diff --git a/.github/workflows/plananvil-codex-qualification.yml b/.github/workflows/plananvil-codex-qualification.yml
index a6b5fe3..dab815d 100644
--- a/.github/workflows/plananvil-codex-qualification.yml
+++ b/.github/workflows/plananvil-codex-qualification.yml
@@ -84,6 +84,20 @@ jobs:
           git --version
           python3 --version
 
+      - name: Validate Linux Codex sandbox prerequisites
+        shell: bash
+        run: |
+          set -euo pipefail
+          if ! command -v bwrap >/dev/null 2>&1; then
+            echo "::error::System bubblewrap is required on the trusted Linux qualification runner. Install the Debian bubblewrap package instead of relying on the bundled Codex helper."
+            exit 2
+          fi
+          bwrap --version
+          if ! bwrap --unshare-user --uid 0 --gid 0 --ro-bind / / /bin/true; then
+            echo "::error::bubblewrap cannot create the user namespace required by the Codex command sandbox. Fix the Podman/host user-namespace policy before running C01-C16."
+            exit 2
+          fi
+
       - name: Create trusted main qualification workspace
         shell: bash
         run: |
@@ -131,7 +145,7 @@ jobs:
 
       - name: Upload sanitized capability evidence
         if: always()
-        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
+        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6
         with:
           name: plananvil-codex-evidence-${{ github.run_id }}
           path: ${{ env.QUALIFICATION_ARTIFACT }}
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7ae4108..2bc375e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,8 @@ All notable changes to PlanAnvil are documented here.
 ### Changed
 
 - update pinned `actions/checkout` and `actions/setup-python` workflow SHAs to the current v7 releases while retaining immutable action pinning and Node 24 compatibility;
+- require the full Linux Codex qualification job to pass a system-`bubblewrap` user-namespace probe before C01-C16, so incompatible Podman runners fail fast instead of timing out capability-by-capability;
+- update the qualification evidence uploader to the Node-24-native `actions/upload-artifact` v6 immutable SHA;
 - require production releases to use a GitHub-verified signed annotated tag whose target is reachable from `main`;
 - fail the production release gate closed when the release worktree is dirty or Git cleanliness cannot be verified;
 - document the controlled self-hosted Codex qualification path and keep the previous sandbox procedure as a manual fallback.
diff --git a/docs/CODEX_SANDBOX_RUNBOOK.md b/docs/CODEX_SANDBOX_RUNBOOK.md
index 64eba7f..98024a2 100644
--- a/docs/CODEX_SANDBOX_RUNBOOK.md
+++ b/docs/CODEX_SANDBOX_RUNBOOK.md
@@ -10,8 +10,19 @@ This is the remaining live-runtime step after deterministic/release hardening is
 - operating system recorded;
 - permission mode and project-trust mode recorded;
 - Git available and target fixture repositories disposable;
+- system `bubblewrap` installed on Linux (`apt install bubblewrap` on Debian/Ubuntu);
+- Linux user-namespace creation must work from the runner process;
 - no real credentials, private repository URLs, personal paths, or proprietary source in fixtures/evidence.
 
+On a Podman-hosted runner, Codex creates an inner Linux command sandbox. The outer container must therefore permit the runner user to create the user namespace required by `bwrap`. Before a full qualification run, this probe must succeed inside the runner container:
+
+```text
+command -v bwrap
+bwrap --unshare-user --uid 0 --gid 0 --ro-bind / / /bin/true
+```
+
+If the probe fails with `setting up uid map: Operation not permitted`, fix the Podman/host user-namespace policy rather than disabling Codex sandboxing. The qualification workflow deliberately does not use `danger-full-access` or `--dangerously-bypass-approvals-and-sandbox`.
+
 ## Controlled GitHub Actions path
 
 The preferred qualification path is `.github/workflows/plananvil-codex-qualification.yml` in `full` mode. The workflow is intentionally `workflow_dispatch`-only, accepts execution only from `main`, uses Environment `plananvil-codex`, and targets `[self-hosted, linux, x64, plananvil, codex]`.
@@ -20,7 +31,7 @@ The controlled runner must provide `plananvil-qualification-workspace`. The work
 
 Raw Codex session streams are not retained. The controller keeps only sanitized final assertions, event-type counts, and relative Git structure required for evaluation. The self-hosted runner has repository read permission only and never pushes qualification changes.
 
-The workflow uploads `plananvil-codex-evidence-<run-id>` as a short-lived artifact. Review that artifact before committing evidence through a normal protected pull request. A full workflow run exits successfully only when every release-gating capability is `REPRODUCED`; partial/failed runs still upload their sanitized evidence artifact for diagnosis.
+The workflow performs the Linux `bubblewrap` user-namespace probe before preparing fixtures, so an incompatible container fails in seconds instead of consuming a full C01-C16 run. It uploads `plananvil-codex-evidence-<run-id>` as a short-lived artifact. Review that artifact before committing evidence through a normal protected pull request. A full workflow run exits successfully only when every release-gating capability is `REPRODUCED`; partial/failed runs still upload their sanitized evidence artifact for diagnosis.
 
 ## Manual fallback
 
diff --git a/tests/test_live_codex_qualification.py b/tests/test_live_codex_qualification.py
index 3212b5d..1d967fe 100644
--- a/tests/test_live_codex_qualification.py
+++ b/tests/test_live_codex_qualification.py
@@ -104,6 +104,21 @@ def test_validate_plan_rejects_duplicate_trials(self) -> None:
         }
         self.assertIn("duplicate", live.validate_plan("C01", plan) or "")
 
+    def test_workflow_fails_fast_without_linux_user_namespace_sandbox(self) -> None:
+        workflow = (ROOT / ".github/workflows/plananvil-codex-qualification.yml").read_text(
+            encoding="utf-8"
+        )
+        self.assertIn("command -v bwrap", workflow)
+        self.assertIn(
+            "bwrap --unshare-user --uid 0 --gid 0 --ro-bind / / /bin/true",
+            workflow,
+        )
+        self.assertIn(
+            "actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6",
+            workflow,
+        )
+        self.assertNotIn("actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02", workflow)
+
 
 if __name__ == "__main__":
     unittest.main()