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feat: answer server/discover for connection-less capability discovery #235

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4 changes: 4 additions & 0 deletions docs/server/capabilities.md
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Expand Up @@ -29,3 +29,7 @@ val server = StreamingMcpServer[Identity]().addStreamingTool(work)
```

Server-wide capabilities are enabled by registering a handler — only what you wire up is advertised: `.withCompletion`, `.withLoggingLevel`, `.withSubscriptions`.

## Discovery

The chimp server is stateless: every request is handled on its own, no session is stored, and no `initialize` handshake is required before other methods. A client (or a proxy) can learn the server's capabilities up front with a `server/discover` request, which returns the same `serverInfo`, `capabilities` and `protocolVersion` as `initialize`, without negotiating a session.
35 changes: 20 additions & 15 deletions server/src/main/scala/chimp/server/McpHandler.scala
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Expand Up @@ -76,6 +76,8 @@ private[server] class McpHandler[F[_], C <: ServerContext[F]](server: McpServerD
method match
case "initialize" =>
jsonResponse(handleInitialize(params, id)).unit
case "server/discover" =>
jsonResponse(JSONRPCMessage.Response(id = id, result = discoveryResult(ProtocolVersion.Latest).asJson)).unit
case "ping" =>
jsonResponse(JSONRPCMessage.Response(id = id, result = Json.obj())).unit
case "tools/list" =>
Expand Down Expand Up @@ -117,24 +119,27 @@ private[server] class McpHandler[F[_], C <: ServerContext[F]](server: McpServerD

private def jsonResponse(message: JSONRPCMessage): McpResponse = McpResponse.JsonResponse(message.asJson)

private def handleInitialize(params: Option[Json], id: RequestId): JSONRPCMessage.Response =
val requested = params.flatMap(_.hcursor.downField("protocolVersion").as[String].toOption)
val negotiated = requested.map(ProtocolVersion.negotiate).getOrElse(ProtocolVersion.Latest)
val capabilities = ServerCapabilities(
logging = Option.when(server.loggingLevel.isDefined)(Json.obj()),
completions = Option.when(server.completion.isDefined)(Json.obj()),
prompts = Option.when(server.prompts.nonEmpty)(ServerPromptsCapability(listChanged = Some(false))),
resources =
Option.when(hasResources)(ServerResourcesCapability(subscribe = Some(server.subscriptions.isDefined), listChanged = Some(false))),
tools = Option.when(server.tools.nonEmpty)(ServerToolsCapability(listChanged = Some(false)))
)
val result = InitializeResult(
protocolVersion = negotiated.name,
capabilities = capabilities,
private def serverCapabilities: ServerCapabilities = ServerCapabilities(
logging = Option.when(server.loggingLevel.isDefined)(Json.obj()),
completions = Option.when(server.completion.isDefined)(Json.obj()),
prompts = Option.when(server.prompts.nonEmpty)(ServerPromptsCapability(listChanged = Some(false))),
resources =
Option.when(hasResources)(ServerResourcesCapability(subscribe = Some(server.subscriptions.isDefined), listChanged = Some(false))),
tools = Option.when(server.tools.nonEmpty)(ServerToolsCapability(listChanged = Some(false)))
)

private def discoveryResult(protocolVersion: ProtocolVersion): InitializeResult =
InitializeResult(
protocolVersion = protocolVersion.name,
capabilities = serverCapabilities,
serverInfo = Implementation(server.name, server.version),
instructions = server.instructions
)
JSONRPCMessage.Response(id = id, result = result.asJson)

private def handleInitialize(params: Option[Json], id: RequestId): JSONRPCMessage.Response =
val requested = params.flatMap(_.hcursor.downField("protocolVersion").as[String].toOption)
val negotiated = requested.map(ProtocolVersion.negotiate).getOrElse(ProtocolVersion.Latest)
JSONRPCMessage.Response(id = id, result = discoveryResult(negotiated).asJson)

private def handleToolsCall(params: Option[Json], id: RequestId, headers: Seq[Header], makeContext: Option[ProgressToken] => C)(using
MonadError[F]
Expand Down
13 changes: 13 additions & 0 deletions server/src/test/scala/chimp/server/McpHandlerSpec.scala
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Expand Up @@ -103,6 +103,19 @@ class McpHandlerSpec extends AnyFlatSpec with Matchers:
// nulls should be dropped
respJson.hcursor.downField("result").downField("instructions").focus shouldBe None

it should "respond to server/discover without an initialize handshake" in:
val req: JSONRPCMessage = Request(method = "server/discover", id = RequestId("disc"))
val response = handler.handleJsonRpc(req.asJson, Seq.empty)
val respJson = extractJsonFromResponse(response)

respJson.as[JSONRPCMessage].getOrElse(fail("Failed to decode response")) match
case Response(_, _, result) =>
val resultObj = result.as[InitializeResult].getOrElse(fail("Failed to decode result"))
resultObj.protocolVersion shouldBe ProtocolVersion.Latest.name
resultObj.capabilities.tools.isDefined shouldBe true
resultObj.serverInfo.name should include("Chimp MCP server")
case _ => fail("Expected Response")

it should "list available tools" in:
val req: JSONRPCMessage = Request(method = "tools/list", id = RequestId("2"))
val json = req.asJson
Expand Down
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