AG-UI .NET Wire Format & Codec Internals

Deep reference for the protobuf transport. The canonical wire contract is the TypeScript proto package; the .NET codec mirrors it byte-for-byte.

Table of contents

1. File map
2. Binary framing
3. The codec surface (AGUIProtobuf)
4. Event oneof mapping
5. JsonElement <-> google.protobuf.Value bridge
6. Schema reference & generation
7. Content negotiation parity
8. Adding a new encoding
9. Native AOT / trim constraints

---

1. File map

2. Binary framing

Per event: uint32 big-endian length, then the protobuf message bytes. No trailing delimiter; the reader loops until the prefix read returns 0 bytes.

+--------- 4 bytes ---------+------------- length bytes -------------+
|  big-endian uint32 length |        protobuf Event message          |
+---------------------------+----------------------------------------+

TS reference (@ag-ui/encoder encodeProtobuf): dataView.setUint32(0, length, false) — the false is big-endian. .NET writes via BinaryPrimitives.WriteUInt32BigEndian in AGUIProtobuf.WriteFramed; reads via ReadUInt32BigEndian in ReadFramedAsync. ReadFramedAsync uses an internal ReadExactlyAsync that distinguishes clean EOF (0 bytes at a frame boundary → yield break) from a truncated payload (EndOfStreamException).

3. The codec surface (AGUIProtobuf)

ProtobufEventStreamFormatter.WriteAsync reuses one PooledBufferWriter across events (Reset() per event) and flushes after each frame so streaming stays incremental.

4. Event oneof mapping

ProtoEventMapper.ToProto/FromProto switch over the .NET event / generated Proto.Event oneof. The per-event reshaping mirrors sdks/typescript/packages/proto/src/proto.ts verbatim — e.g.:

• RunFinishedEvent: polymorphic Outcome is flattened to a string outcome ("success" / "interrupt" / empty) plus a repeated interrupts list; Result (a JsonElement?) becomes a Value.
• StateDeltaEvent: JSON-Patch op strings <-> a generated enum.
• CustomEvent/StateSnapshotEvent/tool args: dynamic payloads via ProtoValueConverter.

The default case throws NotSupportedException listing the supported set (RUN_STARTED, RUN_FINISHED, RUN_ERROR, STEP_STARTED, STEP_FINISHED, TEXT_MESSAGE_START/CONTENT/END, TOOL_CALL_START/ARGS/END, STATE_SNAPSHOT, STATE_DELTA, MESSAGES_SNAPSHOT, RAW, CUSTOM). Coverage is an intentional subset of the full event set.

5. JsonElement <-> google.protobuf.Value bridge

ProtoValueConverter is the only correct way to move dynamic JSON across the wire. It recurses over the generated Struct / ListValue / Value well-known types:

Reverse: ToJsonElement writes via Utf8JsonWriter (through JsonElementFactory) switching on Value.KindCase. Helpers ToValueOrNull / ToJsonElementOrNull handle optional payloads.

Double-only caveat: every number is an IEEE-754 double, so long/decimal magnitudes beyond 2^53 lose precision on the round trip. This intentionally matches the JS @ag-ui/proto implementation, which has the same limitation. Do not add an Any-based or string-encoded escape hatch to "fix" it — that would break cross-language parity.

6. Schema reference & generation

AGUI.Protobuf.csproj does not contain a copy of the schema. It sets:

<AGUIProtoRoot>$(MSBuildThisFileDirectory)..\..\..\typescript\packages\proto\src\proto</AGUIProtoRoot>
<Protobuf Include="$(AGUIProtoRoot)\events.proto;$(AGUIProtoRoot)\types.proto;$(AGUIProtoRoot)\patch.proto"
          ProtoRoot="$(AGUIProtoRoot)" GrpcServices="None" Access="Internal" />

Grpc.Tools generates internal C# types into namespace AGUI.ProtocolBuffers (set via csharp_namespace in the .proto), referenced in code as using Proto = AGUI.ProtocolBuffers;. Generated types are Access="Internal" so they never leak into the public API. Referencing the TS files directly guarantees the codec cannot drift from the canonical contract.

To add a wire-representable event: edit the shared .proto in the TS package (a cross-SDK coordinated change), then add the mapper case. There is no separate .NET schema to keep in sync.

7. Content negotiation parity

AGUIResults.Negotiate (server) reproduces @ag-ui/encoder's preferredMediaTypes(accept, [proto]):

• Protobuf is chosen only when application/vnd.ag-ui.event+proto is explicitly in Accept with non-zero quality and a matching formatter is registered.
• Otherwise SSE is used when text/event-stream, a wildcard (*/*, text/*), or no Accept is acceptable.
• Otherwise 406 Not Acceptable.

Client side: AGUIEventStreamHandler advertises all formatter media types in Accept (order = preference), then CanRead selects the decoder from the response Content-Type. SSE's CanRead also accepts a null/empty content type so a server that omits Content-Type still decodes.

8. Adding a new encoding

1. Implement IAGUIEventStreamFormatter in a new package that references only AGUI.Abstractions
   • AGUI.Formatting (plus any codec dependency). Pick a unique MediaType.
2. Implement CanRead for that media type; implement ReadAsync/WriteAsync over the raw stream.
3. Register the formatter as an IAGUIEventStreamFormatter on the server (for example, services.AddSingleton<IAGUIEventStreamFormatter, XxxEventStreamFormatter>()); on the client, pass it to AGUIEventStreamHandler and wire that handler into the HttpClient. The SDK ships no DI/builder sugar for transports.
4. If the server negotiation must prefer it explicitly (like proto), extend the negotiation logic; otherwise registration order handles preference.
5. Add cross-language parity coverage if the encoding is shared with another SDK.

9. Native AOT / trim constraints

• The package multi-targets net10/9/8/netstandard2.0/net472; Directory.Build.props treats warnings as errors and enables nullable. Use #if NET / #if NET7_0_OR_GREATER for APIs missing on netstandard2.0/net472 (e.g. ArgumentNullException.ThrowIfNull, Stream.ReadAsync(Memory), request.Options).
• SSE/JSON paths serialize exclusively through the source-generated AGUIJsonSerializerContext — never JsonSerializer.Serialize<object> or reflection-based serialization.
• Protobuf dynamic JSON goes only through ProtoValueConverter. Google.Protobuf's JsonFormatter, JsonParser, and descriptor reflection APIs pull in reflection and are not trim/AOT safe — they are banned in this package.