diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index cc7adf11..e9f6a062 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -19,4 +19,22 @@ build:
       - UV_PROJECT_ENVIRONMENT=$READTHEDOCS_VIRTUALENV_PATH uv sync --frozen --all-extras
     build:
       html:
-        - UV_PROJECT_ENVIRONMENT=$READTHEDOCS_VIRTUALENV_PATH uv run --frozen sphinx-build -b html -W -n --keep-going docs $READTHEDOCS_OUTPUT/html
+        # Diagnostic, not for merging: the build writes to a file while the foreground loop
+        # reports progress, so the log survives; at 780 s it dumps the Python stacks and stops.
+        # Read the Docs wraps a command in single quotes, so it must hold none.
+        - nproc; free -m; cat /proc/loadavg
+        - |
+          log=/tmp/sphinx.log
+          : > $log
+          (PYTHONFAULTHANDLER=1 PYTHONUNBUFFERED=1 UV_PROJECT_ENVIRONMENT=$READTHEDOCS_VIRTUALENV_PATH uv run --frozen sphinx-build -b html -W -n --keep-going docs $READTHEDOCS_OUTPUT/html > $log 2>&1; echo "sphinx exit $?" >> $log) &
+          n=0
+          while [ $n -lt 780 ] && ! grep -q "^sphinx exit " $log; do
+            sleep 20
+            n=$((n+20))
+            echo "[$n] load $(cut -d" " -f1-3 /proc/loadavg) mem $(free -m | sed -n 2p | tr -s " " | cut -d" " -f3,7) lines $(wc -l < $log) | $(tail -n 1 $log | tail -c 90)"
+          done
+          ps -eo pid,ppid,stat,wchan:20,etimes,time,pcpu,rss,args --sort=-pcpu | head -12 | cut -c1-190
+          pkill -ABRT -f "[b]in/sphinx-build" && sleep 3
+          echo "=== log"
+          cat $log
+          exit 1