mcp: reject requests with bad IDs (#202)

Add an isRequest field to methodInfo, used it to reject non-notification
requests that lack a valid ID. Furthermore, lift this validation to the
transport layer for HTTP server transports, so that we can preemptively
reject bad HTTP requests.

Fixes #194
Fixes #197

mcp/client.go

@@ -287,16 +287,16 @@ func (c *Client) AddReceivingMiddleware(middleware ...Middleware[*ClientSession]
 
 // clientMethodInfos maps from the RPC method name to serverMethodInfos.
 var clientMethodInfos = map[string]methodInfo{
-	methodComplete:                  newMethodInfo(sessionMethod((*ClientSession).Complete)),
-	methodPing:                      newMethodInfo(sessionMethod((*ClientSession).ping)),
-	methodListRoots:                 newMethodInfo(clientMethod((*Client).listRoots)),
-	methodCreateMessage:             newMethodInfo(clientMethod((*Client).createMessage)),
-	notificationToolListChanged:     newMethodInfo(clientMethod((*Client).callToolChangedHandler)),
-	notificationPromptListChanged:   newMethodInfo(clientMethod((*Client).callPromptChangedHandler)),
-	notificationResourceListChanged: newMethodInfo(clientMethod((*Client).callResourceChangedHandler)),
-	notificationResourceUpdated:     newMethodInfo(clientMethod((*Client).callResourceUpdatedHandler)),
-	notificationLoggingMessage:      newMethodInfo(clientMethod((*Client).callLoggingHandler)),
-	notificationProgress:            newMethodInfo(sessionMethod((*ClientSession).callProgressNotificationHandler)),
+	methodComplete:                  newMethodInfo(sessionMethod((*ClientSession).Complete), true),
+	methodPing:                      newMethodInfo(sessionMethod((*ClientSession).ping), true),
+	methodListRoots:                 newMethodInfo(clientMethod((*Client).listRoots), true),
+	methodCreateMessage:             newMethodInfo(clientMethod((*Client).createMessage), true),
+	notificationToolListChanged:     newMethodInfo(clientMethod((*Client).callToolChangedHandler), false),
+	notificationPromptListChanged:   newMethodInfo(clientMethod((*Client).callPromptChangedHandler), false),
+	notificationResourceListChanged: newMethodInfo(clientMethod((*Client).callResourceChangedHandler), false),
+	notificationResourceUpdated:     newMethodInfo(clientMethod((*Client).callResourceUpdatedHandler), false),
+	notificationLoggingMessage:      newMethodInfo(clientMethod((*Client).callLoggingHandler), false),
+	notificationProgress:            newMethodInfo(sessionMethod((*ClientSession).callProgressNotificationHandler), false),
 }
 
 func (cs *ClientSession) sendingMethodInfos() map[string]methodInfo {
@@ -323,7 +323,7 @@ func (cs *ClientSession) receivingMethodHandler() methodHandler {
 	return cs.client.receivingMethodHandler_
 }
 
-// getConn implements [session.getConn].
+// getConn implements [Session.getConn].
 func (cs *ClientSession) getConn() *jsonrpc2.Connection { return cs.conn }
 
 func (*ClientSession) ping(context.Context, *PingParams) (*emptyResult, error) {

mcp/server.go

@@ -688,22 +688,22 @@ func (s *Server) AddReceivingMiddleware(middleware ...Middleware[*ServerSession]
 
 // serverMethodInfos maps from the RPC method name to serverMethodInfos.
 var serverMethodInfos = map[string]methodInfo{
-	methodComplete:               newMethodInfo(serverMethod((*Server).complete)),
-	methodInitialize:             newMethodInfo(sessionMethod((*ServerSession).initialize)),
-	methodPing:                   newMethodInfo(sessionMethod((*ServerSession).ping)),
-	methodListPrompts:            newMethodInfo(serverMethod((*Server).listPrompts)),
-	methodGetPrompt:              newMethodInfo(serverMethod((*Server).getPrompt)),
-	methodListTools:              newMethodInfo(serverMethod((*Server).listTools)),
-	methodCallTool:               newMethodInfo(serverMethod((*Server).callTool)),
-	methodListResources:          newMethodInfo(serverMethod((*Server).listResources)),
-	methodListResourceTemplates:  newMethodInfo(serverMethod((*Server).listResourceTemplates)),
-	methodReadResource:           newMethodInfo(serverMethod((*Server).readResource)),
-	methodSetLevel:               newMethodInfo(sessionMethod((*ServerSession).setLevel)),
-	methodSubscribe:              newMethodInfo(serverMethod((*Server).subscribe)),
-	methodUnsubscribe:            newMethodInfo(serverMethod((*Server).unsubscribe)),
-	notificationInitialized:      newMethodInfo(serverMethod((*Server).callInitializedHandler)),
-	notificationRootsListChanged: newMethodInfo(serverMethod((*Server).callRootsListChangedHandler)),
-	notificationProgress:         newMethodInfo(sessionMethod((*ServerSession).callProgressNotificationHandler)),
+	methodComplete:               newMethodInfo(serverMethod((*Server).complete), true),
+	methodInitialize:             newMethodInfo(sessionMethod((*ServerSession).initialize), true),
+	methodPing:                   newMethodInfo(sessionMethod((*ServerSession).ping), true),
+	methodListPrompts:            newMethodInfo(serverMethod((*Server).listPrompts), true),
+	methodGetPrompt:              newMethodInfo(serverMethod((*Server).getPrompt), true),
+	methodListTools:              newMethodInfo(serverMethod((*Server).listTools), true),
+	methodCallTool:               newMethodInfo(serverMethod((*Server).callTool), true),
+	methodListResources:          newMethodInfo(serverMethod((*Server).listResources), true),
+	methodListResourceTemplates:  newMethodInfo(serverMethod((*Server).listResourceTemplates), true),
+	methodReadResource:           newMethodInfo(serverMethod((*Server).readResource), true),
+	methodSetLevel:               newMethodInfo(sessionMethod((*ServerSession).setLevel), true),
+	methodSubscribe:              newMethodInfo(serverMethod((*Server).subscribe), true),
+	methodUnsubscribe:            newMethodInfo(serverMethod((*Server).unsubscribe), true),
+	notificationInitialized:      newMethodInfo(serverMethod((*Server).callInitializedHandler), false),
+	notificationRootsListChanged: newMethodInfo(serverMethod((*Server).callRootsListChangedHandler), false),
+	notificationProgress:         newMethodInfo(sessionMethod((*ServerSession).callProgressNotificationHandler), false),
 }
 
 func (ss *ServerSession) sendingMethodInfos() map[string]methodInfo { return clientMethodInfos }

mcp/shared.go

@@ -123,9 +123,9 @@ func defaultReceivingMethodHandler[S Session](ctx context.Context, session S, me
 }
 
 func handleReceive[S Session](ctx context.Context, session S, req *jsonrpc.Request) (Result, error) {
-	info, ok := session.receivingMethodInfos()[req.Method]
-	if !ok {
-		return nil, jsonrpc2.ErrNotHandled
+	info, err := checkRequest(req, session.receivingMethodInfos())
+	if err != nil {
+		return nil, err
 	}
 	params, err := info.unmarshalParams(req.Params)
 	if err != nil {
@@ -141,8 +141,30 @@ func handleReceive[S Session](ctx context.Context, session S, req *jsonrpc.Reque
 	return res, nil
 }
 
+// checkRequest checks the given request against the provided method info, to
+// ensure it is a valid MCP request.
+//
+// If valid, the relevant method info is returned. Otherwise, a non-nil error
+// is returned describing why the request is invalid.
+//
+// This is extracted from request handling so that it can be called in the
+// transport layer to preemptively reject bad requests.
+func checkRequest(req *jsonrpc.Request, infos map[string]methodInfo) (methodInfo, error) {
+	info, ok := infos[req.Method]
+	if !ok {
+		return methodInfo{}, fmt.Errorf("%w: %q unsupported", jsonrpc2.ErrNotHandled, req.Method)
+	}
+	if info.isRequest && !req.ID.IsValid() {
+		return methodInfo{}, fmt.Errorf("%w: %q missing ID", jsonrpc2.ErrInvalidRequest, req.Method)
+	}
+	return info, nil
+}
+
 // methodInfo is information about sending and receiving a method.
 type methodInfo struct {
+	// isRequest reports whether the method is a JSON-RPC request.
+	// Otherwise, the method is treated as a notification.
+	isRequest bool
 	// Unmarshal params from the wire into a Params struct.
 	// Used on the receive side.
 	unmarshalParams func(json.RawMessage) (Params, error)
@@ -169,8 +191,12 @@ type paramsPtr[T any] interface {
 }
 
 // newMethodInfo creates a methodInfo from a typedMethodHandler.
-func newMethodInfo[S Session, P paramsPtr[T], R Result, T any](d typedMethodHandler[S, P, R]) methodInfo {
+//
+// If isRequest is set, the method is treated as a request rather than a
+// notification.
+func newMethodInfo[S Session, P paramsPtr[T], R Result, T any](d typedMethodHandler[S, P, R], isRequest bool) methodInfo {
 	return methodInfo{
+		isRequest: isRequest,
 		unmarshalParams: func(m json.RawMessage) (Params, error) {
 			var p P
 			if m != nil {

mcp/sse.go

@@ -129,6 +129,12 @@ func (t *SSEServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)
 		http.Error(w, "failed to parse body", http.StatusBadRequest)
 		return
 	}
+	if req, ok := msg.(*jsonrpc.Request); ok {
+		if _, err := checkRequest(req, serverMethodInfos); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+	}
 	select {
 	case t.incoming <- msg:
 		w.WriteHeader(http.StatusAccepted)

mcp/sse_test.go

@@ -5,8 +5,10 @@
 package mcp
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"sync/atomic"
@@ -24,10 +26,10 @@ func TestSSEServer(t *testing.T) {
 
 			sseHandler := NewSSEHandler(func(*http.Request) *Server { return server })
 
-			conns := make(chan *ServerSession, 1)
-			sseHandler.onConnection = func(cc *ServerSession) {
+			serverSessions := make(chan *ServerSession, 1)
+			sseHandler.onConnection = func(ss *ServerSession) {
 				select {
-				case conns <- cc:
+				case serverSessions <- ss:
 				default:
 				}
 			}
@@ -54,7 +56,7 @@ func TestSSEServer(t *testing.T) {
 			if err := cs.Ping(ctx, nil); err != nil {
 				t.Fatal(err)
 			}
-			ss := <-conns
+			ss := <-serverSessions
 			gotHi, err := cs.CallTool(ctx, &CallToolParams{
 				Name:      "greet",
 				Arguments: map[string]any{"Name": "user"},
@@ -76,6 +78,39 @@ func TestSSEServer(t *testing.T) {
 				t.Error("Expected custom HTTP client to be used, but it wasn't")
 			}
 
+			t.Run("badrequests", func(t *testing.T) {
+				msgEndpoint := cs.mcpConn.(*sseClientConn).msgEndpoint.String()
+
+				// Test some invalid data, and verify that we get 400s.
+				badRequests := []struct {
+					name             string
+					body             string
+					responseContains string
+				}{
+					{"not a method", `{"jsonrpc":"2.0", "method":"notamethod"}`, "not handled"},
+					{"missing ID", `{"jsonrpc":"2.0", "method":"ping"}`, "missing ID"},
+				}
+				for _, r := range badRequests {
+					t.Run(r.name, func(t *testing.T) {
+						resp, err := http.Post(msgEndpoint, "application/json", bytes.NewReader([]byte(r.body)))
+						if err != nil {
+							t.Fatal(err)
+						}
+						defer resp.Body.Close()
+						if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
+							t.Errorf("Sending bad request %q: got status %d, want %d", r.body, got, want)
+						}
+						result, err := io.ReadAll(resp.Body)
+						if err != nil {
+							t.Fatalf("Reading response: %v", err)
+						}
+						if !bytes.Contains(result, []byte(r.responseContains)) {
+							t.Errorf("Response body does not contain %q:\n%s", r.responseContains, string(result))
+						}
+					})
+				}
+			})
+
 			// Test that closing either end of the connection terminates the other
 			// end.
 			if closeServerFirst {

mcp/streamable.go

@@ -387,8 +387,16 @@ func (t *StreamableServerTransport) servePOST(w http.ResponseWriter, req *http.R
 	}
 	requests := make(map[jsonrpc.ID]struct{})
 	for _, msg := range incoming {
-		if req, ok := msg.(*jsonrpc.Request); ok && req.ID.IsValid() {
-			requests[req.ID] = struct{}{}
+		if req, ok := msg.(*jsonrpc.Request); ok {
+			// Preemptively check that this is a valid request, so that we can fail
+			// the HTTP request. If we didn't do this, a request with a bad method or
+			// missing ID could be silently swallowed.
+			if _, err := checkRequest(req, serverMethodInfos); err != nil {
+				return http.StatusBadRequest, err.Error()
+			}
+			if req.ID.IsValid() {
+				requests[req.ID] = struct{}{}
+			}
 		}
 	}
 

mcp/streamable_test.go

@@ -280,7 +280,7 @@ func TestStreamableServerTransport(t *testing.T) {
 	}
 
 	// Predefined steps, to avoid repetition below.
-	initReq := req(1, "initialize", &InitializeParams{})
+	initReq := req(1, methodInitialize, &InitializeParams{})
 	initResp := resp(1, &InitializeResult{
 		Capabilities: &serverCapabilities{
 			Completions: &completionCapabilities{},
@@ -290,7 +290,7 @@ func TestStreamableServerTransport(t *testing.T) {
 		ProtocolVersion: latestProtocolVersion,
 		ServerInfo:      &Implementation{Name: "testServer", Version: "v1.0.0"},
 	}, nil)
-	initializedMsg := req(0, "initialized", &InitializedParams{})
+	initializedMsg := req(0, notificationInitialized, &InitializedParams{})
 	initialize := step{
 		Method:     "POST",
 		Send:       []jsonrpc.Message{initReq},
@@ -438,6 +438,16 @@ func TestStreamableServerTransport(t *testing.T) {
 					Method:     "DELETE",
 					StatusCode: http.StatusBadRequest,
 				},
+				{
+					Method:     "POST",
+					Send:       []jsonrpc.Message{req(1, "notamethod", nil)},
+					StatusCode: http.StatusBadRequest, // notamethod is an invalid method
+				},
+				{
+					Method:     "POST",
+					Send:       []jsonrpc.Message{req(0, "tools/call", &CallToolParams{Name: "tool"})},
+					StatusCode: http.StatusBadRequest, // tools/call must have an ID
+				},
 				{
 					Method:     "POST",
 					Send:       []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},

mcp/testdata/conformance/server/missing_fields.txtar

@@ -0,0 +1,54 @@
+Check robustness to missing fields: servers should reject and otherwise ignore
+bad requests.
+
+Fixed bugs:
+- No id in 'initialize' should not panic (#197).
+- No id in 'ping' should not panic (#194).
+
+TODO:
+- No params in 'initialize' should not panic (#195).
+
+-- prompts --
+code_review
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+{
+  "jsonrpc": "2.0",
+  "id": 2,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+{"jsonrpc":"2.0", "method":"ping"}
+
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 2,
+	"result": {
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2024-11-05",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}