Summary

HttpServletStreamableServerTransportProvider intermittently answers -32603 "Stream unavailable for session <id>" (wrapped as JSON-RPC internal error / HTTP 500) when it attempts to push a server-initiated request (e.g. roots/list) to a client whose GET /mcp SSE stream has not been registered yet.

Observed failure

• Test: HttpServletStreamableIntegrationTests.testRootsNotificationWithEmptyRootsList(String)[1]
• CI run: https://github.com/modelcontextprotocol/java-sdk/actions/runs/33068920136 (Build and Test)
• Client-side surfacing: java.lang.RuntimeException: Failed to send message: AggregateResponseEvent[...] wrapping
  jsonRpcError: {"code":-32603,"message":"Error processing message: Stream unavailable for session 82454020-4c10-4276-9c30-57c15d49810e"}

Repro shape

1. Client declares the roots capability
2. Client sends initialize; server accepts and creates the session
3. Server immediately pushes roots/list over the session's GET /mcp stream
4. POST traffic racing the first stream registration loses → -32603, stream unavailable

Why we believe it is a race (not environment-specific)

• The same commit passed the parallel "Jackson 2 Integration Tests" job minutes later
• Full local suite (embedded Tomcat 11, JDK 17, macOS) passed repeatedly; only 1 of 43 parameterizations in the class failed on CI
• Related history: revert fb029c8c "Clean up AbstractMcpClientServerIntegrationTests", HttpServletStreamableServerTransportProvider removes session on non-fatal failure #952 (session removed on non-fatal failure), and Stateful streamable server calls listRoots() unconditionally on roots/list_changed, blocking a servlet thread until requestTimeout when no roots consumer is registered #1061 (same roots-push seam in the stateful streamable provider, blocking variant)

Suggested directions

1. Buffer server→client messages per session until a stream registers (short grace window), or
2. Single retry on "stream unavailable" for capability-prompted pushes

A test-side await cannot fix this, since the failure occurs inside the server's push path.

Environment

• main @ b31841ee (via PR feat(transport): add SEP-2243 Mcp-Method / Mcp-Name header mirroring with MCP-Protocol-Version validation #1112 CI), JDK 17, CI ubuntu-latest + local macOS