Add a first-party .NET client (proposal)

.github/workflows/ci.yml

@@ -255,3 +255,58 @@ jobs:
       - name: Test Go module
         run: go test ./...
 
+  dotnet:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: clients/dotnet
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 24
+          cache: npm
+
+      # The .NET 10 SDK builds the net8.0 target and understands the .slnx
+      # solution format; the 8.0.x entry provides the net8.0 runtime so the
+      # net8.0 tests run natively.
+      - uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: |
+            8.0.x
+            9.0.x
+            10.0.x
+
+      - name: Install Node deps
+        working-directory: .
+        run: npm ci
+
+      # Verify the committed C# sources are in sync with the TypeScript
+      # protocol definitions. `git status --porcelain` (like the Kotlin / Go
+      # jobs) so a newly-emitted file also fails the check.
+      - name: Verify generated .NET is up to date
+        working-directory: .
+        run: |
+          npm run generate:dotnet
+          if [ -n "$(git status --porcelain -- clients/dotnet)" ]; then
+            echo "::error::Generated .NET sources are out of date. Run 'npm run generate:dotnet' and commit the result."
+            git status --porcelain -- clients/dotnet
+            git --no-pager diff -- clients/dotnet
+            exit 1
+          fi
+
+      - name: Restore .NET solution
+        run: dotnet restore
+
+      # Whitespace formatting gate — the C# analog of the Go job's gofmt check,
+      # governed by clients/dotnet/.editorconfig.
+      - name: Verify .NET formatting
+        run: dotnet format whitespace --verify-no-changes --no-restore
+
+      - name: Build .NET solution
+        run: dotnet build --no-restore --configuration Release
+
+      - name: Test .NET solution
+        run: dotnet test --no-build --configuration Release
+

AGENTS.md

@@ -2,15 +2,16 @@
 
 Cross-cutting rules for AI coding agents working in this repository. Per-client
 codegen conventions are in `clients/kotlin/AGENTS.md`,
-`clients/swift/AGENTS.md`, and `clients/go/AGENTS.md`. Editorial rules
+`clients/swift/AGENTS.md`, `clients/go/AGENTS.md`, and
+`clients/dotnet/AGENTS.md`. Editorial rules
 for protocol types are in
 `.github/instructions/general-instructions.instructions.md`. Release mechanics
 are in [`RELEASING.md`](RELEASING.md).
 
 ## Updating CHANGELOGs
 
-This repo ships six independently-versioned artifacts (the spec plus
-the Rust / Kotlin / Swift / TypeScript / Go clients), each with its
+This repo ships seven independently-versioned artifacts (the spec plus
+the Rust / Kotlin / Swift / TypeScript / Go / .NET clients), each with its
 own `CHANGELOG.md` in Keep-a-Changelog format. The publish workflows
 refuse to release a tag whose matching `## [X.Y.Z]` heading is
 missing, so every user-visible change should land its CHANGELOG bullet
@@ -50,6 +51,7 @@ Map source paths to changelogs:
 | `clients/swift/**` (non-generated) | `clients/swift/CHANGELOG.md` only. |
 | `clients/typescript/**` (non-generated) | `clients/typescript/CHANGELOG.md` only. |
 | `clients/go/**` (non-generated) | `clients/go/CHANGELOG.md` only. |
+| `clients/dotnet/**` (non-generated) | `clients/dotnet/CHANGELOG.md` only. |
 | `schema/**` | Root `CHANGELOG.md` (the schema is a spec output). |
 | `scripts/generate*.ts` that changes any client's generated output | Every affected client's `CHANGELOG.md`. |
 

CONTRIBUTING.md

@@ -25,6 +25,8 @@ against them.
 | `clients/kotlin/` | Kotlin/JVM library (`com.microsoft.agenthostprotocol:agent-host-protocol`). |
 | `clients/swift/` | Swift package (consumed by SwiftPM at the repo root). |
 | `clients/typescript/` | npm package `@microsoft/agent-host-protocol`. |
+| `clients/go/` | Go module (`ahptypes`, `ahp`, `ahpws`). |
+| `clients/dotnet/` | .NET / NuGet packages (`Microsoft.AgentHostProtocol`, `.Abstractions`, `.WebSockets`). |
 | `.github/workflows/` | CI and per-artifact publish pipelines. |
 
 ## Local dev loop
@@ -42,6 +44,8 @@ cd clients/typescript && npm ci && npm test && npm run build
 cd clients/rust && cargo test --workspace
 cd clients/kotlin && ./gradlew build
 swift build && swift test        # Swift uses the root Package.swift
+cd clients/go && go test ./...
+cd clients/dotnet && dotnet test
 ```
 
 ## Releases
@@ -53,7 +57,7 @@ see [`docs/specification/versioning.md`](docs/specification/versioning.md).
 
 ## Updating CHANGELOGs
 
-This repo ships five independently-versioned artifacts (spec + four clients),
+This repo ships seven independently-versioned artifacts (spec + six clients),
 each with its own `CHANGELOG.md` in [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
 format. The publish workflows refuse to release a tag whose matching
 `## [X.Y.Z]` heading is missing, so every user-visible change should land its
@@ -94,4 +98,5 @@ When iterating on the protocol surface in `types/`, see
 for the project's editorial rules on type changes.
 
 For language-specific code-gen conventions, see the `AGENTS.md` file in each
-client directory (`clients/kotlin/AGENTS.md`, `clients/swift/AGENTS.md`).
+client directory (`clients/go/AGENTS.md`, `clients/kotlin/AGENTS.md`,
+`clients/swift/AGENTS.md`, `clients/dotnet/AGENTS.md`).

README.md

@@ -17,14 +17,15 @@ The Agent Host Protocol (AHP) defines how a portable, standalone sessions server
 - **Kotlin** — Add `com.microsoft.agenthostprotocol:agent-host-protocol` from Maven Central to use from Android or any JVM project. See [`clients/kotlin/`](clients/kotlin/) for the source and [`CHANGELOG`](clients/kotlin/CHANGELOG.md). Released via `kotlin/vX.Y.Z` tags.
 - **TypeScript** — Install `@microsoft/agent-host-protocol` to use the wire types, reducers, `AhpClient`, and the `WebSocketTransport`. See [`clients/typescript/`](clients/typescript/) and [`CHANGELOG`](clients/typescript/CHANGELOG.md). Released via `typescript/vX.Y.Z` tags; the Azure DevOps publish pipeline at [`clients/typescript/pipeline.yml`](clients/typescript/pipeline.yml) picks up the tag, validates it, and publishes to npm.
 - **Go** — `go get github.com/microsoft/agent-host-protocol/clients/go` to use the `ahptypes` wire types, the `ahp` async client (client + pure reducers + pluggable `Transport`), and the `ahpws` WebSocket transport. See [`clients/go/`](clients/go/) and [`CHANGELOG`](clients/go/CHANGELOG.md). Released via `clients/go/vX.Y.Z` tags — the Go module proxy indexes the directory-prefixed tag directly from this repo, so there is no separate package registry.
+- **.NET** — Install `Microsoft.AgentHostProtocol` (and `Microsoft.AgentHostProtocol.WebSockets` for a `ClientWebSocket` transport) to use the wire types, the pure reducers, the async `AhpClient`, and the `MultiHostClient`. The `Microsoft.AgentHostProtocol.Abstractions` package carries the wire types + transport/serializer interfaces alone. See [`clients/dotnet/`](clients/dotnet/) and [`CHANGELOG`](clients/dotnet/CHANGELOG.md). Released to NuGet.org via `dotnet/vX.Y.Z` tags.
 - **[AHPX](https://github.com/TylerLeonhardt/ahpx)** — A command-line and Node.js client for connecting to AHP servers, managing sessions, and sending prompts.
 - **[VS Code](https://github.com/microsoft/vscode)** — VS Code includes Agent Sessions client code for working with AHP hosts.
 
 ### Servers
 
 - **[VS Code agent host](https://github.com/microsoft/vscode)** — The reference AHP server implementation. Start in [`src/vs/platform/agentHost/node/`](https://github.com/microsoft/vscode/tree/main/src/vs/platform/agentHost/node) when browsing the repository.
 
-For consumers that need to talk to two or more hosts at once, the Rust SDK ships a `MultiHostClient` abstraction in [`ahp::hosts`](https://docs.rs/ahp/latest/ahp/hosts/), the Swift SDK ships `MultiHostClient` in `AgentHostProtocolClient`, and the Go SDK ships `MultiHostClient` in [`ahp/hosts`](clients/go/ahp/hosts/). Single-host consumers use the same API via `MultiHostClient::single` in Rust, `MultiHostClient.single(...)` in Swift, or `hosts.Single(...)` in Go. See [Connecting to Multiple Hosts](https://microsoft.github.io/agent-host-protocol/guide/clients-multi-host) for the design and surface.
+For consumers that need to talk to two or more hosts at once, the Rust SDK ships a `MultiHostClient` abstraction in [`ahp::hosts`](https://docs.rs/ahp/latest/ahp/hosts/), the Swift SDK ships `MultiHostClient` in `AgentHostProtocolClient`, the Go SDK ships `MultiHostClient` in [`ahp/hosts`](clients/go/ahp/hosts/), and the .NET SDK ships `MultiHostClient` in `Microsoft.AgentHostProtocol.Hosts`. Single-host consumers use the same API via `MultiHostClient::single` in Rust, `MultiHostClient.single(...)` in Swift, `hosts.Single(...)` in Go, or `MultiHostClient.SingleAsync(...)` in .NET. See [Connecting to Multiple Hosts](https://microsoft.github.io/agent-host-protocol/guide/clients-multi-host) for the design and surface.
 
 ## Versioning and releases
 

RELEASING.md

@@ -23,6 +23,7 @@ and a checked-in `clients/<lang>/release-metadata.json`.
 | TypeScript | `typescript/vX.Y.Z` | `clients/typescript/pipeline.yml` (Azure DevOps) | npm (`@microsoft/agent-host-protocol`) via ESRP. |
 | Swift      | `vX.Y.Z` (bare)     | `.github/workflows/publish-swift.yml` | SwiftPM resolves the tag directly. |
 | Go         | `clients/go/vX.Y.Z` | `.github/workflows/publish-go.yml` | Go module proxy resolves the tag directly. |
+| .NET       | `dotnet/vX.Y.Z`     | maintainer-owned pipeline (see below) | NuGet.org (`Microsoft.AgentHostProtocol`, `.Abstractions`, `.WebSockets`). |
 
 > **Why Swift gets the bare semver tag namespace:** SwiftPM only resolves
 > packages by matching plain `X.Y.Z` / `vX.Y.Z` git tags at the manifest's
@@ -146,6 +147,26 @@ trigger started the run.
    `go get github.com/microsoft/agent-host-protocol/clients/go@vX.Y.Z`;
    no registry push happens.
 
+### .NET (`dotnet/vX.Y.Z`)
+
+1. Update `clients/dotnet/VERSION` to the new bare semver string (no
+   leading `v`).
+2. Run `npm run generate:metadata` and commit the regenerated
+   `clients/dotnet/release-metadata.json`.
+3. Rotate `clients/dotnet/CHANGELOG.md`.
+4. Merge to `main`.
+5. Tag: `git tag dotnet/v0.X.Y && git push origin dotnet/v0.X.Y`.
+6. Publish the libraries (`Microsoft.AgentHostProtocol`,
+   `Microsoft.AgentHostProtocol.Abstractions`,
+   `Microsoft.AgentHostProtocol.WebSockets`) to NuGet.org. This client does
+   not ship its own publish automation — the maintainers wire the
+   `dotnet pack` + `dotnet nuget push` step into their own release pipeline,
+   the same way the Kotlin and TypeScript packages publish through the signed
+   Azure DevOps / ESRP pipelines rather than a GitHub Actions registry push.
+   The per-PR CI job already builds, tests, and runs the test-parity gate for
+   the solution; `npm run verify:changelog` guards the
+   `clients/dotnet/VERSION` ↔ `CHANGELOG.md` heading match.
+
 ### Spec (`spec/vX.Y.Z`)
 
 1. Bump `PROTOCOL_VERSION` in `types/version/registry.ts` (and, if the

clients/dotnet/.editorconfig

@@ -0,0 +1,12 @@
+# Formatting conventions for the .NET client. Gated in CI by
+# `dotnet format whitespace --verify-no-changes` (the C# analog of the Go
+# lane's `gofmt -l` check), so whitespace cannot silently drift.
+root = true
+
+[*.cs]
+indent_style = space
+indent_size = 4
+tab_width = 4
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8

clients/dotnet/.gitignore

@@ -0,0 +1,2 @@
+bin/
+obj/

clients/dotnet/AGENTS.md

@@ -0,0 +1,123 @@
+# Agent Guide — .NET client
+
+Conventions for AI coding agents working on the .NET client. Cross-cutting
+repo rules are in the root [`AGENTS.md`](../../AGENTS.md); release mechanics
+are in [`RELEASING.md`](../../RELEASING.md).
+
+## Layout
+
+| Path | Contents |
+| --- | --- |
+| `src/AgentHostProtocol.Abstractions/Generated/*.generated.cs` | **Generated** wire types. Do not edit. |
+| `src/AgentHostProtocol.Abstractions/Json/`, `Transport/` | Hand-written serialization support (`AhpUnion`, `UnionConverter`, `WireEnumConverter`, `StringOrMarkdown`) and the `ITransport` / `IAhpSerializer` seams. |
+| `src/AgentHostProtocol/` | `AhpClient`, the reducers, the default `SystemTextJsonAhpSerializer`, subscriptions, and the `Hosts/` multi-host runtime. |
+| `src/AgentHostProtocol.WebSockets/` | `ClientWebSocket`-based transport. |
+| `tests/AgentHostProtocol.Tests/` | xUnit tests, including the shared reducer-fixture conformance suite. |
+| `examples/` | Runnable console samples. |
+
+## Code generation
+
+Generated files are produced by `scripts/generate-csharp.ts` (run from the repo
+root via `npm run generate:dotnet`) from the TypeScript definitions in
+`types/`. The generator is modeled on `scripts/generate-go.ts` and shares its
+curated struct / enum / union lists — they are protocol-driven, not
+language-specific. After changing anything under `types/`, regenerate and
+commit; CI fails on any diff between the committed sources and a fresh run.
+
+## Type mapping (TS → C#)
+
+- `number` → `long` (or `double` when the property carries `@format float`).
+- `unknown` / `object` → `System.Text.Json.JsonElement`;
+  `Record<string, unknown>` → `Dictionary<string, JsonElement>`.
+- Optional (`?` / `| undefined` / `| null`) fields → nullable + `[JsonIgnore(
+  Condition = JsonIgnoreCondition.WhenWritingNull)]`. Required fields serialize
+  their value (a required reference left null serializes as `null`, mirroring
+  Go's `nil`-slice semantics).
+- String enums → C# `enum` with `[WireValue("…")]` per member, (de)serialized
+  by `WireEnumConverter<T>`. Bitset enums → `[Flags] enum : uint`, serialized
+  as their numeric value so unknown future bits round-trip.
+- Discriminated unions → a sealed wrapper deriving from `AhpUnion` (carrying
+  `object? Value`) plus a generated `UnionConverter<T>`. Unknown discriminator
+  values are preserved verbatim as a raw `JsonElement`.
+
+## Reducers
+
+The reducers are a faithful port of the Go client's `reducers.go` and mirror
+the canonical TypeScript reducers. They mutate state in place. The shared
+fixtures under `types/test-cases/reducers/*.json` are the cross-language parity
+gate — run them with `dotnet test`. The `resourceWatch` reducer is an
+intentional stub (parity with the Rust and Go clients).
+
+## Testing
+
+Run by `dotnet test` (against `net8.0`), all green (0 skipped):
+
+1. **Shared reducer conformance** — `FixtureDrivenReducerTests` replays the 169
+   cross-language reducer fixtures (`types/test-cases/reducers/*.json`). The
+   whole set counts as a single `[Theory]`.
+2. **Shared wire round-trip corpus** — `TypesRoundTripFixtures` data-drives the
+   language-agnostic round-trip corpus under `types/test-cases/round-trips/*.json`
+   through the REAL serializer, asserting decode → re-encode is a byte-exact
+   fixed point. A `[Theory]` (`CorpusFixture`) iterates every fixture in the dir.
+3. **Native unit tests** — `ClientTests` (full `AhpClient` over an in-memory
+   `MemTransport`, the port of Go's `client_test.go`), `HostsTests`,
+   `MultiHostClientTests`, `MultiHostStateMirrorTests`, `NativeReducerTests`,
+   `ReconnectPolicyTests`, `ClientIdStoreTests`,
+   `FileClientIdStoreTests`, `TransportTests`, `WebSocketTransportTests`. The
+   multi-host / host / client fake servers share one declarative loop helper,
+   `FakeHost`.
+4. **Cross-implementation convergence** — `CrossImplementationConvergenceTests`
+   replays a session trace captured from an INDEPENDENT host (a separate
+   WebSocket host on the canonical TS `sessionReducer`) and asserts byte-identical
+   convergence (`serverSeq` + host-authoritative `modifiedAt`).
+
+Beyond CI, the **full `AhpClient` has been validated LIVE over a real WebSocket**
+against a spec-faithful AHP host built on the canonical `sessionReducer`: the
+real `initialize` request/response handshake, the snapshot in `InitializeResult`,
+and the live `action` notification stream all converge with the host. (No
+client in any language ships a real-socket integration test — they are all
+mock-transport-based; this validation is run out-of-band rather than committed,
+since it needs a Node host + the published package.)
+
+Cross-language parity is verified by the shared fixture corpora the suite
+replays — the 169 reducer fixtures (`types/test-cases/reducers/*.json`) and the
+round-trip corpus (`types/test-cases/round-trips/*.json`), both of which every
+client runs. (A .NET-only grep-based test-count gate used to live here; it was
+retired in favor of relying on the shared corpora, which actually exercise the
+behavior rather than counting method names.)
+
+## Architecture decisions
+
+- [`docs/decisions/sync.md`](docs/decisions/sync.md)
+  — the full menu of .NET synchronization primitives, the distinct concurrency
+  use cases in the client, which primitive each gets (`ConcurrentDictionary`
+  for the collections, `lock` for the `HostEntry` field-bundle, `SemaphoreSlim`
+  only for the WebSocket send path, `Channels`/`Interlocked`/`volatile`
+  elsewhere), and why the client targets `net8.0` only.
+- [`docs/decisions/serialization.md`](docs/decisions/serialization.md)
+  — System.Text.Json (default, in-box, fastest) behind the `IAhpSerializer`
+  seam, versus Newtonsoft / lazy-DOM / validating options, across speed,
+  memory, lazy-vs-eager, validation, dependencies, and AOT.
+- [`docs/decisions/reconnect.md`](docs/decisions/reconnect.md)
+  — hand-rolled exponential backoff (with opt-in jitter) versus
+  Polly / `Microsoft.Extensions.Resilience`, and why the core stays
+  dependency-free.
+
+These decision records live under `docs/decisions/` and are repo-only — they are not packed into any NuGet
+package (only `README.md` is).
+
+## Releasing
+
+Sub-package releases publish the `Microsoft.AgentHostProtocol*` packages to
+NuGet.org. This client does not ship its own publish automation; the
+maintainers wire `dotnet pack` + `dotnet nuget push` into their own release
+pipeline (e.g. the signed Azure DevOps / ESRP pipeline used for the Kotlin and
+TypeScript packages). The `clients/dotnet/VERSION` ↔ `CHANGELOG.md` heading
+match is enforced for every PR by `npm run verify:changelog`.
+
+## Out of scope
+
+JSON-Schema validation (a `Microsoft.AgentHostProtocol.Validation` decorator
+over `IAhpSerializer`) and DI/extension helpers
+(`Microsoft.AgentHostProtocol.Extensions`) are planned follow-ups, not part of
+this client yet.

clients/dotnet/AgentHostProtocol.slnx

@@ -0,0 +1,14 @@
+<Solution>
+  <Folder Name="/examples/">
+    <Project Path="examples/ConnectWs/ConnectWs.csproj" />
+    <Project Path="examples/ReducersDemo/ReducersDemo.csproj" />
+  </Folder>
+  <Folder Name="/src/">
+    <Project Path="src/AgentHostProtocol.Abstractions/AgentHostProtocol.Abstractions.csproj" />
+    <Project Path="src/AgentHostProtocol.WebSockets/AgentHostProtocol.WebSockets.csproj" />
+    <Project Path="src/AgentHostProtocol/AgentHostProtocol.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/AgentHostProtocol.Tests/AgentHostProtocol.Tests.csproj" />
+  </Folder>
+</Solution>