diff --git a/Makefile b/Makefile
index 020d753..55e8a1b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,7 @@
 ACTIONS := \
 	docker \
 	node \
+	claude/check-shared-block \
 	git/create_tag \
 	git/promotion-gate \
 	helm/diff \
diff --git a/README.md b/README.md
index b4aa160..789a40b 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,12 @@ Reusable composite actions for Gitea CI/CD pipelines.
 
 ## Actions
 
+### Claude
+
+| Action | Description |
+|---|---|
+| [claude/check-shared-block](claude/check-shared-block/README.md) | Fails when a repo's shared CLAUDE.md block has drifted from the canonical copy in stat-tackler-infra |
+
 ### Docker
 
 | Action | Description |
diff --git a/claude/check-shared-block/README.md b/claude/check-shared-block/README.md
new file mode 100644
index 0000000..667347b
--- /dev/null
+++ b/claude/check-shared-block/README.md
@@ -0,0 +1,44 @@
+# Check Shared CLAUDE.md Block
+
+<!-- action-docs-description source="action.yml" -->
+## Description
+
+Fails when a repo's shared CLAUDE.md block has drifted from the canonical copy in stat-tackler-infra
+<!-- action-docs-description source="action.yml" -->
+
+<!-- action-docs-inputs source="action.yml" -->
+## Inputs
+
+| name | description | required | default |
+| --- | --- | --- | --- |
+| `TARGET` | <p>Path to the CLAUDE.md file to check</p> | `false` | `CLAUDE.md` |
+| `CANONICAL_URL` | <p>Raw URL of the canonical shared fragment</p> | `false` | `https://gitea.pixelparasol.com/stat-tackler/stat-tackler-infra/raw/branch/main/docs/CLAUDE.shared.md` |
+| `GITEA_TOKEN` | <p>Gitea token with read access to stat-tackler-infra, if it is private</p> | `false` | `""` |
+<!-- action-docs-inputs source="action.yml" -->
+
+<!-- action-docs-runs source="action.yml" -->
+## Runs
+
+This action is a `composite` action.
+<!-- action-docs-runs source="action.yml" -->
+
+## Running locally
+
+The check logic has no dependency on the composite action wrapper beyond
+`curl`, `awk`, and `diff` on `PATH`. From a repo's root:
+
+```bash
+CANONICAL_URL=https://gitea.pixelparasol.com/stat-tackler/stat-tackler-infra/raw/branch/main/docs/CLAUDE.shared.md \
+  TARGET=CLAUDE.md sh -c '
+    extract() { awk "/<!-- SHARED:BEGIN/{f=1} f{print} /SHARED:END -->/{f=0}" "$1"; }
+    tmp=$(mktemp)
+    curl -sfSL "$CANONICAL_URL" -o "$tmp"
+    extract "$tmp" > "$tmp.canon"
+    extract "$TARGET" > "$tmp.mine"
+    diff -u "$tmp.canon" "$tmp.mine" && echo "matches canonical"
+    rm -f "$tmp" "$tmp.canon" "$tmp.mine"
+  '
+```
+
+`GITEA_TOKEN` is only needed if `stat-tackler-infra` becomes private; it is
+otherwise safe to omit.
diff --git a/claude/check-shared-block/action.yml b/claude/check-shared-block/action.yml
new file mode 100644
index 0000000..f084e6a
--- /dev/null
+++ b/claude/check-shared-block/action.yml
@@ -0,0 +1,75 @@
+name: Check Shared CLAUDE.md Block
+description: Fails when a repo's shared CLAUDE.md block has drifted from the canonical copy in stat-tackler-infra
+
+inputs:
+  TARGET:
+    description: "Path to the CLAUDE.md file to check"
+    default: "CLAUDE.md"
+  CANONICAL_URL:
+    description: "Raw URL of the canonical shared fragment"
+    default: "https://gitea.pixelparasol.com/stat-tackler/stat-tackler-infra/raw/branch/main/docs/CLAUDE.shared.md"
+  GITEA_TOKEN:
+    description: "Gitea token with read access to stat-tackler-infra, if it is private"
+    default: ""
+
+runs:
+  using: composite
+  steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Check shared CLAUDE.md block
+      shell: sh
+      env:
+        TARGET: ${{ inputs.TARGET }}
+        CANONICAL_URL: ${{ inputs.CANONICAL_URL }}
+        GITEA_TOKEN: ${{ inputs.GITEA_TOKEN }}
+      run: |
+        set -e
+
+        if [ ! -f "$TARGET" ]; then
+          echo "::error::$TARGET not found"
+          exit 1
+        fi
+
+        extract() {
+          # Everything between the markers, markers included.
+          awk '/<!-- SHARED:BEGIN/{f=1} f{print} /SHARED:END -->/{f=0}' "$1"
+        }
+
+        mine=$(extract "$TARGET")
+        if [ -z "$mine" ]; then
+          echo "::error::$TARGET has no SHARED:BEGIN/SHARED:END block."
+          echo "Copy it from $CANONICAL_URL"
+          exit 1
+        fi
+
+        tmp=$(mktemp)
+        if [ -n "$GITEA_TOKEN" ]; then
+          curl -sfSL -H "Authorization: token $GITEA_TOKEN" "$CANONICAL_URL" -o "$tmp"
+        else
+          curl -sfSL "$CANONICAL_URL" -o "$tmp"
+        fi
+
+        theirs=$(extract "$tmp")
+        if [ -z "$theirs" ]; then
+          echo "::error::Could not read the canonical block from $CANONICAL_URL"
+          exit 1
+        fi
+
+        if [ "$mine" = "$theirs" ]; then
+          echo "Shared CLAUDE.md block matches the canonical copy."
+          rm -f "$tmp"
+          exit 0
+        fi
+
+        echo "::error::The shared CLAUDE.md block has drifted from the canonical copy."
+        echo "Canonical: $CANONICAL_URL"
+        echo
+        echo "--- canonical"
+        echo "+++ $TARGET"
+        printf '%s\n' "$theirs" > "$tmp.canon"
+        printf '%s\n' "$mine" > "$tmp.mine"
+        diff -u "$tmp.canon" "$tmp.mine" || true
+        rm -f "$tmp" "$tmp.canon" "$tmp.mine"
+        exit 1