fix(SEP-2575): introduce a custom error code for unsupported protocol error

Author: yarolegovich

docs/seps/2575-stateless-mcp.mdx

@@ -68,7 +68,7 @@ listing the versions it does support:
   "jsonrpc": "2.0",
   "id": 1,
   "error": {
-    "code": -32602,
+    "code": -32004,
     "message": "Unsupported protocol version",
     "data": {
       "supported": ["DRAFT-2026-v1", "2025-11-25"],

docs/specification/draft/basic/lifecycle.mdx

@@ -2,7 +2,7 @@
   "jsonrpc": "2.0",
   "id": 1,
   "error": {
-    "code": -32602,
+    "code": -32004,
     "message": "Unsupported protocol version",
     "data": {
       "supported": ["DRAFT-2026-v1", "2025-11-25"],

docs/specification/draft/schema.mdx

@@ -361,6 +361,14 @@ export interface InternalError extends Error {
  */
 export const MISSING_REQUIRED_CLIENT_CAPABILITY = -32003;
 
+/**
+ * Error code returned when the request's protocol version is not supported
+ * by the server.
+ *
+ * @category Errors
+ */
+export const UNSUPPORTED_PROTOCOL_VERSION = -32004;
+
 /**
  * Returned when the request's protocol version is unknown to the server or
  * unsupported (e.g., a known experimental or draft version the server has
@@ -377,7 +385,7 @@ export interface UnsupportedProtocolVersionError extends Omit<
   "error"
 > {
   error: Error & {
-    code: typeof INVALID_PARAMS;
+    code: typeof UNSUPPORTED_PROTOCOL_VERSION;
     data: {
       /**
        * Protocol versions the server supports. The client should choose a

schema/draft/examples/UnsupportedProtocolVersionError/unsupported-version.json

@@ -171,12 +171,14 @@ return a JSON-RPC error response. For HTTP, the response status code MUST be
 `400 Bad Request`. The error MUST conform to the following structure:
 
 ```ts
+export const UNSUPPORTED_PROTOCOL_VERSION = -32004;
+
 export interface UnsupportedProtocolVersionError extends Omit<
   JSONRPCErrorResponse,
   "error"
 > {
   error: Error & {
-    code: typeof INVALID_PARAMS;
+    code: typeof UNSUPPORTED_PROTOCOL_VERSION;
     data: {
       /**
        * An array of protocol version strings that the server supports.