From 94f3804b6155cee1b98371b241f3a3bf6ad4dddc Mon Sep 17 00:00:00 2001
From: Jonah Grant <jg@openai.com>
Date: Sat, 14 Nov 2026 09:17:02 -0500
Subject: [PATCH] Return invalid params when typed request decoding fails

JSON-RPC 2.0 section 5.1 assigns -32602 to invalid method parameters. Map typed request decoding failures to that code before invoking the registered handler.
---
 Sources/MCP/Base/Messages.swift  |  9 +++++++--
 Tests/MCPTests/ServerTests.swift | 26 ++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/Sources/MCP/Base/Messages.swift b/Sources/MCP/Base/Messages.swift
index e53bb729..86ad45a7 100644
--- a/Sources/MCP/Base/Messages.swift
+++ b/Sources/MCP/Base/Messages.swift
@@ -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 {
diff --git a/Tests/MCPTests/ServerTests.swift b/Tests/MCPTests/ServerTests.swift
index 4713b278..d31e9519 100644
--- a/Tests/MCPTests/ServerTests.swift
+++ b/Tests/MCPTests/ServerTests.swift
@@ -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()