diff --git a/crawlers/oshwhub/crawler.py b/crawlers/oshwhub/crawler.py
index 9e87a64..9921910 100644
--- a/crawlers/oshwhub/crawler.py
+++ b/crawlers/oshwhub/crawler.py
@@ -1246,13 +1246,16 @@ def main(argv: list[str] | None = None) -> int:
     # --backfill-source: standalone path that scans existing project dirs and
     # only fetches source. No listing/HTML/attachment work.
     if args.backfill_source:
-        return _run_backfill_source(args.out, only_uuids=args.uuids)
+        return _run_backfill_source(
+            args.out, only_uuids=args.uuids, concurrency=args.concurrency
+        )
     if args.backfill_pro_source:
         return _run_backfill_pro_source(
             args.out,
             only_uuids=args.uuids,
             cookie_path=args.pro_cookie,
             max_source_mb=args.max_source_mb,
+            concurrency=args.concurrency,
         )
 
     with make_client() as client:
@@ -1341,49 +1344,128 @@ def main(argv: list[str] | None = None) -> int:
     return 0
 
 
-def _run_backfill_source(out_root: Path, only_uuids: str | None = None) -> int:
-    """Walk existing per-project dirs in out_root and fetch source.json into each.
+def _run_backfill_concurrent(
+    targets: list[Path],
+    fetch_one,
+    *,
+    concurrency: int,
+    label: str,
+    size_unit: str = "MB",
+) -> None:
+    """Shared worker dispatch for std/pro backfill.
 
-    Updates metadata.json in-place to add source_format / source_documents / editor_version.
+    `fetch_one(proj_dir, meta) -> (status, src_meta_or_msg)` where status is one of:
+      "ok"          — `src_meta` is the fetch_*_source dict
+      "oversize"    — Pro-only; ProjectOversizeError already recorded by caller
+      "fail"        — `src_meta_or_msg` is the human-readable error string
+
+    Threads share fetch_one's captured client (httpx.Client is sync-thread-safe).
+    Print + per-target metadata writes are independent (each project owns its
+    metadata.json), so no lock needed there. Only stdout ordering is serialized.
     """
+    print_lock = threading.Lock()
+
+    def _do_one(idx: int, proj_dir: Path) -> None:
+        uuid = proj_dir.name
+        meta_path = proj_dir / "metadata.json"
+        try:
+            meta = json.loads(meta_path.read_text(encoding="utf-8"))
+        except Exception as e:  # noqa: BLE001
+            with print_lock:
+                print(f"[{idx}/{len(targets)}] meta read FAIL {uuid}: {e}", file=sys.stderr)
+            return
+        title = meta.get("title", "?")
+        status, payload = fetch_one(proj_dir, meta)
+        with print_lock:
+            head = f"[{idx}/{len(targets)}] {uuid}"
+            if status == "ok":
+                src_meta = payload
+                meta["source_format"] = src_meta["source_format"]
+                meta["source_path"] = src_meta["source_path"]
+                meta["source_documents"] = src_meta["source_documents"]
+                if src_meta.get("editor_version"):
+                    meta["editor_version"] = src_meta["editor_version"]
+                meta_path.write_text(
+                    json.dumps(meta, ensure_ascii=False, indent=2), encoding="utf-8"
+                )
+                docs = src_meta["source_documents"]
+                total = sum(d["size"] for d in docs)
+                if size_unit == "KB":
+                    sz = f"{total / 1024:.1f} KB"
+                else:
+                    sz = f"{total / 1024 / 1024:.1f} MB"
+                print(
+                    f"{head} OK ({title[:30]}): {len(docs)} docs, {sz}, "
+                    f"editor={src_meta.get('editor_version')}"
+                )
+            elif status == "oversize":
+                print(f"{head} SKIPPED oversize ({title[:30]}): {payload}", file=sys.stderr)
+            else:
+                print(f"{head} FAIL ({title[:30]}): {payload}", file=sys.stderr)
+
+    print(f"Backfill {label} for {len(targets)} projects (concurrency={concurrency})")
+    if concurrency <= 1:
+        for i, d in enumerate(targets, 1):
+            _do_one(i, d)
+    else:
+        from concurrent.futures import ThreadPoolExecutor, as_completed
+        with ThreadPoolExecutor(max_workers=concurrency) as pool:
+            futs = [pool.submit(_do_one, i, d) for i, d in enumerate(targets, 1)]
+            for f in as_completed(futs):
+                f.result()
+
+
+def _discover_backfill_targets(
+    out_root: Path,
+    only_uuids: str | None,
+    *,
+    require_origin_pro: bool = False,
+) -> list[Path]:
+    """Walk out_root for per-project dirs that have metadata.json and match
+    the filter. If `only_uuids` is given, that's the authoritative whitelist;
+    otherwise (Pro only) we fall back to filtering by metadata.raw_fields.origin."""
     wanted: set[str] | None = set(only_uuids.split(",")) if only_uuids else None
     targets: list[Path] = []
     for d in sorted(out_root.iterdir()):
-        if not d.is_dir():
+        if not d.is_dir() or not (d / "metadata.json").exists():
             continue
-        meta_path = d / "metadata.json"
-        if not meta_path.exists():
-            continue
-        if wanted and d.name not in wanted:
-            continue
-        targets.append(d)
-
-    print(f"Backfill source for {len(targets)} projects under {out_root}")
-    src_client = make_source_client()
-    try:
-        for i, proj_dir in enumerate(targets, 1):
-            uuid = proj_dir.name
-            meta_path = proj_dir / "metadata.json"
-            meta = json.loads(meta_path.read_text(encoding="utf-8"))
-            print(f"[{i}/{len(targets)}] {uuid}  ({meta.get('title', '?')})")
-            try:
-                src_meta = fetch_std_source(src_client, uuid, proj_dir)
-            except Exception as e:  # noqa: BLE001
-                print(f"  FAIL: {e}", file=sys.stderr)
+        if wanted is not None:
+            if d.name not in wanted:
                 continue
-            meta["source_format"] = src_meta["source_format"]
-            meta["source_path"] = src_meta["source_path"]
-            meta["source_documents"] = src_meta["source_documents"]
-            if src_meta.get("editor_version"):
-                meta["editor_version"] = src_meta["editor_version"]
-            meta_path.write_text(
-                json.dumps(meta, ensure_ascii=False, indent=2), encoding="utf-8"
-            )
-            total = sum(d["size"] for d in src_meta["source_documents"])
-            print(
-                f"  OK: {len(src_meta['source_documents'])} docs, "
-                f"{total / 1024:.1f} KB, editor={src_meta.get('editor_version')}"
-            )
+        elif require_origin_pro:
+            try:
+                m = json.loads((d / "metadata.json").read_text(encoding="utf-8"))
+            except Exception:  # noqa: BLE001
+                continue
+            if (m.get("raw_fields") or {}).get("origin") != "pro":
+                continue
+        targets.append(d)
+    return targets
+
+
+def _run_backfill_source(
+    out_root: Path,
+    only_uuids: str | None = None,
+    concurrency: int = 1,
+) -> int:
+    """Walk per-project dirs and fetch Std source.json into each.
+
+    Updates metadata.json in-place to add source_format / source_documents /
+    editor_version.
+    """
+    targets = _discover_backfill_targets(out_root, only_uuids)
+    src_client = make_source_client()
+
+    def fetch(proj_dir: Path, meta: dict):
+        try:
+            return ("ok", fetch_std_source(src_client, proj_dir.name, proj_dir))
+        except Exception as e:  # noqa: BLE001
+            return ("fail", str(e))
+
+    try:
+        _run_backfill_concurrent(
+            targets, fetch, concurrency=concurrency, label="std source", size_unit="KB"
+        )
     finally:
         src_client.close()
     return 0
@@ -1394,65 +1476,37 @@ def _run_backfill_pro_source(
     only_uuids: str | None = None,
     cookie_path: str = PRO_COOKIE_PATH_DEFAULT,
     max_source_mb: int | None = None,
+    concurrency: int = 1,
 ) -> int:
-    """Walk per-project dirs in out_root, fetch Pro source for origin=pro projects.
-
-    A project is considered Pro by either: existing metadata.json marks
-    raw_fields.origin == 'pro', OR --uuids was passed and includes this UUID
-    (caller is asserting Pro).
-    """
-    wanted: set[str] | None = set(only_uuids.split(",")) if only_uuids else None
-    targets: list[Path] = []
-    for d in sorted(out_root.iterdir()):
-        if not d.is_dir():
-            continue
-        meta_path = d / "metadata.json"
-        if not meta_path.exists():
-            continue
-        if wanted is not None:
-            if d.name not in wanted:
-                continue
-        else:
-            try:
-                m = json.loads(meta_path.read_text(encoding="utf-8"))
-            except Exception:  # noqa: BLE001
-                continue
-            if (m.get("raw_fields") or {}).get("origin") != "pro":
-                continue
-        targets.append(d)
-
-    print(f"Backfill pro source for {len(targets)} projects under {out_root}")
+    """Walk per-project dirs and fetch Pro source into each (modern + legacy)."""
+    targets = _discover_backfill_targets(
+        out_root, only_uuids, require_origin_pro=(only_uuids is None)
+    )
     pro_client = make_pro_source_client(cookie_path=cookie_path)
-    try:
-        for i, proj_dir in enumerate(targets, 1):
-            uuid = proj_dir.name
-            meta_path = proj_dir / "metadata.json"
-            meta = json.loads(meta_path.read_text(encoding="utf-8"))
-            print(f"[{i}/{len(targets)}] {uuid}  ({meta.get('title', '?')})")
-            try:
-                src_meta = fetch_pro_source(
+    # Multiple threads may trip oversize concurrently; serialize the state-file
+    # append so lines don't interleave.
+    oversize_lock = threading.Lock()
+
+    def fetch(proj_dir: Path, meta: dict):
+        uuid = proj_dir.name
+        try:
+            return (
+                "ok",
+                fetch_pro_source(
                     pro_client, uuid, proj_dir, max_source_mb=max_source_mb
-                )
-            except ProjectOversizeError as e:
-                print(f"  SKIPPED (oversize): {e}", file=sys.stderr)
+                ),
+            )
+        except ProjectOversizeError as e:
+            with oversize_lock:
                 _record_oversize(out_root, uuid, e)
-                continue
-            except Exception as e:  # noqa: BLE001
-                print(f"  FAIL: {e}", file=sys.stderr)
-                continue
-            meta["source_format"] = src_meta["source_format"]
-            meta["source_path"] = src_meta["source_path"]
-            meta["source_documents"] = src_meta["source_documents"]
-            if src_meta.get("editor_version"):
-                meta["editor_version"] = src_meta["editor_version"]
-            meta_path.write_text(
-                json.dumps(meta, ensure_ascii=False, indent=2), encoding="utf-8"
-            )
-            total = sum(d["size"] for d in src_meta["source_documents"])
-            print(
-                f"  OK: {len(src_meta['source_documents'])} docs, "
-                f"{total / 1024 / 1024:.1f} MB plain, editor={src_meta.get('editor_version')}"
-            )
+            return ("oversize", str(e))
+        except Exception as e:  # noqa: BLE001
+            return ("fail", str(e))
+
+    try:
+        _run_backfill_concurrent(
+            targets, fetch, concurrency=concurrency, label="pro source"
+        )
     finally:
         pro_client.close()
     return 0