fix(spawn): use target agent model instead of caller model

When spawn is called with agent_id parameter, the subagent now uses
the target agent's configured model instead of always using the caller's model.

Changes:
- Add modelResolver field to SubagentManager
- Add SetModelResolver method to configure the resolver
- In runTask, resolve target model using the resolver when agent_id is specified
- Set up model resolver in registerSharedTools to lookup agent from registry

Fixes #1322

pkg/agent/loop.go

@@ -224,6 +224,13 @@ func registerSharedTools(
 			if cfg.Tools.IsToolEnabled("subagent") {
 				subagentManager := tools.NewSubagentManager(provider, agent.Model, agent.Workspace)
 				subagentManager.SetLLMOptions(agent.MaxTokens, agent.Temperature)
+				// Set up model resolver to use target agent's configured model
+				subagentManager.SetModelResolver(func(targetAgentID string) string {
+					if targetAgent, ok := registry.GetAgent(targetAgentID); ok {
+						return targetAgent.Model
+					}
+					return ""
+				})
 				spawnTool := tools.NewSpawnTool(subagentManager)
 				currentAgentID := agentID
 				spawnTool.SetAllowlistChecker(func(targetAgentID string) bool {

pkg/tools/subagent.go

@@ -34,6 +34,9 @@ type SubagentManager struct {
 	hasMaxTokens   bool
 	hasTemperature bool
 	nextID         int
+	// modelResolver is called to resolve the target model for a given agent ID.
+	// If nil or returns empty string, defaultModel is used.
+	modelResolver func(agentID string) string
 }
 
 func NewSubagentManager(
@@ -61,6 +64,14 @@ func (sm *SubagentManager) SetLLMOptions(maxTokens int, temperature float64) {
 	sm.hasTemperature = true
 }
 
+// SetModelResolver sets a function to resolve the target model for a given agent ID.
+// This allows subagent to use the target agent's configured model instead of the caller's model.
+func (sm *SubagentManager) SetModelResolver(resolver func(agentID string) string) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+	sm.modelResolver = resolver
+}
+
 // 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) {
@@ -147,8 +158,18 @@ After completing the task, provide a clear summary of what was done.`
 	temperature := sm.temperature
 	hasMaxTokens := sm.hasMaxTokens
 	hasTemperature := sm.hasTemperature
+	modelResolver := sm.modelResolver
+	defaultModel := sm.defaultModel
 	sm.mu.RUnlock()
 
+	// Resolve target model: use target agent's model if agent_id is specified and resolver is available
+	targetModel := defaultModel
+	if task.AgentID != "" && modelResolver != nil {
+		if resolvedModel := modelResolver(task.AgentID); resolvedModel != "" {
+			targetModel = resolvedModel
+		}
+	}
+
 	var llmOptions map[string]any
 	if hasMaxTokens || hasTemperature {
 		llmOptions = map[string]any{}
@@ -162,7 +183,7 @@ After completing the task, provide a clear summary of what was done.`
 
 	loopResult, err := RunToolLoop(ctx, ToolLoopConfig{
 		Provider:      sm.provider,
-		Model:         sm.defaultModel,
+		Model:         targetModel,
 		Tools:         tools,
 		MaxIterations: maxIter,
 		LLMOptions:    llmOptions,