remove-ai-watermarks/.claude/rules/development.md

---
globs: ["src/**/*.py", "tests/**/*.py", "scripts/**/*.py", "pyproject.toml", "uv.lock", "maintain.sh", ".github/workflows/*.yml"]
description: Command contracts, project gate, typing boundaries, model-adjacent test invariants, and the detection-path measurement rule.
---

# Development invariants

## Command contracts

Every single-image command declares `source` with `dir_okay=False`; `batch` declares its directory with `file_okay=False`. Keep `tests/test_cli_robustness.py::TestDirectoryInputIsRejected` as the regression guard.

Exit-code and no-signal behavior is a public contract. Read the command-line section of [`../../docs/module-internals.md`](../../docs/module-internals.md) before changing it.

Do not add an option whose only outcome is an error. Model id, step count and CFG are fixed by the profile, so none of them is a parameter of the CLI, `InvisibleEngine`, or `WatermarkRemover` -- they were accepted-then-rejected for a while, which moved the failure several frames below the caller and advertised choices the pinned stack cannot honor. If a value cannot vary, delete the knob rather than validating it.

`device` is the deliberate exception and stays a library parameter: `None`/`"auto"` detect, `"cuda"` pins without detecting, and everything else raises at construction. It is not a CLI option, because the only value a user could usefully type is the one auto-detection already finds.

The same rule applies to install hints: name the extra that actually makes the command work (`qwen-zimage`, not `diffusion`), and keep the printed command shell-quoted -- bare `pkg[extra]` is a glob in zsh.

## Local gate

Run `bash maintain.sh` from the repository root. The authoritative type gate is scoped to `src/`; full-project Pyright can exhaust Node memory on the ML dependency graph.

Boundary modules for cv2, Torch, and Diffusers may carry narrow per-file relaxations for unknown third-party types. Keep pure-logic files strict, preserve the local piexif stub, and fix real errors before widening a pragma.

From a worktree, `uv run` imports the package from the MAIN checkout -- that is where the editable install points. A script measuring a worktree's edit must insert that worktree's `src` at `sys.path[0]` and assert `module.__file__` resolves inside it, or it silently compares unmodified code against itself.

## Model-adjacent tests

Do not classify an entire module as untestable because its main path downloads a model. Keep pure behavior covered without downloads, including:

- target-size selection in `test_invisible_engine.py`;
- unsharp and adaptive-polish helpers in `test_humanizer.py`;
- tiling geometry and blending in `test_tiling.py`;
- prompt-embedding cache keying, storage round-trip, and the cross-pipeline reuse
  that lets a stack load without its text encoder, in `test_qwen_zimage_pipeline.py`;
- the face stack's dtype, in `test_qwen_zimage_pipeline.py`. A subclass that changes
  the pipeline dtype for its own global model must not change the inherited face
  stage's; `sdxl-zimage` shipped doing exactly that and crashed on every image with a
  face. When one profile inherits another's stage, guard the invariants that stage
  relies on, not just the code path.
- the `InvisibleOptions` defaults, in `test_api.py`. When one signature promises to
  mirror another, compare them field by field rather than pinning the values you happen
  to know about, so the next field added on one side and not the other fails at the
  seam. Two of these defaults drifted in practice and neither needed a GPU to catch;
  the incident is recorded in `docs/module-internals.md`. Keep the comparison free of
  an exception table: a field that needs one is a field that belongs elsewhere, which
  is what `force` turned out to be.

A defaults comparison is not a forwarding test, and the two fail differently. Pin the
VALUE at the seam, not just the name -- `_run_invisible` passed the whole suite with
`controlnet_conditioning_scale` hardcoded, because nothing asserted the caller's value
arrived. `test_every_field_arrives_at_the_engine_with_the_caller_s_value` drives the
real seam with every field set off its default, so one test covers the whole bag
instead of one assertion per knob.

Count the seams before believing a knob is covered. Each of `force` and
`controlnet_conditioning_scale` reaches the engine through TWO paths -- `remove_all`
versus `remove_batch(mode="all")` for the first, `_run_invisible` versus `_batch_engine`
for the second -- and in both cases guarding one path left the other free to hardcode a
constant with a green suite. The mode-parametrized guards in
`TestRemoveBatchLibrary::test_force_reaches_the_scrub_gate_in_every_scrubbing_mode` and
`TestBatchCommand::test_batch_controlnet_scale_flows_to_the_cached_engine` exist because
that is what actually happened.

Use availability checks only for paths that actually load large models.

## One measurement, one gate seam

A detector is split into a trust-level-blind scan and a verdict that applies the
threshold, so `detect` and `detect_both` reach the same numbers by construction. Two
rules follow, and both were broken in practice before they were written down:

- A per-mark demotion goes in the `_post_gate` hook (or, for a whole-scan precondition
  like LibLibAI's size floor, in `_scan`) -- never in a `detect` override. An override
  is invisible to `detect_both`, so the RunningHub and Yuanbao anchor gates silently
  stopped applying on the arbiter's perception path. `TestSinglePassPerception` is the
  guard: it asserts `detect_both` equals two `detect` calls field for field.
- Detection and the removal mask must read ONE sweep. The winning box travels on
  `TextMarkDetection.match_box` and the registry threads the detection into the mask
  builder; a mask path that re-runs its own sweep is how the two drift apart.

The C2PA manifest-store JSON is NOT stable across reads: the reader regenerates manifest
URNs and instance ids. Compare the derived `c2pa_info`, never the raw store.

A third seam reaches the same verdict: `collect_metadata_record` ->
`evidence_from_metadata_record` -> `identify_from_evidence`, the path a caller uses when
collection and verdict run on different machines. Its contract is equality with
`identify(path, check_visible=False, check_invisible=False)` on the same image, and it
can break from EITHER side -- a region the collector stops walking, or a placement the
file path learns to read and the record does not. `tests/test_metadata_record.py` pins
it over the tracked fixtures; a separate local evaluation corpus catches placements the
fixtures do not cover. Change either side and re-run both.

Before changing anything in the detection path, record the detectors' exact verdicts
over a local sample first and diff them after. A refactor here is only correct if that
record is byte-identical, and a green test suite does not establish that on its own. A
change that is meant to FIX detection is the exception that proves the rule: the diff
must then be exactly the files you intended to change, named in advance.

Two corollaries in `dwt_dct.py`, where "close enough" has an exact meaning. The bit test
is `peak % 36 > 18.0` and for uint8 input the exact Haar value is a multiple of 0.5, so it
lands ON the threshold once per 72 blocks and a 1-ulp difference flips real bits: leave the
transform to `pywt` however slow it looks, because its C convolution contracts into an FMA
that numpy has no ufunc for (`docs/module-internals.md` carries what that costs). And
`tests/test_invisible_watermark.py` is `skipif(not is_available())` -- without the `detect`
extra the upstream-parity test never runs, so a green suite is not evidence here and the
verdict record is not optional.

## A certified operating point is data, not a constant

The video SynthID default is only meaningful as a row in
`data/evaluations/video-synthid-oracle.csv`, so
`test_shipped_defaults_match_a_certified_manifest_row` derives the pin from that
manifest instead of restating literals. Pinning `noise_std` alone had let `long_side`
and `fps` -- two thirds of what the oracle was actually shown -- move with a green
suite.

The certified profile is a perturbation-to-signal ratio, not a bare `noise_std`, so
the latent scaling factor is gated in `load_video_vae_runtime`, carried on
`VideoVaeRuntime`, and passed into encode and decode: the validated value and the
applied value are one measurement. Anything that produces oracle evidence loads
through that function. `video_synthid_sweep.py` hand-rolled the load and was the one
path exempt from the gate it exists to feed, which is exactly backwards.

Prove a video-path refactor the same way the detection path is proven, and without
needing an oracle carrier: build a clip from a tracked fixture with ffmpeg, run the
engine before and after, and require an identical output sha256. Keep the generated
media outside the repository.

Frame sampling is compared as `timestamp + 1e-9 >= next_sample_time`, so mutating
that `>=` to `>` changes nothing and a green suite proves nothing. Mutate the phase
or the period instead -- starting the accumulator half a period late shifts the
selection by one frame while keeping the count identical, which is the drift a
frame-count check cannot see and what
`test_pairing_follows_the_engine_sampling_rule_not_just_the_frame_count` exists to
catch.

Environment setup, dependency recovery, CI behavior, and fixture policy: [`../../docs/development.md`](../../docs/development.md).