A flaky external probe in a tool's check_fn (e.g. check_terminal_requirements
running `docker version` with a 5s timeout, momentarily timing out under load)
would return False for a single get_tool_definitions() call. Because file
tools delegate their check_fn to the terminal check, that one flake silently
stripped read_file/write_file/patch/search_files AND terminal from whatever
agent was being constructed at that instant — most visibly a delegate_task
subagent, which then reported "Tool read_file does not exist". This explains
both the intermittent (~80% success) user-session failures and the
deterministic cron failures in #21658 / #5304.
The existing _check_fn TTL cache made this worse: it cached the transient
False for the full 30s window, poisoning every subagent spawned in that span.
Fix: remember the last time each check_fn returned True; when a fresh probe
fails within a short grace window of that success, treat it as a flake —
serve the last-good True and do NOT cache the failure (so the next call
re-probes). A failure with no recent success, or past the grace window, is
honored normally so a backend that genuinely went down stops advertising its
tools. Probe failures now log at WARNING regardless of quiet mode, making the
previously-silent tool loss diagnosable in subagent (quiet) sessions.
Co-authored-by: Stuart Horner <5261694+djstunami@users.noreply.github.com>
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