feat: add ResolveModel to SubagentManager with 3-level fallback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/tools/subagent.go

@@ -14,6 +14,7 @@ type SubagentTask struct {
 	ID            string
 	Task          string
 	Label         string
+	Model         string // model_name override for this task
 	AgentID       string
 	OriginChannel string
 	OriginChatID  string
@@ -27,6 +28,8 @@ type SubagentManager struct {
 	mu             sync.RWMutex
 	provider       providers.LLMProvider
 	defaultModel         string
+	subagentDefaultModel string            // from SubagentsConfig.Model
+	modelValidator       func(string) bool // validates model_name exists
 	bus                  *bus.MessageBus
 	workspace      string
 	tools          *ToolRegistry
@@ -65,6 +68,41 @@ func (sm *SubagentManager) SetLLMOptions(maxTokens int, temperature float64) {
 	sm.hasTemperature = true
 }
 
+// SetSubagentDefaultModel sets the default model for subagents from config.
+func (sm *SubagentManager) SetSubagentDefaultModel(model string) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+	sm.subagentDefaultModel = model
+}
+
+// SetModelValidator sets the function used to validate model names.
+func (sm *SubagentManager) SetModelValidator(validator func(string) bool) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+	sm.modelValidator = validator
+}
+
+// ResolveModel resolves the model to use for a subagent task.
+// Priority: requested > subagentDefaultModel > defaultModel (parent).
+func (sm *SubagentManager) ResolveModel(requested string) (string, error) {
+	sm.mu.RLock()
+	validator := sm.modelValidator
+	subDefault := sm.subagentDefaultModel
+	parentModel := sm.defaultModel
+	sm.mu.RUnlock()
+
+	if requested != "" {
+		if validator != nil && !validator(requested) {
+			return "", fmt.Errorf("model %q not found in model_list", requested)
+		}
+		return requested, nil
+	}
+	if subDefault != "" {
+		return subDefault, nil
+	}
+	return parentModel, nil
+}
+
 // SetTools sets the tool registry for subagent execution.
 // If not set, subagent will have access to the provided tools.
 func (sm *SubagentManager) SetTools(tools *ToolRegistry) {

pkg/tools/subagent_tool_test.go

@@ -300,6 +300,68 @@ func TestSubagentTool_Execute_ContextPassing(t *testing.T) {
 	// but execution success indicates context was handled properly
 }
 
+func TestSubagentManager_ResolveModel(t *testing.T) {
+	provider := &MockLLMProvider{}
+	alwaysValid := func(name string) bool { return true }
+
+	t.Run("uses requested model when valid", func(t *testing.T) {
+		manager := NewSubagentManager(provider, "parent-model", "/tmp/test", nil)
+		manager.SetModelValidator(alwaysValid)
+		model, err := manager.ResolveModel("requested-model")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if model != "requested-model" {
+			t.Errorf("got %q, want %q", model, "requested-model")
+		}
+	})
+
+	t.Run("returns error for invalid requested model", func(t *testing.T) {
+		manager := NewSubagentManager(provider, "parent-model", "/tmp/test", nil)
+		manager.SetModelValidator(func(name string) bool { return name != "bad-model" })
+		_, err := manager.ResolveModel("bad-model")
+		if err == nil {
+			t.Fatal("expected error for invalid model")
+		}
+	})
+
+	t.Run("falls back to subagent default model", func(t *testing.T) {
+		manager := NewSubagentManager(provider, "parent-model", "/tmp/test", nil)
+		manager.SetSubagentDefaultModel("subagent-default")
+		manager.SetModelValidator(alwaysValid)
+		model, err := manager.ResolveModel("")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if model != "subagent-default" {
+			t.Errorf("got %q, want %q", model, "subagent-default")
+		}
+	})
+
+	t.Run("falls back to parent model", func(t *testing.T) {
+		manager := NewSubagentManager(provider, "parent-model", "/tmp/test", nil)
+		manager.SetModelValidator(alwaysValid)
+		model, err := manager.ResolveModel("")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if model != "parent-model" {
+			t.Errorf("got %q, want %q", model, "parent-model")
+		}
+	})
+
+	t.Run("works without validator set (no validation)", func(t *testing.T) {
+		manager := NewSubagentManager(provider, "parent-model", "/tmp/test", nil)
+		model, err := manager.ResolveModel("any-model")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if model != "any-model" {
+			t.Errorf("got %q, want %q", model, "any-model")
+		}
+	})
+}
+
 // TestSubagentTool_ForUserTruncation verifies long content is truncated for user
 func TestSubagentTool_ForUserTruncation(t *testing.T) {
 	// Create a mock provider that returns very long content