feat(ldai): stamp modelKey and modelVersion on AI usage events (AIC-2850) (#414)

<!-- CURSOR_AGENT_PR_BODY_BEGIN -->
## Summary

- Read `modelKey` and `modelVersion` from the AI Config variation
payload (`model.modelKey` / `model.modelVersion`) and expose them on
`Config` via `ModelKey()` / `ModelVersion()` accessors and builder
methods
- Stamp `modelKey` (when present) and `modelVersion` on all `Tracker`
metric event payloads, alongside existing `modelName`/`providerName`
fields
- Default `modelVersion` to `1` when absent, matching variation version
handling; exclude both fields from the resumption token
- Additive/backward compatible — older payloads without the new fields
continue to work

Part of [AIC-2850](https://launchdarkly.atlassian.net/browse/AIC-2850) /
[AIC-2849](https://launchdarkly.atlassian.net/browse/AIC-2849). Depends
on backend payload work
([AIC-2876](https://launchdarkly.atlassian.net/browse/AIC-2876))
shipping `modelKey`/`modelVersion` on variations.

## Test plan

- [x] `go build ./...` in `ldai/`
- [x] `go vet ./...` in `ldai/`
- [x] `go test ./...` in `ldai/` (all passed)
- [ ] Verify against a staging environment once AIC-2876 payload is
available

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's pull request submission
guidelines
- [x] I have validated my changes against all supported platform
versions

**Related issues**

- [AIC-2850](https://launchdarkly.atlassian.net/browse/AIC-2850)
- Parent: [AIC-2849](https://launchdarkly.atlassian.net/browse/AIC-2849)
- Depends on:
[AIC-2876](https://launchdarkly.atlassian.net/browse/AIC-2876)

**Describe the solution you've provided**

The Go AI SDK now parses `modelKey` and `modelVersion` from the
variation's `model` object, exposes them through `Config` accessors, and
includes them in the `trackData` stamped on every metric event.
`modelVersion` is always emitted (defaulting to 1); `modelKey` is
omitted when empty. Neither field is included in the resumption token.

**Describe alternatives you've considered**

Threading `modelKey`/`modelVersion` as explicit constructor parameters
to `newTracker` (as in the Python SDK PR) was considered but rejected in
favor of sourcing from `Config`, matching the existing
`modelName`/`providerName` pattern.

**Additional context**

Mirrors
[python-server-sdk-ai#208](launchdarkly/python-server-sdk-ai#208).
<!-- CURSOR_AGENT_PR_BODY_END -->

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[AIC-2850]:
https://launchdarkly.atlassian.net/browse/AIC-2850?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[AIC-2849]:
https://launchdarkly.atlassian.net/browse/AIC-2849?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[AIC-2876]:
https://launchdarkly.atlassian.net/browse/AIC-2876?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[AIC-2850]:
https://launchdarkly.atlassian.net/browse/AIC-2850?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Anthony Torns II <atornsii@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

ldai/client.go

@@ -114,7 +114,7 @@ func (c *Client) CreateTracker(token string, context ldcontext.Context) (*Tracke
 // returns the resulting Config. Used for all error-path returns in evaluateConfig.
 func (c *Client) returnDefault(key string, context ldcontext.Context, def Config) Config {
 	def.trackerFactory = func() *Tracker {
-		return newTracker(c.sdk, newRunID(), key, "", 1, context, &def, c.logger)
+		return newTracker(c.sdk, newRunID(), key, "", 1, "", 1, context, &def, c.logger)
 	}
 	return def
 }
@@ -189,9 +189,15 @@ func (c *Client) evaluateConfig(
 		version = *parsed.Meta.Version
 	}
 
+	modelVersion := 1
+	if parsed.Meta.ModelVersion != nil {
+		modelVersion = *parsed.Meta.ModelVersion
+	}
+
 	variationKey := parsed.Meta.VariationKey
+	modelKey := parsed.Meta.ModelKey
 	cfg.trackerFactory = func() *Tracker {
-		return newTracker(c.sdk, newRunID(), key, variationKey, version, context, &cfg, c.logger)
+		return newTracker(c.sdk, newRunID(), key, variationKey, version, modelKey, modelVersion, context, &cfg, c.logger)
 	}
 
 	return cfg

ldai/client_test.go

@@ -150,6 +150,82 @@ func TestParseModelName(t *testing.T) {
 	}
 }
 
+func TestParseModelKeyAndVersion(t *testing.T) {
+	// modelKey/modelVersion are intentionally not exposed on Config (they'd read as properties of
+	// the LLM itself, e.g. a version like "5.4"); the only place they surface is the tracker's
+	// stamped event data, mirroring variationKey/version.
+	tests := []struct {
+		name            string
+		json            []byte
+		expectedKey     string
+		expectedVersion int
+	}{
+		{
+			name:            "missing",
+			json:            []byte(`{"model": {"name": "gpt-4"}}`),
+			expectedKey:     "",
+			expectedVersion: 1,
+		},
+		{
+			name:            "modelKey and modelVersion set",
+			json:            []byte(`{"model": {"name": "gpt-4"}, "_ldMeta": {"modelKey": "my-model", "modelVersion": 2}}`),
+			expectedKey:     "my-model",
+			expectedVersion: 2,
+		},
+		{
+			name:            "modelVersion only",
+			json:            []byte(`{"model": {"name": "gpt-4"}, "_ldMeta": {"modelVersion": 3}}`),
+			expectedKey:     "",
+			expectedVersion: 3,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			mockSDK := newMockSDK(test.json, nil)
+			client, err := NewClient(mockSDK)
+			require.NoError(t, err)
+			require.NotNil(t, client)
+			mockSDK.events = nil
+
+			defaultVal := NewConfig().Enable().WithMessage("hello", datamodel.User).Build()
+			cfg := client.CompletionConfig("key", ldcontext.New("user"), defaultVal, nil)
+			tracker := cfg.CreateTracker()
+			require.NotNil(t, tracker)
+			assert.NoError(t, tracker.TrackSuccess())
+
+			require.NotEmpty(t, mockSDK.events)
+			data := mockSDK.events[len(mockSDK.events)-1].data
+			assert.Equal(t, test.expectedKey, data.GetByKey("modelKey").StringValue())
+			assert.Equal(t, test.expectedVersion, data.GetByKey("modelVersion").IntValue())
+		})
+	}
+}
+
+func TestCreateTrackerStampsModelKeyAndVersionOnTrackData(t *testing.T) {
+	configJSON := []byte(`{
+		"_ldMeta": {"variationKey": "var-1", "enabled": true, "version": 1, "modelKey": "my-model", "modelVersion": 2},
+		"model": {"name": "gpt-4"},
+		"provider": {"name": "openai"},
+		"messages": [{"content": "hello", "role": "user"}]
+	}`)
+
+	mockSDK := newMockSDK(configJSON, nil)
+	client, err := NewClient(mockSDK)
+	require.NoError(t, err)
+	mockSDK.events = nil
+
+	cfg := client.CompletionConfig("my-config", ldcontext.New("user"), Disabled(), nil)
+	tracker := cfg.CreateTracker()
+	require.NotNil(t, tracker)
+	assert.NoError(t, tracker.TrackSuccess())
+
+	require.NotEmpty(t, mockSDK.events)
+	data := mockSDK.events[len(mockSDK.events)-1].data
+	assert.Equal(t, "my-model", data.GetByKey("modelKey").StringValue())
+	assert.Equal(t, 2, data.GetByKey("modelVersion").IntValue())
+}
+
 func TestParseProviderName(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -1009,6 +1085,8 @@ func TestClient_CreateTracker_RoundTrip(t *testing.T) {
 	// modelName and providerName should be empty on reconstructed tracker
 	assert.Equal(t, "", feedbackEvent.data.GetByKey("modelName").StringValue())
 	assert.Equal(t, "", feedbackEvent.data.GetByKey("providerName").StringValue())
+	assert.False(t, feedbackEvent.data.GetByKey("modelKey").IsDefined())
+	assert.Equal(t, 1, feedbackEvent.data.GetByKey("modelVersion").IntValue())
 }
 
 func TestClient_CreateTracker_InvalidToken(t *testing.T) {

ldai/datamodel/datamodel.go

@@ -12,6 +12,12 @@ type Meta struct {
 
 	// Version is the version of the Variation.
 	Version *int `json:"version,omitempty"`
+
+	// ModelKey is the model's stable, unique key (distinct from Model.Name, which is not guaranteed unique).
+	ModelKey string `json:"modelKey,omitempty"`
+
+	// ModelVersion is the pinned version of the model that the variation references.
+	ModelVersion *int `json:"modelVersion,omitempty"`
 }
 
 // Model defines the serialization format for a model.

ldai/tracker.go

@@ -174,11 +174,14 @@ func newTracker(
 	key string,
 	variationKey string,
 	version int,
+	modelKey string,
+	modelVersion int,
 	ctx ldcontext.Context,
 	config *Config,
 	loggers interfaces.LDLoggers,
 ) *Tracker {
-	return newTrackerWithStopwatch(events, runID, key, variationKey, version, ctx, config, loggers, &defaultStopwatch{})
+	return newTrackerWithStopwatch(
+		events, runID, key, variationKey, version, modelKey, modelVersion, ctx, config, loggers, &defaultStopwatch{})
 }
 
 // newTrackerWithStopwatch creates a new Tracker with the specified runID, key, event sink, config, context, loggers,
@@ -189,6 +192,8 @@ func newTrackerWithStopwatch(
 	key string,
 	variationKey string,
 	version int,
+	modelKey string,
+	modelVersion int,
 	ctx ldcontext.Context,
 	config *Config,
 	loggers interfaces.LDLoggers,
@@ -203,7 +208,11 @@ func newTrackerWithStopwatch(
 		Set("configKey", ldvalue.String(key)).
 		Set("version", ldvalue.Int(version)).
 		Set("providerName", ldvalue.String(config.ProviderName())).
-		Set("modelName", ldvalue.String(config.ModelName()))
+		Set("modelName", ldvalue.String(config.ModelName())).
+		Set("modelVersion", ldvalue.Int(modelVersion))
+	if modelKey != "" {
+		builder.Set("modelKey", ldvalue.String(modelKey))
+	}
 	if variationKey != "" {
 		builder.Set("variationKey", ldvalue.String(variationKey))
 	}
@@ -230,7 +239,7 @@ func (t *Tracker) logWarning(format string, args ...interface{}) {
 
 // ResumptionToken returns a URL-safe Base64-encoded token that can be used to reconstruct a tracker
 // in a different process (e.g., for deferred feedback). The token contains the runId, configKey,
-// variationKey, and version. It does not contain modelName or providerName.
+// variationKey, and version. It does not contain modelName, providerName, modelKey, or modelVersion.
 func (t *Tracker) ResumptionToken() string {
 	payload := resumptionPayload{
 		RunID:        t.runID,
@@ -245,8 +254,8 @@ func (t *Tracker) ResumptionToken() string {
 // TrackerFromResumptionToken reconstructs a Tracker from a resumption token and the given context.
 // This is used for cross-process scenarios (e.g., deferred feedback) where the original tracker
 // is no longer available but its runId must be reused. The token is obtained from Tracker.ResumptionToken().
-// The reconstructed tracker will have empty modelName and providerName since these are not included
-// in the token.
+// The reconstructed tracker will have empty modelName, providerName, and modelKey, and modelVersion
+// defaults to 1, since these are not included in the token.
 func TrackerFromResumptionToken(token string, sdk ServerSDK, context ldcontext.Context) (*Tracker, error) {
 	decoded, err := base64.RawURLEncoding.DecodeString(token)
 	if err != nil {
@@ -262,7 +271,8 @@ func TrackerFromResumptionToken(token string, sdk ServerSDK, context ldcontext.C
 		Set("configKey", ldvalue.String(payload.ConfigKey)).
 		Set("version", ldvalue.Int(payload.Version)).
 		Set("providerName", ldvalue.String("")).
-		Set("modelName", ldvalue.String(""))
+		Set("modelName", ldvalue.String("")).
+		Set("modelVersion", ldvalue.Int(1))
 	if payload.VariationKey != "" {
 		builder.Set("variationKey", ldvalue.String(payload.VariationKey))
 	}