Both are first-class rows in the database (agents/src/sqlite-schema.sql:70 and :123), pointing at each other:

runs.session_id — the transcript session a run drives. The type comment is explicit: "Transcript session for agent runs; workflow runs have none" (agents/protocol/src/index.ts:897). A script run has no session at all.

sessions.run_id — the run that spawned this session, for sessions created as run children.

sessions.parent_session_id — the parent session, for sub-sessions.

One session accumulates many runs over its life: every turn of conversation creates a new run row. RunInfo.kind is 'agent' | 'workflow', and only agent runs carry a session.

What did change is the naming at the edges. The M1 verb migration deleted the session-named tools — sub_session and start_session folded into delegate, and set_session_title was removed outright (a test asserts it never returns: api-service.test.ts:2624). The UI's pinned card is a run card. So the word "session" largely vanished from what you type and see, while remaining the backbone of what you talk to.

agents/src/sqlite-schema.sql:70-86: id, account_id, agent_id, title, title_source, status, parent_session_id, run_id, plan_cbor, created_at, updated_at. Indexed by agent (updated_at DESC) and by parent.

Status is a derived mirror, not a source of truth. #syncSessionStatusFromRuns (agents/src/api-service.ts:3250-3269) recomputes it from run state: streaming when a live agent run references the session, error when the latest run failed, else idle. Liveness truth lives in the runs table; the column exists so older clients keep working. A subtle and deliberate detail: sessionHasLiveRun (agents/src/runs.ts:302-310) counts only queued, claimed, and running — a run parked on its children is not "streaming", because a parent waiting on background work can still hold a conversation.

title_source is system, user, or agent. A user-set title always wins — the update is guarded WHERE title_source <> 'user'.

Titling happens asynchronously rather than in the turn. #ensureSessionTitled (agents/src/api-service.ts:2999-3023) fires when a run starts, parks, or finalizes; if the session is still untitled and the source is system, #nameSessionWithModel (:3026) builds a digest from the first twelve events and makes one minimal, tool-less model call whose entire system prompt is: "You are a session-titling assistant. Reply with ONLY a concise one-line title (at most eight words)…" (:3095). The point, per the code's own comment, is that the agent names its sessions — the title is not an echo of the user's first words.

A wrinkle worth knowing: #setSessionTitleFromAgent (:2310) still exists but has zero call sites on this branch — a leftover of the deleted in-turn titling tool.

#piMessages (agents/src/api-service.ts:4852-5065) rebuilds the provider's message list from durable events on every turn, in two passes. It does more than concatenate:

Tool results are reattached directly behind the assistant message that called them, regardless of where the durable event actually landed, because providers require that adjacency.

A child still running gets a synthetic result — {status: 'running', note: 'Still running in the background…'} — so the model sees an honest placeholder instead of a gap.

User actions are not replayed as tool calls. A verb the user ran through the wrench palette is rendered as tagged user text: <user_action verb="…"> and <user_action_result verb="…">, with payloads passed through escapeActionFraming() so injected content cannot close the frame early (see Security).

The checklist is injected fresh, never stored. planStateBlock() (:411-435) renders <plan_state> from session state into each turn — see Plans.

Window context from the client rides as <window_context> and is deliberately kept out of the durable content field.

Note that transcript compaction is not active: sessions are created with compaction: {enabled: false} (:4420). Comments elsewhere about surviving compaction describe a property the design preserves, not a running code path.

A delegated model child gets its own session, linked to the parent by parent_session_id, and its run records parent_tool_call_id — the id of the tool call that spawned it. That field is what lets a delegate row in a transcript find its child while the child is still working, before any result exists: the desktop looks up run.parentToolCallId === toolCallId across the run tree (assistant-message-rendering.tsx:1790-1812).

Session lifecycle actions in the protocol: CreateSession, ListSessions (keyset pagination on (updatedAt, id) — the id is in the cursor because a trigger batch can create several sessions in the same millisecond), GetSession, UpdateSession, DeleteSession, MessageSession, StopSession, RetrySession, and InvokeSessionTool.

RetrySession re-enters from the durable transcript without appending a new user message, and deliberately does not replay error events to the provider. It is refused if a run is live or the latest run did not fail.