Switch the session's model
Points the session at another model, and records that model as this installation’s default for every session opened afterwards. No host restart and no new session: the next turn runs on the chosen model, and a turn already running keeps the model it started on — a switch never mutates a turn in flight. Note what that means for the message a person sends next while a turn is running: it is admitted as steering into that turn (see createMessage), so it is answered by the model the turn started on, not by the one just chosen. A client that tells them “the next message runs on it” is telling them something untrue. The switch is announced in the transcript as a system entry, like every other model change nobody typed — except when a turn is in flight, where an entry cannot be placed without separating a tool call from its result, which would fail that turn and quarantine the session on recovery; the switch still takes effect, and this response is the confirmation.
The variant travels WITH the model as one choice, never as a setting beside it: a body naming no variant runs the chosen model at its own defaults, so switching model always clears a level the previous model was running at rather than carrying it onto one that may not offer it. The variant is recorded with the model as this installation’s default for sessions opened afterwards, on the same terms the model itself is.
Path Parameters
The session's id.
Body
The model to point a session at. Both fields are required: a bare model id is a client-side convenience, never a wire shape, so this operation never has to guess which provider was meant.
The provider serving the model, as the catalogue names it.
128The model's id, as the catalogue names it.
256The preset of request options the chosen model is to run under — one of the names Model.variants publishes for it, which includes the reasoning-effort levels every model whose family carries that knob offers for free. Absent means the model's own defaults, so a switch that omits it clears whatever the session was running at. A name the chosen model does not offer is refused by name, with the ones it does offer, rather than ignored: a level silently dropped is a week of turns at a setting somebody believes they chose.
64Response
The session, now on the chosen model.
A durable conversation container. Survives the process that created it; archiving hides it from active lists but its transcript remains queryable and exportable forever.
The session's id.
Absolute path of the repository the session works in.
4096Lifecycle: running while a turn executes or normal follow-ups chain; waiting after interrupt when admitted steering or follow-up inputs remain queued; idle only when neither execution nor queued input exists; archived accepts no new work.
idle, running, waiting, archived Which kind of client opened this session, recorded when it was created and never rewritten. Always present: unknown for a session created before this host recorded the datum, which is the honest answer rather than a guessed frontend.
inline, tui, oneshot, api, unknown Human-readable title.
512The model provider currently in use.
The model currently in use.
The preset of request options this session's turns run under — the name, not the options it stands for, since what a name means is the catalogue's answer and can change while the session's own choice cannot. Absent on a session that never chose one, which is every session running its model at the model's own defaults.
The named response style this session's answers are shaped by — the name, not the prompt it appends, since what a name means is configuration's answer and changes when somebody edits that file while the session's own choice does not. Absent on a session that never chose one, which is every session answering in narya's own voice. Unlike variant it survives a model switch: a style is a fact about how the conversation reads rather than about the model answering it. A client renders this and derives nothing — the names that exist are the host's answer (setSessionStyle refuses the rest).
64One sentence stating what a client could not otherwise know about the model this session opened on or was just moved to, present on the responses to createSession and setSessionModel. Two facts earn it. The first is a model narya chose itself rather than being told to use: no default_model configured and no model previously chosen, so the first provider holding a usable credential answered — or a named model the catalogue no longer carries, for which the provider default was substituted. It names the alternatives that also qualified and how to choose another, because a choice nobody made has to be stated to be a choice at all rather than something that merely happened. The second is a level this session will run without: a model reference may carry one (see the variant field), and an endpoint that does not take the field runs at its own default instead — so a level named at creation or at a switch, or configured beside default_model, is said here rather than quoted in a picker while every request drops it. A client that renders the live event stream rather than the session's transcript has no other road to it. Otherwise absent, including for a model that was simply configured, chosen or named; absent on getSession and listSessions, which report state rather than how it came about.
1024What this session has consumed and cost across every turn it has run — a running total, not the last turn's figure. Carried by getSession and by every row of listSessions, so a client drawing a list of sessions can say what each one has cost without asking per row. Absent on a session that has never completed a turn, and its costUsd is absent whenever any one of that session's turns could not be priced: the sum over only the priced turns is smaller than what the session actually cost, and a figure that quietly understates money is worse than an admitted unknown.
What the newest round of this session's own conversation sent and produced — the LAST turn's figure, not the running total above. Carried by getSession only. It exists because the two are not interchangeable for the one question a reader reopening a long conversation asks: how much room is left before this compacts. That answer is the size of the prompt last sent, and usage above is a sum over every turn the session ever ran, which is a larger number growing without bound and means nothing as a fraction of a context window. The live event stream carries this figure on turn-finished, so a client watching a session has always had it; a client that READ the session had no source for it at all and drew no context figure until it spent a turn of its own. Restoring a recorded reading rather than deriving one is the whole of it — nothing here is computed from the transcript. The newest round of the MAIN lane: a delegation's rounds are another conversation's prompts, and the figure is about the reader's. Absent on a session that has never completed a round, and on one whose rounds all ran on delegates. Its costUsd is that one round's price, absent when the round could not be priced.
Pending steering and follow-up references in durable operation order. Content is intentionally absent; fetch the transcript by entryId.
When the session was branched, the session it branched from.
What a model said this conversation tried and concluded. Absent until somebody asks for it (summarizeSession) — nothing summarises a branch because a rewind moved away from it. A model that answers past this length is trimmed to it, with a marker saying so rather than a sentence that just stops.
4096When the summary was taken. Read against updatedAt it answers whether the summary predates the last thing that happened here.
Whether somebody is there to answer a permission prompt raised here. Absent on a session nobody ever said either way about, which reads as attended — the posture is only ever recorded because somebody stated it (setSessionPosture), never inferred from whether a client happens to be connected.
attended, detached Whether this session pays to keep its provider cache alive while nobody is using it, and the most it may ever spend doing so. Absent on a session nobody ever turned it on for, which is every session until somebody does — warming is off unless somebody asked (setSessionWarming).
When a reader last had this session in front of them (acknowledgeSessionViewed). Read against updatedAt it answers whether anything has happened here since somebody looked, which is what lets a client mark a background conversation as carrying news — and, because the fact is held here rather than in a window, what makes two clients agree about it and makes the answer survive a relaunch. ABSENT on a session nobody has ever viewed, which is not the same as viewed long ago: one has nothing to report, the other has everything that ever happened in it.
How the last turn here ended. It is what lets a client that was not running at the time tell a conversation that finished from one that failed. Absent on a session that has never run a turn. An interrupt is not among the values: somebody was there and stopped it themselves, so it is not news to bring back to them.
finished, failed What this session's provider prompt cache has cost it, from both sides: what letting it expire cost, and what keeping it alive cost. Present only on getSession, and only once one of the two has happened — a session that has never paid for either carries nothing here rather than a pair of zeros.
The agent's own task list for this session — every item it has written, in the order it wrote them, whatever is in the store right now.
It is here rather than behind a listing of its own because the list is a stored row and every client already reads the session when it attaches, so one optional field costs no round trip and no second operation. A change to the list is announced as task-list, which carries the same whole list for the same reason; this is where a client that was not attached at the time gets it.
Present on getSession only, like lastTurnUsage and cacheSpend and for their reason: a listing that carried it would read every row's tasks to draw a page nobody asked for.
ABSENT MEANS EMPTY, and it cannot mean anything narrower: the store holds rows, so a list nobody ever wrote and a list somebody cleared are the same read. The live stream is where those two differ — a clearing write announces task-list carrying an empty array — and a read that returned [] would be claiming to know which of the two it was looking at.