Reject empty metadata keys in MCP validation

src/core/mcp/validation.h

@@ -314,7 +314,7 @@ inline auto metadata_key(const JSON::StringView key, const bool prefix_required)
   const auto slash{key.find('/')};
   JSON::StringView name{key};
   if (slash == JSON::StringView::npos) {
-    if (prefix_required) {
+    if (prefix_required || key.empty()) {
       return false;
     }
   } else {

test/mcp/mcp_headers_test.cc

@@ -3738,6 +3738,15 @@ TEST(request_meta_validation_rejects_a_prefix_closing_on_a_dot) {
             sourcemeta::core::MCPRequestMetaStatus::InvalidMetaKey);
 }
 
+TEST(request_meta_validation_rejects_an_empty_metadata_key) {
+  const auto envelope{sourcemeta::core::parse_json(R"JSON({
+    "jsonrpc": "2.0", "id": 1, "method": "tools/list",
+    "params": { "_meta": { "": true } }
+  })JSON")};
+  EXPECT_EQ(sourcemeta::core::mcp_validate_request_meta(envelope).first,
+            sourcemeta::core::MCPRequestMetaStatus::InvalidMetaKey);
+}
+
 // A value that cannot travel in a header as it stands is carried base64
 // encoded between the markers, so what the comparison sees is what the
 // encoding spells out rather than the encoded form itself

test/mcp/mcp_notifications_test.cc

@@ -817,6 +817,19 @@ TEST(make_notification_rejects_malformed_metadata_key_on_2026_07_28) {
                      "Invalid notification metadata");
 }
 
+TEST(make_notification_rejects_empty_metadata_key_on_2026_07_28) {
+  EXPECT_MCP_REJECTS(sourcemeta::core::mcp_make_notification(
+                         sourcemeta::core::MCPProtocolVersion::V_2026_07_28,
+                         false, "notifications/cancelled",
+                         sourcemeta::core::parse_json(
+                             R"JSON({
+                               "requestId": 1,
+                               "_meta": { "": 1 }
+                             })JSON"),
+                         std::nullopt),
+                     "Invalid notification metadata");
+}
+
 TEST(make_notification_rejects_subscription_identifier_on_2025_11_25) {
   EXPECT_MCP_REJECTS(sourcemeta::core::mcp_make_notification(
                          sourcemeta::core::MCPProtocolVersion::V_2025_11_25,

test/mcp/mcp_results_test.cc

@@ -6725,6 +6725,24 @@ TEST(result_writer_metadata_rejections) {
                      "Discovery must include its serving revision");
 }
 
+TEST(result_writer_rejects_empty_metadata_key) {
+  std::ostringstream stream;
+  const auto bad{sourcemeta::core::parse_json(
+      R"JSON({
+        "resources": [],
+        "_meta": {
+          "": 1
+        }
+      })JSON")};
+  EXPECT_MCP_REJECTS(sourcemeta::core::mcp_write_result(
+                         stream,
+                         sourcemeta::core::MCPProtocolVersion::V_2026_07_28,
+                         "resources/list", sourcemeta::core::JSON{0}, bad,
+                         sourcemeta::core::MCPCachePolicy{}),
+                     "Invalid precomputed metadata");
+  EXPECT_TRUE(stream.str().empty());
+}
+
 // An elicitation schema may describe a single or a multiple choice, and the
 // builder carries the schema through untouched
 // https://modelcontextprotocol.io/specification/2026-07-28/schema#titledsingleselectenumschema