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The API changes by adding, never by breaking. This page says exactly what that promise covers, which parts are still in preview, and where the current baseline began. Read it before you decide how tightly to bind your integration to a field.

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The additive-only promise

Everything in the API reference is additive-only from the current baseline, except the groups marked as previews. Additive-only means a documented field or route never changes its meaning or its type, and never disappears. We add new fields and new routes. Your side of the promise is to ignore fields you don’t recognize. In return, the fields you do recognize keep working, and you never have to watch for a breaking-change announcement, because there are none to watch for.

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What’s in preview

A preview can still change shape, and it’s outside the promise until its preview label comes off. The Legacy Partner API is under the promise: nothing in it is removed while a firm runs on it, and Keys and authentication says which of its calls have venue routes. Partner routes and trdrs_sk_… keys keep their current contracts. The broker query parameter keeps its spelling, even though these docs call the thing it names a provider.

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Where the baseline began

The current baseline began on 2026-09-25, before the first partner integration. That day a reset of an issued account became an audited operation on a settled account: POST /api/partner/accounts/reset now requires the reason for the reset, and refuses while the account holds a position, a working order, a command with an unknown outcome, unresolved risk or a negative balance. It never closes or cancels anything itself. That was a breaking change, so the baseline moved rather than the promise bending. No client or integrator used the route, so there’s no compatibility path, and the changelog says exactly what a test client written before then must change. Earlier baselines:

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How the promise is enforced

The promise is checked on every build, not just stated. The build reads the engine’s own router and fails if a route a key can reach is missing from the reference, and every documented schema is checked against the types the engine itself compiles against. A shape can’t drift from what’s on the wire, and a route can’t ship undocumented, without the build failing.