diff --git a/packages/aws-durable-execution-sdk-python-microvm-worker/src/aws_durable_execution_sdk_python_microvm_worker/worker.py b/packages/aws-durable-execution-sdk-python-microvm-worker/src/aws_durable_execution_sdk_python_microvm_worker/worker.py
index 8ffbe8305..66cdcb35b 100644
--- a/packages/aws-durable-execution-sdk-python-microvm-worker/src/aws_durable_execution_sdk_python_microvm_worker/worker.py
+++ b/packages/aws-durable-execution-sdk-python-microvm-worker/src/aws_durable_execution_sdk_python_microvm_worker/worker.py
@@ -101,7 +101,9 @@
 
 _UNKNOWN_MICROVM_ID = "unknown"
 
-_CONTENT_LENGTH = re.compile(r"[0-9]+")
+# At most 19 digits, so that int() stays under Python's limit on digits, and
+# any declared length still exceeds every body limit.
+_CONTENT_LENGTH = re.compile(r"[0-9]{1,19}")
 
 _CHUNK_SIZE = re.compile(rb"[0-9A-Fa-f]+")
 
@@ -624,13 +626,24 @@ def on_callback_gone(error: BaseException) -> None:
                 gone.append(error)
                 cancelled.set()
 
-            heartbeats = Heartbeats.start(
-                reporter,
-                heartbeat_timeout_seconds=job.heartbeat_timeout_seconds,
-                on_callback_gone=on_callback_gone,
-                heartbeat_interval_seconds=self._heartbeat_interval_seconds,
-                logger=self._logger,
-            )
+            try:
+                heartbeats = Heartbeats.start(
+                    reporter,
+                    heartbeat_timeout_seconds=job.heartbeat_timeout_seconds,
+                    on_callback_gone=on_callback_gone,
+                    heartbeat_interval_seconds=self._heartbeat_interval_seconds,
+                    logger=self._logger,
+                )
+            except Exception as error:  # noqa: BLE001
+                # The job has its answer, so no later delivery runs it. Fail
+                # the callback, instead of leaving the job to its heartbeat
+                # timeout.
+                self._logger.error(
+                    "could not start the job's heartbeats",
+                    extra={**fields, "error": describe(error)},
+                )
+                self._report(reporter, _Outcome(error=error), fields)
+                return
             try:
                 self._logger.info("job started", extra=fields)
                 context = MicrovmJobContext(
diff --git a/packages/aws-durable-execution-sdk-python-microvm-worker/tests/worker_test.py b/packages/aws-durable-execution-sdk-python-microvm-worker/tests/worker_test.py
index 18c87e039..a3e24cd0f 100644
--- a/packages/aws-durable-execution-sdk-python-microvm-worker/tests/worker_test.py
+++ b/packages/aws-durable-execution-sdk-python-microvm-worker/tests/worker_test.py
@@ -580,6 +580,24 @@ def test_answers_400_to_a_body_over_the_limit_without_reading_it(start: Any) ->
     assert b"request body exceeds 65536 bytes" in answer
 
 
+def test_answers_400_to_a_content_length_with_too_many_digits(start: Any) -> None:
+    worker, _ = start(Held())
+
+    for expect in ("", "Expect: 100-continue\r\n"):
+        answer = raw_request(
+            worker,
+            (
+                f"POST {RUN} HTTP/1.1\r\nHost: localhost\r\n{expect}"
+                f"Content-Length: {'9' * 5000}\r\n\r\n"
+            ).encode(),
+        )
+
+        # With Expect, the length is not known to be over the limit. So the
+        # worker sends 100 Continue, and then the 400 for the invalid header.
+        assert b"HTTP/1.1 400" in answer, expect
+        assert answer.endswith(b'{"error":"request Content-Length is not valid"}')
+
+
 def test_answers_expect_100_continue_before_it_reads_the_body(start: Any) -> None:
     worker, client = start(routes={"/build": lambda job_input, context: "ok"})
     data = json.dumps(job_body()).encode()
@@ -1319,6 +1337,36 @@ def failing_start(self: threading.Thread) -> None:
     assert "could not start the run hook job" in logger.messages("error")
 
 
+def test_fails_a_job_whose_heartbeats_cannot_start(
+    start: Any, logger: Any, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    handler = Held()
+    handler.release.set()
+    worker, client = start(routes={"/build": handler})
+    original = threading.Thread.start
+
+    def failing_start(self: threading.Thread) -> None:
+        if self.name.startswith("heartbeats-"):
+            msg = "can't start new thread"
+            raise RuntimeError(msg)
+        original(self)
+
+    monkeypatch.setattr(threading.Thread, "start", failing_start)
+    # The job already has its 202, so no delivery follows. The worker fails
+    # the callback, instead of leaving the job to its heartbeat timeout.
+    assert request(worker, "/build", job_body(heartbeatTimeoutSeconds=60))[0] == 202
+    assert worker.idle(WAIT)
+
+    [(name, kwargs)] = completions(client)
+    assert name == "failure"
+    assert kwargs["Error"] == {
+        "ErrorType": "RuntimeError",
+        "ErrorMessage": "can't start new thread",
+    }
+    assert handler.inputs == []
+    assert "could not start the job's heartbeats" in logger.messages("error")
+
+
 def test_counts_no_report_whose_thread_cannot_start(
     start: Any, monkeypatch: pytest.MonkeyPatch
 ) -> None: