AG-UI .NET Cross-Language Interop Tests

How to add tests proving the .NET AG-UI SDK interoperates with the TypeScript SDK on the wire. The harness lives under sdks/dotnet/tests/. Read sdks/dotnet/docs/cross-language-testing.md (design plus the "Harness reference" section) for full background.

Interop philosophy

• The TypeScript SDK is the reference (oracle). A round-trip through the TS codec — proto.decode(proto.encode(event)) — is the canonical normalised form both SDKs must agree on. Assert .NET output against that, not against the raw fixture object.
• Round-trip semantic equivalence is the real guarantee, in both directions (TS→.NET and .NET→TS). Byte-for-byte parity is a stronger claim asserted only where deterministic.
• Tests exercise our code via project references (AGUI.Abstractions, AGUI.Server, AGUI.Protobuf), never the published NuGet packages.

Harness architecture

Two directions, two server processes (see the "Harness reference" section of sdks/dotnet/docs/cross-language-testing.md):

Phase 1 mechanism: Vitest globalSetup (CrossLanguage.Vitest/helpers/global-setup.ts) starts LLMock (@copilotkit/aimock, deterministic LLM responses), then helpers/dotnet-server.ts runs dotnet build and spawns the produced CrossLanguage.TestServer.exe (not dotnet run — single PID for clean Windows teardown) with OPENAI_BASE_URL pointed at LLMock. It waits for the HTTP listener, then test files fetch the routes. Both server and LLMock stay alive across the whole run; vitest.config.ts sets fileParallelism: false so concurrent SDK builds don't contend for artifact locks.

Phase 2 mechanism: TsServerFixture (IAsyncLifetime, shared via [Collection(nameof(TsServerCollection))]) shells out pnpm run server (tsx server/main.ts) which emits canned AG-UI events via @ag-ui/encoder; AGUIChatClient consumes them.

Adding a TS → .NET scenario (Phase 1)

1. Add a server-side route under CrossLanguage.TestServer/ following the endpoint pattern (MapPost → input.ToChatRequestContext(...) → GetStreamingResponseAsync → .AsAGUIEventStreamAsync(ctx) → TypedResults.ServerSentEvents(AgenticChatRoute.WrapAsSseItems(...))). See ParallelToolCallsRoute.cs. For a route that must negotiate the transport (SSE or protobuf from the Accept header), end with AGUIResults.Events(events, httpContext, cancellationToken) instead and make sure Program.cs registers ProtobufEventStreamFormatter as an IAGUIEventStreamFormatter — see AgenticChatRoute.cs.
2. Register it in Program.cs (e.g. app.MapParallelToolCalls("/parallel_tool_calls")).
3. Register any new payload types in CrossLanguageJsonSerializerContext.cs ([JsonSerializable(typeof(YourReport))]). This context is snake_case to match the TS wire shape.
4. Add an aimock fixture in fixtures/*.json keyed by userMessage / toolCallId so LLM responses are deterministic (see fixtures/parallel-tool-calls.json).
5. Add a tests/<scenario>.test.ts that drives the route with HttpAgent and asserts on the collected event stream (see parallel-tool-calls.test.ts).

Adding a .NET → TS scenario (Phase 2)

1. Add a fake agent ((RunAgentInput) => BaseEvent[]) in server/fakeAgents.ts.
2. Mount its route in server/main.ts.
3. Add a *.cs test in AGUI.CrossLanguage.IntegrationTests/, decorated [Collection(nameof(TsServerCollection))], driving it with AGUIChatClient (see AgenticChatTests.cs).

Protobuf wire-parity pattern

ProtobufParityRoute.cs exposes three codec routes backed by AGUIProtobuf:

tests/protobuf-parity.test.ts proves parity against @ag-ui/proto (sdks/typescript/packages/proto/src/proto.ts) and @ag-ui/encoder framing (sdks/typescript/packages/encoder/src/encoder.ts). For each fixture (fixtures/protobuf-events.ts):

• tsBytes = proto.encode(event); reference = proto.decode(tsBytes).
• TS encode → .NET decode: expect(netDecode(tsBytes)).toEqual(reference).
• .NET encode → TS decode: expect(proto.decode(netEncode(event))).toEqual(reference).
• Byte parity depends on the fixture's byteParity flag:
  • "strict" — scalar-only events (only string/number fields, serialised in field-number order): assert the TS and .NET bytes are byte-for-byte equal.
  • "roundtrip" — payloads mapping to google.protobuf.Struct (map<string, Value>): map-entry ordering is not canonical across encoders, so do NOT assert byte equality. Assert both byte streams decode (via the TS codec) to the same event instead.

A framing test concatenates encoder.encodeProtobuf(event) frames (4-byte BE prefix) and posts them to /protobuf/decode-framed to exercise ReadFramedAsync. When adding a new protobuf event, add a fixture with the correct byteParity flag — scalar-only ⇒ strict, any object/array payload ⇒ roundtrip.

Transport parity (parameterize scenarios over SSE + protobuf)

The codec-parity pattern above isolates the codecs — it never goes through the TS HttpAgent or Accept-header negotiation. To prove the full transport path (TS client negotiates a protocol → .NET server encodes it → TS client decodes it), parameterize a scenario suite over both transports — the cross-language analogue of the .NET integration tests' TransportFormat {Json, Protobuf} [Theory].

helpers/transport.ts is the shared mechanism: TRANSPORTS = ["sse", "protobuf"], TRANSPORT_MEDIA_TYPE, and createTransportAgent(config, transport) which returns an HttpAgent that requests the transport (it opts into protobuf via the public fetch hook — the default agent hardcodes Accept: text/event-stream after spreading headers, so a headers option can't override it) and captures the response Content-Type. Drive the suite with describe.each(TRANSPORTS) and assert lastResponseContentType() === TRANSPORT_MEDIA_TYPE[transport] plus the usual decoded-event assertions — see tests/agentic-chat.test.ts.

describe.each(TRANSPORTS)("… [%s]", (transport) => {
  it("…", async () => {
    const { agent, lastResponseContentType } = createTransportAgent(
      { url: `${baseUrl()}/agentic_chat`, threadId: `t-${transport}` }, transport);
    agent.messages = [{ id: "u", role: "user", content: "Hi, I am duaa" }];
    const events: BaseEvent[] = [];
    await agent.runAgent({}, { onEvent: ({ event }) => events.push(event) });
    expect(lastResponseContentType()).toBe(TRANSPORT_MEDIA_TYPE[transport]);
    // …assert decoded events (identical regardless of transport)…
  });
});

The route under test must negotiate (AGUIResults.Events, with ProtobufEventStreamFormatter registered as an IAGUIEventStreamFormatter in Program.cs). tests/agentic-chat.test.ts and tests/state-events.test.ts are parameterized this way today.

Only parameterize protobuf-safe scenarios. ToolCallResult, Reasoning*, and Activity* events have no message/oneof entry in the shared events.proto (the one schema referenced by both @ag-ui/proto and .NET AGUI.Protobuf), so neither SDK can protobuf-encode them — the codec throwing NotSupportedException mirrors the schema, it is not a .NET gap. A suite that emits any of those must stay SSE-only. Adding protobuf support for them is an upstream schema change (extend the canonical TS events.proto, regenerate both SDKs, add mappers) — see agui-cross-sdk-parity.

AGUI_MEDIA_TYPE (@ag-ui/proto) and ProtobufEventStreamFormatter.ProtobufMediaType are the identical exact string, and AGUIEventStreamResult sets Content-Type to it with no charset — the client's exact === content-type match depends on that.

Running the suites

# TS → .NET + protobuf parity (builds + spawns the C# server automatically)
cd sdks/dotnet/tests/CrossLanguage.Vitest
pnpm test
pnpm exec vitest run tests/protobuf-parity.test.ts   # just the parity suite

# .NET → TS (shells out `pnpm run server`)
cd sdks/dotnet/tests/AGUI.CrossLanguage.IntegrationTests
dotnet test

pnpm install from the repo root once first. Requires .NET 10 SDK, Node 20+, pnpm 10+.

❌ Critical anti-patterns

1. Don't assert byte-equality for object/Struct payloads. google.protobuf.Struct map ordering is non-canonical; byte parity is only valid for scalar-only ("strict") fixtures. For object payloads, assert round-trip equivalence.
2. Don't make the .NET fixture the oracle. The TS codec is the reference — compare against proto.decode(proto.encode(event)), not the raw event object.
3. Don't add a route without registering it in BOTH Program.cs AND CrossLanguageJsonSerializerContext.cs (when it introduces new payload types) — the server won't map the route or will fail AOT-safe serialization.
4. Don't depend on real LLM calls. Phase 1 LLM responses come from aimock fixtures keyed by userMessage/toolCallId; add a fixture for every new prompt. Use fixed/deterministic tool outputs (see ParallelToolCallsRoute's frozen clock).
5. Don't use dotnet run to start the server in helpers, and don't assume parallel test files. The harness spawns the built .exe for a single killable PID and runs files sequentially (fileParallelism: false) to avoid build-lock contention and Windows port orphans.