diff --git a/hooks/pre-tool-scope-guard b/hooks/pre-tool-scope-guard
index 22d9515..a9c3e72 100755
--- a/hooks/pre-tool-scope-guard
+++ b/hooks/pre-tool-scope-guard
@@ -10,7 +10,8 @@
 #              — git push / gh pr create when locked plan hash mismatches
 #              — git push / git commit to main or master       (unless SUPERPOWERS_ALLOW_DEFAULT_BRANCH=1)
 #   Write / Edit / MultiEdit tool
-#              — *.scope-lock files                            (unless SUPERPOWERS_SCOPE_LOCK_WRITE=1)
+#              — *.scope-lock files                            (agent path: hooks/scope-lock-apply via Bash;
+#                                                              operator path: SUPERPOWERS_SCOPE_LOCK_WRITE=1)
 #              — docs/plans/*.md when plan is Locked           (unless SUPERPOWERS_PLAN_LOCK_WRITE=1)
 #
 # Global opt-out: set SUPERPOWERS_HOOKS_DISABLE=1 in the *operator's* terminal
@@ -141,10 +142,23 @@ case "$tool_name" in
     while IFS= read -r fpath; do
         [ -z "$fpath" ] && continue
 
-        # ── 5. scope-lock files (always blocked unless sentinel env var) ─────
+        # ── 5. scope-lock files ──────────────────────────────────────────────
+        # Agent path:   bash "${CLAUDE_PLUGIN_ROOT}/hooks/scope-lock-apply" <plan-path>
+        # Operator path (pre-session): export SUPERPOWERS_SCOPE_LOCK_WRITE=1
         if printf '%s' "$fpath" | grep -qE '\.scope-lock$'; then
             if [ "${SUPERPOWERS_SCOPE_LOCK_WRITE:-}" != "1" ]; then
-                block "Writing to '$(basename "$fpath")' is blocked — .scope-lock files are written exclusively by the scope-lock skill during alignment-check PASS. Direct edits break the manifest integrity guarantee and allow silent scope tampering. To update the lock legitimately: go through the unlock path (recording-decisions → update manifest → re-run alignment-check), which will regenerate the lock file."
+                _reason="Writing to '$(basename "$fpath")' is blocked —"
+                _reason="${_reason} .scope-lock files must be written by the scope-lock skill's helper,"
+                _reason="${_reason} not via the Write tool."
+                _reason="${_reason} Run: bash \"\${CLAUDE_PLUGIN_ROOT}/hooks/scope-lock-apply\" <plan-path>."
+                _reason="${_reason} The helper extracts the Scope Manifest section, computes its sha256,"
+                _reason="${_reason} and writes the lock file via shell redirection (which is not blocked)."
+                _reason="${_reason} Direct Write/Edit calls break the manifest integrity guarantee"
+                _reason="${_reason} and allow silent scope tampering."
+                _reason="${_reason} To update the lock legitimately after a scope reduction:"
+                _reason="${_reason} use the unlock path (recording-decisions → update manifest"
+                _reason="${_reason} → re-run alignment-check → re-run scope-lock-apply)."
+                block "$_reason"
             fi
         fi
 
diff --git a/hooks/scope-lock-apply b/hooks/scope-lock-apply
new file mode 100755
index 0000000..5dc858a
--- /dev/null
+++ b/hooks/scope-lock-apply
@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# hooks/scope-lock-apply
+# Compute and write the .scope-lock file for a plan.
+#
+# Usage: scope-lock-apply <plan-path>
+#
+# Extracts the "## Scope Manifest" section from <plan-path>, computes its
+# sha256, and writes the digest to <plan-path>.scope-lock via shell
+# redirection.
+#
+# This helper exists because the Write/Edit/MultiEdit tools are blocked for
+# *.scope-lock paths by hooks/pre-tool-scope-guard (to prevent silent scope
+# tampering by implementation skills or subagents). The scope-lock skill is
+# the only legitimate writer of these files, and calling this helper via the
+# Bash tool is the sanctioned path — it uses shell redirection rather than
+# the Write tool, so the guard never fires.
+#
+# The hash format is a single bare hex sha256 digest, which is exactly what
+# tests/plan-scope-check.sh --verify-lock expects:
+#   expected="$(awk 'NF && !/^#/ {print; exit}' "$lock")"
+
+set -euo pipefail
+
+plan="${1:-}"
+
+if [ -z "$plan" ]; then
+    printf 'Usage: scope-lock-apply <plan-path>\n' >&2
+    exit 3
+fi
+
+if [ ! -f "$plan" ]; then
+    printf 'Error: plan file not found: %s\n' "$plan" >&2
+    exit 1
+fi
+
+# Verify the plan has a Scope Manifest section before attempting to hash it.
+if ! grep -qE '^## Scope Manifest[[:space:]]*$' "$plan"; then
+    printf 'Error: no "## Scope Manifest" section found in %s\n' "$plan" >&2
+    printf 'The manifest section is required before locking. See skills/scope-lock/SKILL.md.\n' >&2
+    exit 1
+fi
+
+# Compute sha256 of the manifest section.
+# The awk pattern below is intentionally identical to extract_manifest() in
+# tests/plan-scope-check.sh — the byte stream must match exactly so that
+# tests/plan-scope-check.sh --verify-lock produces the same hash.
+# Extract to a temp file to avoid duplicating the awk script across sha256
+# backend branches.
+tmpfile=$(mktemp)
+trap 'rm -f "$tmpfile"' EXIT
+
+awk '
+    /^## Scope Manifest[[:space:]]*$/ { in_section = 1; print; next }
+    in_section && /^## / { in_section = 0 }
+    in_section { print }
+' "$plan" > "$tmpfile"
+
+if command -v sha256sum >/dev/null 2>&1; then
+    hash=$(sha256sum < "$tmpfile" | awk '{print $1}')
+elif command -v shasum >/dev/null 2>&1; then
+    hash=$(shasum -a 256 < "$tmpfile" | awk '{print $1}')
+else
+    printf 'Error: sha256sum or shasum is required but neither was found\n' >&2
+    exit 1
+fi
+
+if [ -z "$hash" ]; then
+    printf 'Error: failed to compute sha256 for %s\n' "$plan" >&2
+    exit 1
+fi
+
+lock_file="${plan}.scope-lock"
+printf '%s\n' "$hash" > "$lock_file"
+printf 'scope-lock-apply: %s written (sha256=%s…)\n' "$lock_file" "${hash:0:12}"
diff --git a/skills/alignment-check/SKILL.md b/skills/alignment-check/SKILL.md
index 082407e..879b131 100644
--- a/skills/alignment-check/SKILL.md
+++ b/skills/alignment-check/SKILL.md
@@ -130,7 +130,7 @@ Re-run alignment check after revision. **Max 2 revision cycles** before escalati
 After alignment passes, **lock the plan's scope** so subsequent execution cannot silently rescope. Invoke `superpowers:scope-lock` with the plan path. The scope-lock skill:
 
 1. Stamps the plan's `**Status:**` line with `Locked <UTC ISO-8601 timestamp>`.
-2. Computes the manifest's sha256 and writes `<plan-path>.scope-lock`.
+2. Writes the lock file by running `bash "${CLAUDE_PLUGIN_ROOT}/hooks/scope-lock-apply" <plan-path>` (do **not** use the Write tool — it is blocked for `*.scope-lock` paths). The helper computes the manifest's sha256 and writes `<plan-path>.scope-lock` via shell redirection.
 3. Commits both files (`chore: lock scope for <feature> (alignment passed)`).
 
 After the lock is in place, proceed to execution:
diff --git a/skills/scope-lock/SKILL.md b/skills/scope-lock/SKILL.md
index 2634c64..3e0edbd 100644
--- a/skills/scope-lock/SKILL.md
+++ b/skills/scope-lock/SKILL.md
@@ -104,7 +104,11 @@ The unlock path is intentionally heavyweight. Cheap unlock = no lock at all.
 
 **`alignment-check` (pre-lock and re-lock):**
 - After PASS, edit the plan's `**Status:**` line to `Locked <UTC ISO-8601 timestamp>`.
-- Compute `sha256` of the manifest section (from `## Scope Manifest` to the next `##` heading) and write it to `<plan-path>.scope-lock`. This is the lock file.
+- Write the lock file by running the helper via Bash (do **not** use the Write tool — the Write tool is blocked for `*.scope-lock` paths by the scope guard hook):
+  ```
+  bash "${CLAUDE_PLUGIN_ROOT}/hooks/scope-lock-apply" <plan-path>
+  ```
+  The helper extracts the `## Scope Manifest` section, computes its sha256, and writes `<plan-path>.scope-lock` via shell redirection. It prints the path and hash prefix on success, or an error message and exits non-zero on failure.
 - Commit both files in the same commit: `chore: lock scope for <feature> (alignment passed)`.
 
 **`subagent-driven-development` (per-task checkpoint):**