fix: honor filters.holdout in local flag evaluation

[rubychilds]

💡 Motivation and Context

A user in an experiment holdout still gets bucketed into a regular variant when their flag is evaluated locally. The team running that experiment believes a population is excluded from it, and it is not.

• The server resolves the holdout before release conditions and returns holdout-<id> (flag_matching.rs).
• Local evaluation ignored filters.holdout and returned control or test instead.
• Nothing surfaced the difference. The SDK raised no error and did not fall back to /flags, so the holdout failed silently.
• /local_evaluation already sends the holdout inside the flag's filters, so no server-side data was missing.

🔧 What changed

• match_feature_flag_properties resolves the holdout before release conditions and returns holdout-<id>, so a held-out value never reaches the flag's targeting.
• _holdout_hash hashes holdout-<bucketing_value>. The existing _hash helper cannot be reused because it joins key and value with a dot, which selects a different population while still looking uniform and deterministic.
• Three server behaviors are mirrored deliberately, because each one is silent when wrong: an exclusion percentage of 100 short-circuits before hashing, the threshold comparison is inclusive, and the percentage is clamped to 0-100.
• Both helpers are underscore-private, so the public API snapshot does not change.

💚 How did you test it?

Four tests in TestLocalEvaluation. Three of them fail on main and pass with this change; the fourth (a 0% holdout excludes nobody) passes either way and is there as a control.

The parity test is the one that matters. It pins the exact membership set for a 20% holdout against hashes computed from the server's algorithm, and asserts that the dot-joined variant produces a different set. Without that second assertion the test would pass with the dot bug present.

Suites run

posthog/test/test_feature_flags.py and posthog/test/test_evaluate_flags.py, 276 passed.

Not run: mypy, make public_api_check, and the rest of the suite.

📝 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

Note

The changeset was written by hand into .sampo/changesets/, not generated by sampo add.

Warning

This changes bucketing for flags that carry a holdout. On upgrade, users a holdout should always have excluded start being excluded, so variant assignment shifts for that share of traffic. The new behavior matches the server, so it removes a divergence rather than creating one.

Docs are unchanged here, but the holdouts page does not say which SDK versions honor holdouts locally. Worth adding once this ships.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Investigated and written with Claude Code, model claude-opus-5[1m]. Skill invoked: writing-pr-descriptions.

The first approach considered was the opposite: omit holdout-bearing flags from /local_evaluation so every SDK version falls back to the server immediately, without an SDK release. That was rejected. A holdout needs no server-side data, unlike a static cohort or experience continuity, so falling back would trade correct local behavior for a network round trip.

The bug was confirmed by running local evaluation against a flag with a 100% holdout and comparing it to the same flag without one. The outputs were identical, which rules out both "holdout honored" and "fell back to the server".

Other server SDKs were not checked. Their local evaluators are separate implementations and may or may not have the same gap.

🤖 Generated with Claude Code

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posthog-python Compliance Report

Date: 2026-09-22T22:46:23.975053+00:00
Duration: 259819ms

✅ 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	✅	517ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	511ms
Required Headers.Has Authorization Bearer Header	✅	511ms
Required Headers.Has Content Type Json	✅	511ms
Required Headers.Has Posthog Sdk Info Format	✅	511ms
Required Headers.Has Posthog Attempt Header	✅	510ms
Required Headers.Has Posthog Request Id	✅	510ms
Required Headers.Has Posthog Request Timestamp	✅	510ms
Required Headers.Has User Agent	✅	511ms
Body Format.Body Has Created At And Batch	✅	511ms
Body Format.No Api Key In Body	✅	510ms
Body Format.No Sent At In Body	✅	510ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	511ms
Event Format.Event Timestamp Is Rfc3339	✅	510ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	516ms
Event Format.Distinct Id Is String	✅	511ms
Event Format.Distinct Id At Root Not Properties	✅	510ms
Event Format.Custom Properties Preserved	✅	510ms
Event Format.Set Properties Preserved	✅	510ms
Event Format.Set Once Properties Preserved	✅	510ms
Event Format.Groups Properties Preserved	✅	511ms
Event Format.Sdk Generates Uuid If Not Provided	✅	510ms
Event Format.Event Has Required Root Fields Batch	✅	513ms
Event Format.Event Uuid Is Valid Batch	✅	514ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	514ms
Event Format.Distinct Id Is String Batch	✅	513ms
Event Format.Distinct Id At Root Not Properties Batch	✅	513ms
Event Format.Custom Properties Preserved Batch	✅	513ms
Event Format.Set Properties Preserved Batch	✅	514ms
Event Format.Set Once Properties Preserved Batch	✅	513ms
Event Format.Groups Properties Preserved Batch	✅	514ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	514ms
Batch Behavior.Multiple Events In Single Batch	✅	517ms
Batch Behavior.Batch Envelope Smoke	✅	515ms
Batch Behavior.Flush With No Events Sends Nothing	✅	506ms
Batch Behavior.Flush At Triggers Batch	✅	1011ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	511ms
Deduplication.Generates Unique Uuids	✅	517ms
Deduplication.Different Events Same Content Different Uuids	✅	512ms
Deduplication.Preserves Uuid On Retry	✅	6520ms
Deduplication.Preserves Timestamp On Retry	✅	6519ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6523ms
Deduplication.No Duplicate Events In Batch	✅	518ms
Header Behavior On Retry.Attempt Header Starts At One	✅	510ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13528ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6521ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3019ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6519ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	511ms
Response Format Validation.Success Response Has Ok For Each Event	✅	514ms
Response Format Validation.Success No Retry After When All Ok	✅	513ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1516ms
Response Format Validation.Success No Retry After When Drop Only	✅	512ms
Response Format Validation.Response Echoes Request Id	✅	511ms
Retry Behavior.Retries On 408	✅	6520ms
Retry Behavior.Retries On 500	✅	6520ms
Retry Behavior.Retries On 503	✅	8519ms
Retry Behavior.Retries On 504	✅	6521ms
Retry Behavior.Retryable Errors Have Retry After	✅	3517ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11523ms
Retry Behavior.Does Not Retry On 400	✅	2514ms
Retry Behavior.Does Not Retry On 401	✅	2514ms
Retry Behavior.Does Not Retry On 402	✅	2514ms
Retry Behavior.Does Not Retry On 413	✅	2514ms
Retry Behavior.Does Not Retry On 415	✅	2512ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2514ms
Retry Behavior.Implements Backoff	✅	22532ms
Retry Behavior.Max Retries Respected	✅	22530ms
Partial Batch Handling.Handles 200 Full Success	✅	2511ms
Partial Batch Handling.Handles 200 With All Ok	✅	3515ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3516ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3517ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6517ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6518ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6522ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6517ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6527ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6519ms
Partial Batch Handling.Respects Retry After On Partial	✅	8517ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3517ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3520ms
Compression.Sends Gzip Content Encoding	✅	512ms
Compression.No Content Encoding When Disabled	✅	510ms
Compression.Compressed Body Is Decompressible	✅	511ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2512ms
Event Options.Cookieless Mode Override	✅	510ms
Event Options.Disable Skew Correction Override	✅	510ms
Event Options.Process Person Profile Override	✅	511ms
Event Options.Product Tour Id Override	✅	510ms
Event Options.Unset Options Omitted	✅	510ms
Event Options.Options Override In Batch	✅	513ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	510ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	511ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	510ms
Routing.Capture Does Not Reroute Ai Named Events	✅	510ms
Identity.Capture Ai Event Has Uuid	✅	510ms
Identity.Capture Ai Keeps Supplied Uuid	✅	510ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	511ms

Feature_Flags Tests

✅ 17/17 tests passed

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	✅	9ms
Request Payload.Token In Flags Body Matches Init	✅	9ms
Request Payload.Groups Round Trip	✅	9ms
Request Payload.Groups Default To Empty Object	✅	10ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	9ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	509ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	14ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Retry Behavior.Retries Flags On 502	✅	313ms
Retry Behavior.Retries Flags On 504	✅	314ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	512ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	68ms
Versioned Boolean Matching.Matching Version 1	✅	68ms
Versioned Boolean Matching.Matching Version 2	✅	67ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	36ms

[marandaneto]

The code looks good; no qualifying findings. Verdict: correct.