Always add subagent tool

README.md

@@ -205,7 +205,7 @@ mcpServers:
 # Extensions and tools
 --extension, -e          Load additional extension file(s) (repeatable)
 --no-extensions          Disable all extensions
---no-core-tools          Disable all built-in core tools (bash, read, write, edit, grep, find, ls, subagent)
+--no-core-tools          Disable built-in core tools except subagent: (bash, read, write, edit, grep, find, ls)
 --prompt-template        Load a specific prompt template by name
 --no-prompt-templates    Disable prompt template loading
 
@@ -602,7 +602,8 @@ host, err := kit.New(ctx, &kit.Options{
     // Tool options
     Tools:            []kit.Tool{...},     // Replace default tool set entirely
     ExtraTools:       []kit.Tool{...},     // Add tools alongside defaults
-    DisableCoreTools: true,                // Disable all built-in core tools; also controllable via
+    DisableCoreTools: true,                // Disable all built-in core tools except for the
+                                           // subagent tool; also controllable via
                                            // --no-core-tools flag, KIT_NO_CORE_TOOLS env var,
                                            // or no-core-tools: true in .kit.yml
 

internal/agent/agent.go

@@ -42,9 +42,10 @@ type AgentConfig struct {
 	// CodingTools or tools with a custom WorkDir).
 	CoreTools []fantasy.AgentTool
 
-	// DisableCoreTools, when true, prevents loading any core tools.
+	// DisableCoreTools, when true, prevents loading any core tools except for
+	// the subagent tool.
 	// If both DisableCoreTools is true and CoreTools is empty, the agent
-	// will have no tools (useful for simple chat completions).
+	// will only have the subagent tool (useful for simple chat completions).
 	DisableCoreTools bool
 
 	// ToolWrapper is an optional function that wraps the combined tool list
@@ -288,11 +289,12 @@ func NewAgent(ctx context.Context, agentConfig *AgentConfig) (*Agent, error) {
 
 	// Register core tools (direct AgentTool implementations, no MCP overhead).
 	// Use caller-provided tools if set, otherwise default to all core tools.
-	// DisableCoreTools allows explicitly having zero tools (for chat-only mode).
+	// DisableCoreTools allows explicitly having zero tools except for the
+	// subagent tool (for chat-only mode).
 	var coreTools []fantasy.AgentTool
 	if agentConfig.DisableCoreTools && len(agentConfig.CoreTools) == 0 {
-		// Explicitly zero tools - chat-only mode
-		coreTools = nil
+		// Explicitly subagent tool only - chat-only mode
+		coreTools = core.SubagentTool()
 	} else if len(agentConfig.CoreTools) > 0 {
 		// Custom tools provided - use them
 		coreTools = agentConfig.CoreTools

internal/agent/factory.go

@@ -45,9 +45,10 @@ type AgentCreationOptions struct {
 	// CoreTools overrides the default core tool set. If empty, core.AllTools()
 	// is used.
 	CoreTools []fantasy.AgentTool
-	// DisableCoreTools, when true, prevents loading any core tools.
+	// DisableCoreTools, when true, prevents loading any core tools except for
+	// the subagent tool.
 	// If both DisableCoreTools is true and CoreTools is empty, the agent
-	// will have no tools (useful for simple chat completions).
+	// will only have the subagent tool (useful for simple chat completions).
 	DisableCoreTools bool
 	// ToolWrapper wraps the combined tool list before agent creation.
 	ToolWrapper func([]fantasy.AgentTool) []fantasy.AgentTool

internal/core/tools.go

@@ -100,6 +100,12 @@ func SubagentTools(opts ...ToolOption) []fantasy.AgentTool {
 	}
 }
 
+func SubagentTool(opts ...ToolOption) []fantasy.AgentTool {
+	return []fantasy.AgentTool{
+		NewSubagentTool(opts...),
+	}
+}
+
 // AllTools returns all available core tools.
 func AllTools(opts ...ToolOption) []fantasy.AgentTool {
 	return append(SubagentTools(opts...), NewSubagentTool(opts...))

internal/kitsetup/setup.go

@@ -33,9 +33,10 @@ type AgentSetupOptions struct {
 	// CoreTools overrides the default core tool set. If empty, core.AllTools()
 	// is used. Allows SDK users to pass custom tools (e.g. with WithWorkDir).
 	CoreTools []fantasy.AgentTool
-	// DisableCoreTools, when true, prevents loading any core tools.
+	// DisableCoreTools, when true, prevents loading any core tools except for
+	// the subagent tool.
 	// If both DisableCoreTools is true and CoreTools is empty, the agent
-	// will have no tools (useful for simple chat completions).
+	// will only have the subagent tool (useful for simple chat completions).
 	DisableCoreTools bool
 	// ExtraTools are additional tools added alongside core, MCP, and extension
 	// tools. They do not replace the defaults — they extend them.

pkg/kit/README.md

@@ -100,7 +100,7 @@ host, err := kit.New(ctx, &kit.Options{
     // Tool options
     Tools:            []kit.Tool{kit.NewBashTool()}, // Replace default tool set
     ExtraTools:       []kit.Tool{myTool},            // Add alongside defaults
-    DisableCoreTools: true,                        // Use no core tools (0 tools)
+    DisableCoreTools: true,                        // Use no core tools except for the subagent tool
 
     // Configuration
     SkipConfig:   true,                        // Skip .kit.yml files (viper defaults + env vars still apply)
@@ -398,7 +398,7 @@ Key `Options` fields for SDK usage:
 | `SkipConfig` | Skip `.kit.yml` loading (defaults + env vars still apply) |
 | `Tools` | Replace core tools with custom set |
 | `ExtraTools` | Add tools alongside defaults |
-| `DisableCoreTools` | Use no core tools (0 tools, for chat-only) |
+| `DisableCoreTools` | Use no core tools (except subagent tool - for chat-only) |
 | `NoSession` | Ephemeral mode (no session persistence) |
 | `SessionPath` | Open specific session file |
 | `Continue` | Resume most recent session |

pkg/kit/kit_test.go

@@ -375,6 +375,7 @@ func TestNewWithSkillsOptions(t *testing.T) {
 	ctx := context.Background()
 
 	t.Run("NoSkills disables skill loading", func(t *testing.T) {
+		defer resetViper()
 		host, err := kit.New(ctx, &kit.Options{
 			Model:     "anthropic/claude-sonnet-4-5-20250929",
 			Quiet:     true,
@@ -392,6 +393,7 @@ func TestNewWithSkillsOptions(t *testing.T) {
 	})
 
 	t.Run("SkillsDir propagates", func(t *testing.T) {
+		defer resetViper()
 		// Use a non-existent dir — no skills will load but the option must be
 		// accepted without error and result in zero skills.
 		dir := t.TempDir()
@@ -411,6 +413,7 @@ func TestNewWithSkillsOptions(t *testing.T) {
 	})
 
 	t.Run("explicit Skills paths load correctly", func(t *testing.T) {
+		defer resetViper()
 		// Write a minimal skill file to a temp dir.
 		dir := t.TempDir()
 		skillFile := dir + "/my-skill.md"
@@ -472,3 +475,32 @@ func TestNewSystemPromptInline(t *testing.T) {
 		t.Errorf("composed system-prompt missing inline content; got %q", composed)
 	}
 }
+
+// TestDisableCoreTools verifies that setting Options.DisableCoreTools to true
+// limits the available tools to only the 'subagent' tool.
+func TestDisableCoreTools(t *testing.T) {
+	if os.Getenv("ANTHROPIC_API_KEY") == "" {
+		t.Skip("Skipping test: ANTHROPIC_API_KEY not set")
+	}
+	defer resetViper()
+
+	ctx := context.Background()
+	host, err := kit.New(ctx, &kit.Options{
+		Model:            "anthropic/claude-sonnet-4-5-20250929",
+		Quiet:            true,
+		NoSession:        true,
+		NoExtensions:     true,
+		DisableCoreTools: true,
+	})
+	if err != nil {
+		t.Fatalf("Failed to create Kit with DisableCoreTools: %v", err)
+	}
+	defer func() { _ = host.Close() }()
+
+	tools := host.GetToolNames()
+	if len(tools) != 1 {
+		t.Errorf("Expected 1 tool when DisableCoreTools is true, got %d: %v", len(tools), tools)
+	} else if len(tools) > 0 && tools[0] != "subagent" {
+		t.Errorf("Expected only 'subagent' tool, got %q", tools[0])
+	}
+}

skills/kit-sdk/SKILL.md

@@ -107,7 +107,7 @@ host, err := kit.New(ctx, &kit.Options{
     // Tools
     Tools:            []kit.Tool{kit.NewBashTool()}, // REPLACES entire default tool set
     ExtraTools:       []kit.Tool{myTool},            // ADDS alongside core/MCP/extension tools
-    DisableCoreTools: true,                        // Use no core tools (0 tools, for chat-only)
+    DisableCoreTools: true,                        // Use no core tools except for the subagent tool (for chat-only)
 
     // Configuration
     SkipConfig:   true,                        // Skip .kit.yml files (viper defaults + env vars still apply)

www/pages/sdk/options.md

@@ -55,7 +55,7 @@ host, err := kit.New(ctx, &kit.Options{
     // Tools
     Tools:            []kit.Tool{...},     // Replace default tool set entirely
     ExtraTools:       []kit.Tool{...},     // Add tools alongside defaults
-    DisableCoreTools: true,                // Use no core tools (0 tools, for chat-only)
+    DisableCoreTools: true,                // Use no core tools except for the subagent tool (for chat-only)
 
     // Configuration
     SkipConfig:   true,                   // Skip .kit.yml files (viper defaults + env vars still apply)
@@ -158,7 +158,7 @@ when embedding Kit as a library.
 |-------|------|---------|-------------|
 | `Tools` | `[]Tool` | — | Replace the entire default tool set |
 | `ExtraTools` | `[]Tool` | — | Additional tools alongside core/MCP/extension tools |
-| `DisableCoreTools` | `bool` | `false` | Use no core tools (0 tools, for chat-only) |
+| `DisableCoreTools` | `bool` | `false` | Use no core tools except for the subagent tool (for chat-only) |
 | `NoExtensions` | `bool` | `false` | Disable Yaegi extension loading |
 | `NoContextFiles` | `bool` | `false` | Disable automatic AGENTS.md loading |