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Posted to notifications@superset.apache.org by "AurimasNav (via GitHub)" <gi...@apache.org> on 2026/08/20 06:42:46 UTC
[I] MCP: failed tool calls return isError: false, so denials look like successes [superset]
AurimasNav opened a new issue, #43358:
URL: https://github.com/apache/superset/issues/43358
> _Drafted with AI assistance._
### Bug description
Every failed MCP tool call returns a **successful** result with the error as plain text. `isError` is never set, so a client following the MCP spec reads a failure — including a permission denial — as a success.
Observed on 6.1.0 against a read-only service user whose role forbids the tool:
```json
{"jsonrpc":"2.0","id":7,"result":{"content":[{"type":"text","text":
"Error: Error calling tool 'save_sql_query': Permission denied: can_write on SavedQuery for user mcp-svc (tool: save_sql_query)"}],
"isError":false}}
```
The message itself is good — it names the permission, view, user and tool. The problem is purely the flag: an agent that checks `isError` concludes the write succeeded, and may report it as done.
### Root cause, and why this isn't a simple fix
`StructuredContentStripperMiddleware.on_call_tool` catches every exception and returns an ordinary `ToolResult`:
https://github.com/apache/superset/blob/master/superset/mcp_service/middleware.py#L767-L821
The catch is deliberate, and the comment explains why:
> When exceptions propagate past the middleware chain to the MCP SDK layer, they become `CallToolResult(isError=True)`. Some transports (Claude.ai's MCP bridge) cannot encode these error responses, producing "encoding without a string argument". Catch ALL exceptions (not just specific types) because any unhandled exception … will cause encoding failures on the wire.
So this is a workaround for a client-side encoding bug, traded against the protocol contract for every other client. That trade is invisible to callers and currently silent.
A fix has to keep the workaround working for that transport while restoring the contract for conforming clients. Possible directions, in rough order of preference:
1. Set the error flag while keeping the same encodable text payload — if the bridge's failure is specifically about the SDK's error *object* rather than a result carrying `isError`, this may satisfy both.
2. Gate the behaviour on transport or on a config flag, defaulting to spec-conformant.
3. Leave as-is but document it prominently, so integrators know `isError` is not usable against Superset.
**A question I can't answer from outside:** does the Claude.ai bridge encoding bug still reproduce on current `fastmcp` (3.4.7 here)? If it has since been fixed upstream, the workaround can simply be removed and this becomes a one-line change. Maintainers with access to that transport are better placed to check than I am.
### How to reproduce
1. Give the MCP identity a role lacking `can_write on SavedQuery`.
2. Call `save_sql_query` with valid arguments (`database_id`, `label`, `sql`) so it reaches the RBAC check rather than failing validation.
3. Inspect the JSON-RPC result: the text describes a permission denial, and `isError` is `false`.
Any failing tool call shows it; a denial is just the clearest case, since the operation definitively did not happen.
### Why it matters to us
We proxy an internal agent to a deliberately read-only Superset MCP. The proxy forwards caller tokens and does not pattern-match Superset's error strings to synthesise the flag, since that would couple us to another service's message wording. So a forbidden write currently returns a success-shaped response to an agent.
### Not part of this report
Two things I checked that are already handled on `master`, noted so nobody re-investigates:
- **Error IDs are no longer collision-prone.** 6.1.0 builds them as `f"err_{int(time.time())}"`, so two failures in the same second share an ID; `master` uses `mcp_call_id or f"err_{secrets.token_hex(8)}"`.
- **Denials are not mislabelled.** They render with a specific permission message, as above. The generic `Internal error … Error ID: …` text appears for *validation* errors (e.g. a missing required field), which is a separate and much less serious cosmetic issue.
### Superset version
master / latest-dev
### Python version
3.11
### Node version
Not applicable
### Browser
Not applicable
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Re: [I] MCP: failed tool calls return isError: false, so denials look like successes [superset]
Posted by "rusackas (via GitHub)" <gi...@apache.org>.
rusackas closed issue #43358: MCP: failed tool calls return isError: false, so denials look like successes
URL: https://github.com/apache/superset/issues/43358
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Re: [I] MCP: failed tool calls return isError: false, so denials look like successes [superset]
Posted by "dosubot[bot] (via GitHub)" <gi...@apache.org>.
dosubot[bot] commented on issue #43358:
URL: https://github.com/apache/superset/issues/43358#issuecomment-5352402643
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<!-- Answer -->
@AurimasNav good analysis. I can confirm the root cause and add some context from the current `master` code.
**What the current code does**
In `StructuredContentStripperMiddleware.on_call_tool`, the `except` block catches all exceptions and returns a plain `ToolResult` with the error text as content — but never sets `isError: true` [[1]](https://github.com/apache/superset/blob/master/superset/mcp_service/middleware.py#L767-L821). The comment explicitly explains the trade-off:
> When exceptions propagate past the middleware chain to the MCP SDK layer, they become `CallToolResult(isError=True)`. Some transports (Claude.ai's MCP bridge) cannot encode these error responses, producing "encoding without a string argument". Catch ALL exceptions…
The workaround is still present on `master` and `isError` is never set in this path.
**On your three proposed directions**
Option 1 (set `isError` while keeping the text payload) is the most tractable. The original `StructuredContentStripperMiddleware` was introduced in [PR #38786](https://github.com/apache/superset/pull/38786) specifically to strip `structured_content` and catch encoding failures from the Claude.ai bridge. The bridge's problem was triggered by `structuredContent` dicts and unencoded exception objects — not by a `ToolResult` that happens to carry `isError: true` in its serialized JSON [[2]](https://github.com/apache/superset/pull/38786). A `ToolResult(content=[TextContent(...)], isError=True)` serializes as a plain dict and should not trigger the bridge's encoding failure. That hypothesis is worth verifying against the bridge before merging, but it's the right starting point.
**On the fastmcp encoding bug question**
The `master` comment still says the workaround is needed — nobody has removed it, which suggests nobody has verified it's safe to drop against the current bridge version. Maintainers with access to the Claude.ai transport are better placed to test this.
**No existing PR covers this**
There's been a cluster of MCP middleware fixes recently — [PR #39159](https://github.com/apache/superset/pull/39159) (success flag always true in logs), [PR #41921](https://github.com/apache/superset/pull/41921) (observability + error ID collision), [PR #40098](https://github.com/apache/superset/pull/40098) (RBAC tool visibility) — but none address the `isError` protocol conformance issue you've filed here.
**Suggested path forward**
If you want to send a fix, option 1 is lowest risk: change the `return ToolResult(...)` in the `except` block to `return ToolResult(content=..., isError=True, meta=...)` and run it against the Claude.ai bridge to confirm no encoding regression. If the bridge does break, that points toward a bridge-specific config flag (option 2). Either way, a test covering the `isError` value in the returned `ToolResult` from that except block would be a good addition alongside the existing middleware chain tests in `test_middleware_logging.py` [[3]](https://github.com/apache/superset/pull/39159).
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Re: [I] MCP: failed tool calls return isError: false, so denials look like successes [superset]
Posted by "sadpandajoe (via GitHub)" <gi...@apache.org>.
sadpandajoe commented on issue #43358:
URL: https://github.com/apache/superset/issues/43358#issuecomment-5359183925
@AurimasNav feel free to open a PR to address the issue you are seeing.
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Re: [I] MCP: failed tool calls return isError: false, so denials look like successes [superset]
Posted by "AurimasNav (via GitHub)" <gi...@apache.org>.
AurimasNav commented on issue #43358:
URL: https://github.com/apache/superset/issues/43358#issuecomment-5352455197
> _Drafted with AI assistance._
Thanks — I verified the suggestion against fastmcp 3.4.7 in a running 6.1.0 instance. It works, with one correction, and the evidence supports the hypothesis that the bridge is unaffected.
**The suggested code as written raises `TypeError`.** `ToolResult` takes **`is_error`**, not `isError`:
```
ToolResult.__init__(self, content=..., structured_content=..., meta=..., is_error: bool = False)
>>> ToolResult(content=[], isError=True)
TypeError: ToolResult.__init__() got an unexpected keyword argument 'isError'
```
**With the right name it produces exactly the wire result we want:**
```python
ToolResult(content=[mt.TextContent(type="text", text="Error: denied")], is_error=True).to_mcp_result()
# CallToolResult(meta=None,
# content=[TextContent(type='text', text='Error: denied', ...)],
# structuredContent=None,
# isError=True)
```
**That output is also evidence for the bridge question.** `structuredContent` stays `None` and the content list is plain `TextContent` — the only delta versus today's behaviour is the boolean. Since #38786 introduced this middleware to strip `structuredContent`/`outputSchema` and to stop unencoded exception *objects* reaching the SDK, and neither changes here, the encoding path the workaround protects is untouched. That is consistent with your read, though it still wants a real check against the bridge, which I can't run.
So the change is:
```python
return ToolResult(
content=[mt.TextContent(type="text", text=error_text)],
is_error=True,
meta={"mcp_call_id": mcp_call_id} if mcp_call_id else None,
)
```
Happy to send that with a test asserting `is_error` on the returned `ToolResult` from that `except` block, alongside the existing middleware tests. I can't validate against the Claude.ai bridge myself, so that verification would need to happen in review — if it does regress, option 2 (a transport or config gate) is the fallback.
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