feat: extend OpenFeature numeric, payload, and reason support

[marandaneto]

💡 Motivation and Context

Stacked on #214. This PR targets posthog/openfeature-provider, not main, so #214 can merge independently. After #214 merges, retarget this follow-up to main.

The cross-provider comparison in #214 identified three additions:

• Accept unsigned hexadecimal numeric variants such as 0x10 and 0Xff. Invalid and signed hexadecimal strings remain type mismatches.
• Return JSON array payloads, including empty and nested arrays. The upstream OpenFeature SDK still requires a map default (%{}), even when get_map_value/4 returns an array. This restriction is documented in the module docs and a code comment beside resolve_map_value/4, and tested through the real client.
• Expose flag_metadata["posthog_reason"], using the PostHog description, code, or existing string reason. Explicit server flag_disabled codes map off results to :disabled. Other off results stay :default. Type-mismatch details retain available reason metadata while preserving the error code and caller default. This uses a stable code rather than Python's description substring check.

Error-hook behavior is unchanged and tracked separately in #215. There is no OpenFeature-specific SDK spec.

Related work: PostHog/posthog-python#980 and PostHog/posthog-js#5099.

💚 How did you test it?

• Five additional regression tests reproduced missing metadata on type mismatches before the fix. All 75 provider tests now pass.
• Added regression tests first. Nineteen tests failed before implementation for hexadecimal parsing, array payloads, and reason metadata.
• mix test: 596 passed, 18 integration tests excluded.
• mix format --check-formatted, MIX_ENV=test mix compile --warnings-as-errors, and mix credo --strict passed.
• Isolated autoreview of the final branch against origin/posthog/openfeature-provider reported no actionable findings.

Checks ran on Elixir 1.20.4 / OTP 29. No live PostHog project was used.

📝 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

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with Pi using local file editing, shell tests, and the isolated autoreview helper. The user requested a separate follow-up so the initial provider PR can merge without waiting for these additions. The upstream map-default restriction is documented rather than changing the OpenFeature dependency. No shared session link is available.

[greptile-apps]

[Medium risk] Extends OpenFeature flag resolution to handle more data types.

The new change appears safe to merge, though the previously reported missing reason metadata on type mismatches remains.

_{Reviews (2) · Last reviewed commit: "docs: explain upstream OpenFeature array..."}

[github-actions]

posthog-elixir Compliance Report

Date: 2026-09-25T06:33:47.152904+00:00
Duration: 108660ms

⚠️ Some Tests Failed

40/47 tests passed, 7 failed

---

Capture Tests

⚠️ 29/30 tests passed, 1 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	609ms
Format Validation.Event Has Uuid	✅	610ms
Format Validation.Event Has Lib Properties	✅	610ms
Format Validation.Distinct Id Is String	✅	610ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	610ms
Format Validation.Event Has Timestamp	✅	610ms
Format Validation.Non Utc Event Timestamp Is Converted To Utc	❌	610ms
Retry Behavior.Retries On 503	✅	5616ms
Retry Behavior.Does Not Retry On 400	✅	2611ms
Retry Behavior.Does Not Retry On 401	✅	2611ms
Retry Behavior.Respects Retry After Header	✅	5617ms
Retry Behavior.Implements Backoff	✅	15624ms
Retry Behavior.Retries On 500	✅	5616ms
Retry Behavior.Retries On 502	✅	5616ms
Retry Behavior.Retries On 504	✅	5616ms
Retry Behavior.Max Retries Respected	✅	15625ms
Deduplication.Generates Unique Uuids	✅	623ms
Deduplication.Preserves Uuid On Retry	✅	5615ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10611ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5618ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	612ms
Compression.Sends Gzip When Enabled	✅	610ms
Batch Format.Uses Proper Batch Structure	✅	610ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	615ms
Error Handling.Does Not Retry On 403	✅	2612ms
Error Handling.Does Not Retry On 413	✅	2613ms
Error Handling.Retries On 408	✅	5616ms

Failures

format_validation.non_utc_event_timestamp_is_converted_to_utc

Event 0 field 'timestamp' instant '2026-09-25T06:32:03.085966Z' != expected '2025-01-02T03:04:05Z'

Feature_Flags Tests

⚠️ 11/17 tests passed, 6 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	8ms
Request Payload.Flags Request Uses V2 Query Param	✅	7ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	6ms
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	✅	7ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	609ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	12ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	6ms
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	❌	609ms

Failures

request_payload.groups_default_to_empty_object

Field 'groups' not found in /flags request body at path 'groups'. Available keys: ['api_key', 'distinct_id', 'flag_keys_to_evaluate']

request_payload.disable_geoip_omitted_defaults_to_false

Field 'geoip_disable' not found in /flags request body at path 'geoip_disable'. Available keys: ['api_key', 'distinct_id', 'flag_keys_to_evaluate']

request_lifecycle.mock_response_value_is_returned_to_caller

Last action result missing field 'value'. Keys: ['error', 'success']

retry_behavior.retries_flags_on_502

Last action result missing field 'value'. Keys: ['error', 'success']

retry_behavior.retries_flags_on_504

Last action result missing field 'value'. Keys: ['error', 'success']

side_effect_events.get_feature_flag_captures_feature_flag_called_event

Expected 1 events with name '$feature_flag_called', got 0

[marandaneto]

raised open-feature/elixir-sdk#22