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Return invalid params when typed request decoding fails #330

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9 changes: 7 additions & 2 deletions Sources/MCP/Base/Messages.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -186,10 +186,15 @@ final class TypedRequestHandler<M: Method>: RequestHandlerBox, @unchecked Sendab

// Create a concrete request from the type-erased one
let data = try encoder.encode(request)
let request = try decoder.decode(Request<M>.self, from: data)
let typedRequest: Request<M>
do {
typedRequest = try decoder.decode(Request<M>.self, from: data)
} catch is DecodingError {
throw MCPError.invalidParams("Invalid parameters for \(request.method)")
}

// Handle with concrete type
let response = try await _handle(request)
let response = try await _handle(typedRequest)

// Convert result to AnyMethod response
switch response.result {
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26 changes: 26 additions & 0 deletions Tests/MCPTests/ServerTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,32 @@ import Testing

@Suite("Server Tests")
struct ServerTests {
@Test("Invalid tool call parameters return a JSON-RPC invalid params error", arguments: [
#"{}"#, #"{"name":"echo","arguments":[]}"#,
])
func testInvalidToolCallParameters(_ params: String) async throws {
let (clientTransport, serverTransport) = await InMemoryTransport.createConnectedPair()
let server = Server(name: "TestServer", version: "1.0", capabilities: .init(tools: .init()))
await server.withMethodHandler(CallTool.self) { _ in
Issue.record("Malformed parameters reached the tool handler")
return .init(content: [])
}

try await server.start(transport: serverTransport)
try await clientTransport.connect()
var replies = await clientTransport.receive().makeAsyncIterator()
let request = #"{"jsonrpc":"2.0","id":7,"method":"tools/call","params":\#(params)}"#
try await clientTransport.send(Data(request.utf8))

let data = try #require(try await replies.next())
let response = try #require(JSONSerialization.jsonObject(with: data) as? [String: Any])
#expect(response["id"] as? Int == 7)
#expect((response["error"] as? [String: Any])?["code"] as? Int == -32602)

await clientTransport.disconnect()
await server.stop()
}

@Test("Start and stop server")
func testServerStartAndStop() async throws {
let transport = MockTransport()
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