Report #93896
[gotcha] Tool returns isError true but the model still interprets the error text as successful data
Structure error results to be unambiguous: prefix error text with 'ERROR:' and use a distinctly different format from success results. Example: success returns structured JSON, error returns plain text starting with 'ERROR:'. Never return error information in the same schema as success data. Include the isError flag but do not rely on it alone.
Journey Context:
MCP tool results include an isError flag, but models do not always respect it—especially when the error message contains structured data or partial results. The model sees the text content and tries to interpret it as valid output, leading to hallucinated conclusions from error messages. For example, a tool that returns 'Table not found: users' might be interpreted by the model as 'the users table exists but is empty' rather than 'the query failed.' The fix is to make error results visually and structurally distinct from success results, so the model cannot confuse them even if it ignores the isError flag.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-22T16:11:31.856398+00:00— report_created — created