BerriAI/litellm · error · HTTPException

Content blocked: MCP tool result matched a masking rule on a

Error message

Content blocked: MCP tool result matched a masking rule on a non-rewritable field (a structuredContent key or numeric value), which cannot be redacted without changing the payload contract

What it means

Error "Content blocked: MCP tool result matched a masking rule on a non-rewritable field (a structuredContent key or numeric value), which cannot be redacted without changing the payload contract" thrown in BerriAI/litellm.

Source

Thrown at litellm/proxy/_experimental/mcp_server/guardrail_translation/handler.py:175

        if masked_texts is None:
            return response
        if len(masked_texts) != len(originals):
            verbose_proxy_logger.warning(
                "MCP Guardrail: guardrail returned %d texts for %d tool result texts; leaving the result unmasked",
                len(masked_texts),
                len(originals),
            )
            return response

        split: Final = len(text_blocks)
        if content is not None:
            for (index, original), masked in zip(text_blocks, masked_texts[:split]):
                if masked != original:
                    content[index] = with_mcp_content_item_text(content[index], masked)

        label_start: Final = split + len(structured_leaves)
        if any(masked != original for original, masked in zip(structured_labels, masked_texts[label_start:])):
            raise HTTPException(
                status_code=400,
                detail={
                    "error": (
                        "Content blocked: MCP tool result matched a masking rule on a non-rewritable field "
                        "(a structuredContent key or numeric value), which cannot be redacted without changing "
                        "the payload contract"
                    )
                },
            )

        structured_replacements: Final = {
            path: masked
            for (path, original), masked in zip(structured_leaves, masked_texts[split:label_start])
            if masked != original
        }
        if structured_replacements:
            set_mcp_tool_result_structured_content(
                response, with_json_string_leaves(structured, structured_replacements)

View on GitHub (pinned to 77b7c6c40c)

Solutions

1. Adjust the masking rule to target rewritable text fields only.
2. Or change the payload so matched entities appear in string fields that can be redacted.

Example fix

Move sensitive values into string content fields.

When it happens

Trigger: Thrown at litellm/proxy/_experimental/mcp_server/guardrail_translation/handler.py:175 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/88200688dd5144ac. Report an issue: GitHub.