03 API Specification

API Specification Skill

Use When

• Consumers and providers need one testable API contract before parallel implementation.

Do Not Use When

• Do not use for internal method design, database schema design or undocumented endpoint invention.

Required Inputs

Workflow

1. Read the named inputs and confirm their approval, version and unresolved decisions.
2. Apply the decision rules below before drafting; stop on a missing authority, unsafe assumption or unresolved scope driver.
3. Produce the API specification and valid OpenAPI 3 artefact through the existing domain procedure and load only the references needed for the chosen branch.
4. Trace each material statement in the API specification and valid OpenAPI 3 artefact to an input, decision or explicitly qualified assumption.
5. Verify the observable acceptance conditions, record unassessed checks, and hand the artefacts to their named consumers.
6. If validation fails, recover by correcting the source decision or artefact and rerun the affected check; do not weaken the acceptance condition.

Outputs

Evidence Produced

Capability and permission boundaries

Read-only is the default for analysis, review, evaluation and planning. Read and search access to authorised project artefacts are required. Editing is limited to an explicitly requested project deliverable. Execution may run document, syntax or validation checks. Network access is used only for facts that require current verification. Do not publish, spend, change production, approve policy, or claim certification without explicit authority.

Degraded mode

If any required capability is unavailable, return the narrowest useful qualified API specification and valid OpenAPI 3 artefact draft plus a gap register showing the missing item, affected sections, risk and owner. Never convert an unassessed check into a pass.

Decision Rules

Quality Standards

• Preserve repository terminology and trace every material choice to project context.
• Use deterministic acceptance conditions; replace vague quality claims with an observable check, threshold or named approval.
• Cover error, empty, edge, recovery and operational cases relevant to this skill.
• Verify standards, citations, APIs and package names before relying on them; qualify what cannot be checked.
• Stop release for a failed safety, security, legal, financial, accessibility or data-integrity gate.

Anti-Patterns

• Listing routes without schemas. Fix: define request, response and error models.
• Returning 200 for every outcome. Fix: use stable status and domain error semantics.
• Adding pagination after launch. Fix: specify it for every unbounded collection.
• Putting secrets in examples. Fix: use inert placeholders and auth schemes.
• Publishing OpenAPI that was not parsed. Fix: run syntax and contract checks.

References

• Advanced API patterns
• Long-running operations
• Practical API architecture

Overview

This skill generates comprehensive API documentation and a machine-readable OpenAPI 3.0 specification. It translates functional requirements from the SRS and architectural decisions from the HLD into a complete, implementation-ready API contract. The skill can run after 01-high-level-design completes and operates in parallel with 02-low-level-design and 04-database-design.

When to Use

• After 01-high-level-design has produced HLD.md in projects/<ProjectName>/<phase>/<document>/, which identifies system components and integration points.
• When SRS_Draft.md Section 3.2 provides the functional requirements that map to API endpoints.
• When the team needs a formal API contract before backend development begins.

Quick Reference

Input Files

Output Files

Core Instructions

Follow these eleven steps in order. Halt and notify the user if a required input file is missing.

Step 1: Read Context Files

Read SRS_Draft.md and HLD.md from projects/<ProjectName>/<phase>/<document>/, and tech_stack.md from projects/<ProjectName>/_context/. Log every file path read. If any required file is missing, halt execution and report the gap.

Step 2: Extract API Resources

Extract entities and operations from SRS Section 3.2 to identify API resources. Each entity that the system manages (e.g., User, Order, Product) becomes a resource. Each operation on that entity becomes an endpoint.

For public, partner, workflow-heavy, or long-lived APIs, load references/practical-api-architecture.md before locking the resource model. Apply its consumer contract, idempotency, lifecycle, error-code, and observability checks.

Step 3: Map CRUD to HTTP Methods

Map CRUD operations to HTTP methods following REST conventions: Create = POST, Read = GET, Update (full) = PUT, Update (partial) = PATCH, Delete = DELETE. Use HLD component boundaries to determine resource grouping and URL namespace.

Step 4: Define Authentication Scheme

Extract the authentication mechanism from SRS Section 3.5.3 (Security Requirements). Define the scheme as one of: JWT Bearer Token, Session Cookies, or API Key. Reference skills/dual-auth-rbac/ if the project uses role-based access control with multiple authentication strategies.

Step 5: Define Endpoint Details

For each endpoint, specify: path (RESTful URL), HTTP method, description, path parameters with data types, query parameters with defaults, request body schema (JSON with field types and constraints), response schema (JSON with field types), and HTTP status codes (200, 201, 400, 401, 403, 404, 422, 500).

Step 6: Define Error Response Format

Define a standardized error response format for all endpoints. Reference skills/api-error-handling/ for the canonical pattern:

{
  "success": false,
  "error": {
    "code": "string",
    "message": "string",
    "details": []
  }
}

Every error status code shall return this structure.

Step 7: Define Rate Limiting

Extract performance constraints from SRS Section 3.3 (Performance Requirements). Define rate limits per endpoint tier: public endpoints, authenticated endpoints, and administrative endpoints. Specify the rate limit headers: X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset.

Step 8: Define Pagination

Define pagination strategy for all list endpoints: cursor-based or offset-based depending on data characteristics. Include standard parameters page, per_page and response envelope total, data[]. Reference skills/api-pagination/ if available.

Step 9: Apply Advanced API Patterns (Optional)

For APIs with operations beyond standard CRUD, apply patterns from references/advanced-api-patterns.md:

• Long-Running Operations: If any endpoint processes >10 seconds, implement the LRO pattern from references/long-running-operations.md (POST returns 202 + operation resource)
• Batch Operations: If bulk create/update/delete is needed, implement batch endpoints from references/batch-operations.md (POST /resources:batchCreate)
• Custom Methods: For non-CRUD actions (cancel, approve, archive), use POST /resource:action pattern
• Field Masks: For large resources, implement partial response via fields parameter

Source: API Design Patterns (JJ Geewax)

Step 10: Generate API_Specification.md

Write the human-readable specification to projects/<ProjectName>/<phase>/<document>/API_Specification.md with all sections defined in the Output Format below.

Step 11: Generate openapi.yaml

Generate a valid OpenAPI 3.0 document at projects/<ProjectName>/<phase>/<document>/openapi.yaml. The document shall include: openapi: "3.0.3", info block, servers block, paths with all endpoints, components/schemas with all request/response models, and components/securitySchemes with the authentication definition.

Output Format

The generated API_Specification.md shall follow this template structure:

# API Specification: [Project Name]

## Document Header
## 1. API Overview
### 1.1 Base URL and Versioning
### 1.2 Content Types
### 1.3 Common Headers
## 2. Authentication and Authorization
### 2.1 Authentication Scheme
### 2.2 Authorization Model
### 2.3 Token Lifecycle
## 3. Endpoint Reference
### 3.x [Resource Name]
#### 3.x.1 [METHOD] /api/v1/resource
## 4. Request and Response Schemas
### 4.1 Common Models
### 4.2 Resource-Specific Models
## 5. Error Response Format
### 5.1 Standard Error Envelope
### 5.2 Error Code Registry
## 6. Rate Limiting
## 7. Pagination
## 8. Versioning Strategy
## 9. CORS and Security Headers
## 10. Traceability Matrix

Section 10 (Traceability Matrix) shall map each endpoint to its originating SRS requirement ID and the HLD component that owns it.

Common Pitfalls

1. Missing error codes: Every endpoint shall document all possible HTTP status codes, not just the success case.
2. Unprotected routes: Every non-public endpoint shall specify its authentication and authorization requirements.
3. Inconsistent naming: Use consistent casing (snake_case or camelCase) and pluralization across all endpoints and schema fields.
4. No pagination on list endpoints: Every endpoint that returns a collection shall include pagination parameters and metadata.
5. Missing request validation: Document required fields, data types, and constraints for every request body.

Verification Checklist

• All required input files were read and logged.
• Every entity in SRS Section 3.2 maps to at least one API resource with CRUD endpoints.
• Every endpoint specifies authentication requirements, request/response schemas, and all applicable status codes.
• The error response format is consistent across all endpoints.
• List endpoints include pagination parameters and response metadata.
• The openapi.yaml file is valid OpenAPI 3.0 and contains all endpoints defined in API_Specification.md.
• For long-lived or side-effecting APIs, the specification includes a consumer contract matrix, idempotency map, stable error-code registry with retryability, versioning/deprecation policy, and contract-test obligations.

Integration

Standards

• OpenAPI 3.0: The OpenAPI Specification defines a standard, language-agnostic interface to HTTP APIs. Governs the structure of openapi.yaml.
• IEEE 29148-2018: Systems and software engineering -- Life cycle processes -- Requirements engineering. Ensures traceability from requirements to API endpoints.
• RFC 7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content. Defines HTTP method semantics and status code meanings.

Resources

• logic.prompt -- executable prompt for automated API specification generation.
• README.md -- quick-start guide for this skill.