diff --git a/tools/probe_media_403.py b/tools/probe_media_403.py
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+++ b/tools/probe_media_403.py
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+"""One-off diagnostic: why do some ChatGPT media assets 403?
+
+The API answers a refused download with a bare {"detail":"Forbidden"}, so the
+cause has to be narrowed by experiment. This script runs the experiments that
+distinguish the plausible causes and prints a table.
+
+  Hypothesis A — auth is fine, the asset is the problem.
+      Probe: fetch an asset that DID download alongside one that 403'd. If the
+      good one still 200s in the same session, the session is not at fault.
+
+  Hypothesis B — the asset is scoped to a workspace/account we don't name.
+      The exporter never sends ChatGPT-Account-Id. Resources belonging to a
+      workspace (vs the personal account) can require it.
+      Probe: retry the 403 with each account id the session reports.
+
+  Hypothesis C — only the signed-URL step is restricted.
+      Probe: hit /files/{id} (metadata, no /download) and see if that differs.
+
+  Also, with no API call at all: what KIND of asset are the failures? The
+  exported placeholders record source=user_upload | model_generated, so
+  scanning exports/ classifies the failures for free.
+
+Run from the project root with the venv active:
+
+    python tools/probe_media_403.py
+
+Delete this file once the cause is known.
+"""
+
+import os
+import re
+import sys
+from collections import Counter
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from dotenv import load_dotenv
+
+load_dotenv()
+
+from src.providers.chatgpt import BASE_URL, ChatGPTProvider  # noqa: E402
+
+# The IDs that 403'd in the 2026-08-17 runs.
+FAILED_IDS = [
+    "file_00000000c18471f6a9b8f726c599b15d",
+    "file_00000000667071f694d8aa00b4d42c7b",
+    "file_00000000a53c71f6ba82c859cdfc9659",
+    "file_00000000dccc71f69472e8526e6c7e0c",
+    "file_00000000f34071f6af231eaf1c655f6d",
+    "file_00000000d5e471f6bbbac75623c0cc3d",
+    "file_000000003454722f9481506b96aed510",
+    "file_00000000f550722f990b7c3227f2cb59",
+    "file_0000000094c8722fa508cfe647750137",
+    "file_000000000000722f8c9757d63f45679b",
+    "file_000000007a4081f58297ab692fb625e5",
+    "file_000000002f5081f5bc459887eb6f494f",
+    "file_000000001b3071f59e30d3213f8fcca5",
+    "file_000000009478822faec2289717d9f628",
+    "file_00000000f0bc81f5933009bec614088c",
+    "file_00000000902881f59dca2333091a59cc",
+    "file_0000000017e0820cb269596ec8a79497",
+    "file_00000000762c81f5ac85d9fdcae20061",
+]
+
+ACCOUNT_ENDPOINTS = [
+    f"{BASE_URL}/accounts/check/v4-2023-04-27",
+    f"{BASE_URL}/accounts/check",
+]
+
+# "> 🖼️ **Image attached** — `sediment://file_x` (model_generated, image/png, …)"
+PLACEHOLDER_RE = re.compile(
+    r"\*\*(?:Image|File) attached\*\* — `([^`]+)`\s*\(([^)]*)\)"
+)
+
+
+def classify_from_exports(export_dir: Path) -> None:
+    """Offline: what kind of asset were the failures, and where did they live?"""
+    print("=" * 78)
+    print("A. What the exports already say (no API calls)")
+    print("=" * 78)
+    if not export_dir.is_dir():
+        print(f"  exports dir not found: {export_dir} — skipping\n")
+        return
+
+    wanted = set(FAILED_IDS)
+    hits: dict[str, list[tuple[str, str]]] = {}
+    all_sources: Counter = Counter()
+    failed_sources: Counter = Counter()
+
+    for md in export_dir.rglob("*.md"):
+        try:
+            text = md.read_text(encoding="utf-8", errors="replace")
+        except OSError:
+            continue
+        for ref, meta in PLACEHOLDER_RE.findall(text):
+            source = meta.split(",")[0].strip()
+            all_sources[source] += 1
+            for fid in wanted:
+                if fid in ref:
+                    hits.setdefault(fid, []).append((source, str(md.relative_to(export_dir))))
+                    failed_sources[source] += 1
+
+    print(f"  placeholders still unresolved across exports: {sum(all_sources.values())}")
+    print(f"    by source: {dict(all_sources)}")
+    print(f"  of those, matching a known 403 ID: {sum(failed_sources.values())}")
+    print(f"    by source: {dict(failed_sources)}")
+    print()
+    for fid, places in sorted(hits.items()):
+        source, path = places[0]
+        print(f"  {fid[:28]}…  source={source:<16} {path}")
+    if not hits:
+        print("  (no matches — exports may live elsewhere; set EXPORT_DIR)")
+    print()
+
+
+def find_good_ids(export_dir: Path, limit: int = 3) -> list[str]:
+    """IDs that downloaded successfully — media/ files are named by file ID."""
+    good = []
+    for media_file in export_dir.rglob("media/*"):
+        if media_file.is_file() and media_file.stem.startswith("file"):
+            good.append(media_file.stem)
+            if len(good) >= limit:
+                break
+    return good
+
+
+def get_account_ids(provider) -> list[str]:
+    print("=" * 78)
+    print("B. Account / workspace IDs this session reports")
+    print("=" * 78)
+    ids: list[str] = []
+    for url in ACCOUNT_ENDPOINTS:
+        try:
+            resp = provider._session.request("GET", url, timeout=30)
+        except Exception as e:  # noqa: BLE001 - diagnostic
+            print(f"  {url} → error {e}")
+            continue
+        print(f"  {url} → {resp.status_code}")
+        if resp.status_code != 200:
+            continue
+        try:
+            data = resp.json()
+        except Exception:  # noqa: BLE001 - diagnostic
+            continue
+        accounts = data.get("accounts") if isinstance(data, dict) else None
+        if isinstance(accounts, dict):
+            for key, value in accounts.items():
+                acct = (value or {}).get("account", {}) if isinstance(value, dict) else {}
+                acct_id = acct.get("account_id")
+                plan = acct.get("structure") or acct.get("plan_type")
+                if acct_id:
+                    ids.append(acct_id)
+                    print(f"      key={key!r:<28} account_id={acct_id} ({plan})")
+        if ids:
+            break
+    if not ids:
+        print("  (none discovered — hypothesis B untestable)")
+    print()
+    return ids
+
+
+def probe(provider, file_id: str, account_ids: list[str]) -> None:
+    def call(url: str, headers: dict | None = None) -> str:
+        try:
+            resp = provider._session.request("GET", url, headers=headers, timeout=30)
+        except Exception as e:  # noqa: BLE001 - diagnostic
+            return f"error {type(e).__name__}"
+        detail = ""
+        if resp.status_code != 200:
+            try:
+                detail = f" {resp.json().get('detail', '')}"
+            except Exception:  # noqa: BLE001 - diagnostic
+                detail = f" {resp.text[:60]}"
+        return f"{resp.status_code}{detail}"
+
+    dl = f"{BASE_URL}/files/{file_id}/download"
+    meta = f"{BASE_URL}/files/{file_id}"
+
+    print(f"  {file_id}")
+    print(f"      /download                    → {call(dl)}")
+    print(f"      /files/{{id}} (metadata)       → {call(meta)}")
+    for acct in account_ids:
+        header = {"ChatGPT-Account-Id": acct}
+        print(f"      /download + Account-Id       → {call(dl, header)}  [{acct[:8]}…]")
+
+
+def main() -> None:
+    export_dir = Path(os.getenv("EXPORT_DIR", "./exports")).expanduser()
+    classify_from_exports(export_dir)
+
+    provider = ChatGPTProvider()
+    account_ids = get_account_ids(provider)
+
+    print("=" * 78)
+    print("C. Live probes")
+    print("=" * 78)
+
+    good_ids = find_good_ids(export_dir)
+    print("-- assets that downloaded fine (control group) --")
+    if not good_ids:
+        print("  (none found under exports/**/media — control group unavailable)")
+    for fid in good_ids:
+        probe(provider, fid, account_ids)
+
+    print()
+    print("-- assets that 403'd --")
+    for fid in FAILED_IDS[:4]:
+        probe(provider, fid, account_ids)
+
+
+if __name__ == "__main__":
+    main()