feat(rails): expose flag_definition_cache_provider in PostHog.init block

[seanpem]

Summary

Rails apps couldn't set flag_definition_cache_provider from the PostHog.init block. The core client has accepted the option since #114, but the Rails InitConfig wrapper is an explicit allowlist of setters written in #81 and was never extended, so config.flag_definition_cache_provider = ... raised NoMethodError. The only workaround was the hash form of PostHog.init.

This PR:

• Adds a flag_definition_cache_provider= setter to PostHog::Rails::InitConfig
• Documents the option in the generated initializer template and the mirrored example, pointing at examples/redis_flag_cache.rb
• Regenerates public_api_snapshot.txt
• Adds the first specs covering the block form of PostHog.init: one asserts the provider receives fetched definitions, one asserts the core interface validation still fires
• Adds a posthog-rails minor changeset

Scope is intentionally limited to the cache provider. Other newer core options (feature_flags_async_load, feature_flag_request_max_retries, etc.) are still missing from InitConfig and could get the same treatment in a follow-up.

Test plan

• bundle exec rspec (1263 examples, 0 failures)
• bundle exec rubocop on changed files
• bundle exec rake public_api:check

🤖 Generated with Claude Code

[github-actions]

posthog-ruby-async Compliance Report

Date: 2026-10-13T21:26:38.352677+00:00
Duration: 98355ms

⚠️ 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	✅	111ms
Format Validation.Event Has Uuid	✅	107ms
Format Validation.Event Has Lib Properties	✅	109ms
Format Validation.Distinct Id Is String	✅	107ms
Format Validation.Token Is Present	✅	109ms
Format Validation.Custom Properties Preserved	✅	108ms
Format Validation.Event Has Timestamp	✅	108ms
Format Validation.Non Utc Event Timestamp Is Converted To Utc	✅	10ms
Retry Behavior.Retries On 503	✅	5311ms
Retry Behavior.Does Not Retry On 400	✅	2109ms
Retry Behavior.Does Not Retry On 401	✅	2108ms
Retry Behavior.Respects Retry After Header	✅	8017ms
Retry Behavior.Implements Backoff	✅	15624ms
Retry Behavior.Retries On 500	✅	5212ms
Retry Behavior.Retries On 502	✅	5213ms
Retry Behavior.Retries On 504	✅	5212ms
Retry Behavior.Max Retries Respected	✅	15523ms
Deduplication.Generates Unique Uuids	✅	112ms
Deduplication.Preserves Uuid On Retry	✅	5213ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10412ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5213ms
Deduplication.No Duplicate Events In Batch	✅	112ms
Deduplication.Different Events Have Different Uuids	✅	108ms
Compression.Sends Gzip When Enabled	✅	106ms
Batch Format.Uses Proper Batch Structure	✅	107ms
Batch Format.Flush With No Events Sends Nothing	✅	5ms
Batch Format.Multiple Events Batched Together	✅	110ms
Error Handling.Does Not Retry On 403	✅	2106ms
Error Handling.Does Not Retry On 413	✅	2110ms
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	✅	108ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	109ms
Request Payload.Flags Request Omits Authorization Header	✅	109ms
Request Payload.Token In Flags Body Matches Init	✅	108ms
Request Payload.Groups Round Trip	✅	107ms
Request Payload.Groups Default To Empty Object	✅	108ms
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	✅	107ms
Retry Behavior.Retries Flags On 502	✅	244ms
Retry Behavior.Retries Flags On 504	✅	246ms
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: 2026-10-13T21:26:49.596813+00:00
Duration: 93952ms

⚠️ 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	✅	6ms
Format Validation.Event Has Lib Properties	✅	6ms
Format Validation.Distinct Id Is String	✅	8ms
Format Validation.Token Is Present	✅	6ms
Format Validation.Custom Properties Preserved	✅	6ms
Format Validation.Event Has Timestamp	✅	6ms
Format Validation.Non Utc Event Timestamp Is Converted To Utc	✅	6ms
Retry Behavior.Retries On 503	✅	5258ms
Retry Behavior.Does Not Retry On 400	✅	2008ms
Retry Behavior.Does Not Retry On 401	✅	2010ms
Retry Behavior.Respects Retry After Header	✅	8016ms
Retry Behavior.Implements Backoff	✅	15322ms
Retry Behavior.Retries On 500	✅	5114ms
Retry Behavior.Retries On 502	✅	5141ms
Retry Behavior.Retries On 504	✅	5148ms
Retry Behavior.Max Retries Respected	✅	15542ms
Deduplication.Generates Unique Uuids	✅	15ms
Deduplication.Preserves Uuid On Retry	✅	5145ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10260ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5156ms
Deduplication.No Duplicate Events In Batch	✅	15ms
Deduplication.Different Events Have Different Uuids	✅	7ms
Compression.Sends Gzip When Enabled	✅	4ms
Batch Format.Uses Proper Batch Structure	✅	5ms
Batch Format.Flush With No Events Sends Nothing	✅	3ms
Batch Format.Multiple Events Batched Together	❌	18ms
Error Handling.Does Not Retry On 403	✅	2007ms
Error Handling.Does Not Retry On 413	✅	2006ms
Error Handling.Retries On 408	✅	5115ms

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	✅	7ms
Request Payload.Flags Request Uses V2 Query Param	✅	5ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	6ms
Request Payload.Flags Request Omits Authorization Header	✅	6ms
Request Payload.Token In Flags Body Matches Init	✅	5ms
Request Payload.Groups Round Trip	✅	5ms
Request Payload.Groups Default To Empty Object	✅	5ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	7ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	5ms
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	✅	157ms
Retry Behavior.Retries Flags On 504	✅	112ms
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']

[greptile-apps]

[Medium impact] Adds optional feature flag cache configuration to Rails SDK.

The PR appears safe to merge, with a non-blocking cleanup issue in the new provider test.

_{Reviews (1) · Last reviewed commit: "feat(rails): expose flag_definition_cach..." · Reviewed by Greptile}

[marandaneto]

thanks @seanpem