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A tool can finish without raising and still fail to do what it was asked.
Slack answers HTTP 200 with `{"ok": false, "error": "channel_not_found"}`;
an MCP server sets `isError`; a CrewAI AMP action returns
`API request failed: ...`. In every case the call "worked", so the error
text reached the agent as an ordinary result, the agent narrated the
problem in prose, and the run was recorded as a success.
Concretely: five failed `slackbot_send_message` calls each rendered as
"Tool Execution Completed", the task passed, and the crew passed -- with
the only evidence being a sentence in the final answer. Nothing
downstream could tell the difference, and an agent that keeps going on a
step that silently did nothing builds the rest of its work on it.
Give that outcome a type and a reaction:
- `ToolFailure` -- what a tool returns instead of an error string. The
agent still reads prose via `as_agent_message()`, so model behavior is
unchanged; the framework now knows the call failed.
- `ToolFailurePolicy` -- `ignore` (previous behavior), `warn` (default:
record + emit, keep going), `raise` (abort with
`ToolExecutionFailedError`). Resolved most-specific-first: tool, task,
agent, crew.
- `ToolFailureDetectedEvent` -- emitted before a `raise` aborts, so
subscribers always observe the failure. `ToolUsageFinishedEvent` also
carries a `failure` field so a trace UI can mark the call failed
without correlating two events.
- `tool_failures` on `TaskOutput`, `CrewOutput` and `LiteAgentOutput`,
plus `has_tool_failures`, so consumers never parse a string.
Detection is strictly declarative -- no string sniffing, so a tool that
legitimately returns text about an error is never misread as failing.
Failures come from a returned `ToolFailure`, a raised exception, MCP
`isError`, a spent `max_usage_count`, or an unknown tool.
Wired into all four tool-execution paths (the ReAct path and the three
native function-calling implementations). Sources updated to report
structurally: `MCPClient.call_tool_result()` preserves `isError` that
`call_tool()` dropped, and `CrewAIPlatformActionTool` returns a
`ToolFailure` for non-2xx and for caught exceptions.
Two latent bugs fixed along the way: `ToolUsage` assumed every agent has
a `fingerprint` (LiteAgent does not), and policy resolution now tolerates
malformed values rather than letting telemetry take down a tool call.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ETacm2dMASfpMAYUiDu5YG