Found by a live browser E2E of the coding workflow in test mode: every
scripted full-task run failed at `steps#0:step-execute` with `Step 4 out
of range (task has 4 steps)`, rebounding through recovery forever.
**Root cause:** `fn_task_update.step` has been **0-based since FN-6607**
(executor.ts FNXC:StepNumbering — the old `step - 1` conversion made
Step 0 impossible to mark). `mock-provider.ts` still sent `index + 1`,
so test mode marked steps 1..N instead of 0..N-1: Step 0 (Preflight)
never completed and step N threw out-of-range. Test mode's full-task
path has been broken since June.
**Also fixes the test that pinned the bug:** `mock-provider.test.ts`
expected `{ step: 1 }` for a fixture whose first unfinished step is
index 0 — the expectation encoded the 1-based off-by-one.
Verified: 12/12 mock-provider tests; the live E2E instance completes the
task after this patch (see follow-up screenshot in the session).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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