API Surface Review

API Surface Review (Polar)

The handle is the diff. The evidence is what this change does to the API contract, and whether any of it is breaking.

This is where Polar's most expensive review mistakes live. A schema change does not stay in the repo: it flows into docs/openapi.json, the generated TypeScript client, the Speakeasy SDKs under sdk/, and out to merchants who already wrote code against it.

Scope

Diffs touching **/schemas.py, **/endpoints.py, polar/openapi.py, docs/openapi.json, sdk/, clients/packages/client/.

CI already does part of this job. The OpenAPI Diff workflow posts the schema delta as a PR comment, and OpenAPI Client Regeneration Check fails if the client is stale. Do not recompute either by hand. Read the diff comment if it exists and judge whether the delta is acceptable; that judgement is the thing CI cannot make.

Owned elsewhere: ADR-0002 (status-coded PolarError), ADR-0007 (no default in output schemas) → adr-check. Endpoint conventions written in server/AGENTS.md — response_model and ORM returns, POST/PATCH/DELETE status codes, ListResource, responses=, the PolarRequestValidationError boundary → conventions-check. Existing shared types → reuse-check.

Checks

1. Public or private

APITag has two values, public and private (polar/openapi.py). Public means documented and in the SDKs.

• Set it deliberately on every new router and route. Never hide something with include_in_schema=False; use tags=[APITag.private].
• Tag the route, not the router, when only one route should change. The pattern: keep APITag.public on what stays, add APITag.private to the deprecated one.
• Private endpoints (payouts are the standing example) are not in the SDK. Do not reason about them as public API.
• On every new field, ask: does a merchant need to see this? Internal Organization settings keep leaking into the public API this way.

2. Schema shape

• Output fields are required, nullable where needed. A default makes them optional in OpenAPI, which generates a wrongly-optional TS field. (ADR-0007 — name it, do not restate it.)
• Type fields with their enum, not str.
• Reuse a related resource's base schema rather than redefining its fields: class DisputeCustomer(CustomerBase): ....
• A value that already exists as a model property is a plain field, not a reimplemented computed_field.
• Descriptions are user-facing: no implementation detail, and write None not null.
• Any field writing to a SQL INT column is Int32, so out-of-range input is rejected at validation instead of overflowing.
• Constrain enums by listing allowed values (reason: Literal[RefundReason.foo, ...]) rather than a hand-rolled validator. Pydantic validates, OpenAPI documents.
• Discriminated unions get a plain Discriminator plus SetSchemaReference, so the union lands in OpenAPI properly.
• Do not over-constrain fields fed by a payment processor. "We'll never know what Stripe or other payment processor will send us."

3. Naming

The union takes the clean name, the variants get qualified: CustomerCreate is the union, CustomerIndividualCreate and CustomerTeamCreate are members. Same for CustomerState.

Fields are snake_case. Prefer explicit over clever (payment_method_type, not payment_method). Do not overload an existing concept with a new meaning.

4. Breaking changes

Breaking = removing a field, flipping required/optional, changing a type, renaming anything, or changing a status code a client branches on.

Before removing a field or behaviour:

1. Check Logfire for real usage. This is what François actually does. Merchants create things programmatically that nobody expects.
2. If it is used, keep a compatibility layer and use SkipJsonSchema so runtime still accepts the value while it disappears from future SDKs.
3. If it is genuinely breaking, say so plainly. That is a human decision, not something to wave through.

5. Route shape

Only the parts AGENTS.md does not already cover:

• Trailing slash on root endpoints.
• Errors on the same route need distinct status codes, or their schemas collide in the OpenAPI output.
• A query parameter must act as a pure filter. If it changes the response shape, the resource is wrong and it wants its own route.
• Removing an endpoint and updating its frontend caller in one PR is a deploy hazard → ship-safety owns the split.

Output

## API Surface

### 🔴 Blocking
- `file:line` — <what breaks, and for whom>. Fix: <fix>

### 🟠 Should fix
- `file:line` — <claim>. Fix: <fix>

### 🟡 Question
- `file:line` — <question>

### Notes
- public surface delta: <one line, or "see the OpenAPI Diff PR comment">

### Verdict
✅ Clean  |  ❌ n blocking, n should-fix

Anything blocking needs a human decision before merge.