diff --git a/tests/test_cancel_stream_owner_guard.py b/tests/test_cancel_stream_owner_guard.py
index 6b85b66f4..e149dd420 100644
--- a/tests/test_cancel_stream_owner_guard.py
+++ b/tests/test_cancel_stream_owner_guard.py
@@ -329,7 +329,16 @@ def _run_cancel_stream_scenarios() -> dict:
             f"--- stderr ---\n{completed.stderr}"
         )
     try:
-        return json.loads(completed.stdout)
+        # The function under test legitimately emits diagnostic lines on stdout
+        # (e.g. cancelStream's '[stream] cancel requested' provenance log, added
+        # for #5345). Those fire DURING runAll(); the result JSON is written by
+        # `console.log(JSON.stringify(r))` only after runAll() resolves, so it is
+        # always the LAST non-empty stdout line. Parse that rather than assuming
+        # stdout is pure JSON.
+        lines = [ln for ln in completed.stdout.splitlines() if ln.strip()]
+        if not lines:
+            raise json.JSONDecodeError("empty stdout", completed.stdout or "", 0)
+        return json.loads(lines[-1])
     except json.JSONDecodeError as e:
         raise AssertionError(
             f"node subprocess returned non-JSON output:\n"
diff --git a/tests/test_sprint36.py b/tests/test_sprint36.py
index 97b5d6fe3..89bc6f9f0 100644
--- a/tests/test_sprint36.py
+++ b/tests/test_sprint36.py
@@ -46,8 +46,11 @@ class TestCancelStreamCleanup:
     def _get_cancel_block(self):
         """Extract the cancelStream function body from boot.js."""
         src = read("static/boot.js")
-        idx = src.find("async function cancelStream()")
-        assert idx != -1, "cancelStream not found in boot.js"
+        # Signature-tolerant: cancelStream now takes a `reason` param (#5345), so
+        # match the declaration regardless of its parameter list.
+        m = re.search(r"async function cancelStream\s*\(", src)
+        assert m is not None, "cancelStream not found in boot.js"
+        idx = m.start()
         # Find the closing brace — scan for the matching }
         depth = 0
         end = idx
@@ -122,8 +125,13 @@ class TestCancelStreamErrorPath:
         The status is cleared by setStatus('') unconditionally.
         """
         src = read("static/boot.js")
-        idx = src.find("async function cancelStream()")
-        block = src[idx:idx + 400]
+        # Signature-tolerant match (cancelStream now takes a `reason` param, #5345).
+        m = re.search(r"async function cancelStream\s*\(", src)
+        assert m is not None, "cancelStream not found in boot.js"
+        idx = m.start()
+        # Widen the window: the provenance log + comments added for #5345 sit
+        # before the try/catch, so 400 chars no longer reaches the catch block.
+        block = src[idx:idx + 1200]
         # The old pattern was setStatus inside catch; new pattern has it outside
         # Look for the catch block specifically
         catch_idx = block.find("}catch(")