feat(mcp): set $mcp_interface on mcp analytics events

[gesh]

💡 Motivation and Context

Why: WebMCP events identify their interface, but MCP server events do not. This gap prevents one consistent interface breakdown.

This change sets $mcp_interface to mcp on MCP server events and their $exception siblings. It also adds a patch changeset.

The shared MCP Analytics specification permits this property but does not define it yet.

💚 How did you test it?

• Ran the MCP pipeline test file. All 157 tests passed.
• Ran Ruff format and lint checks.
• Updated and checked the public API snapshot.
• Imported posthog with warnings treated as errors.

📝 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

• Added a Sampo changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Codex in PostHog Desktop implemented and tested this change. The value mcp pairs with the existing webmcp value.

---

Created with PostHog Desktop

[greptile-apps]

[Medium risk] Adds a property to MCP server analytics events.

The PR appears safe to merge.

_{Reviews (1) · Last reviewed commit: "feat(mcp): add interface property to ser..." · Reviewed by Greptile}

[github-actions]

posthog-python Compliance Report

Date: 2026-10-07T12:36:45.119958+00:00
Duration: 259415ms

✅ All Tests Passed!

121/121 tests passed

---

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	515ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	509ms
Required Headers.Has Authorization Bearer Header	✅	508ms
Required Headers.Has Content Type Json	✅	510ms
Required Headers.Has Posthog Sdk Info Format	✅	509ms
Required Headers.Has Posthog Attempt Header	✅	508ms
Required Headers.Has Posthog Request Id	✅	509ms
Required Headers.Has Posthog Request Timestamp	✅	508ms
Required Headers.Has User Agent	✅	509ms
Body Format.Body Has Created At And Batch	✅	508ms
Body Format.No Api Key In Body	✅	509ms
Body Format.No Sent At In Body	✅	508ms
Event Format.Event Has Required Root Fields	✅	508ms
Event Format.Event Uuid Is Valid	✅	509ms
Event Format.Event Timestamp Is Rfc3339	✅	508ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	513ms
Event Format.Distinct Id Is String	✅	508ms
Event Format.Distinct Id At Root Not Properties	✅	509ms
Event Format.Custom Properties Preserved	✅	508ms
Event Format.Set Properties Preserved	✅	509ms
Event Format.Set Once Properties Preserved	✅	508ms
Event Format.Groups Properties Preserved	✅	509ms
Event Format.Sdk Generates Uuid If Not Provided	✅	508ms
Event Format.Event Has Required Root Fields Batch	✅	511ms
Event Format.Event Uuid Is Valid Batch	✅	511ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	511ms
Event Format.Distinct Id Is String Batch	✅	511ms
Event Format.Distinct Id At Root Not Properties Batch	✅	511ms
Event Format.Custom Properties Preserved Batch	✅	511ms
Event Format.Set Properties Preserved Batch	✅	511ms
Event Format.Set Once Properties Preserved Batch	✅	511ms
Event Format.Groups Properties Preserved Batch	✅	511ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	511ms
Batch Behavior.Multiple Events In Single Batch	✅	514ms
Batch Behavior.Batch Envelope Smoke	✅	513ms
Batch Behavior.Flush With No Events Sends Nothing	✅	505ms
Batch Behavior.Flush At Triggers Batch	✅	1009ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	509ms
Deduplication.Generates Unique Uuids	✅	514ms
Deduplication.Different Events Same Content Different Uuids	✅	510ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6514ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6520ms
Deduplication.No Duplicate Events In Batch	✅	515ms
Header Behavior On Retry.Attempt Header Starts At One	✅	509ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13517ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6517ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3018ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6518ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	509ms
Response Format Validation.Success Response Has Ok For Each Event	✅	511ms
Response Format Validation.Success No Retry After When All Ok	✅	511ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1513ms
Response Format Validation.Success No Retry After When Drop Only	✅	511ms
Response Format Validation.Response Echoes Request Id	✅	508ms
Retry Behavior.Retries On 408	✅	6518ms
Retry Behavior.Retries On 500	✅	6514ms
Retry Behavior.Retries On 503	✅	8520ms
Retry Behavior.Retries On 504	✅	6518ms
Retry Behavior.Retryable Errors Have Retry After	✅	3514ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11519ms
Retry Behavior.Does Not Retry On 400	✅	2512ms
Retry Behavior.Does Not Retry On 401	✅	2512ms
Retry Behavior.Does Not Retry On 402	✅	2512ms
Retry Behavior.Does Not Retry On 413	✅	2512ms
Retry Behavior.Does Not Retry On 415	✅	2512ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2511ms
Retry Behavior.Implements Backoff	✅	22522ms
Retry Behavior.Max Retries Respected	✅	22527ms
Partial Batch Handling.Handles 200 Full Success	✅	2511ms
Partial Batch Handling.Handles 200 With All Ok	✅	3516ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3513ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3511ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6519ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6515ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6516ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6515ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6518ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6519ms
Partial Batch Handling.Respects Retry After On Partial	✅	8520ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3514ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3516ms
Compression.Sends Gzip Content Encoding	✅	510ms
Compression.No Content Encoding When Disabled	✅	509ms
Compression.Compressed Body Is Decompressible	✅	509ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2511ms
Event Options.Cookieless Mode Override	✅	508ms
Event Options.Disable Skew Correction Override	✅	509ms
Event Options.Process Person Profile Override	✅	509ms
Event Options.Product Tour Id Override	✅	508ms
Event Options.Unset Options Omitted	✅	509ms
Event Options.Options Override In Batch	✅	511ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	508ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	509ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	508ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	508ms
Routing.Capture Does Not Reroute Ai Named Events	✅	509ms
Identity.Capture Ai Event Has Uuid	✅	509ms
Identity.Capture Ai Keeps Supplied Uuid	✅	508ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	510ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	9ms
Request Payload.Flags Request Uses V2 Query Param	✅	8ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	7ms
Request Payload.Flags Request Omits Authorization Header	✅	8ms
Request Payload.Token In Flags Body Matches Init	✅	7ms
Request Payload.Groups Round Trip	✅	7ms
Request Payload.Groups Default To Empty Object	✅	8ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	7ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	8ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	7ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	508ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	12ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	8ms
Retry Behavior.Retries Flags On 502	✅	311ms
Retry Behavior.Retries Flags On 504	✅	311ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	509ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	54ms
Versioned Boolean Matching.Matching Version 1	✅	51ms
Versioned Boolean Matching.Matching Version 2	✅	52ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	29ms

[stamphog]

Not approved — this change needs a human reviewer.

Re-adding the stamphog label gives the same result unless the changed files change.

Stamphog can't auto-review this pull request because two gates refused it. The deny-list gate matched public_api, since the change adds a new INTERFACE attribute in posthog/mcp/constants.py and a matching line in references/public_api_snapshot.txt. The tier gate classified it as T2-never (a cross-cutting feature across 5 files), which is a category that is never handled automatically.

Size was not the problem, so splitting the PR won't change the outcome. The author should ask a human reviewer, ideally someone who owns the public API surface of the MCP module, to review it.

• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✗	matches: public_api
size	✓	8L, 2F substantive, 16L/5F incl. docs/generated/snapshots — within ceiling
tier	✗	classified as T2-never: T2-never (16L, 5F, cross-cutting, feat)
stamphog 2.4.1		.stamphog/policy.yml @ unknown · reviewed head 5ad2cb9