Add Model Parity

Goal

Create parity tests as early as possible in a FastVideo port. The first pass can land before conversion or component implementation as an executable scaffold; handoff is blocked until the same tests become non-skip PASS with real weights.

When To Run

Follow ../add-model/shared/common_rules.md for token/auth safety, state files, escape hatches, and skip/pass semantics.

Run immediately after /add-model Phase 1 has identified:

• official component classes and call signatures;
• FastVideo target component buckets/classes/configs;
• local reference clone or import path from add-model-01-prep;
• local raw or Diffusers weight path;
• official_env_status=imports_ok, or private deps that will be stubbed locally in tests;
• local_tests_readme documenting setup and planned review/test commands;
• expected component inputs and output tensors.

Do not wait for all FastVideo components to be implemented. Write the tests first, then let component-porting subagents make them pass.

Outputs

• One component parity test per required component, including reused components: tests/local_tests/<bucket>/test_<family>_<component>_parity.py.
• Optional helper for upstream private deps: tests/local_tests/helpers/<family>_upstream.py.
• Pipeline parity is owned later by ../add-model-09-pipeline/SKILL.md after all component parity tests pass non-skip.
• A parity status block for the /add-model parity verification phase.

Early Scaffold Rules

• A scaffold may skip while the FastVideo class, converted weights, or official import is missing.
• A scaffold must already encode the real official load path, FastVideo load path, deterministic inputs, expected output extraction, and tolerance target.
• Each parity test must declare its coverage scope in the file docstring or a module constant: production_loader, implementation_subcomponent, or both. Implementation/subcomponent parity may bypass production loaders deliberately, but final handoff still needs production-loader coverage somewhere before the pipeline depends on that component.
• Official reference imports must run in the current FastVideo environment; do not create or assume a separate upstream venv/conda env.
• A scaffold is not evidence of correctness. It becomes evidence only after a local non-skip PASS.
• Prefer env-var path overrides with repo-relative defaults.
• Keep tests local-only under tests/local_tests/; package/CI quality tests are added later.
• Update shared state files as described in ../add-model/shared/common_rules.md whenever adding or activating parity tests.

Component Template

Copy templates/component_parity_test.py and fill every TODO marker. The template is distilled from:

• tests/local_tests/transformers/test_ltx2.py
• tests/local_tests/transformers/test_gamecraft_parity.py
• tests/local_tests/encoders/test_ltx2_gemma_parity.py
• tests/local_tests/vaes/test_oobleck_vae_parity.py
• tests/local_tests/sd35/test_sd35_component_parity.py

The template supports three states:

Subagent Dispatch Pattern

After Phase 1, dispatch one parity subagent per component before or alongside component implementation:

Create a local parity test scaffold for <family> <component>.

Use the prep handoff:
- official_ref_dir/import: <...>
- local_weights_dir: <...>
- source_layout: <...>
- needs_conversion: <yes/no>
- official_env_status: <imports_ok | private_deps_need_stubs>
- local_tests_readme: tests/local_tests/<model_family>/README.md
- port_state_file: tests/local_tests/<model_family>/PORT_STATUS.md
- official_definition_files: <paths + classes/functions>
- official_instantiation_files: <paths + factory/pipeline/config call sites + args>
- concerns_or_unknowns: <known ambiguous inputs, outputs, deps, or args>

The complete per-component packet must match
`../add-model/contracts/component_context.md`.

Read the official component call path and the planned FastVideo component API.
Add tests/local_tests/<bucket>/test_<family>_<component>_parity.py based on
add-model-02-parity/templates/component_parity_test.py.

The scaffold must load the official model with real weights when available,
load the FastVideo model through the standardized config/class/loader path when
available, create deterministic inputs, compare concrete outputs, and skip only
when a dependency is genuinely missing. Do not make an unconditional skip or a
shape-only test.

FastVideo Load Patterns

Pick the narrowest load path that matches the component:

For early scaffolds, an import of the planned FastVideo class may be inside a helper that calls pytest.skip if the class does not exist yet. Replace that skip with a real import once the component PR adds the class.

Direct class/config construction is allowed for implementation or subcomponent parity, such as connector-only encoder checks or official monolithic-checkpoint mapping tests. Label that scope explicitly and add separate production-loader coverage when converted component dirs are available.

Official Load Patterns

• Clone/reference repo path: add its source dir to sys.path before imports.
• HF/Diffusers reference: import only inside the test, not production code.
• Private deps: add a helper under tests/local_tests/helpers/ to install stubs before importing upstream modules; do not rely on an external upstream environment.
• Gated HF repos: resolve HF_TOKEN, HUGGINGFACE_HUB_TOKEN, or HF_API_KEY under the token rules in ../add-model/shared/common_rules.md.

Non-Skip Activation Checklist

Before /add-model handoff, each scaffolded test must be activated:

[ ] Official side imports and loads real weights.
[ ] FastVideo side imports and loads the converted or original weights.
[ ] Test executes at least one real forward call on both sides.
[ ] Test compares output tensors, not only shapes or state-dict keys.
[ ] Local pytest output contains PASSED, not SKIPPED or XFAIL.
[ ] Tolerance is justified for the component scope and kernel alignment.

Component Parity Details

Reference imports:

• Import from official_ref_dir or the recorded package/import path.
• If upstream has private deps, add a helper under tests/local_tests/helpers/<family>_upstream.py that installs minimal stubs before importing upstream modules.
• Common stubs: identity compile/op-registration decorators, CP world size set to 1, identity scatter/gather, and test-friendly custom-op kernels.
• Stub decorators that register torch.ops.<ns>.<op> must preserve the torch.library registration side-effect. Identity decorators alone are not enough.
• Delete stub helpers and every install_stubs() call as soon as the real deps become required installs. No-op shims are dead code.

Kernel and wrapper pitfalls:

• If parity routes flash-attn GQA through SDPA, expand KV heads manually on the SDPA side with repeat_interleave along the head axis.
• If upstream VAE decode() denormalizes internally but FastVideo/Diffusers expects pre-denormalized latents, apply z = z * std + mean only on the FastVideo side in the parity test.
• Per-channel VAE latents_mean / latents_std must be reshaped explicitly, e.g. .view(1, z_dim, 1, 1, 1) for 5D video latents.

Tolerance guide:

Element-wise assert_close alone is not enough for deep full-DiT parity. Also log global abs-mean drift and per-modality summaries.

When a non-skip component parity run is numerically red after weight/input checks, invoke ../add-model-08-trace/SKILL.md before adding bespoke forward hooks. Use docs/contributing/activation_trace.md to keep FASTVIDEO_TRACE_LAYERS, FASTVIDEO_TRACE_STATS, and FASTVIDEO_TRACE_STEPS identical across FastVideo and upstream traces.

Useful local commands:

pytest tests/local_tests/<bucket>/test_<family>_*parity*.py -v -s
pytest tests/local_tests -k "<family> and parity" -v -s

Escape Hatches

Follow ../add-model/shared/common_rules.md. Parity-specific ask cases include private dependency approval, choosing between incompatible official references, accepting a shape-only substitute, or loosening required tolerances.

Pipeline Parity

Pipeline parity is later than component parity because it needs stages, presets, registry wiring, converted weights, and green component parity. Record official pipeline call notes in local_tests_readme, but do not treat pipeline parity as owned by this skill.

Use ../add-model-09-pipeline/SKILL.md and its templates/pipeline_parity_test.py for pipeline parity scaffolding and debugging. Compare denoised latents or decoded media, not just successful generation.

Handoff Status Block

Return ../add-model/contracts/parity_status.md to /add-model and update the shared state files before handoff.