Treat JSON-null MCP result/error members as omitted

CHANGELOG.md

@@ -7,6 +7,13 @@ loosely while pre-1.0 (breaking changes can land on minor bumps).
 
 ## [Unreleased]
 
+### Fixed
+- **MCP JSON-RPC null members.** `parse_response` now treats `"error": null`
+  / `"result": null` as omitted, matching A2A and serializers that emit
+  optional fields as null. A success envelope with `"error": null` (or an
+  error envelope with `"result": null`) no longer throws and no longer
+  burns the client's 5-parse-failure budget.
+
 ## [0.13.4] — 2026-09-10
 
 ### Fixed

src/mcp/types.cpp

@@ -59,11 +59,17 @@ Response parse_response(const std::string& line) {
 
     auto res_v = v->get("result");
     auto err_v = v->get("error");
-    if (res_v && err_v)
+    // JSON-RPC 2.0 allows only one of result/error. Serializers (and
+    // A2A's parser) still emit the other member as JSON null. Treat
+    // null as omitted so `"result":{...},"error":null` is success and
+    // `"result":null,"error":{...}` is an error. Otherwise Client::rpc
+    // counts the throw as a parse failure and kills the MCP subprocess
+    // after five such lines.
+    const bool have_result = res_v && !res_v->is_null();
+    const bool have_error  = err_v && !err_v->is_null();
+    if (have_result && have_error)
         throw std::runtime_error("response has both 'result' and 'error'");
-    if (res_v) {
-        r.result = res_v;
-    } else if (err_v) {
+    if (have_error) {
         if (!err_v->is_object())
             throw std::runtime_error("'error' is not a JSON object");
         RpcError e;
@@ -73,8 +79,12 @@ Response parse_response(const std::string& line) {
             e.message = m->as_string();
         if (auto d = err_v->get("data")) e.data = d;
         r.error = std::move(e);
+    } else if (res_v) {
+        // Keep JSON-null `result` so a successful method that returns
+        // null is distinct from a notification (no result member).
+        r.result = res_v;
     }
-    // Empty result + empty error is allowed (e.g. notifications/initialized
+    // Empty result + empty error is allowed (e.g. notifications/initialized)
     // ack from some servers); leaves result null and error empty.
     return r;
 }

tests/test_mcp.cpp

@@ -220,6 +220,36 @@ TEST_CASE("parse_response rejects malformed envelopes") {
     CHECK_THROWS(parse_response(R"({"jsonrpc":"2.0","id":1,"result":{},"error":{"code":1,"message":"x"}})")); // both
 }
 
+TEST_CASE("parse_response treats JSON-null result/error as omitted") {
+    auto ok = parse_response(
+        R"({"jsonrpc":"2.0","id":5,"result":{"x":1},"error":null})");
+    CHECK(ok.id == 5);
+    REQUIRE(ok.result);
+    CHECK_FALSE(ok.result->is_null());
+    CHECK(static_cast<int>(ok.result->get_number("x", 0)) == 1);
+    CHECK_FALSE(ok.error.has_value());
+
+    auto err = parse_response(
+        R"({"jsonrpc":"2.0","id":6,"result":null,"error":{"code":-32601,"message":"no such method"}})");
+    CHECK(err.id == 6);
+    CHECK_FALSE(static_cast<bool>(err.result));
+    REQUIRE(err.error.has_value());
+    CHECK(err.error->code == -32601);
+    CHECK(err.error->message == "no such method");
+
+    auto null_result = parse_response(
+        R"({"jsonrpc":"2.0","id":7,"result":null})");
+    CHECK(null_result.id == 7);
+    REQUIRE(null_result.result);
+    CHECK(null_result.result->is_null());
+    CHECK_FALSE(null_result.error.has_value());
+
+    auto empty = parse_response(R"({"jsonrpc":"2.0","id":8,"error":null})");
+    CHECK(empty.id == 8);
+    CHECK_FALSE(static_cast<bool>(empty.result));
+    CHECK_FALSE(empty.error.has_value());
+}
+
 TEST_CASE("parse_tools_list extracts name + description + schema") {
     auto resp_v = json_parse(R"({"jsonrpc":"2.0","id":1,"result":{"tools":[
         {"name":"navigate","description":"Open a URL","inputSchema":{"type":"object"}},