-initialized session, tearing down the connection. Fix: make `start_mcp_server` idempotent — skip if the server is already running or a start is already in flight (tracked via a new `mcp_starting` set on AppState), clearing the in-flight marker when the attempt completes. The reconnect path (`schedule_mcp_start_task`) is intentionally left unguarded so auto-reconnect can still re-serve. Fixes janhq#8411
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qnixsynapse
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Address review: the idempotency-guard skip messages are routine, not noteworthy, so log them at debug instead of info.
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Problem
Fixes #8411
Remote streamable-HTTP MCP servers (
type: "http") intermittently fail to connect with:or connect (UI turns green) then drop a few seconds later. Server-side logs show Jan sending the
initializeJSON-RPC request twice with the same request id0; the first succeeds, the second is rejected as a duplicate on an already-initialized session.Root cause
start_mcp_serverhad no idempotency guard, so a server could beserve()'d twice. rmcp'sserve()creates a fresh client that sendsinitializeexactly once (request id starts at 0 per client) — so twoinitializewith id 0 means twoserve()calls, not an rmcp retry.Two starts happen in practice because
restart_active_mcp_servers(helpers.rs) restarts every active server without clearing it from the running map first, and several frontend flows fireactivateand a restart-all for one action (e.g. renaming a server:activate+syncServersAndRestart). The twoserve()calls each POSTinitialize; the second gets400.Fix
Make
start_mcp_serveridempotent (3 files, +32/-2):mcp_starting: HashSet<String>onAppState; cleared when the attempt finishes (success or failure).The reconnect path (
schedule_mcp_start_task) is intentionally left unguarded so health-check auto-reconnect can still re-serve.Verification
Tested locally against a mock streamable-HTTP server that mimics the remote (first
initialize→ 200 + session id, duplicate → 400), with injected latency to widen the race window:initialize→200+400 DUPLICATE.MCP server <name> start already in progress; skipping duplicate start; the mock receives exactly oneinitialize, no400, connection stays up.initializeand remain connected.🤖 Generated with Claude Code