Fret Fixture Driven Harnesses

Fixture-driven harnesses (Fret)

This skill is for turning large, repetitive Rust test matrices into data-driven fixtures with a thin harness. It reduces merge conflicts, makes review easier, and keeps scenario intent visible.

When to use fixtures (and when not to)

Good fits:

• conformance tables (many cases, same runner, different inputs/expected)
• geometry/layout placement matrices
• policy matrices using the same arbitration rules
• any test file that keeps growing because new behavior means “add another case”

Keep in Rust instead:

• highly procedural interactions where the logic is the test
• cases requiring closures/async hooks or bespoke host wiring
• tests where compile-time types are the primary safety net

Inputs to collect (ask the user)

• What is the repeated dimension (inputs, expected outputs, environment knobs)?
• Does each case share the same runner/harness, or do you need multiple harnesses?
• What must be stable IDs (case id, scenario id, golden id)?
• Do you need deterministic geometry (avoid floats) or will tolerances be required?
• How should failures be reported (case-id-addressable, diff-friendly output)?

Defaults if unclear:

• Start with a single fixture file with schema_version + stable cases[].id, and a thin harness that prints failing ids.

Smallest starting point (one command)

• cargo nextest run -p fret-ui-shadcn

Quick start

1. Pick a single “god test” to extract first.
2. Read the fixture reference note before defining the schema.
3. Mirror the old test with a thin harness before deleting anything.
4. Keep failures case-id-addressable and review-friendly.

Workflow

0) Read the reference note first

Use this note to keep the main skill lean:

• .agents/skills/fret-fixture-driven-harnesses/references/fixture-schema-and-harness.md

1) Decide whether fixtures are the right tool

Use fixtures when the runner stays the same and only the data changes.

If the behavior is a procedural multi-frame interaction, prefer fret-diag-workflow instead.

2) Extract one matrix incrementally

• copy the existing Rust matrix into a fixture file
• keep the old test temporarily
• mirror the assertions in a thin harness

3) Keep the harness thin and deterministic

The harness should mostly be:

• parse fixture,
• iterate cases,
• call run_case,
• assert with stable case ids in failure output.

4) Gate first, delete second

Only remove the old “god test” after:

• the fixture harness is green,
• failures are clearly case-id-addressable,
• reviewers can inspect the fixture diff without decoding harness internals.

Definition of done (what to leave behind)

• Minimum deliverables (3-pack): Repro (fixture suite), Gate (nextest), Evidence (case-id failures). See fret-skills-playbook.
• Fixture suite has schema_version and stable case id keys.
• Harness is thin (parse → run_case → asserts) and does not depend on cwd.
• Failing output is case-id-addressable.
• Old matrix tests are removed only after the fixture harness is green and reviewed.

Evidence anchors

• Fixture-driven test examples:
  • ecosystem/fret-ui-shadcn/tests/web_vs_fret_layout.rs
  • ecosystem/fret-ui-shadcn/tests/data_grid_layout.rs
  • ecosystem/fret-ui-shadcn/tests/resizable_panel_group_layout.rs
• Goldens (overview + conventions): goldens/README.md
• Layering boundaries: tools/check_layering.py
• Diagnostics scripts for procedural cases: tools/diag-scripts/ui-gallery-intro-idle-screenshot.json
• This skill’s reference:
  • .agents/skills/fret-fixture-driven-harnesses/references/fixture-schema-and-harness.md

Examples

• Example: replace a growing matrix test with fixtures
  • User says: "This test file keeps growing—every change adds another case."
  • Actions: extract cases into fixtures/*.json, keep a thin harness, and make failures case-id-addressable.
  • Result: smaller diffs, fewer conflicts, clearer intent.

Common pitfalls

• Using fixtures for procedural state machines.
• Making fixtures too clever (unstable IDs, floats everywhere, hard-to-diff shape).
• Letting the harness grow beyond parsing + run_case + asserts.

Troubleshooting

• Symptom: fixtures depend on cwd and fail in CI.
  • Fix: load with include_str! + env!("CARGO_MANIFEST_DIR").
• Symptom: fixtures are hard to review.
  • Fix: keep stable ids, prefer integers/enums, and split large suites by subsystem.

Related skills

• fret-diag-workflow
• fret-shadcn-source-alignment
• fret-boundary-checks