feat(event): add reactive runtime event plumbing

internal/assistant/client.go

@@ -42,6 +42,7 @@ const (
 	jsonTextKey             = "text"
 	jsonThinkingKey         = "thinking"
 	jsonDisplayKey          = "display"
+	jsonUsageKey            = "usage"
 	jsonUserRole            = "user"
 	functionToolType        = "function"
 	functionCallType        = "function_call"

internal/assistant/lifecycle.go

@@ -126,6 +126,18 @@ func (runtime *Runtime) dispatchTurnErrorLifecycle(
 	runtime.dispatchLifecycle(ctx, extension.LifecycleAgentEnd, payload)
 }
 
+func (runtime *Runtime) dispatchContextBuild(
+	ctx context.Context,
+	sessionID string,
+	cwd string,
+	systemPrompt string,
+	messages []database.MessageEntity,
+	usage model.TokenUsage,
+) {
+	payload := contextBuildLifecyclePayload(sessionID, cwd, systemPrompt, messages, usage)
+	runtime.dispatchLifecycle(ctx, extension.LifecycleContextBuild, payload)
+}
+
 func promptLifecyclePayload(request *PromptRequest) map[string]any {
 	if request == nil {
 		return map[string]any{}
@@ -185,7 +197,7 @@ func turnEndLifecyclePayload(
 		"cached":                     cached,
 		lifecycleErrorKey:            "",
 		jsonSessionIDKey:             sessionID,
-		"usage":                      tokenUsageLifecyclePayload(usage),
+		jsonUsageKey:                 tokenUsageLifecyclePayload(usage),
 		lifecycleUserEntryIDKey:      userEntryID,
 	}
 	if turnErr != nil {
@@ -195,6 +207,43 @@ func turnEndLifecyclePayload(
 	return payload
 }
 
+func contextBuildLifecyclePayload(
+	sessionID string,
+	cwd string,
+	systemPrompt string,
+	messages []database.MessageEntity,
+	usage model.TokenUsage,
+) map[string]any {
+	return map[string]any{
+		lifecycleCWDKey:      cwd,
+		jsonSessionIDKey:     sessionID,
+		"message_count":      len(messages),
+		"system_tokens":      estimateTokens(systemPrompt),
+		"message_tokens":     estimateMessageTokens(messages),
+		jsonUsageKey:         tokenUsageLifecyclePayload(usage),
+		"model_facing_roles": modelFacingRoleCounts(messages),
+	}
+}
+
+func estimateMessageTokens(messages []database.MessageEntity) int {
+	tokens := 0
+	for index := range messages {
+		tokens += estimateTokens(messages[index].Content)
+	}
+
+	return tokens
+}
+
+func modelFacingRoleCounts(messages []database.MessageEntity) map[string]int {
+	counts := map[string]int{}
+	for index := range messages {
+		role := string(messages[index].Role)
+		counts[role]++
+	}
+
+	return counts
+}
+
 func entryLifecyclePayload(entry *database.EntryEntity) map[string]any {
 	return map[string]any{
 		lifecycleCreatedAtKey:            entry.CreatedAt.Format(timeFormatRFC3339Nano),

internal/assistant/runtime.go

@@ -650,6 +650,7 @@ func (runtime *Runtime) modelResponse(
 	}
 
 	estimatedUsage := estimateTokenUsage(systemPrompt, messages, &selectedModel)
+	runtime.dispatchContextBuild(ctx, sessionID, cwd, systemPrompt, messages, estimatedUsage)
 	runtime.emitUsage(ctx, onEvent, estimatedUsage)
 	request := &CompletionRequest{
 		Model:         selectedModel,

internal/assistant/runtime_lifecycle_test.go

@@ -25,6 +25,7 @@ local names = {
   "before_agent_start",
   "agent_start",
   "turn_start",
+  "context_build",
   "message_append",
   "turn_end",
   "agent_end",
@@ -75,6 +76,7 @@ func TestRuntime_PromptEmitsOrderedSessionTurnLifecycleEvents(t *testing.T) {
 				"before_agent_start|||session",
 				"agent_start|||session",
 				"turn_start|||session",
+				"context_build|||session",
 				"message_append|assistant||session",
 				"turn_end|||session",
 				"agent_end|||session",
@@ -92,6 +94,7 @@ func TestRuntime_PromptEmitsOrderedSessionTurnLifecycleEvents(t *testing.T) {
 				"before_agent_start|||session",
 				"agent_start|||session",
 				"turn_start|||session",
+				"context_build|||session",
 				"turn_end||error|session",
 				"agent_end||error|session",
 			}},

internal/assistant/usage_events.go

@@ -22,9 +22,9 @@ func (runtime *Runtime) emitUsage(ctx context.Context, onEvent func(StreamEvent)
 		jsonInputTokensKey:   usage.InputTokens,
 		jsonOutputTokensKey:  usage.OutputTokens,
 	}
-	runtime.emit(ctx, "usage", payload)
+	runtime.emit(ctx, jsonUsageKey, payload)
 	if runtime.extensions != nil {
-		if err := runtime.extensions.Emit(ctx, "usage", payload); err != nil {
+		if err := runtime.extensions.Emit(ctx, jsonUsageKey, payload); err != nil {
 			runtime.logger.Debug("emit usage extension event failed", "error", err)
 		}
 	}