feat(mcp): emit $mcp_protocol_version on events

[gesh]

What

Emit $mcp_protocol_version (the MCP spec version, e.g. 2025-06-18) on MCP analytics events — closing a parity gap with the TypeScript SDK, which already emits it.

Why it was missing

The session token already carried the protocol version (pv), but it never reached events. Root cause: capture_event's normalizer rebuilds the event from a fixed key list that omitted protocol_version, so it was silently dropped before reaching the property builder.

Changes

• constants / _posthog_events: add $mcp_protocol_version, emitted on the primary $mcp_* event and the $exception sibling.
• _capture: the event normalizer now keeps protocol_version (the missing link).
• instrument() adapters: read the live protocol version off client_params, and backfill from the replayed token on stateless pods (via resolve_session_and_client), threading it through prepare_request / record_*.
• PostHogMCP: capture_tool_call / capture_initialize / capture_tools_list / capture_missing_capability gain a protocol_version argument.

Validation

141 unit tests (incl. new primary+$exception emission tests), mypy + ruff clean, public-API snapshot updated. Verified on the 2-pod stateless cluster: $mcp_protocol_version=2025-06-18 lands on $mcp_initialize and every $mcp_tool_call across both pods (recovered from the token on the pod that never saw initialize).

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Created with PostHog Code

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posthog-python Compliance Report

Date: 2026-07-28 10:08:51 UTC
Duration: 338856ms

✅ All Tests Passed!

111/111 tests passed

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Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	516ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	1009ms
Required Headers.Has Authorization Bearer Header	✅	1008ms
Required Headers.Has Content Type Json	✅	1009ms
Required Headers.Has Posthog Sdk Info Format	✅	1008ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
Required Headers.Has Posthog Request Id	✅	1008ms
Required Headers.Has Posthog Request Timestamp	✅	1007ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1009ms
Body Format.No Api Key In Body	✅	1008ms
Body Format.No Sent At In Body	✅	1008ms
Event Format.Event Has Required Root Fields	✅	1009ms
Event Format.Event Uuid Is Valid	✅	1008ms
Event Format.Event Timestamp Is Rfc3339	✅	1008ms
Event Format.Distinct Id Is String	✅	1009ms
Event Format.Distinct Id At Root Not Properties	✅	1008ms
Event Format.Custom Properties Preserved	✅	1009ms
Event Format.Set Properties Preserved	✅	1008ms
Event Format.Set Once Properties Preserved	✅	1008ms
Event Format.Groups Properties Preserved	✅	1010ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1008ms
Event Format.Event Has Required Root Fields Batch	✅	1013ms
Event Format.Event Uuid Is Valid Batch	✅	1012ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1013ms
Event Format.Distinct Id Is String Batch	✅	1012ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1012ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1012ms
Event Format.Set Once Properties Preserved Batch	✅	1013ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1011ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1011ms
Deduplication.Generates Unique Uuids	✅	1506ms
Deduplication.Different Events Same Content Different Uuids	✅	1507ms
Deduplication.Preserves Uuid On Retry	✅	7513ms
Deduplication.Preserves Timestamp On Retry	✅	7514ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7511ms
Deduplication.No Duplicate Events In Batch	✅	1504ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1008ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14523ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7505ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3517ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7514ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1007ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1507ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2510ms
Response Format Validation.Success No Retry After When Drop Only	✅	1507ms
Response Format Validation.Response Echoes Request Id	✅	1009ms
Retry Behavior.Retries On 408	✅	7513ms
Retry Behavior.Retries On 500	✅	7512ms
Retry Behavior.Retries On 503	✅	9517ms
Retry Behavior.Retries On 504	✅	7512ms
Retry Behavior.Retryable Errors Have Retry After	✅	4509ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12518ms
Retry Behavior.Does Not Retry On 400	✅	3503ms
Retry Behavior.Does Not Retry On 401	✅	3508ms
Retry Behavior.Does Not Retry On 402	✅	3508ms
Retry Behavior.Does Not Retry On 413	✅	3507ms
Retry Behavior.Does Not Retry On 415	✅	3508ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3506ms
Retry Behavior.Implements Backoff	✅	23525ms
Retry Behavior.Max Retries Respected	✅	23523ms
Partial Batch Handling.Handles 200 Full Success	✅	3007ms
Partial Batch Handling.Handles 200 With All Ok	✅	4505ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4509ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4509ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7510ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7511ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7516ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7512ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7512ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7512ms
Partial Batch Handling.Respects Retry After On Partial	✅	9511ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4506ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4506ms
Compression.Sends Gzip Content Encoding	✅	1010ms
Compression.No Content Encoding When Disabled	✅	1010ms
Compression.Compressed Body Is Decompressible	✅	1009ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1007ms
Event Options.Disable Skew Correction Override	✅	1009ms
Event Options.Process Person Profile Override	✅	1008ms
Event Options.Product Tour Id Override	✅	1008ms
Event Options.Unset Options Omitted	✅	1009ms
Event Options.Options Override In Batch	✅	1012ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1009ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1009ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1008ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1007ms
Request Payload.Flags Request Uses V2 Query Param	✅	1007ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1007ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1007ms
Request Payload.Groups Round Trip	✅	1006ms
Request Payload.Groups Default To Empty Object	✅	1006ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1006ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1007ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1007ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1010ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1002ms
Retry Behavior.Retries Flags On 502	✅	1007ms
Retry Behavior.Retries Flags On 504	✅	1006ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1509ms

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