Agent Skills: Stripe-Inspired API Design Best Practices

Stripe-Inspired API Design Best Practices

A reference distillation of the design conventions behind Stripe's API — the most widely admired and copied JSON HTTP API in the industry. Contains 52 actionable rules across 8 categories, prioritised by how irreversibly a wrong decision cascades through every endpoint, every SDK, and every client integration. Each rule explains the WHY, shows incorrect-vs-correct code, and links to the canonical source.

When to Apply

Reach for this skill when:

• Designing a new JSON HTTP API or a new endpoint on an existing one
• Reviewing an API design proposal, OpenAPI spec, or PR that adds/changes endpoints
• Debugging an integration where the "wrong" shape of the API is causing client bugs
• Auditing an API for naming consistency, error shape uniformity, or compatibility risks
• Producing an API design report (the kind your inspector tool emits — Critical / Warning / Suggestion / Positive)
• Picking between two designs and looking for an authoritative source to back the choice
• Onboarding to API design — these are the canonical patterns to internalise first

The rules are general — they apply to any JSON HTTP API, not just APIs imitating Stripe. Triggers include "API design", "OpenAPI", "endpoint", "schema", "webhook", "idempotency", "pagination", "API versioning", and reviews of YAML/JSON spec files.

Rule Categories by Priority

| Priority | Category | Impact | Prefix | |----------|----------|--------|--------| | 1 | Resource Modeling & Identifiers | CRITICAL | resource- | | 2 | URL Structure & HTTP Semantics | CRITICAL | url- | | 3 | Request & Response Format | HIGH | format- | | 4 | Errors & Status Codes | HIGH | error- | | 5 | Idempotency & Safe Retries | HIGH | idem- | | 6 | Versioning & Backwards Compatibility | HIGH | ver- | | 7 | Naming, Polymorphism & Metadata | MEDIUM-HIGH | naming- | | 8 | Authentication, Webhooks & Search | MEDIUM-HIGH | ops- |

Earlier categories cascade harder: a wrong choice in resource modeling (numeric IDs, no object discriminator) propagates to every endpoint forever; a wrong choice in webhook event naming is a single category to fix.

Quick Reference

1. Resource Modeling & Identifiers (CRITICAL)

• resource-prefixed-string-ids — Use Prefixed String IDs for Every Resource
• resource-object-discriminator — Include a Read-Only object Discriminator on Every Resource
• resource-opaque-ids — Treat IDs as Opaque Strings up to 255 Characters
• resource-unix-seconds-timestamps — Use Unix Seconds (Integer) for All Datetimes
• resource-iso-date-only — Use ISO 8601 Date Strings for Date-Only Values
• resource-birthdate-hash — Represent Birth Dates as {day, month, year} Hashes
• resource-integer-minor-units — Use Integer Minor Units for Money, Never Floats
• resource-currency-field-not-name — Colocate a currency Field; Never Bake Currency into Field Names
• resource-decimal-suffix-strings — Use _decimal String Suffix for Precise Decimals That Can't Be Integers

2. URL Structure & HTTP Semantics (CRITICAL)

• url-plural-collections — Pluralize Collection URLs; Singularize Object Types
• url-post-for-updates — Use POST for Updates (Not PUT or PATCH)
• url-action-verbs-as-subpaths — Express Non-CRUD Actions as Imperative Sub-Paths
• url-no-bulk-endpoints — One Object Per Request — No Bulk Endpoints
• url-version-in-path-and-header — Version in URL Path and Stripe-Version Header
• url-dedicated-search-endpoint — Use a Dedicated /search Endpoint for Complex Queries

3. Request & Response Format (HIGH)

• format-form-encoded-requests — Accept Form-Encoded Requests, Always Return JSON
• format-bracket-notation-nesting — Use Bracket Notation for Nested Fields in Form Bodies
• format-list-envelope — Return Lists in a {object, url, has_more, data} Envelope
• format-cursor-pagination — Paginate by Cursor (starting_after/ending_before), Not by Offset
• format-no-total-counts — Use has_more Boolean; Never Return Total Counts
• format-expand-parameter — Expand Related Objects with expand[] in One Round Trip
• format-dot-notation-expansion — Allow Dot-Notation for Nested Expansion (Max Depth 4)

4. Errors & Status Codes (HIGH)

• error-top-level-object — Always Wrap Failures in a Top-Level error Object
• error-four-type-enum — Use a Small Fixed type Enum, Don't Proliferate Types
• error-message-mandatory-code-optional — Require message; Make code Optional and Only for Programmatic Handling
• error-lowercase-snake-case-codes — Use Lowercase snake_case for Error Codes, Not SCREAMING_SNAKE_CASE
• error-http-status-mapping — Map Error Types to HTTP Status Codes Consistently
• error-doc-url-on-every-error — Include doc_url Links and Request IDs on Every Error

5. Idempotency & Safe Retries (HIGH)

• idem-key-header — Accept Idempotency-Key Header on All Mutating Requests
• idem-scoped-per-account — Scope Idempotency Keys per Account, Not Globally
• idem-24h-ttl — Keep Idempotency Keys for 24 Hours, Reap at 72
• idem-fail-on-key-reuse — Return 409 When a Key Is Reused with Different Params
• idem-recovery-points — Use Recovery Points for Multi-Step Idempotent Operations

6. Versioning & Backwards Compatibility (HIGH)

• ver-dated-versions — Use Dated Versions (YYYY-MM-DD), Not v1/v2/v3
• ver-account-pinning — Pin Each Account to Its First-Request Version
• ver-additive-changes — Define What Counts as a Backwards-Compatible Change
• ver-version-change-modules — Encapsulate Each Breaking Change in a Version-Change Module
• ver-tolerate-unknown — Document That Clients Must Tolerate Unknown Fields, Events, and Enum Values

7. Naming, Polymorphism & Metadata (MEDIUM-HIGH)

• naming-snake-case-wire-format — Use snake_case for All Wire Identifiers
• naming-american-english — Use American English Spelling (canceled, Not cancelled)
• naming-simple-unambiguous — Names — Simple, Unambiguous, No Leading Digits, No Jargon
• naming-type-discriminator-polymorphism — Discriminate Polymorphic Types with a type Field and Sibling Objects
• naming-metadata-pattern — Provide a metadata Pass-Through with Strict Limits
• naming-boolean-past-tense — Booleans — Past-Tense Verbs and Plain Adjectives, Not is_/has_ Prefixes
• naming-enums-over-booleans — Prefer Enums over Booleans for New Status/Flag Fields

8. Authentication, Webhooks & Search (MEDIUM-HIGH)

• ops-prefixed-api-keys — Prefix API Keys with Scope and Mode (sk_live_, pk_test_, rk_)
• ops-https-only-basic-auth — Enforce HTTPS Only and Use HTTP Basic Auth with the Key as Username
• ops-on-behalf-of-header — Use a Dedicated On-Behalf-Of Header for Multi-Tenant Calls
• ops-webhook-event-envelope — Webhook Events Use a Fixed {id, object, type, data, created} Envelope
• ops-webhook-event-naming — Event Type Naming — <resource>.<past_tense_action>
• ops-webhook-signature — Sign Webhook Deliveries with HMAC and a Timestamp Tolerance Window
• ops-webhook-at-least-once-handlers-idempotent — Document At-Least-Once Delivery; Handlers Must Dedupe on event.id

How to Use

For a focused question ("should this field be a boolean or an enum?"), jump directly to the relevant rule (naming-enums-over-booleans) — each rule is self-contained with the WHY, code examples, and the canonical source.

For a full API review or audit, work through the categories top-to-bottom. The order matches Stripe's own design priority: get resource modeling and URL structure right first; format, errors, idempotency, and versioning are the next layer; naming and operational surface come last because they're the easiest to evolve.

For producing a structured findings report (the kind an inspector tool emits), cite rules by their slug — resource-unix-seconds-timestamps, format-no-total-counts — so each finding traces back to a specific, defensible source.

Read section definitions for the cascade-impact rationale behind the category ordering, or the rule template when adding a new rule.

Reference Files

| File | Description | |------|-------------| | references/_sections.md | Category definitions and ordering by design-propagation impact | | assets/templates/_template.md | Template for authoring new rules | | metadata.json | Version and reference URLs | | gotchas.md | Failure points discovered when applying the rules |