fix(mcp): don't crash stdio servers when rack is missing

[lucasheriques]

Why

`PostHog::MCP.instrument` crashed stdio-only MCP servers at boot when `rack` was not installed. The `mcp` gem autoloads its Streamable HTTP transport, and that file raises `LoadError` without rack. `instrument` only rescues `StandardError`, so the error escaped. Reproduced with posthog-ruby 3.26.2 and mcp 1.7.0.

What changed

• `install_extensions!` now installs the HTTP transport extension in its own method that rescues `LoadError` and logs through `Log.debug`. The server extension is always installed, so stdio capture is unaffected.
• Adds a patch changeset for `posthog-ruby`.

Scope

• Covers only the missing-rack boot crash.
• Does not change how the HTTP transport or `RackMiddleware` behave when rack is present.

Tradeoffs

• The skip logs at debug, not warn. A stdio server without rack is a valid setup, so a warning on every boot would be noise. HTTP users without rack still get the gem's own `LoadError` when they use the transport.

Blast Radius

Only the boot path of `instrument`. Servers with rack installed take the same path as before.

Verification

• New spec runs `instrument` plus a stdio `tools/call` in a subprocess where `require 'rack'` raises. It fails on main with the gem's `LoadError` and passes here.
• `bundle exec rspec`: 1254 examples, 0 failures. `bundle exec rubocop`: clean.

Follow-up: once released, the context-mill `mcp-analytics` skill can drop its `rack` Gemfile workaround and revisit its `posthog-ruby >= 3.26.0` floor.

[github-actions]

posthog-ruby-async Compliance Report

Date: 2026-10-07T17:25:19.525707+00:00
Duration: 98341ms

⚠️ Some Tests Failed

46/47 tests passed, 1 failed

---

Capture Tests

✅ 30/30 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	110ms
Format Validation.Event Has Uuid	✅	107ms
Format Validation.Event Has Lib Properties	✅	111ms
Format Validation.Distinct Id Is String	✅	155ms
Format Validation.Token Is Present	✅	108ms
Format Validation.Custom Properties Preserved	✅	107ms
Format Validation.Event Has Timestamp	✅	107ms
Format Validation.Non Utc Event Timestamp Is Converted To Utc	✅	9ms
Retry Behavior.Retries On 503	✅	5312ms
Retry Behavior.Does Not Retry On 400	✅	2108ms
Retry Behavior.Does Not Retry On 401	✅	2109ms
Retry Behavior.Respects Retry After Header	✅	8017ms
Retry Behavior.Implements Backoff	✅	15622ms
Retry Behavior.Retries On 500	✅	5212ms
Retry Behavior.Retries On 502	✅	5213ms
Retry Behavior.Retries On 504	✅	5212ms
Retry Behavior.Max Retries Respected	✅	15623ms
Deduplication.Generates Unique Uuids	✅	111ms
Deduplication.Preserves Uuid On Retry	✅	5212ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10317ms
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	✅	106ms
Batch Format.Flush With No Events Sends Nothing	✅	5ms
Batch Format.Multiple Events Batched Together	✅	110ms
Error Handling.Does Not Retry On 403	✅	2108ms
Error Handling.Does Not Retry On 413	✅	2109ms
Error Handling.Retries On 408	✅	5212ms

Feature_Flags Tests

⚠️ 16/17 tests passed, 1 failed

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	✅	107ms
Request Payload.Groups Round Trip	✅	107ms
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	✅	108ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	106ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	112ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	108ms
Retry Behavior.Retries Flags On 502	✅	212ms
Retry Behavior.Retries Flags On 504	✅	209ms
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']

[github-actions]

posthog-ruby-sync Compliance Report

Date: 2026-10-07T17:25:21.017382+00:00
Duration: 93948ms

⚠️ Some Tests Failed

45/47 tests passed, 2 failed

---

Capture Tests

⚠️ 29/30 tests passed, 1 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	9ms
Format Validation.Event Has Uuid	✅	5ms
Format Validation.Event Has Lib Properties	✅	5ms
Format Validation.Distinct Id Is String	✅	8ms
Format Validation.Token Is Present	✅	6ms
Format Validation.Custom Properties Preserved	✅	5ms
Format Validation.Event Has Timestamp	✅	6ms
Format Validation.Non Utc Event Timestamp Is Converted To Utc	✅	5ms
Retry Behavior.Retries On 503	✅	5344ms
Retry Behavior.Does Not Retry On 400	✅	2011ms
Retry Behavior.Does Not Retry On 401	✅	2011ms
Retry Behavior.Respects Retry After Header	✅	8017ms
Retry Behavior.Implements Backoff	✅	15297ms
Retry Behavior.Retries On 500	✅	5149ms
Retry Behavior.Retries On 502	✅	5118ms
Retry Behavior.Retries On 504	✅	5145ms
Retry Behavior.Max Retries Respected	✅	15514ms
Deduplication.Generates Unique Uuids	✅	20ms
Deduplication.Preserves Uuid On Retry	✅	5155ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10269ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5126ms
Deduplication.No Duplicate Events In Batch	✅	22ms
Deduplication.Different Events Have Different Uuids	✅	11ms
Compression.Sends Gzip When Enabled	✅	8ms
Batch Format.Uses Proper Batch Structure	✅	8ms
Batch Format.Flush With No Events Sends Nothing	✅	5ms
Batch Format.Multiple Events Batched Together	❌	15ms
Error Handling.Does Not Retry On 403	✅	2006ms
Error Handling.Does Not Retry On 413	✅	2013ms
Error Handling.Retries On 408	✅	5119ms

Failures

batch_format.multiple_events_batched_together

Expected 1 requests, got 5

Feature_Flags Tests

⚠️ 16/17 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	11ms
Request Payload.Flags Request Uses V2 Query Param	✅	9ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	9ms
Request Payload.Flags Request Omits Authorization Header	✅	8ms
Request Payload.Token In Flags Body Matches Init	✅	10ms
Request Payload.Groups Round Trip	✅	10ms
Request Payload.Groups Default To Empty Object	✅	8ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	5ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	6ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	5ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	5ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	8ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	5ms
Retry Behavior.Retries Flags On 502	✅	109ms
Retry Behavior.Retries Flags On 504	✅	111ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	7ms

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']

[greptile-apps]

[Medium risk] Handles missing rack dependency in MCP instrumentation.

The PR appears safe to merge; no actionable issues were found.

_{Reviews (1) · Last reviewed commit: "test(mcp): exercise the real rack LoadEr..." · Reviewed by Greptile}