From 1c456eaacd1601f7506734771b194633e39ca63f Mon Sep 17 00:00:00 2001
From: Jonathan Amsterdam <jba@google.com>
Date: Wed, 23 Jul 2025 05:33:26 -0400
Subject: [PATCH 1/2] mcp: panic if AddTool is given a nil InputSchema

The spec requires an input schema for every tool.
---
 mcp/mcp_test.go    | 2 +-
 mcp/server.go      | 4 +---
 mcp/server_test.go | 6 ++++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/mcp/mcp_test.go b/mcp/mcp_test.go
index 032181a1..3c222eb4 100644
--- a/mcp/mcp_test.go
+++ b/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()
 
diff --git a/mcp/server.go b/mcp/server.go
index 75e66dbc..c91a9723 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -12,7 +12,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"iter"
-	"log"
 	"maps"
 	"net/url"
 	"path/filepath"
@@ -137,8 +136,7 @@ func (s *Server) RemovePrompts(names ...string) {
 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))
+		panic(fmt.Sprintf("adding tool %q: nil input schema", t.Name))
 	}
 	if err := addToolErr(s, t, h); err != nil {
 		panic(err)
diff --git a/mcp/server_test.go b/mcp/server_test.go
index bb539772..9a36c962 100644
--- a/mcp/server_test.go
+++ b/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 {

From d28233011a1d8d33134c392f7cc59adf2c703dc2 Mon Sep 17 00:00:00 2001
From: Jonathan Amsterdam <jba@google.com>
Date: Wed, 23 Jul 2025 14:58:56 -0400
Subject: [PATCH 2/2] mcp: panic if AddTool ... add comment

---
 mcp/server.go | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mcp/server.go b/mcp/server.go
index c91a9723..2a2b4b64 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -131,11 +131,17 @@ 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 {
+		// 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 {