diff --git a/.github/workflows/pr-auto-rebase.yml b/.github/workflows/pr-auto-rebase.yml
index 4992224926..45b0d7f7ae 100644
--- a/.github/workflows/pr-auto-rebase.yml
+++ b/.github/workflows/pr-auto-rebase.yml
@@ -46,9 +46,9 @@ on:
         default: ""
         type: string
       canonical_ref:
-        description: Ref of ContextualWisdomLab/.github to use for scheduler code
+        description: Deprecated compatibility input; accepted and ignored because privileged source is bound to the called workflow SHA
         required: false
-        default: "main"
+        default: ""
         type: string
   repository_dispatch:
     types: [pr-auto-rebase]
@@ -84,7 +84,6 @@ jobs:
       MAX_PRS: ${{ github.event.client_payload.max_prs || inputs.max_prs || '100' }}
       AUTO_REBASE_MAX_PER_RUN: ${{ github.event.client_payload.max_per_run || inputs.max_per_run || vars.AUTO_REBASE_MAX_PER_RUN || '10' }}
       AUTO_REBASE_HUMAN_WINDOW_MINUTES: ${{ github.event.client_payload.human_window_minutes || inputs.human_window_minutes || vars.AUTO_REBASE_HUMAN_WINDOW_MINUTES || '30' }}
-      CANONICAL_REF: main
     steps:
       - name: Exchange OpenCode app token for cross-repo git writes
         id: scheduler_app_token
@@ -152,14 +151,67 @@ jobs:
             echo "token=$app_token"
           } >>"$GITHUB_OUTPUT"
 
-      - name: Checkout canonical scheduler
+      - name: Resolve immutable called-workflow source
+        id: trusted_source
+        env:
+          # github.workflow_sha is the commit GitHub loaded for this workflow.
+          # Same-repository callers share that commit. A caller-supplied ref
+          # must not select it.
+          CALLER_REPOSITORY: ${{ github.repository }}
+          WORKFLOW_SHA: ${{ github.workflow_sha }}
+        run: |
+          set -euo pipefail
+          expected_repository="ContextualWisdomLab/.github"
+          expected_file=".github/workflows/pr-auto-rebase.yml"
+
+          if [ "$CALLER_REPOSITORY" != "$expected_repository" ]; then
+            printf '::error::Called workflow was invoked from an untrusted repository: %s (expected %s).\n' \
+              "${CALLER_REPOSITORY:-<empty>}" "$expected_repository"
+            exit 1
+          fi
+
+          if ! [[ "$WORKFLOW_SHA" =~ ^[0-9a-f]{40}$ ]]; then
+            printf '::error::Called workflow SHA is missing or malformed: %s.\n' \
+              "${WORKFLOW_SHA:-<empty>}"
+            exit 1
+          fi
+
+          {
+            printf 'repository=%s\n' "$expected_repository"
+            printf 'sha=%s\n' "$WORKFLOW_SHA"
+            printf 'workflow_file_path=%s\n' "$expected_file"
+          } >>"$GITHUB_OUTPUT"
+          printf 'Resolved immutable called-workflow source repository=%s file=%s sha=%s.\n' \
+            "$expected_repository" "$expected_file" "$WORKFLOW_SHA"
+
+      - name: Checkout immutable called-workflow source
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
-          repository: ContextualWisdomLab/.github
-          ref: ${{ env.CANONICAL_REF }}
+          repository: ${{ steps.trusted_source.outputs.repository }}
+          ref: ${{ steps.trusted_source.outputs.sha }}
           fetch-depth: 1
           persist-credentials: false
 
+      - name: Verify immutable called-workflow checkout
+        env:
+          EXPECTED_SHA: ${{ steps.trusted_source.outputs.sha }}
+          EXPECTED_FILE: ${{ steps.trusted_source.outputs.workflow_file_path }}
+        run: |
+          set -euo pipefail
+          actual_sha="$(git rev-parse HEAD)"
+          if [ "$actual_sha" != "$EXPECTED_SHA" ]; then
+            printf '::error::Checked-out scheduler SHA %s does not match called-workflow SHA %s.\n' \
+              "$actual_sha" "$EXPECTED_SHA"
+            exit 1
+          fi
+          if [ ! -f "$EXPECTED_FILE" ] || [ -L "$EXPECTED_FILE" ]; then
+            printf '::error::Called workflow source file is missing or symlinked: %s.\n' \
+              "$EXPECTED_FILE"
+            exit 1
+          fi
+          printf 'Verified immutable scheduler checkout at %s (%s).\n' \
+            "$actual_sha" "$EXPECTED_FILE"
+
       - name: Self-test auto-rebase scheduler contract
         run: python3 scripts/ci/pr_auto_rebase.py --self-test
 
diff --git a/tests/test_pr_auto_rebase_source_pin.py b/tests/test_pr_auto_rebase_source_pin.py
new file mode 100644
index 0000000000..48dc036ead
--- /dev/null
+++ b/tests/test_pr_auto_rebase_source_pin.py
@@ -0,0 +1,66 @@
+"""Supply-chain contract for the PR auto-rebase scheduler source checkout."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+
+_WORKFLOW = (
+    Path(__file__).resolve().parents[1]
+    / ".github"
+    / "workflows"
+    / "pr-auto-rebase.yml"
+)
+
+
+def _workflow_text() -> str:
+    """Read the auto-rebase workflow as UTF-8 text."""
+    return _WORKFLOW.read_text(encoding="utf-8")
+
+
+def test_auto_rebase_validates_called_workflow_sha_before_checkout() -> None:
+    """A floating branch tip must not be checked out before SHA validation."""
+    workflow = _workflow_text()
+    guard = workflow.index("Resolve immutable called-workflow source")
+    checkout = workflow.index("Checkout immutable called-workflow source")
+
+    assert guard < checkout
+    assert "CALLER_REPOSITORY: ${{ github.repository }}" in workflow
+    assert "WORKFLOW_SHA: ${{ github.workflow_sha }}" in workflow
+    assert '[ "$CALLER_REPOSITORY" != "$expected_repository" ]' in workflow
+    assert 'expected_repository="ContextualWisdomLab/.github"' in workflow
+    assert 'expected_file=".github/workflows/pr-auto-rebase.yml"' in workflow
+    assert '[[ "$WORKFLOW_SHA" =~ ^[0-9a-f]{40}$ ]]' in workflow
+    assert "repository: ${{ steps.trusted_source.outputs.repository }}" in workflow
+    assert "ref: ${{ steps.trusted_source.outputs.sha }}" in workflow
+    assert "ref: main" not in workflow
+    assert "CANONICAL_REF: main" not in workflow
+    assert "ref: ${{ env.CANONICAL_REF }}" not in workflow
+    assert "ref: ${{ github.sha }}" not in workflow
+
+
+def test_auto_rebase_verifies_checked_out_sha_matches_runtime_parameter() -> None:
+    """The commit that executes must equal the validated 40-character SHA."""
+    workflow = _workflow_text()
+    verification = workflow.index("Verify immutable called-workflow checkout")
+    self_test = workflow.index("Self-test auto-rebase scheduler contract")
+
+    assert verification < self_test
+    assert 'actual_sha="$(git rev-parse HEAD)"' in workflow
+    assert '[ "$actual_sha" != "$EXPECTED_SHA" ]' in workflow
+    assert '[ ! -f "$EXPECTED_FILE" ] || [ -L "$EXPECTED_FILE" ]' in workflow
+
+
+def test_auto_rebase_canonical_ref_input_cannot_select_source() -> None:
+    """Callers may still send canonical_ref, but it never selects the checkout."""
+    workflow = _workflow_text()
+    declaration = workflow.split("canonical_ref:", 1)[1].split(
+        "repository_dispatch:", 1
+    )[0]
+
+    assert "Deprecated compatibility input" in declaration
+    assert "ignored" in declaration
+    assert 'default: ""' in declaration
+    assert workflow.count("canonical_ref") == 1
+    assert "inputs.canonical_ref" not in workflow
+    assert "github.event.client_payload.canonical_ref" not in workflow