Backplane.AiProtocol

backplane_ai_protocol is the Backplane umbrella's pure protocol-contract and provider-codec application. It contains canonical request, response, content, tool, usage, error, provider-state, and stream-event values together with bounded validation, serialization, SSE framing, and provider wire codecs.

It does not own HTTP transport, credentials, retries, cancellation, process lifecycle, persistence, Phoenix, database access, or network startup. A client such as Synapsis owns those concerns and feeds bytes to the codec. The Backplane gateway separately owns proxy routing, authorization, native forwarding, provider presets, and durable observation.

Provider codec API

Backplane.AiProtocol.Codec is a pure facade with three supported selectors:

The facade exposes:

Codec.encode_request(protocol, request, opts)

Codec.decode_response(protocol, status, headers, body, opts)

Codec.decode_error(protocol, status, headers, body, opts)

Codec.stream_new(protocol, opts)

Codec.stream_feed(protocol, state, bytes)

Codec.stream_finish(protocol, state, reason)

Request encoding returns {:ok, string_keyed_wire_map} or a structured {:error, %Backplane.AiProtocol.Error{}}. Response and error decoding are also tagged. Stream state is an ordinary caller-owned value; the package starts no processes.

stream_new/2 returns the initial state directly. stream_feed/3 and stream_finish/3 use these tagged shapes:

{:ok, next_state, [%Backplane.AiProtocol.StreamEvent{}]}

{:error, %Backplane.AiProtocol.Error{}, next_state}

Stream events can represent text/reasoning deltas, content, tool-call lifecycle, opaque provider state, usage, terminal state, or a structured error. Hosts must decide how those events become runtime messages and must not treat protocol terminal observation as transport/process ownership.

The OpenAI Responses observer remains an observation helper for Backplane's native proxy path. It is not an OpenAI Responses client codec, and :responses is not a supported Codec selector.

Opaque provider state

Signed Anthropic thinking and Google thought signatures are represented as opaque %Backplane.AiProtocol.ProviderState{} values. The host is responsible for persisting and restoring them without interpreting the payload.

Replay is origin-bound. Encoding requires matching source protocol and profile, plus complete matching origin/destination affinity for profile, endpoint, and model; optional account and workspace values must also match when present. Missing or changed origin metadata is rejected as incompatible. Google content that carries a thought signature without thought: true is also rejected because the package cannot preserve it faithfully.

Unsupported response content, multiple outputs where only one can be preserved, and other lossy conversions return structured incompatibility errors rather than being dropped.

Backplane proxy boundary

Same-protocol gateway traffic continues to use native passthrough. Native selection is based on the concrete client wire protocol and the selected provider API's declared native_protocols, not its provider name or broad compatibility family. Native requests, responses, streaming events, and upstream errors bypass canonical conversion. Model alias mapping is the sole routing exception: when needed, only the semantic model value changes; otherwise the original request bytes are forwarded.

A cross-protocol gateway route may be enabled only when the package has complete request, non-streaming response, streaming-event lifecycle, tool, usage, and error codecs for that directed pair. The current gateway has no complete cross-protocol HTTP codec pair, so the host rejects mismatches with unsupported_protocol_translation before submission. Adding or consuming the client codec API does not alter native passthrough, provider presets, route selection, or host observation behavior.

Consumption status

Synapsis currently consumes this application from a local sibling checkout path. The package is published independently, but this integration does not imply a standalone-package or independent-CI portability guarantee.

Source notices

See SOURCE_NOTES.md for the pinned extraction candidates and current non-extraction boundary.