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fix(mcp): warn when stateless session middleware never attached - #856

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fix(mcp): warn when stateless session middleware never attached#856
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@posthog

@posthogposthogBot commented Aug 7, 2026

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💡 Motivation and Context

  • A customer on an older MCP spec lost weeks of analytics sessions because PostHogMcpStatelessSessionMiddlewaresilently never attached — and nothing in the SDK said so. It cost two support round trips and a full SDK release that didn't fix their issue.
  • The mint is zero-config only for an ASGI app built afterinstrument() runs. autowire_stateless_mint monkey-patches the app factories on the server, so an app built or mounted beforeinstrument() (the common FastAPI-mounts-at-import case) gets no middleware at all.
  • The failure was completely dark: wiring errors were swallowed into a log line, and a missing session just fell through resolve_session_id to the per-process generated branch. Classic works-in-dev (mcp.run() calls the patched factory), dark-in-prod (FastAPI mounts its own app).
  • This is a wiring problem, not the separate MCP 2026-07-28 spec break — that one is covered by draft PR feat(mcp): support the MCP 2026-07-28 spec and mcp 2.x SDK #830. This PR is additive to feat(mcp): support the MCP 2026-07-28 spec and mcp 2.x SDK #830 and touches asgi.py (which feat(mcp): support the MCP 2026-07-28 spec and mcp 2.x SDK #830 does not); the only shared file is session.py, which this PR deliberately leaves untouched to avoid conflict.

💚 How did you test it?

Added unit + end-to-end tests, and reproduced the customer's ordering trap against a real stateless FastMCP streamable-HTTP transport.

  • Runtime signal, warn-once — HTTP request, no token, no Mcp-Session-Id → generated session fires exactly one warning; repeat requests stay quiet.
  • No false positives — stdio (no HTTP request) and a correctly-wired server that replays our token both stay silent.
  • instrument-time signal — building streamable_http_app() before instrument() triggers the warning.
  • Reproduced end-to-end: with the app built before instrument(), a tools/call with no session header fires both warnings; the correctly-ordered path fires neither.
  • Full MCP suite: 147 passed, 1 skipped. ruff check / ruff format clean.

Two independent signals now surface the failure (either would have ended the ticket on day one):

SignalWhenPoints to
instrument() warningstreamable_http_app() was already called before instrument() ranmanual add_middleware + posthog/mcp/README.md
Runtime warn-oncefirst HTTP tool call resolves with no session on a stateless serversame

Docs: the manual app.add_middleware(PostHogMcpStatelessSessionMiddleware) path now lives in a findable posthog/mcp/README.md, plus a sharpened note in examples/mcp_stateless.py.

📝 Checklist

  • I reviewed the submitted code.
  • I added tests to verify the changes.
  • I updated the docs if needed.
  • No breaking change or entry added to the changelog.

If releasing new changes

  • Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

  • Authored by Claude Code (Opus 4.8) driving the PostHog self-driving harness, from an inbox report.
  • Root-caused the silent failure to autowire only affecting factories consumed after instrument(). Chose two complementary detectors: an instrument-time probe of FastMCP's lazily-created _session_manager (set once streamable_http_app() has run) and a runtime warn-once gated on http_request AND no token AND no mcp_session_id AND session_source == "generated" — the precise fingerprint that can't fire for stdio or correctly-wired servers.
  • Deliberately did not touch posthog/mcp/session.py so this stays conflict-free with draft feat(mcp): support the MCP 2026-07-28 spec and mcp 2.x SDK #830; plumbed a new http_request flag through prepare_request and the adapters instead.

Created with PostHog Desktop from this inbox report.

@posthog
fix(mcp): warn when stateless session middleware never attached
32c8acd
The stateless-session mint (PostHogMcpStatelessSessionMiddleware) is
zero-config only when the ASGI app is built after instrument() runs.
An app built or mounted earlier (the common FastAPI case) silently gets
no middleware, so every session falls back to a fragmented per-process
id with nothing in the SDK saying so.
Make the failure loud with two independent signals:
- instrument() warns when streamable_http_app() was already called
before it ran (FastMCP's cached _session_manager is the tell).
- A one-time runtime warning fires when a tool call arrives over HTTP
with no session id and resolution falls back to a generated session.
Both point to the manual fix, app.add_middleware(...), now documented
in posthog/mcp/README.md. No behavior change on correctly-wired servers
or on stdio.
Generated-By: PostHog Code
Task-Id: 9efb38cb-2672-4236-8da6-208e4f83f686
@github-actions

github-actionsBot commented Aug 7, 2026•
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posthog-python Compliance Report

Date: 2026-08-07 13:25:20 UTC
Duration: 256384ms

✅ All Tests Passed!

111/111 tests passed

Capture_V1 Tests

✅ 94/94 tests passed

View Details
TestStatusDuration
Endpoint And Method.Targets V1 Endpoint✅516ms
Endpoint And Method.Does Not Use Legacy Endpoints✅510ms
Required Headers.Has Authorization Bearer Header✅510ms
Required Headers.Has Content Type Json✅510ms
Required Headers.Has Posthog Sdk Info Format✅511ms
Required Headers.Has Posthog Attempt Header✅510ms
Required Headers.Has Posthog Request Id✅510ms
Required Headers.Has Posthog Request Timestamp✅510ms
Required Headers.Has User Agent✅510ms
Body Format.Body Has Created At And Batch✅510ms
Body Format.No Api Key In Body✅510ms
Body Format.No Sent At In Body✅510ms
Event Format.Event Has Required Root Fields✅511ms
Event Format.Event Uuid Is Valid✅510ms
Event Format.Event Timestamp Is Rfc3339✅510ms
Event Format.Distinct Id Is String✅510ms
Event Format.Distinct Id At Root Not Properties✅511ms
Event Format.Custom Properties Preserved✅510ms
Event Format.Set Properties Preserved✅510ms
Event Format.Set Once Properties Preserved✅511ms
Event Format.Groups Properties Preserved✅511ms
Event Format.Sdk Generates Uuid If Not Provided✅510ms
Event Format.Event Has Required Root Fields Batch✅514ms
Event Format.Event Uuid Is Valid Batch✅514ms
Event Format.Event Timestamp Is Rfc3339 Batch✅513ms
Event Format.Distinct Id Is String Batch✅514ms
Event Format.Distinct Id At Root Not Properties Batch✅514ms
Event Format.Custom Properties Preserved Batch✅513ms
Event Format.Set Properties Preserved Batch✅514ms
Event Format.Set Once Properties Preserved Batch✅513ms
Event Format.Groups Properties Preserved Batch✅514ms
Event Format.Sdk Generates Uuid If Not Provided Batch✅514ms
Batch Behavior.Multiple Events In Single Batch✅518ms
Batch Behavior.Batch Envelope Smoke✅516ms
Batch Behavior.Flush With No Events Sends Nothing✅506ms
Batch Behavior.Flush At Triggers Batch✅1011ms
Batch Behavior.Created At Reflects Batch Creation Time✅511ms
Deduplication.Generates Unique Uuids✅518ms
Deduplication.Different Events Same Content Different Uuids✅512ms
Deduplication.Preserves Uuid On Retry✅6519ms
Deduplication.Preserves Timestamp On Retry✅6519ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry✅6519ms
Deduplication.No Duplicate Events In Batch✅518ms
Header Behavior On Retry.Attempt Header Starts At One✅510ms
Header Behavior On Retry.Attempt Header Increments On Retry✅13524ms
Header Behavior On Retry.Request Id Preserved On Retry✅6520ms
Header Behavior On Retry.Different Requests Have Different Request Ids✅3020ms
Header Behavior On Retry.Request Timestamp Changes On Retry✅6520ms
Response Format Validation.Success Response Has Uuid Keyed Results✅511ms
Response Format Validation.Success Response Has Ok For Each Event✅514ms
Response Format Validation.Success No Retry After When All Ok✅513ms
Response Format Validation.Success Retry After Present When Retry Events✅1515ms
Response Format Validation.Success No Retry After When Drop Only✅513ms
Response Format Validation.Response Echoes Request Id✅510ms
Retry Behavior.Retries On 408✅6518ms
Retry Behavior.Retries On 500✅6520ms
Retry Behavior.Retries On 503✅8523ms
Retry Behavior.Retries On 504✅6519ms
Retry Behavior.Retryable Errors Have Retry After✅3518ms
Retry Behavior.Respects Retry After On Retryable Error✅11516ms
Retry Behavior.Does Not Retry On 400✅2512ms
Retry Behavior.Does Not Retry On 401✅2513ms
Retry Behavior.Does Not Retry On 402✅2513ms
Retry Behavior.Does Not Retry On 413✅2513ms
Retry Behavior.Does Not Retry On 415✅2514ms
Retry Behavior.Non Retryable Errors Have No Retry After✅2513ms
Retry Behavior.Implements Backoff✅22528ms
Retry Behavior.Max Retries Respected✅22526ms
Partial Batch Handling.Handles 200 Full Success✅2512ms
Partial Batch Handling.Handles 200 With All Ok✅3518ms
Partial Batch Handling.Does Not Retry Dropped Events✅3516ms
Partial Batch Handling.Does Not Retry Limited Events✅3516ms
Partial Batch Handling.Prunes Ok Events On Partial Retry✅6522ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry✅6516ms
Partial Batch Handling.Retries Only Retry Events From Partial✅6523ms
Partial Batch Handling.Partial Retry Preserves Uuids✅6517ms
Partial Batch Handling.Partial Retry Attempt Header Increments✅6521ms
Partial Batch Handling.Partial Retry Request Id Preserved✅6521ms
Partial Batch Handling.Respects Retry After On Partial✅8522ms
Partial Batch Handling.Unknown Result Treated As Terminal✅3516ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry✅3520ms
Compression.Sends Gzip Content Encoding✅511ms
Compression.No Content Encoding When Disabled✅510ms
Compression.Compressed Body Is Decompressible✅510ms
Error Handling.Does Not Retry On Unknown 4Xx✅2511ms
Event Options.Cookieless Mode Override✅511ms
Event Options.Disable Skew Correction Override✅510ms
Event Options.Process Person Profile Override✅510ms
Event Options.Product Tour Id Override✅510ms
Event Options.Unset Options Omitted✅509ms
Event Options.Options Override In Batch✅513ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties✅510ms
Geoip And Historical Migration.Historical Migration Set In Body✅510ms
Geoip And Historical Migration.Historical Migration Absent By Default✅509ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details
TestStatusDuration
Request Payload.Request With Person Properties Device Id✅11ms
Request Payload.Flags Request Uses V2 Query Param✅8ms
Request Payload.Flags Request Hits Flags Path Not Decide✅9ms
Request Payload.Flags Request Omits Authorization Header✅10ms
Request Payload.Token In Flags Body Matches Init✅8ms
Request Payload.Groups Round Trip✅9ms
Request Payload.Groups Default To Empty Object✅9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False✅9ms
Request Payload.Disable Geoip Omitted Defaults To False✅9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key✅9ms
Request Lifecycle.No Flags Request On Init Alone✅4ms
Request Lifecycle.No Flags Request On Normal Capture✅508ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests✅14ms
Request Lifecycle.Mock Response Value Is Returned To Caller✅10ms
Retry Behavior.Retries Flags On 502✅312ms
Retry Behavior.Retries Flags On 504✅312ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event✅511ms

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