fix(handoff): preserve GT_AGENT across session restarts (#788)

Adds GT_AGENT env var to track agent override when using --agent flag.
Handoff reads and preserves GT_AGENT so non-default agents persist across restarts.

Co-authored-by: joshuavial <git@codewithjv.com>

internal/cmd/handoff.go

@@ -384,7 +384,20 @@ func buildRestartCommand(sessionName string) (string, error) {
 	// 3. export Claude-related env vars (not inherited by fresh shell)
 	// 4. run claude with the startup beacon (triggers immediate context loading)
 	// Use exec to ensure clean process replacement.
-	runtimeCmd := config.GetRuntimeCommandWithPrompt("", beacon)
+	//
+	// Check if current session is using a non-default agent (GT_AGENT env var).
+	// If so, preserve it across handoff by using the override variant.
+	currentAgent := os.Getenv("GT_AGENT")
+	var runtimeCmd string
+	if currentAgent != "" {
+		var err error
+		runtimeCmd, err = config.GetRuntimeCommandWithPromptAndAgentOverride("", beacon, currentAgent)
+		if err != nil {
+			return "", fmt.Errorf("resolving agent config: %w", err)
+		}
+	} else {
+		runtimeCmd = config.GetRuntimeCommandWithPrompt("", beacon)
+	}
 
 	// Build environment exports - role vars first, then Claude vars
 	var exports []string
@@ -398,6 +411,11 @@ func buildRestartCommand(sessionName string) (string, error) {
 		}
 	}
 
+	// Preserve GT_AGENT across handoff so agent override persists
+	if currentAgent != "" {
+		exports = append(exports, "GT_AGENT="+currentAgent)
+	}
+
 	// Add Claude-related env vars from current environment
 	for _, name := range claudeEnvVars {
 		if val := os.Getenv(name); val != "" {

internal/config/loader.go

@@ -1353,6 +1353,10 @@ func BuildStartupCommandWithAgentOverride(envVars map[string]string, rigPath, pr
 	if rc.Session != nil && rc.Session.SessionIDEnv != "" {
 		resolvedEnv["GT_SESSION_ID_ENV"] = rc.Session.SessionIDEnv
 	}
+	// Record agent override so handoff can preserve it
+	if agentOverride != "" {
+		resolvedEnv["GT_AGENT"] = agentOverride
+	}
 
 	// Build environment export prefix
 	var exports []string

internal/config/loader_test.go

@@ -2670,3 +2670,63 @@ func TestQuoteForShell(t *testing.T) {
 		})
 	}
 }
+
+func TestBuildStartupCommandWithAgentOverride_SetsGTAgent(t *testing.T) {
+	t.Parallel()
+	townRoot := t.TempDir()
+	rigPath := filepath.Join(townRoot, "testrig")
+
+	// Create necessary config files
+	townSettings := NewTownSettings()
+	if err := SaveTownSettings(TownSettingsPath(townRoot), townSettings); err != nil {
+		t.Fatalf("SaveTownSettings: %v", err)
+	}
+	if err := SaveRigSettings(RigSettingsPath(rigPath), NewRigSettings()); err != nil {
+		t.Fatalf("SaveRigSettings: %v", err)
+	}
+
+	cmd, err := BuildStartupCommandWithAgentOverride(
+		map[string]string{"GT_ROLE": constants.RoleWitness},
+		rigPath,
+		"",
+		"gemini",
+	)
+	if err != nil {
+		t.Fatalf("BuildStartupCommandWithAgentOverride: %v", err)
+	}
+
+	// Should include GT_AGENT=gemini in export so handoff can preserve it
+	if !strings.Contains(cmd, "GT_AGENT=gemini") {
+		t.Errorf("expected GT_AGENT=gemini in command, got: %q", cmd)
+	}
+}
+
+func TestBuildStartupCommandWithAgentOverride_NoGTAgentWhenNoOverride(t *testing.T) {
+	t.Parallel()
+	townRoot := t.TempDir()
+	rigPath := filepath.Join(townRoot, "testrig")
+
+	// Create necessary config files
+	townSettings := NewTownSettings()
+	if err := SaveTownSettings(TownSettingsPath(townRoot), townSettings); err != nil {
+		t.Fatalf("SaveTownSettings: %v", err)
+	}
+	if err := SaveRigSettings(RigSettingsPath(rigPath), NewRigSettings()); err != nil {
+		t.Fatalf("SaveRigSettings: %v", err)
+	}
+
+	cmd, err := BuildStartupCommandWithAgentOverride(
+		map[string]string{"GT_ROLE": constants.RoleWitness},
+		rigPath,
+		"",
+		"", // No override
+	)
+	if err != nil {
+		t.Fatalf("BuildStartupCommandWithAgentOverride: %v", err)
+	}
+
+	// Should NOT include GT_AGENT when no override is used
+	if strings.Contains(cmd, "GT_AGENT=") {
+		t.Errorf("expected no GT_AGENT in command when no override, got: %q", cmd)
+	}
+}