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Protocol semantics are identical on every transport. A transport is a binding: it defines how the run input is delivered, how events are framed and encoded, and how a stream terminates or fails. It does not define what events mean — the event patterns and the processing model are the same on every binding.

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The binding contract

A binding MUST provide:
  • Ordered, complete delivery of a run’s events, in the order the producer emitted them. The protocol’s order is arrival order; a transport that can reorder or drop events cannot carry AG-UI without a layer that restores both.
  • Delivery of the RunAgentInput that opens the exchange, before any events. A stream that goes on to carry further runs — a replayed thread — delivers no further input for them: those runs are the producer restating history, and each RUN_STARTED MAY carry its own input echo.
  • A termination signal a consumer can tell from truncation: a stream that ends cleanly after a terminal event is a closed run, and a connection that dies without one is a truncated run.
  • An error path for rejected input — a structurally invalid RunAgentInput is refused before RUN_STARTED, outside the stream.
Authentication and authorization are properties of the binding and the application, not of the protocol: AG-UI defines no credential, and a binding carries whatever its channel uses (HTTP authentication, ambient process identity, or nothing).

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Standard bindings

  1. HTTP + Server-Sent Events: the run input is an HTTP POST; the events stream back as SSE frames carrying JSON.
  2. HTTP + Protobuf: the same POST, negotiated to a binary response of length-prefixed protobuf frames.
Both bindings share the request half; they differ only in the response encoding, selected by content negotiation. An implementation that speaks HTTP MUST support the SSE binding; the protobuf binding is OPTIONAL.

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Truncation

A consumer whose stream ends without a terminal event has a truncated run: the producer may have kept going, but this consumer will never see the rest. A truncated run has no outcome. A consumer MUST NOT synthesize a RUN_FINISHED for it and MUST NOT report it as having succeeded; everything the run delivered before the break remains delivered. Whether and how to surface the truncation beyond that — leaving the run unresolved, or raising a synthetic failure — is the consumer’s business; re-running is a new run with a new runId.

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Custom transports

Implementations MAY carry AG-UI over other channels — WebSockets, message buses, in-process pipes. A custom transport MUST preserve the event model, the event patterns, and the processing rules, and MUST satisfy the binding contract above. It SHOULD document its framing, its input delivery, and its termination and error signals, to aid interoperability. A custom transport that carries JSON SHOULD frame events exactly as the SSE binding does — one event object per frame — rather than inventing a new envelope: the SSE binding’s framing is the protocol’s JSON framing, and only its HTTP mechanics are specific to HTTP.