mcp: enforce input schema type "object" (#349)

The spec makes it clear that input schemas must have type "object".
Enforce that.
Allow "any" as an input argument type by special-casing it.

Fixes #283.

Author: jba

mcp/mcp_test.go

@@ -98,7 +98,7 @@ func TestEndToEnd(t *testing.T) {
 		Name:        "greet",
 		Description: "say hi",
 	}, sayHi)
-	AddTool(s, &Tool{Name: "fail", InputSchema: &jsonschema.Schema{}},
+	AddTool(s, &Tool{Name: "fail", InputSchema: &jsonschema.Schema{Type: "object"}},
 		func(context.Context, *CallToolRequest, map[string]any) (*CallToolResult, any, error) {
 			return nil, nil, errTestFailure
 		})
@@ -257,7 +257,7 @@ func TestEndToEnd(t *testing.T) {
 			t.Errorf("tools/call 'fail' mismatch (-want +got):\n%s", diff)
 		}
 
-		s.AddTool(&Tool{Name: "T", InputSchema: &jsonschema.Schema{}}, nopHandler)
+		s.AddTool(&Tool{Name: "T", InputSchema: &jsonschema.Schema{Type: "object"}}, nopHandler)
 		waitForNotification(t, "tools")
 		s.RemoveTools("T")
 		waitForNotification(t, "tools")
@@ -697,7 +697,7 @@ func TestCancellation(t *testing.T) {
 		return nil, nil, nil
 	}
 	cs, _ := basicConnection(t, func(s *Server) {
-		AddTool(s, &Tool{Name: "slow", InputSchema: &jsonschema.Schema{}}, slowRequest)
+		AddTool(s, &Tool{Name: "slow", InputSchema: &jsonschema.Schema{Type: "object"}}, slowRequest)
 	})
 	defer cs.Close()
 
@@ -1496,8 +1496,8 @@ func TestAddTool_DuplicateNoPanicAndNoDuplicate(t *testing.T) {
 		// Use two distinct Tool instances with the same name but different
 		// descriptions to ensure the second replaces the first
 		// This case was written specifically to reproduce a bug where duplicate tools where causing jsonschema errors
-		t1 := &Tool{Name: "dup", Description: "first", InputSchema: &jsonschema.Schema{}}
-		t2 := &Tool{Name: "dup", Description: "second", InputSchema: &jsonschema.Schema{}}
+		t1 := &Tool{Name: "dup", Description: "first", InputSchema: &jsonschema.Schema{Type: "object"}}
+		t2 := &Tool{Name: "dup", Description: "second", InputSchema: &jsonschema.Schema{Type: "object"}}
 		s.AddTool(t1, nopHandler)
 		s.AddTool(t2, nopHandler)
 	})

mcp/server.go

@@ -162,6 +162,9 @@ func (s *Server) AddTool(t *Tool, h ToolHandler) {
 		// discovered until runtime, when the LLM sent bad data.
 		panic(fmt.Errorf("AddTool %q: missing input schema", t.Name))
 	}
+	if t.InputSchema.Type != "object" {
+		panic(fmt.Errorf(`AddTool %q: input schema must have type "object"`, t.Name))
+	}
 	st := &serverTool{tool: t, handler: h}
 	// Assume there was a change, since add replaces existing tools.
 	// (It's possible a tool was replaced with an identical one, but not worth checking.)
@@ -190,6 +193,12 @@ func ToolFor[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHandle
 // TODO(v0.3.0): test
 func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHandler, error) {
 	tt := *t
+
+	// Special handling for an "any" input: treat as an empty object.
+	if reflect.TypeFor[In]() == reflect.TypeFor[any]() && t.InputSchema == nil {
+		tt.InputSchema = &jsonschema.Schema{Type: "object"}
+	}
+
 	var inputResolved *jsonschema.Resolved
 	if _, err := setSchema[In](&tt.InputSchema, &inputResolved); err != nil {
 		return nil, nil, fmt.Errorf("input schema: %w", err)

mcp/server_test.go

@@ -232,7 +232,7 @@ func TestServerPaginateVariousPageSizes(t *testing.T) {
 }
 
 func TestServerCapabilities(t *testing.T) {
-	tool := &Tool{Name: "t", InputSchema: &jsonschema.Schema{}}
+	tool := &Tool{Name: "t", InputSchema: &jsonschema.Schema{Type: "object"}}
 	testCases := []struct {
 		name             string
 		configureServer  func(s *Server)

mcp/streamable_test.go

@@ -219,7 +219,7 @@ func testClientReplay(t *testing.T, test clientReplayTest) {
 	// proxy-killing action.
 	serverReadyToKillProxy := make(chan struct{})
 	serverClosed := make(chan struct{})
-	AddTool(server, &Tool{Name: "multiMessageTool", InputSchema: &jsonschema.Schema{}},
+	AddTool(server, &Tool{Name: "multiMessageTool", InputSchema: &jsonschema.Schema{Type: "object"}},
 		func(ctx context.Context, req *CallToolRequest, args map[string]any) (*CallToolResult, any, error) {
 			// Send one message to the request context, and another to a background
 			// context (which will end up on the hanging GET).
@@ -353,7 +353,7 @@ func TestServerInitiatedSSE(t *testing.T) {
 		t.Fatalf("client.Connect() failed: %v", err)
 	}
 	defer clientSession.Close()
-	AddTool(server, &Tool{Name: "testTool", InputSchema: &jsonschema.Schema{}},
+	AddTool(server, &Tool{Name: "testTool", InputSchema: &jsonschema.Schema{Type: "object"}},
 		func(context.Context, *CallToolRequest, map[string]any) (*CallToolResult, any, error) {
 			return &CallToolResult{}, nil, nil
 		})
@@ -658,7 +658,7 @@ func TestStreamableServerTransport(t *testing.T) {
 			// behavior, if any.
 			server := NewServer(&Implementation{Name: "testServer", Version: "v1.0.0"}, nil)
 			server.AddTool(
-				&Tool{Name: "tool", InputSchema: &jsonschema.Schema{}},
+				&Tool{Name: "tool", InputSchema: &jsonschema.Schema{Type: "object"}},
 				func(ctx context.Context, req *CallToolRequest) (*CallToolResult, error) {
 					if test.tool != nil {
 						test.tool(t, ctx, req.Session)