chore(app-server): document experimental API opt-in (#10667)

Add a section on how to opt in to the experimental API.

Author: owenlin0

codex-rs/app-server/README.md

@@ -15,7 +15,7 @@
 - [Skills](#skills)
 - [Apps](#apps)
 - [Auth endpoints](#auth-endpoints)
-- [Adding an experimental field](#adding-an-experimental-field)
+- [Experimental API Opt-in](#experimental-api-opt-in)
 
 ## Protocol
 
@@ -827,7 +827,67 @@ Field notes:
 - `windowDurationMins` is the quota window length.
 - `resetsAt` is a Unix timestamp (seconds) for the next reset.
 
-## Adding an experimental field
+## Experimental API Opt-in
+
+Some app-server methods and fields are intentionally gated behind an experimental capability with no backwards-compatible guarantees. This lets clients choose between:
+
+- Stable surface only (default): no opt-in, no experimental methods/fields exposed.
+- Experimental surface: opt in during `initialize`.
+
+### Generating stable vs experimental client schemas
+
+`codex app-server` schema generation defaults to the stable API surface (experimental fields and methods filtered out). Pass `--experimental` to include experimental methods/fields in generated TypeScript or JSON schema:
+
+```bash
+# Stable-only output (default)
+codex app-server generate-ts --out DIR
+codex app-server generate-json-schema --out DIR
+
+# Include experimental API surface
+codex app-server generate-ts --out DIR --experimental
+codex app-server generate-json-schema --out DIR --experimental
+```
+
+### How clients opt in at runtime
+
+Set `capabilities.experimentalApi` to `true` in your single `initialize` request:
+
+```json
+{
+  "method": "initialize",
+  "id": 1,
+  "params": {
+    "clientInfo": {
+      "name": "my_client",
+      "title": "My Client",
+      "version": "0.1.0"
+    },
+    "capabilities": {
+      "experimentalApi": true
+    }
+  }
+}
+```
+
+Then send the standard `initialized` notification and proceed normally.
+
+Notes:
+
+- If `capabilities` is omitted, `experimentalApi` is treated as `false`.
+- This setting is negotiated once at initialization time for the process lifetime (re-initializing is rejected with `"Already initialized"`).
+
+### What happens without opt-in
+
+If a request uses an experimental method or sets an experimental field without opting in, app-server rejects it with a JSON-RPC error. The message is:
+
+`<descriptor> requires experimentalApi capability`
+
+Examples of descriptor strings:
+
+- `mock/experimentalMethod` (method-level gate)
+- `thread/start.mockExperimentalField` (field-level gate)
+
+### For maintainers: Adding experimental fields and methods
 Use this checklist when introducing a field/method that should only be available when the client opts into experimental APIs.
 
 At runtime, clients must send `initialize` with `capabilities.experimentalApi = true` to use experimental methods or fields.