diff --git a/Makefile b/Makefile
index f9ce0f9..15883e9 100644
--- a/Makefile
+++ b/Makefile
@@ -149,10 +149,14 @@ activate:
 	@echo "To activate the virtual environment, run:"
 	@echo "  source $(VENV_DIR)/bin/activate"
 
-# Run the test suite (installs test dependencies if needed)
+# Run the test suite (installs test dependencies if needed).
+# --dist=loadgroup pins tests sharing an xdist_group marker to a single
+# worker — used by test_pandoc.py to serialize ~14 pandoc subprocesses
+# that would otherwise race cold-start CPU contention on CI and exceed
+# the 5s/30s subprocess timeouts.
 test: install-source-dev-and-test
 	@echo "Running tests in parallel..."
-	$(VENV_DIR)/bin/py.test -n auto
+	$(VENV_DIR)/bin/py.test -n auto --dist=loadgroup
 
 # Start a simple HTTP server (for serving static files like index.html)
 http: venv
diff --git a/remarkbox/lib/pandoc.py b/remarkbox/lib/pandoc.py
index 1e9385d..f1e2e53 100644
--- a/remarkbox/lib/pandoc.py
+++ b/remarkbox/lib/pandoc.py
@@ -8,6 +8,7 @@ import logging
 import subprocess
 import tempfile
 import os
+from functools import lru_cache
 
 log = logging.getLogger(__name__)
 
@@ -155,12 +156,23 @@ def resolve_format(ext):
     return EXTENSION_ALIASES.get(ext, ext)
 
 
+# 30s — generous on cold-start contended runners (xdist N-worker fan-out).
+# The result is cached, so this cost gets paid at most once per process.
+_FORMAT_QUERY_TIMEOUT = 30
+
+
+@lru_cache(maxsize=1)
 def get_available_output_formats():
-    """Return the set of output formats pandoc supports on this system."""
+    """Return the set of output formats pandoc supports on this system.
+
+    Result cached per process — pandoc's format list is static for a given
+    binary, so subsequent callers (and parallel pytest workers each in their
+    own Python process) skip the subprocess entirely after first call.
+    """
     try:
         result = subprocess.run(
             ["pandoc", "--list-output-formats"],
-            capture_output=True, text=True, timeout=5,
+            capture_output=True, text=True, timeout=_FORMAT_QUERY_TIMEOUT,
         )
         return frozenset(result.stdout.strip().split("\n"))
     except Exception:
@@ -168,12 +180,16 @@ def get_available_output_formats():
         return frozenset()
 
 
+@lru_cache(maxsize=1)
 def get_available_input_formats():
-    """Return the set of input formats pandoc supports on this system."""
+    """Return the set of input formats pandoc supports on this system.
+
+    Result cached per process — see get_available_output_formats() above.
+    """
     try:
         result = subprocess.run(
             ["pandoc", "--list-input-formats"],
-            capture_output=True, text=True, timeout=5,
+            capture_output=True, text=True, timeout=_FORMAT_QUERY_TIMEOUT,
         )
         return frozenset(result.stdout.strip().split("\n"))
     except Exception:
diff --git a/remarkbox/tests/test_pandoc.py b/remarkbox/tests/test_pandoc.py
index 37f77bf..7829576 100644
--- a/remarkbox/tests/test_pandoc.py
+++ b/remarkbox/tests/test_pandoc.py
@@ -1,4 +1,14 @@
-"""Unit tests for remarkbox.lib.pandoc — pandoc subprocess wrapper."""
+"""Unit tests for remarkbox.lib.pandoc — pandoc subprocess wrapper.
+
+xdist_group("pandoc-subprocess") on every class that fires pandoc as a
+subprocess. With -n auto on a 32-core CI runner, ~14 of these tests would
+otherwise race for `pandoc` cold-starts simultaneously, exceeding 5s/30s
+subprocess timeouts under GHC-runtime + filter-loading contention.
+Grouping pins every pandoc-subprocess test onto one worker, so each pandoc
+invocation completes before the next starts. Other workers keep parallel-
+processing the rest of the suite. Requires Makefile to pass
+--dist=loadgroup so pytest-xdist honors the marker.
+"""
 
 import shutil
 import unittest
@@ -19,6 +29,7 @@ from remarkbox.lib.pandoc import (
 )
 
 
+@pytest.mark.xdist_group("pandoc-subprocess")
 class TestConvert(unittest.TestCase):
     """Unit tests for the pandoc convert() function."""
 
@@ -95,6 +106,7 @@ class TestConvert(unittest.TestCase):
             convert("test", "markdown", "not_a_real_format_xyz")
 
 
+@pytest.mark.xdist_group("pandoc-subprocess")
 class TestAvailableFormats(unittest.TestCase):
     """Test format discovery."""