diff --git a/.github/workflows/mirror.yml b/.github/workflows/mirror.yml
new file mode 100644
index 0000000..4f682a7
--- /dev/null
+++ b/.github/workflows/mirror.yml
@@ -0,0 +1,52 @@
+# Mirrors a release's engine and models to dl.godwit.email/plugins/startlux/ and
+# prints its registry assets. Manual only: without "upload" it downloads, checks
+# and prints, writing nothing to the bucket. Needs the R2_* secrets to upload.
+name: Mirror assets
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: Release tag, e.g. v0.1.0
+        required: true
+      upload:
+        description: Upload to the bucket (otherwise a dry run)
+        type: boolean
+        default: false
+
+permissions:
+  contents: read
+
+jobs:
+  mirror:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: refs/tags/${{ inputs.tag }}
+      # The script from this branch: a tag made before it existed can still be mirrored.
+      - uses: actions/checkout@v4
+        with:
+          path: current
+          sparse-checkout: scripts/mirror.py
+          sparse-checkout-cone-mode: false
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+      - name: Mirror
+        env:
+          AWS_ACCESS_KEY_ID: ${{ inputs.upload && secrets.R2_ACCESS_KEY_ID || '' }}
+          AWS_SECRET_ACCESS_KEY: ${{ inputs.upload && secrets.R2_SECRET_ACCESS_KEY || '' }}
+          AWS_DEFAULT_REGION: auto
+          R2: https://9c578ed9cad837fb4a2564db7b36b483.r2.cloudflarestorage.com
+        run: |
+          cp current/scripts/mirror.py scripts/mirror.py
+          python scripts/mirror.py "$RUNNER_TEMP/mirror" ${{ inputs.upload && '--upload' || '' }}
+      - name: Assets
+        run: |
+          { echo '```json'; cat "$RUNNER_TEMP/mirror/assets.json"; echo '```'; } >> "$GITHUB_STEP_SUMMARY"
+      - uses: actions/upload-artifact@v4
+        with:
+          name: assets-${{ inputs.tag }}
+          path: ${{ runner.temp }}/mirror/assets.json
diff --git a/scripts/mirror.py b/scripts/mirror.py
new file mode 100755
index 0000000..39996ff
--- /dev/null
+++ b/scripts/mirror.py
@@ -0,0 +1,122 @@
+#!/usr/bin/env python3
+"""Mirror the engine and models to dl.godwit.email and print the registry assets.
+
+    mirror.py WORK_DIR [--upload]
+
+scripts/engine.py lays out the pinned llama.cpp build (third_party/llama/VERSION)
+per platform, as Godwit stores the engine asset. The models come from startlux-models on Hugging Face at
+pinned revisions and must have the hashes below. Without --upload nothing is
+written to the bucket. With it, each file goes to s3://godwit-downloads/plugins/
+startlux/... once: a path that already holds other bytes fails the run, since a
+published release's files never change. WORK_DIR/assets.json is the release's
+"assets" for registry.json.
+"""
+import argparse
+import hashlib
+import json
+import os
+from pathlib import Path
+import subprocess
+import sys
+import urllib.request
+
+BUCKET = "godwit-downloads"
+PUBLIC = "https://dl.godwit.email"
+PLATFORMS = ["darwin-arm64", "darwin-amd64", "linux-amd64", "linux-arm64", "windows-amd64", "windows-arm64"]
+GPU = ["darwin-arm64"]  # only Apple silicon has a GPU back end; elsewhere only 2B is offered (CPU)
+
+HF = "https://huggingface.co/startlux-models/StartLux-Decision-v1.1-"
+MODELS = {
+    "4b-q4": {
+        "model.gguf": (HF + "4B-Instant-GGUF/resolve/a9ab4199fdff586f86e3baf8c9557f6c5ca0fdc0/StartLux-Decision-v1.1-4B-Instant-Q4_K_M.gguf",
+                       "5fcfe9771db996a868f78215c08955c21cf5e8188609e17e9c653183d946e84d"),
+        "decision_config.json": (HF + "4B-Instant/resolve/416088f77cfce09633906a3675c7cd2e5dabcf10/decision_config.json",
+                                 "9ca014791b5c71996c212c7aed6275405c4d9b682a6b6eba325a3ab17bcccc08"),
+    },
+    "2b-q8": {
+        "model.gguf": (HF + "2B-Instant-GGUF/resolve/afb95dbba67e648f4e1b5ad3f770a91f799f9496/StartLux-Decision-v1.1-2B-Instant-Q8_0.gguf",
+                       "170cf86482a3340c7bd2000f921aa482820a992c14587f496f43fc57009f9529"),
+        "decision_config.json": (HF + "2B-Instant/resolve/f1e6cd147dda82c9ffa3e8e725d5c1f175566fb1/decision_config.json",
+                                 "3bd3c0c3854b18be967c7a660d38b8340aa2b4c14648d8ce0260cd32f6e89c1a"),
+    },
+}
+# Physical memory each needs, measured (docs/parity.md): Windows commits the
+# 4K-token compute buffer that macOS and Linux only reserve.
+MEMORY = {"4b-q4": {"default": 8}, "2b-q8": {"default": 6, "windows-*": 12}}
+
+
+def sha256(path):
+    h = hashlib.sha256()
+    with open(path, "rb") as f:
+        for chunk in iter(lambda: f.read(1 << 20), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def download(url, dest, want):
+    if not (dest.exists() and sha256(dest) == want):
+        dest.parent.mkdir(parents=True, exist_ok=True)
+        with urllib.request.urlopen(url, timeout=600) as r, open(dest, "wb") as f:
+            while chunk := r.read(1 << 20):
+                f.write(chunk)
+    if (got := sha256(dest)) != want:
+        sys.exit(f"{url}: sha256 {got}, not {want}")
+
+
+def aws(*args, check=True):
+    return subprocess.run(["aws", *args, "--endpoint-url", os.environ["R2"]], check=check, capture_output=True, text=True)
+
+
+def publish(local, key, digest, upload):
+    """The registry file for local at key: uploaded once, never replaced."""
+    if upload:
+        head = aws("s3api", "head-object", "--bucket", BUCKET, "--key", key, check=False)
+        if head.returncode == 0:
+            have = json.loads(head.stdout).get("Metadata", {}).get("sha256")
+            if have != digest:
+                sys.exit(f"{key} already holds other bytes ({have}); a published file never changes")
+            print(f"{key}: already there")
+        else:
+            aws("s3", "cp", str(local), f"s3://{BUCKET}/{key}", "--metadata", f"sha256={digest}",
+                "--content-type", "application/octet-stream", "--cache-control", "public, max-age=31536000, immutable")
+            print(f"{key}: uploaded")
+    return {"name": local.name, "url": f"{PUBLIC}/{key}", "sha256": digest, "size": local.stat().st_size}
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("work", type=Path)
+    ap.add_argument("--upload", action="store_true")
+    args = ap.parse_args()
+    if args.upload and not os.environ.get("R2"):
+        sys.exit("--upload needs R2 (the bucket's endpoint) and the AWS_* credentials")
+    root = Path(__file__).resolve().parent.parent
+    version = (root / "third_party" / "llama" / "VERSION").read_text().strip()
+
+    assets = []
+    for platform in PLATFORMS:
+        out = args.work / "engine" / platform
+        if not out.exists():
+            subprocess.run([sys.executable, str(root / "scripts" / "engine.py"), str(out), "--platform", platform], check=True)
+        files = [publish(p, f"plugins/startlux/engine/{version}/{platform}/{p.name}", sha256(p), args.upload)
+                 for p in sorted(out.iterdir())]
+        assets.append({"name": "engine", "platforms": [platform], "variants": [{"id": version, "files": files}]})
+
+    variants = {}
+    for variant, files in MODELS.items():
+        published = []
+        for name, (url, want) in files.items():
+            local = args.work / "models" / variant / name
+            download(url, local, want)
+            published.append(publish(local, f"plugins/startlux/models/v1.1/{variant}/{name}", want, args.upload))
+        variants[variant] = {"id": variant, "minMemoryGB": MEMORY[variant], "files": published}
+    cpu = [p for p in PLATFORMS if p not in GPU]
+    assets.append({"name": "model", "platforms": GPU, "variants": [variants["4b-q4"], variants["2b-q8"]]})
+    assets.append({"name": "model", "platforms": cpu, "variants": [variants["2b-q8"]]})
+
+    (args.work / "assets.json").write_text(json.dumps(assets, indent=2) + "\n")
+    print(f"assets: {args.work / 'assets.json'}")
+
+
+if __name__ == "__main__":
+    main()