serializable: Flow PT field type reference

Data type serializable, probed on Task.splits (stock, editable) and Project.tracking_settings (stock, editable, read only here: a site-wide configuration object). The schema declares no structure: properties holds default_value (null) and summary_default. On the probed site, a scan of all 114 entity types found six serializable fields and no others.

field	editable	on a project-scoped entity
`Task.splits`	yes	yes
`Task.split_durations`	no	yes
`Project.tracking_settings`	yes	it is the project
`EventLogEntry.meta`	no	no
`SavedFilter.filters`	yes	no
`RvLicense.meta`	yes	no

Read A decoded JSON object or array under attributes, never a string holding JSON, never under relationships. One decode, not two.

GET /entity/projects/<id>?fields=tracking_settings -> 200
{"data":{"type":"Project","attributes":{"tracking_settings":{"navchains":{"Asset":"Asset.sg_asset_type",
 "Shot":"Shot.sg_sequence","Cut":"Cut.entity","CutItem":"CutItem.cut.entity"}}},
 "relationships":{},"id":<id>,"links":{"self":"/api/v1/entity/projects/<id>"}}}

EventLogEntry.meta, a deeper stored value; python types str, int, bool, NoneType, list:
{"type": "attribute_change", "attribute_name": "sg_versions", "entity_type": "Task",
 "entity_id": <id>, "in_create": true, "field_data_type": "multi_entity", "platform_id": null,
 "added": [{"name": "charA.jpg", "type": "Version", "id": <id>, "status": "rev", "uuid": "<uuid>"}],
 "removed": []}

Write PUT /entity/tasks/<id>, Content-Type: application/json. Task.splits answers a well-formed write with 200 and stores null.

sent	status	stored
`[]`, `[{}]`, `[{"foo": "bar"}]`	200	`null`
`[{"start_date": "2026-01-01", "end_date": "2026-01-02", "duration": 480}]`	200	`null`
`[{"note": "café 日本語 ✓"}]`, `[{"clé": 1}]`	200	`null`
two or more hashes in the array	400 code 104, `undefined method '<' for nil`	n/a
`{"a": {"b": [1,2,3]}}`	400 code 104, `no implicit conversion of String into Integer`	n/a
`[1,2,3]`, `[1,"two",null,true]`, 12 levels of `[[…]]`	400 code 104, same message	n/a
`42`, `2.5`, `true`, `"hello"`, `'[{"a": 1}]'`	400 code 103, wrong class	n/a

code 103, source {} — the type layer, before the field sees the value:
 "API update() Task.splits expected [Hash,\n ActiveSupport::HashWithIndifferentAccess,\n
  ActionDispatch::Http::Parameters,\n ActionDispatch::Http::ParamsHashWithIndifferentAccess,\n
  Array,\n NilClass] data type(s) but got Integer: 42"

code 104, source null — the field's setter, on a value of an accepted class:
 "Update failed for [Task.splits]: no implicit conversion of String into Integer"
 "Update failed for [Task.splits]: undefined method '<' for nil"

Key order, numeric type and empty-container fidelity stay unproven on this site: a structure of an int, a float, true, false, null, a nested list, {}, [], a unicode value and an empty string was answered 200 and stored nothing.

Size One hash holding one long string, as [{"note": "x" * n}]. No request ceiling found; none of it is stored, so this measures the request path and not the column.

body	result
1026 bytes	200 in 0.4s
100026 bytes	200 in 0.4s
1000026 bytes	200 in 0.7s
5000026 bytes	200 in 1.0s
7312 bytes, 100 well-formed split hashes	400 code 104, `undefined method '<' for nil`

Clear

sent	result
`null`	200, reads back `null`
`{}`	200, reads back `null`
`[]`	200, reads back `null`
`""`	400 code 103, `... Array,\n NilClass] data type(s) but got String: ""`
never written	`null`

There is no cleared-versus-empty distinction to read back, and on Task.splits no written state either.

Filter The type cannot appear in a filter. A bogus relation lists no Valid relations; there is no list.

[["splits", "definitely_not_an_operator", null]] -> 400
 {"status": 400, "code": 103,
  "title": "API read() Task.splits's 'serializable' data type cannot be used in a filter.",
  "source": {"Task.splits": " data type cannot be used in a filter. Value: {\"path\" => \"splits\",
             \"relation\" => \"definitely_not_an_operator\", \"values\" => [nil]}"}}

filter	result
`is null`, `is_not null`, `is {}`, `contains "x"`, `in [[]]`	400, the same title
the same on `Project.tracking_settings`	400, the same title

Traps

A client cannot query into a blob. No operator means no is None to rank fill rate by (probe 007) and no way to find the rows holding a marker; _summarize (probe 020) has nothing to group on. Fetch rows on some other filter and inspect the decoded value client-side.
Task.splits answers 200 and stores nothing. A client that writes state there and re-reads it cannot tell a discarded write from an empty field. Read back every serializable write.
Two failure layers, two error shapes. A wrong Ruby class gives code 103 with the accepted-class list and an empty source; a right class the setter chokes on gives code 104 with a raw Ruby message and source: null. Neither names the key at fault.
The column takes arbitrary JSON; the field refuses it. EventLogEntry.meta holds nested objects, lists, booleans and nulls, so the storage is a blob. Of the four editable fields three are outside a project: Project.tracking_settings is site configuration, SavedFilter.filters and RvLicense.meta have no project field. A project-scoped blob to write does not exist here.