mcp: panic if AddTool method is given a nil InputSchema (#157)

The spec requires an input schema for every tool.

Author: jba

mcp/mcp_test.go

@@ -659,7 +659,7 @@ func TestCancellation(t *testing.T) {
 		return nil, nil
 	}
 	_, cs := basicConnection(t, func(s *Server) {
-		s.AddTool(&Tool{Name: "slow"}, slowRequest)
+		s.AddTool(&Tool{Name: "slow", InputSchema: &jsonschema.Schema{}}, slowRequest)
 	})
 	defer cs.Close()
 

mcp/server.go

@@ -12,7 +12,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"iter"
-	"log"
 	"maps"
 	"net/url"
 	"path/filepath"
@@ -132,13 +131,18 @@ func (s *Server) RemovePrompts(names ...string) {
 }
 
 // AddTool adds a [Tool] to the server, or replaces one with the same name.
-// The tool's input schema must be non-nil.
 // The Tool argument must not be modified after this call.
+//
+// The tool's input schema must be non-nil. For a tool that takes no input,
+// or one where any input is valid, set [Tool.InputSchema] to the empty schema,
+// &jsonschema.Schema{}.
 func (s *Server) AddTool(t *Tool, h ToolHandler) {
-	// TODO(jba): This is a breaking behavior change. Add before v0.2.0?
 	if t.InputSchema == nil {
-		log.Printf("mcp: tool %q has a nil input schema. This will panic in a future release.", t.Name)
-		// panic(fmt.Sprintf("adding tool %q: nil input schema", t.Name))
+		// This prevents the tool author from forgetting to write a schema where
+		// one should be provided. If we papered over this by supplying the empty
+		// schema, then every input would be validated and the problem wouldn't be
+		// discovered until runtime, when the LLM sent bad data.
+		panic(fmt.Sprintf("adding tool %q: nil input schema", t.Name))
 	}
 	if err := addToolErr(s, t, h); err != nil {
 		panic(err)

mcp/server_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/modelcontextprotocol/go-sdk/jsonschema"
 )
 
 type testItem struct {
@@ -230,6 +231,7 @@ func TestServerPaginateVariousPageSizes(t *testing.T) {
 }
 
 func TestServerCapabilities(t *testing.T) {
+	tool := &Tool{Name: "t", InputSchema: &jsonschema.Schema{}}
 	testCases := []struct {
 		name             string
 		configureServer  func(s *Server)
@@ -299,7 +301,7 @@ func TestServerCapabilities(t *testing.T) {
 		{
 			name: "With tools",
 			configureServer: func(s *Server) {
-				s.AddTool(&Tool{Name: "t"}, nil)
+				s.AddTool(tool, nil)
 			},
 			wantCapabilities: &serverCapabilities{
 				Completions: &completionCapabilities{},
@@ -313,7 +315,7 @@ func TestServerCapabilities(t *testing.T) {
 				s.AddPrompt(&Prompt{Name: "p"}, nil)
 				s.AddResource(&Resource{URI: "file:///r"}, nil)
 				s.AddResourceTemplate(&ResourceTemplate{URITemplate: "file:///rt"}, nil)
-				s.AddTool(&Tool{Name: "t"}, nil)
+				s.AddTool(tool, nil)
 			},
 			serverOpts: ServerOptions{
 				SubscribeHandler: func(ctx context.Context, sp *SubscribeParams) error {