From fb2498f50827f0283dabc06c5a5aee0b3b3c0374 Mon Sep 17 00:00:00 2001
From: systemBlue <290303321+systemblueio@users.noreply.github.com>
Date: Sun, 7 Jun 2026 11:00:49 -0400
Subject: [PATCH] fix: preserve internal error message when decoding from a
 peer

MCPError's decoder dropped the message for code -32603 by passing nil as
the detail fallback, while every other standard JSON-RPC error case falls
back to the message. A peer sending {"code":-32603,"message":"..."} with
no data.detail decoded to .internalError(nil), losing the message. Use the
message fallback so internalError matches the other cases.
---
 Sources/MCP/Base/Error.swift       |  2 +-
 Tests/MCPTests/ResponseTests.swift | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Sources/MCP/Base/Error.swift b/Sources/MCP/Base/Error.swift
index b7196f07..2d1cde75 100644
--- a/Sources/MCP/Base/Error.swift
+++ b/Sources/MCP/Base/Error.swift
@@ -237,7 +237,7 @@ extension MCPError: Codable {
         case -32602:
             self = .invalidParams(unwrapDetail(message))
         case -32603:
-            self = .internalError(unwrapDetail(nil))
+            self = .internalError(unwrapDetail(message))
         case -32042:
             // Extract elicitations array from data
             var elicitations: [URLElicitationInfo] = []
diff --git a/Tests/MCPTests/ResponseTests.swift b/Tests/MCPTests/ResponseTests.swift
index cb918521..165ede71 100644
--- a/Tests/MCPTests/ResponseTests.swift
+++ b/Tests/MCPTests/ResponseTests.swift
@@ -84,6 +84,29 @@ struct ResponseTests {
         }
     }
 
+    @Test("Internal error preserves the message from an external server")
+    func testInternalErrorPreservesMessage() throws {
+        // A peer sends a -32603 error with a message and no `data.detail`, like
+        // every other JSON-RPC error. Decoding should keep the message instead
+        // of dropping it, matching the other standard error cases.
+        let jsonString = """
+            {"jsonrpc":"2.0","id":"test-id","error":{"code":-32603,"message":"Database connection failed"}}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(Response<TestMethod>.self, from: data)
+
+        if case .failure(let decodedError) = decoded.result {
+            #expect(decodedError.code == -32603)
+            #expect(
+                decodedError.localizedDescription
+                    == "Internal error: Database connection failed")
+        } else {
+            #expect(Bool(false), "Expected error result")
+        }
+    }
+
     @Test("Empty result response encoding")
     func testEmptyResultResponseEncoding() throws {
         let response = EmptyMethod.response(id: "test-id")