Slicing Into Tracer Bullets

Slicing into tracer bullets

Stage contract

• Stage: 4. Breakdown · Kind: method
• Inputs: the canonical spec (stage 2) + the module boundaries (stage 3).
• Outputs: an ordered list of vertical slices, each with acceptance criteria, scope boundaries, and Blocked by: links — one slice = one build-loop run.
• Entry gate: the spec is build-ready and it is bigger than one slice. A single-slice job skips this stage — build it directly.
• Done when: every slice is end-to-end (touches all layers it needs), demoable/verifiable alone, and its dependencies are stated. No horizontal-only ("schema-only", "API-only") items remain.
• Next: two engines. Sequential (default): the converging-and-polishing skill — run the build loop once per slice, each in its own fresh session seeded from the spec + that one slice. Fleet (gate G3F: parallelizable across independent nodes AND budget for a worker fleet): the delivering-mvp-fleet skill — it builds its own board from the spec + slices.
• Maintains: —

Why vertical, not horizontal

A tracer bullet is a thin slice that cuts through all the layers it touches — schema, backend, adapter, UI, tests — and delivers one small end-to-end behavior that you can actually run and see. It is the opposite of a horizontal layer ("build the whole DB schema", then "build the whole API"), which produces nothing demoable until the last layer lands and hides integration risk until the end.

Why it matters for an agent build: each vertical slice is small enough to build, review, and converge in one focused session without the context filling up. The integration seams — the places things actually break — get exercised on slice #1, not discovered at the end. And each slice merges independently.

What makes a good slice

• End-to-end. It reaches every layer its behavior needs. If a slice can't be demoed or verified without a future slice, it isn't a slice yet — it's a layer.
• Demoable / verifiable alone. There's a concrete way to see it work: a request that returns the right thing, a screen that renders, a test that passes.
• Small. Sized to one build-loop run. If it won't converge in one session, split it.
• Scoped. State what it explicitly does NOT cover (goes to a later slice), so the builder doesn't gold-plate.
• Behavioral, not procedural. Describe what the slice must do (interfaces, contracts, acceptance criteria) — not how to code it, and not by pointing at file paths/line numbers that will have drifted by the time it's built.

Ordering

• Dependency-first. A slice that another needs comes earlier; record it as Blocked by: <slice>. The first slice should be the thinnest complete path through the system (the "walking skeleton") — it proves the seams end to end.
• Value-and-risk next. After the skeleton, order by what de-risks or delivers most.
• One slice per build session. Each converging-and-polishing run starts fresh, seeded with the spec + exactly one slice — not the whole conversation. Keeping the design→spec→breakdown work unbroken in one context window, then handing each slice to a clean session, is the context hygiene that keeps every build sharp.

Slice template

### Slice <n>: <short behavioral title>
- **Delivers:** <the one end-to-end behavior, in user/behavior terms>
- **Layers touched:** <schema · API · adapter · UI · tests — only what it needs>
- **Acceptance criteria:** <concrete, testable, independently verifiable>
- **Out of scope:** <what a later slice covers — do not build here>
- **Blocked by:** <earlier slice(s), or "none">

Smells — you sliced it wrong if…

• A slice is named after a layer (schema.md, "the API work") → horizontal; re-slice by behavior.
• A slice can't be demoed until a later slice lands → it's a layer, not a tracer bullet.
• Slice #1 doesn't touch the UI or the outermost adapter → the walking skeleton doesn't actually walk; the integration seams stay unproven.
• The list has no Blocked by: anywhere on a multi-slice build → dependencies are hidden and the build order is guesswork.
• A single slice clearly won't converge in one session → split it before building.

Source: Serge Shima (@aostrikov_agents_chat), Breakdown into slices.