diff --git a/.github/gpu-runner/Dockerfile b/.github/gpu-runner/Dockerfile
index b10ba6f7..01cc4028 100644
--- a/.github/gpu-runner/Dockerfile
+++ b/.github/gpu-runner/Dockerfile
@@ -9,8 +9,10 @@ ENV DEBIAN_FRONTEND=noninteractive \
     RUNNER_ALLOW_RUNASROOT=1 \
     UV_PYTHON_INSTALL_DIR=/opt/uv/python
 
+# gcc/libc6-dev: triton >=3.7 JIT-compiles launcher stubs at runtime and
+# needs a host C compiler (same reason the conversion image ships gcc).
 RUN apt-get update && apt-get install -y --no-install-recommends \
-        ca-certificates curl git jq tar \
+        ca-certificates curl git jq tar gcc libc6-dev \
     && rm -rf /var/lib/apt/lists/*
 
 COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /usr/local/bin/
@@ -23,7 +25,7 @@ RUN mkdir /runner && cd /runner \
     && rm -rf /var/lib/apt/lists/*
 
 # Bake the interpreter so job-time `uv sync` only resolves wheels.
-RUN uv python install 3.11
+RUN uv python install 3.12
 
 COPY entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
diff --git a/.github/workflows/gpu-ci.yml b/.github/workflows/gpu-ci.yml
index e2e1fb51..e5014969 100644
--- a/.github/workflows/gpu-ci.yml
+++ b/.github/workflows/gpu-ci.yml
@@ -125,7 +125,20 @@ jobs:
 
       - name: Sync deps (cu129 lock)
         id: sync
-        run: uv sync --extra dev --extra torch --extra images
+        # Pin the interpreter to the fleet floor: without this, a runner whose
+        # baked python predates requires-python makes uv download the newest
+        # CPython (3.14 incident, gw#502).
+        run: uv sync --python 3.12 --extra dev --extra torch --extra images
+
+      # triton >=3.7 JIT-compiles launcher stubs at runtime and needs a host
+      # C compiler. Baked into the runner image; the apt fallback covers pods
+      # booted from a pre-gcc image (rebuild races this same push).
+      - name: Ensure C compiler (triton JIT)
+        run: |
+          command -v cc >/dev/null 2>&1 || {
+            apt-get update -qq
+            apt-get install -y -qq --no-install-recommends gcc libc6-dev
+          }
 
       - name: Run tests
         run: uv run --extra dev --extra torch --extra images pytest tests/ -q --junitxml=gpu-junit.xml