feat: Add device_id parameter to /flags requests

[andehen]

Problem

Need consistent feature-flag evaluation and reporting when flags use device-based bucketing; local evaluation and event metadata were missing device_id support.

Changes

• Add device_id to flags requests and SendFeatureFlagsOptions.
• Use device_id for local evaluation bucketing when configured.
• Include device_id in events and the feature-flag dedupe key.
• Refactor remote flags request tests.
• User-facing: no code changes required unless you want device_id bucketing; to opt in, pass device_id in feature-flag evaluation (payload/options) so server + local evaluation can bucket by device_id.

Testing

• Existing and updated unit tests for flags/local evaluation.
• Remote flags request tests refactored.

Tested the go SDK locally:

• verified that getFeatureFlag sends device_id to /flags when being passed. It's also being set as a property on the $feature_flag_called events when passed
• local evaluation works, and returns the same result. $feature_flag_called events are being sent as before the first time it is evaluated. Subsequent evaluation doesn't trigger a new event.

[rodrigoi]

dusting off my C, with is as close as Go as I can ... go.

Approving to unblock. The only things I've found are those little things on the tests and the getFeatureVariantsWithOptions situation. Otherwise, looks consistent and well tested.

[andehen]

@dmarticus @rodrigoi I first planned to skip support for local evaluation, but realized it's better to just support it from the beginning (it's not that much code, really).

Great if you can take another look!

[dmarticus]

looks good! No notes from my end

[github-actions]

posthog-go Compliance Report

Date: 2026-02-04 11:59:52 UTC
Duration: 105783ms

⚠️ Some Tests Failed

22/29 tests passed, 7 failed

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Capture Tests

⚠️ 22/29 tests passed, 7 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	607ms
Format Validation.Event Has Uuid	✅	607ms
Format Validation.Event Has Lib Properties	✅	607ms
Format Validation.Distinct Id Is String	✅	608ms
Format Validation.Token Is Present	✅	607ms
Format Validation.Custom Properties Preserved	✅	607ms
Format Validation.Event Has Timestamp	✅	607ms
Retry Behavior.Retries On 503	✅	5608ms
Retry Behavior.Does Not Retry On 400	❌	2609ms
Retry Behavior.Does Not Retry On 401	❌	2610ms
Retry Behavior.Respects Retry After Header	❌	5613ms
Retry Behavior.Implements Backoff	✅	15623ms
Retry Behavior.Retries On 500	✅	5612ms
Retry Behavior.Retries On 502	✅	5613ms
Retry Behavior.Retries On 504	✅	5613ms
Retry Behavior.Max Retries Respected	❌	15622ms
Deduplication.Generates Unique Uuids	✅	616ms
Deduplication.Preserves Uuid On Retry	✅	5611ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10618ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5614ms
Deduplication.No Duplicate Events In Batch	✅	610ms
Deduplication.Different Events Have Different Uuids	✅	609ms
Compression.Sends Gzip When Enabled	❌	607ms
Batch Format.Uses Proper Batch Structure	✅	607ms
Batch Format.Flush With No Events Sends Nothing	✅	605ms
Batch Format.Multiple Events Batched Together	✅	609ms
Error Handling.Does Not Retry On 403	❌	2609ms
Error Handling.Does Not Retry On 413	❌	2609ms
Error Handling.Retries On 408	✅	5610ms

Failures

retry_behavior.does_not_retry_on_400

Expected 1 requests, got 2

retry_behavior.does_not_retry_on_401

Expected 1 requests, got 2

retry_behavior.respects_retry_after_header

Retry delay too short: 102ms < 2500ms

retry_behavior.max_retries_respected

Expected 4 requests, got 6

compression.sends_gzip_when_enabled

Header 'Content-Encoding' with value 'gzip' not found in requests

error_handling.does_not_retry_on_403

Expected 1 requests, got 2

error_handling.does_not_retry_on_413

Expected 1 requests, got 2