fix(flags): align local string matching with the flags service

[marandaneto]

💡 Motivation and Context

Local feature flag evaluation must match the currently released /flags service until its behavior changes. The service applies boolean-like truthiness before ordinary array membership for exact and is_not. This includes all-boolean arrays and the vacuous truthiness of an empty array.

The separate backend proposal in PostHog/posthog#90694 would replace those semantics. This PR intentionally does not adopt that draft behavior.

This fixes PostHog/posthog#78019 by:

• applying the service's boolean-like filter precedence before ordinary array membership
• preserving is_not as the direct complement of exact
• using ordinary ANY membership only for arrays that are not entirely boolean-like
• keeping serde_json stringification and Rust Unicode lowercase for exact and is_not
• keeping ASCII-only lowercase for contains, prefix, and suffix operators
• adding final-sigma, dotted-I, boolean-array, and empty-array regression vectors

The patch changeset for cargo/posthog-rs describes the complete matching update.

💚 How did you test it?

• Red: cargo test test_exact_boolean_coercion_matches_flags_service -- --nocapture failed at exact with filter false and property "banana".
• Green: the same focused test passed after porting the service truthiness helpers.
• cargo test test_property_case_folding_matches_flags_service -- --nocapture
• cargo test
• cargo fmt --all -- --check
• cargo clippy -- -D warnings
• scripts/check-public-api.sh
• Autoreview with autoreview --mode local

The test suite emits existing deprecation warnings from httpmock-based tests. No test failed.

📝 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 the address-pr-comments, check-pr, autoreview, and karpathy-guidelines skills. Verification showed that Rust and serde_json already match the service's Unicode and JSON behavior, so production changes remain limited to boolean-like precedence.

[github-actions]

posthog-rs-v0 Compliance Report

Date: 2026-08-28 06:16:28 UTC
Duration: 15513ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	142ms
Format Validation.Event Has Uuid	✅	148ms
Format Validation.Event Has Lib Properties	✅	149ms
Format Validation.Distinct Id Is String	✅	145ms
Format Validation.Token Is Present	✅	153ms
Format Validation.Custom Properties Preserved	✅	152ms
Format Validation.Event Has Timestamp	✅	151ms
Retry Behavior.Retries On 503	✅	5147ms
Retry Behavior.Does Not Retry On 400	✅	2154ms
Retry Behavior.Does Not Retry On 401	✅	2148ms
Retry Behavior.Respects Retry After Header	✅	5092ms
Retry Behavior.Implements Backoff	✅	15097ms
Retry Behavior.Retries On 500	✅	5096ms
Retry Behavior.Retries On 502	✅	5091ms
Retry Behavior.Retries On 504	✅	5087ms
Retry Behavior.Max Retries Respected	✅	15088ms
Deduplication.Generates Unique Uuids	✅	95ms
Deduplication.Preserves Uuid On Retry	✅	5024ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10033ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5036ms
Deduplication.No Duplicate Events In Batch	✅	26ms
Deduplication.Different Events Have Different Uuids	✅	23ms
Compression.Sends Gzip When Enabled	✅	23ms
Batch Format.Uses Proper Batch Structure	✅	24ms
Batch Format.Flush With No Events Sends Nothing	✅	25ms
Batch Format.Multiple Events Batched Together	✅	110ms
Error Handling.Does Not Retry On 403	✅	2090ms
Error Handling.Does Not Retry On 413	✅	2062ms
Error Handling.Retries On 408	✅	5066ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	74ms
Request Payload.Flags Request Uses V2 Query Param	✅	56ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	22ms
Request Payload.Flags Request Omits Authorization Header	✅	51ms
Request Payload.Token In Flags Body Matches Init	✅	34ms
Request Payload.Groups Round Trip	✅	36ms
Request Payload.Groups Default To Empty Object	✅	42ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	42ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	41ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	41ms
Request Lifecycle.No Flags Request On Init Alone	✅	25ms
Request Lifecycle.No Flags Request On Normal Capture	✅	68ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	45ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	33ms
Retry Behavior.Retries Flags On 502	✅	246ms
Retry Behavior.Retries Flags On 504	✅	245ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	47ms

[github-actions]

posthog-rs-v1 Compliance Report

Date: 2026-08-28 06:16:44 UTC
Duration: 22819ms

✅ All Tests Passed!

111/111 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	118ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	119ms
Required Headers.Has Authorization Bearer Header	✅	119ms
Required Headers.Has Content Type Json	✅	117ms
Required Headers.Has Posthog Sdk Info Format	✅	118ms
Required Headers.Has Posthog Attempt Header	✅	123ms
Required Headers.Has Posthog Request Id	✅	117ms
Required Headers.Has Posthog Request Timestamp	✅	116ms
Required Headers.Has User Agent	✅	116ms
Body Format.Body Has Created At And Batch	✅	116ms
Body Format.No Api Key In Body	✅	115ms
Body Format.No Sent At In Body	✅	115ms
Event Format.Event Has Required Root Fields	✅	115ms
Event Format.Event Uuid Is Valid	✅	115ms
Event Format.Event Timestamp Is Rfc3339	✅	114ms
Event Format.Distinct Id Is String	✅	114ms
Event Format.Distinct Id At Root Not Properties	✅	114ms
Event Format.Custom Properties Preserved	✅	114ms
Event Format.Set Properties Preserved	✅	114ms
Event Format.Set Once Properties Preserved	✅	112ms
Event Format.Groups Properties Preserved	✅	106ms
Event Format.Sdk Generates Uuid If Not Provided	✅	106ms
Event Format.Event Has Required Root Fields Batch	✅	134ms
Event Format.Event Uuid Is Valid Batch	✅	148ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	134ms
Event Format.Distinct Id Is String Batch	✅	133ms
Event Format.Distinct Id At Root Not Properties Batch	✅	145ms
Event Format.Custom Properties Preserved Batch	✅	134ms
Event Format.Set Properties Preserved Batch	✅	148ms
Event Format.Set Once Properties Preserved Batch	✅	149ms
Event Format.Groups Properties Preserved Batch	✅	129ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	124ms
Batch Behavior.Multiple Events In Single Batch	✅	173ms
Batch Behavior.Batch Envelope Smoke	✅	153ms
Batch Behavior.Flush With No Events Sends Nothing	✅	76ms
Batch Behavior.Flush At Triggers Batch	✅	1113ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	88ms
Deduplication.Generates Unique Uuids	✅	158ms
Deduplication.Different Events Same Content Different Uuids	✅	105ms
Deduplication.Preserves Uuid On Retry	✅	5103ms
Deduplication.Preserves Timestamp On Retry	✅	5081ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5096ms
Deduplication.No Duplicate Events In Batch	✅	123ms
Header Behavior On Retry.Attempt Header Starts At One	✅	73ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	10073ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	5068ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	2057ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	5052ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	53ms
Response Format Validation.Success Response Has Ok For Each Event	✅	29ms
Response Format Validation.Success No Retry After When All Ok	✅	31ms
Response Format Validation.Success Retry After Present When Retry Events	✅	24ms
Response Format Validation.Success No Retry After When Drop Only	✅	26ms
Response Format Validation.Response Echoes Request Id	✅	22ms
Retry Behavior.Retries On 408	✅	5031ms
Retry Behavior.Retries On 500	✅	5020ms
Retry Behavior.Retries On 503	✅	5023ms
Retry Behavior.Retries On 504	✅	5020ms
Retry Behavior.Retryable Errors Have Retry After	✅	2023ms
Retry Behavior.Respects Retry After On Retryable Error	✅	8021ms
Retry Behavior.Does Not Retry On 400	✅	2024ms
Retry Behavior.Does Not Retry On 401	✅	2025ms
Retry Behavior.Does Not Retry On 402	✅	2028ms
Retry Behavior.Does Not Retry On 413	✅	2025ms
Retry Behavior.Does Not Retry On 415	✅	2026ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2022ms
Retry Behavior.Implements Backoff	✅	15028ms
Retry Behavior.Max Retries Respected	✅	15021ms
Partial Batch Handling.Handles 200 Full Success	✅	2032ms
Partial Batch Handling.Handles 200 With All Ok	✅	3037ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3029ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3025ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	5027ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	5021ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	5023ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	5022ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	5027ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	5029ms
Partial Batch Handling.Respects Retry After On Partial	✅	5023ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3021ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3029ms
Compression.Sends Gzip Content Encoding	✅	30ms
Compression.No Content Encoding When Disabled	✅	20ms
Compression.Compressed Body Is Decompressible	✅	20ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2021ms
Event Options.Cookieless Mode Override	✅	20ms
Event Options.Disable Skew Correction Override	✅	20ms
Event Options.Process Person Profile Override	✅	19ms
Event Options.Product Tour Id Override	✅	19ms
Event Options.Unset Options Omitted	✅	20ms
Event Options.Options Override In Batch	✅	20ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	20ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	20ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	20ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	13ms
Request Payload.Flags Request Uses V2 Query Param	✅	13ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	13ms
Request Payload.Flags Request Omits Authorization Header	✅	13ms
Request Payload.Token In Flags Body Matches Init	✅	14ms
Request Payload.Groups Round Trip	✅	13ms
Request Payload.Groups Default To Empty Object	✅	14ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	13ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	13ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	14ms
Request Lifecycle.No Flags Request On Init Alone	✅	10ms
Request Lifecycle.No Flags Request On Normal Capture	✅	20ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	13ms
Retry Behavior.Retries Flags On 502	✅	217ms
Retry Behavior.Retries Flags On 504	✅	217ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	19ms

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
src/feature_flags.rs:1720-1742
**Parameterize comparison cases**

The new comparison matrix repeats individual assertions instead of using parameterized cases, making additions more repetitive and failures less descriptive than the existing table-driven operator-test pattern.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix(flags): align local string matching ..." | Re-trigger Greptile}