Module apply 

Turning a stream of events into materialised state.

Consuming AG-UI is not “read a response”. The agent sends deltas — a message opens, text arrives a fragment at a time, tool arguments accumulate as unparseable JSON fragments, state moves by RFC 6902 patch. Applier is the state machine that folds all of that back into something a UI can draw: a message list, a JSON state document, reasoning text, activities.

It is a plain synchronous struct. No async, no runtime, no I/O — feed it events from anywhere.

use ag_ui::client::apply::Applier;
use ag_ui::{Event, TextMessageRole};

let mut applier = Applier::new();
for event in [
    Event::run_started("thread-1", "run-1"),
    Event::text_message_start("msg-1", TextMessageRole::Assistant),
    Event::text_message_content("msg-1", "Hello, "),
    Event::text_message_content("msg-1", "world"),
    Event::text_message_end("msg-1"),
    Event::run_finished_success("thread-1", "run-1"),
] {
    applier.apply(&event)?;
}

assert_eq!(applier.messages().len(), 1);
assert_eq!(applier.text_of("msg-1"), Some("Hello, world"));

What it does not do

The applier is tolerant: an orphan TEXT_MESSAGE_CONTENT opens a message rather than failing, because a half-drawn conversation beats a blank screen. Catching the producer’s mistake is crate::client::verify’s job, and crate::client::Session runs both. The one thing the applier refuses to do quietly is corrupt state: a patch that does not apply is an error.

Structs

Applier

The materialised view of a run.

MessageChange

Which message changed, and how.

ReasoningChange

Which reasoning message changed, and how.

Subagent

One subagent invocation the run has announced, as the applier tracks it.

SubagentChange

Which subagent changed, and how.

Enums

Changed

What one event changed.

MessageChangeKind

What happened to a message.

ReasoningChangeKind

What happened to a reasoning message.

SubagentChangeKind

What happened to a subagent invocation.

SubagentStatus

Where a subagent invocation stands.