Contract

Skill — contract

Produce or update a domain contract from a validated feature spec. The contract normalizes the backend ↔ frontend interface for a bounded context — commands, shared types, and error variants — so both sides can implement against the same shape.

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Required tools

Read, Glob, Write, Edit, AskUserQuestion. Interactive — cannot complete in a non-interactive shell.

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When to use

• Third step of Workflow A — after spec-reviewer green-lights the spec, before contract-reviewer validates the contract
• When introducing a new bounded-context aggregate — first-time contract for a domain
• When extending an existing contract — adding commands to an aggregate that already has a contract (this skill is upsert-aware and patches in place)

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When NOT to use

• Validating an existing contract — use the contract-reviewer agent; this skill produces, it does not validate
• Producing the spec — use spec-writer first; the contract is derived from the spec
• Amending a single field — edit docs/contracts/{domain}-contract.md directly; do not re-run the skill for trivial fixes
• Generating the implementation plan — /feature-planner consumes the contract; this skill stops at the contract

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Output format

Produces:

• docs/contracts/{domain}-contract.md — the domain contract file (created if missing; patched in place if existing, per step 5)
• A ## Changelog entry appended inside that file (step 6)

If no frontend-callable commands are derived (step 3 — frontend-only or backend-internal-only feature), a duplicate command is detected across contracts (step 4), or the user rejects the diff (step 5), do not write or modify any file. Report ❌ Aborted — {reason}. and exit.

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Execution Steps

1. Load spec

Ask for the spec path if not provided. Read docs/spec/{feature}.md in full.

If no spec path is given, list files in docs/spec/ and ask the user which spec to use.

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2. Identify domain

Extract the domain name from the spec's ## Context section. The domain must name a bounded context (aggregate root) — the context/{domain}/ service that owns these commands. Use-case folders (use_cases/{domain}/) are implementation details; their commands belong in the aggregate contract they primarily mutate. The domain must NOT be named after a frontend feature, page, or UI concern.

If it cannot be inferred, ask the user: "Which bounded context (aggregate) does this feature belong to? (e.g. user, portfolio, payment — must match a context/ folder)"

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3. Extract contract data from spec

From the spec, derive:

Commands — one per frontend-callable backend operation. A rule describes a frontend-callable operation if it specifies an action triggered from the UI or an external caller. Internal-only logic (background jobs, startup tasks, scheduled workers, system monitors) is not a command — it has no frontend caller and no interface to normalize, so it does not appear in the contract.

If a backend rule is ambiguous about who triggers it (e.g., "transactions are reconciled on demand" — UI-triggered or system-internal?), use AskUserQuestion to ask before deciding. Never silently classify a borderline rule as internal-only — the omission is harder to recover from than a false-positive command.

For each command identify:

• command: snake_case name matching the spec rule's operation
• args: struct name and fields from the Entity Definition section
• return: the entity or value the command returns (from Entity Definition)
• errors: every failure condition described in the spec rule

Shared Types — every entity struct involved in args or return values. Use Rust field naming (snake_case fields, PascalCase struct names). Describe business meaning only — no storage types, no Option<>, no derives. Those are implementation details for /feature-planner.

Events — any named events implied by state-transition rules.

If no frontend-callable commands are derived — either because the spec is frontend-only (no backend rules) or because all backend rules describe internal-only logic (no frontend caller) — the feature has no backend ↔ frontend interface to normalize. Stop and report:

❌ Aborted — no frontend ↔ backend interface to normalize, no contract needed.

Skip steps 4–7. The user can re-run /contract later if frontend-callable backend operations are added to the spec.

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4. Check for cross-contract command duplication

Before writing anything, glob all existing contracts and scan for command name conflicts:

• Run Glob docs/contracts/*-contract.md to collect every existing contract.
• For each contract found (excluding {domain}-contract.md itself), read it and collect its command names.
• If any command you are about to write already exists in another contract, stop and report:

`` Command {name} already exists in docs/contracts/{other}-contract.md. Each command must belong to exactly one backend boundary. Resolve the overlap before proceeding. ``

• Do not write the contract until all conflicts are resolved.

5. Check for existing contract

Run Glob docs/contracts/{domain}-contract.md.

If the file does not exist:

• Compose the full contract (see format below)
• Show it to the user and ask: "Does this contract look correct? Any changes before I create it?"
• On approval, write docs/contracts/{domain}-contract.md

If the file already exists:

• Read the current content
• Identify what is new (commands not yet present) and what would be modified (changed args/return/errors)
• Present the diff to the user:

`` New commands: create_user, update_user Modified: get_user — adding NotFound error variant Unchanged: delete_user ``

• Ask: "Does this look correct? Any changes before I update the contract?"
• On approval, patch the file — append new commands, update modified rows, never remove existing commands silently

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6. Write changelog entry

After writing or patching, append to the ## Changelog section:

- {YYYY-MM-DD} — Added by `{feature-name}` spec: {comma-separated list of new/modified commands}

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7. Confirm and hand off

Report using this shape:

✅ Contract written — docs/contracts/{domain}-contract.md
   Commands: create_user, update_user, delete_user
   Next: run `contract-reviewer` to validate, then `/feature-planner` for the implementation plan.

For an upsert (existing contract patched), prefix Updated instead of Written and list only the new/modified commands.

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Contract file format

# Contract — {Domain}

> Domain: {domain}
> Last updated by: {spec-name}

## Commands

| Command           | Args                        | Return       | Errors                       |
| ----------------- | --------------------------- | ------------ | ---------------------------- |
| `snake_case_name` | `ArgStruct { field: Type }` | `ReturnType` | `ErrorVariant`, `OtherError` |

## Shared Types

\`\`\`rust
// Business-level struct — no derives, no Option, no storage detail
struct EntityName {
field_name: FieldType,
}
\`\`\`

## Events

| Event        | Payload       |
| ------------ | ------------- |
| `event_name` | `PayloadType` |

## Changelog

- {YYYY-MM-DD} — Added by `{spec-name}`: {command list}

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Critical Rules

1. Never silently overwrite existing commands — always diff and confirm with the user first
2. One contract = one bounded context (aggregate root). The domain must name a

context/{domain}/ bounded context, not a use case or module folder. Commands from use_cases/ that primarily mutate one aggregate belong in that aggregate's contract. A frontend gateway may call commands from multiple contracts — that is expected. What is forbidden is the same command appearing in two contracts.

1. No cross-contract command duplication. If a command name already exists in another contract,

stop and report before writing — do not proceed until the overlap is resolved.

1. Features with no frontend ↔ backend interface get no contract. The contract normalizes

a frontend ↔ backend interface; a feature with no frontend-callable backend operations has nothing to normalize. This covers both frontend-only features (no backend rules) and backend-internal-only features (cron jobs, startup tasks, schedulers — backend rules with no frontend caller). Step 3 detects this and exits without writing a file.

1. Types use Rust naming conventions: snake_case fields, PascalCase structs
2. Errors must be exhaustive. Every failure path described in a spec rule must appear as a

named error variant. A command whose only error variant is a generic catch-all (DbError, InternalError, Unknown) is rejected — name the specific failure modes from the spec (e.g., NotFound, AlreadyExists, InvalidStatus). Catch-all variants are acceptable only as a final fallback alongside named variants, never alone.

1. Do not invent commands not backed by a spec rule — traceability is mandatory
2. If the docs/contracts/ directory does not exist, create it before writing
3. This skill produces the contract shape — contract-reviewer validates correctness

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: phileggel
• Source: phileggel/claude-kit
• License: MIT

Install and usage instructions live in the source repository linked above.