diff --git a/extensions/ai-providers/cursor/protobuf.ts b/extensions/ai-providers/cursor/protobuf.ts
index 7c04ae79..956907b9 100644
--- a/extensions/ai-providers/cursor/protobuf.ts
+++ b/extensions/ai-providers/cursor/protobuf.ts
@@ -447,13 +447,12 @@ function compileMapField(desc: MapFieldDesc): CompiledField {
     decode(message, reader, wireType, _fieldNumber) {
       assertWireType(wireType, 2);
       const target = mapField(message, name);
-      const limit = reader.uint32();
-      const end = reader.pos + limit;
+      const entryReader = new Reader(reader.bytes());
       let entryKey = "";
       let entryValue: unknown = valCodec.defaultValue;
 
-      while (reader.pos < end) {
-        const tag = reader.uint32();
+      while (entryReader.pos < entryReader.len) {
+        const tag = entryReader.uint32();
         const entryNumber = tag >>> 3;
         const entryWireType = tag & 7;
         if (!isWireType(entryWireType)) {
@@ -463,12 +462,12 @@ function compileMapField(desc: MapFieldDesc): CompiledField {
         }
         if (entryNumber === 1) {
           assertWireType(entryWireType, key.wireType);
-          entryKey = requireString(key.decode(reader));
+          entryKey = requireString(key.decode(entryReader));
         } else if (entryNumber === 2) {
           assertWireType(entryWireType, valCodec.wireType);
-          entryValue = valCodec.decode(reader);
+          entryValue = valCodec.decode(entryReader);
         } else {
-          reader.skip(entryWireType);
+          entryReader.skip(entryWireType);
         }
       }
 
diff --git a/tests/extensions/ai-providers/cursor-protobuf.test.ts b/tests/extensions/ai-providers/cursor-protobuf.test.ts
new file mode 100644
index 00000000..87f8a178
--- /dev/null
+++ b/tests/extensions/ai-providers/cursor-protobuf.test.ts
@@ -0,0 +1,28 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import {
+  pb,
+  type ProtoMessage,
+} from "../../../extensions/ai-providers/cursor/protobuf.ts";
+
+interface StringMapMessage extends ProtoMessage {
+  values: Record<string, string>;
+}
+
+const StringMap = pb<StringMapMessage>("test.StringMap", [
+  { kind: "map", no: 1, name: "values", K: "string", V: "string" },
+]);
+
+test("protobuf maps do not decode beyond a declared entry boundary", () => {
+  const valid = Uint8Array.from([
+    0x0a, 0x0a, 0x0a, 0x03, 0x75, 0x72, 0x6c, 0x12, 0x03, 0x66, 0x6f, 0x6f,
+  ]);
+  const malformed = Uint8Array.from(valid);
+  malformed[1] = 9;
+
+  assert.equal(StringMap.decode(valid).values.url, "foo");
+  assert.throws(
+    () => StringMap.decode(malformed),
+    /Unexpected EOF reading bytes/,
+  );
+});