@@ -181,28 +181,40 @@ substitute. See the [Data API](/docs/api/data-api).
 ### `MISSING_REQUIRED_FIELD`
 **Cause:** A required field was not provided in the request body.  
 **Fix:** Include the missing field. Check `fields` for the field name — **except** on the
-`controlled_by_parent` stored-master path below, where the status is `422` and no `fields`
+`controlled_by_parent` stored required-master path below, where the status is `422` and no `fields`
 array is sent; read the field name out of the message there.  
 **Retry:** `no_retry`
 
 <Callout type="warn">
 **`MISSING_REQUIRED_FIELD` is `400` with one exception: a `controlled_by_parent` detail
-whose stored master reference is null answers `422`, without `fields`.**
+whose stored reference to a required master is null answers `422`, without `fields`.**
 
 An object whose `sharingModel` is `controlled_by_parent` derives its access from a master
 record, so the gate authorizing writes to it resolves that master *before* the executor —
 and the executor is where required-field validation runs. An update or delete **by id** of
-a detail record whose *stored* master reference is null answers
-`422 MISSING_REQUIRED_FIELD`, whatever the declaration: the caller sent no such field, so
-nothing could be named in `fields` and no payload would fix it.
+a detail record whose *stored* master reference is null, where the master is declared
+`required: true`, answers `422 MISSING_REQUIRED_FIELD`, whether or not the master is marked
+`readonly` or `system`: the caller sent no such field, so nothing could be named in `fields`
+and no payload would fix it.
+
+An optional master never answers this code. When the object's only `master_detail` is
+declared `required: false` (an optional master, ADR-0055 §1 as amended), a stored empty
+reference is not an error: the row belongs to no master and is its owner's. Its owner and a
+holder of View All on the detail read it; its owner and a holder of Modify All on the detail
+update or delete it. Everyone else who reaches the gate, View All included, is refused
+`403 PERMISSION_DENIED`; a principal who cannot read the row meets the read door first
+(`404 RECORD_NOT_FOUND`), as for every hidden row.
 
 A request that leaves the master reference out, or empties it, does not answer this code.
-On a `master_detail` that is `required` and neither `readonly` nor `system`, validation
-answers it with the documented `400 VALIDATION_FAILED` and `fields`. A master reference
-marked `readonly` or `system` is not one an author's write supplies, so it cannot be the
-master (ADR-0055 §1): `ObjectSchema` refuses that declaration at parse
-(`master-not-writable`), and on an object that reaches the runtime without the parse, every
-write that sets the reference answers `422 INVALID_METADATA`:
+On a `master_detail` that is `required: true` and neither `readonly` nor `system`, validation
+answers it with the documented `400 VALIDATION_FAILED` and `fields`. On an optional master
+an insert that leaves it out is not refused, and its creator owns the row; an update that
+empties it is judged by the master it leaves, asking edit access on it
+(`403 PERMISSION_DENIED` without it). A master reference marked `readonly` or `system`,
+required or optional, is not one an author's write supplies, so it cannot be the master
+(ADR-0055 §1): `ObjectSchema` refuses that declaration at parse (`master-not-writable`), and
+on an object that reaches the runtime without the parse, every write that sets the
+reference answers `422 INVALID_METADATA`:
 
 - every insert, single or batch, whether or not it names a master;
 - every update whose body carries the reference key, whatever the value, `null` included.
@@ -211,19 +223,24 @@ Its message starts `[Security] Configuration defect:` and carries the parse's ow
 naming the object, the field, the flag and the fix. A delete, and an update that leaves the
 key out, are answered as on any detail.
 
-A `master_detail` with no `required`, and a `required` `lookup` on an object that declares no
-`master_detail`, are not master references at all: the master is exactly one `master_detail`
-declared `required: true` (ADR-0055 §1). `ObjectSchema` refuses such an object at parse, and
-one that reaches the runtime unparsed answers every write `422 INVALID_METADATA`.
+The master is exactly one `master_detail`: the one declared `required: true`, or, when the
+object declares none, its only `master_detail` declared `required: false` (ADR-0055 §1 as
+amended). Two `master_detail` fields with none required, two required ones, and a
+`required` `lookup` on an object that declares no `master_detail` name no master at all:
+`ObjectSchema` refuses such an object at parse, and one that reaches the runtime unparsed
+answers every write `422 INVALID_METADATA`. Write `required` on the master either way:
+`ObjectSchema.create()` forces an omitted one to `true` and `os lint` reports the omission
+at `error`, while a raw parse reads it as `false`, the optional master.
 
 The full matrix is on the protocol page:
 [`MISSING_REQUIRED_FIELD`](/docs/protocol/kernel/error-handling#missing_required_field).
 
 The `INVALID_METADATA` refusal is not going to be relaxed: required-field validation skips
 `system` and `readonly` fields before its required check, so on those two declarations the
-gate is the only thing standing between the request and a detail record outside its master
-— which the `controlled_by_parent` read filter (`fk IN (readable masters)`) can never match,
-leaving a record readable by nobody.
+gate is the only thing standing between the request and, under a required master, a detail
+record outside its master — which the read filter of a required master
+(`fk IN (readable masters)`, with no arm for an empty reference) can never match, leaving a
+record readable by nobody.
 
 **Branch on `code`, treat `fields` as optional, and read the status off the response**
 rather than deriving it from the table at the end of this page.
@@ -1011,7 +1028,7 @@ async function handleApiCall() {
 | 404 | `not_found` | `RECORD_NOT_FOUND`, `OBJECT_NOT_FOUND`, `ENDPOINT_NOT_FOUND` |
 | 409 | `conflict` | `CONCURRENT_MODIFICATION`, `UNIQUE_VIOLATION`, `DELETE_RESTRICTED` |
 | 413 | `validation` | `PAYLOAD_TOO_LARGE` — an upload over the storage settings' `max_upload_mb`, or an import over its row ceiling (see [above](#payload_too_large)) |
-| 422 | `validation` | `MISSING_REQUIRED_FIELD` on a `controlled_by_parent` detail whose stored master reference is null (see [above](#missing_required_field)) — this row is an exception to the 400 row, not a second home for the code |
+| 422 | `validation` | `MISSING_REQUIRED_FIELD` on a `controlled_by_parent` detail whose stored reference to a required master is null; an optional master's empty reference is not an error (see [above](#missing_required_field)) — this row is an exception to the 400 row, not a second home for the code |
 | 429 | `rate_limit` | `RATE_LIMIT_EXCEEDED`, `QUOTA_EXCEEDED` |
 | 500 | `server` | `INTERNAL_ERROR`, `DATABASE_ERROR` |
 | 501 | `server` | `NOT_IMPLEMENTED` — the storage routes answer it when the adapter cannot issue presigned URLs |
 
@@ -2139,7 +2139,7 @@ can produce, because both supply a live one.
 | Types with no repository write path (`agent`) | Their write path records no history and would force a draft live — a half-write is worse than leaving the row to the read path |
 | Rows that still fail the current schema after conversion | That is a genuine contract violation, not chain-owned history. The write path's rejection is correct; fix the row in Studio |
 | A site the conversion chain leaves as stored because no lossless rewrite exists — above all a page filter carrying `$and` / `$or` / `$not` | Flattening a combinator changes which rows the page selects, so it is never done. Each site is printed as a `TODO` line under its row — path, block, and what blocks the rewrite — whatever the row's outcome; a row with nothing but TODOs is reported `skipped`. It does not fail the run, since no run of this pass can clear it: rewrite each site by hand |
-| A stored shape protocol 18 refuses and no conversion rewrites — a `controlled_by_parent` object without exactly one required `master_detail` (two masters, none, a required `lookup` only, an explicit `required: false`), a view bound to the retired view-level API data source or a page component whose `data` prop binds it (`object-grid`, `object-map`, `object-gantt`, `object-tree`), a flow value slot carrying a `{…}` token (a `get_record` / `update_record` / `delete_record` filter operand among them) | Each would need a choice only its owner can make — which master, which page, what an absent value should do — so it is never rewritten. Each site is printed as a `TODO` line under its row, naming the step-18 entry that refuses it, the object, view, page component or flow node, and the owner's choices; a binding's request headers are withheld. Until it is rewritten by hand, the row is refused: an object registers but derives no master, a view registers under `[metadata_spec_invalid]`, a page still loads, its next save is refused at the metadata save door (422), and `os validate`, `os build` and `os lint` refuse the binding in source, and a flow is not registered |
+| A stored shape protocol 18 refuses and no conversion rewrites — a `controlled_by_parent` object without exactly one master, which is one `master_detail` declared `required: true` or, with none, its only `master_detail` declared `required: false` (two masters, none, a required `lookup` only; an explicit `required: false` on the only `master_detail` is that optional master, admitted by `cbp-optional-master-admitted` and not listed), a view bound to the retired view-level API data source or a page component whose `data` prop binds it (`object-grid`, `object-map`, `object-gantt`, `object-tree`), a flow value slot carrying a `{…}` token (a `get_record` / `update_record` / `delete_record` filter operand among them) | Each would need a choice only its owner can make — which master, which page, what an absent value should do — so it is never rewritten. Each site is printed as a `TODO` line under its row, naming the step-18 entry that refuses it, the object, view, page component or flow node, and the owner's choices; a binding's request headers are withheld. Until it is rewritten by hand, the row is refused: an object registers but derives no master, a view registers under `[metadata_spec_invalid]`, a page still loads, its next save is refused at the metadata save door (422), and `os validate`, `os build` and `os lint` refuse the binding in source, and a flow is not registered |
 | Any other site `registerFlow` refuses in a stored flow the parse accepts — a text slot's template hole, a decision's `mode`, a malformed predicate, a plugin node's undeclared config key, an `api` flow with no secret | The flow's own definition is wrong, and no conversion rewrites it. Each site is printed as a `TODO registerFlow` line under its row, in the words `registerFlow` refuses it with; a site already listed under a step-18 entry is not repeated. Until it is fixed, the flow is not registered at boot and never runs. A flow the parse itself refuses is reported among the rows that could not be rewritten, in the parse's words |
 
 **One thing it lists and never writes: decision nodes that changed meaning at
@@ -2706,10 +2706,11 @@ os verify --dashboard-windows                  # Also prove each dashboard date
    reported inconclusive, never proven.
    With `--parent-write-reach`, another fresh boot proves ADR-0055's write rule
    for every object declared `controlled_by_parent` (exactly one
-   `master_detail` field names its master): editing, deleting or creating a
-   detail is admitted exactly when the caller holds that verb on the detail's
-   object **and** can edit its master. The first half is read from the
-   server's effective permissions for the caller (`/auth/me/permissions`); the
+   `master_detail` field names its master, required or optional): editing,
+   deleting or creating a detail is admitted exactly when the caller holds
+   that verb on the detail's object **and** can edit its master. The first
+   half is read from the server's effective permissions for the caller
+   (`/auth/me/permissions`); the
    second is measured, by an empty update of the master as the caller, which
    runs every write gate the master has — for a master that is itself
    controlled by its parent, the gate above it too. The writes go through the
@@ -2726,9 +2727,15 @@ os verify --dashboard-windows                  # Also prove each dashboard date
    answer disagrees fails the run, and so does a persona that could not be
    provisioned; a refusal that is not the platform's `403 PERMISSION_DENIED`
    naming the detail and the operation (a rule, a hook's own code, a refusal
-   on another object) is reported inconclusive, never proven. An object
-   with two `master_detail` fields, none, or a chain at the write gate's depth
-   bound is reported `excluded`. A detail two levels below its root master (a
+   on another object) is reported inconclusive, never proven. For an optional
+   master (its only `master_detail` declared `required: false`) the run also
+   proves the records that name no master: such a record is its owner's, so
+   its edit or delete is admitted exactly when the caller holds that verb on
+   the detail's object **and** owns the record (`owner_id`) or holds Modify All
+   on that object, and a create that names no master exactly when the caller
+   holds create on it; a null-master record the persona cannot read is not
+   probed. An object with two `master_detail` fields, none, or a chain at the
+   write gate's depth bound is reported `excluded`. A detail two levels below its root master (a
    line under a quote under an account) is answered the same way: its write is
    admitted exactly when the caller holds the verb on the detail and may update
    the middle master directly, which also needs edit access to the root, and