TDD Plan — Slice & Plan (Phase 1)
Turn a request into a set of thin vertical slices, each captured as a markdown execution plan in plans/. A good slice is the smallest change in system behaviour that is independently valuable and testable end-to-end. The plan you write here is the contract the red-green-refactor skill executes next, and that tdd-ci and safe-pr later read — so make it precise.
Slicing is the hardest and most valuable judgement in TDD. "Sequencing the tests properly is a skill — pick tests that drive you quickly to the salient points in the design." Take your time here.
For the full slicing toolkit — vertical-vs-horizontal, INVEST, the walking skeleton, and nine concrete splitting patterns with a worked example — read references/slicing-guide.md.
Procedure
Understand the request. Restate the feature/outcome in one or two sentences. Ask only the questions that change the slicing: who the user is, the externally observable behaviour, the boundary it goes through (web UI, HTTP API, CLI), and any hard constraints. Don't over-interrogate.
Detect the context. Is this a brand-new system or a change to an existing one? Look at the repo (build tooling, existing tests, frameworks). If there is no working build/test/deploy path yet, the first slice must be a walking skeleton: the thinnest end-to-end thread that builds, runs, and is tested through real infrastructure — it de-risks architecture and CI before any real feature content.
Also note where the app lives relative to the repo root — the project directory. It is . for a single app at the root, or a subfolder for a monorepo package / services/<x> / a sandbox/ smoke-test. Record it in every plan (see the template field): the later phases run install/test/build there and CI keys working-directory + cache-dependency-path off it. The git branch is always cut at the repo root regardless.
Slice vertically. Decompose into an ordered list of slices, each cutting through all the layers it needs (UI → logic → persistence) to deliver one observable behaviour. Never slice horizontally (a "build the DB layer" slice has no independent value and can't be tested end-to-end — reject it). Use the splitting patterns in the guide. Order slices so the earliest ones de-risk the most and each builds on the last.
Validate every slice against INVEST — Independent, Negotiable, Valuable, Estimable, Small, Testable. If a slice isn't Small and Testable, split it again. If it has no discernible value, drop it. Aim for slices a developer could finish in well under a day.
Write the plans. Create plans/<feature-slug>/:
- A
README.md index/status board from the Feature index template in assets/plan-template.md, listing all slices in order with status.
- One
<NN>-<slice-slug>.md execution plan per slice from the Slice plan template in the same file. <NN> is the zero-padded order (01, 02, …).
- Fill in every section. The crucial ones: the acceptance criterion written as a Given/When/Then that becomes the slice's failing outer e2e test, and the initial unit test list that seeds the inner loop. These don't have to be exhaustive — the test list is living and
red-green-refactor will add to it — but they must pin down "done".
Confirm and hand off. Show the user the slice list (titles + one-line goals + the proposed first slice). Get sign-off before any code is written. Then tell them the next step: run red-green-refactor on slice 01. If they're using the tdd-harness orchestrator, return control to it.
What makes a plan good
- One behaviour per slice. If you can't state the slice's value in a single sentence, it's too big.
- The acceptance criterion is concrete and observable from outside the system — it names the real endpoint (a URL, a CLI invocation, an API call), not an internal function.
- Out-of-scope is explicit. Listing what a slice deliberately defers is how you keep it thin and stop gold-plating.
- The plan is executable by someone else. The
red-green-refactor skill should be able to start solely from the plan file.
Output
Plans only. Do not write production or test code in this phase — that is red-green-refactor's job. The deliverable is plans/<feature-slug>/ populated with a README index and one plan per slice, plus a short summary to the user of the slices and the recommended first one.
1---2name: tdd-plan3description: Use at the START of any new feature, product, bugfix, or change, before writing code, to decompose it into thin vertical slices (each the smallest end-to-end testable behaviour that delivers value) and write a markdown execution plan per slice into the plans/ folder. Phase 1 of the TDD harness. Trigger on 'slice this up', 'what is the smallest first step', 'plan this test-first', 'write a TDD plan', or any request to build something where the slices should be identified first. Each plan becomes the contract the red-green-refactor skill then executes.4---56# TDD Plan — Slice & Plan (Phase 1)78Turn a request into a set of **thin vertical slices**, each captured as a markdown execution plan in `plans/`. A good slice is the smallest change in system behaviour that is independently valuable and testable end-to-end. The plan you write here is the contract the `red-green-refactor` skill executes next, and that `tdd-ci` and `safe-pr` later read — so make it precise.910**Slicing is the hardest and most valuable judgement in TDD.** "Sequencing the tests properly is a skill — pick tests that drive you quickly to the salient points in the design." Take your time here.1112For the full slicing toolkit — vertical-vs-horizontal, INVEST, the walking skeleton, and nine concrete splitting patterns with a worked example — read `references/slicing-guide.md`.1314## Procedure15161. **Understand the request.** Restate the feature/outcome in one or two sentences. Ask only the questions that change the slicing: who the user is, the externally observable behaviour, the boundary it goes through (web UI, HTTP API, CLI), and any hard constraints. Don't over-interrogate.17182. **Detect the context.** Is this a brand-new system or a change to an existing one? Look at the repo (build tooling, existing tests, frameworks). If there is **no working build/test/deploy path yet**, the first slice must be a **walking skeleton**: the thinnest end-to-end thread that builds, runs, and is tested through real infrastructure — it de-risks architecture and CI before any real feature content.1920 Also note **where the app lives** relative to the repo root — the *project directory*. It is `.` for a single app at the root, or a subfolder for a monorepo package / `services/<x>` / a `sandbox/` smoke-test. Record it in every plan (see the template field): the later phases run install/test/build there and CI keys `working-directory` + `cache-dependency-path` off it. The git branch is always cut at the repo root regardless.21223. **Slice vertically.** Decompose into an ordered list of slices, each cutting through all the layers it needs (UI → logic → persistence) to deliver one observable behaviour. **Never slice horizontally** (a "build the DB layer" slice has no independent value and can't be tested end-to-end — reject it). Use the splitting patterns in the guide. Order slices so the earliest ones de-risk the most and each builds on the last.23244. **Validate every slice against INVEST** — Independent, Negotiable, Valuable, Estimable, Small, Testable. If a slice isn't Small and Testable, split it again. If it has no discernible value, drop it. Aim for slices a developer could finish in well under a day.25265. **Write the plans.** Create `plans/<feature-slug>/`:27 - A `README.md` index/status board from the **Feature index template** in `assets/plan-template.md`, listing all slices in order with status.28 - One `<NN>-<slice-slug>.md` execution plan per slice from the **Slice plan template** in the same file. `<NN>` is the zero-padded order (`01`, `02`, …).29 - Fill in every section. The crucial ones: the **acceptance criterion** written as a Given/When/Then that becomes the slice's failing outer e2e test, and the **initial unit test list** that seeds the inner loop. These don't have to be exhaustive — the test list is living and `red-green-refactor` will add to it — but they must pin down "done".30316. **Confirm and hand off.** Show the user the slice list (titles + one-line goals + the proposed first slice). Get sign-off before any code is written. Then tell them the next step: run `red-green-refactor` on slice `01`. If they're using the `tdd-harness` orchestrator, return control to it.3233## What makes a plan good3435- **One behaviour per slice.** If you can't state the slice's value in a single sentence, it's too big.36- **The acceptance criterion is concrete and observable** from outside the system — it names the real endpoint (a URL, a CLI invocation, an API call), not an internal function.37- **Out-of-scope is explicit.** Listing what a slice deliberately defers is how you keep it thin and stop gold-plating.38- **The plan is executable by someone else.** The `red-green-refactor` skill should be able to start solely from the plan file.3940## Output4142Plans only. Do not write production or test code in this phase — that is `red-green-refactor`'s job. The deliverable is `plans/<feature-slug>/` populated with a README index and one plan per slice, plus a short summary to the user of the slices and the recommended first one.
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