@@ -645,10 +645,18 @@ def handle_request(request, method, session: nil, related_request_id: nil, auth_
       begin
         Methods.ensure_capability!(method, capabilities)
       rescue Methods::MissingRequiredCapabilityError => e
-        # Re-raise through RequestHandlerError, the one channel whose message is
-        # deliberately surfaced to clients: `JsonRpcHandler`'s blind `rescue StandardError`
-        # no longer echoes exception messages into the error `data` member (CWE-209).
-        raise RequestHandlerError.new(e.message, request, error_type: :internal_error, original_error: e)
+        # A method of a capability the server does not declare is one the server does not implement, so it is answered as Method not found,
+        # which the TypeScript and Python servers return for a handler they never registered, and which the Streamable HTTP transport maps to
+        # 404 on the modern lifecycle. Re-raised through RequestHandlerError, the one channel whose message is deliberately surfaced to clients:
+        # `JsonRpcHandler`'s blind `rescue StandardError` no longer echoes exception messages into the error `data` member (CWE-209).
+        raise RequestHandlerError.new(
+          "Method not found",
+          request,
+          error_type: :method_not_found,
+          error_code: JsonRpcHandler::ErrorCode::METHOD_NOT_FOUND,
+          error_data: e.message,
+          original_error: e,
+        )
       end
 
       # `initialize` MUST NOT be cancelled (MCP spec 2025-11-25, cancellation item 2),
 
@@ -6047,6 +6047,55 @@ def string
           assert_equal(-32601, JSON.parse(response[2][0]).dig("error", "code"))
         end
 
+        test "modern POST maps a method of an undeclared capability to 404 with -32601" do
+          server = Server.new(name: "tools_only", capabilities: { tools: {} })
+          transport = StreamableHTTPTransport.new(server)
+
+          response = transport.handle_request(modern_rack_request(modern_body("prompts/list", {})))
+
+          assert_equal 404, response[0]
+
+          body = JSON.parse(response[2][0])
+
+          assert_equal(-32601, body.dig("error", "code"))
+          assert_equal "Server does not support prompts (required for prompts/list)", body.dig("error", "data")
+          assert_equal 1, body["id"]
+        ensure
+          transport.close
+        end
+
+        test "legacy POST answers a method of an undeclared capability with 200 and -32601" do
+          server = Server.new(name: "tools_only", capabilities: { tools: {} })
+          transport = StreamableHTTPTransport.new(server)
+          init_response = transport.handle_request(create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
+          ))
+          session_id = init_response[1]["mcp-session-id"]
+
+          response = transport.handle_request(create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => session_id },
+            { jsonrpc: "2.0", method: "prompts/list", id: 1 }.to_json,
+          ))
+
+          assert_equal 200, response[0]
+
+          io = StringIO.new
+          response[2].call(io)
+          body = JSON.parse(io.string.match(/^data: (.+)$/)[1])
+
+          assert_equal(-32601, body.dig("error", "code"))
+          assert_equal "Method not found", body.dig("error", "message")
+          assert_equal "Server does not support prompts (required for prompts/list)", body.dig("error", "data")
+          assert_equal 1, body["id"]
+        ensure
+          transport.close
+        end
+
         test "modern POST requires the _meta envelope" do
           response = @transport.handle_request(modern_rack_request(
             { jsonrpc: "2.0", method: "tools/list", id: 1 }.to_json,
 
@@ -1880,23 +1880,32 @@ def read_resource_request(uri)
 
       assert_equal "2.0", response[:jsonrpc]
       assert_equal 1, response[:id]
-      assert_equal(-32603, response[:error][:code])
+      assert_equal(-32601, response[:error][:code])
+      assert_equal "Method not found", response[:error][:message]
       assert_includes response[:error][:data], "Server does not support logging"
     end
 
-    test "#handle method with missing required top-level capability returns an error" do
+    test "#handle method with missing required top-level capability returns Method not found" do
+      # A method of an undeclared capability is one the server does not implement, as the TypeScript and Python
+      # servers answer for a handler they never registered; -32603 would read as a crash.
       @server.capabilities = {}
 
       response = @server.handle({ jsonrpc: "2.0", method: "prompts/list", id: 1 })
+      assert_equal(-32601, response[:error][:code])
+      assert_equal "Method not found", response[:error][:message]
       assert_equal "Server does not support prompts (required for prompts/list)", response[:error][:data]
 
       response = @server.handle({ jsonrpc: "2.0", method: "resources/list", id: 1 })
+      assert_equal(-32601, response[:error][:code])
+      assert_equal "Method not found", response[:error][:message]
       assert_equal "Server does not support resources (required for resources/list)", response[:error][:data]
     end
 
-    test "#handle method with missing required nested capability returns an error" do
+    test "#handle method with missing required nested capability returns Method not found" do
       @server.capabilities = { resources: {} }
       response = @server.handle({ jsonrpc: "2.0", method: "resources/subscribe", id: 1 })
+      assert_equal(-32601, response[:error][:code])
+      assert_equal "Method not found", response[:error][:message]
       assert_equal "Server does not support resources.subscribe (required for resources/subscribe)",
         response[:error][:data]
     end