diff --git a/docs/architecture/14-api-contract/openapi.yaml b/docs/architecture/14-api-contract/openapi.yaml
index 25b370b9..5d6007f3 100644
--- a/docs/architecture/14-api-contract/openapi.yaml
+++ b/docs/architecture/14-api-contract/openapi.yaml
@@ -1574,8 +1574,11 @@ components:
         Machine reason a run is paused; the client draws the phrase. **TWO values since the
         order-form minor (0.11.0), and the distinction is load-bearing because the remedies are
         OPPOSITE:** `credit_exhausted` — the ACCOUNT has no money, the remedy is to top up;
-        `run_limit_reached` — the account is fine and the RUN delivered everything that was ordered,
-        the remedy is to order more. Reading the second as the first sends a paying user to a
+        `run_limit_reached` — the account is fine and the RUN spent the order it was
+        sold, the remedy is to order more. ⚠ **The mechanism is the run's own ceiling, set flush
+        against its hold — NOT a proof that everything ordered was delivered:** an order whose
+        projection ran low reaches this state before the last chapter, and the honest reading is
+        "this run has spent what it was given", not "your order is complete". Reading the second as the first sends a paying user to a
         payment page they do not need.
 
         A client MUST tolerate an unknown one under a minor bump and MUST render a paused run whose
@@ -1807,8 +1810,13 @@ components:
             `resume` clears nothing — nothing was withdrawn.
         ordered_chapters:
           type: integer
-          minimum: 1
+          minimum: 0
           description: |
+            ⚠ **`0` is legal and is not "nothing was ordered":** a re-pass buys no chapters, and an
+            order phrased in CHARACTERS spans no whole chapter — both report `0` here while being
+            perfectly ordinary runs. A client that treats this as at-least-one will reject valid
+            answers.
+
             What this run was ORDERED to deliver, in chapters — a property of the RUN. Present so a
             reloaded screen can name what the user bought and read `progress` against it.
 
@@ -2535,20 +2543,45 @@ components:
             run does not stop for it and uses the bank as it stands. Nothing on this surface marks a
             book as having been translated against an unsigned bank: what is observable is the bank
             itself — `BankPage.signed` against `BankPage.total`.
-        ceiling_chapters:
+        chapters:
           type: integer
           minimum: 1
           description: |
-            Limit of THIS run, in chapters, within the bounds from
-            `GET /books/{bookId}/run-options`.
+            Order this many chapters, counted from the first not yet delivered. Bounds come from
+            `GET /books/{bookId}/run-options` (`order.chapters_left`, `order.affordable_chapters`).
 
-            Required for an ordinary run, and a run started without a declared limit would spend
-            past the boundary the user is entitled to set BEFORE it begins rather than learn about
-            after. `0` is not legal.
+            ⚠ **OPTIONAL, and its absence is not "no limit" — it is the order of the WHOLE BOOK**,
+            which is the ratified default. `0` is not legal: an order that buys nothing is not an
+            order.
 
-            ⚠ **Absent — and refused — when `re_pass` is true:** a re-pass does not buy chapters, so
-            a chapter limit would describe nothing. The two members are mutually exclusive, and a
-            request carrying both is `400`.
+            Mutually exclusive with `characters`; a request carrying both is `400`.
+        characters:
+          type: integer
+          minimum: 1
+          description: |
+            Order by size instead of by chapters, for a book whose chapter boundaries this build
+            does not trust — `order.chapter_orders: false` says when that is the case.
+
+            ⚠ **Such an order stops INSIDE a chapter**, so the run it starts reports
+            `delivered_chapters: null` rather than `0`: it closed no whole chapter, and `0` would
+            read as "nothing happened" when work was done and paid for.
+
+            Mutually exclusive with `chapters`; a request carrying both is `400`.
+        ceiling_chapters:
+          deprecated: true
+          type: integer
+          description: |
+            ⛔ **RETIRED BY THE ORDER-FORM MINOR (0.11.0) AND REFUSED ON THE WAY IN — `400` with a
+            pointer to `/chapters`.** It is NOT silently reinterpreted: the old name said "limit"
+            when the figure is a PURCHASE, and a client that kept sending it would be buying
+            something it did not mean.
+
+            ⚠ Named here only so a reader of a 0.10.0 client can find out what became of it. It is
+            not accepted, and it is not the same field as `chapters` — one bounded a run, the other
+            orders work.
+
+            ⚠ **A re-pass (`re_pass: true`) carries NEITHER volume member:** it does not buy
+            chapters, so a volume would describe nothing.
 
     RunOptions:
       type: object