Technical Specifications: System Design, APIs & Architecture

Why Technical Specs Get Skipped (And Why That's Expensive)

Technical specifications get skipped for the same reason all documentation gets skipped: they feel like overhead when you're in the flow of building. The cost of skipping them compounds over time — misaligned implementations, re-architecture after launch, and knowledge loss when team members leave.

Claude makes technical specs fast enough to write that the excuse of "we don't have time" becomes untenable.

System Design Document

A System Design Document (SDD) describes what the system does, how it's structured, and why key architectural decisions were made.

What it must contain:

• Overview and scope
• System architecture diagram description
• Component breakdown and responsibilities
• Data flow description
• External integrations
• Non-functional requirements (performance, scalability, availability)
• Security model
• Deployment architecture
• Open questions and decisions pending

Claude prompt:

Generate a System Design Document for a real-time collaborative document editor.
The system must support: 100 concurrent users per document, offline editing
with sync on reconnect, and full edit history.
Tech stack: React frontend, Node.js backend, PostgreSQL, Redis for pub/sub.
Include architecture overview, component responsibilities, data flow,
conflict resolution strategy, and top 3 technical risks with mitigations.

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API Specification

An API spec defines every endpoint, its parameters, responses, and error codes — before a line of code is written.

What it must contain:

• Base URL and versioning strategy
• Authentication method
• Each endpoint: method, path, description, request body, query params, response schema, error codes
• Rate limiting policy
• Pagination pattern

Claude prompt:

Generate an OpenAPI-style specification for a task management API.
Endpoints needed: create task, update task, delete task, list tasks (with filtering),
get task by ID, assign task to user, mark complete.
For each endpoint, define: HTTP method, path, request body schema,
success response (200/201), and relevant error responses (400, 401, 403, 404).
Use RESTful conventions.

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Architecture Decision Record (ADR)

An ADR documents a specific technical decision — why it was made, what alternatives were considered, and what the trade-offs are.

What it must contain:

• Status (Proposed / Accepted / Deprecated / Superseded)
• Context — what situation forced this decision
• Decision — what was decided
• Alternatives considered — other options with their trade-offs
• Consequences — what this decision makes easier and harder going forward

Claude prompt:

Write an Architecture Decision Record for the decision to use PostgreSQL instead
of MongoDB for our primary data store.

Context: We're building a B2B SaaS with complex relational data (users, teams,
projects, tasks, permissions). Schema is mostly known upfront.

Document: the decision, 3 alternatives we considered (MongoDB, DynamoDB, CockroachDB),
trade-off analysis for each, and the consequences of this choice on future scalability
and team skill requirements.

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Data Model Documentation

Generate a data model document for a multi-tenant SaaS application.
Entities: Organization, User, Project, Task, Comment, Attachment.
For each entity, provide: fields with data types, constraints, and descriptions;
primary and foreign key relationships; indexing recommendations;
and a short explanation of the multi-tenancy isolation strategy.
Format as a table for each entity followed by a relationship summary.

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Technical Spec for a Feature

The most common technical spec is a per-feature document that engineers use to implement a specific capability.

What it must contain:

• Feature description and acceptance criteria
• Technical approach and implementation plan
• Data changes required
• API changes required
• Frontend changes required
• Dependencies and blockers
• Testing requirements
• Rollout strategy (feature flag, gradual rollout, etc.)
• Estimated complexity

Claude prompt:

Write a technical specification for implementing two-factor authentication (2FA)
via TOTP (authenticator app) in an existing web application.

Stack: React, Node.js/Express, PostgreSQL, JWT-based auth.

Include: database schema changes, backend API endpoints, frontend flow,
library recommendations, security considerations, recovery code implementation,
and QA checklist.

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Integration Specification

Write an integration specification for connecting our application to Stripe
for subscription billing.

Include: webhook events we must handle (payment_succeeded, payment_failed,
subscription_cancelled, trial_ending), idempotency requirements,
local database schema changes for subscription state,
error handling and retry strategy, and testing approach (Stripe test mode,
event simulation).

Key Takeaways

• Technical specs prevent the expensive rework that comes from misaligned implementations
• ADRs are a lightweight but high-value habit — document decisions as they're made, not after
• Claude generates comprehensive spec structures quickly; engineers fill in the implementation-specific details
• Always include acceptance criteria and testing requirements in feature specs

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Try It Yourself: Choose a feature you're building or planning. Prompt Claude: "Write a technical specification for [feature]. Stack: [your stack]. Include database changes, API changes, frontend changes, and a QA checklist." Use the output as a pre-flight checklist before you start building.