Emit JSON-RPC error code for error.type on validation failures

Align the ValidationError branch with the MCPError branch and the C# SDK:
set error.type and rpc.response.status_code to the numeric JSON-RPC code
(INVALID_PARAMS) as a string, not the exception class name.

Author: Kludex

src/mcp/server/_otel.py

@@ -8,7 +8,7 @@
 from mcp.server.context import CallNext, HandlerResult, ServerMiddleware, ServerRequestContext
 from mcp.shared._otel import extract_trace_context, otel_span
 from mcp.shared.exceptions import MCPError
-from mcp.types import CallToolResult
+from mcp.types import INVALID_PARAMS, CallToolResult
 
 
 class OpenTelemetryMiddleware(ServerMiddleware[Any]):
@@ -20,8 +20,9 @@ class OpenTelemetryMiddleware(ServerMiddleware[Any]):
 
     Tool and prompt operations additionally carry the GenAI semantic-convention attributes `gen_ai.tool.name` /
     `gen_ai.prompt.name`, and `gen_ai.operation.name` is set to `execute_tool` for `tools/call`. Failures set
-    `error.type` and `rpc.response.status_code` to the JSON-RPC error code, or `error.type` to `tool_error` for a
-    `tools/call` result carrying `is_error`.
+    `error.type` and `rpc.response.status_code` to the JSON-RPC error code as a string (e.g. `-32602`), or
+    `error.type` to `tool_error` for a `tools/call` result carrying `is_error`; a non-`MCPError` handler exception
+    sets `error.type` to its type name.
     """
 
     async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNext) -> HandlerResult:
@@ -59,7 +60,8 @@ async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNex
                 raise
             except ValidationError:
                 # Mirror the sanitized wire response; pydantic messages carry client input.
-                span.set_attribute("error.type", "ValidationError")
+                code = str(INVALID_PARAMS)
+                span.set_attributes({"error.type": code, "rpc.response.status_code": code})
                 span.set_status(StatusCode.ERROR, "Invalid request parameters")
                 raise
             except Exception as e:

tests/server/test_otel.py

@@ -14,6 +14,7 @@
 from mcp.shared._otel import inject_trace_context
 from mcp.shared.exceptions import MCPError
 from mcp.types import (
+    INVALID_PARAMS,
     CallToolRequestParams,
     CallToolResult,
     GetPromptRequestParams,
@@ -258,7 +259,8 @@ async def test_validation_failure_sets_sanitized_status(server: SrvT, spans: Spa
     assert span.status.status_code == StatusCode.ERROR
     assert span.status.description == "Invalid request parameters"
     assert span.attributes is not None
-    assert span.attributes["error.type"] == "ValidationError"
+    assert span.attributes["error.type"] == str(INVALID_PARAMS)
+    assert span.attributes["rpc.response.status_code"] == str(INVALID_PARAMS)
     assert not span.events