diff --git a/src/adapters/cursor/protobuf-events.ts b/src/adapters/cursor/protobuf-events.ts
index 98c8779ca7..0484bf373f 100644
--- a/src/adapters/cursor/protobuf-events.ts
+++ b/src/adapters/cursor/protobuf-events.ts
@@ -335,13 +335,15 @@ function normalizeJsonText(text: string, toolName: string | undefined, state: Cu
  * streamed onward), and/or as a structured protobuf map on `toolCallCompleted`. We emit the args
  * exactly once, at completion, so they can always be schema-normalized regardless of which form
  * arrived. The completed map wins when present (canonical); otherwise the buffered streamed text is
- * used. Returns an empty string when there are no args (the bridge serializes that as `{}`).
+ * used. Preserve malformed buffered text so the downstream client rejects the call instead of
+ * silently treating it as a no-arg call. Returns an empty string only when there were no args (the
+ * bridge serializes that as `{}`).
  */
 function resolveCompletedArgs(buffered: string, args: McpArgs | undefined, state: CursorProtobufEventState): string {
   if (hasMcpArgBytes(args)) return decodeMcpArgsNormalized(args, state);
   const name = mcpWireNameFromArgs(args);
   if (isCompleteJson(buffered)) return normalizeJsonText(buffered, name, state);
-  return "";
+  return buffered;
 }
 
 const PATCH_BEGIN = "*** Begin Patch";
diff --git a/tests/cursor-protobuf-events.test.ts b/tests/cursor-protobuf-events.test.ts
index 779d0df0b0..6430107011 100644
--- a/tests/cursor-protobuf-events.test.ts
+++ b/tests/cursor-protobuf-events.test.ts
@@ -309,6 +309,30 @@ describe("Cursor protobuf tool-call events", () => {
     expect(delta && delta.type === "tool_call_delta" ? JSON.parse(delta.arguments) : null).toEqual({ path: "a.txt" });
   });
 
+  test("preserves incomplete streamed args when completion has no argument map", () => {
+    const state = createCursorProtobufEventState();
+    const toolCall = mcpToolCall("mcp__fs__read_file", {});
+
+    expect(mapCursorProtobufServerMessage(interaction({
+      case: "partialToolCall",
+      value: create(PartialToolCallUpdateSchema, {
+        callId: "call_1",
+        modelCallId: "model_1",
+        toolCall,
+        argsTextDelta: "{\"path\":",
+      }),
+    }), state)).toEqual([]);
+
+    expect(mapCursorProtobufServerMessage(interaction({
+      case: "toolCallCompleted",
+      value: create(ToolCallCompletedUpdateSchema, { callId: "call_1", modelCallId: "model_1", toolCall }),
+    }), state)).toEqual([
+      { type: "tool_call_start", id: "call_1", name: "mcp__fs__read_file" },
+      { type: "tool_call_delta", arguments: "{\"path\":" },
+      { type: "tool_call_end", id: "call_1" },
+    ]);
+  });
+
   test("commits an advertised no-arg tool call instead of dropping it", () => {
     // A completed client tool call with no args and no streamed text must still reach Codex when the
     // tool is advertised (e.g. a no-arg list/status tool). The bridge serializes empty args as "{}".