program-design

name

program-design

description

Designing a program by the discipline of rational development. Use when an FRD (from the `requirements-intake` skill) and a frozen API contract (OpenAPI/AsyncAPI) exist and a design package is needed for later implementation (vertical slices, module contracts, antecedents/consequents, unit tests by formula, component scenarios). Do NOT use if the FRD, the API contract, or the README failure-mode map is missing — run `requirements-intake` first. Tier-agnostic: stepwise steps, router tables, checklists and STOP rules carry the essence without bloat — the optimum, not the minimum.

program-design.skill — design a program by the discipline of rational development

Purpose

The planner's skill. In: a functional-requirements document (FRD) produced by the requirements-intake skill (problem statement, Cockburn use cases, interfaces, draft contract + failure-mode map). Out: a design package the implementer uses to build the program.

Roles are work modes, not necessarily different models. One model can play both roles in sequence; what matters is the mode switch and the artifact handoff (the design package), not a model switch.

How you reason — the frame the steps derive from

Rational design = correctness by construction, not by exhaustive test:

• Stepwise refinement (Wirth) — decompose top-down, one decision at a time, until each module is trivially implementable.
• Information hiding (Parnas) — a module is a secret: one design decision hidden behind a contract, so it changes without rippling.
• Design by Contract (Hoare/Meyer) — every module carries an antecedent (what must hold in) and a consequent (what it guarantees out); that pair is its correctness.
• Compositional correctness — the whole is correct because the contracts compose (each antecedent met by its caller's consequent), established by the call-graph reconciliation (Step 9), not by testing every path. Unit tests confirm the formula; they don't establish correctness.
• Vertical slice — organize every artifact by behaviour (one external input → outcome), never by technical layer.

The 12 steps below are this frame made procedural.

Scope

DO:

• Design the module schema and contracts.
• Iteratively discuss decision points with the operator.
• Prepare the ticket backlog for the implementer.

DON'T:

• Write implementation code (that's the implementer's job).
• Make architecture decisions without operator approval.
• Add dependencies or technologies without explicit justification.

Steps (procedure map + progressive disclosure)

Hold the whole process by this table. Keep only the current step in context, not the whole skill — find the step number, open its detail file (reference/step-NN-*.md); each file has its own In/Out. If you get lost, return here.

Step	Does	Main out	Detail file (reference/)
0	Check mandatory input artifacts (FRD, contract, failure table, Gherkin)	decision "design / stop"	step-00-input.md
1	Confirm the FRD from requirements-intake (problem statement + use cases + interfaces)	verified requirements/<slug>.md	step-01-requirements.md
2	List the external inputs of the slices	slice table	step-02-inputs.md
3	Design each slice's module tree (hard rules, C4, head-pipe pseudocode)	tree + head-pipe pseudocode	step-03-module-tree.md
4	Describe the message catalog + error model	messages.md (types, Result<T, Error>)	step-04-messages.md
5	Describe module contracts by template (Dependencies: checklist)	contracts (Input/Deps/antecedent/consequent)	step-05-contracts.md
6	Isolate I/O into autonomous objects	I/O objects (Store/Client/Publisher)	step-06-isolate-io.md
7	Describe the app infrastructure module	infrastructure.md	step-07-infrastructure.md
8	Count unit tests by formula + reconcile design with Gherkin	unit-test table + Gherkin-mapping	step-08-tests-gherkin.md
9	Reconcile contract consistency via the call graph	contracts-graph.md	step-09-contracts-graph.md
10	Assemble the design package	folder .agent/planner/design/<slug>/	step-10-package.md
11	Build the ticket backlog (one per slice; + )

Hard rules and STOP (consolidated — this is the skill's conformance-gate)

plan-reviewer checks the design against this list as the reference standard (asymmetry: do not trust Step 12's self-fill). Any violation = STOP, return to the named step, handoff forbidden. Anti-gaming: you MUST NOT tick [x] without an existing artifact. The full text of each rule is in its step's file.

• Correctness by composition, not coverage (P1, foundational). A program is correct because each module's consequent ⊆ the next's antecedent (Step 9) — not because tests pass ("testing never proves the absence of errors; program better, don't test more"). Module bodies are structured: nested blocks, one entry / one exit, only sequence/selection/iteration, no goto.
• Input gate (Step 0). No contract (OpenAPI/AsyncAPI), no "failure-mode map" in the README, or no Gherkin scenarios for the slice endpoints → design does not start.
• Single Request (Step 3, lesson D1). 1 slice = 1 external input = exactly one Request; flag/option = a Request field; no side-injections past Request; no test-only I/O method; a branch on a field = logic (unit), not a component scenario.
• One data argument (Step 3). Each tree node takes exactly one data entity; 2+ → introduce a domain entity and a constructor node NewT(...).
• Invariant — subtype, not guard (Step 3). Checking an invariant over a domain struct = a subtype constructor, not a guard function -> ().
• I/O isolation (Step 5/6). No raw *sql.DB/*http.Client/broker-conn in Dependencies: or in the head's Deps — only autonomous Store/Client/Publisher/Consumer/FileStore.
• Head/I/O/adapter are not unit-covered (Step 8.1). Unit tests only for constructors and pure logic functions, by the formula 1 happy + Σ antecedent branches. Every boundary / equivalence class / decision-table case is a unit — never a component scenario (test-level split, docs/04_PLANNING_PIPELINE.md §5).
• Mandatory io: field (Step 5). Every module contract carries io: none|http|llm|queue|db — the sole routing key for io sub-skills at ticket-writing (Step 11 / §4). io: none ⇒ no external client in Dependencies:.
• Component count by adapters (Step 8.6). #component_failure_scenarios == #distinguishable_I/O-adapter_branches == #error.codes of the adapter branches; input-validation Extensions map to unit boundaries, not component scenarios (their codes are NOT counted here). Source for .

Definition of Done of the skill

• All 13 steps (0–12) passed.
• Folder .agent/planner/design/<slug>/ (working package) created and filled.
• Durable docs published to docs/design/<slice>/: module-tree.md, contracts.md (this skill), c4.md (c4), use-case.md (cockburn-use-case) — committed, reviewed at Gate #1 (Step 10 "Two locations").
• backlog.md contains one ticket per slice.
• The handoff checklist in backlog.md is fully [x] (including the last line with the handle and the PR creation date).
• The design PR is open; operator review awaited. Merging the PR = approval = the implementer may start.