diff --git a/pkg/agentic/commands_session_test.go b/pkg/agentic/commands_session_test.go
index e296a28..c7deae5 100644
--- a/pkg/agentic/commands_session_test.go
+++ b/pkg/agentic/commands_session_test.go
@@ -102,17 +102,17 @@ func TestCommandsSession_CmdSessionStart_Good(t *testing.T) {
 		var payload map[string]any
 		parseResult := core.JSONUnmarshalString(bodyResult.Value.(string), &payload)
 		require.True(t, parseResult.OK)
-		assert.Equal(t, "codex", payload["agent_type"])
+		assert.Equal(t, "opus", payload["agent_type"])
 		assert.Equal(t, "ax-follow-up", payload["plan_slug"])
 
-		_, _ = w.Write([]byte(`{"data":{"session_id":"ses-start","plan_slug":"ax-follow-up","agent_type":"codex","status":"active"}}`))
+		_, _ = w.Write([]byte(`{"data":{"session_id":"ses-start","plan_slug":"ax-follow-up","agent_type":"opus","status":"active"}}`))
 	}))
 	defer server.Close()
 
 	subsystem := testPrepWithPlatformServer(t, server, "secret-token")
 	result := subsystem.cmdSessionStart(core.NewOptions(
 		core.Option{Key: "_arg", Value: "ax-follow-up"},
-		core.Option{Key: "agent_type", Value: "codex"},
+		core.Option{Key: "agent_type", Value: "opus"},
 	))
 	require.True(t, result.OK)
 
@@ -120,19 +120,32 @@ func TestCommandsSession_CmdSessionStart_Good(t *testing.T) {
 	require.True(t, ok)
 	assert.Equal(t, "ses-start", output.Session.SessionID)
 	assert.Equal(t, "ax-follow-up", output.Session.PlanSlug)
-	assert.Equal(t, "codex", output.Session.AgentType)
+	assert.Equal(t, "opus", output.Session.AgentType)
 }
 
 func TestCommandsSession_CmdSessionStart_Bad_MissingPlanSlug(t *testing.T) {
 	subsystem := testPrepWithPlatformServer(t, nil, "secret-token")
 
-	result := subsystem.cmdSessionStart(core.NewOptions(core.Option{Key: "agent_type", Value: "codex"}))
+	result := subsystem.cmdSessionStart(core.NewOptions(core.Option{Key: "agent_type", Value: "opus"}))
 
 	assert.False(t, result.OK)
 	require.Error(t, result.Value.(error))
 	assert.Contains(t, result.Value.(error).Error(), "plan_slug is required")
 }
 
+func TestCommandsSession_CmdSessionStart_Bad_InvalidAgentType(t *testing.T) {
+	subsystem := testPrepWithPlatformServer(t, nil, "secret-token")
+
+	result := subsystem.cmdSessionStart(core.NewOptions(
+		core.Option{Key: "_arg", Value: "ax-follow-up"},
+		core.Option{Key: "agent_type", Value: "codex"},
+	))
+
+	assert.False(t, result.OK)
+	require.Error(t, result.Value.(error))
+	assert.Contains(t, result.Value.(error).Error(), "opus, sonnet, or haiku")
+}
+
 func TestCommandsSession_CmdSessionStart_Ugly_InvalidResponse(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, _ = w.Write([]byte(`{"data":`))
diff --git a/pkg/agentic/session.go b/pkg/agentic/session.go
index daa3774..4b690ab 100644
--- a/pkg/agentic/session.go
+++ b/pkg/agentic/session.go
@@ -364,6 +364,9 @@ func (s *PrepSubsystem) sessionStart(ctx context.Context, _ *mcp.CallToolRequest
 	if input.AgentType == "" {
 		return nil, SessionOutput{}, core.E("sessionStart", "agent_type is required", nil)
 	}
+	if !validSessionAgentType(input.AgentType) {
+		return nil, SessionOutput{}, core.E("sessionStart", "agent_type must be opus, sonnet, or haiku", nil)
+	}
 
 	body := map[string]any{
 		"agent_type": input.AgentType,
@@ -1137,3 +1140,12 @@ func resultErrorValue(action string, result core.Result) error {
 
 	return core.E(action, "request failed", nil)
 }
+
+func validSessionAgentType(agentType string) bool {
+	switch core.Lower(core.Trim(agentType)) {
+	case "opus", "sonnet", "haiku":
+		return true
+	default:
+		return false
+	}
+}
diff --git a/pkg/agentic/session_test.go b/pkg/agentic/session_test.go
index 7719595..feb4d2b 100644
--- a/pkg/agentic/session_test.go
+++ b/pkg/agentic/session_test.go
@@ -25,16 +25,16 @@ func TestSession_HandleSessionStart_Good(t *testing.T) {
 		var payload map[string]any
 		parseResult := core.JSONUnmarshalString(bodyResult.Value.(string), &payload)
 		require.True(t, parseResult.OK)
-		require.Equal(t, "codex", payload["agent_type"])
+		require.Equal(t, "opus", payload["agent_type"])
 		require.Equal(t, "ax-follow-up", payload["plan_slug"])
 
-		_, _ = w.Write([]byte(`{"data":{"id":1,"session_id":"ses_abc123","plan_slug":"ax-follow-up","agent_type":"codex","status":"active","context_summary":{"repo":"core/go"}}}`))
+		_, _ = w.Write([]byte(`{"data":{"id":1,"session_id":"ses_abc123","plan_slug":"ax-follow-up","agent_type":"opus","status":"active","context_summary":{"repo":"core/go"}}}`))
 	}))
 	defer server.Close()
 
 	subsystem := testPrepWithPlatformServer(t, server, "secret-token")
 	result := subsystem.handleSessionStart(context.Background(), core.NewOptions(
-		core.Option{Key: "agent_type", Value: "codex"},
+		core.Option{Key: "agent_type", Value: "opus"},
 		core.Option{Key: "plan_slug", Value: "ax-follow-up"},
 		core.Option{Key: "context", Value: `{"repo":"core/go"}`},
 	))
@@ -44,7 +44,7 @@ func TestSession_HandleSessionStart_Good(t *testing.T) {
 	require.True(t, ok)
 	assert.Equal(t, "ses_abc123", output.Session.SessionID)
 	assert.Equal(t, "active", output.Session.Status)
-	assert.Equal(t, "codex", output.Session.AgentType)
+	assert.Equal(t, "opus", output.Session.AgentType)
 }
 
 func TestSession_HandleSessionStart_Bad(t *testing.T) {
@@ -54,6 +54,16 @@ func TestSession_HandleSessionStart_Bad(t *testing.T) {
 	assert.False(t, result.OK)
 }
 
+func TestSession_HandleSessionStart_Bad_InvalidAgentType(t *testing.T) {
+	subsystem := testPrepWithPlatformServer(t, nil, "secret-token")
+
+	result := subsystem.handleSessionStart(context.Background(), core.NewOptions(
+		core.Option{Key: "agent_type", Value: "codex"},
+	))
+	assert.False(t, result.OK)
+	require.Contains(t, result.Value.(error).Error(), "opus, sonnet, or haiku")
+}
+
 func TestSession_HandleSessionStart_Ugly(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, _ = w.Write([]byte(`{"data":`))