diff --git a/README.md b/README.md
index a389425..899a9bb 100644
--- a/README.md
+++ b/README.md
@@ -12,19 +12,28 @@ Rootstock Admin provides a central dashboard for monitoring Rootstock installati
 |-------------|----------|-----|
 | Dev | Dashboard | https://garden-ai-dev--rootstock-admin-dashboard.modal.run |
 | Dev | Manifest API | https://garden-ai-dev--rootstock-admin-manifest.modal.run |
+| Dev | Usage API | https://garden-ai-dev--rootstock-admin-usage.modal.run |
 | Prod | Dashboard | https://garden-ai-prod--rootstock-admin-dashboard.modal.run |
 | Prod | Manifest API | https://garden-ai-prod--rootstock-admin-manifest.modal.run |
+| Prod | Usage API | https://garden-ai-prod--rootstock-admin-usage.modal.run |
 
 ### Manifest API `POST`
 Receives and stores environment manifests from Rootstock clusters. Requires proxy authentication.
 
 **Payload:** JSON manifest containing cluster name, rootstock version, Python version, maintainer info, and environment details.
 
+### Usage API `POST`
+Receives anonymous usage rollups from Rootstock clusters ([rootstock#47](https://github.com/Garden-AI/rootstock/issues/47)). Requires proxy authentication.
+
+**Payload:** `{"cluster": "<name>", "rows": [...]}` where each row is a monthly rollup row as produced by `rootstock usage report` — keyed by `month`/`cluster`/`env`/`checkpoint`/`device`/`client` with `sessions`, `n_calculations`, `duration_s`, and `unique_users` counts. Rollups are stored per cluster per month; a push replaces only the months it contains, so previously reported months persist even if the cluster's local spool has been purged. User hashes never leave the cluster — only derived counts — and any `users` field sent by mistake is dropped on ingest.
+
 ### Dashboard `GET`
 Public dashboard displaying all registered clusters and their environments. Shows:
 - Cluster metadata (name, version, root path, maintainer)
 - Environment status, dependencies, source code, and checkpoints
 
+`GET /usage` returns every stored usage rollup: `{"usage": [{"cluster": ..., "month": ..., "rows": [...]}, ...]}`.
+
 ## Development
 
 ### Prerequisites
diff --git a/modal_app.py b/modal_app.py
index 0797ac0..1a61606 100644
--- a/modal_app.py
+++ b/modal_app.py
@@ -1,6 +1,7 @@
-import modal
 import os
 
+import modal
+
 app = modal.App("rootstock-admin")
 app.image = modal.Image.debian_slim().pip_install("fastapi[standard]")
 
@@ -22,6 +23,52 @@ def manifest(manifest: dict):
     return manifest
 
 
+@app.function(volumes={"/data": vol})
+@modal.fastapi_endpoint(method="POST", requires_proxy_auth=True)
+def usage(payload: dict):
+    """Ingest a cluster's usage rollups: {"cluster": str, "rows": [...]}.
+
+    Rows are the aggregated rollup rows produced by `rootstock usage report`
+    (keyed by month/cluster/env/checkpoint/device/client, with sessions,
+    n_calculations, duration_s, unique_users). Rows are stored per month and
+    a push only replaces the months it contains, so history survives even if
+    a cluster's spool loses old months (e.g. scratch purge policies).
+    """
+    import json
+    import os
+    import re
+    from collections import defaultdict
+
+    from fastapi import HTTPException
+
+    cluster = payload.get("cluster")
+    rows = payload.get("rows")
+    if not isinstance(cluster, str) or not re.fullmatch(r"[A-Za-z0-9_-]+", cluster):
+        raise HTTPException(422, "invalid or missing 'cluster'")
+    if not isinstance(rows, list):
+        raise HTTPException(422, "'rows' must be a list of rollup rows")
+
+    by_month = defaultdict(list)
+    for row in rows:
+        if not isinstance(row, dict) or not re.fullmatch(
+            r"\d{4}-\d{2}", str(row.get("month"))
+        ):
+            raise HTTPException(422, "every row needs a 'month' of the form YYYY-MM")
+        # Only derived counts ever leave a cluster; drop user hashes if a
+        # client ever sends them by mistake.
+        row.pop("users", None)
+        by_month[row["month"]].append(row)
+
+    vol.reload()
+    os.makedirs(f"/data/usage/{cluster}", exist_ok=True)
+    for month, month_rows in by_month.items():
+        with open(f"/data/usage/{cluster}/{month}.json", "w") as f:
+            json.dump({"cluster": cluster, "month": month, "rows": month_rows}, f)
+
+    vol.commit()
+    return {"cluster": cluster, "months": sorted(by_month)}
+
+
 @app.function(volumes={"/data": vol}, min_containers=1)
 @modal.asgi_app()
 def dashboard():
@@ -45,4 +92,16 @@ def get_manifests():
         manifests = [json.loads(p.read_text()) for p in Path("/data").glob("*.json")]
         return {"manifests": manifests}
 
+    @web_app.get("/usage")
+    def get_usage():
+        import json
+        from pathlib import Path
+
+        vol.reload()
+        rollups = [
+            json.loads(p.read_text())
+            for p in sorted(Path("/data/usage").glob("*/*.json"))
+        ]
+        return {"usage": rollups}
+
     return web_app