Merge remote-tracking branch 'origin/master' into session-query-trace

# Conflicts:
#	packages/session-query/session-query/README.md
#	packages/support/invariants/src/index.ts

packages/core/session/README.md

@@ -79,6 +79,29 @@ Every `SessionEvent` carries two optional top-level fields (structural metadata)
 - Replay/fork: `ctx.sessions.create(id, { seed })` seeds a new session with an existing event log. The surface rebuilds deterministically from `surfaceOp` markers in the seeded events. The constructor reads each seed entry once and uses the same one-pass lossless-JSON snapshot and exact surface-metadata shape checks as `append`, then enforces contiguous seqs and deep-freezes every accepted record; a stateful caller, exotic nested value, marker-less or malformed surface event, metadata on a non-surface event, or retained seed reference therefore cannot silently change the reconstructed history. Broader turn-enclosure checks stay in `dsh-invariants` and persistence repair. Ordinary live-session forks use `ctx.sessions.fork(source, boundary?, childSessionId?)`, where `boundary` is the inclusive source event seq to fork through.
 - Compaction: the `dsh-compact-basic` plugin appends a `user/message` with `surfaceOp: { op: 'replace', start, end }` to shadow old surface nodes behind a summary checkpoint.
 
-### What is NOT here (TODO)
+## Model Experience
+
+### Derived message history
+
+**What the model sees**: The model receives projections of `user/message`, `assistant/message`, and `tool/result` surface nodes verbatim. A `context/message` is a user-role message containing exactly `<context source="<source-kind>">`, its content blocks, and `</context>`; `steering/message` uses the identical `<steering source="<source-kind>">` / `</steering>` wrapper. Tool calls live inside assistant messages. Chunks, boundaries, usage, hook records, todo records, and other log-only events add no message.
+
+**Token effect**: Appended surface nodes are resent on later steps. A `replace` surface operation removes the shadowed nodes from future inputs without deleting their raw log records.
+
+### Crash-repair result
+
+**What the model sees**: If a persisted turn ended with unanswered tool calls, each synthetic error result contains exactly `Tool call interrupted by a crash; no result was recorded.`
+
+**Token effect**: Zero tokens in an intact session. Each repaired call adds this retained error text on resume.
+
+### Logged request header
+
+**What the model sees**: The session reconstructs the system prompt, tool schemas, call config, and session prefix that the loop actually sent. Header events do not add a second copy to message history; the prefix is prepended outside `deriveMessages()`.
+
+**Token effect**: Zero duplicate tokens from logging. The reconstructed prefix, system text, and schemas still incur their normal per-request cost.
+
+## Known Limitations and Deferred Work
 
 - **Session branching/tree** (pi-style entry tree) — deferred unless needed beyond boundary-based `fork()`.
+- **`fork()` cuts only at closed-turn boundaries of live sessions** — the boundary must be a `turn/end` event and the source must be in the store; forking a persisted-but-unloaded session is excluded from the [fork API](../../../docs/rfc/implemented/feature/2026-06-30-session-store-fork-api.md).
+- **`SESSION_FORMAT_VERSION` stays pinned at `0`** — pre-release, no compatibility implied: a backend rejects any other version, and no migration path exists until the first release ([policy](../../../AGENTS.md)).
+- **`TurnEndReasonMap` omits the ACP-named `refusal` / `max_turn_requests` variants** — producer-gated: they land when an adapter or the loop first emits them.