diff --git a/pkg/agent/instance.go b/pkg/agent/instance.go
index 0ac3d8894..c0ede9a9c 100644
--- a/pkg/agent/instance.go
+++ b/pkg/agent/instance.go
@@ -9,7 +9,6 @@ import (
 	"strings"
 
 	"github.com/sipeed/picoclaw/pkg/config"
-	"github.com/sipeed/picoclaw/pkg/git"
 	"github.com/sipeed/picoclaw/pkg/providers"
 	"github.com/sipeed/picoclaw/pkg/routing"
 	"github.com/sipeed/picoclaw/pkg/session"
@@ -141,23 +140,10 @@ func NewAgentInstance(
 
 	agentID := routing.DefaultAgentID
 	agentName := ""
-	var subagents *config.SubagentsConfig
-	var skillsFilter []string
 
 	if agentCfg != nil {
 		agentID = routing.NormalizeAgentID(agentCfg.ID)
 		agentName = agentCfg.Name
-		subagents = agentCfg.Subagents
-		skillsFilter = agentCfg.Skills
-	}
-
-	// Apply defaults.Orchestration: if the flag is set, ensure orchestration is enabled.
-	if defaults.Orchestration {
-		if subagents == nil {
-			subagents = &config.SubagentsConfig{Enabled: true}
-		} else {
-			subagents.Enabled = true
-		}
 	}
 
 	maxIter := defaults.MaxToolIterations
@@ -243,19 +229,6 @@ func NewAgentInstance(
 
 	candidates := providers.ResolveCandidatesWithLookup(modelCfg, defaults.Provider, resolveFromModelList)
 
-	// Resolve plan model (for interviewing/review phases)
-	planModel := resolvePlanModel(agentCfg, defaults)
-	planFallbacks := resolvePlanFallbacks(agentCfg, defaults)
-
-	var planCandidates []providers.FallbackCandidate
-	if planModel != "" {
-		planModelCfg := providers.ModelConfig{
-			Primary:   planModel,
-			Fallbacks: planFallbacks,
-		}
-		planCandidates = providers.ResolveCandidates(planModelCfg, defaults.Provider)
-	}
-
 	// Model routing setup: pre-resolve light model candidates at creation time
 	// to avoid repeated model_list lookups on every incoming message.
 	var router *routing.Router
@@ -275,17 +248,7 @@ func NewAgentInstance(
 		}
 	}
 
-	// Startup cleanup: prune orphaned worktrees
-	worktreesDir := filepath.Join(workspace, ".worktrees")
-	if repoRoot := git.FindRepoRoot(workspace); repoRoot != "" {
-		git.PruneOrphaned(repoRoot, worktreesDir)
-	}
-
-	return &AgentInstance{
-		instanceExt: instanceExt{
-			Subagents:    subagents,
-			SkillsFilter: skillsFilter,
-		},
+	agent := &AgentInstance{
 		ID:                        agentID,
 		Name:                      agentName,
 		Model:                     model,
@@ -304,12 +267,14 @@ func NewAgentInstance(
 		ContextBuilder:            contextBuilder,
 		Tools:                     toolsRegistry,
 		Candidates:                candidates,
-		PlanModel:                 planModel,
-		PlanFallbacks:             planFallbacks,
-		PlanCandidates:            planCandidates,
 		Router:                    router,
 		LightCandidates:           lightCandidates,
 	}
+
+	// Initialize fork-specific fields (subagents, plan model, worktree pruning).
+	agent.initInstanceExt(agentCfg, defaults, cfg)
+
+	return agent
 }
 
 // resolveAgentWorkspace determines the workspace directory for an agent.
diff --git a/pkg/agent/instance_ext.go b/pkg/agent/instance_ext.go
index 10aa835e9..4a76a819e 100644
--- a/pkg/agent/instance_ext.go
+++ b/pkg/agent/instance_ext.go
@@ -2,10 +2,12 @@ package agent
 
 import (
 	"fmt"
+	"path/filepath"
 	"sync"
 
 	"github.com/sipeed/picoclaw/pkg/config"
 	"github.com/sipeed/picoclaw/pkg/git"
+	"github.com/sipeed/picoclaw/pkg/providers"
 	"github.com/sipeed/picoclaw/pkg/tools"
 )
 
@@ -28,6 +30,47 @@ type instanceExt struct {
 	worktreeMu sync.RWMutex
 }
 
+// initInstanceExt initializes fork-specific fields: subagents config,
+// skills filter, plan model resolution, and worktree pruning.
+func (ai *AgentInstance) initInstanceExt(
+	agentCfg *config.AgentConfig,
+	defaults *config.AgentDefaults,
+	cfg *config.Config,
+) {
+	// Extract subagents and skills filter from agent config
+	if agentCfg != nil {
+		ai.Subagents = agentCfg.Subagents
+		ai.SkillsFilter = agentCfg.Skills
+	}
+
+	// Apply defaults.Orchestration: if the flag is set, ensure orchestration is enabled.
+	if defaults.Orchestration {
+		if ai.Subagents == nil {
+			ai.Subagents = &config.SubagentsConfig{Enabled: true}
+		} else {
+			ai.Subagents.Enabled = true
+		}
+	}
+
+	// Resolve plan model (for interviewing/review phases)
+	ai.PlanModel = resolvePlanModel(agentCfg, defaults)
+	ai.PlanFallbacks = resolvePlanFallbacks(agentCfg, defaults)
+
+	if ai.PlanModel != "" {
+		planModelCfg := providers.ModelConfig{
+			Primary:   ai.PlanModel,
+			Fallbacks: ai.PlanFallbacks,
+		}
+		ai.PlanCandidates = providers.ResolveCandidates(planModelCfg, defaults.Provider)
+	}
+
+	// Startup cleanup: prune orphaned worktrees
+	worktreesDir := filepath.Join(ai.Workspace, ".worktrees")
+	if repoRoot := git.FindRepoRoot(ai.Workspace); repoRoot != "" {
+		git.PruneOrphaned(repoRoot, worktreesDir)
+	}
+}
+
 // ActivateWorktree creates a worktree for a session.
 // projectDir is the git repository to create the worktree in.
 // If empty, falls back to ai.Workspace.