diff --git a/internal/scaffold/fullsend-repo/scripts/pre-retro-test.sh b/internal/scaffold/fullsend-repo/scripts/pre-retro-test.sh
new file mode 100644
index 000000000..b70b37157
--- /dev/null
+++ b/internal/scaffold/fullsend-repo/scripts/pre-retro-test.sh
@@ -0,0 +1,129 @@
+#!/usr/bin/env bash
+# pre-retro-test.sh — Test pre-retro.sh with mock gh to verify token validation.
+#
+# Uses a mock gh command to capture calls without hitting GitHub.
+# Run from the repo root: bash internal/scaffold/fullsend-repo/scripts/pre-retro-test.sh
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PRE_SCRIPT="${SCRIPT_DIR}/pre-retro.sh"
+FAILURES=0
+
+# Create a temp directory for mock state.
+TMPDIR="$(mktemp -d)"
+trap 'rm -rf "${TMPDIR}"' EXIT
+
+# --- Helpers ---
+
+# build_mock creates a mock gh binary that returns a preconfigured exit code
+# for "gh auth status" calls.
+# Arguments:
+#   $1 — exit code for "gh auth status" (0 = valid token, 1 = invalid token).
+build_mock() {
+  local auth_exit="$1"
+  local mock_bin="${TMPDIR}/bin"
+
+  rm -rf "${mock_bin}"
+  mkdir -p "${mock_bin}"
+
+  cat > "${mock_bin}/gh" <<MOCKEOF
+#!/usr/bin/env bash
+if [[ "\$1" == "auth" && "\$2" == "status" ]]; then
+  exit ${auth_exit}
+fi
+MOCKEOF
+
+  chmod +x "${mock_bin}/gh"
+  echo "${mock_bin}"
+}
+
+run_test_stdout() {
+  local test_name="$1"
+  local auth_exit="$2"
+  local expected_stdout="$3"
+  local expect_exit="$4"
+  local extra_env="${5:-}"
+
+  local mock_bin
+  mock_bin="$(build_mock "${auth_exit}")"
+
+  local env_cmd=(
+    env -i
+    PATH="${mock_bin}:/usr/bin:/bin"
+    HOME="${TMPDIR}"
+    ORIGINATING_URL="https://github.com/test-org/test-repo/pull/42"
+  )
+
+  # Add extra env vars if provided.
+  if [[ -n "${extra_env}" ]]; then
+    while IFS= read -r kv; do
+      [[ -n "${kv}" ]] && env_cmd+=("${kv}")
+    done <<< "${extra_env}"
+  fi
+
+  local exit_code=0
+  "${env_cmd[@]}" bash "${PRE_SCRIPT}" > "${TMPDIR}/stdout.log" 2>&1 || exit_code=$?
+
+  if [[ ${exit_code} -ne ${expect_exit} ]]; then
+    echo "FAIL: ${test_name} — expected exit ${expect_exit}, got ${exit_code}"
+    cat "${TMPDIR}/stdout.log"
+    FAILURES=$((FAILURES + 1))
+    return
+  fi
+
+  if ! grep -qF "${expected_stdout}" "${TMPDIR}/stdout.log" 2>/dev/null; then
+    echo "FAIL: ${test_name} — expected stdout '${expected_stdout}' not found"
+    echo "Actual stdout:"
+    cat "${TMPDIR}/stdout.log"
+    FAILURES=$((FAILURES + 1))
+    return
+  fi
+
+  echo "PASS: ${test_name}"
+}
+
+# --- Test cases ---
+
+# Valid token → auth status exits 0 → script succeeds.
+run_test_stdout "valid-token-succeeds" \
+  0 \
+  "GH_TOKEN validated successfully." \
+  0 \
+  "GH_TOKEN=fake-token"
+
+# Invalid token → auth status exits 1 → script fails with error annotation.
+run_test_stdout "invalid-token-fails" \
+  1 \
+  "::error::GH_TOKEN is invalid" \
+  1 \
+  "GH_TOKEN=bad-token"
+
+# Absent token → script fails with error annotation (no sandbox wasted).
+run_test_stdout "absent-token-fails" \
+  0 \
+  "::error::GH_TOKEN is not set" \
+  1
+
+# Valid token with retro comment → script succeeds with on-demand message.
+run_test_stdout "retro-comment-on-demand" \
+  0 \
+  "Retro triggered on-demand with comment." \
+  0 \
+  "$(printf '%s\n%s' 'GH_TOKEN=fake-token' 'RETRO_COMMENT=look at this')"
+
+# Valid token without retro comment → script succeeds with automatic message.
+run_test_stdout "retro-automatic-trigger" \
+  0 \
+  "Retro triggered automatically (PR close)." \
+  0 \
+  "GH_TOKEN=fake-token"
+
+# --- Summary ---
+
+echo ""
+if [[ ${FAILURES} -gt 0 ]]; then
+  echo "${FAILURES} test(s) failed"
+  exit 1
+fi
+echo "All tests passed"
diff --git a/internal/scaffold/fullsend-repo/scripts/pre-retro.sh b/internal/scaffold/fullsend-repo/scripts/pre-retro.sh
index 9ee8b9eab..c64a902ad 100755
--- a/internal/scaffold/fullsend-repo/scripts/pre-retro.sh
+++ b/internal/scaffold/fullsend-repo/scripts/pre-retro.sh
@@ -6,6 +6,7 @@
 #
 # Required env vars:
 #   ORIGINATING_URL — HTML URL of the PR or issue that triggered retro
+#   GH_TOKEN        — GitHub token; validated before sandbox starts
 #
 # Optional env vars:
 #   RETRO_COMMENT   — The /retro comment text (empty for automatic triggers)
@@ -22,6 +23,23 @@ fi
 
 echo "::notice::Retro target: ${ORIGINATING_URL}"
 
+# ---------------------------------------------------------------------------
+# Validate GH_TOKEN before starting the sandbox
+# ---------------------------------------------------------------------------
+# ponytail: gh auth status validates auth, not scopes — a token with only
+# contents:read would pass here but fail in post-retro.sh. Still catches the
+# #256 failure mode (expired/revoked token). Scope checks if needed later.
+if [[ -z "${GH_TOKEN:-}" ]]; then
+  echo "::error::GH_TOKEN is not set — retro agent requires GitHub API access"
+  exit 1
+fi
+echo "::add-mask::${GH_TOKEN}"
+if ! gh auth status >/dev/null 2>&1; then
+  echo "::error::GH_TOKEN is invalid — retro agent requires GitHub API access"
+  exit 1
+fi
+echo "GH_TOKEN validated successfully."
+
 if [[ -n "${RETRO_COMMENT:-}" ]]; then
   echo "Retro triggered on-demand with comment."
 else