Technical design doc

Technical Design Document

A technical design document (TDD) captures the architecture, trade-offs, and rollout plan for a significant feature before implementation begins. Not every feature needs a full TDD — apply this methodology when a feature is complex enough that undocumented architectural decisions would slow or block implementation.

Write the prose this skill governs at a seventh-grade reading level and in ASD-STE100 Simplified Technical English — short sentences, common words, one instruction per sentence, no unexplained jargon. Full methodology: skills/writing-prose/SKILL.md.

When to Write a TDD

Write a TDD when the feature:

• Introduces a new architectural pattern not already established in the codebase (new data store, new service boundary, new async pattern)
• Has multiple valid approaches with real trade-offs worth documenting before committing to one
• Touches multiple subsystems and requires coordination across components
• Has a non-trivial rollout (schema migration, backward compatibility requirements, phased rollout, feature flag)
• Has significant performance or security implications that need deliberate design

When in doubt: if the plan would benefit from having the architectural decisions documented alongside the steps, write the TDD section.

TDD Structure

Problem Statement

One to three sentences: what problem does this feature solve, and why does it need to be solved now? This grounds every subsequent decision in context.

Goals and Non-Goals

Goals: What the feature must accomplish. State measurable outcomes where possible. This section defines scope.

Non-Goals: What the feature explicitly does NOT do. Non-goals are as important as goals — they prevent scope creep and answer "why didn't you also do X?" questions in advance.

Background

Context a reader needs to understand the design: relevant existing components, prior art in the codebase, constraints inherited from other systems. Keep this to what a new contributor would need to evaluate the design choices.

Design

The core of the TDD. Describe the architecture of the solution:

Data Model

If the feature introduces new data structures, describe them. For databases: table names, columns, types, indexes, foreign keys. For in-memory: type definitions, invariants, lifecycle.

API / Interface

What new interfaces does this feature expose? HTTP endpoints, function signatures, event schemas, CLI commands. Be precise — these become the contract downstream agents implement against.

Key Components

Which existing components are modified? Which new components are introduced? For each:

• What is its single responsibility?
• What does it consume and produce?
• What are its dependencies?

Sequence / Flow

For features with non-obvious control flow, describe the sequence of operations from trigger to completion. A numbered list or ASCII sequence diagram works well.

Edge Cases and Failure Modes

Enumerate the boundary conditions, error paths, and unusual inputs the feature must handle. For each, name the scenario and the chosen behavior. Walk these categories so none gets skipped:

• Boundary values: empty inputs, zero, one, max size, off-by-one.
• Invalid inputs: malformed payloads, wrong types, missing fields.
• Failure paths: downstream errors, timeouts, partial writes, network failures, retries.
• Concurrency: simultaneous requests, idempotency, races.
• Authorization: unauthenticated, unauthorized, expired credentials.
• Resource limits: rate exhaustion, quota, memory pressure.

Edge cases that are intentionally out of scope belong in Non-Goals, so downstream agents do not silently expand coverage. A TDD with no edge-case section is incomplete.

Trade-offs Considered

Every design choice implies rejected alternatives. For each major decision:

**Decision:** What was chosen.

**Alternatives considered:**
- Alternative A — why rejected
- Alternative B — why rejected

**Risk:** What could go wrong with the chosen approach, and how it is mitigated.

Rollout Plan

How will this feature be deployed safely?

• Migration strategy: If the feature changes data or APIs, how is compatibility maintained during the transition?
• Feature flags: If the feature should be enabled gradually, what flags exist and what gates them?
• Rollback plan: If the feature needs to be reverted, what does that require? Is it reversible?
• Monitoring: What metrics or logs will confirm the feature is working correctly in production?

Open Questions

Unresolved decisions that must be answered before implementation begins. List them explicitly so they are not silently assumed. Each open question should include who must answer it and when.

When the Planner Should Use This Methodology

The planner agent loads this methodology when:

1. The research artifact identifies multiple valid approaches or significant architectural decisions.
2. The feature introduces new patterns or touches multiple subsystems.
3. The plan artifact warrants richer documentation than the standard plan format provides.

In these cases, the planner produces an enhanced plan that includes TDD sections (trade-offs, data model, rollout) alongside the standard phases, steps, and done criteria.

For smaller features, the standard plan format is sufficient — do not add TDD sections for their own sake. A well-structured plan without TDD overhead is better than an incomplete TDD that delays implementation.

Integration With the Standard Plan Format

When producing an enhanced plan, add TDD sections after the Context section and before the Steps section:

### Context
...

### Trade-offs
...

### Data Model (if applicable)
...

### Rollout Plan (if applicable)
...

### Steps
...

### Tests
...

### Done Criteria
...

Keep the total plan under 300 lines. If the TDD content would push the plan over that limit, extract the TDD to a separate file in docs/plans/ and reference it from the plan.