Tech Spec

---
name: tech-spec
description: Write a typed call-stack architecture handoff.
disable-model-invocation: true
---

# Tech Spec

A tech spec is a **typed call-stack architecture handoff**: code-shaped contracts plus execution flows. Prefer TypeScript pseudocode over prose wherever precision matters.

This skill is design-only. Do not implement. Save a file only when the user asks for a file; otherwise return the spec inline.

## Branch selection

1. Use **Path A: Convert context to spec** when the conversation, docs, or codebase already contain enough background to describe the change.
2. Use **Path B: Grill first** when the user wants a new spec but has not provided enough problem, constraints, design direction, affected code, or acceptance criteria.

If a question can be answered by exploring the codebase, inspect the codebase instead of asking.

Completion criterion: the branch is chosen from actual available context; missing architectural decisions are not invented.

## Path A: Convert context to spec

### 1. Load standards and local context

Inspect existing code and docs for local vocabulary, module layout, domain concepts, errors, adapters, observability, runtime patterns, and test style.

Completion criterion: the spec uses project vocabulary and does not introduce a pattern, library, adapter, schema style, or test strategy before checking local precedent.

### 2. Extract the design problem

Capture:

- current state;
- problem;
- users/callers;
- goals;
- non-goals;
- constraints;
- invariants;
- affected systems;
- likely entrypoints;
- operational/runtime concerns;
- risks;
- open questions.

Mark unknowns as open questions instead of filling gaps with plausible design.

Completion criterion: every claimed requirement or constraint is grounded in conversation, code, docs, or an explicit open question.

### 3. Explore design alternatives

Produce materially different alternatives before choosing the recommended design. Alternatives should differ in interface shape, seam placement, ownership, call stack, runtime topology, or module boundaries — not just names.

For each alternative, sketch:

- domain types and state model;
- public/module interfaces and APIs;
- input/output types;
- expected failure types;
- seams, boundaries, and adapters;
- entrypoint-to-side-effect call stack;
- parsing/projection strategy;
- authorization, observability, cancellation, idempotency, and transaction flow when reachable;
- test seam strategy;
- tradeoffs.

Compare alternatives on:

- caller burden;
- module depth and leverage;
- locality of invariants and change;
- seam placement;
- boundary parsing and projections;
- error and cancellation model;
- testability through real seams;
- operational/runtime fit;
- implementation complexity.

Completion criterion: the recommendation is chosen after comparing alternatives, not before.

### 4. Specify the recommended typed contracts

For the recommended design, outline every new, changed, or deleted:

- domain value;
- branded/refined type;
- state machine variant;
- input/output type;
- request/response shape;
- function signature;
- class or module interface;
- expected-failure/custom-error type;
- adapter interface;
- protocol DTO;
- persistence DTO/projection;
- runtime-boundary codec;
- public API.

Name seams, adapters, implementations, ownership boundaries, and what crosses each boundary. State what each layer may know and what must not leak across the seam.

Completion criterion: every new or changed boundary has a concrete type/interface/API sketch, or an explicit reason no new contract is needed.

### 5. Specify call stacks and data flow

For every new, changed, or deleted behavior, show the call stack from entrypoint to side effects and response.

Include type/data flow:

```txt
raw input
  -> boundary DTO / unknown
  -> parser
  -> canonical domain/application input
  -> service/module interface
  -> adapter call
  -> typed result/error
  -> projection
  -> serialized output
```

Include current vs proposed flow when changing existing behavior. Include failure, retry, cancellation, transactionality, idempotency, observability, authorization, and runtime-hop flow when reachable.

Completion criterion: every affected behavior has an end-to-end call stack and type/data-flow trace.

### 6. Map files and modules

List:

- files/modules to add;
- files/modules to change;
- files/modules to delete, if any;
- test files;
- config/migration/runtime files, if any.

For each file, state the contract, code path, boundary, adapter, domain concept, or test responsibility it owns.

Completion criterion: every contract and call-stack step maps to a file/module or an open question.

### 7. Write the RGR TDD test plan

Use the sibling TDD workflow and testing standards. Plan vertical Red-Green-Refactor slices: one failing behavior test, minimal implementation, repeat. Do not write a horizontal "all tests first, all code later" plan.

Favor behavior through public interfaces and real seams over implementation-coupled mocks.

Cover proportionately:

- happy paths;
- failure paths;
- parser rejection and accepted shapes;
- domain invariants and state transitions;
- adapter contracts;
- persistence/runtime semantics;
- cancellation/retry/idempotency paths;
- observability and safe summaries where relevant;
- end-to-end flows for high-consequence behavior.

Completion criterion: every public behavior, invariant, important failure path, changed boundary, and changed seam has a red test slice or an explicit reason not to test it.

### 8. Produce the spec

Return the spec inline unless the user requested a file path. If a file was requested, save it there.

Do not implement and do not ask to implement by default.

Completion criterion: the output follows the outline below and is implementation-ready for another engineer.

## Path B: Grill first

1. Do not write a full spec yet.
   - State that there is not enough context for an implementation-ready tech spec.
   - Completion criterion: the agent has not invented requirements, APIs, files, or call stacks.
2. Start a grilling interview.
   - Ask one question at a time and provide the recommended answer with each question.
   - If a question can be answered by exploring the codebase, inspect the codebase instead of asking.
   - Completion criterion: the interview has enough context for Path A: problem, users/callers, constraints, affected systems, desired behavior, boundaries, likely APIs, invariants, risks, and acceptance tests.
3. Convert to the spec.
   - Once grilling context is sufficient, run Path A.
   - Completion criterion: the final artifact is a typed call-stack architecture handoff, not interview notes.

## Required spec outline

Use this shape unless the task is tiny enough to compress without losing contracts or call stacks:

```md
# <Title>

## Summary

## Context / Current State

## Goals

## Non-Goals

## Invariants

## Design Constraints

## Alternatives Considered

### Option 1: <name>

### Option 2: <name>

### Option 3: <name>

## Recommendation

## Proposed Design

## Domain Model and Types

## Types, Interfaces, and APIs

## Seams, Boundaries, Adapters, and Implementations

## Call Stacks and Data Flow

### Current / Old Flow

### Proposed / New Flow

### Failure Flow

### Retry / Cancellation / Idempotency Flow

### Observability Flow

## Files to Add / Change / Delete

## RGR TDD Test Plan

## Risks and Open Questions
```

Omit sections that truly do not apply, but do not omit typed contracts, seams, call stacks, or tests merely because they are hard to specify.

## Writing rules

- Code first: TypeScript pseudocode defines contracts, APIs, and data flow.
- Prose explains why; types and call stacks define what changes.
- Focus on types, interfaces, APIs, inputs/outputs, seams, boundaries, adapters, domain modules, service modules, external adapters, and call stacks.
- Prefer precise domain values over strings, booleans, nullable bags, and loosely shaped objects.
- Keep seams real: adapters translate framework, persistence, network, time, randomness, telemetry, runtime, or platform boundaries.
- Avoid speculative abstraction; every seam earns its existence through invariants, locality, leverage, testing, or a real boundary.
- Keep a single source of truth; do not restate the same rule in multiple sections unless one section points to the other.
- Unknowns stay open questions. Do not invent product requirements, domain rules, APIs, or call stacks to make the spec feel complete.