diff --git a/.github/actions/pytest-skip-summary/action.yml b/.github/actions/pytest-skip-summary/action.yml
index 94e2359e..59bf8968 100644
--- a/.github/actions/pytest-skip-summary/action.yml
+++ b/.github/actions/pytest-skip-summary/action.yml
@@ -17,6 +17,14 @@ runs:
           echo '```'
           if [ ! -f pytest-output.log ]; then
             echo "pytest did not run -- see the failed step above"
+          elif ! grep -qE ' in [0-9]+\.[0-9]+s' pytest-output.log; then
+            # pytest always prints a final "N passed/failed... in X.XXs" line
+            # on a normal exit, pass or fail. Its absence means the process
+            # was killed mid-run (e.g. pytest-timeout's `thread` method
+            # force-exiting via os._exit()) rather than that nothing was
+            # skipped -- distinguish the two so a hung run doesn't get a
+            # falsely reassuring summary.
+            echo "pytest did not finish normally (killed, crashed, or timed out) -- see the failed step above"
           elif grep -q "short test summary info" pytest-output.log; then
             awk '/short test summary info/,0' pytest-output.log
           else
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8460ce9a..69198691 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -94,6 +94,13 @@ jobs:
   test:
     name: Test - Python ${{ matrix.python-version }}
     runs-on: ubuntu-latest
+    # Hard backstop behind pytest-timeout's per-test enforcement (pyproject.toml's
+    # [tool.pytest.ini_options] `timeout`): a hang during collection, before any
+    # test has started, or inside the Rust/maturin build in the install step
+    # below is outside pytest's control entirely. PR #308 hung here for ~55
+    # minutes (a fork-safety deadlock) with nothing to stop it until a human
+    # noticed and cancelled the run.
+    timeout-minutes: 20
     strategy:
       fail-fast: false
       matrix:
diff --git a/changelog.d/309.changed.md b/changelog.d/309.changed.md
new file mode 100644
index 00000000..fc53700a
--- /dev/null
+++ b/changelog.d/309.changed.md
@@ -0,0 +1 @@
+**CI now fails fast on a hung test run instead of burning an hour of compute.** `pytest-timeout` caps every test at 120s, using the `thread` method rather than the platform-default `signal` one — a watchdog thread dumps every stack and force-exits the process, which also catches a hang stuck in a C-level lock that a `SIGALRM` can't interrupt. `.github/workflows/ci.yml`'s test job additionally carries a 20-minute job-level `timeout-minutes` backstop for hangs outside pytest's control (test collection, the pre-test Rust build). Prompted by PR #308's "Run tests" job hanging for ~55 minutes with nothing to stop it until a human noticed and cancelled it.
diff --git a/pyproject.toml b/pyproject.toml
index 96ba6cbf..d1c4dc8e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -76,6 +76,7 @@ onnx = [
 test = [
   "pytest>=8.0.0",
   "pytest-cov>=4.1.0",
+  "pytest-timeout>=2.4.0",
   "pytest-xdist>=3.5.0",
 ]
 # Full local dev env: test runners + lint/type/QA tooling.
@@ -127,6 +128,27 @@ addopts = "-v"
 python_files = "test_*.py"
 python_functions = "test_*"
 testpaths = ["tests"]
+# Per-test hard cap so a hung test fails loudly instead of running out the
+# clock -- see .github/workflows/ci.yml's job-level `timeout-minutes: 20` for
+# the backstop behind collection-time and pre-test hangs this can't catch.
+# The full suite's slowest test currently runs in ~22s
+# (test_distributed_training.py's two-rank checkpoint/gradient-parity tests,
+# which spin up a real 2-process DDP group); 120s leaves >5x headroom for a
+# slower or more contended CI runner while still failing well inside the
+# job-level backstop. Measured via `srun -p rna -c 8 --mem 32G -t 45 --
+# uv run pytest --durations=10` against the extras CI installs
+# (test,rust,onnx): 1707 passed, 45 skipped in 322.89s.
+timeout = 120
+# pytest-timeout's own platform default is "signal" (SIGALRM) wherever
+# available, which is Linux/CI here -- but a SIGALRM only interrupts the
+# interpreter at the next bytecode boundary, which is exactly what a hang
+# stuck in a C-level lock (a forked multiprocessing.Pool deadlocking against
+# libtorch's already-initialized thread pool, as in rnabioco/leech#308) will
+# never reach. "thread" runs a separate watchdog thread that dumps every
+# thread's stack and force-exits the whole process via os._exit() regardless
+# of what the main thread is blocked on, so it is set explicitly here rather
+# than left to the per-platform default.
+timeout_method = "thread"
 markers = ["slow: slow tests (torch.export, bundles) — run with --slow"]
 
 [tool.uv]
diff --git a/tests/test_inference.py b/tests/test_inference.py
index fea11343..55814e59 100644
--- a/tests/test_inference.py
+++ b/tests/test_inference.py
@@ -1176,6 +1176,13 @@ def test_rust_path_writes_tags(self, tmp_path):
         rust_predicted = self._predict(tmp_path, bundle_path, "rust", backend="rust")
         assert rust_predicted
 
+    # Longer than pyproject.toml's global 120s: the subprocess.run(...,
+    # timeout=120) below is this test's own hang guard for the exact #308
+    # deadlock, and must be the one to fire (raising subprocess.TimeoutExpired
+    # with a clean assertion message) rather than racing the global per-test
+    # timeout, which would instead kill the whole pytest process via
+    # os._exit() before the subprocess's own timeout could report cleanly.
+    @pytest.mark.timeout(150)
     def test_parallel_path_alone_writes_tags(self, tmp_path):
         """``num_workers > 0`` (mp.Pool, always Python extraction) --
         previously untested here (all prior bundle tests ran serial only).
@@ -1784,6 +1791,12 @@ class TestSequentialThenParallelInProcess:
 print("OK")
 """
 
+    # Longer than pyproject.toml's global 120s -- same reasoning as
+    # test_parallel_path_alone_writes_tags above: the subprocess.run(...,
+    # timeout=120) inside this test is the intended hang guard ("the bug
+    # under test IS a hang"), and needs room to fire before the global
+    # per-test timeout would otherwise race it and kill the whole process.
+    @pytest.mark.timeout(150)
     def test_a_fork_after_a_waited_shutdown_completes(self, tmp_path):
         """Runs in a subprocess with a hard timeout: the bug under test IS a hang."""
         import subprocess
diff --git a/tests/test_parallel_prep.py b/tests/test_parallel_prep.py
index 65d264a0..f5d5c4de 100644
--- a/tests/test_parallel_prep.py
+++ b/tests/test_parallel_prep.py
@@ -806,6 +806,13 @@ def test_rust_backend_end_to_end(self):
         assert all(c["label"] == "Ala" for c in chunks)
         assert all(int(c["label_int"]) == 1 for c in chunks)
 
+    # Longer than pyproject.toml's global 120s: this test's own
+    # subprocess.run(..., timeout=120) below IS the hang guard (a real fork
+    # deadlock must raise subprocess.TimeoutExpired with a clean assertion
+    # message, not have the whole pytest process killed out from under it by
+    # the global timeout firing at the same 120s mark). See CLAUDE.md's "A
+    # test that exercises the real mp.Pool" note.
+    @pytest.mark.timeout(150)
     def test_python_backend_real_pool_matches_rust_chunk_set(self):
         """``backend_choice="python", num_workers=2`` -- the real mp.Pool path.
 
diff --git a/uv.lock b/uv.lock
index 302fe0d1..179b5538 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1274,6 +1274,7 @@ dev = [
     { name = "onnxscript", marker = "sys_platform != 'emscripten'" },
     { name = "pytest", marker = "sys_platform != 'emscripten'" },
     { name = "pytest-cov", marker = "sys_platform != 'emscripten'" },
+    { name = "pytest-timeout", marker = "sys_platform != 'emscripten'" },
     { name = "pytest-xdist", marker = "sys_platform != 'emscripten'" },
     { name = "ruff", marker = "sys_platform != 'emscripten'" },
     { name = "towncrier", marker = "sys_platform != 'emscripten'" },
@@ -1303,6 +1304,7 @@ rust = [
 test = [
     { name = "pytest", marker = "sys_platform != 'emscripten'" },
     { name = "pytest-cov", marker = "sys_platform != 'emscripten'" },
+    { name = "pytest-timeout", marker = "sys_platform != 'emscripten'" },
     { name = "pytest-xdist", marker = "sys_platform != 'emscripten'" },
 ]
 
@@ -1334,6 +1336,7 @@ requires-dist = [
     { name = "pysam", specifier = ">=0.22.0" },
     { name = "pytest", marker = "extra == 'test'", specifier = ">=8.0.0" },
     { name = "pytest-cov", marker = "extra == 'test'", specifier = ">=4.1.0" },
+    { name = "pytest-timeout", marker = "extra == 'test'", specifier = ">=2.4.0" },
     { name = "pytest-xdist", marker = "extra == 'test'", specifier = ">=3.5.0" },
     { name = "pyyaml", specifier = ">=6.0" },
     { name = "rich", specifier = ">=13.0.0" },
@@ -2634,6 +2637,18 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/9d/7a/d968e294073affff457b041c2be9868a40c1c71f4a35fcc1e45e5493067b/pytest_cov-7.1.0-py3-none-any.whl", hash = "sha256:a0461110b7865f9a271aa1b51e516c9a95de9d696734a2f71e3e78f46e1d4678", size = 22876, upload-time = "2026-03-21T20:11:14.438Z" },
 ]
 
+[[package]]
+name = "pytest-timeout"
+version = "2.4.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "pytest", marker = "sys_platform != 'emscripten'" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/ac/82/4c9ecabab13363e72d880f2fb504c5f750433b2b6f16e99f4ec21ada284c/pytest_timeout-2.4.0.tar.gz", hash = "sha256:7e68e90b01f9eff71332b25001f85c75495fc4e3a836701876183c4bcfd0540a", size = 17973, upload-time = "2025-05-05T19:44:34.99Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/fa/b6/3127540ecdf1464a00e5a01ee60a1b09175f6913f0644ac748494d9c4b21/pytest_timeout-2.4.0-py3-none-any.whl", hash = "sha256:c42667e5cdadb151aeb5b26d114aff6bdf5a907f176a007a30b940d3d865b5c2", size = 14382, upload-time = "2025-05-05T19:44:33.502Z" },
+]
+
 [[package]]
 name = "pytest-xdist"
 version = "3.8.0"