feat(flags): accept a caller default in FeatureFlagEvaluations.IsEnabled

[posthog]

💡 Motivation and Context

The cross-SDK is-feature-enabled spec states a hard requirement with no server-SDK carve-out:

The SDK SHALL accept a caller-supplied boolean default (defaultValue; parameter placement per platform idiom) and SHALL return it whenever the flag has no value: flags not loaded yet, a failed flags request, or no flag with that key in the loaded flags. A flag that has a value — including false and variant strings — always wins over the caller-supplied default.

The compliance matrix records this as a ❌ Fail for posthog-go:

The canonical path, FeatureFlagEvaluations.IsEnabled(key string) bool, takes only a key argument and hardcodes return false for an unknown flag — no default-value parameter. Both collapse "known false" and "unknown/missing" to the identical false result with no caller-supplied override.

Remediation: Add an equivalent optional default parameter to FeatureFlagEvaluations.IsEnabled.

Today a caller cannot tell "the flag is off" apart from "we never got an answer", and cannot choose to fail open when flag data is unavailable (empty snapshot, failed /flags request, quota-limited response).

Explanation of the change

FeatureFlagEvaluations.IsEnabled gains an optional variadic defaultValue, Go's idiomatic spelling of an optional parameter:

flags, _ := client.EvaluateFlags(posthog.EvaluateFlagsPayload{DistinctId: "user-1"})
if flags.IsEnabled("new-checkout", true) { // fail open when we have no answer
    // ...
}

The default is returned only when the key is absent from the snapshot, or when the snapshot pointer itself is nil (the IsEnabled receiver is already nil-safe, which is how "flags were never loaded" surfaces in Go). Any flag that has a value — true, false, or a variant string — still wins over it, and omitting the argument preserves the historical false result for an unknown flag.

$feature_flag_called reporting is deliberately untouched: the event still carries the real evaluated response (nil plus $feature_flag_error: flag_missing for a miss), not the caller's default, so exposure data keeps reflecting what the server actually said.

Scope is limited to this one contract. The deprecated Client.IsFeatureEnabled/Client.GetFeatureFlag legacy path has the same gap and is not changed here — both already point callers at EvaluateFlags, which is now the compliant surface. The matrix's other open gaps for this SDK are left alone.

Why this is backwards-compatible

Purely additive. A variadic parameter keeps every existing IsEnabled(key) call site compiling and behaving identically, and the miss path is only diverted when a caller explicitly passes a value. api/public-api.txt is regenerated to record the new optional argument. The one source-level caveat worth noting: a method value previously typed func(string) bool is now func(string, ...bool) bool, which is vanishingly rare in practice but is why this ships as a minor, not a patch.

💚 How did you test it?

Four new tests in feature_flag_evaluations_test.go, mapping onto the spec's acceptance scenarios (acceptance/public/is-feature-enabled.feature):

• a missing flag resolves to the caller default (true and false), and stays false with no default
• an existing value wins over the default (disabled flag, variant flag, boolean flag)
• a nil snapshot uses the default
• $feature_flag_called still reports the evaluated response and flag_missing, not the default

Locally, on Go 1.23:

• go vet ./... — clean
• gofmt -l . — clean
• go test -race -count=1 ./... — all packages pass
• make api-update — snapshot regenerated, so make api-diff is clean

No manual testing against a live project.

Follow-up work

• The same default-value gap is recorded for posthog-node, posthog-php and posthog-dotnet; posthog-ruby has PR #285 open for its equivalent.
• The spec's own "Surface variants" table has no Go row, and the SDK compliance adapter (sdk_compliance_adapter) exposes no is_feature_enabled route, so the harness cannot exercise these scenarios end-to-end yet.
• The legacy FeatureFlagPayload path could gain a DefaultValue *bool field for parity, if maintainers want the deprecated surface brought along too.

📝 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 pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

Opened by a scheduled PostHog agent run that reads the PostHog/sdk-specs compliance matrices and implements one safe, well-scoped gap per run. This gap was picked over the other open ❌ rows because it is a core, user-facing flag contract, the remediation is concrete, the backwards-compatibility verdict is additive, and no PR was already open for it in this repo.

Decisions along the way: a variadic defaultValue was chosen over a separate IsEnabledWithDefault method (one way to do a thing, and the spec asks for "parameter placement per platform idiom") and over touching the deprecated FeatureFlagPayload path (out of scope for a single-contract change). The nil-receiver case was included because Go's nil-safe snapshot is how the spec's "flags not loaded yet / failed flags request" states actually reach IsEnabled.

Per AGENTS.md, the published sdk-spec is the public-API agreement, so no separate issue was opened.

---

Created with PostHog Desktop

🤖 Generated with Claude Code

[github-actions]

posthog-go Compliance Report

Date: 2026-10-02 06:10:43 UTC
Duration: 168946ms

✅ 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	✅	16ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	5ms
Required Headers.Has Authorization Bearer Header	✅	6ms
Required Headers.Has Content Type Json	✅	4ms
Required Headers.Has Posthog Sdk Info Format	✅	5ms
Required Headers.Has Posthog Attempt Header	✅	4ms
Required Headers.Has Posthog Request Id	✅	5ms
Required Headers.Has Posthog Request Timestamp	✅	4ms
Required Headers.Has User Agent	✅	5ms
Body Format.Body Has Created At And Batch	✅	4ms
Body Format.No Api Key In Body	✅	5ms
Body Format.No Sent At In Body	✅	5ms
Event Format.Event Has Required Root Fields	✅	4ms
Event Format.Event Uuid Is Valid	✅	5ms
Event Format.Event Timestamp Is Rfc3339	✅	4ms
Event Format.Distinct Id Is String	✅	5ms
Event Format.Distinct Id At Root Not Properties	✅	4ms
Event Format.Custom Properties Preserved	✅	5ms
Event Format.Set Properties Preserved	✅	4ms
Event Format.Set Once Properties Preserved	✅	5ms
Event Format.Groups Properties Preserved	✅	5ms
Event Format.Sdk Generates Uuid If Not Provided	✅	4ms
Event Format.Event Has Required Root Fields Batch	✅	16ms
Event Format.Event Uuid Is Valid Batch	✅	7ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	7ms
Event Format.Distinct Id Is String Batch	✅	6ms
Event Format.Distinct Id At Root Not Properties Batch	✅	6ms
Event Format.Custom Properties Preserved Batch	✅	7ms
Event Format.Set Properties Preserved Batch	✅	6ms
Event Format.Set Once Properties Preserved Batch	✅	7ms
Event Format.Groups Properties Preserved Batch	✅	6ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	6ms
Batch Behavior.Multiple Events In Single Batch	✅	28ms
Batch Behavior.Batch Envelope Smoke	✅	8ms
Batch Behavior.Flush With No Events Sends Nothing	✅	3ms
Batch Behavior.Flush At Triggers Batch	✅	1006ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	5ms
Deduplication.Generates Unique Uuids	✅	11ms
Deduplication.Different Events Same Content Different Uuids	✅	6ms
Deduplication.Preserves Uuid On Retry	✅	5132ms
Deduplication.Preserves Timestamp On Retry	✅	5133ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5135ms
Deduplication.No Duplicate Events In Batch	✅	29ms
Header Behavior On Retry.Attempt Header Starts At One	✅	15ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	10140ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	5134ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	2011ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	5135ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	6ms
Response Format Validation.Success Response Has Ok For Each Event	✅	26ms
Response Format Validation.Success No Retry After When All Ok	✅	26ms
Response Format Validation.Success Retry After Present When Retry Events	✅	128ms
Response Format Validation.Success No Retry After When Drop Only	✅	26ms
Response Format Validation.Response Echoes Request Id	✅	5ms
Retry Behavior.Retries On 408	✅	5133ms
Retry Behavior.Retries On 500	✅	5133ms
Retry Behavior.Retries On 503	✅	5135ms
Retry Behavior.Retries On 504	✅	5134ms
Retry Behavior.Retryable Errors Have Retry After	✅	2131ms
Retry Behavior.Respects Retry After On Retryable Error	✅	8131ms
Retry Behavior.Does Not Retry On 400	✅	2131ms
Retry Behavior.Does Not Retry On 401	✅	2131ms
Retry Behavior.Does Not Retry On 402	✅	2132ms
Retry Behavior.Does Not Retry On 413	✅	2132ms
Retry Behavior.Does Not Retry On 415	✅	2131ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2132ms
Retry Behavior.Implements Backoff	✅	15133ms
Retry Behavior.Max Retries Respected	✅	15142ms
Partial Batch Handling.Handles 200 Full Success	✅	2008ms
Partial Batch Handling.Handles 200 With All Ok	✅	3028ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3027ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3030ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	5133ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	5134ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	5134ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	5133ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	5133ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	5133ms
Partial Batch Handling.Respects Retry After On Partial	✅	5136ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3029ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3033ms
Compression.Sends Gzip Content Encoding	✅	15ms
Compression.No Content Encoding When Disabled	✅	5ms
Compression.Compressed Body Is Decompressible	✅	15ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2130ms
Event Options.Cookieless Mode Override	✅	6ms
Event Options.Disable Skew Correction Override	✅	4ms
Event Options.Process Person Profile Override	✅	5ms
Event Options.Product Tour Id Override	✅	4ms
Event Options.Unset Options Omitted	✅	5ms
Event Options.Options Override In Batch	✅	7ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	4ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	4ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	5ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	14ms
Request Payload.Flags Request Uses V2 Query Param	✅	14ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	15ms
Request Payload.Flags Request Omits Authorization Header	✅	14ms
Request Payload.Token In Flags Body Matches Init	✅	14ms
Request Payload.Groups Round Trip	✅	15ms
Request Payload.Groups Default To Empty Object	✅	14ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	14ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	15ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	15ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	5ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	27ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	15ms
Retry Behavior.Retries Flags On 502	✅	16ms
Retry Behavior.Retries Flags On 504	✅	15ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	16ms