diff --git a/CLAUDE.md b/CLAUDE.md
index 4c65ea22fbf3..36685e43a6a7 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -79,8 +79,9 @@ becomes a multi-week project. The schedule below keeps drift bounded.
 
 **Monthly** (first Monday):
 
-- Run `scripts/upstream-sync-check.sh` (TODO) to list commits on
-  upstream's current stable line and on `upstream/main` since our pin.
+- Run `scripts/upstream-sync-check.sh` to list missed patches on the
+  pinned line, newer minor lines, `upstream/main` commit volume, and
+  security-flagged subjects since our pin.
 - Triage security fixes (filter for `[SECURITY]`, `fix(auth)`, CVE
   labels). Open `fix/*` PRs for each must-backport item.
 - 30-minute review with one other engineer.
diff --git a/scripts/upstream-sync-check.sh b/scripts/upstream-sync-check.sh
new file mode 100755
index 000000000000..1e586663cf3b
--- /dev/null
+++ b/scripts/upstream-sync-check.sh
@@ -0,0 +1,262 @@
+#!/usr/bin/env bash
+# upstream-sync-check.sh — list upstream commits we haven't ingested.
+#
+# Reads the current pin from CLAUDE.md "Current pinning" block, fetches
+# the upstream BerriAI/litellm remote, and reports:
+#   1. Missed backports on our minor line (.patch.N + newer vX.Y.W)
+#   2. Newer minor lines stabilized since our pin
+#   3. Commit volume on upstream/main since our pin
+#   4. Security-flagged commit subjects since our pin
+#
+# Read-only: never modifies branches, tags, or remote state.
+#
+# Usage:
+#   scripts/upstream-sync-check.sh            # fetch then check
+#   scripts/upstream-sync-check.sh --no-fetch # skip fetch (offline)
+#
+# Run monthly per CLAUDE.md → "Upstream sync cadence".
+
+set -euo pipefail
+
+# ---------- Args ----------
+
+NO_FETCH=0
+for arg in "$@"; do
+  case "${arg}" in
+    --no-fetch) NO_FETCH=1 ;;
+    -h|--help)
+      sed -n '2,18p' "$0" | sed 's/^# \{0,1\}//'
+      exit 0 ;;
+    *)
+      echo "Unknown argument: ${arg}" >&2
+      exit 2 ;;
+  esac
+done
+
+# ---------- Locate repo ----------
+
+REPO_ROOT="$(git rev-parse --show-toplevel 2>/dev/null)" || {
+  echo "Error: not inside a git repository." >&2
+  exit 2
+}
+cd "${REPO_ROOT}"
+
+CLAUDE_MD="${REPO_ROOT}/CLAUDE.md"
+UPSTREAM_REMOTE="upstream"
+EXPECTED_UPSTREAM_FRAGMENT="BerriAI/litellm"
+SECURITY_RE='(\[SECURITY\]|fix\(auth\)|fix\(security\)|CVE-[0-9]|SSRF|IDOR|bypass|injection|sandbox|hijack|traversal|disclosure|VERIA-)'
+
+# ---------- Output helpers ----------
+
+if [[ -t 1 ]]; then
+  C_BOLD=$'\033[1m'; C_DIM=$'\033[2m'
+  C_GREEN=$'\033[32m'; C_YELLOW=$'\033[33m'
+  C_RED=$'\033[31m'; C_BLUE=$'\033[34m'
+  C_RESET=$'\033[0m'
+else
+  C_BOLD=''; C_DIM=''; C_GREEN=''; C_YELLOW=''
+  C_RED=''; C_BLUE=''; C_RESET=''
+fi
+
+section() { printf '\n%s== %s ==%s\n' "${C_BOLD}${C_BLUE}" "$*" "${C_RESET}"; }
+ok() { printf '%s✓%s %s\n' "${C_GREEN}" "${C_RESET}" "$*"; }
+warn() { printf '%s⚠%s %s\n' "${C_YELLOW}" "${C_RESET}" "$*"; }
+err() { printf '%s✗%s %s\n' "${C_RED}" "${C_RESET}" "$*" >&2; }
+dim() { printf '%s%s%s\n' "${C_DIM}" "$*" "${C_RESET}"; }
+
+# ---------- Parse pin from CLAUDE.md ----------
+
+if [[ ! -f "${CLAUDE_MD}" ]]; then
+  err "CLAUDE.md not found at ${CLAUDE_MD}"
+  exit 2
+fi
+
+# Expected line: "- **Upstream pin**: `v1.83.10-stable`"
+PIN_TAG="$(grep -m1 -oE '\*\*Upstream pin\*\*: `[^`]+`' "${CLAUDE_MD}" \
+           | sed -E 's/.*`([^`]+)`.*/\1/' || true)"
+
+if [[ -z "${PIN_TAG}" ]]; then
+  err "Could not find the 'Upstream pin' line in CLAUDE.md."
+  err "Expected a line like:  - **Upstream pin**: \`v1.83.10-stable\`"
+  exit 2
+fi
+
+if ! [[ "${PIN_TAG}" =~ ^v([0-9]+)\.([0-9]+)\.([0-9]+)(-stable)?$ ]]; then
+  err "Pin tag does not match vMAJOR.MINOR.PATCH[-stable]: ${PIN_TAG}"
+  exit 2
+fi
+PIN_MAJOR="${BASH_REMATCH[1]}"
+PIN_MINOR="${BASH_REMATCH[2]}"
+PIN_PATCH="${BASH_REMATCH[3]}"
+
+# ---------- Verify upstream remote ----------
+
+UPSTREAM_URL="$(git config --get "remote.${UPSTREAM_REMOTE}.url" 2>/dev/null || true)"
+if [[ -z "${UPSTREAM_URL}" ]]; then
+  err "Git remote '${UPSTREAM_REMOTE}' is not configured."
+  err "Add it with: git remote add ${UPSTREAM_REMOTE} https://github.com/BerriAI/litellm.git"
+  exit 2
+fi
+
+if [[ "${UPSTREAM_URL}" != *"${EXPECTED_UPSTREAM_FRAGMENT}"* ]]; then
+  warn "Remote '${UPSTREAM_REMOTE}' URL does not contain '${EXPECTED_UPSTREAM_FRAGMENT}':"
+  warn "  ${UPSTREAM_URL}"
+fi
+
+# ---------- Fetch ----------
+
+if [[ "${NO_FETCH}" -eq 1 ]]; then
+  dim "Skipping fetch (--no-fetch)."
+else
+  dim "Fetching ${UPSTREAM_REMOTE} (tags + heads) ..."
+  git fetch "${UPSTREAM_REMOTE}" --tags --quiet
+fi
+
+# ---------- Verify pin reachable ----------
+
+if ! git rev-parse --verify --quiet "${PIN_TAG}^{commit}" >/dev/null; then
+  err "Pin tag ${PIN_TAG} is not reachable in this repo."
+  err "Make sure 'git fetch ${UPSTREAM_REMOTE} --tags' has completed."
+  exit 2
+fi
+
+# ---------- Header ----------
+
+echo
+printf '%sUpstream sync check%s — pinned to %s%s%s\n' \
+  "${C_BOLD}" "${C_RESET}" "${C_BOLD}" "${PIN_TAG}" "${C_RESET}"
+dim "Generated: $(date '+%Y-%m-%d %H:%M:%S')"
+
+# ---------- 1. Missed backports on our minor line ----------
+
+section "1. Missed backports on v${PIN_MAJOR}.${PIN_MINOR}.x"
+
+# Collect candidates:
+#   - .patch.N suffix on the exact pin tag (old convention)
+#   - vX.Y.Z / vX.Y.Z-stable with same MAJOR.MINOR and higher PATCH
+MISSED=()
+
+while IFS= read -r tag; do
+  [[ -n "${tag}" ]] && MISSED+=("${tag}")
+done < <(git tag -l "${PIN_TAG}.patch.*" --sort=version:refname)
+
+while IFS= read -r tag; do
+  if [[ "${tag}" =~ ^v([0-9]+)\.([0-9]+)\.([0-9]+)(-stable)?$ ]]; then
+    M="${BASH_REMATCH[1]}"; N="${BASH_REMATCH[2]}"; P="${BASH_REMATCH[3]}"
+    if (( M == PIN_MAJOR && N == PIN_MINOR && P > PIN_PATCH )); then
+      MISSED+=("${tag}")
+    fi
+  fi
+done < <(git tag -l "v${PIN_MAJOR}.${PIN_MINOR}.*" --sort=version:refname \
+         | grep -E '^v[0-9]+\.[0-9]+\.[0-9]+(-stable)?$' || true)
+
+# Plus .patch.N on any newer same-minor stable tag
+while IFS= read -r tag; do
+  if [[ "${tag}" =~ ^v${PIN_MAJOR}\.${PIN_MINOR}\.([0-9]+)-stable\.patch\.[0-9]+$ ]]; then
+    P="${BASH_REMATCH[1]}"
+    if (( P > PIN_PATCH )); then
+      MISSED+=("${tag}")
+    fi
+  fi
+done < <(git tag -l "v${PIN_MAJOR}.${PIN_MINOR}.*-stable.patch.*" --sort=version:refname || true)
+
+if [[ ${#MISSED[@]} -eq 0 ]]; then
+  ok "Nothing new on v${PIN_MAJOR}.${PIN_MINOR}.x."
+else
+  warn "${#MISSED[@]} tag(s) on the pinned line we have not synced:"
+  for tag in "${MISSED[@]}"; do
+    COMMIT_DATE="$(git log -1 --format=%ai "${tag}" 2>/dev/null | head -c 10)"
+    echo "    ${tag}  (${COMMIT_DATE})"
+  done
+  echo
+  dim "  Inspect a tag:   git log ${PIN_TAG}..<tag> --oneline --no-merges"
+fi
+
+# ---------- 2. Newer minor lines stabilized ----------
+
+section "2. Newer minor lines stabilized since pin"
+
+declare -A SEEN_MINOR=()
+NEWER_MINORS=()
+
+while IFS= read -r tag; do
+  if [[ "${tag}" =~ ^v([0-9]+)\.([0-9]+)\.([0-9]+)(-stable)?$ ]]; then
+    M="${BASH_REMATCH[1]}"; N="${BASH_REMATCH[2]}"
+    if (( M == PIN_MAJOR && N > PIN_MINOR )); then
+      KEY="${M}.${N}"
+      if [[ -z "${SEEN_MINOR[$KEY]:-}" ]]; then
+        SEEN_MINOR[$KEY]=1
+        NEWER_MINORS+=("${KEY}")
+      fi
+    fi
+  fi
+done < <(git tag -l "v${PIN_MAJOR}.*" --sort=version:refname \
+         | grep -E '^v[0-9]+\.[0-9]+\.[0-9]+(-stable)?$' || true)
+
+if [[ ${#NEWER_MINORS[@]} -eq 0 ]]; then
+  ok "No newer minor lines published a stable tag."
+else
+  warn "${#NEWER_MINORS[@]} newer minor line(s) available:"
+  for minor in "${NEWER_MINORS[@]}"; do
+    LATEST="$(git tag -l "v${minor}.*" --sort=-version:refname \
+              | grep -E '^v[0-9]+\.[0-9]+\.[0-9]+(-stable)?$' \
+              | head -1)"
+    COMMIT_DATE="$(git log -1 --format=%ai "${LATEST}" 2>/dev/null | head -c 10)"
+    echo "    v${minor}.x  →  latest stable: ${LATEST}  (${COMMIT_DATE})"
+  done
+  echo
+  dim "  Consider quarterly bump per CLAUDE.md → Upstream sync cadence."
+fi
+
+# ---------- 3. upstream/main volume ----------
+
+section "3. Volume on ${UPSTREAM_REMOTE}/main since ${PIN_TAG}"
+
+if ! git rev-parse --verify --quiet "${UPSTREAM_REMOTE}/main" >/dev/null; then
+  warn "${UPSTREAM_REMOTE}/main is not fetched. Re-run without --no-fetch."
+else
+  MAIN_COUNT="$(git rev-list --count "${PIN_TAG}..${UPSTREAM_REMOTE}/main" 2>/dev/null || echo "?")"
+  printf '%sCommits on %s/main not in %s: %s%s%s\n' \
+    "" "${UPSTREAM_REMOTE}" "${PIN_TAG}" "${C_BOLD}" "${MAIN_COUNT}" "${C_RESET}"
+  if [[ "${MAIN_COUNT}" != "?" && "${MAIN_COUNT}" -gt 0 ]]; then
+    echo
+    dim "  Sample (5 most recent, no-merges):"
+    git log --no-merges --format="    %h %s" -5 "${PIN_TAG}..${UPSTREAM_REMOTE}/main"
+  fi
+fi
+
+# ---------- 4. Security-flagged commits ----------
+
+section "4. Security-flagged commits on ${UPSTREAM_REMOTE}/main since ${PIN_TAG}"
+
+if ! git rev-parse --verify --quiet "${UPSTREAM_REMOTE}/main" >/dev/null; then
+  warn "${UPSTREAM_REMOTE}/main not fetched — skipping."
+else
+  SEC_TMP="$(mktemp)"
+  trap 'rm -f "${SEC_TMP}"' EXIT
+  git log --no-merges --format='%h %s' \
+      "${PIN_TAG}..${UPSTREAM_REMOTE}/main" \
+    | grep -iE "${SECURITY_RE}" > "${SEC_TMP}" || true
+
+  COUNT="$(wc -l < "${SEC_TMP}" | tr -d ' ')"
+  if [[ "${COUNT}" -eq 0 ]]; then
+    ok "No security-flagged subjects matched."
+  else
+    warn "${COUNT} security-flagged commit(s) (regex: ${SECURITY_RE}):"
+    sed 's/^/    /' "${SEC_TMP}"
+    echo
+    dim "  Triage per CLAUDE.md → Fork tier classification (Tier C → upstream PR or carry)."
+  fi
+fi
+
+# ---------- Footer ----------
+
+section "Next steps"
+cat <<EOF
+  • Open fix/* branches for must-backport security items.
+  • Re-classify the carried ship-branch delta — drop fixes upstreamed by others.
+  • Quarterly: evaluate whether to bump to a newer minor line.
+
+EOF
+dim "  Read-only check. To act, see CLAUDE.md → Upstream sync cadence."
+echo