## Description
This PR adds a new `historyMode = "legacy" | "paginated"` to `Thread`.
This will be stored in `SessionMeta` in the JSONL rollout file and as a
new column in the SQLite thread_metadata table, and exposed on
`thread/start` and on the `Thread` object in app-server.
## What changed
- Added canonical `ThreadHistoryMode` with `legacy` and `paginated`,
defaulting old and new SessionMeta to `legacy`.
- Carried `history_mode` through core session config, ThreadStore stored
metadata, local/in-memory stores, rollout metadata extraction, and the
existing SQLite `threads` table.
- Added experimental `historyMode` to app-server v2 `Thread` and
`thread/start`.
- Made paginated stored threads metadata-discoverable but unsupported
for legacy full-history reads, `load_history`, live resume, and create
paths.
- Regenerated app-server schema fixtures and added
protocol/state/thread-store/app-server coverage for persistence and
fail-closed behavior.
## Compatibility floor
Because users may be running various versions of Codex binaries on the
same machine (TUI, Codex App, etc.), we will need to establish a
compatibility floor for upcoming paginated threads, which will change
how thread storage reads and writes work.
The overall plan here:
```
Release N:
- Add historyMode to SessionMeta / Thread / SQLite metadata.
- Teach binaries to understand paginated threads.
- If a binary sees `historyMode="paginated"` but does not support the paginated contract, it refuses to resume/mutate the thread.
- Default remains `"legacy"`.
Release N+1:
- First-party clients start opting into paginated threads where appropriate.
- Internal dogfood / staged rollout.
- Measure old-client usage and paginated-thread unsupported errors.
Release N+2:
- Only after Release N+ is overwhelmingly deployed, make paginated the default.
- Accept that a small tail of N-1-or-older binaries may not understand paginated threads.
```
The important behavior change is fail-closed handling for a binary that
encounters a persisted `paginated` thread before it knows how to fully
support paginated history. In app-server, if a thread is `paginated`, we
will:
- allow metadata-only discovery paths like `thread/list` and
`thread/read(includeTurns=false)`, so clients can still see the thread
and inspect its `historyMode`
- reject legacy full-history/live-thread paths like
`thread/read(includeTurns=true)` and `thread/resume` with an unsupported
JSON-RPC error
- avoid silently treating an unknown or future `historyMode` as `legacy`
Under the hood, the ThreadStore layer also rejects legacy operations
that would need to load or replay the full thread history for a
paginated thread. That gives us the behavior we want for Release N:
future paginated threads are visible, but this binary fails closed
instead of trying to operate on them as if they were legacy threads.
## Why
The goal of this PR is to align on app-server and `ThreadStore` API
updates for paginating through large threads.
#### app-server
##### `thread/turns/list`
- Updates `thread/turns/list` to support `itemsView?: "notLoaded" |
"summary" | "full" | null`, defaulting to `summary`.
- Implements the current `thread/turns/list` behavior over the existing
persisted rollout-history fallback:
- `notLoaded` returns turn envelopes with empty `items`.
- `summary` returns the first user message and final assistant message
when available.
- `full` preserves the existing full item behavior.
Note that this method still uses the naive approach of loading the
entire rollout file, and returns just the filtered slice of the data.
Real pagination will come later by leveraging SQLite.
##### `thread/turns/items/list`
- Adds the experimental `thread/turns/items/list` protocol, schema,
dispatcher, and processor stub. The app-server currently returns
JSON-RPC `-32601` with `thread/turns/items/list is not supported yet`.
#### ThreadStore
- Adds the experimental `thread/turns/items/list` protocol, schema,
dispatcher, and processor stub. The app-server currently returns
JSON-RPC `-32601` with `thread/turns/items/list is not supported yet`.
- Adds `ThreadStore` contract types and stubbed methods for listing
thread turns and listing items within a turn.
- Adds a typed `StoredTurnStatus` and `StoredTurnError` to avoid baking
app-server API enums or lossy string status values into the store-facing
turn contract.
- Adds a typed `StoredTurnStatus` and `StoredTurnError` to avoid baking
app-server API enums or lossy string status values into the store-facing
turn contract.
This also sketches the storage abstraction we expect to need once turns
are indexed/stored. In particular, `notLoaded` is useful only if
ThreadStore can eventually list turn metadata without loading every
persisted item for each turn.
## Validation
- Added/updated protocol serialization coverage for the new request and
response shapes.
- Added app-server integration coverage for `thread/turns/list` default
summary behavior and all three `itemsView` modes.
- Added app-server integration coverage that `thread/turns/items/list`
returns the expected unsupported JSON-RPC error when experimental APIs
are enabled.
- Added thread-store coverage that the default trait methods return
`ThreadStoreError::Unsupported`.
No developers.openai.com documentation update is needed for this
internal experimental app-server API surface.
Introduce a ThreadStore interface for mediating access to the filesystem
(rollout jsonl files + sqlite db) based thread storage.
In later PRs we'll move the existing fs code behind a "local"
implementation of this ThreadStore interface.
This PR should be a no-op behaviorally, it only introduces the
interface.
