diff --git a/mem0/client/main.py b/mem0/client/main.py
index 65f2212b0..3a82d9e5a 100644
--- a/mem0/client/main.py
+++ b/mem0/client/main.py
@@ -164,10 +164,8 @@ def _client_headers(api_key: str, user_id: str) -> Dict[str, str]:
 def _client_stack() -> str:
     """This SDK appended to any stack an outer layer already declared."""
     existing = os.getenv("MEM0_CLIENT_STACK", "").strip()
-    mine = f"mem0-python/{_sdk_version()}"
     entries = [part.strip() for part in existing.split(",") if part.strip()] if existing else []
-    entries.append(mine)
-    return _bounded_stack(entries)
+    return _bounded_stack(entries, f"mem0-python/{_sdk_version()}")
 
 
 class MemoryClient:
diff --git a/tests/test_client_surface_headers.py b/tests/test_client_surface_headers.py
new file mode 100644
index 000000000..a2602a5bc
--- /dev/null
+++ b/tests/test_client_surface_headers.py
@@ -0,0 +1,65 @@
+"""Surface-identity headers, and that a client can be constructed at all.
+
+The construction test exists because it was not there: a signature change to
+_bounded_stack missed the _client_stack call site, every MemoryClient(...) raised
+TypeError, and the whole suite stayed green because nothing built one.
+"""
+
+import os
+from unittest.mock import patch
+
+import pytest
+
+from mem0.client.main import _bounded_stack, _client_headers, _client_stack
+
+
+def test_a_client_can_be_constructed():
+    from mem0 import MemoryClient
+
+    # _validate_api_key normally populates org/project from the API response;
+    # stubbing it leaves them None, which a later accessor rejects. Set them the
+    # way a real validation would. This test is about construction reaching the
+    # header stage at all.
+    def _stub(self):
+        self.org_id, self.project_id = "org", "proj"
+
+    with patch.object(MemoryClient, "_validate_api_key", _stub):
+        client = MemoryClient(api_key="m0-test")
+
+    assert client.client.headers["X-Mem0-Client"].startswith("mem0-python/")
+
+
+# AsyncMemoryClient is deliberately not constructed here: its validation path
+# makes a real request to /v1/ping/, and a unit test that needs the network is
+# worse than none. It shares _client_headers with the sync client, which is the
+# code the construction test above actually guards.
+def test_headers_carry_this_sdk():
+    headers = _client_headers("m0-test", "u1")
+    assert headers["X-Mem0-Client"].startswith("mem0-python/")
+
+
+def test_our_entry_survives_a_caller_that_already_filled_the_stack():
+    # Appending first and trimming to four dropped exactly the entry the
+    # function exists to add.
+    stack = _bounded_stack(["a/1", "b/2", "c/3", "d/4"], "mem0-python/9.9.9")
+
+    assert "mem0-python/9.9.9" in stack
+    assert len(stack.split(",")) <= 4
+
+
+def test_the_character_cap_drops_whole_entries_not_characters():
+    long_entries = [f"{'n' * 90}/1.0", f"{'m' * 90}/1.0", "c/3"]
+    stack = _bounded_stack(long_entries, "mem0-python/9.9.9")
+
+    assert len(stack) <= 200
+    assert stack.endswith("mem0-python/9.9.9")
+    for entry in stack.split(","):
+        assert entry.strip().count("/") == 1, f"severed entry: {entry!r}"
+
+
+def test_an_outer_stack_is_appended_to_not_replaced():
+    with patch.dict(os.environ, {"MEM0_CLIENT_STACK": "openclaw/2.1.0"}):
+        stack = _client_stack()
+
+    assert stack.startswith("openclaw/2.1.0")
+    assert "mem0-python/" in stack