Vakyartha documentation

1use std::collections::VecDeque;
2use std::pin::Pin;
3use std::task::{Context, Poll};
4
5use futures::Stream;
6use tokio::sync::mpsc;
7
8use crate::error::LlmError;
9use crate::types::AssistantMessage;
10
11#[derive(Debug, Clone, serde::Serialize)]
12pub enum StreamEvent {
13 Start {
14 partial: AssistantMessage,
15 },
16 TextDelta {
17 delta: String,
18 partial: AssistantMessage,
19 },
20 ThinkingDelta {
21 delta: String,
22 partial: AssistantMessage,
23 },
24 ToolUseStart {
25 index: usize,
26 id: String,
27 name: String,
28 partial: AssistantMessage,
29 },
30 ToolInputDelta {
31 index: usize,
32 delta: String,
33 partial: AssistantMessage,
34 },
35 End {
36 message: AssistantMessage,
37 },
38}
39
40impl StreamEvent {
41 pub fn partial(&self) -> &AssistantMessage {
42 match self {
43 StreamEvent::Start { partial }
44 | StreamEvent::TextDelta { partial, .. }
45 | StreamEvent::ThinkingDelta { partial, .. }
46 | StreamEvent::ToolUseStart { partial, .. }
47 | StreamEvent::ToolInputDelta { partial, .. } => partial,
48 StreamEvent::End { message } => message,
49 }
50 }
51
52 /// Merges `next` into `self` when they are consecutive deltas of the
53 /// same kind (and, for `ToolInputDelta`, the same block `index`):
54 /// concatenates `delta` and keeps `next`'s `partial`, since `partial`
55 /// is already the full accumulated snapshot and the newer one subsumes
56 /// the older. Returns `next` back unmerged for anything else -- a
57 /// different kind, a different tool-input index, or a structural event
58 /// (`Start`/`ToolUseStart`/`End`) that is never itself a delta. The one
59 /// merge rule every lossless forwarding hop uses (`EventSink::push`
60 /// here, and `Agent::complete_with_reliability`'s listener forward) to
61 /// coalesce a backlog under backpressure instead of dropping events.
62 pub fn try_merge(&mut self, next: StreamEvent) -> Option<StreamEvent> {
63 match self {
64 StreamEvent::TextDelta { delta, partial } => match next {
65 StreamEvent::TextDelta {
66 delta: next_delta,
67 partial: next_partial,
68 } => {
69 delta.push_str(&next_delta);
70 *partial = next_partial;
71 None
72 }
73 other => Some(other),
74 },
75 StreamEvent::ThinkingDelta { delta, partial } => match next {
76 StreamEvent::ThinkingDelta {
77 delta: next_delta,
78 partial: next_partial,
79 } => {
80 delta.push_str(&next_delta);
81 *partial = next_partial;
82 None
83 }
84 other => Some(other),
85 },
86 StreamEvent::ToolInputDelta {
87 index,
88 delta,
89 partial,
90 } => match next {
91 StreamEvent::ToolInputDelta {
92 index: next_index,
93 delta: next_delta,
94 partial: next_partial,
95 } if next_index == *index => {
96 delta.push_str(&next_delta);
97 *partial = next_partial;
98 None
99 }
100 other => Some(other),
101 },
102 _ => Some(next),
103 }
104 }
105}
106
107enum Terminal {
108 Message(AssistantMessage),
109 Error(LlmError),
110}
111
112enum Wire {
113 Event(StreamEvent),
114 Terminal(Terminal),
115}
116
117pub struct EventSink {
118 tx: mpsc::Sender<Wire>,
119 closed: bool,
120 /// Events `push` could not deliver immediately because the channel was
121 /// full, coalesced via `StreamEvent::try_merge` -- never dropped. Never
122 /// awaited-send from `push` itself (that would block the provider's own
123 /// read loop on a slow listener); flushed opportunistically on the next
124 /// `push` and unconditionally, with an awaited send, at close.
125 pending: VecDeque<StreamEvent>,
126}
127
128impl EventSink {
129 /// Best-effort, non-blocking delivery that never drops a still-
130 /// deliverable event: a full channel means the listener is behind, not
131 /// gone, so `event` is queued locally instead and retried ahead of the
132 /// next call. Never awaits -- callers run inside the provider's own
133 /// spawned stream-reading task, and a slow listener must not stall that
134 /// read loop.
135 pub fn push(&mut self, event: StreamEvent) {
136 if self.closed {
137 return;
138 }
139 self.drain_pending();
140 if !self.pending.is_empty() {
141 queue_or_merge(&mut self.pending, event);
142 return;
143 }
144 if let Err(mpsc::error::TrySendError::Full(wire)) = self.tx.try_send(Wire::Event(event))
145 && let Wire::Event(event) = wire
146 {
147 self.pending.push_back(event);
148 }
149 // `Closed`: nothing could ever be delivered from here on; the event
150 // is simply undeliverable, not "dropped under backpressure".
151 }
152
153 /// Drains as much of the local backlog as the channel currently
154 /// accepts, oldest first, without blocking.
155 fn drain_pending(&mut self) {
156 while let Some(event) = self.pending.pop_front() {
157 match self.tx.try_send(Wire::Event(event)) {
158 Ok(()) => {}
159 Err(mpsc::error::TrySendError::Full(wire)) => {
160 if let Wire::Event(event) = wire {
161 self.pending.push_front(event);
162 }
163 break;
164 }
165 Err(mpsc::error::TrySendError::Closed(_)) => {
166 self.pending.clear();
167 break;
168 }
169 }
170 }
171 }
172
173 /// Terminal delivery is awaited, never `try_send`: a full buffer at
174 /// message_stop must not turn a completed model turn into
175 /// "stream ended without a terminal event". Any backlog `push` queued
176 /// under backpressure is flushed first, in order, so the terminal event
177 /// is always the last thing the listener sees. Callers run inside the
178 /// provider's spawned stream task, so awaiting here is safe.
179 pub async fn close_message(&mut self, message: AssistantMessage) {
180 self.flush_and_close(Terminal::Message(message)).await;
181 }
182
183 pub async fn close_error(&mut self, error: LlmError) {
184 self.flush_and_close(Terminal::Error(error)).await;
185 }
186
187 async fn flush_and_close(&mut self, terminal: Terminal) {
188 self.closed = true;
189 for event in self.pending.drain(..) {
190 let _ = self.tx.send(Wire::Event(event)).await;
191 }
192 let _ = self.tx.send(Wire::Terminal(terminal)).await;
193 }
194}
195
196/// Queues `event`, merging it into the last still-pending event when
197/// `StreamEvent::try_merge` allows it -- shared by `push` and `drain_pending`
198/// so a slow listener's backlog stays one entry per in-progress block
199/// instead of growing one entry per delta.
200fn queue_or_merge(pending: &mut VecDeque<StreamEvent>, event: StreamEvent) {
201 match pending.back_mut() {
202 Some(last) => {
203 if let Some(event) = last.try_merge(event) {
204 pending.push_back(event);
205 }
206 }
207 None => pending.push_back(event),
208 }
209}
210
211pub struct EventStream {
212 rx: mpsc::Receiver<Wire>,
213 terminal: Option<Terminal>,
214 guard: Option<Box<dyn Send>>,
215}
216
217pub fn channel(buffer: usize) -> (EventSink, EventStream) {
218 let (tx, rx) = mpsc::channel(buffer.max(1));
219 (
220 EventSink {
221 tx,
222 closed: false,
223 pending: VecDeque::new(),
224 },
225 EventStream {
226 rx,
227 terminal: None,
228 guard: None,
229 },
230 )
231}
232
233impl EventStream {
234 pub(crate) fn with_guard<T: Send + 'static>(mut self, guard: T) -> Self {
235 self.guard = Some(Box::new(guard));
236 self
237 }
238
239 pub async fn result(mut self) -> Result<AssistantMessage, LlmError> {
240 use futures::StreamExt;
241 while let Some(_event) = self.next().await {}
242 match self.terminal.take() {
243 Some(Terminal::Message(m)) => Ok(m),
244 Some(Terminal::Error(e)) => Err(e),
245 None => Err(LlmError::Parse(
246 "stream ended without a terminal event".into(),
247 )),
248 }
249 }
250}
251
252impl Stream for EventStream {
253 type Item = StreamEvent;
254
255 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
256 loop {
257 if self.terminal.is_some() {
258 return Poll::Ready(None);
259 }
260 match self.rx.poll_recv(cx) {
261 Poll::Ready(Some(Wire::Event(e))) => return Poll::Ready(Some(e)),
262 Poll::Ready(Some(Wire::Terminal(t))) => {
263 self.terminal = Some(t);
264 continue;
265 }
266 Poll::Ready(None) => return Poll::Ready(None),
267 Poll::Pending => return Poll::Pending,
268 }
269 }
270 }
271}
272
273#[cfg(test)]
274#[allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
275mod tests {
276 use super::*;
277 use crate::types::{ContentBlock, StopReason, Usage};
278
279 fn msg(text: &str) -> AssistantMessage {
280 AssistantMessage {
281 content: vec![ContentBlock::text(text.to_string())],
282 stop_reason: StopReason::EndTurn,
283 usage: Usage::default(),
284 model: "m".into(),
285 response_id: None,
286 }
287 }
288
289 fn text_delta(delta: &str, text_so_far: &str) -> StreamEvent {
290 StreamEvent::TextDelta {
291 delta: delta.to_string(),
292 partial: msg(text_so_far),
293 }
294 }
295
296 #[test]
297 fn try_merge_concatenates_consecutive_text_deltas_and_keeps_newest_partial() {
298 let mut a = text_delta("Hel", "Hel");
299 let b = text_delta("lo", "Hello");
300 assert!(a.try_merge(b).is_none());
301 match a {
302 StreamEvent::TextDelta { delta, partial } => {
303 assert_eq!(delta, "Hello");
304 assert_eq!(partial.text_content(), "Hello");
305 }
306 other => panic!("expected TextDelta, got {other:?}"),
307 }
308 }
309
310 #[test]
311 fn try_merge_refuses_different_kinds_and_leaves_self_untouched() {
312 let mut a = text_delta("Hel", "Hel");
313 let b = StreamEvent::ToolUseStart {
314 index: 0,
315 id: "1".into(),
316 name: "t".into(),
317 partial: AssistantMessage::empty("m"),
318 };
319 let unmerged = a.try_merge(b);
320 assert!(matches!(unmerged, Some(StreamEvent::ToolUseStart { .. })));
321 match a {
322 StreamEvent::TextDelta { delta, .. } => assert_eq!(delta, "Hel"),
323 other => panic!("self must be untouched, got {other:?}"),
324 }
325 }
326
327 #[test]
328 fn try_merge_refuses_tool_input_deltas_at_different_indices() {
329 let mut a = StreamEvent::ToolInputDelta {
330 index: 0,
331 delta: "a".into(),
332 partial: AssistantMessage::empty("m"),
333 };
334 let b = StreamEvent::ToolInputDelta {
335 index: 1,
336 delta: "b".into(),
337 partial: AssistantMessage::empty("m"),
338 };
339 assert!(a.try_merge(b).is_some());
340 match a {
341 StreamEvent::ToolInputDelta { delta, .. } => assert_eq!(delta, "a"),
342 other => panic!("self must be untouched, got {other:?}"),
343 }
344 }
345
346 #[test]
347 fn try_merge_concatenates_tool_input_deltas_at_the_same_index() {
348 let mut a = StreamEvent::ToolInputDelta {
349 index: 2,
350 delta: "{\"a\":".into(),
351 partial: AssistantMessage::empty("m"),
352 };
353 let b = StreamEvent::ToolInputDelta {
354 index: 2,
355 delta: "1}".into(),
356 partial: AssistantMessage::empty("m"),
357 };
358 assert!(a.try_merge(b).is_none());
359 match a {
360 StreamEvent::ToolInputDelta { delta, .. } => assert_eq!(delta, "{\"a\":1}"),
361 other => panic!("expected ToolInputDelta, got {other:?}"),
362 }
363 }
364
365 /// The lossless-streaming contract (docs/design/68-context-engine.md
366 /// §6): a channel with room for exactly one event, pushed far faster
367 /// than a slow consumer drains it, still delivers every character —
368 /// concatenating every `TextDelta` the consumer receives (in order)
369 /// reconstructs the exact text that was pushed, and the terminal
370 /// message carries the same text.
371 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
372 async fn a_slow_consumer_over_a_tiny_channel_still_receives_every_delta_losslessly() {
373 use futures::StreamExt;
374
375 let (mut sink, mut rx) = channel(1);
376 let words: Vec<String> = (0..300).map(|n| format!("w{n} ")).collect();
377 let expected: String = words.concat();
378 let pieces = words.clone();
379 tokio::spawn(async move {
380 let mut acc = String::new();
381 for word in &pieces {
382 acc.push_str(word);
383 sink.push(StreamEvent::TextDelta {
384 delta: word.clone(),
385 partial: msg(&acc),
386 });
387 }
388 sink.close_message(msg(&acc)).await;
389 });
390
391 let result = tokio::time::timeout(std::time::Duration::from_secs(10), async {
392 let mut received = String::new();
393 while let Some(event) = rx.next().await {
394 // A deliberately slow consumer: sleep after every receive
395 // so the producer races far ahead and backpressure bites.
396 tokio::time::sleep(std::time::Duration::from_micros(200)).await;
397 if let StreamEvent::TextDelta { delta, .. } = event {
398 received.push_str(&delta);
399 }
400 }
401 received
402 })
403 .await
404 .expect("a lossless slow consumer must still finish promptly");
405
406 assert_eq!(result, expected, "no delta was dropped or reordered");
407 let message = rx.result().await.expect("terminal message");
408 assert_eq!(message.text_content(), expected);
409 }
410}
411

1use std::collections::VecDeque;
2use std::pin::Pin;
3use std::task::{Context, Poll};
4
5use futures::Stream;
6use tokio::sync::mpsc;
7
8use crate::error::LlmError;
9use crate::types::AssistantMessage;
10
11#[derive(Debug, Clone, serde::Serialize)]
12pub enum StreamEvent {
13 Start {
14 partial: AssistantMessage,
15 },
16 TextDelta {
17 delta: String,
18 partial: AssistantMessage,
19 },
20 ThinkingDelta {
21 delta: String,
22 partial: AssistantMessage,
23 },
24 ToolUseStart {
25 index: usize,
26 id: String,
27 name: String,
28 partial: AssistantMessage,
29 },
30 ToolInputDelta {
31 index: usize,
32 delta: String,
33 partial: AssistantMessage,
34 },
35 End {
36 message: AssistantMessage,
37 },
38}
39
40impl StreamEvent {
41 pub fn partial(&self) -> &AssistantMessage {
42 match self {
43 StreamEvent::Start { partial }
44 | StreamEvent::TextDelta { partial, .. }
45 | StreamEvent::ThinkingDelta { partial, .. }
46 | StreamEvent::ToolUseStart { partial, .. }
47 | StreamEvent::ToolInputDelta { partial, .. } => partial,
48 StreamEvent::End { message } => message,
49 }
50 }
51
52 /// Merges `next` into `self` when they are consecutive deltas of the
53 /// same kind (and, for `ToolInputDelta`, the same block `index`):
54 /// concatenates `delta` and keeps `next`'s `partial`, since `partial`
55 /// is already the full accumulated snapshot and the newer one subsumes
56 /// the older. Returns `next` back unmerged for anything else -- a
57 /// different kind, a different tool-input index, or a structural event
58 /// (`Start`/`ToolUseStart`/`End`) that is never itself a delta. The one
59 /// merge rule every lossless forwarding hop uses (`EventSink::push`
60 /// here, and `Agent::complete_with_reliability`'s listener forward) to
61 /// coalesce a backlog under backpressure instead of dropping events.
62 pub fn try_merge(&mut self, next: StreamEvent) -> Option<StreamEvent> {
63 match self {
64 StreamEvent::TextDelta { delta, partial } => match next {
65 StreamEvent::TextDelta {
66 delta: next_delta,
67 partial: next_partial,
68 } => {
69 delta.push_str(&next_delta);
70 *partial = next_partial;
71 None
72 }
73 other => Some(other),
74 },
75 StreamEvent::ThinkingDelta { delta, partial } => match next {
76 StreamEvent::ThinkingDelta {
77 delta: next_delta,
78 partial: next_partial,
79 } => {
80 delta.push_str(&next_delta);
81 *partial = next_partial;
82 None
83 }
84 other => Some(other),
85 },
86 StreamEvent::ToolInputDelta {
87 index,
88 delta,
89 partial,
90 } => match next {
91 StreamEvent::ToolInputDelta {
92 index: next_index,
93 delta: next_delta,
94 partial: next_partial,
95 } if next_index == *index => {
96 delta.push_str(&next_delta);
97 *partial = next_partial;
98 None
99 }
100 other => Some(other),
101 },
102 _ => Some(next),
103 }
104 }
105}
106
107enum Terminal {
108 Message(AssistantMessage),
109 Error(LlmError),
110}
111
112enum Wire {
113 Event(StreamEvent),
114 Terminal(Terminal),
115}
116
117pub struct EventSink {
118 tx: mpsc::Sender<Wire>,
119 closed: bool,
120 /// Events `push` could not deliver immediately because the channel was
121 /// full, coalesced via `StreamEvent::try_merge` -- never dropped. Never
122 /// awaited-send from `push` itself (that would block the provider's own
123 /// read loop on a slow listener); flushed opportunistically on the next
124 /// `push` and unconditionally, with an awaited send, at close.
125 pending: VecDeque<StreamEvent>,
126}
127
128impl EventSink {
129 /// Best-effort, non-blocking delivery that never drops a still-
130 /// deliverable event: a full channel means the listener is behind, not
131 /// gone, so `event` is queued locally instead and retried ahead of the
132 /// next call. Never awaits -- callers run inside the provider's own
133 /// spawned stream-reading task, and a slow listener must not stall that
134 /// read loop.
135 pub fn push(&mut self, event: StreamEvent) {
136 if self.closed {
137 return;
138 }
139 self.drain_pending();
140 if !self.pending.is_empty() {
141 queue_or_merge(&mut self.pending, event);
142 return;
143 }
144 if let Err(mpsc::error::TrySendError::Full(wire)) = self.tx.try_send(Wire::Event(event))
145 && let Wire::Event(event) = wire
146 {
147 self.pending.push_back(event);
148 }
149 // `Closed`: nothing could ever be delivered from here on; the event
150 // is simply undeliverable, not "dropped under backpressure".
151 }
152
153 /// Drains as much of the local backlog as the channel currently
154 /// accepts, oldest first, without blocking.
155 fn drain_pending(&mut self) {
156 while let Some(event) = self.pending.pop_front() {
157 match self.tx.try_send(Wire::Event(event)) {
158 Ok(()) => {}
159 Err(mpsc::error::TrySendError::Full(wire)) => {
160 if let Wire::Event(event) = wire {
161 self.pending.push_front(event);
162 }
163 break;
164 }
165 Err(mpsc::error::TrySendError::Closed(_)) => {
166 self.pending.clear();
167 break;
168 }
169 }
170 }
171 }
172
173 /// Terminal delivery is awaited, never `try_send`: a full buffer at
174 /// message_stop must not turn a completed model turn into
175 /// "stream ended without a terminal event". Any backlog `push` queued
176 /// under backpressure is flushed first, in order, so the terminal event
177 /// is always the last thing the listener sees. Callers run inside the
178 /// provider's spawned stream task, so awaiting here is safe.
179 pub async fn close_message(&mut self, message: AssistantMessage) {
180 self.flush_and_close(Terminal::Message(message)).await;
181 }
182
183 pub async fn close_error(&mut self, error: LlmError) {
184 self.flush_and_close(Terminal::Error(error)).await;
185 }
186
187 async fn flush_and_close(&mut self, terminal: Terminal) {
188 self.closed = true;
189 for event in self.pending.drain(..) {
190 let _ = self.tx.send(Wire::Event(event)).await;
191 }
192 let _ = self.tx.send(Wire::Terminal(terminal)).await;
193 }
194}
195
196/// Queues `event`, merging it into the last still-pending event when
197/// `StreamEvent::try_merge` allows it -- shared by `push` and `drain_pending`
198/// so a slow listener's backlog stays one entry per in-progress block
199/// instead of growing one entry per delta.
200fn queue_or_merge(pending: &mut VecDeque<StreamEvent>, event: StreamEvent) {
201 match pending.back_mut() {
202 Some(last) => {
203 if let Some(event) = last.try_merge(event) {
204 pending.push_back(event);
205 }
206 }
207 None => pending.push_back(event),
208 }
209}
210
211pub struct EventStream {
212 rx: mpsc::Receiver<Wire>,
213 terminal: Option<Terminal>,
214 guard: Option<Box<dyn Send>>,
215}
216
217pub fn channel(buffer: usize) -> (EventSink, EventStream) {
218 let (tx, rx) = mpsc::channel(buffer.max(1));
219 (
220 EventSink {
221 tx,
222 closed: false,
223 pending: VecDeque::new(),
224 },
225 EventStream {
226 rx,
227 terminal: None,
228 guard: None,
229 },
230 )
231}
232
233impl EventStream {
234 pub(crate) fn with_guard<T: Send + 'static>(mut self, guard: T) -> Self {
235 self.guard = Some(Box::new(guard));
236 self
237 }
238
239 pub async fn result(mut self) -> Result<AssistantMessage, LlmError> {
240 use futures::StreamExt;
241 while let Some(_event) = self.next().await {}
242 match self.terminal.take() {
243 Some(Terminal::Message(m)) => Ok(m),
244 Some(Terminal::Error(e)) => Err(e),
245 None => Err(LlmError::Parse(
246 "stream ended without a terminal event".into(),
247 )),
248 }
249 }
250}
251
252impl Stream for EventStream {
253 type Item = StreamEvent;
254
255 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
256 loop {
257 if self.terminal.is_some() {
258 return Poll::Ready(None);
259 }
260 match self.rx.poll_recv(cx) {
261 Poll::Ready(Some(Wire::Event(e))) => return Poll::Ready(Some(e)),
262 Poll::Ready(Some(Wire::Terminal(t))) => {
263 self.terminal = Some(t);
264 continue;
265 }
266 Poll::Ready(None) => return Poll::Ready(None),
267 Poll::Pending => return Poll::Pending,
268 }
269 }
270 }
271}
272
273#[cfg(test)]
274#[allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
275mod tests {
276 use super::*;
277 use crate::types::{ContentBlock, StopReason, Usage};
278
279 fn msg(text: &str) -> AssistantMessage {
280 AssistantMessage {
281 content: vec![ContentBlock::text(text.to_string())],
282 stop_reason: StopReason::EndTurn,
283 usage: Usage::default(),
284 model: "m".into(),
285 response_id: None,
286 }
287 }
288
289 fn text_delta(delta: &str, text_so_far: &str) -> StreamEvent {
290 StreamEvent::TextDelta {
291 delta: delta.to_string(),
292 partial: msg(text_so_far),
293 }
294 }
295
296 #[test]
297 fn try_merge_concatenates_consecutive_text_deltas_and_keeps_newest_partial() {
298 let mut a = text_delta("Hel", "Hel");
299 let b = text_delta("lo", "Hello");
300 assert!(a.try_merge(b).is_none());
301 match a {
302 StreamEvent::TextDelta { delta, partial } => {
303 assert_eq!(delta, "Hello");
304 assert_eq!(partial.text_content(), "Hello");
305 }
306 other => panic!("expected TextDelta, got {other:?}"),
307 }
308 }
309
310 #[test]
311 fn try_merge_refuses_different_kinds_and_leaves_self_untouched() {
312 let mut a = text_delta("Hel", "Hel");
313 let b = StreamEvent::ToolUseStart {
314 index: 0,
315 id: "1".into(),
316 name: "t".into(),
317 partial: AssistantMessage::empty("m"),
318 };
319 let unmerged = a.try_merge(b);
320 assert!(matches!(unmerged, Some(StreamEvent::ToolUseStart { .. })));
321 match a {
322 StreamEvent::TextDelta { delta, .. } => assert_eq!(delta, "Hel"),
323 other => panic!("self must be untouched, got {other:?}"),
324 }
325 }
326
327 #[test]
328 fn try_merge_refuses_tool_input_deltas_at_different_indices() {
329 let mut a = StreamEvent::ToolInputDelta {
330 index: 0,
331 delta: "a".into(),
332 partial: AssistantMessage::empty("m"),
333 };
334 let b = StreamEvent::ToolInputDelta {
335 index: 1,
336 delta: "b".into(),
337 partial: AssistantMessage::empty("m"),
338 };
339 assert!(a.try_merge(b).is_some());
340 match a {
341 StreamEvent::ToolInputDelta { delta, .. } => assert_eq!(delta, "a"),
342 other => panic!("self must be untouched, got {other:?}"),
343 }
344 }
345
346 #[test]
347 fn try_merge_concatenates_tool_input_deltas_at_the_same_index() {
348 let mut a = StreamEvent::ToolInputDelta {
349 index: 2,
350 delta: "{\"a\":".into(),
351 partial: AssistantMessage::empty("m"),
352 };
353 let b = StreamEvent::ToolInputDelta {
354 index: 2,
355 delta: "1}".into(),
356 partial: AssistantMessage::empty("m"),
357 };
358 assert!(a.try_merge(b).is_none());
359 match a {
360 StreamEvent::ToolInputDelta { delta, .. } => assert_eq!(delta, "{\"a\":1}"),
361 other => panic!("expected ToolInputDelta, got {other:?}"),
362 }
363 }
364
365 /// The lossless-streaming contract (docs/design/68-context-engine.md
366 /// §6): a channel with room for exactly one event, pushed far faster
367 /// than a slow consumer drains it, still delivers every character —
368 /// concatenating every `TextDelta` the consumer receives (in order)
369 /// reconstructs the exact text that was pushed, and the terminal
370 /// message carries the same text.
371 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
372 async fn a_slow_consumer_over_a_tiny_channel_still_receives_every_delta_losslessly() {
373 use futures::StreamExt;
374
375 let (mut sink, mut rx) = channel(1);
376 let words: Vec<String> = (0..300).map(|n| format!("w{n} ")).collect();
377 let expected: String = words.concat();
378 let pieces = words.clone();
379 tokio::spawn(async move {
380 let mut acc = String::new();
381 for word in &pieces {
382 acc.push_str(word);
383 sink.push(StreamEvent::TextDelta {
384 delta: word.clone(),
385 partial: msg(&acc),
386 });
387 }
388 sink.close_message(msg(&acc)).await;
389 });
390
391 let result = tokio::time::timeout(std::time::Duration::from_secs(10), async {
392 let mut received = String::new();
393 while let Some(event) = rx.next().await {
394 // A deliberately slow consumer: sleep after every receive
395 // so the producer races far ahead and backpressure bites.
396 tokio::time::sleep(std::time::Duration::from_micros(200)).await;
397 if let StreamEvent::TextDelta { delta, .. } = event {
398 received.push_str(&delta);
399 }
400 }
401 received
402 })
403 .await
404 .expect("a lossless slow consumer must still finish promptly");
405
406 assert_eq!(result, expected, "no delta was dropped or reordered");
407 let message = rx.result().await.expect("terminal message");
408 assert_eq!(message.text_content(), expected);
409 }
410}
411

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