fix(docs): the schema-extension FAQ rotted in reverse — pin the claim in the checked block this time

#3890 taught that FieldSchema/ObjectSchema/ActionSchema are ZodPipes and that
FieldSchema.extend throws. True when written; protocol 17 (#3855, acbf364)
then retired the aliases whose lowering was the whole reason for the
field/object transforms, the pipes collapsed to plain ZodObjects, and both
prose claims inverted within days: .extend works, and the recommended
FieldSchema.in is now undefined — following the FAQ was once again the only
way to hit an error. Only ActionSchema (requiresFeature -> visible lowering,
still live per the ledger) remains a pipe. All verified against a clean build:

    FieldSchema   bound ZodObject  .extend: function   .in: undefined
    ObjectSchema  bound ZodObject  .extend: function   .in: undefined
    ActionSchema  bound ZodPipe    .extend: undefined  .in: ZodObject

The example gate never noticed because #3890's checked block used only
.parse() — deliberately shape-agnostic after CI rejected the .in.extend
attempt. That made the code durable and left the PROSE as the only
load-bearing surface, which is exactly where the rot settled.

The rewrite moves the claim into the checked block: it calls
FieldSchema.extend({ ... }) directly, so a future shape flip fails CI loudly
instead of the prose going quietly wrong. Composition stays as the
shape-agnostic default; ActionSchema is the documented pipe case; and the FAQ
teaches the one-line probe (typeof SomeSchema.extend === 'function') instead
of a table of shapes that history says will not stay true.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ajwvrmd1hDC9RBofYBhGuR

Author: claude

.changeset/docs-extend-faq-reversed-rot.md

@@ -0,0 +1,27 @@
+---
+"@objectstack/spec": patch
+---
+
+fix(docs): the schema-extension FAQ rotted in reverse — `.extend()` works on `FieldSchema` again since protocol 17
+
+The #3890 fix taught that `FieldSchema` / `ObjectSchema` / `ActionSchema` are all
+`ZodPipe`s and that `FieldSchema.extend` throws. True when written; protocol 17
+(#3855) then retired the deprecated aliases whose lowering was the whole reason
+for the field/object transforms, the pipes collapsed to plain `ZodObject`s, and
+both prose claims inverted within days: `.extend` works, and the recommended
+`FieldSchema.in` is now `undefined` — following the FAQ was once again the only
+way to hit an error. Only `ActionSchema` (whose `requiresFeature` → `visible`
+lowering is still live) remains a pipe.
+
+The example gate never noticed because the checked block used only `.parse()` —
+deliberately shape-agnostic after CI rejected the first #3890 attempt. That made
+the code durable and left the PROSE as the only load-bearing surface, which is
+where the rot settled.
+
+So the rewrite moves the claim into the checked block: it now calls
+`FieldSchema.extend({ … })` directly, so if the schema ever grows a transform
+again the gate goes red instead of the prose going quietly wrong. Composition
+stays as the shape-agnostic default, `ActionSchema` is documented as the pipe
+case with the `.in.extend` caveat, and the FAQ teaches the one-line probe
+(`typeof SomeSchema.extend === 'function'`) instead of a table of shapes that
+history says will not stay true.

content/docs/deployment/troubleshooting.mdx

@@ -364,34 +364,44 @@ Yes. All schemas work with plain JavaScript. You lose compile-time type checking
 
 ### How do I extend a built-in schema?
 
-**Not with `.extend()` on the schema itself.** Several built-in schemas —
-`FieldSchema`, `ObjectSchema`, `ActionSchema` — carry a `.transform()` that lowers
-author-facing sugar at parse time, which makes the exported value a **`ZodPipe`,
-not a `ZodObject`**. `FieldSchema.extend` is `undefined`, so calling it throws
-`is not a function`.
-
-Compose instead: parse with the built-in schema, and validate your additions
-alongside it. This keeps the transform running, which `.extend()` would discard.
+It depends on whether the schema you are extending carries a parse-time
+`.transform()`. A plain schema (`FieldSchema`, `ObjectSchema` since protocol 17
+retired their alias-lowering transforms, #3855) is a `ZodObject` and takes
+`.extend()` directly. The composition form works in **either** case, and is the
+safe default when you don't want to track which is which:
 
 {/* os:check */}
 ```typescript
 import { z } from 'zod';
 import { FieldSchema } from '@objectstack/spec/data';
 
-const CustomProps = z.object({ customProperty: z.string().optional() });
+// Plain object schema: .extend() works directly. (This line is what pins the
+// claim — if FieldSchema ever grows a transform again and becomes a ZodPipe,
+// .extend vanishes and this example fails CI instead of the prose going stale.)
+const CustomFieldSchema = FieldSchema.extend({
+  customProperty: z.string().optional(),
+});
+CustomFieldSchema.parse({ name: 'code', type: 'text', customProperty: 'x' });
 
+// Composition: parse with the built-in schema, validate additions alongside.
+// Shape-agnostic — works for plain schemas AND transform-carrying pipes.
+const CustomProps = z.object({ customProperty: z.string().optional() });
 function parseCustomField(input: unknown) {
   return { ...FieldSchema.parse(input), ...CustomProps.parse(input) };
 }
-
 parseCustomField({ name: 'code', type: 'text', customProperty: 'x' });
 ```
 
-If you genuinely need one merged schema object, `FieldSchema.in` is the
-`ZodObject` the pipe wraps, so `FieldSchema.in.extend({ … })` builds one — but
-the result **skips the transform**, so author-facing sugar the pipe would have
-lowered stays raw. Prefer the composition above unless you specifically want the
-untransformed shape.
+A schema that still lowers author-facing sugar at parse time — `ActionSchema`
+(its `requiresFeature` → `visible` lowering) is one, as of protocol 17 — is a
+**`ZodPipe`**, where `.extend` does not exist: prefer the composition form.
+Reaching the inner object via `.in.extend({ … })` builds a merged schema but
+**skips the transform**, so the sugar the pipe would have lowered stays raw.
+
+A quick check when unsure: `typeof SomeSchema.extend === 'function'` — a pipe
+reports `undefined`. This page once asserted the shapes the other way around;
+the schemas moved under it within days (#3890 → #3855), which is why the
+load-bearing claim above now lives in a CI-checked block rather than prose.
 
 ### Where are the JSON Schemas for IDE autocomplete?