diff --git a/scripts/stop_a_calibration.py b/scripts/stop_a_calibration.py
new file mode 100644
index 0000000..ad27489
--- /dev/null
+++ b/scripts/stop_a_calibration.py
@@ -0,0 +1,128 @@
+#!/usr/bin/env python3
+"""Stop-A calibration: print the offered-L-C-A convergence curve for a raw trace window.
+
+The convergence of prefix-vs-full L-C-A is a deterministic property of the trace
+metadata (lengths, hash_ids, arrivals), so this runs on CPU without serving the
+model. Use it to pick tau / tau_c / stable_checks and to compare how late the C
+dimension converges across workloads (e.g. low-reuse chat vs high-reuse coder).
+
+Example:
+  PYTHONPATH=src python3 scripts/stop_a_calibration.py \
+    --jsonl /dashscope/.../qwen_chat_blksz_64_032109-032111.jsonl \
+    --block-size 64 --window-start 3600 --window-end 4200 --gpu-count 8 --label chat
+"""
+from __future__ import annotations
+
+import argparse
+import json
+from pathlib import Path
+
+from aituner.lca import find_convergence_prefix, resolve_length_mode
+from aituner.trace import TraceRequest, WindowRecord
+
+
+def _session_root(row: dict, root_of: dict) -> object:
+    chat_id = row.get("chat_id")
+    parent = row.get("parent_chat_id")
+    parent_is_root = parent is None or (
+        isinstance(parent, (int, float)) and not isinstance(parent, bool) and int(parent) < 0
+    )
+    root = chat_id if parent_is_root else root_of.get(parent, parent)
+    if chat_id is not None:
+        root_of[chat_id] = root
+    return root
+
+
+def load_window(jsonl: Path, *, window_start: float, window_end: float) -> list[TraceRequest]:
+    root_of: dict = {}
+    requests: list[TraceRequest] = []
+    with jsonl.open(encoding="utf-8") as handle:
+        for line in handle:
+            line = line.strip()
+            if not line:
+                continue
+            row = json.loads(line)
+            _session_root(row, root_of)  # keep chain complete even outside the window
+            ts = float(row.get("timestamp") or 0.0)
+            if not (window_start <= ts < window_end):
+                continue
+            hash_ids = row.get("hash_ids")
+            requests.append(
+                TraceRequest(
+                    row_id=str(row.get("chat_id")),
+                    arrival_s=ts - window_start,
+                    sampling_u=1.0,
+                    body={},
+                    prompt_tokens_hint=int(row.get("input_length") or 0),
+                    completion_tokens_hint=int(row.get("output_length") or 0),
+                    metadata={"hash_ids": hash_ids if isinstance(hash_ids, list) else None},
+                )
+            )
+    requests.sort(key=lambda item: item.arrival_s)
+    return requests
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--jsonl", type=Path, required=True)
+    ap.add_argument("--block-size", type=int, required=True)
+    ap.add_argument("--window-start", type=float, default=3600.0)
+    ap.add_argument("--window-end", type=float, default=4200.0)
+    ap.add_argument("--gpu-count", type=int, default=8)
+    ap.add_argument("--length-mode", default="total")
+    ap.add_argument("--label", default="")
+    ap.add_argument("--tau", type=float, default=0.9)
+    ap.add_argument("--max-checks", type=int, default=20)
+    args = ap.parse_args()
+
+    requests = load_window(
+        args.jsonl, window_start=args.window_start, window_end=args.window_end
+    )
+    window = WindowRecord(
+        window_id=args.label or args.jsonl.stem,
+        trace_path=args.jsonl,
+        trace_type=args.label or "chat",
+        window_start=0.0,
+        window_end=float(args.window_end - args.window_start),
+        source_payload={"block_size": args.block_size},
+    )
+    mode = resolve_length_mode(length_mode=args.length_mode)
+    rows_with_hash = sum(1 for r in requests if r.metadata.get("hash_ids"))
+    print(
+        f"[{args.label}] requests={len(requests)} rows_with_hash_ids={rows_with_hash} "
+        f"window={args.window_start:.0f}-{args.window_end:.0f}s block_size={args.block_size}"
+    )
+
+    # Full curve (tau_c high so it never short-circuits; we read the curve directly).
+    point = find_convergence_prefix(
+        requests, window, gpu_count=args.gpu_count, length_mode=mode,
+        tau=args.tau, tau_c=1.01, stable_checks=10_000, max_checks=args.max_checks,
+        min_fraction=0.05,
+    )
+    print("  frac   time_s     L      C      A")
+    for c in point.checks:
+        s = c["family_similarity"]
+        print(
+            f"  {c['fraction']:.2f}  {c['time_s']:7.1f}  "
+            f"{s['L']:.3f}  {s['C']:.3f}  {s['A']:.3f}"
+        )
+
+    # Stop fraction at candidate tau_c values (L,A >= tau, C >= tau_c, stable for W=3).
+    print("  -- stop fraction (tau_L=tau_A=%.2f, W=3) --" % args.tau)
+    for tau_c in (0.85, 0.90, 0.92, 0.95):
+        p = find_convergence_prefix(
+            requests, window, gpu_count=args.gpu_count, length_mode=mode,
+            tau=args.tau, tau_c=tau_c, stable_checks=3, max_checks=args.max_checks,
+            min_fraction=0.05,
+        )
+        verdict = (
+            f"stop@frac={p.fraction:.2f} t={p.stop_time_s:.0f}s"
+            if p.converged
+            else "NEVER (replays full window)"
+        )
+        print(f"  tau_c={tau_c:.2f}: {verdict}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())