## Summary
Documents the two lower-level chat backend APIs and restructures the
Building agents section so it has a sane reading order.
**Custom agents page.** `chat.customAgent()` was effectively
undocumented (one passing mention) and `chat.createSession()` was buried
at the bottom of the Backend page, prompted by a customer asking whether
dropping down a level was supported at all. Both now live on one
dedicated page framed as a composition: register with `customAgent`,
then drive turns with the managed `createSession` iterator or a
hand-rolled primitives loop. The page covers the patterns the managed
lifecycle otherwise handles for you, each verified against a running
agent: seeding history on continuation runs (and why the seed must go
through the turn-0 `addIncoming`, which replaces the accumulator),
persisting the user message before streaming so a mid-stream reload
keeps it, racing `totalUsage` after a stop so the loop cannot wedge, and
the single-message wire shape.
**Backend page.** Now leads with a decision table across the three
abstraction levels and focuses on `chat.agent()`, routing to the new
page. Stale examples that read a plural `messages` field off the wire
payload are fixed (copy-pasting them broke turn accumulation), and the
ChatSessionOptions / ChatTurn reference tables gain their missing rows
(`compaction`, `pendingMessages`, usage fields, `setMessages`,
`prepareStep`).
**Anatomy page + reorder.** The Building agents group opened with the
long How it works mechanics page, a wall right after the Quick Start. A
short Anatomy page now leads the group: the three moving parts, one
annotated example where each region names the page that covers it, and a
routing table. How it works moves to the end of the group as the depth
payoff, matching where peer docs put their internals pages.
All pages visually verified against a local Mintlify build; cross-links
and anchors updated across the section.
Eric Allam
3bc3a1796f