Implement batch related utilities for JSON-RPC and MCP

src/core/jsonrpc/include/sourcemeta/core/jsonrpc.h

@@ -66,6 +66,37 @@ constexpr std::int64_t JSONRPC_CODE_SERVER_ERROR_MAX = -32000;
 SOURCEMETA_CORE_JSONRPC_EXPORT
 auto jsonrpc_is_server_error(const std::int64_t code) -> bool;
 
+/// @ingroup jsonrpc
+/// Check whether the given JSON value is a JSON-RPC 2.0 batch envelope. For
+/// example:
+///
+/// ```cpp
+/// #include <sourcemeta/core/json.h>
+/// #include <sourcemeta/core/jsonrpc.h>
+/// #include <cassert>
+///
+/// const auto payload{sourcemeta::core::parse_json(R"([])")};
+/// assert(sourcemeta::core::jsonrpc_is_batch(payload));
+/// ```
+SOURCEMETA_CORE_JSONRPC_EXPORT
+auto jsonrpc_is_batch(const sourcemeta::core::JSON &payload) -> bool;
+
+/// @ingroup jsonrpc
+/// Check whether the given JSON value is a non-empty JSON-RPC 2.0 batch
+/// envelope. For example:
+///
+/// ```cpp
+/// #include <sourcemeta/core/json.h>
+/// #include <sourcemeta/core/jsonrpc.h>
+/// #include <cassert>
+///
+/// const auto payload{sourcemeta::core::parse_json(
+///     R"([ { "jsonrpc": "2.0", "method": "ping" } ])")};
+/// assert(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+/// ```
+SOURCEMETA_CORE_JSONRPC_EXPORT
+auto jsonrpc_is_valid_batch(const sourcemeta::core::JSON &payload) -> bool;
+
 /// @ingroup jsonrpc
 /// Extract the request identifier from a JSON-RPC 2.0 envelope. Returns a
 /// pointer to the identifier (string, number, or null per the specification)

src/core/jsonrpc/jsonrpc.cc

@@ -29,6 +29,14 @@ auto jsonrpc_is_server_error(const std::int64_t code) -> bool {
          code <= JSONRPC_CODE_SERVER_ERROR_MAX;
 }
 
+auto jsonrpc_is_batch(const sourcemeta::core::JSON &payload) -> bool {
+  return payload.is_array();
+}
+
+auto jsonrpc_is_valid_batch(const sourcemeta::core::JSON &payload) -> bool {
+  return jsonrpc_is_batch(payload) && !payload.empty();
+}
+
 auto jsonrpc_request_id(const sourcemeta::core::JSON &request)
     -> const sourcemeta::core::JSON * {
   if (!request.is_object()) {

src/core/mcp/include/sourcemeta/core/mcp.h

@@ -206,6 +206,14 @@ constexpr auto mcp_supports_implementation_website_url(
   return version == MCPProtocolVersion::V_2025_11_25;
 }
 
+/// @ingroup mcp
+/// Whether the given protocol version supports JSON-RPC 2.0 batching.
+constexpr auto
+mcp_supports_jsonrpc_batching(const MCPProtocolVersion version) noexcept
+    -> bool {
+  return version == MCPProtocolVersion::V_2025_03_26;
+}
+
 /// @ingroup mcp
 /// Build an MCP `text` content block carrying the given text payload. For
 /// example:

test/jsonrpc/jsonrpc_test.cc

@@ -51,6 +51,70 @@ TEST(JSONRPC, is_server_error_positive_application_code) {
   EXPECT_FALSE(sourcemeta::core::jsonrpc_is_server_error(1));
 }
 
+TEST(JSONRPC, is_batch_empty_array) {
+  const auto payload{sourcemeta::core::parse_json(R"([])")};
+  EXPECT_TRUE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_batch_non_empty_array) {
+  const auto payload{sourcemeta::core::parse_json(
+      R"([ { "jsonrpc": "2.0", "method": "ping" } ])")};
+  EXPECT_TRUE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_batch_object) {
+  const auto payload{sourcemeta::core::parse_json(
+      R"({ "jsonrpc": "2.0", "id": 1, "method": "ping" })")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_batch_string) {
+  const auto payload{sourcemeta::core::parse_json(R"("hello")")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_batch_null) {
+  const auto payload{sourcemeta::core::parse_json(R"(null)")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_batch_integer) {
+  const auto payload{sourcemeta::core::parse_json(R"(42)")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_non_empty_array) {
+  const auto payload{sourcemeta::core::parse_json(
+      R"([ { "jsonrpc": "2.0", "method": "ping" } ])")};
+  EXPECT_TRUE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_empty_array) {
+  const auto payload{sourcemeta::core::parse_json(R"([])")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_object) {
+  const auto payload{sourcemeta::core::parse_json(
+      R"({ "jsonrpc": "2.0", "id": 1, "method": "ping" })")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_string) {
+  const auto payload{sourcemeta::core::parse_json(R"("hello")")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_null) {
+  const auto payload{sourcemeta::core::parse_json(R"(null)")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
+TEST(JSONRPC, is_valid_batch_integer) {
+  const auto payload{sourcemeta::core::parse_json(R"(42)")};
+  EXPECT_FALSE(sourcemeta::core::jsonrpc_is_valid_batch(payload));
+}
+
 TEST(JSONRPC, request_id_integer) {
   const auto request{sourcemeta::core::parse_json(
       R"({ "jsonrpc": "2.0", "id": 7, "method": "ping" })")};

test/mcp/mcp_test.cc

@@ -240,6 +240,21 @@ TEST(MCP, supports_implementation_website_url_2025_11_25) {
       sourcemeta::core::MCPProtocolVersion::V_2025_11_25));
 }
 
+TEST(MCP, supports_jsonrpc_batching_2025_03_26) {
+  EXPECT_TRUE(sourcemeta::core::mcp_supports_jsonrpc_batching(
+      sourcemeta::core::MCPProtocolVersion::V_2025_03_26));
+}
+
+TEST(MCP, supports_jsonrpc_batching_2025_06_18) {
+  EXPECT_FALSE(sourcemeta::core::mcp_supports_jsonrpc_batching(
+      sourcemeta::core::MCPProtocolVersion::V_2025_06_18));
+}
+
+TEST(MCP, supports_jsonrpc_batching_2025_11_25) {
+  EXPECT_FALSE(sourcemeta::core::mcp_supports_jsonrpc_batching(
+      sourcemeta::core::MCPProtocolVersion::V_2025_11_25));
+}
+
 TEST(MCP, make_text_block) {
   const auto block{sourcemeta::core::mcp_make_text_block("hello")};
   const auto expected{sourcemeta::core::parse_json(R"JSON({