diff --git a/docs/gpu_plan.md b/docs/gpu_plan.md
index 0741261..b72431b 100644
--- a/docs/gpu_plan.md
+++ b/docs/gpu_plan.md
@@ -17,6 +17,27 @@ the inputs are *tiny*, so the design optimises for zero idle cost, not for a per
 The 27 GB LINCS data is **not** part of this track — nothing big to upload. The only thing worth
 persisting is the model-weights cache (so we don't re-download = re-pay GPU time every run).
 
+## How the model weights persist (the cost-saver)
+
+A `modal.Volume` is a **named, cloud-backed filesystem that lives independently of any container
+or GPU** — it survives every teardown. Mounted into the function at `/weights`:
+
+- **Run 1:** `/weights` is empty → the model downloads weights there (the one-time slow cost).
+- **Run 2+:** the same Volume mounts with the files already present → download skipped → **no
+  GPU-billed seconds wasted re-fetching 5 GB.**
+
+Two things make it actually cache:
+1. **Point the downloader at the mount** (weights only persist if written under `/weights`):
+   `HF_HOME=/weights/hf` (HuggingFace), `TORCH_HOME=/weights/torch`, `boltz --cache /weights/boltz`.
+2. **Commit semantics:** writes persist on `weights.commit()` (modern Modal also auto-commits on a
+   clean exit); other containers see them after `weights.reload()`. Pattern: `reload()` → run →
+   `commit()`.
+
+The Volume itself costs pennies (~$/GB-month of storage), *separate from the GPU* — so caching ~5 GB
+of weights is near-free and saves real GPU time on every subsequent run.
+(Alternative: bake weights into the image at build time via `image.run_function(download)` — fastest
+cold start, but the image rebuilds when weights change. The skeleton uses the Volume approach.)
+
 ## Provider choice
 
 | Option | Billing | Idle cost | "Kill" model | Best for |
diff --git a/gpu/modal_app.py b/gpu/modal_app.py
index 94b5a6f..f735828 100644
--- a/gpu/modal_app.py
+++ b/gpu/modal_app.py
@@ -37,17 +37,37 @@ KNOWN = {
 }
 
 
-@app.function(gpu="L4", image=image, volumes={"/weights": weights}, timeout=3600)
-def cofold(protein_seq: str, ligand_smiles: str, weights_dir: str = "/weights") -> dict:
+# Cache locations on the persistent Volume — the model downloads here ONCE and reuses forever.
+WEIGHTS = "/weights"
+
+
+@app.function(gpu="L4", image=image, volumes={WEIGHTS: weights}, timeout=3600)
+def cofold(protein_seq: str, ligand_smiles: str) -> dict:
     """Co-fold one protein+ligand complex and return predicted affinity + pose (PDB string).
 
-    Runs on the GPU only for this call, then the GPU is released. TODO: replace the stub with the
-    actual Boltz-2 invocation (write the YAML/FASTA input spec, call `boltz predict
-    --use_msa_server --out_dir ... --cache /weights`, parse the predicted structure + affinity).
+    Runs on the GPU only for this call, then the GPU is released. Model weights persist on the
+    mounted Volume across runs (see HF_HOME / --cache below), so we never re-pay GPU time to
+    re-download them.
     """
+    import os
     import subprocess  # noqa: F401  (used once boltz is wired)
 
-    # TODO: build boltz input (protein_seq + ligand_smiles), run, parse pose+affinity.
+    # Point every weight downloader at the persistent Volume so the cache survives teardown.
+    os.environ["HF_HOME"] = f"{WEIGHTS}/hf"          # huggingface_hub cache
+    os.environ["TORCH_HOME"] = f"{WEIGHTS}/torch"    # torch.hub cache
+    boltz_cache = f"{WEIGHTS}/boltz"                  # boltz --cache target
+    os.makedirs(boltz_cache, exist_ok=True)
+
+    # See what's already cached (run 2+ finds weights here and skips the download).
+    weights.reload()
+
+    # TODO: build boltz input (protein_seq + ligand_smiles), then:
+    #   subprocess.run(["boltz", "predict", input_yaml, "--use_msa_server",
+    #                   "--cache", boltz_cache, "--out_dir", "/tmp/out"], check=True)
+    # parse predicted structure + affinity from /tmp/out.
+
+    # Persist anything newly downloaded into the cache so the NEXT run reuses it.
+    weights.commit()
     raise NotImplementedError("Wire Boltz-2 here; see docs/gpu_plan.md Phase 1.")