Omit the id of an error response when it is unknown

lib/json_rpc_handler.rb

@@ -202,6 +202,6 @@ def error_response(id:, id_validation_pattern:, error:)
       jsonrpc: Version::V2_0,
       id: valid_id?(id, id_validation_pattern) ? id : nil,
       error: error.compact,
-    } unless id.nil?
+    }.compact unless id.nil?
   end
 end

lib/mcp/server/transports/streamable_http_transport.rb

@@ -2033,7 +2033,7 @@ def validate_protocol_version_header(request)
         def json_rpc_error_response(status:, code:, message:, data: nil, id: nil)
           error = { code: code, message: message }
           error[:data] = data if data
-          body = { jsonrpc: "2.0", id: id, error: error }
+          body = { jsonrpc: "2.0", id: id, error: error }.compact
           [status, { "content-type" => "application/json" }, [body.to_json]]
         end
 
@@ -2486,7 +2486,7 @@ def invalid_request_response(message, request_id: nil)
               code: JsonRpcHandler::ErrorCode::INVALID_REQUEST,
               message: message,
             },
-          }
+          }.compact
           [400, { "content-type" => "application/json" }, [body.to_json]]
         end
 

test/json_rpc_handler_test.rb

@@ -290,6 +290,8 @@
     #   If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be
     #   Null.
     #
+    # MCP leaves the id out instead: an error response has an optional string or integer id, never null.
+    #
     # Either the result member or error member MUST be included, but both members MUST NOT be included.
 
     it "returns a result object and no error object on success" do
@@ -315,7 +317,7 @@
       assert_nil @response[:result]
     end
 
-    it "returns nil for id when there is an error and and error detecting the id" do
+    it "omits the id when there is an error and an error detecting the id" do
       register("ping") { "pong" }
 
       handle jsonrpc: "2.0", id: {}, method: "ping"
@@ -325,7 +327,7 @@
         message: "Invalid Request",
         data: "Request ID must match validation pattern, or be an integer or null",
       }
-      assert_nil @response[:id]
+      refute @response.key?(:id)
     end
 
     it "returns the same request id on an Invalid Request error when the id is detectable" do
@@ -339,37 +341,37 @@
       assert_equal 3, @response[:id]
     end
 
-    it "returns nil for id on an Invalid Request error when the request has no id" do
+    it "omits the id on an Invalid Request error when the request has no id" do
       handle jsonrpc: "1.0", method: "ping", params: {}
 
       assert_rpc_error expected_error: {
         code: -32600,
         message: "Invalid Request",
         data: "JSON-RPC version must be 2.0",
       }
-      assert_nil @response[:id]
+      refute @response.key?(:id)
     end
 
-    it "returns nil for id on an Invalid Request error when the request id is explicitly null" do
+    it "omits the id on an Invalid Request error when the request id is explicitly null" do
       handle jsonrpc: "1.0", id: nil, method: "ping", params: {}
 
       assert_rpc_error expected_error: {
         code: -32600,
         message: "Invalid Request",
         data: "JSON-RPC version must be 2.0",
       }
-      assert_nil @response[:id]
+      refute @response.key?(:id)
     end
 
-    it "returns nil for id on an Invalid Request error when the id fails validation" do
+    it "omits the id on an Invalid Request error when the id fails validation" do
       handle jsonrpc: "1.0", id: "<script>alert('xss')</script>", method: "ping", params: {}
 
       assert_rpc_error expected_error: {
         code: -32600,
         message: "Invalid Request",
         data: "JSON-RPC version must be 2.0",
       }
-      assert_nil @response[:id]
+      refute @response.key?(:id)
     end
 
     # 5.1 Error object
@@ -405,6 +407,7 @@
         message: "Parse error",
         data: "Invalid JSON",
       }
+      refute @response.key?(:id)
     end
 
     it "returns an error with code set to -32600 when the request is not an array or a hash" do

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -171,7 +171,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::PARSE_ERROR, body["error"]["code"]
           assert_equal "Parse error: Invalid JSON", body["error"]["message"]
         end
@@ -300,7 +300,7 @@ def string
           assert_equal 404, response[0]
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Session not found", body["error"]["message"]
         end
@@ -388,7 +388,7 @@ def string
           assert_equal 400, response[0]
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_match(/single request object/i, body["error"]["message"])
         end
@@ -553,7 +553,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Missing session ID", body["error"]["message"]
         end
@@ -711,7 +711,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Conflict: Only one SSE stream is allowed per session", body["error"]["message"]
         end
@@ -1618,7 +1618,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Method not allowed", body["error"]["message"]
         end
@@ -1636,7 +1636,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Unsupported Media Type: Content-Type must be application/json", body["error"]["message"]
         end
@@ -1831,7 +1831,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
           assert_equal "Not Acceptable: Accept header must include text/event-stream", body["error"]["message"]
         end
@@ -2996,7 +2996,7 @@ def string
 
           body = JSON.parse(response[2][0])
           assert_equal "2.0", body["jsonrpc"]
-          assert_nil body["id"]
+          refute body.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INTERNAL_ERROR, body["error"]["code"]
           assert_equal "Internal server error", body["error"]["message"]
         end
@@ -6758,9 +6758,8 @@ def string
 
           parsed = JSON.parse(response[2][0])
 
-          # The member is present and null, as JSON-RPC 2.0 requires when the id cannot be determined.
-          assert parsed.key?("id")
-          assert_nil parsed["id"]
+          # The member is left out: MCP allows no null id, where JSON-RPC 2.0 would set it to null.
+          refute parsed.key?("id")
           assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, parsed.dig("error", "code")
           assert_empty @transport.instance_variable_get(:@listen_subscriptions)
         end