fix(mcp): record only the tools/list response envelope - #286
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$mcp_tools_list events no longer copy the tool descriptors into $mcp_response. Only the result's non-tools keys with a value (nextCursor, ttlMs, cacheScope, _meta) are recorded, and nothing when there are none. Names stay in $mcp_listed_tool_names. Avoids sanitizing and repeatedly re-truncating large catalogues. Mirrors posthog-js#5160. Tested: bundle exec rspec spec/posthog/mcp (178 examples, 0 failures); bundle exec rspec (1252 examples, 0 failures, 2 pending); bundle exec rubocop (129 files, no offenses); LANG=en_US.UTF-8 bundle exec rake public_api:check (passed). Notes: new specs fail without the change (4 failures). Manual Client#capture_tools_list unchanged. No lib/posthog/mcp/README.md exists and no in-repo doc claims descriptors are captured.
posthog-ruby-sync Compliance ReportDate: 2026-09-30T17:53:22.899318+00:00
|
| Test | Status | Duration |
|---|---|---|
| Format Validation.Event Has Required Fields | ✅ | 10ms |
| Format Validation.Event Has Uuid | ✅ | 6ms |
| Format Validation.Event Has Lib Properties | ✅ | 6ms |
| Format Validation.Distinct Id Is String | ✅ | 9ms |
| Format Validation.Token Is Present | ✅ | 7ms |
| Format Validation.Custom Properties Preserved | ✅ | 7ms |
| Format Validation.Event Has Timestamp | ✅ | 7ms |
| Format Validation.Non Utc Event Timestamp Is Converted To Utc | ✅ | 7ms |
| Retry Behavior.Retries On 503 | ✅ | 5330ms |
| Retry Behavior.Does Not Retry On 400 | ✅ | 2010ms |
| Retry Behavior.Does Not Retry On 401 | ✅ | 2011ms |
| Retry Behavior.Respects Retry After Header | ✅ | 8016ms |
| Retry Behavior.Implements Backoff | ✅ | 15285ms |
| Retry Behavior.Retries On 500 | ✅ | 5116ms |
| Retry Behavior.Retries On 502 | ✅ | 5151ms |
| Retry Behavior.Retries On 504 | ✅ | 5158ms |
| Retry Behavior.Max Retries Respected | ✅ | 15620ms |
| Deduplication.Generates Unique Uuids | ✅ | 23ms |
| Deduplication.Preserves Uuid On Retry | ✅ | 5112ms |
| Deduplication.Preserves Uuid And Timestamp On Retry | ✅ | 10318ms |
| Deduplication.Preserves Uuid And Timestamp On Batch Retry | ✅ | 5121ms |
| Deduplication.No Duplicate Events In Batch | ✅ | 18ms |
| Deduplication.Different Events Have Different Uuids | ✅ | 8ms |
| Compression.Sends Gzip When Enabled | ✅ | 8ms |
| Batch Format.Uses Proper Batch Structure | ✅ | 6ms |
| Batch Format.Flush With No Events Sends Nothing | ✅ | 4ms |
| Batch Format.Multiple Events Batched Together | ❌ | 16ms |
| Error Handling.Does Not Retry On 403 | ✅ | 2007ms |
| Error Handling.Does Not Retry On 413 | ✅ | 2010ms |
| Error Handling.Retries On 408 | ✅ | 5149ms |
Failures
batch_format.multiple_events_batched_together
Expected 1 requests, got 5
Feature_Flags Tests
View Details
| Test | Status | Duration |
|---|---|---|
| Request Payload.Request With Person Properties Device Id | ✅ | 9ms |
| Request Payload.Flags Request Uses V2 Query Param | ✅ | 9ms |
| Request Payload.Flags Request Hits Flags Path Not Decide | ✅ | 8ms |
| Request Payload.Flags Request Omits Authorization Header | ✅ | 7ms |
| Request Payload.Token In Flags Body Matches Init | ✅ | 7ms |
| Request Payload.Groups Round Trip | ✅ | 6ms |
| Request Payload.Groups Default To Empty Object | ✅ | 7ms |
| Request Payload.Disable Geoip False Propagates As Geoip Disable False | ✅ | 6ms |
| Request Payload.Disable Geoip Omitted Defaults To False | ❌ | 7ms |
| Request Payload.Flag Keys To Evaluate Contains Only Requested Key | ✅ | 6ms |
| Request Lifecycle.No Flags Request On Init Alone | ✅ | 3ms |
| Request Lifecycle.No Flags Request On Normal Capture | ✅ | 6ms |
| Request Lifecycle.Two Flag Calls Produce Two Remote Requests | ✅ | 11ms |
| Request Lifecycle.Mock Response Value Is Returned To Caller | ✅ | 6ms |
| Retry Behavior.Retries Flags On 502 | ✅ | 139ms |
| Retry Behavior.Retries Flags On 504 | ✅ | 111ms |
| Side Effect Events.Get Feature Flag Captures Feature Flag Called Event | ✅ | 8ms |
Failures
request_payload.disable_geoip_omitted_defaults_to_false
Field 'geoip_disable' not found in /flags request body at path 'geoip_disable'. Available keys: ['distinct_id', 'groups', 'person_properties', 'group_properties', 'flag_keys_to_evaluate', 'token']
posthog-ruby-async Compliance ReportDate: 2026-09-30T17:53:31.232122+00:00
|
| Test | Status | Duration |
|---|---|---|
| Format Validation.Event Has Required Fields | ✅ | 109ms |
| Format Validation.Event Has Uuid | ✅ | 106ms |
| Format Validation.Event Has Lib Properties | ✅ | 109ms |
| Format Validation.Distinct Id Is String | ✅ | 106ms |
| Format Validation.Token Is Present | ✅ | 108ms |
| Format Validation.Custom Properties Preserved | ✅ | 107ms |
| Format Validation.Event Has Timestamp | ✅ | 106ms |
| Format Validation.Non Utc Event Timestamp Is Converted To Utc | ✅ | 9ms |
| Retry Behavior.Retries On 503 | ✅ | 5311ms |
| Retry Behavior.Does Not Retry On 400 | ✅ | 2109ms |
| Retry Behavior.Does Not Retry On 401 | ✅ | 2109ms |
| Retry Behavior.Respects Retry After Header | ✅ | 8115ms |
| Retry Behavior.Implements Backoff | ✅ | 15613ms |
| Retry Behavior.Retries On 500 | ✅ | 5212ms |
| Retry Behavior.Retries On 502 | ✅ | 5213ms |
| Retry Behavior.Retries On 504 | ✅ | 5212ms |
| Retry Behavior.Max Retries Respected | ✅ | 15522ms |
| Deduplication.Generates Unique Uuids | ✅ | 111ms |
| Deduplication.Preserves Uuid On Retry | ✅ | 5211ms |
| Deduplication.Preserves Uuid And Timestamp On Retry | ✅ | 10314ms |
| Deduplication.Preserves Uuid And Timestamp On Batch Retry | ✅ | 5215ms |
| Deduplication.No Duplicate Events In Batch | ✅ | 113ms |
| Deduplication.Different Events Have Different Uuids | ✅ | 109ms |
| Compression.Sends Gzip When Enabled | ✅ | 107ms |
| Batch Format.Uses Proper Batch Structure | ✅ | 107ms |
| Batch Format.Flush With No Events Sends Nothing | ✅ | 4ms |
| Batch Format.Multiple Events Batched Together | ✅ | 110ms |
| Error Handling.Does Not Retry On 403 | ✅ | 2109ms |
| Error Handling.Does Not Retry On 413 | ✅ | 2109ms |
| Error Handling.Retries On 408 | ✅ | 5212ms |
Feature_Flags Tests
View Details
| Test | Status | Duration |
|---|---|---|
| Request Payload.Request With Person Properties Device Id | ✅ | 108ms |
| Request Payload.Flags Request Uses V2 Query Param | ✅ | 107ms |
| Request Payload.Flags Request Hits Flags Path Not Decide | ✅ | 108ms |
| Request Payload.Flags Request Omits Authorization Header | ✅ | 107ms |
| Request Payload.Token In Flags Body Matches Init | ✅ | 106ms |
| Request Payload.Groups Round Trip | ✅ | 106ms |
| Request Payload.Groups Default To Empty Object | ✅ | 107ms |
| Request Payload.Disable Geoip False Propagates As Geoip Disable False | ✅ | 107ms |
| Request Payload.Disable Geoip Omitted Defaults To False | ❌ | 107ms |
| Request Payload.Flag Keys To Evaluate Contains Only Requested Key | ✅ | 107ms |
| Request Lifecycle.No Flags Request On Init Alone | ✅ | 2ms |
| Request Lifecycle.No Flags Request On Normal Capture | ✅ | 107ms |
| Request Lifecycle.Two Flag Calls Produce Two Remote Requests | ✅ | 111ms |
| Request Lifecycle.Mock Response Value Is Returned To Caller | ✅ | 107ms |
| Retry Behavior.Retries Flags On 502 | ✅ | 211ms |
| Retry Behavior.Retries Flags On 504 | ✅ | 254ms |
| Side Effect Events.Get Feature Flag Captures Feature Flag Called Event | ✅ | 109ms |
Failures
request_payload.disable_geoip_omitted_defaults_to_false
Field 'geoip_disable' not found in /flags request body at path 'geoip_disable'. Available keys: ['distinct_id', 'groups', 'person_properties', 'group_properties', 'flag_keys_to_evaluate', 'token']
Contributor
|
[Medium risk] Changes what telemetry data gets recorded for tool listings. The PR appears safe to merge, though pagination coverage should be restored. Reviews (2) · Last reviewed commit: "fix(mcp): drop resultType from the tools..." |
![greptile-apps[bot]](https://avatars.githubusercontent.com/in/867647?s=60&v=4){kind=small}
…specs Review follow-up. On the 2026-07-28 wire the gem stamps resultType (a lifecycle discriminator) on every reply before PostHog's hook sees it, so it reached $mcp_response on every event. It is now dropped with tools. Key matching uses key.to_s, which also covers a hash holding both :tools and 'tools'. The four tools/list specs are one table: cache hints with symbol and string keys, only tools, and the modern wire, where the gem's ttlMs/cacheScope defaults stay because they are what the client receives. Tested: bundle exec rspec (see PR), rubocop clean, public_api:check passed.
lucasheriques
marked this pull request as ready for review
September 30, 2026 16:51
pauldambra
approved these changes
Sep 30, 2026
Review follow-up (Greptile). The spec table lost the only check that a paginated tools/list records its nextCursor and lists only the current page's names. Added it back as a row. Tested: bundle exec rspec spec/posthog/mcp passes; rubocop clean.
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Why
$mcp_tools_listevents copied the wholetools/listresult into$mcp_response, including every tool descriptor with its full input schema. On a large catalogue that copy goes through sanitization and the truncation ladder on every list, and nothing in PostHog reads it.$mcp_listed_tool_namesalready carries every tool name.This mirrors PostHog/posthog-js#5160 and PostHog/posthog-python#994.
Scope
dispatch_tools_listpassestools_list_envelope(result)as the response. The envelope is every key with a value excepttoolsandresultType(nextCursor,ttlMs,cacheScope,_meta), for string or symbol keys.$mcp_response.$mcp_listed_tool_names, and the manualClient#capture_tools_listare unchanged.Tradeoffs
On the 2026-07-28 wire the gem stamps
resultType,ttlMs: 0andcacheScope: "private"before PostHog's hook sees the result.resultTypeis a lifecycle discriminator, so it is dropped. The cache hints stay because they are what the client receives, so an unconfigured Ruby server on the modern wire records{ttlMs: 0, cacheScope: "private"}where Python records nothing. Filtering them would mean copying the gem's default values into this SDK.Blast Radius
Only the
$mcp_responseproperty of$mcp_tools_listchanges. No PostHog query or insight reads the tool list from it.Verification
bundle exec rspec: 1252 examples, 0 failures, 2 pending. The tools/list table covers symbol keys, string keys, only tools, and the modern wire.bundle exec rubocop: no offenses.bundle exec rake public_api:check: passed.