mcp: preserve existing Content in SetError

mcp/mcp_test.go

@@ -2244,4 +2244,54 @@ func TestToolErrorMiddleware(t *testing.T) {
 	}
 }
 
+func TestSetErrorPreservesContent(t *testing.T) {
+	t.Run("empty content is populated", func(t *testing.T) {
+		var res CallToolResult
+		res.SetError(errors.New("internal failure"))
+		if !res.IsError {
+			t.Fatal("want IsError=true")
+		}
+		text := res.Content[0].(*TextContent).Text
+		if text != "internal failure" {
+			t.Errorf("got %q, want %q", text, "internal failure")
+		}
+		if res.GetError() == nil || res.GetError().Error() != "internal failure" {
+			t.Errorf("GetError() = %v, want 'internal failure'", res.GetError())
+		}
+	})
+
+	t.Run("empty slice content is populated", func(t *testing.T) {
+		res := CallToolResult{
+			Content: []Content{},
+		}
+		res.SetError(errors.New("internal failure"))
+		if !res.IsError {
+			t.Fatal("want IsError=true")
+		}
+		text := res.Content[0].(*TextContent).Text
+		if text != "internal failure" {
+			t.Errorf("got %q, want %q", text, "internal failure")
+		}
+	})
+
+	t.Run("existing content is preserved", func(t *testing.T) {
+		res := CallToolResult{
+			Content: []Content{&TextContent{Text: "Something went wrong, please try again."}},
+		}
+		res.SetError(errors.New("db timeout on query SELECT * FROM users"))
+		if !res.IsError {
+			t.Fatal("want IsError=true")
+		}
+		// User-facing content should be preserved.
+		text := res.Content[0].(*TextContent).Text
+		if text != "Something went wrong, please try again." {
+			t.Errorf("Content was overwritten: got %q", text)
+		}
+		// Internal error should still be accessible via GetError.
+		if res.GetError() == nil || res.GetError().Error() != "db timeout on query SELECT * FROM users" {
+			t.Errorf("GetError() = %v, want 'db timeout on query SELECT * FROM users'", res.GetError())
+		}
+	})
+}
+
 var ctrCmpOpts = []cmp.Option{cmp.AllowUnexported(CallToolResult{})}

mcp/protocol.go

@@ -119,10 +119,15 @@ type CallToolResult struct {
 	err error
 }
 
-// SetError sets the error for the tool result and populates the Content field
-// with the error text. It also sets IsError to true.
+// SetError sets the error for the tool result and sets IsError to true.
+// If Content has not already been populated, it is set to the error text.
+// If Content has already been populated, it is left unchanged, allowing callers
+// to provide a user-friendly message while still recording the underlying error
+// for inspection via [GetError] in server middleware.
 func (r *CallToolResult) SetError(err error) {
-	r.Content = []Content{&TextContent{Text: err.Error()}}
+	if len(r.Content) == 0 {
+		r.Content = []Content{&TextContent{Text: err.Error()}}
+	}
 	r.IsError = true
 	r.err = err
 }