diff --git a/lexicons/space.notjack.thunderstorm.catalogBaseline.json b/lexicons/space.notjack.thunderstorm.catalogBaseline.json
new file mode 100644
index 0000000..f0df3f1
--- /dev/null
+++ b/lexicons/space.notjack.thunderstorm.catalogBaseline.json
@@ -0,0 +1,26 @@
+{
+  "lexicon": 1,
+  "id": "space.notjack.thunderstorm.catalogBaseline",
+  "defs": {
+    "main": {
+      "type": "record",
+      "description": "A marker that a particular package catalog snapshot was chosen to be a baseline snapshot, where Thunderstorm recomputes the package catalog index from scratch.",
+      "key": "tid",
+      "record": {
+        "type": "object",
+        "required": ["snapshot", "packageCount"],
+        "properties": {
+          "snapshot": {
+            "type": "string",
+            "format": "at-uri",
+            "description": "The snapshot this baseline is for."
+          },
+          "packageCount": {
+            "type": "integer",
+            "description": "The total number of packages in this baseline's catalog snapshot."
+          }
+        }
+      }
+    }
+  }
+}
diff --git a/lexicons/space.notjack.thunderstorm.catalogDelta.json b/lexicons/space.notjack.thunderstorm.catalogDelta.json
index 2430961..417b9e6 100644
--- a/lexicons/space.notjack.thunderstorm.catalogDelta.json
+++ b/lexicons/space.notjack.thunderstorm.catalogDelta.json
@@ -8,7 +8,7 @@
       "key": "tid",
       "record": {
         "type": "object",
-        "required": ["preceding", "result", "packageChanges"],
+        "required": ["preceding", "result", "packageChanges", "packageCountDelta"],
         "properties": {
           "preceding": {
             "type": "string",
@@ -20,6 +20,10 @@
             "format": "at-uri",
             "description": "A reference to the space.notjack.thunderstorm.catalogSnapshot record representing the state of the package catalog after this delta's changes occurred. The result snapshot's `.previous` field MUST point to the same record as `preceding` for this delta record to be considered valid."
           },
+          "packageCountDelta": {
+            "type": "integer",
+            "description": "The change in the number of packages in the catalog as a result of this delta's changes. This should be equal to the number of packages in .packageChanges that have a null contentsBefore field (newly-created packages) minus the number that have a null contentsAfter field (deleted packages)."
+          },
           "packageChanges": {
             "type": "array",
             "items": {
diff --git a/lexicons/space.notjack.thunderstorm.catalogSnapshot.json b/lexicons/space.notjack.thunderstorm.catalogSnapshot.json
index e750c14..c892a3c 100644
--- a/lexicons/space.notjack.thunderstorm.catalogSnapshot.json
+++ b/lexicons/space.notjack.thunderstorm.catalogSnapshot.json
@@ -25,10 +25,6 @@
             "format": "at-uri",
             "description": "Must reference another space.notjack.thunderstorm.catalogSnapshot record: the one for the snapshot immediately following this one, under the same strict adjacency rule as previous, and likewise a plain at-uri. Absent for the newest snapshot, and on either side of a gap."
           },
-          "packageCount": {
-            "type": "integer",
-            "description": "Packages listed in the upstream catalog, which may exceed the number Thunderstorm has ingested. Optional because whatever creates this record need not have fetched the package list; a later service fills it in."
-          },
           "publishedAt": {
             "type": "string",
             "format": "datetime",
diff --git a/services/racksnaps-baseliner/README.md b/services/racksnaps-baseliner/README.md
new file mode 100644
index 0000000..d347d42
--- /dev/null
+++ b/services/racksnaps-baseliner/README.md
@@ -0,0 +1,17 @@
+# Racksnaps Baseliner
+
+This service is responsible for nominating certain catalog snapshots as "baselines" and creating individual package snapshot records for every package in every catalog baseline.
+
+## Manifest
+
+This service is allowed to interact with the following data sources:
+
+- https://racksnaps.defn.io/ (read-only)
+- at://{THUNDERSTORM_DID}/space.notjack.thunderstorm.catalogSnapshot (read-only)
+- at://{THUNDERSTORM_DID}/space.notjack.thunderstorm.catalogBaseline (read, create)
+- at://{THUNDERSTORM_DID}/space.notjack.thunderstorm.packageSnapshot (read, create)
+
+It can be configured via the following environment variables:
+
+- `THUNDERSTORM_DID`: the DID of the central Thunderstorm account
+- `THUNDERSTORM_DID_APP_PASSWORD_FILE_PATH`: an absolute path to a file containing an app password that the service can use to publish writes under the Thunderstorm account