Designing Module Interfaces and API Contracts for Agent Workflows

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When breaking a feature into modules for agent teams, the interface between modules is the coordination point. A clear, machine‑readable contract prevents ambiguity, reduces handoffs, and lets each agent work end‑to‑end with confidence. This guide gives concrete conventions you can copy into your spec.

1) Choose contract formats

Prefer machine‑readable specs that match your architecture:

  • REST: OpenAPI (JSON/YAML) for endpoints, parameters, status codes.
  • gRPC/microservices: .proto files for RPC signatures and types.
  • Event or async flows: AsyncAPI for topics, payloads, and delivery semantics.
  • Data payloads: JSON Schema (or Protobuf schema) for request/response bodies and validation rules.

2) Define request and response schemas (minimal required fields)

For every interface include:

  • Shape: explicit field names, types, required vs optional.
  • Examples: one minimal and one full example payload for both request and response.
  • Constraints: value ranges, string formats (ISO 8601, email), max lengths.
  • Error model: structure for errors (code, message, details) and common error codes.

3) File, folder and naming conventions

Make artifacts predictable so agents can locate and parse them automatically:

  • Spec location: /specs/{feature}/openapi.yaml or /specs/{feature}/schema.json
  • Examples: /specs/{feature}/examples/request.min.json and /response.full.json
  • Artifacts: outputs saved as /artifacts/{module}/{version}/{artifact-name}.ndjson or .jsonl when streaming is used.
  • Names: use kebab-case for files and resource-based paths for endpoints (e.g., /orders/{order_id}/fulfillments).

4) Versioning and changelog

Require semver and a changelog link in every module header so consumers can detect breaking changes:

  • Spec header: “version”: “2.1.0” and “spec_version”: “openapi:3.1”
  • Breaking changes: increment major, document migration steps and deprecation windows (e.g., 90 days).
  • Automated tooling: CI validates that new code conforms to the declared spec version before merge.

5) Contract testing and mock servers

Include tests agents can run locally or in CI:

  • Schema validation tests: fail if responses violate JSON Schema or OpenAPI types.
  • Behavioral acceptance tests: a small set of request/response pairs that must pass.
  • Mock server: generated from the spec (e.g., Prism, mockoon) with deterministic responses for the examples directory.

6) Orchestration signals and handoff artifacts

Define what constitutes a completed handoff and how agents declare it:

  • Handoff artifact: a named file (e.g., /handoffs/{module}/{ticket-id}.json) containing status, produced artifacts, and validation report.
  • Signal conventions: status values (ready, blocked, deprecated) and timestamps in ISO 8601.
  • Retries and idempotency: document idempotency keys for repeated requests and recommended retry/backoff behavior.

7) Security and rate limits

Make auth explicit in the contract:

  • Authentication method (Bearer, API key, mTLS) and required scopes per endpoint.
  • Rate limits, quotas, and expected error codes for limit breaches.
  • Data sensitivity: mark fields that must be redacted or encrypted at rest/in transit.

8) Documentation and discoverability

Ensure agents can find the contract and examples automatically:

  • Expose machine spec at a canonical URL (e.g., /specs/{feature}/openapi.json).
  • Include a short human description and one‑line mission for the module in the spec metadata.
  • List sample commands to validate the module (curl or CLI examples) in /specs/{feature}/README.md.

Applying these conventions turns interfaces into executable agreements: agents can generate types, run mocks, validate outputs, and hand off artifacts without extra coordination. Copy the checklist into your feature spec as the interface section so every module ships with a usable contract.

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