diff --git a/tests/test_api_gpu.py b/tests/test_api_gpu.py
index db51353..e36c490 100644
--- a/tests/test_api_gpu.py
+++ b/tests/test_api_gpu.py
@@ -362,19 +362,33 @@ def test_the_agent_heartbeats_whether_or_not_it_holds_a_lease():
 
     Read from the source rather than by running the loop: it is a `while True`
     with a sleep, so exercising it means threads and timing, and the property
-    is simply that the call is not behind a `if ids:`.
+    is simply that the call is not behind a guard on `ids`.
+
+    Parsed rather than grepped, and that is not fussiness — the first cut
+    searched the raw function text and failed on its own explanation. The
+    docstring of the fix quotes the construct the fix removed, because that is
+    what a docstring explaining a fix DOES. A source-text assertion cannot
+    tell the code from the prose about the code; the AST can, so the body is
+    unparsed with its docstring dropped.
     """
+    import ast
     from pathlib import Path
 
     src = (
         Path(__file__).resolve().parents[1] / "agent" / "fc_agent" / "worker.py"
     ).read_text()
-    loop = src[src.index("def _heartbeat_loop"):]
-    loop = loop[:loop.index("\n    def ")]
+    fn = next(
+        n for n in ast.walk(ast.parse(src))
+        if isinstance(n, ast.FunctionDef) and n.name == "_heartbeat_loop"
+    )
+    code = "\n".join(
+        ast.unparse(n) for n in fn.body
+        if not (isinstance(n, ast.Expr) and isinstance(n.value, ast.Constant))
+    )
 
-    assert "self.client.heartbeat(ids)" in loop
-    assert "if ids:" not in loop, (
+    assert "self.client.heartbeat(ids)" in code
+    assert "if ids" not in code, (
         "the heartbeat is gated on holding leases again; an idle agent then "
         "reads as stopped after 300s while sleep mode backs its lease poll "
-        "off to 900s"
+        f"off to 900s\n\n{code}"
     )