fix(streamable_http): handle ClientDisconnect during POST at WARNING level by wiggzz · Pull Request #2 · dbt-labs/mcp-python-sdk

wiggzz · 2026-05-05T16:10:56Z

Problem

When a client disconnects mid-POST (network timeout, cancel, LB drop), the MCP SDK's _handle_post_request catches ClientDisconnect in its generic except Exception handler, which:

Logs an ERROR ("Error handling POST request") to Datadog — this is pattern 1, 67% of all prod ERRORs in ai-codegen-api
Synthesizes a 500 response and sends it to the already-closed socket
Does not notify the inner session writer, so stateful session tasks can hang

Solution

Catch ClientDisconnect before the generic except Exception handler in _handle_post_request:

Log at WARNING — honest level: we aborted handling a request, but it wasn't our fault
Notify the session writer with ClientDisconnect() so inner session tasks unblock cleanly (stateful sessions)
Skip sending a response — the socket is closed; sending to it is just noise
Scope: POST only — Datadog confirms 100% of pattern 1 stacks originate from _handle_post_request

Tests

4 new unit tests in tests/server/streamable_http/test_client_disconnect_post.py:

test_client_disconnect_logs_warning_not_error — verifies WARNING, not ERROR
test_client_disconnect_does_not_send_response — verifies no ASGI sends to closed socket
test_client_disconnect_notifies_writer — verifies writer receives ClientDisconnect
test_client_disconnect_writer_suppresses_errors — verifies broken writer doesn't crash us

All existing streamable_http tests (49 + 18) pass.

Upstream context

This patch combines the approach of two open upstream PRs (neither merged):

modelcontextprotocol/python-sdk#1647 — right scope (POST only) but wrong semantics (still sends response, doesn't notify writer)
modelcontextprotocol/python-sdk#1947 — right semantics (notify writer, skip response) but broader scope than needed (covers GET/replay paths)

This is a tight ~10-line change taking the spirit of modelcontextprotocol#1647 + semantics of modelcontextprotocol#1947. Tracking issue: modelcontextprotocol/python-sdk#1648.

Follow-up

After this lands in the fork and is verified in prod via ai-codegen-api bump:

Open mirror PR upstream
Consider dropping ClientDisconnectHandlerMiddleware in ai-codegen-api (no longer load-bearing for POST path)

…otocol#1766)

…tocol#1804)

…#1481) Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

…col#1503) Co-authored-by: Paul Carleton <paulcarletonjr@gmail.com>

…textprotocol#1808) Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

…ntextprotocol#1760)

…otocol#1651) Co-authored-by: Claude <noreply@anthropic.com>

modelcontextprotocol#1827)

…contextprotocol#1828)

…ocol#1833)

…xtprotocol#1826) Co-authored-by: Jay Hemnani <jayhemnani9910@gmail.com> Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

…n) (modelcontextprotocol#1634)

…lcontextprotocol#1829)

…col#1840) Co-authored-by: Jacem Elwaar <jacem@mcpappsbuilders.com>

…delcontextprotocol#1854) Co-authored-by: Claude <noreply@anthropic.com>

…modelcontextprotocol#1866)

…ndling (modelcontextprotocol#1868)

…ntextprotocol#1871)

…xtprotocol#2006)

…client (modelcontextprotocol#2005)

modelcontextprotocol#2010)

…across 1 directory (modelcontextprotocol#2031) Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

…protocol#2035)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

…ross 1 directory (modelcontextprotocol#2036) Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

…args (modelcontextprotocol#1985)

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

…odelcontextprotocol#2048)

…tprotocol#2050)

…protocol#2055)

…ontextprotocol#2047)

… in SSE tests (modelcontextprotocol#2073)

…tprotocol#2056)

…_allowed (modelcontextprotocol#2074)

…tprotocol#2022)

…delcontextprotocol#2038) Co-authored-by: Lee Hubbard <hubbard.zlee@unknowncyber.com> Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

…tocol#2080)

…otocol#2082)

…elcontextprotocol#2089)

…level Catch ClientDisconnect in _handle_post_request so client disconnections (network timeout, cancel, LB drop) log at WARNING instead of ERROR and do not attempt to send a response to the closed socket. This fixes pattern 1 (67% of all prod ERRORs in ai-codegen-api): when the client disconnects mid-POST, the current except-Exception handler synthesizes a 500 response to a closed socket and logs ERROR-level noise to Datadog. Changes: - Catch ClientDisconnect before the generic except-Exception handler - Log at WARNING (honest level: we aborted but it wasn't our fault) - Notify the session writer so inner session tasks unblock cleanly - Suppress errors from writer.send() since the writer may already be closed - Skip sending a response (the socket is gone) Upstream context: - modelcontextprotocol#1647 (POST-only scope, right approach) - modelcontextprotocol#1947 (writer-notification semantics, skip-response) - modelcontextprotocol#1648 (tracking issue, still open, no PRs merged) This patch takes the spirit of modelcontextprotocol#1647 (POST-only scope) + the semantics of modelcontextprotocol#1947 (notify writer, skip response) in a tight ~10-line change. Github-Issue:modelcontextprotocol#1648

wiggzz · 2026-05-05T16:15:47Z

Closing to fix base branch — re-opening with only our commit.

yugannkt and others added 30 commits December 19, 2025 11:17

Add workflow to comment on PRs when released (modelcontextprotocol#1772)

4807eb5

feat: standardize timeout values to floats in seconds (modelcontextpr…

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…otocol#1766)

fix: Include extra field for context log (modelcontextprotocol#1535)

06748eb

docs: update CONTRIBUTING with v2 branching strategy (modelcontextpro…

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…tocol#1804)

fix: Token endpoint response for invalid_client (modelcontextprotocol…

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…#1481) Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

fix: accept HTTP 201 status code in token exchange (modelcontextproto…

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…col#1503) Co-authored-by: Paul Carleton <paulcarletonjr@gmail.com>

fix: return HTTP 404 for unknown session IDs instead of 400 (modelcon…

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…textprotocol#1808) Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

docs: add guidance on discussing features before opening PRs (modelco…

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…ntextprotocol#1760)

Make refresh_token grant type optional in DCR handler (modelcontextpr…

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…otocol#1651) Co-authored-by: Claude <noreply@anthropic.com>

fix: raise clear error for server-to-client requests in stateless mode (

bb6cb02

modelcontextprotocol#1827)

fix: add StatelessModeNotSupported exception and improve tests (model…

37de501

…contextprotocol#1828)

tests: add missing init files (modelcontextprotocol#1831)

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types: add missing py.typed from examples package (modelcontextprot…

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…ocol#1833)

refactor: use __future__.annotations (modelcontextprotocol#1832)

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fix: add explicit type annotation for call_tool decorator (modelconte…

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…xtprotocol#1826) Co-authored-by: Jay Hemnani <jayhemnani9910@gmail.com> Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

Server initialize response update to last spec (add title, descriptio…

3863f20

…n) (modelcontextprotocol#1634)

Add type checker to examples/client (modelcontextprotocol#1837)

1fd557a

Support Python 3.14 (modelcontextprotocol#1834)

3ffe142

docs: fix simple-auth README references to non-existent scripts (mode…

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…lcontextprotocol#1829)

Support for Resource and ResourceTemplate metadata (modelcontextproto…

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…col#1840) Co-authored-by: Jacem Elwaar <jacem@mcpappsbuilders.com>

Add missing TasksCallCapability to enable proper MCP task support (mo…

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…delcontextprotocol#1854) Co-authored-by: Claude <noreply@anthropic.com>

Drop deprecated streamablehttp_client (modelcontextprotocol#1836)

8893b02

chore: drop as _annotations (modelcontextprotocol#1858)

32a4587

docs: add migrations page (modelcontextprotocol#1859)

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Add v2 development warning to README (modelcontextprotocol#1862)

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Drop Content and args parameter in ClientSessionGroup.call_tool (…

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…modelcontextprotocol#1866)

fix(ci): use bash shell for pytest step to ensure consistent error ha…

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…ndling (modelcontextprotocol#1868)

Remove deprecated cursor parameter and ResourceReference (modelco…

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…ntextprotocol#1871)

ci: add weekly lockfile update workflow (modelcontextprotocol#1874)

fc5cb6a

ci: pin all GitHub Actions to commit SHAs (modelcontextprotocol#1875)

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Kludex and others added 27 commits February 7, 2026 14:12

docs: clarify test file structure convention in CLAUDE.md (modelconte…

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…xtprotocol#2006)

fix: send JSONRPCError instead of bare exceptions in streamable HTTP …

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…client (modelcontextprotocol#2005)

fix: pass conformance auth scenarios, add RFC 8707 resource validation (

239d682

modelcontextprotocol#2010)

chore(deps): bump cryptography from 45.0.5 to 46.0.5 in the uv group …

e39038d

…across 1 directory (modelcontextprotocol#2031) Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

feat: add /review-pr Claude Code command for PR reviews (modelcontext…

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…protocol#2035)

Fix leaked anyio streams in streamable_http (modelcontextprotocol#1991)

f049c8e

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

chore(deps-dev): bump pillow from 12.1.0 to 12.1.1 in the uv group ac…

d6d3ad9

…ross 1 directory (modelcontextprotocol#2036) Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

refactor: replace lowlevel Server decorators with on_* constructor kw…

0a22a9d

…args (modelcontextprotocol#1985)

fix: revert README.md to v1 documentation (modelcontextprotocol#2045)

1e0b5c0

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

fix: skip readme-v1-frozen in CI and add diff-based README.md check (m…

29a14ab

…odelcontextprotocol#2048)

docs: add coverage verification instruction to CLAUDE.md (modelcontex…

a287a40

…tprotocol#2050)

fix: explicitly load required pytest plugins in addopts (modelcontext…

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…protocol#2055)

fix: handle HTTP error status codes in streamable HTTP client (modelc…

2fe56e5

…ontextprotocol#2047)

fix: add HTTP readiness check to wait_for_server and remove dead code…

3b53fb9

… in SSE tests (modelcontextprotocol#2073)

fix: allow null id in JSONRPCError per JSON-RPC 2.0 spec (modelcontex…

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…tprotocol#2056)

fix: normalize trailing slashes before length check in check_resource…

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…_allowed (modelcontextprotocol#2074)

Add idle session timeout to StreamableHTTPSessionManager (modelcontex…

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test: fix progress notification assertions for related_request_id (mo…

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…delcontextprotocol#2038) Co-authored-by: Lee Hubbard <hubbard.zlee@unknowncyber.com> Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

refactor: remove unused mcp.shared.progress module (modelcontextpro…

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fix: prevent command injection in example URL opening (modelcontextpr…

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…otocol#2082)

fix: use exact match for loopback hosts in issuer URL validation (mod…

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…elcontextprotocol#2089)

ci: pin all GitHub Actions to commit SHAs (modelcontextprotocol#2088)

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Update SECURITY.md to use GitHub Security Advisories (modelcontextpro…

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docs: fix docstrings across public API surface (modelcontextprotocol#…

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docs: add code fences to Example: docstring blocks (modelcontextpro…

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fix: silence mkdocs social plugin warnings in strict mode (modelconte…

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…xtprotocol#2109)

wiggzz closed this May 5, 2026

wiggzz deleted the wj/DI-3218-clientdisconnect-fix branch May 5, 2026 16:34

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fix(streamable_http): handle ClientDisconnect during POST at WARNING level#2

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