feat: emit $is_server property on captured events

[turnipdabeets]

Adds $is_server: true to every event captured by this SDK so PostHog ingestion can identify server-side events.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
capture_test.go:59-94
**Unnecessary JSON round-trip in test**

The test marshals `apiMsg` to JSON and back into `map[string]interface{}` only to read `properties`. `apiMsg.Properties` is already a `map[string]interface{}` (`Properties` type), so the round-trip adds complexity with no benefit. A direct assertion like `apiMsg.Properties["$is_server"] != true` achieves the same check without superfluous steps.

_{Reviews (1): Last reviewed commit: "feat: emit $is_server property on captur..." | Re-trigger Greptile}

[github-actions]

posthog-go Compliance Report

Date: 2026-06-03 14:55:28 UTC
Duration: 107308ms

⚠️ Some Tests Failed

39/45 tests passed, 6 failed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	609ms
Format Validation.Event Has Uuid	✅	608ms
Format Validation.Event Has Lib Properties	✅	608ms
Format Validation.Distinct Id Is String	✅	609ms
Format Validation.Token Is Present	✅	608ms
Format Validation.Custom Properties Preserved	✅	608ms
Format Validation.Event Has Timestamp	✅	608ms
Retry Behavior.Retries On 503	✅	5613ms
Retry Behavior.Does Not Retry On 400	✅	2610ms
Retry Behavior.Does Not Retry On 401	✅	2611ms
Retry Behavior.Respects Retry After Header	✅	5613ms
Retry Behavior.Implements Backoff	✅	15623ms
Retry Behavior.Retries On 500	✅	5613ms
Retry Behavior.Retries On 502	✅	5614ms
Retry Behavior.Retries On 504	✅	5613ms
Retry Behavior.Max Retries Respected	✅	15624ms
Deduplication.Generates Unique Uuids	✅	616ms
Deduplication.Preserves Uuid On Retry	✅	5610ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10614ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5614ms
Deduplication.No Duplicate Events In Batch	✅	611ms
Deduplication.Different Events Have Different Uuids	✅	611ms
Compression.Sends Gzip When Enabled	✅	608ms
Batch Format.Uses Proper Batch Structure	✅	608ms
Batch Format.Flush With No Events Sends Nothing	✅	605ms
Batch Format.Multiple Events Batched Together	✅	611ms
Error Handling.Does Not Retry On 403	✅	2610ms
Error Handling.Does Not Retry On 413	✅	2611ms
Error Handling.Retries On 408	✅	5613ms

Feature_Flags Tests

⚠️ 10/16 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	✅	5ms
Request Payload.Flags Request Omits Authorization Header	✅	7ms
Request Payload.Token In Flags Body Matches Init	✅	5ms
Request Payload.Groups Round Trip	✅	7ms
Request Payload.Groups Default To Empty Object	✅	5ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	❌	6ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	❌	6ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	6ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	❌	6ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	607ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	10ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	5ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	609ms

Failures

request_payload.request_with_person_properties_device_id

Field 'distinct_id' not found in /flags request body at path 'person_properties.distinct_id'. Available keys: ['$device_id']

request_payload.person_properties_distinct_id_auto_populated_when_caller_omits_it

Field 'distinct_id' not found in /flags request body at path 'person_properties.distinct_id'. Available keys: ['email']

request_payload.disable_geoip_false_propagates_as_geoip_disable_false

Expected geoip_disable=False, got True

request_payload.disable_geoip_omitted_defaults_to_false

Expected geoip_disable=False, got True

request_payload.flag_keys_to_evaluate_contains_only_requested_key

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

side_effect_events.get_feature_flag_captures_feature_flag_called_event

Expected 1 events with name '$feature_flag_called', got 2

[marandaneto]

if we decide to add $is_server to all SDKs, there should be a way to turn it off. Some SDKs can also be used as a client, e.g., the Go SDK could be a CLI and wouldn't be a server-side SDK
same for java, node, etc

[turnipdabeets]

if we decide to add $is_server to all SDKs, there should be a way to turn it off. Some SDKs can also be used as a client, e.g., the Go SDK could be a CLI and wouldn't be a server-side SDK same for java, node, etc

Made these configurable now, but defaulted to true.