Gremlins MVP: build report

Date: 2026-10-16/17 · Orchestrator model: claude-opus-5-5 · Gremlin model: claude-opus-5-5 (all calls)
Branch gremlins-mvp (worktree .claude/worktrees/gremlins-mvp), from main at b8affd8. Not pushed, not merged.
Spec: 2026-10-17-00-gremlins-mvp-spec.md. Tool: the prototype at .claude/worktrees/headless-forks/plugins/spire/skills/gremlins/gremlins.py, unmodified.
Wall time: about 21 minutes, 23:12:58 to 23:33:35, against a 90-minute timebox. Fallback: not needed; the prototype did not misbehave once.

How it ran

One base (B1) was trained on the spec, the verdict and the prototype source. It wrote no code. It wrote 70 lines of trunk notes, which pinned every cross-module seam: signatures, return shapes, the state-dir layout, the ledger and record keys, and its own decisions where the spec was open. Then eight gremlins were forked from B1, at most two running at a time (spawn --mode parallel --paths …, then wait --any / --all with Bash timeout 600000):

Wave 1: stream + launch, then store + call, each pair started as soon as a slot freed up. Committed in aff29d6.
Wave 2: cli + entry. entry is gremlins.py plus SKILL.md. Committed in 9209e7d and fd75777.
Wave 3: tests-unit (started in entry's freed slot, since it does not depend on cli), then tests-cli. Committed in d662790.
Integration fix: 79df026.

Every gremlin finished ok on its first attempt. No gremlin needed a follow-up turn, and none touched another gremlin's files. Each one wrote a note that listed every function with its box.

Gremlin metrics (from the prototype's results and ledger)

Gremlin	Module	Wall s	Cost delta $	Cache	Cache read	Lines written
base B1 (train)	trunk notes only	407.8	1.9725	n/a	1,085,902	0 code (70 notes)
stream	`grm/stream.py` (+`__init__.py`)	104.3	0.5175	hit	924,301	215
launch	`grm/launch.py`	66.7	0.4196	hit	783,210	182
store	`grm/store.py`	62.2	0.3815	hit	903,383	116
call	`grm/call.py`	80.0	0.4198	hit	908,838	86
cli	`grm/cli.py`	230.2	1.2211	hit	1,667,879	359
entry	`gremlins.py`, `SKILL.md`	52.2	0.3836	hit	916,241	14 + 54
tests-unit	`tests/test_{store,launch,stream,call}.py`	80.0	0.5441	hit	834,361	218
tests-cli	`tests/fake_claude.py`, `tests/test_cli.py`	197.3	1.1388	hit	1,982,799	275

Base cost: $1.9725. The eight spawns together cost $5.0260. Build total (prototype usage): $6.9985, 9 calls, 0 cold starts, 8 of 8 cache hits.
The live smoke of the MVP cost another $0.2204 (4 calls). That brings the grand total to about $7.22.

Final line counts against the spec's estimates

Module	Lines	Estimate	Reason for the difference
`grm/store.py`	116	70	docstrings; `init_state` and `SUBDIRS` added (trunk decision)
`grm/launch.py`	182	110	the kept scrub constants and the tool lists; `binary_version` added
`grm/stream.py`	215	170	`iso`/`parse_iso` moved here; `TOKEN_KEYS`; `mismatch` kept (4 lines, not used)
`grm/call.py`	86	60	feeder and reader threads; the child is killed if `on_start` raises; a second signal escalates to `kill()`
`grm/cli.py`	359	200	`delta_header` verbatim (~25); JSON argparse errors (~30); explicit job, ledger and base dicts (~40); status; docstrings
`gremlins.py`	14	(in cli)
Code total	972	610	The verdict estimated 600–800. No failure path was dropped, and nothing outside spec §4 was built.
`tests/`	493	about 300	per-file boilerplate; the fake has 7 scenarios; B4 needs a subprocess and per-platform signalling; one extra test (below)
`SKILL.md`	54	not in the spec	see Deviations

Test and smoke results

Unit tests: uv run --no-project --python 3.12 python -m unittest discover -s tests, run from the skill dir: 24 tests, OK, 7.0 s. That is 14 module tests and 10 CLI tests, which cover every §6 case from B1 to B8.
Live smoke (MVP; real claude.exe 2.1.299; scratch git repo; model claude-opus-5-5):
- init: run.json was written with the fingerprint, the argv and fp_hash, and a frozen preamble copy was made.
- train B1: ok, 5.0 s, $0.0969, session id taken from --session-id.
- spawn s1: ok, cache: hit (69,651 tokens read, 1,664 written), 36.5 s, cost delta $0.0431, base_state: warm. The note was written and the repo was left untouched.
- The ledger had two lines, each with model, models_used, six token columns, cost_delta_usd, cost_cumulative_usd and hit_at.
- status: B1 warm, ttl_s 3600, ttl_left_s 3566.
Parallel test (B3): two spawns, p1 and p2, ran as separate Bash run_in_background calls. Both started at 23:30:33 and ended at 23:30:44 and 23:30:45. Both completion notices arrived with the result JSON and exit 0, and both reported cache: hit. The run ended with two new job files, two new whole ledger lines (four in total) and two notes.
Processes afterwards: Get-CimInstance Win32_Process shows no claude -p, python or uv process left. The only claude.exe processes are the Desktop app and its own sessions. The prototype's list showed nothing running, and expire --all retired its base B1, so no keep-warm ping can fire later.
Not run live: the B4 TaskStop on a backgrounded spawn (spec §5 step 4), a B7 rebase, and the B8 gate on WSL. These paths are covered only by the fake-claude tests: CTRL_BREAK to the helper subprocess, and rebase argv and session id.

Deviations from the spec, each with its reason

SKILL.md was added, although it is not in the module table. Reason: B3 and B4 only work if the orchestrator is told to run spawn via run_in_background and to stop it with TaskStop, and the skill file is where it is told.
templates/rebase.md was reworded ("a fork … becomes the new base ${new_base} (phase …)" became "base ${base} itself, resumed"). cli stopped passing new_base and phase. Reason: §3 said to use the templates as is, but the old text contradicts B7, which resumes without a fork and keeps the same base. The cli gremlin flagged it, and the fix is in 79df026.
spawn.md: ${commit_rule} became the fixed line "Commit only if the brief below says so", and ${mode} was removed. This is per spec §3; the fixed line is so that the rule is still visible.
Trunk-note decisions where the spec was open:
- build_fingerprint takes mcp_config and returns {fingerprint, argv, fp_hash}.
- The argv builders take the stored run dict, so that a test's fake argv is honoured.
- resume_argv has no --session-id, because claude refuses it on a resume without a fork.
- The fingerprint adds allowed_tools and disallowed_tools, because dontAsk denies every tool that is not allowed.
- resolve_binary returns a str, not the prototype's tuple.
models_used is ordered by usage, largest first, not by name. Reason: by name, a haiku side-call could become model (= models_used[0]).
The ledger line adds hit_at. Reason: B6 derives warmth from the ledger, and the line's ts is the end of the call, not the time the cache was read.
Job ids:
- train is <B>-train; it can be re-run only while there is no base file.
- rebase is <B>-rebase<n>; a failed rebase re-run overwrites its own job and raw files.
- spawn takes --job J, and an existing J is refused.
The base's cost_cumulative_usd is stored unrounded. Ledger and printed costs are still rounded once, at 6 decimals. Reason: a rounded basis made the deltas drift by 1e-6 (0.020001 instead of 0.02).
StreamState prefers the result's session_id over the init event's. Reason: spec §7, B7.
Exit codes are 0 / 1 / 2: 0 ok, 1 a call that ended error or stopped, 2 refused. status refuses a run with no run.json.
No SPIRE_GREMLIN_CHILD refusal. Reason: no box needs it, and it would break the helper's own tests when they run inside a gremlin.
One extra test, test_call.test_tee_and_parse. Reason: it is the only check that the prompt goes in on stdin as UTF-8 and that the stream is teed to the raw file and parsed.

Observations on the dogfooding

The prototype raised rebase_due: phase_boundary on every wave-2 and wave-3 spawn, because --phase was used as a wave label. The flag is advisory, so it was ignored. The delta header gave the forks the new commits instead.
The trunk notes were what made the parallel work fit together: four modules written blind against each other imported cleanly on the first try. The only integration fix was the template wording.
Each fork read 0.8 to 2.0 M tokens from cache, at about $0.38 to $1.22 per module. The base's $1.97 was spent once.

Open items

Live B4 check: a TaskStop on a backgrounded MVP spawn.
Live B7 check: does a plain --resume report a cumulative total_cost_usd? The cost basis assumes it does.
The B8 gate on WSL.
Cross-process ledger appends on Windows are not strictly atomic (CRT seek + write). This is accepted per the trunk notes. The live parallel run gave whole lines.

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