diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 72915fbc3..4e2fbce92 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -21350,6 +21350,9 @@ components:
           type: string
           minLength: 1
           maxLength: 60
+          description: Apple's `HKUnit` string for `value`. The server converts the reading into the unit it stores the identifier
+            in when the two differ and the factor is known (`km` to `m`, `lb` to `kg`, `degF` to `degC`, and their
+            siblings); a unit the server cannot place leaves the reading untouched.
         startDate:
           type: string
           format: date-time
diff --git a/src/lib/openapi/routes/measurements.ts b/src/lib/openapi/routes/measurements.ts
index ff655f406..756618059 100644
--- a/src/lib/openapi/routes/measurements.ts
+++ b/src/lib/openapi/routes/measurements.ts
@@ -53,7 +53,13 @@ const batchEntrySchema = z
       .describe(
         "v1.19.2 (iOS #34 extension) — the bucket's MAXIMUM bpm for a 10-minute heart-rate bucket (see `externalId`). Persisted ONLY on a well-formed `stats:HKQuantityTypeIdentifierHeartRate:<bucket-start>` row; ignored (stored null) on every other entry. Omit on a pre-v1.19.2 client — the bucket keeps the avg-only contract.",
       ),
-    unit: z.string().min(1).max(60),
+    unit: z
+      .string()
+      .min(1)
+      .max(60)
+      .describe(
+        "Apple's `HKUnit` string for `value`. The server converts the reading into the unit it stores the identifier in when the two differ and the factor is known (`km` to `m`, `lb` to `kg`, `degF` to `degC`, and their siblings); a unit the server cannot place leaves the reading untouched.",
+      ),
     startDate: z.iso.datetime({ offset: true }),
     endDate: z.iso.datetime({ offset: true }),
     sleepStage: z