diff --git a/.github/workflows/thesis-facts-append.yml b/.github/workflows/thesis-facts-append.yml
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+name: Thesis facts append gate
+
+# Deterministic review for every change to the observation ledger. Resolver
+# appends arrive as pull requests targeting codex/thesis-ledger-facts; this
+# gate enforces the immutable frozen prefix, an append-only diff against the
+# PR base, the witnessed release chain, per-row schema and binding requirements,
+# and explicit supersede semantics for corrections. Direct pushes get the same
+# full-file checks.
+#
+# No paths filter, on purpose: branch protection lists "Append gate" as a
+# required status check, so it must report on every pull request into the
+# branch. With a paths filter, a PR that touches none of the gated files
+# (tests, CI config) never receives the check and deadlocks on "Expected".
+# For such PRs the append-only diff is trivially clean and the gate stays
+# cheap.
+
+on:
+  pull_request:
+    branches:
+      - codex/thesis-ledger-facts
+  # When this definition is installed on the repository default branch, this
+  # base-owned event can be the trust root for a required PR workflow. The
+  # ordinary pull_request workflow below still tests the merge ref, but its YAML
+  # is candidate-controlled and cannot by itself prevent a PR from stubbing the
+  # job that invokes the detached base gate. No paths filter here either: a
+  # required check that a paths filter can skip deadlocks on "Expected" for
+  # out-of-scope PRs.
+  pull_request_target:
+    branches:
+      - codex/thesis-ledger-facts
+  push:
+    branches:
+      - codex/thesis-ledger-facts
+
+permissions:
+  contents: read
+
+jobs:
+  trusted-base-append-gate:
+    name: Trusted base append gate
+    if: github.event_name == 'pull_request_target'
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Judge the PR merge tree with the base gate
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          MERGE_SHA: ${{ github.event.pull_request.merge_commit_sha }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          set -euo pipefail
+          [[ "$BASE_SHA" =~ ^[0-9a-f]{40,64}$ ]]
+          [[ "$MERGE_SHA" =~ ^[0-9a-f]{40,64}$ ]]
+          [[ "$PR_NUMBER" =~ ^[0-9]+$ ]]
+
+          repository_url="$GITHUB_SERVER_URL/$GITHUB_REPOSITORY.git"
+          candidate="$RUNNER_TEMP/thesis-facts-candidate"
+          base_gate="$RUNNER_TEMP/thesis-facts-base-gate"
+
+          git clone --no-checkout "$repository_url" "$candidate"
+          git -C "$candidate" fetch --no-tags origin \
+            "+refs/pull/${PR_NUMBER}/merge:refs/remotes/origin/pr-merge"
+          git -C "$candidate" checkout --detach refs/remotes/origin/pr-merge
+          test "$(git -C "$candidate" rev-parse HEAD)" = "$MERGE_SHA"
+          git -C "$candidate" cat-file -e "${BASE_SHA}^{commit}"
+
+          git clone --no-checkout "$repository_url" "$base_gate"
+          git -C "$base_gate" fetch --no-tags origin "$BASE_SHA"
+          git -C "$base_gate" checkout --detach "$BASE_SHA"
+          test "$(git -C "$base_gate" rev-parse HEAD)" = "$BASE_SHA"
+
+          cd "$RUNNER_TEMP"
+          PYTHONPATH="$base_gate/scripts" \
+            PYTHONNOUSERSITE=1 \
+            python3 "$base_gate/scripts/check_thesis_facts_append.py" \
+              --root "$candidate" \
+              --base-ref "$BASE_SHA"
+
+  append-gate:
+    name: Append gate
+    if: github.event_name != 'pull_request_target'
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Enforce append-only ledger invariants
+        run: |
+          set -euo pipefail
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            base_sha="${{ github.event.pull_request.base.sha }}"
+            base_gate="$RUNNER_TEMP/thesis-facts-base-gate"
+
+            git -C "$GITHUB_WORKSPACE" cat-file -e "${base_sha}^{commit}"
+            git clone --no-checkout \
+              "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY.git" \
+              "$base_gate"
+            git -C "$base_gate" fetch --no-tags --depth 1 origin "$base_sha"
+            git -C "$base_gate" checkout --detach "$base_sha"
+            test "$(git -C "$base_gate" rev-parse HEAD)" = "$base_sha"
+
+            cd "$RUNNER_TEMP"
+            PYTHONPATH="$base_gate/scripts" \
+              PYTHONNOUSERSITE=1 \
+              python3 "$base_gate/scripts/check_thesis_facts_append.py" \
+                --root "$GITHUB_WORKSPACE" \
+                --base-ref "$base_sha"
+          else
+            python3 scripts/check_thesis_facts_append.py
+          fi
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.14"
+
+      - name: Install dependencies
+        run: uv sync --locked --all-extras
+
+      - name: Ledger observation invariants
+        run: >
+          uv run pytest
+          tests/test_policyengine_ledger.py
+          tests/test_release_chain.py
+          tests/test_thesis_append_adversarial.py
+          -q