aminghadersohi commented on code in PR #42244:
URL: https://github.com/apache/superset/pull/42244#discussion_r3616599986
##########
superset/mcp_service/middleware.py:
##########
@@ -1331,6 +1333,141 @@ def _try_truncate_info_response(
return truncated
+ def _try_truncate_data_query_response(
+ self,
+ tool_name: str,
+ response: Any,
+ estimated_tokens: int,
+ ) -> Any | None:
+ """Attempt to truncate a data-query tool response by dropping tail
rows.
+
+ Returns the truncated response if successful, None otherwise.
+ """
+ extracted = self._extract_payload_from_tool_result(response)
+ truncation_target = extracted if extracted is not None else response
+
+ try:
+ truncated, was_truncated, notes = truncate_query_result(
+ truncation_target, self.token_limit, tool_name=tool_name
+ )
+ except Exception as trunc_error: # noqa: BLE001
+ logger.warning(
+ "Query result truncation failed for %s due to %s: %s",
+ tool_name,
+ type(trunc_error).__name__,
+ trunc_error,
+ )
+ return None
+
+ if not was_truncated:
+ return None
+
+ # Mirror the info-tool path: if truncation couldn't bring the
+ # response back under the limit (e.g. a single row/scalar field
+ # alone exceeds it), fall back to the hard size-limit error instead
+ # of shipping an over-budget response.
+ truncated_tokens = estimate_response_tokens(truncated)
+ if truncated_tokens > self.token_limit:
+ return None
+
+ logger.warning(
+ "Query result for %s truncated from ~%d to ~%d tokens (limit: %d).
%s",
+ tool_name,
+ estimated_tokens,
+ truncated_tokens,
+ self.token_limit,
+ "; ".join(notes),
+ )
+
+ try:
+ user_id = get_user_id()
+ event_logger.log(
+ user_id=user_id,
+ action="mcp_response_truncated",
+ curated_payload={
+ "tool": tool_name,
+ "original_tokens": estimated_tokens,
+ "truncated_tokens": truncated_tokens,
+ "token_limit": self.token_limit,
+ "truncation_notes": notes,
+ },
+ )
+ except Exception as log_error: # noqa: BLE001
+ logger.warning("Failed to log truncation event: %s", log_error)
+
+ if extracted is not None and isinstance(truncated, dict):
+ return self._rewrap_as_tool_result(truncated, response)
+
Review Comment:
Confirmed — this was a real bug, not just a theoretical edge case.
`estimate_response_tokens` is called on the unwrapped payload dict, but
`ToolResult` (the object actually returned) adds envelope overhead — the
`content`/`type`/`text`/`meta` wrapper plus JSON-escaping of the embedded
payload string — that isn't reflected in that check. Measured empirically: for
a 20k-token payload the wrapped object came out ~440 tokens larger (~2.2%), and
the overhead is proportionally worse for small payloads (~40 tokens fixed
baseline).
Fix (pushed shortly) in `superset/mcp_service/middleware.py` and
`superset/mcp_service/utils/token_utils.py`:
1. In both `_try_truncate_info_response` and
`_try_truncate_data_query_response`, the size limit is now re-checked against
the final re-wrapped `ToolResult`, not the bare payload.
2. More importantly, the underlying binary search that decides *how many
rows to keep* (`_bisect_row_limit`, `_bisect_csv_row_limit`, and the phase
checks in `truncate_oversized_response`) now accepts an optional `size_fn`
callable. The middleware passes a `size_fn` that re-wraps each candidate into a
`ToolResult` before measuring it, so the search itself converges on a row count
that still fits *after* re-wrapping, instead of just re-validating the wrong
measurement afterward and giving up (hard-blocking) if it doesn't fit.
Added a regression test (`test_data_query_truncation_fits_after_rewrap` in
`tests/unit_tests/mcp_service/test_middleware.py`) that asserts
`estimate_response_tokens(result) <= token_limit` on the actual object returned
from `on_call_tool` — this fails on the pre-fix code (proportional wrapper
overhead pushes ~461 unwrapped tokens to >500 wrapped) and passes now.
All 157 mcp_service/middleware + token_utils tests pass, plus the full
mcp_service suite (3089/3090, the one pre-existing failure is an unrelated
e2e/auth test that fails identically on main).
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