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Posted to notifications@superset.apache.org by "eudaimos (via GitHub)" <gi...@apache.org> on 2026/07/29 19:57:01 UTC
[I] fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients [superset]
eudaimos opened a new issue, #42578:
URL: https://github.com/apache/superset/issues/42578
### Bug
`GlobalErrorHandlerMiddleware` in `superset/mcp_service/middleware.py` imports pydantic's `ValidationError`:
```python
from pydantic import ValidationError # line 29
```
and dispatches on it:
```python
elif isinstance(error, ValidationError):
# Pydantic validation errors
validation_details = []
for err in error.errors():
field = " -> ".join(str(loc) for loc in err["loc"])
validation_details.append(f"{field}: {err['msg']}")
raise ToolError(
f"Validation error in {tool_name}: {'; '.join(validation_details)}"
) from error
```
But FastMCP 3.x raises its **own** `fastmcp.exceptions.ValidationError` when tool arguments fail to validate — it is not a subclass of pydantic's, and not a subclass of `ToolError`:
```
fastmcp.ValidationError MRO: ['ValidationError', 'FastMCPError', 'Exception', 'BaseException', 'object']
is subclass of ToolError: False
is subclass of pydantic.ValidationError: False
```
So the `isinstance` check never matches. Every malformed-arguments call falls through to the generic branch:
```python
else:
# Generic internal errors
error_id = f"err_{int(time.time())}"
logger.error("Unexpected error [%s] in %s: %s", error_id, tool_name, error)
raise ToolError(
f"Internal error in {tool_name}: An unexpected error occurred. "
f"Error ID: {error_id}. Please contact support if this persists."
) from error
```
A client-side 400-class error is reported to the client as an opaque internal server error.
### Reproduction
Confirmed in-process against a running MCP service (Superset 6.1.0, fastmcp 3.4.3, pydantic 2.11.7). Calling `execute_sql` with arguments that omit the required `request` wrapper:
```python
t = await mcp.get_tool('execute_sql')
try:
await t.run({'database_id': '1', 'sql': 'SHOW TABLES'})
except Exception as e:
print(type(e).__module__ + '.' + type(e).__name__)
print(isinstance(e, PydanticValidationError))
print(isinstance(e, FastMCPValidationError))
```
Output:
```
fastmcp.exceptions.ValidationError
False
True
```
Resulting server log — note `error_type=ValidationError` is logged, then the *same* `_handle_error` invocation falls through to the generic branch:
```
ERROR:superset.mcp_service.middleware:MCP tool error: tool=execute_sql, user_id=17, duration_ms=2, error_type=ValidationError, error=3 validation errors for call[execute_sql]
request
Missing required argument [type=missing_argument, input_value={'database_id': '1', 'sql': 'SHOW TABLES'}, input_type=dict]
ERROR:superset.mcp_service.middleware:Unexpected error [err_1785349449] in execute_sql: 3 validation errors for call[execute_sql]
```
What the client receives:
```
Internal error in execute_sql: An unexpected error occurred. Error ID: err_1785349449. Please contact support if this persists.
```
### Impact
This is actively harmful with LLM clients, which are the primary consumers of this service. In our case a user's agent made three malformed calls (`execute_sql`, `list_datasets`, `list_dashboards`), received three "Internal error… contact support" responses, observed that `health_check` still passed (it is the only tool taking no arguments), and concluded:
> Health check passes, but every data endpoint still returns internal server errors — it looks like a backend outage on the Superset MCP's data layer, not something I can work around from here.
It then stopped retrying and the user escalated it as an outage. The service was entirely healthy.
The `ValidationError` branch that already exists would have returned:
```
Validation error in execute_sql: request: Missing required argument
```
which is self-correcting — an LLM client reading that will retry with the wrapper. Correct error classification is the difference between a client recovering on its own and a false outage report.
### Suggested fix
Match both exception types:
```python
from fastmcp.exceptions import ToolError, ValidationError as FastMCPValidationError
from pydantic import ValidationError as PydanticValidationError
...
elif isinstance(error, (PydanticValidationError, FastMCPValidationError)):
```
`error.errors()` only exists on the pydantic exception, so the detail-formatting loop needs to be guarded — `fastmcp.exceptions.ValidationError` carries its message in `str(error)`, which already contains the per-field pydantic detail (as visible in the log above).
Since `fastmcp.exceptions.ValidationError` derives from `FastMCPError`, it may be worth handling `FastMCPError` generally so future FastMCP exception types don't silently land in the internal-error branch too.
### Environment
- Superset 6.1.0 (`apache/superset:6.1.0`)
- fastmcp 3.4.3
- pydantic 2.11.7
- Python 3.10.20, Linux
- Transport: streamable-http
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Re: [I] fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients [superset]
Posted by "eudaimos (via GitHub)" <gi...@apache.org>.
eudaimos commented on issue #42578:
URL: https://github.com/apache/superset/issues/42578#issuecomment-5122874882
Also related: #42580 — a third bug in the same file (`LoggingMiddleware._is_error_response`) that inverts the `success` flag on successful `execute_sql` calls. Taken together, #42578, #42579 and #42580 mean MCP tool errors are misreported to clients, never written to the audit log, and successes are recorded as failures.
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Re: [I] fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients [superset]
Posted by "dosubot[bot] (via GitHub)" <gi...@apache.org>.
dosubot[bot] commented on issue #42578:
URL: https://github.com/apache/superset/issues/42578#issuecomment-5122936785
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<!-- Answer -->
Good summary of the three issues together. It looks like [PR #41921](https://github.com/apache/superset/pull/41921) ("feat(mcp): add observability to MCP service") is currently open and addresses all three bugs in `superset/mcp_service/middleware.py`:
- **#42578** — fixes exception dispatch by unwrapping `ToolError.__cause__` before classification, so `fastmcp.exceptions.ValidationError` is correctly identified instead of falling through to the generic "internal error" branch [[1]](https://github.com/apache/superset/pull/41921)
- **#42579** — adds the missing `dashboard_id`, `slice_id`, and `referrer` arguments to the `event_logger.log()` call in `_handle_error`, so `mcp_tool_error` events actually get written to the audit log [[2]](https://github.com/apache/superset/issues/42579#issuecomment-5122874991)
- **#42580** — replaces the substring check (`'"error_type"' in result.content[0].text`) with a proper `_extract_error_type_from_response()` parser, so successful `execute_sql` responses containing `"error_type": null` aren't misclassified as failures [[3]](https://github.com/apache/superset/issues/42580)
That PR also adds per-tool success/error counters, timing metrics, and a configurable `MCP_ERROR_HOOK` for external error tracking [[1]](https://github.com/apache/superset/pull/41921).
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Re: [I] fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients [superset]
Posted by "eudaimos (via GitHub)" <gi...@apache.org>.
eudaimos commented on issue #42578:
URL: https://github.com/apache/superset/issues/42578#issuecomment-5122828784
Related: #42579 — a second bug in the same `GlobalErrorHandlerMiddleware._handle_error` function (the `event_logger.log()` call a few lines above this dispatch). Both are visible in the same log excerpt quoted above and could reasonably be fixed together.
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Re: [I] fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients [superset]
Posted by "rusackas (via GitHub)" <gi...@apache.org>.
rusackas closed issue #42578: fix(mcp): FastMCP ValidationError misreported as "Internal error" — invalid tool arguments surface as a 500 to clients
URL: https://github.com/apache/superset/issues/42578
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