diff --git a/codex-rs/api-client/src/aggregate.rs b/codex-rs/api-client/src/aggregate.rs
new file mode 100644
index 0000000000..cee283dd2e
--- /dev/null
+++ b/codex-rs/api-client/src/aggregate.rs
@@ -0,0 +1,177 @@
+use std::collections::VecDeque;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ResponseItem;
+use futures::Stream;
+
+use crate::error::Result;
+use crate::stream::ResponseEvent;
+
+#[derive(Clone, Copy, Debug)]
+pub enum ChatAggregationMode {
+    AggregatedOnly,
+    Streaming,
+}
+
+pub trait AggregateStreamExt: Stream<Item = Result<ResponseEvent>> + Sized {
+    fn aggregate(self) -> AggregatedChatStream<Self>
+    where
+        Self: Unpin,
+    {
+        AggregatedChatStream::new(self, AggregateMode::AggregatedOnly)
+    }
+
+    fn streaming_mode(self) -> AggregatedChatStream<Self>
+    where
+        Self: Unpin,
+    {
+        AggregatedChatStream::new(self, AggregateMode::Streaming)
+    }
+}
+
+impl<S> AggregateStreamExt for S where S: Stream<Item = Result<ResponseEvent>> + Sized + Unpin {}
+
+enum AggregateMode {
+    AggregatedOnly,
+    Streaming,
+}
+
+pub struct AggregatedChatStream<S> {
+    inner: S,
+    cumulative: String,
+    cumulative_reasoning: String,
+    pending: VecDeque<ResponseEvent>,
+    mode: AggregateMode,
+}
+
+impl<S> AggregatedChatStream<S>
+where
+    S: Stream<Item = Result<ResponseEvent>> + Unpin,
+{
+    fn new(inner: S, mode: AggregateMode) -> Self {
+        Self {
+            inner,
+            cumulative: String::new(),
+            cumulative_reasoning: String::new(),
+            pending: VecDeque::new(),
+            mode,
+        }
+    }
+}
+
+impl<S> Stream for AggregatedChatStream<S>
+where
+    S: Stream<Item = Result<ResponseEvent>> + Unpin,
+{
+    type Item = Result<ResponseEvent>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if let Some(ev) = self.pending.pop_front() {
+            return Poll::Ready(Some(Ok(ev)));
+        }
+
+        loop {
+            match Pin::new(&mut self.inner).poll_next(cx) {
+                std::task::Poll::Pending => return Poll::Pending,
+                std::task::Poll::Ready(None) => return std::task::Poll::Ready(None),
+                std::task::Poll::Ready(Some(Err(err))) => {
+                    return std::task::Poll::Ready(Some(Err(err)));
+                }
+                std::task::Poll::Ready(Some(Ok(ResponseEvent::OutputItemDone(item)))) => {
+                    let is_assistant_message = matches!(
+                        &item,
+                        ResponseItem::Message { role, .. } if role == "assistant"
+                    );
+
+                    if is_assistant_message {
+                        if let ResponseItem::Message { role, content, .. } = item {
+                            let mut text = String::new();
+                            for c in content {
+                                match c {
+                                    ContentItem::InputText { text: t }
+                                    | ContentItem::OutputText { text: t } => text.push_str(&t),
+                                    ContentItem::InputImage { image_url } => {
+                                        text.push_str(&image_url)
+                                    }
+                                }
+                            }
+                            self.cumulative.push_str(&text);
+                            if matches!(self.mode, AggregateMode::Streaming) {
+                                let output_item =
+                                    ResponseEvent::OutputItemDone(ResponseItem::Message {
+                                        id: None,
+                                        role,
+                                        content: vec![ContentItem::OutputText {
+                                            text: self.cumulative.clone(),
+                                        }],
+                                    });
+                                self.pending.push_back(output_item);
+                            }
+                        }
+                    } else {
+                        return std::task::Poll::Ready(Some(Ok(ResponseEvent::OutputItemDone(
+                            item,
+                        ))));
+                    }
+                }
+                std::task::Poll::Ready(Some(Ok(ResponseEvent::OutputItemAdded(item)))) => {
+                    if !matches!(
+                        &item,
+                        ResponseItem::Message { role, .. } if role == "assistant"
+                    ) {
+                        return std::task::Poll::Ready(Some(Ok(ResponseEvent::OutputItemAdded(
+                            item,
+                        ))));
+                    }
+                }
+                std::task::Poll::Ready(Some(Ok(ResponseEvent::ReasoningContentDelta(delta)))) => {
+                    self.cumulative_reasoning.push_str(&delta);
+                    if matches!(self.mode, AggregateMode::Streaming) {
+                        let ev =
+                            ResponseEvent::ReasoningContentDelta(self.cumulative_reasoning.clone());
+                        self.pending.push_back(ev);
+                    }
+                }
+                std::task::Poll::Ready(Some(Ok(ResponseEvent::ReasoningSummaryDelta(delta)))) => {
+                    if matches!(self.mode, AggregateMode::Streaming) {
+                        let ev = ResponseEvent::ReasoningSummaryDelta(delta);
+                        self.pending.push_back(ev);
+                    }
+                }
+                std::task::Poll::Ready(Some(Ok(ResponseEvent::Completed {
+                    response_id,
+                    token_usage,
+                }))) => {
+                    let assistant_event = ResponseEvent::OutputItemDone(ResponseItem::Message {
+                        id: None,
+                        role: "assistant".to_string(),
+                        content: vec![ContentItem::OutputText {
+                            text: self.cumulative.clone(),
+                        }],
+                    });
+                    let completion_event = ResponseEvent::Completed {
+                        response_id,
+                        token_usage,
+                    };
+
+                    if matches!(self.mode, AggregateMode::Streaming) {
+                        self.pending.push_back(assistant_event);
+                        self.pending.push_back(completion_event);
+                    } else {
+                        return std::task::Poll::Ready(Some(Ok(assistant_event)));
+                    }
+                }
+                std::task::Poll::Ready(Some(Ok(ev))) => {
+                    return std::task::Poll::Ready(Some(Ok(ev)));
+                }
+            }
+
+            if let Some(ev) = self.pending.pop_front() {
+                return std::task::Poll::Ready(Some(Ok(ev)));
+            }
+        }
+    }
+}
diff --git a/codex-rs/api-client/src/common.rs b/codex-rs/api-client/src/common.rs
new file mode 100644
index 0000000000..0a08f42326
--- /dev/null
+++ b/codex-rs/api-client/src/common.rs
@@ -0,0 +1,31 @@
+use std::time::Duration;
+
+use codex_protocol::protocol::SessionSource;
+use codex_protocol::protocol::SubAgentSource;
+
+/// Exponential backoff with a 100ms base and a cap on the exponent to avoid
+/// unbounded growth. The attempt number is clamped to [0, 6].
+pub(crate) fn backoff(attempt: i64) -> Duration {
+    let capped = attempt.clamp(0, 6) as u32;
+    Duration::from_millis(100 * 2_i64.pow(capped) as u64)
+}
+
+/// Apply the `x-openai-subagent` header when the session source indicates a
+/// subagent. Returns the original builder unchanged when not applicable.
+pub(crate) fn apply_subagent_header(
+    mut builder: reqwest::RequestBuilder,
+    session_source: Option<&SessionSource>,
+) -> reqwest::RequestBuilder {
+    if let Some(SessionSource::SubAgent(sub)) = session_source {
+        let subagent = if let SubAgentSource::Other(label) = sub {
+            label.clone()
+        } else {
+            serde_json::to_value(sub)
+                .ok()
+                .and_then(|v| v.as_str().map(std::string::ToString::to_string))
+                .unwrap_or_else(|| "other".to_string())
+        };
+        builder = builder.header("x-openai-subagent", subagent);
+    }
+    builder
+}