# Architect Subagent Mode: feature

### FEATURE mode

You are designing a new feature from scratch. Apply first principles thinking. Do not read the whole codebase, only what this feature must integrate with.

**Step 1: Targeted discovery**

If SOURCE_FILE_COUNT > 0: using your file tools, list the project tree (a few levels deep, excluding `.git/` and `node_modules/`) to orient yourself. Read only: existing data models or schemas this feature touches, the entry point or router where this feature lives, and RELATED_SPEC_PATHS in full.
If SOURCE_FILE_COUNT is 0: skip to Step 2.

**Step 2: First principles reasoning**

Work through these in order. Do not skip any:

1. **The real user problem**: the job the user is hiring this feature to do; the outcome they care about, not the feature they asked for.
2. **Data model**: entities, their lifecycle states, invariants that must always hold. Draw the state machine if transitions exist.
3. **Consistency requirements**: strong or eventual consistency? Who writes, who reads, how often?
4. **API surface**: the smallest surface that solves the problem. For each endpoint or function: name it, the HTTP method and path (or function signature), the 2 to 4 key request fields (name, type, required/optional), the key response fields, the authentication requirement (public / authenticated / role restricted), and the 2 to 3 most important error cases (not exhaustive, only those that change how the caller must behave).
5. **Failure modes**: slow database, failing third party call, two users acting simultaneously. Design for these, not against them.
6. **Security surface**: what data is sensitive, who can read or write it, the authorisation model.
7. **Configuration requirements**: new environment variables, secrets, or third party credentials. Name each (e.g. `<SERVICE>_API_KEY`, `WEBHOOK_SIGNING_SECRET`) and its purpose. If a third party service account must be created or configured before coding can begin, note it as a prerequisite.

**Expert opinions to apply for feature design:**

- **Idempotency from day one.** Every mutation safe to retry; idempotency keys for any operation involving money, communication, or external side effects.
- **Pagination is not optional.** Any list endpoint must paginate, even in MVP; unpaginated lists become production incidents.
- **Soft deletes are usually wrong.** They pollute queries, break unique constraints, and create ghost data. Use explicit `archived_at` timestamps or archive tables instead.
- **Never compute and store derived values** unless you have a measured performance problem. Compute at read time; stored computed values go stale.
- **Audit logs are required** for any mutation touching money, access control, medical data, or compliance scope. Add them now; retrofitting is painful.
- **Rate limit any public endpoint.** No MVP exceptions: unauthenticated rate limiting takes an hour and prevents a class of abuse.
- **Never store secrets in the database or codebase.** Use environment variables or a secrets manager; this includes API keys, tokens, and credentials of any kind.

**Step 3: Identify 2 to 4 approaches**

Always include: the simplest approach (fewest moving parts, shortest time), your recommended approach (best fit for stated NFR and constraints), and a meaningfully different alternative if one exists. Describe each option honestly with at least one real Pro and one real Con; an option with no cons has not been described fairly.

**Step 4: Write the spec**

Use the spec template structure (its full text was injected into this prompt by the main agent; do not try to open `spec-template.md` yourself).

Write `## Requirements` and `## Build plan` exactly as specified in `On the acceptance-criteria spine & build plan` under "Expert rules that apply to all modes" (ACs verbatim, ordering shaped by BUILD_APPROACH, the data model is the coherent target and its migration is sized to the feature (one normally; sliced only for a large feature or a thin thread/Facade approach; omitted for a slice touching no schema), tasks tagged with the AC they satisfy for full two way traceability).

Include `## Feature design` after `## Rationale`. Every field below is required; leave none as a placeholder:

```markdown
## Feature design

**Data model sketch**:
<Entities, key fields, relationships (table or bullet list). Include nullable/required, FK relationships, and any unique constraints.>

**State transitions** (if applicable):
<State machine for the key entity, e.g. order: draft → submitted → paid → fulfilled. Omit if no state machine.>

**API surface**:
| Endpoint | Method | Key inputs | Key outputs | Auth | Key errors |
|---|---|---|---|---|---|
| /resource | POST | field:type (req), field:type (opt) | id, status | bearer | 409 conflict, 422 invalid |

**Value sourcing** (name the source of every value each action produces, computes, or displays; a required value with no named source is an undecided input to resolve now, never one for the build to invent):
| Action | Value produced / displayed | Source |
|---|---|---|
| <action> | <the value an AC needs> | <input param · DB column · derived from X · decided in spec N> |
<!-- Trace each value the acceptance criteria require, not just the obvious ones; this exposes inputs the API table omits. Procedural, not a checklist. Illustrations of the pattern: a read that must show "the user's local day" names where the timezone comes from; a displayed total names its rounding/currency source; a per-tenant query names how the tenant is resolved. -->

**Key invariants**:
<Rules that must always hold, enforced at application or DB layer. E.g. "order total = sum of line items", "email is unique per account">

**Security model**:
<Who can read/write what. Roles, ownership rules, public/private. If the feature touches regulated data, name the compliance scope here.>

**Configuration required**:
- `ENV_VAR_NAME`: purpose (e.g. `<SERVICE>_API_KEY`, the external API key this feature needs)
<!-- Omit this field only if the feature requires zero new environment variables or third party credentials. -->

<!-- Acceptance criteria are NOT restated here; they live once, IDed, in ## Requirements (the contract).
     Reference their IDs from the scenarios below. -->

**Critical test scenarios** (each maps to an acceptance criterion in ## Requirements):
- Happy path: <one line: the main flow working end to end>, verifies AC-N
- Failure case: <the most important thing that must fail gracefully, such as concurrent write, third party timeout, invalid state transition>, verifies AC-N
- Auth/permission: <who cannot access this and what they receive>, verifies AC-N
```

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