Define endpoints, schemas, and error codes — get a ready-to-use API spec. Copy, download Markdown, or export OpenAPI YAML.
The generator output is meant to be pasted into a ticket, pull request, or design review. It should name the owner, decision, evidence, and follow-up work before implementation starts.
Endpoint: POST /refunds Compatibility: additive response field Request: - idempotency_key: required - order_id: required Errors: - REFUND_ALREADY_PENDING - ORDER_NOT_REFUNDABLE
Treat the generated API contract as a starting artifact. Replace example values, attach evidence, assign reviewers, and remove sections your team will not maintain.
An API spec is a structured contract that defines endpoints, request/response schemas, authentication, and error codes — all before writing any implementation code. It serves as the single source of truth for frontend and backend teams, third-party integrators, and automated testing tools.
Writing API specs before coding prevents integration mismatches, reduces back-and-forth between teams, and enables parallel development. With a clear contract in place, frontend teams can mock endpoints while backend teams implement them — and both sides know exactly what to expect.
Learn more: What is Spec-First Development? or browse our API Spec Template.
Name the consumer and provider before filling in fields. A useful API spec answers who calls the endpoint, what business event it represents, which version owns it, and what compatibility promise future changes must respect.
Examples should include real-looking identifiers, timestamps, enum values, pagination fields, and error shapes. A schema without examples leaves client teams guessing about nullability, defaults, and field naming conventions.
Error responses are part of the contract. Document stable error codes, retry guidance, validation messages, authentication failures, rate limits, and what clients should show users or log for support.
Use the Markdown output for discussion first. Once endpoint behavior is agreed, export OpenAPI YAML so client generation, mock servers, contract tests, and documentation can share the same source.
Can the endpoint add fields later without breaking clients? Are enum changes, required fields, default values, and deprecations explicitly handled?
If the client retries after a timeout, will the server create duplicates? For mutating endpoints, specify idempotency keys, duplicate detection, and safe retry behavior.
Does the spec include correlation IDs, audit events, metrics, and enough error detail for support to diagnose failures without exposing sensitive data?
POST /orders creates an order. Returns success or an error.
This leaves client teams guessing about idempotency, validation failures, payment states, partial success, and what fields are stable.
POST /orders accepts an idempotency key, validates inventory before payment capture, returns ORDER_ALREADY_EXISTS for duplicate keys, and exposes payment_status as authorized or declined.
This gives frontend, backend, QA, and support teams enough detail to build and test the contract consistently.
Put the generated Markdown beside the work item, not in a private note. The ticket should link to the endpoint owner, reviewer, version decision, and any client teams that must approve the contract before implementation starts.
Use the sample requests, responses, and error bodies as fixtures for CI. A good API spec is not finished when it is copied; it is finished when an incompatible response fails before the release reaches a consumer.
The first real client often exposes missing pagination behavior, retry guidance, or field semantics. Update the spec before shipping so the published contract matches the behavior the team actually integrated.
A finished spec lets a client team handle success, validation failure, authorization failure, rate limiting, timeout retry, and duplicate submission without asking the producer for hidden rules.
The spec should name which changes are additive, which require a version boundary, which require consumer notification, and which are not allowed without a migration plan.
If a client team still needs a Slack explanation to integrate the endpoint, the contract is not finished.
The spec should carry the decision history far enough that a future maintainer can change the endpoint without rediscovering the original assumptions.
Send the generated Markdown with one real success example and one failure example. Client teams usually find missing fields, unstable enums, and ambiguous retry behavior faster when they can compare expected payloads instead of reading endpoint prose alone.
If a reviewer asks "what happens when this field is missing?" or "can this request be retried?", turn that question into a contract test before implementation merges. The spec should collect these decisions, not leave them scattered across comments.
Use Markdown first when the team is still deciding behavior. Move to OpenAPI when the contract is stable enough for generated clients, mocks, and automated contract tests.
Put rollout details, deprecation plans, rate limits, cross-service dependencies, security assumptions, and testing expectations there. Notes are where hidden integration risk becomes visible.
The fields focus on the API decisions that most often break consumers: request shape, response shape, error taxonomy, idempotency, authentication, and compatibility. The goal is to surface integration risk before code starts.
Generated output is checked against Spec Coding's editorial standard: a reviewer should be able to turn the Markdown into contract tests, implementation tasks, and client handoff notes without relying on private explanation.
Author and editor: Daniel Marsh. Review process: Editorial Policy. Corrections and tool issues: Contact. Last reviewed: April 28, 2026.