diff --git a/README.md b/README.md
index bc92e5f..8c578fc 100644
--- a/README.md
+++ b/README.md
@@ -148,7 +148,9 @@ scripts/check-repo-compliance.sh /path/to/sim-repo
 (`displayName`, `lineage`, `upstream`, framework, deployed URL, physics topics, `status`, etc.).
 Schema: [`structure/repos.schema.json`](structure/repos.schema.json) (validated by
 [`scripts/check-repos-catalog.sh`](scripts/check-repos-catalog.sh)). The compliance workflow,
-Pages landing page, and the catalog scripts consume this file. See
+Pages landing page, and the catalog scripts consume this file. Ground-truth upstream references
+for sims live in the sibling [`Baseline`](https://github.com/OpenPhysics/Baseline) repo (also
+listed here as `type: tool`). See
 [`scripts/README.md`](scripts/README.md) for the tooling:
 
 ```bash
diff --git a/structure/repos.json b/structure/repos.json
index dedfede..982cfb2 100644
--- a/structure/repos.json
+++ b/structure/repos.json
@@ -38,6 +38,25 @@
       "screens": null,
       "status": "active"
     },
+    {
+      "name": "Baseline",
+      "displayName": "Baseline",
+      "type": "tool",
+      "isSimulation": false,
+      "lineage": null,
+      "upstream": null,
+      "language": [
+        "Shell",
+        "JSON",
+        "Markdown"
+      ],
+      "framework": null,
+      "description": "Immutable upstream baselines / ground truth for OpenPhysics sims: pinned git clones (via fetch-baselines.sh) and committed non-git snapshots (NAAP dumps, SPINS, etc.).",
+      "deployedUrl": null,
+      "physicsTopics": [],
+      "screens": null,
+      "status": "active"
+    },
     {
       "name": "Baton",
       "displayName": "Baton",