diff --git a/crates/agen/src/engine.rs b/crates/agen/src/engine.rs
index da3cead9d..2235614bc 100644
--- a/crates/agen/src/engine.rs
+++ b/crates/agen/src/engine.rs
@@ -2275,6 +2275,9 @@ impl<C: LlmClient, S: EngineState, A: Send + Sync> Engine<C, S, A> {
                         info!("Paused during response stream");
                     }
                     self.timeline.abort_current_block();
+                    // Preserve any UsageEvent already received for billing, but
+                    // do not treat this paused request as a normal completion.
+                    self.timeline.flush_usage();
                     return Err(EngineError::PauseRequested);
                 }
                 cancel = self.cancel_rx.recv() => {
diff --git a/crates/agen/tests/engine_state_test.rs b/crates/agen/tests/engine_state_test.rs
index 5060015d5..051f94394 100644
--- a/crates/agen/tests/engine_state_test.rs
+++ b/crates/agen/tests/engine_state_test.rs
@@ -7,6 +7,7 @@ mod common;
 
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
+use std::time::Duration;
 
 use agen::Item;
 use agen::interceptor::{
@@ -18,16 +19,67 @@ use agen::interceptor::{
 };
 use agen::llm_client::{
     ClientError, LlmClient, Request, ResponseStream,
-    event::{Event, ResponseStatus, StatusEvent},
+    event::{Event, ResponseStatus, StatusEvent, UsageEvent},
 };
 use agen::tool::{Tool, ToolDefinition, ToolError, ToolMeta, ToolOutput};
 use agen::{Engine, EngineError, EngineRunExit, History, RunInterruptionReason};
 use async_trait::async_trait;
 use common::MockLlmClient;
+use futures::StreamExt;
 
-// =============================================================================
-// Mutable State Tests
-// =============================================================================
+#[derive(Clone)]
+struct UsageThenPendingClient;
+
+#[async_trait]
+impl LlmClient for UsageThenPendingClient {
+    fn clone_boxed(&self) -> Box<dyn LlmClient> {
+        Box::new(self.clone())
+    }
+
+    async fn stream(&self, _request: Request) -> Result<ResponseStream, ClientError> {
+        let usage = Event::Usage(UsageEvent {
+            input_tokens: Some(90),
+            output_tokens: Some(2),
+            total_tokens: Some(92),
+            cache_read_input_tokens: Some(0),
+            cache_creation_input_tokens: Some(0),
+        });
+        Ok(Box::pin(
+            futures::stream::once(async move { Ok(usage) })
+                .chain(futures::stream::pending::<Result<Event, ClientError>>()),
+        ))
+    }
+}
+
+#[tokio::test]
+async fn pause_during_response_stream_flushes_observed_usage() {
+    let observed = Arc::new(Mutex::new(Vec::new()));
+    let observed_for_callback = Arc::clone(&observed);
+    let mut engine = Engine::new(UsageThenPendingClient);
+    engine.on_usage(move |usage| {
+        observed_for_callback.lock().unwrap().push(usage.clone());
+    });
+    let pause = engine.pause_sender();
+    let mut history = History::new();
+    let mut engine = engine.lock(&history);
+
+    tokio::spawn(async move {
+        tokio::time::sleep(Duration::from_millis(20)).await;
+        pause.send(()).await.unwrap();
+    });
+
+    let exit = tokio::time::timeout(
+        Duration::from_secs(2),
+        engine.run(&mut history, "pause after usage"),
+    )
+    .await
+    .expect("run should observe pause");
+    assert!(matches!(exit, EngineRunExit::Paused));
+    let observed = observed.lock().unwrap();
+    assert_eq!(observed.len(), 1);
+    assert_eq!(observed[0].input_tokens, Some(90));
+    assert_eq!(observed[0].output_tokens, Some(2));
+}
 
 /// Verify that system prompt can be set in Mutable state
 #[test]
diff --git a/crates/worker/src/compact/metrics_tracker.rs b/crates/worker/src/compact/metrics_tracker.rs
index c0e81033c..dcea2131a 100644
--- a/crates/worker/src/compact/metrics_tracker.rs
+++ b/crates/worker/src/compact/metrics_tracker.rs
@@ -2,9 +2,8 @@
 //! Engine callbacks (which run synchronously and cannot themselves
 //! perform `async` store writes).
 //!
-//! Worker drains this buffer in `persist_turn` and writes each metric via
-//! `session_metrics::record_metric`, alongside the regular `LlmUsage`
-//! entries.
+//! Worker drains this buffer at the next successful request accounting
+//! boundary, with terminal `persist_turn` as a fallback.
 
 use std::sync::Mutex;
 
@@ -21,12 +20,12 @@ impl MetricsTracker {
         }
     }
 
-    /// Queue a metric for the next `persist_turn` flush.
+    /// Queue a metric for the next request or terminal accounting flush.
     pub(crate) fn push(&self, metric: Metric) {
         self.pending.lock().unwrap().push(metric);
     }
 
-    /// Drain all queued metrics. Called by Worker after a run completes.
+    /// Drain all queued metrics at an accounting boundary.
     pub(crate) fn drain(&self) -> Vec<Metric> {
         std::mem::take(&mut *self.pending.lock().unwrap())
     }
diff --git a/crates/worker/src/compact/usage_tracker.rs b/crates/worker/src/compact/usage_tracker.rs
index 3310ca338..b0d068f5c 100644
--- a/crates/worker/src/compact/usage_tracker.rs
+++ b/crates/worker/src/compact/usage_tracker.rs
@@ -7,17 +7,24 @@
 //! - **`on_usage` callback** (sync closure): receives the aggregated final
 //!   `UsageEvent` for that request after the stream completes.
 //!
-//! Pairing the two yields one `UsageRecord` per LLM call. Worker drains them
-//! in `persist_turn` and writes them as `LogEntry::LlmUsage` entries.
+//! Pairing the two yields one `UsageRecord` per LLM call. Worker commits
+//! successful requests from the assistant-turn boundary and uses terminal
+//! `persist_turn` only for records that have not already crossed that boundary.
 //!
 //! Multiple LLM calls per Worker run (tool loop) are supported: each call
 //! produces its own `(history_len, UsageEvent)` pair, and the records are
 //! buffered in chronological order.
 
+use std::collections::VecDeque;
 use std::sync::Mutex;
 
 use agen::UsageRecord;
 use agen::timeline::event::UsageEvent;
+use session_metrics::Metric;
+use session_store::{LogEntry, StoreError, segment_log};
+
+use crate::compact::state::CompactState;
+use crate::compact::telemetry::correlated_post_request_metric;
 
 /// The metric emitted after the next measured provider request.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -41,11 +48,15 @@ pub(crate) struct PostRequestLink {
     pub(crate) metric: PostRequestMetric,
 }
 
-/// One drained measurement and its causal metric links.
+/// One measured request and its causal metric links.
 #[derive(Debug, Clone)]
 pub(crate) struct RecordedUsage {
     pub(crate) record: UsageRecord,
     pub(crate) post_requests: Vec<PostRequestLink>,
+    /// True only when the provider stream completed normally and reported an
+    /// input occupancy. Partial/error/cancelled streams remain billable usage
+    /// records, but cannot satisfy a post-compaction rearm.
+    pub(crate) completed_with_occupancy: bool,
 }
 
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
@@ -56,29 +67,52 @@ pub(crate) struct UsageSnapshot {
     pub(crate) output_tokens: u64,
 }
 
+#[derive(Debug)]
+struct PendingRequest {
+    id: u64,
+    history_len: usize,
+    usage_recorded: bool,
+}
+
+#[derive(Debug)]
+struct PendingRecordedUsage {
+    request_id: u64,
+    has_input_occupancy: bool,
+    usage: RecordedUsage,
+}
+
+#[derive(Debug, Default)]
+struct UsageTrackerState {
+    next_request_id: u64,
+    pending_request: Option<PendingRequest>,
+    pending_correlations: Vec<PostRequestLink>,
+    pending_records: VecDeque<PendingRecordedUsage>,
+}
+
 /// Shared between the pre-request hook, the `on_usage` callback, and Worker.
 pub(crate) struct UsageTracker {
-    /// `history.len()` captured at the most recent `pre_llm_request`.
-    /// Cleared when paired with an incoming `on_usage` event.
-    pending_history_len: Mutex<Option<usize>>,
-    /// Optional causal link consumed by the next measured request.
-    pending_correlations: Mutex<Vec<PostRequestLink>>,
-    /// Records accumulated during the current run; drained by Worker.
-    pending_records: Mutex<Vec<RecordedUsage>>,
+    state: Mutex<UsageTrackerState>,
 }
 
 impl UsageTracker {
     pub(crate) fn new() -> Self {
         Self {
-            pending_history_len: Mutex::new(None),
-            pending_correlations: Mutex::new(Vec::new()),
-            pending_records: Mutex::new(Vec::new()),
+            state: Mutex::new(UsageTrackerState::default()),
         }
     }
 
     /// Called from a `pre_llm_request` hook with the current history length.
+    /// Replacing an older pending request classifies that request as incomplete;
+    /// any partial usage already recorded for it remains available for billing.
     pub(crate) fn note_request(&self, history_len: usize) {
-        *self.pending_history_len.lock().unwrap() = Some(history_len);
+        let mut state = self.state.lock().unwrap();
+        let id = state.next_request_id;
+        state.next_request_id = state.next_request_id.wrapping_add(1);
+        state.pending_request = Some(PendingRequest {
+            id,
+            history_len,
+            usage_recorded: false,
+        });
     }
 
     /// Pair a prune event with the next provider request.
@@ -92,25 +126,31 @@ impl UsageTracker {
     }
 
     fn note_post_request(&self, id: String, metric: PostRequestMetric) {
-        let mut pending = self.pending_correlations.lock().unwrap();
-        pending.retain(|link| link.metric != metric);
-        pending.push(PostRequestLink {
+        let mut state = self.state.lock().unwrap();
+        state
+            .pending_correlations
+            .retain(|link| link.metric != metric);
+        state.pending_correlations.push(PostRequestLink {
             correlation_id: id,
             metric,
         });
     }
 
     /// Called from the `on_usage` callback with the aggregated final
-    /// UsageEvent. If a `history_len` was previously stashed via
-    /// `note_request`, builds a `RecordedUsage` and pushes it onto the
-    /// buffer. If not (e.g. test code that fires Usage outside a request),
-    /// drops the event.
+    /// UsageEvent. The record remains associated with the current request until
+    /// [`Self::request_completed`] proves that the provider stream completed
+    /// normally. Error/cancel paths still retain the measurement for billing.
     pub(crate) fn record_usage(&self, event: &UsageEvent) {
-        let history_len = match self.pending_history_len.lock().unwrap().take() {
-            Some(n) => n,
-            None => return,
+        let mut state = self.state.lock().unwrap();
+        let Some(pending) = state.pending_request.as_mut() else {
+            return;
         };
-        let post_requests = std::mem::take(&mut *self.pending_correlations.lock().unwrap());
+        if pending.usage_recorded {
+            return;
+        }
+        pending.usage_recorded = true;
+        let request_id = pending.id;
+        let history_len = pending.history_len;
         // UsageEvent.input_tokens は scheme 層で「占有量（プロンプト全長）」に
         // 正規化済みである前提（Anthropic は cache_read + cache_creation を
         // 加算して emit する）。
@@ -118,35 +158,166 @@ impl UsageTracker {
         let cache_read = event.cache_read_input_tokens.unwrap_or(0);
         let cache_write = event.cache_creation_input_tokens.unwrap_or(0);
         let output = event.output_tokens.unwrap_or(0);
-        self.pending_records.lock().unwrap().push(RecordedUsage {
-            record: UsageRecord {
-                history_len,
-                input_total_tokens: input_total,
-                cache_read_tokens: cache_read,
-                cache_write_tokens: cache_write,
-                output_tokens: output,
+        state.pending_records.push_back(PendingRecordedUsage {
+            request_id,
+            has_input_occupancy: event.input_tokens.is_some(),
+            usage: RecordedUsage {
+                record: UsageRecord {
+                    history_len,
+                    input_total_tokens: input_total,
+                    cache_read_tokens: cache_read,
+                    cache_write_tokens: cache_write,
+                    output_tokens: output,
+                },
+                post_requests: Vec::new(),
+                completed_with_occupancy: false,
             },
-            post_requests,
         });
     }
 
-    /// Return a clone of the accumulated `UsageRecord`s without clearing them.
+    /// Mark the current request as normally completed. A post-request causal
+    /// link is consumed only when that exact request also reported a valid
+    /// input occupancy; callbacks from partial/error/cancelled streams cannot
+    /// rearm the compaction guard.
+    pub(crate) fn request_completed(&self) {
+        let mut state = self.state.lock().unwrap();
+        let Some(pending) = state.pending_request.take() else {
+            return;
+        };
+        let Some(recorded_index) = state
+            .pending_records
+            .iter()
+            .rposition(|recorded| recorded.request_id == pending.id)
+        else {
+            return;
+        };
+        let has_occupancy = state.pending_records[recorded_index].has_input_occupancy;
+        if !has_occupancy {
+            return;
+        }
+        let post_requests = std::mem::take(&mut state.pending_correlations);
+        let recorded = &mut state.pending_records[recorded_index].usage;
+        recorded.completed_with_occupancy = true;
+        recorded.post_requests = post_requests;
+    }
+
+    /// Return a clone of accumulated measurements without clearing them.
     /// Used by request-time circuit breakers that need the same occupancy
     /// projection as Worker persistence while the run is still active.
     pub(crate) fn records(&self) -> Vec<UsageRecord> {
-        self.pending_records
+        self.state
             .lock()
             .unwrap()
+            .pending_records
             .iter()
-            .map(|r| r.record.clone())
+            .map(|r| r.usage.record.clone())
             .collect()
     }
 
-    /// Drain accumulated records. Called by Worker after a run completes,
-    /// before persisting the turn.
-    pub(crate) fn drain(&self) -> Vec<RecordedUsage> {
-        std::mem::take(&mut *self.pending_records.lock().unwrap())
+    /// Number of measurements that have not crossed the durable commit
+    /// boundary. Session rewrites must not activate a replacement Segment while
+    /// this is non-zero, because their history coordinates belong to the current
+    /// Segment.
+    pub(crate) fn pending_record_count(&self) -> usize {
+        self.state.lock().unwrap().pending_records.len()
     }
+
+    /// Remove the oldest measurement for one durable commit attempt.
+    pub(crate) fn take_next(&self) -> Option<RecordedUsage> {
+        self.state
+            .lock()
+            .unwrap()
+            .pending_records
+            .pop_front()
+            .map(|recorded| recorded.usage)
+    }
+
+    /// Restore a measurement after its durable commit failed.
+    pub(crate) fn restore_front(&self, usage: RecordedUsage) {
+        self.state
+            .lock()
+            .unwrap()
+            .pending_records
+            .push_front(PendingRecordedUsage {
+                // Persist ordering is the only remaining identity requirement.
+                request_id: u64::MAX,
+                has_input_occupancy: usage.completed_with_occupancy,
+                usage,
+            });
+    }
+
+    /// Drain accumulated records at a terminal Worker boundary.
+    pub(crate) fn drain(&self) -> Vec<RecordedUsage> {
+        let mut state = self.state.lock().unwrap();
+        state.pending_request = None;
+        state
+            .pending_records
+            .drain(..)
+            .map(|recorded| recorded.usage)
+            .collect()
+    }
+}
+
+/// Commit all currently measured usage records in request order.
+///
+/// A record is removed from the tracker only after its `LlmUsage` append
+/// succeeds. This is the single accounting boundary shared by the in-run
+/// interceptor and terminal `persist_turn`, preventing both loss on append
+/// failure and duplicate terminal writes. Correlated metrics are handed to the
+/// same commit boundary so they stay on the usage record's Segment; the return
+/// value is only useful as test/diagnostic evidence and must not be re-emitted.
+pub(crate) fn persist_pending_usage(
+    tracker: &UsageTracker,
+    usage_history: &Mutex<Vec<UsageRecord>>,
+    compact_state: Option<&CompactState>,
+    mut commit: impl FnMut(LogEntry, &mut dyn FnMut(), &[Metric]) -> Result<(), StoreError>,
+) -> Result<Vec<Metric>, StoreError> {
+    let mut metrics = Vec::new();
+    while let Some(recorded) = tracker.take_next() {
+        let RecordedUsage {
+            record,
+            post_requests,
+            completed_with_occupancy,
+        } = recorded;
+        let post_metrics = post_requests
+            .iter()
+            .map(|link| correlated_post_request_metric(link.metric, &link.correlation_id, &record))
+            .collect::<Vec<_>>();
+        let usage_entry = LogEntry::LlmUsage {
+            ts: segment_log::now_millis(),
+            history_len: record.history_len,
+            input_total_tokens: record.input_total_tokens,
+            cache_read_tokens: record.cache_read_tokens,
+            cache_write_tokens: record.cache_write_tokens,
+            output_tokens: record.output_tokens,
+        };
+        let mut publish_committed_usage = || {
+            usage_history
+                .lock()
+                .expect("usage_history poisoned")
+                .push(record.clone());
+
+            if completed_with_occupancy
+                && post_requests
+                    .iter()
+                    .any(|link| link.metric == PostRequestMetric::Compaction)
+            {
+                if let Some(state) = compact_state {
+                    state.post_compact_request_committed();
+                }
+            }
+        };
+        if let Err(error) = commit(usage_entry, &mut publish_committed_usage, &post_metrics) {
+            tracker.restore_front(RecordedUsage {
+                record,
+                post_requests,
+                completed_with_occupancy,
+            });
+            return Err(error);
+        }
+        metrics.extend(post_metrics);
+    }
+    Ok(metrics)
 }
 
 #[cfg(test)]
@@ -230,6 +401,7 @@ mod tests {
         tracker.note_correlation_id("prune-id".into());
         tracker.note_request(5);
         tracker.record_usage(&make_event(100, 10, 2, 20));
+        tracker.request_completed();
 
         let records = tracker.drain();
         assert_eq!(records[0].post_requests.len(), 2);
@@ -248,9 +420,11 @@ mod tests {
         tracker.note_correlation_id("abc".into());
         tracker.note_request(5);
         tracker.record_usage(&make_event(100, 0, 0, 20));
+        tracker.request_completed();
         // Next request without a fresh stash → no correlation_id.
         tracker.note_request(10);
         tracker.record_usage(&make_event(200, 50, 0, 30));
+        tracker.request_completed();
 
         let records = tracker.drain();
         assert_eq!(records.len(), 2);
@@ -259,4 +433,130 @@ mod tests {
         assert_eq!(records[0].post_requests[0].metric, PostRequestMetric::Prune);
         assert!(records[1].post_requests.is_empty());
     }
+
+    #[test]
+    fn partial_usage_does_not_consume_post_compact_link() {
+        let tracker = UsageTracker::new();
+        tracker.note_compaction_correlation_id("compact-id".into());
+        tracker.note_request(5);
+        tracker.record_usage(&make_event(90, 0, 0, 2));
+
+        // Starting another request proves the prior stream did not complete.
+        tracker.note_request(6);
+        tracker.record_usage(&make_event(100, 0, 0, 3));
+        tracker.request_completed();
+
+        let records = tracker.drain();
+        assert_eq!(records.len(), 2);
+        assert!(!records[0].completed_with_occupancy);
+        assert!(records[0].post_requests.is_empty());
+        assert!(records[1].completed_with_occupancy);
+        assert_eq!(records[1].post_requests.len(), 1);
+        assert_eq!(records[1].post_requests[0].correlation_id, "compact-id");
+    }
+
+    #[test]
+    fn completed_request_without_input_occupancy_does_not_consume_link() {
+        let tracker = UsageTracker::new();
+        tracker.note_compaction_correlation_id("compact-id".into());
+        tracker.note_request(5);
+        let mut missing_input = make_event(0, 0, 0, 2);
+        missing_input.input_tokens = None;
+        tracker.record_usage(&missing_input);
+        tracker.request_completed();
+
+        tracker.note_request(6);
+        tracker.record_usage(&make_event(100, 0, 0, 3));
+        tracker.request_completed();
+
+        let records = tracker.drain();
+        assert!(!records[0].completed_with_occupancy);
+        assert!(records[0].post_requests.is_empty());
+        assert!(records[1].completed_with_occupancy);
+        assert_eq!(records[1].post_requests[0].correlation_id, "compact-id");
+    }
+
+    #[test]
+    fn completed_request_without_usage_does_not_complete_previous_partial_record() {
+        let tracker = UsageTracker::new();
+        tracker.note_compaction_correlation_id("compact-id".into());
+        tracker.note_request(5);
+        tracker.record_usage(&make_event(90, 0, 0, 2));
+
+        tracker.note_request(6);
+        tracker.request_completed();
+
+        let records = tracker.drain();
+        assert_eq!(records.len(), 1);
+        assert!(!records[0].completed_with_occupancy);
+        assert!(records[0].post_requests.is_empty());
+    }
+
+    #[test]
+    fn failed_commit_can_restore_record_without_reordering() {
+        let tracker = UsageTracker::new();
+        tracker.note_request(1);
+        tracker.record_usage(&make_event(10, 0, 0, 1));
+        tracker.request_completed();
+        tracker.note_request(2);
+        tracker.record_usage(&make_event(20, 0, 0, 2));
+        tracker.request_completed();
+
+        let first = tracker.take_next().expect("first record");
+        tracker.restore_front(first);
+        let records = tracker.drain();
+        assert_eq!(records.len(), 2);
+        assert_eq!(records[0].record.input_total_tokens, 10);
+        assert_eq!(records[1].record.input_total_tokens, 20);
+    }
+
+    #[test]
+    fn compaction_rearms_only_after_usage_append_succeeds() {
+        use crate::compact::state::{AutomaticCompactDecision, CompactionOutcome};
+
+        let tracker = UsageTracker::new();
+        let state = CompactState::new(None, Some(10), 0);
+        assert!(matches!(
+            state.evaluate_request(11),
+            AutomaticCompactDecision::Start(_)
+        ));
+        assert!(state.complete_automatic(CompactionOutcome::Succeeded));
+
+        tracker.note_compaction_correlation_id("compact-id".into());
+        tracker.note_request(1);
+        tracker.record_usage(&make_event(5, 0, 0, 1));
+        tracker.request_completed();
+        let usage_history = Mutex::new(Vec::new());
+
+        let error = persist_pending_usage(&tracker, &usage_history, Some(&state), |_, _, _| {
+            Err(StoreError::Io(std::io::Error::other("synthetic failure")))
+        })
+        .expect_err("usage append must fail");
+        assert!(error.to_string().contains("synthetic failure"));
+        assert!(usage_history.lock().unwrap().is_empty());
+        assert!(matches!(
+            state.evaluate_request(11),
+            AutomaticCompactDecision::Block(_)
+        ));
+
+        let mut entries = Vec::new();
+        let metrics = persist_pending_usage(
+            &tracker,
+            &usage_history,
+            Some(&state),
+            |entry, after_commit, _post_metrics| {
+                entries.push(entry);
+                after_commit();
+                Ok(())
+            },
+        )
+        .expect("retry should commit");
+        assert_eq!(entries.len(), 1);
+        assert_eq!(usage_history.lock().unwrap().len(), 1);
+        assert_eq!(metrics.len(), 1);
+        assert!(matches!(
+            state.evaluate_request(11),
+            AutomaticCompactDecision::Start(_)
+        ));
+    }
 }
diff --git a/crates/worker/src/ipc/interceptor.rs b/crates/worker/src/ipc/interceptor.rs
index 773c2298a..802d0c7d7 100644
--- a/crates/worker/src/ipc/interceptor.rs
+++ b/crates/worker/src/ipc/interceptor.rs
@@ -23,11 +23,12 @@ use agen::interceptor::{
 use agen::tool::ToolOutput;
 use arc_swap::ArcSwap;
 use async_trait::async_trait;
-use tracing::info;
+use tracing::{info, warn};
 
+use crate::compact::metrics_tracker::MetricsTracker;
 use crate::compact::state::{AutomaticCompactDecision, CompactState};
-use crate::compact::usage_tracker::UsageTracker;
-use session_store::SystemItem;
+use crate::compact::usage_tracker::{UsageTracker, persist_pending_usage};
+use session_store::{SystemItem, segment_log};
 
 use crate::hook::{
     HookEventKind, HookPostToolAction, HookPreRequestAction, HookPreToolAction, HookPromptAction,
@@ -81,6 +82,10 @@ pub(crate) struct WorkerInterceptor {
     /// persisted into `usage_history`. Subsequent tool-loop LLM calls must
     /// see these records during pre-request safety accounting.
     usage_tracker: Option<Arc<UsageTracker>>,
+    /// Metrics produced synchronously while projecting the request (notably
+    /// prune.fire/skip). Flushed before the matching request usage so metric
+    /// correlation order remains stable at the in-run commit boundary.
+    metrics_tracker: Option<Arc<MetricsTracker>>,
     /// Pending-notification buffer drained into `worker.history`
     /// via [`Self::pending_history_appends`] just before the next LLM
     /// request. The Engine `extend`s these into its persistent history
@@ -152,6 +157,7 @@ impl WorkerInterceptor {
             compact_state,
             usage_history,
             usage_tracker: None,
+            metrics_tracker: None,
             pending_notifies,
             pending_attachments,
             prompts,
@@ -168,6 +174,11 @@ impl WorkerInterceptor {
         self
     }
 
+    pub(crate) fn with_metrics_tracker(mut self, metrics_tracker: Arc<MetricsTracker>) -> Self {
+        self.metrics_tracker = Some(metrics_tracker);
+        self
+    }
+
     pub(crate) fn with_prompt_workspace_id(mut self, workspace_id: Option<String>) -> Self {
         self.prompt_workspace_id = workspace_id;
         self
@@ -272,6 +283,56 @@ impl WorkerInterceptor {
             }
         }
     }
+    fn try_persist_metric(writer: &Arc<dyn SystemItemCommitter>, metric: &session_metrics::Metric) {
+        let payload = serde_json::to_value(metric).expect("Metric is Serialize");
+        if writer
+            .commit_log_entry(session_store::LogEntry::Extension {
+                ts: segment_log::now_millis(),
+                domain: session_metrics::DOMAIN.into(),
+                payload,
+            })
+            .is_err()
+        {
+            warn!(name = %metric.name, "failed to record session metric; dropping");
+        }
+    }
+
+    fn persist_completed_request_usage(&self) -> InterceptorResult<()> {
+        let Some(tracker) = self.usage_tracker.as_ref() else {
+            return Ok(());
+        };
+        tracker.request_completed();
+
+        let (Some(writer), Some(usage_history)) =
+            (self.log_writer.as_ref(), self.usage_history.as_ref())
+        else {
+            // Low-level Worker tests may omit the type-erased writer. Their
+            // terminal persist_turn remains the durable fallback.
+            return Ok(());
+        };
+        if let Some(metrics_tracker) = self.metrics_tracker.as_ref() {
+            for metric in metrics_tracker.drain() {
+                Self::try_persist_metric(writer, &metric);
+            }
+        }
+
+        persist_pending_usage(
+            tracker,
+            usage_history,
+            self.compact_state.as_deref(),
+            |entry, after_commit, post_metrics| {
+                writer.commit_usage_boundary(entry, after_commit, post_metrics)
+            },
+        )
+        .map_err(|error| {
+            InterceptorError::new(
+                InterceptorErrorCategory::Dependency,
+                format!("usage persistence failed: {error}"),
+            )
+        })?;
+        Ok(())
+    }
+
     fn attach_prompt_provenance(&self, items: &mut [SystemItem]) {
         let prompts = self.prompts.load();
         let projection = prompts.projection();
@@ -596,6 +657,11 @@ impl Interceptor<SessionHistoryMetadata> for WorkerInterceptor {
         &self,
         context: AssistantTurnEndContext<'_, SessionHistoryMetadata>,
     ) -> InterceptorResult<TurnEndAction> {
+        // `on_assistant_turn_end` is reached only after a normal provider
+        // stream completed and its usage callback was flushed. Commit that
+        // exact request before tools can extend the same logical run.
+        self.persist_completed_request_usage()?;
+
         let history = context.history;
         let final_text_preview = history
             .iter()
@@ -752,6 +818,25 @@ mod tests {
         }
     }
 
+    struct FailingUsageCommitter {
+        attempts: Arc<AtomicUsize>,
+    }
+
+    impl SystemItemCommitter for FailingUsageCommitter {
+        fn commit_log_entry(
+            &self,
+            entry: session_store::LogEntry,
+        ) -> Result<(), session_store::StoreError> {
+            if matches!(entry, session_store::LogEntry::LlmUsage { .. }) {
+                self.attempts.fetch_add(1, Ordering::SeqCst);
+                return Err(session_store::StoreError::Io(std::io::Error::other(
+                    "synthetic usage failure",
+                )));
+            }
+            Ok(())
+        }
+    }
+
     struct AppendingPreRequestHook {
         saw_handle: Arc<AtomicBool>,
     }
@@ -955,6 +1040,68 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn completed_request_usage_failure_stops_before_rearm() {
+        use crate::compact::state::{AutomaticCompactDecision, CompactionOutcome};
+
+        let registry = Arc::new(HookRegistryBuilder::new().build());
+        let state = Arc::new(CompactState::new(None, Some(10), 0));
+        assert!(matches!(
+            state.evaluate_request(11),
+            AutomaticCompactDecision::Start(_)
+        ));
+        assert!(state.complete_automatic(CompactionOutcome::Succeeded));
+
+        let usage_history = Arc::new(Mutex::new(Vec::new()));
+        let tracker = Arc::new(UsageTracker::new());
+        tracker.note_compaction_correlation_id("compact-id".into());
+        tracker.note_request(1);
+        tracker.record_usage(&agen::event::UsageEvent {
+            input_tokens: Some(5),
+            output_tokens: Some(1),
+            total_tokens: Some(6),
+            cache_read_input_tokens: Some(0),
+            cache_creation_input_tokens: Some(0),
+        });
+        let attempts = Arc::new(AtomicUsize::new(0));
+        let interceptor = WorkerInterceptor::new(
+            registry,
+            Some(Arc::clone(&state)),
+            Some(Arc::clone(&usage_history)),
+            NotifyBuffer::new(),
+            Arc::new(Mutex::new(Vec::new())),
+            test_prompts(),
+            Some(Arc::new(FailingUsageCommitter {
+                attempts: Arc::clone(&attempts),
+            })),
+        )
+        .with_usage_tracker(Arc::clone(&tracker));
+
+        let error = interceptor
+            .on_assistant_turn_end(AssistantTurnEndContext {
+                invocation: Default::default(),
+                assistant_entries: &[],
+                history: &[],
+                tool_calls: &[],
+            })
+            .await
+            .expect_err("usage storage failure must stop the request boundary");
+
+        assert_eq!(error.category(), InterceptorErrorCategory::Dependency);
+        assert!(error.diagnostic().contains("usage persistence failed"));
+        assert_eq!(attempts.load(Ordering::SeqCst), 1);
+        assert_eq!(
+            tracker.records().len(),
+            1,
+            "failed record must remain pending"
+        );
+        assert!(usage_history.lock().unwrap().is_empty());
+        assert!(matches!(
+            state.evaluate_request(11),
+            AutomaticCompactDecision::Block(_)
+        ));
+    }
+
     #[tokio::test]
     async fn pre_llm_request_counts_in_flight_usage_records() {
         let registry = Arc::new(HookRegistryBuilder::new().build());
diff --git a/crates/worker/src/worker.rs b/crates/worker/src/worker.rs
index e1a90d770..9463dcede 100644
--- a/crates/worker/src/worker.rs
+++ b/crates/worker/src/worker.rs
@@ -45,9 +45,9 @@ use crate::compact::state::{
 };
 use crate::compact::telemetry::{
     CompactAttempt, CompactFailureCategory, CompactMode, CompactSuccessStats,
-    CompactThresholdPolicy, correlated_post_request_metric, new_compact_metric_correlation_id,
+    CompactThresholdPolicy, new_compact_metric_correlation_id,
 };
-use crate::compact::usage_tracker::UsageTracker;
+use crate::compact::usage_tracker::{UsageTracker, persist_pending_usage};
 use crate::feature::background::{BackgroundTaskRewriteGuard, FeatureBackgroundTaskRegistry};
 use crate::feature::builtin::memory::WorkspaceMemoryBackendError;
 use crate::feature::builtin::{TaskFeature, WorkerObservationProvider};
@@ -1947,6 +1947,28 @@ impl PendingSubmissionHandle<session_store::FsStore> {
 pub trait SystemItemCommitter: Send + Sync {
     fn commit_log_entry(&self, entry: LogEntry) -> Result<(), StoreError>;
 
+    /// Commit one authoritative usage record while holding the same append
+    /// barrier across its in-memory publication, guard rearm, and correlated
+    /// post-request metrics. Metric appends remain best-effort.
+    fn commit_usage_boundary(
+        &self,
+        entry: LogEntry,
+        after_commit: &mut dyn FnMut(),
+        post_metrics: &[session_metrics::Metric],
+    ) -> Result<(), StoreError> {
+        self.commit_log_entry(entry)?;
+        after_commit();
+        for metric in post_metrics {
+            let payload = serde_json::to_value(metric).expect("Metric is Serialize");
+            let _ = self.commit_log_entry(LogEntry::Extension {
+                ts: segment_log::now_millis(),
+                domain: session_metrics::DOMAIN.into(),
+                payload,
+            });
+        }
+        Ok(())
+    }
+
     fn commit_system_item_with_extensions(
         &self,
         item: SystemItem,
@@ -1978,6 +2000,30 @@ where
     fn commit_log_entry(&self, entry: LogEntry) -> Result<(), StoreError> {
         self.append_entry(entry)
     }
+
+    fn commit_usage_boundary(
+        &self,
+        entry: LogEntry,
+        after_commit: &mut dyn FnMut(),
+        post_metrics: &[session_metrics::Metric],
+    ) -> Result<(), StoreError> {
+        let _append_guard = self
+            .state
+            .append_lock
+            .lock()
+            .expect("segment append lock poisoned");
+        self.append_entry_locked(entry)?;
+        after_commit();
+        for metric in post_metrics {
+            let payload = serde_json::to_value(metric).expect("Metric is Serialize");
+            let _ = self.append_entry_locked(LogEntry::Extension {
+                ts: segment_log::now_millis(),
+                domain: session_metrics::DOMAIN.into(),
+                payload,
+            });
+        }
+        Ok(())
+    }
 }
 
 struct SessionFlowRuntimeStateCommitter<St: Store + Clone> {
@@ -2155,16 +2201,16 @@ pub struct Worker<C: LlmClient, St: Store> {
     /// Shared compaction state (present when threshold is configured).
     compact_state: Option<Arc<CompactState>>,
     /// Per-LLM-request Usage tracker. Always present after construction.
-    /// Captures `(history_len, UsageEvent)` pairs during a run; drained
-    /// in `persist_turn` and persisted as `LogEntry::LlmUsage` entries.
+    /// Captures `(history_len, UsageEvent)` pairs until their authoritative
+    /// request-boundary or terminal `LlmUsage` append succeeds.
     usage_tracker: Arc<UsageTracker>,
     /// Sync-side buffer for `Metric` values queued from inside Engine
-    /// callbacks (currently the prune observer). Drained in `persist_turn`
-    /// and written via `session_metrics::record_metric` alongside
-    /// `LogEntry::LlmUsage`. Always present after construction.
+    /// callbacks (currently the prune observer). Drained at the same request
+    /// boundary as usage, with terminal `persist_turn` as the fallback.
     metrics_tracker: Arc<crate::compact::metrics_tracker::MetricsTracker>,
     /// Cumulative Usage measurement timeline, one entry per LLM call.
-    /// Restored from session log on `restore`, appended on each persist.
+    /// Restored from session log on `restore`, appended after each durable
+    /// request-boundary usage commit.
     /// Read by token-accounting APIs (`Worker::total_tokens`, etc.).
     ///
     /// Wrapped in `Arc<Mutex>` so that callbacks injected into the
@@ -3330,8 +3376,8 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
     /// Handle to the synchronous `MetricsTracker` buffer.
     ///
     /// Engine callbacks (e.g. the prune observer) clone this `Arc` and
-    /// `.push(metric)` into it; Worker drains it in `persist_turn` and
-    /// writes each metric via `session_metrics::record_metric`.
+    /// `.push(metric)` into it; Worker drains it at the next request accounting
+    /// boundary, with terminal persistence as fallback.
     pub(crate) fn metrics_tracker_handle(
         &self,
     ) -> Arc<crate::compact::metrics_tracker::MetricsTracker> {
@@ -3567,6 +3613,7 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
                 self.pending_committed_history.clone(),
             )
             .with_usage_tracker(self.usage_tracker.clone())
+            .with_metrics_tracker(self.metrics_tracker.clone())
             .with_prompt_workspace_id(
                 self.workspace_context
                     .workspace_id()
@@ -4452,6 +4499,15 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
         self.handle_worker_result(result, history_before).await
     }
 
+    fn ensure_no_pending_usage_for_segment_replacement(&self) -> Result<(), WorkerError> {
+        let pending_records = self.usage_tracker.pending_record_count();
+        if pending_records == 0 {
+            Ok(())
+        } else {
+            Err(WorkerError::PendingUsagePersistence { pending_records })
+        }
+    }
+
     /// Ensure the session exists and the writer's tally still matches
     /// the on-disk entry count.
     ///
@@ -4493,6 +4549,7 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
         if store_count == entries_written {
             return Ok(());
         }
+        self.ensure_no_pending_usage_for_segment_replacement()?;
         // Auto-fork within the same Session: mint a fresh Segment and
         // switch to it. The source segment is left immutable (no terminal
         // marker is written back); the fork relationship is recorded
@@ -5029,45 +5086,38 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
             self.try_record_metric(&metric);
         }
 
-        // Persist any LLM Usage measurements collected during this run.
-        // One LogEntry::LlmUsage per LLM call (the tool loop may have run
-        // many calls within a single Worker::run). Each is also appended to
-        // the in-memory `usage_history` so token-accounting APIs see it
-        // before the next run. Records carrying a `correlation_id` (set
-        // by an upstream observer such as the prune projection) also get
-        // a paired `prune.post_request` metric so cache_read/write can be
-        // joined back to the originating event.
-        let usage_records = self.usage_tracker.drain();
-        for recorded in usage_records {
-            let crate::compact::usage_tracker::RecordedUsage {
-                record,
-                post_requests,
-            } = recorded;
-            let committed_post_compact_request = post_requests.iter().any(|link| {
-                link.metric == crate::compact::usage_tracker::PostRequestMetric::Compaction
-            });
-            self.commit_entry(LogEntry::LlmUsage {
-                ts: segment_log::now_millis(),
-                history_len: record.history_len,
-                input_total_tokens: record.input_total_tokens,
-                cache_read_tokens: record.cache_read_tokens,
-                cache_write_tokens: record.cache_write_tokens,
-                output_tokens: record.output_tokens,
-            })?;
-            if committed_post_compact_request {
-                if let Some(state) = &self.compact_state {
-                    state.post_compact_request_committed();
-                }
-            }
-            for link in post_requests {
-                let metric =
-                    correlated_post_request_metric(link.metric, &link.correlation_id, &record);
+        // Persist any request measurements that did not already cross the
+        // in-run request boundary (for example partial usage from an errored or
+        // cancelled stream, or low-level Workers without an attached writer).
+        // `persist_pending_usage` removes each record only after its authoritative
+        // LlmUsage append succeeds, so the in-run and terminal paths cannot
+        // double-write or silently lose a failed append.
+        let usage_tracker = self.usage_tracker.clone();
+        let usage_history = self.usage_history.clone();
+        let compact_state = self.compact_state.clone();
+        if let Some(writer) = self.log_writer.clone() {
+            let _ = persist_pending_usage(
+                &usage_tracker,
+                &usage_history,
+                compact_state.as_deref(),
+                |entry, after_commit, post_metrics| {
+                    writer.commit_usage_boundary(entry, after_commit, post_metrics)
+                },
+            )?;
+        } else {
+            let post_request_metrics = persist_pending_usage(
+                &usage_tracker,
+                &usage_history,
+                compact_state.as_deref(),
+                |entry, after_commit, _post_metrics| {
+                    self.commit_entry(entry)?;
+                    after_commit();
+                    Ok(())
+                },
+            )?;
+            for metric in post_request_metrics {
                 self.try_record_metric(&metric);
             }
-            self.usage_history
-                .lock()
-                .expect("usage_history poisoned")
-                .push(record);
         }
 
         let interrupted = matches!(
@@ -5127,6 +5177,7 @@ impl<C: LlmClient + 'static, St: Store> Worker<C, St> {
         mut cancel: Option<tokio::sync::watch::Receiver<bool>>,
         trigger: CompactionTrigger,
     ) -> Result<SegmentId, WorkerError> {
+        self.ensure_no_pending_usage_for_segment_replacement()?;
         self.release_pending_compaction_service().await?;
         let _rewrite_guard = self
             .prepare_session_rewrite(SessionRewriteKind::Compact)
@@ -7438,7 +7489,9 @@ fn compact_failure_category(error: &WorkerError) -> CompactFailureCategory {
         | WorkerError::SegmentActivationIncomplete { .. } => {
             CompactFailureCategory::ActiveSegmentCommit
         }
-        WorkerError::Store(_) => CompactFailureCategory::Storage,
+        WorkerError::Store(_) | WorkerError::PendingUsagePersistence { .. } => {
+            CompactFailureCategory::Storage
+        }
         WorkerError::InvalidState(_) | WorkerError::Engine(_) => {
             CompactFailureCategory::InternalWorker
         }
@@ -7502,6 +7555,11 @@ pub enum WorkerError {
     #[error(transparent)]
     Provider(#[from] crate::model_client::ProviderError),
 
+    #[error(
+        "cannot replace the active Segment while {pending_records} usage record(s) await durable persistence"
+    )]
+    PendingUsagePersistence { pending_records: usize },
+
     #[error("active Segment changed before compaction commit")]
     CompactActiveSegmentChanged,
 
@@ -8567,6 +8625,117 @@ mod build_summary_prompt_tests {
         }
     }
 
+    #[tokio::test]
+    async fn failed_usage_append_fences_segment_rotation_until_retry_commits_once() {
+        let dir = tempfile::tempdir().unwrap();
+        let store = session_store::FsStore::new(dir.path().join("sessions")).unwrap();
+        let mut worker = Worker::new(
+            minimal_manifest(),
+            Engine::<_, Mutable, SessionHistoryMetadata>::new_annotated(NoopClient),
+            store.clone(),
+            WorkerWorkspaceContext::no_workspace(),
+            WorkerFilesystemAuthority::None,
+            Scope::empty(),
+        )
+        .await
+        .unwrap();
+        worker.ensure_segment_head().await.unwrap();
+        let session_id = worker.session_id();
+        let source_segment = worker.segment_id();
+
+        worker
+            .usage_tracker
+            .note_compaction_correlation_id("compact-id".into());
+        worker.usage_tracker.note_request(1);
+        worker.usage_tracker.record_usage(&agen::event::UsageEvent {
+            input_tokens: Some(10),
+            output_tokens: Some(1),
+            total_tokens: Some(11),
+            cache_read_input_tokens: Some(0),
+            cache_creation_input_tokens: Some(0),
+        });
+        worker.usage_tracker.request_completed();
+
+        let append_error = persist_pending_usage(
+            &worker.usage_tracker,
+            &worker.usage_history,
+            worker.compact_state.as_deref(),
+            |_, _, _| Err(StoreError::Io(std::io::Error::other("synthetic failure"))),
+        )
+        .expect_err("first usage append must fail");
+        assert!(append_error.to_string().contains("synthetic failure"));
+        assert_eq!(worker.usage_tracker.pending_record_count(), 1);
+
+        // Simulate another writer extending the active Segment. The normal run
+        // prelude must not auto-fork while the failed record still belongs to
+        // this Segment.
+        store
+            .append(
+                session_id,
+                source_segment,
+                &LogEntry::TurnEnd {
+                    ts: segment_log::now_millis(),
+                    turn_count: 99,
+                },
+            )
+            .unwrap();
+        let run_error = worker
+            .run_text("must stop before provider ownership")
+            .await
+            .expect_err("head drift must not fork past pending usage");
+        assert!(matches!(
+            run_error,
+            WorkerError::PendingUsagePersistence { pending_records: 1 }
+        ));
+        assert_eq!(worker.segment_id(), source_segment);
+        assert_eq!(
+            store.list_segments(session_id).unwrap(),
+            vec![source_segment]
+        );
+
+        let error = worker
+            .compact(0)
+            .await
+            .expect_err("pending usage must fence Segment replacement");
+        assert!(matches!(
+            error,
+            WorkerError::PendingUsagePersistence { pending_records: 1 }
+        ));
+        assert_eq!(worker.segment_id(), source_segment);
+        assert_eq!(
+            store.list_segments(session_id).unwrap(),
+            vec![source_segment]
+        );
+
+        let metrics = persist_pending_usage(
+            &worker.usage_tracker,
+            &worker.usage_history,
+            worker.compact_state.as_deref(),
+            |entry, after_commit, _| {
+                store.append(session_id, source_segment, &entry)?;
+                after_commit();
+                Ok(())
+            },
+        )
+        .expect("retry should commit on the originating Segment");
+        assert_eq!(worker.usage_tracker.pending_record_count(), 0);
+        assert_eq!(worker.usage_history.lock().unwrap().len(), 1);
+        assert_eq!(metrics.len(), 1, "correlation must be emitted once");
+        assert_eq!(metrics[0].name, "compact.post_request");
+
+        let usage_entries = store
+            .read_all(session_id, source_segment)
+            .unwrap()
+            .into_iter()
+            .filter(|entry| matches!(entry, LogEntry::LlmUsage { .. }))
+            .count();
+        assert_eq!(usage_entries, 1, "retry must not duplicate accounting");
+        assert_eq!(
+            store.list_segments(session_id).unwrap(),
+            vec![source_segment]
+        );
+    }
+
     #[tokio::test]
     async fn fresh_run_clears_interrupted_budget_before_pre_run_compaction() {
         let dir = tempfile::tempdir().unwrap();
diff --git a/crates/worker/tests/compact_events_test.rs b/crates/worker/tests/compact_events_test.rs
index c6c37367c..7e086533f 100644
--- a/crates/worker/tests/compact_events_test.rs
+++ b/crates/worker/tests/compact_events_test.rs
@@ -7,15 +7,16 @@
 //!   (driven by `compact_request_threshold` → `PreRequestAction::Yield`)
 
 use std::pin::Pin;
-use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex};
 
 use agen::Engine;
 use agen::llm_client::event::{Event as LlmEvent, ResponseStatus, StatusEvent, UsageEvent};
 use agen::llm_client::types::Item;
 use agen::llm_client::{ClientError, LlmClient, Request};
+use agen::tool::{Tool, ToolDefinition, ToolError, ToolMeta, ToolOutput};
 use async_trait::async_trait;
-use futures::Stream;
+use futures::{Stream, StreamExt};
 use protocol::{Event, Method, RunResult};
 use session_store::{
     CombinedStore, FsStore, FsWorkerStore, LogEntry, Store, WorkerMetadata, WorkerMetadataStore,
@@ -125,10 +126,13 @@ fn annotated(item: Item) -> session_store::LoggedHistoryEntry {
     }
 }
 
+type CallObserver = Arc<dyn Fn(usize) + Send + Sync>;
+
 #[derive(Clone)]
 struct MockClient {
     responses: Arc<Vec<Vec<LlmEvent>>>,
     call_count: Arc<AtomicUsize>,
+    observer: Arc<Mutex<Option<CallObserver>>>,
 }
 
 impl MockClient {
@@ -136,8 +140,13 @@ impl MockClient {
         Self {
             responses: Arc::new(responses),
             call_count: Arc::new(AtomicUsize::new(0)),
+            observer: Arc::new(Mutex::new(None)),
         }
     }
+
+    fn set_observer(&self, observer: impl Fn(usize) + Send + Sync + 'static) {
+        *self.observer.lock().unwrap() = Some(Arc::new(observer));
+    }
 }
 
 #[async_trait]
@@ -152,6 +161,9 @@ impl LlmClient for MockClient {
     ) -> Result<Pin<Box<dyn Stream<Item = Result<LlmEvent, ClientError>> + Send>>, ClientError>
     {
         let count = self.call_count.fetch_add(1, Ordering::SeqCst);
+        if let Some(observer) = self.observer.lock().unwrap().clone() {
+            observer(count);
+        }
         if count >= self.responses.len() {
             return Err(ClientError::Config("mock client exhausted".into()));
         }
@@ -161,6 +173,90 @@ impl LlmClient for MockClient {
     }
 }
 
+#[derive(Clone)]
+struct PauseAfterUsageClient {
+    responses: Arc<Vec<Vec<LlmEvent>>>,
+    call_count: Arc<AtomicUsize>,
+    hanging_call: usize,
+}
+
+impl PauseAfterUsageClient {
+    fn new(responses: Vec<Vec<LlmEvent>>, hanging_call: usize) -> Self {
+        Self {
+            responses: Arc::new(responses),
+            call_count: Arc::new(AtomicUsize::new(0)),
+            hanging_call,
+        }
+    }
+}
+
+#[async_trait]
+impl LlmClient for PauseAfterUsageClient {
+    fn clone_boxed(&self) -> Box<dyn LlmClient> {
+        Box::new(self.clone())
+    }
+
+    async fn stream(
+        &self,
+        _request: Request,
+    ) -> Result<Pin<Box<dyn Stream<Item = Result<LlmEvent, ClientError>> + Send>>, ClientError>
+    {
+        let count = self.call_count.fetch_add(1, Ordering::SeqCst);
+        let events = self
+            .responses
+            .get(count)
+            .cloned()
+            .ok_or_else(|| ClientError::Config("pause mock client exhausted".into()))?;
+        let finite = futures::stream::iter(events.into_iter().map(Ok));
+        if count == self.hanging_call {
+            Ok(Box::pin(finite.chain(futures::stream::pending())))
+        } else {
+            Ok(Box::pin(finite))
+        }
+    }
+}
+
+#[derive(Clone)]
+struct GrowTool;
+
+#[async_trait]
+impl Tool for GrowTool {
+    async fn execute(
+        &self,
+        _input: &str,
+        _ctx: agen::tool::ToolExecutionContext,
+    ) -> Result<ToolOutput, ToolError> {
+        Ok(ToolOutput {
+            summary: "grew context".into(),
+            content: Some("x".repeat(400_000)),
+            attachments: Vec::new(),
+        })
+    }
+}
+
+fn grow_tool_definition() -> ToolDefinition {
+    Arc::new(|| {
+        (
+            ToolMeta::new("grow")
+                .description("test tool that grows request context")
+                .input_schema(serde_json::json!({"type": "object"})),
+            Arc::new(GrowTool) as Arc<dyn Tool>,
+        )
+    })
+}
+
+fn grow_tool_use_events(call_id: &str, input_tokens: u64) -> Vec<LlmEvent> {
+    vec![
+        LlmEvent::tool_use_start(0, call_id, "grow"),
+        LlmEvent::tool_input_delta(0, "{}"),
+        LlmEvent::tool_use_stop(0),
+        LlmEvent::usage(input_tokens, 1),
+        LlmEvent::Status(StatusEvent {
+            status: ResponseStatus::Completed,
+        }),
+    ]
+}
+
 #[derive(Clone)]
 struct BlockingCompactClient {
     calls: Arc<AtomicUsize>,
@@ -365,12 +461,13 @@ where
     let scope = worker::Scope::writable(&pwd).unwrap();
     std::mem::forget(pwd_tmp);
 
-    let worker =
+    let mut engine =
         Engine::<_, agen::state::Mutable, worker::SessionHistoryMetadata>::new_annotated(client);
+    engine.register_tool(grow_tool_definition());
     let observed_store = store.clone();
     let mut worker = Worker::new(
         manifest,
-        worker,
+        engine,
         store,
         worker::WorkerWorkspaceContext::local_filesystem(None),
         worker::WorkerFilesystemAuthority::local(pwd.clone(), pwd.clone()),
@@ -379,6 +476,7 @@ where
     .await
     .unwrap();
     worker.enable_worker_metadata_write_through().unwrap();
+    worker.attach_log_writer(Arc::new(worker.log_writer_handle()));
     (worker, observed_store)
 }
 
@@ -1057,6 +1155,289 @@ async fn request_threshold_compact_publishes_runtime_progress() {
     assert_eq!(post.metric.correlation_id.as_deref(), Some(correlation_id));
 }
 
+#[tokio::test]
+async fn completed_post_compact_usage_rearms_within_one_tool_loop() {
+    let client = MockClient::new(vec![
+        text_events_with_usage("seed", 100_000),
+        write_summary_tool_use_events("compact-1", "first summary"),
+        single_text_events("done"),
+        grow_tool_use_events("grow-1", 1_000),
+        write_summary_tool_use_events("compact-2", "second summary"),
+        single_text_events("done"),
+        text_events_with_usage("finished", 800),
+    ]);
+    let call_count = Arc::clone(&client.call_count);
+    let observed_client = client.clone();
+    let manifest = MID_TURN_MANIFEST_TOML.replace(
+        "compact_request_threshold = 100",
+        "compact_threshold = 80000\ncompact_request_threshold = 50000",
+    );
+    let (mut worker, store) = make_worker_with_manifest_and_store(&manifest, client).await;
+    let session_id = worker.session_id();
+    let durable_at_next_request = Arc::new(AtomicBool::new(false));
+    let observed_barrier = Arc::clone(&durable_at_next_request);
+    let observed_store = store.clone();
+    observed_client.set_observer(move |call_index| {
+        if call_index != 4 {
+            return;
+        }
+        let usage_is_durable = observed_store
+            .list_segments(session_id)
+            .unwrap()
+            .into_iter()
+            .flat_map(|segment_id| observed_store.read_all(session_id, segment_id).unwrap())
+            .any(|entry| {
+                matches!(
+                    entry,
+                    LogEntry::LlmUsage {
+                        input_total_tokens: 1_000,
+                        ..
+                    }
+                )
+            });
+        assert!(
+            usage_is_durable,
+            "the first post-compact usage must be durable before the next provider request"
+        );
+        observed_barrier.store(true, Ordering::SeqCst);
+    });
+
+    worker.run_text("seed input").await.unwrap();
+    worker
+        .run_text("continue through two compactions")
+        .await
+        .expect("a successful post-compact request must rearm before the tool-loop threshold");
+
+    assert_eq!(
+        call_count.load(Ordering::SeqCst),
+        7,
+        "both compactor cycles and both normal post-compact requests must run"
+    );
+    assert!(durable_at_next_request.load(Ordering::SeqCst));
+
+    let metrics = session_metrics::read_session_metrics(&store, session_id).unwrap();
+    let compact_starts = metrics
+        .iter()
+        .filter(|record| record.metric.name == "compact.start")
+        .collect::<Vec<_>>();
+    assert_eq!(compact_starts.len(), 2);
+    assert!(compact_starts.iter().any(|record| {
+        record.metric.dimensions.get("trigger").map(String::as_str) == Some("pre_run")
+    }));
+    assert!(compact_starts.iter().any(|record| {
+        record.metric.dimensions.get("trigger").map(String::as_str) == Some("request_threshold")
+    }));
+    let compact_posts = metrics
+        .iter()
+        .filter(|record| record.metric.name == "compact.post_request")
+        .collect::<Vec<_>>();
+    assert_eq!(compact_posts.len(), 2);
+    assert_ne!(
+        compact_posts[0].metric.correlation_id,
+        compact_posts[1].metric.correlation_id
+    );
+
+    let usage = store
+        .list_segments(session_id)
+        .unwrap()
+        .into_iter()
+        .flat_map(|segment_id| store.read_all(session_id, segment_id).unwrap())
+        .filter_map(|entry| match entry {
+            LogEntry::LlmUsage {
+                input_total_tokens, ..
+            } => Some(input_total_tokens),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(usage.iter().filter(|tokens| **tokens == 1_000).count(), 1);
+    assert_eq!(usage.iter().filter(|tokens| **tokens == 800).count(), 1);
+
+    let restored = session_store::restore(&store, session_id, worker.segment_id()).unwrap();
+    assert_eq!(restored.usage_history.len(), 1);
+    assert_eq!(restored.usage_history[0].input_total_tokens, 800);
+}
+
+#[tokio::test]
+async fn post_compact_pause_bills_partial_without_rearming_or_consuming_correlation() {
+    let client = PauseAfterUsageClient::new(
+        vec![
+            text_events_with_usage("seed", 100_000),
+            write_summary_tool_use_events("manual-compact", "pause test summary"),
+            single_text_events("done"),
+            vec![
+                LlmEvent::text_block_start(0),
+                LlmEvent::text_delta(0, "partial"),
+                LlmEvent::usage(90, 2),
+            ],
+            grow_tool_use_events("grow-after-resume", 80),
+            write_summary_tool_use_events("automatic-compact", "rearmed summary"),
+            single_text_events("done"),
+            text_events_with_usage("finished", 50),
+        ],
+        3,
+    );
+    let call_count = Arc::clone(&client.call_count);
+    let manifest = MID_TURN_MANIFEST_TOML.replace(
+        "compact_request_threshold = 100",
+        "compact_threshold = 80000\ncompact_request_threshold = 50000",
+    );
+    let (worker, store) = make_worker_with_manifest_and_store(&manifest, client).await;
+    let session_id = worker.session_id();
+
+    let runtime_tmp = tempfile::tempdir().unwrap();
+    let bash_output_dir = runtime_tmp.path().join("bash-output");
+    let (handle, shutdown_receiver) =
+        WorkerController::spawn(worker, runtime_tmp.path(), &bash_output_dir)
+            .await
+            .unwrap();
+    let mut rx = handle.subscribe();
+    handle
+        .send(Method::submit_text(
+            protocol::new_submission_request_id(),
+            "seed history",
+        ))
+        .await
+        .unwrap();
+    loop {
+        if matches!(
+            tokio::time::timeout(std::time::Duration::from_secs(2), rx.recv())
+                .await
+                .expect("timeout waiting for seed")
+                .expect("event"),
+            Event::RunEnd {
+                result: RunResult::Finished
+            }
+        ) {
+            break;
+        }
+    }
+    handle
+        .send(Method::submit_text(
+            protocol::new_submission_request_id(),
+            "pause the first post-compact request",
+        ))
+        .await
+        .unwrap();
+
+    tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        while call_count.load(Ordering::SeqCst) < 4 {
+            tokio::task::yield_now().await;
+        }
+    })
+    .await
+    .expect("post-compact provider request should start");
+    handle
+        .send(Method::Pause {
+            command: protocol::WorkerCommandEnvelope::new(1),
+        })
+        .await
+        .unwrap();
+    loop {
+        let event = tokio::time::timeout(std::time::Duration::from_secs(2), rx.recv())
+            .await
+            .expect("timeout waiting for pause")
+            .expect("event");
+        if matches!(
+            event,
+            Event::RunEnd {
+                result: RunResult::Paused
+            }
+        ) {
+            break;
+        }
+    }
+
+    let paused_usage = store
+        .list_segments(session_id)
+        .unwrap()
+        .into_iter()
+        .flat_map(|segment_id| store.read_all(session_id, segment_id).unwrap())
+        .filter_map(|entry| match entry {
+            LogEntry::LlmUsage {
+                input_total_tokens, ..
+            } => Some(input_total_tokens),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(
+        paused_usage.iter().filter(|tokens| **tokens == 90).count(),
+        1
+    );
+    let paused_metrics = session_metrics::read_session_metrics(&store, session_id).unwrap();
+    assert!(
+        paused_metrics
+            .iter()
+            .all(|record| record.metric.name != "compact.post_request"),
+        "partial Pause usage must not consume compact correlation or rearm"
+    );
+
+    handle
+        .send(Method::Resume {
+            command: protocol::WorkerCommandEnvelope::new(2),
+        })
+        .await
+        .unwrap();
+    loop {
+        if matches!(
+            tokio::time::timeout(std::time::Duration::from_secs(2), rx.recv())
+                .await
+                .expect("timeout waiting for resume completion")
+                .expect("event"),
+            Event::RunEnd {
+                result: RunResult::Finished
+            }
+        ) {
+            break;
+        }
+    }
+
+    assert_eq!(
+        call_count.load(Ordering::SeqCst),
+        8,
+        "the completed resumed request must rearm and permit request-threshold compaction"
+    );
+    let metrics = session_metrics::read_session_metrics(&store, session_id).unwrap();
+    let compact_starts = metrics
+        .iter()
+        .filter(|record| record.metric.name == "compact.start")
+        .collect::<Vec<_>>();
+    assert_eq!(compact_starts.len(), 2);
+    let pre_run_correlation = compact_starts
+        .iter()
+        .find(|record| {
+            record.metric.dimensions.get("trigger").map(String::as_str) == Some("pre_run")
+        })
+        .and_then(|record| record.metric.correlation_id.as_deref())
+        .expect("pre-run automatic compact correlation");
+    let pre_run_posts = metrics
+        .iter()
+        .filter(|record| {
+            record.metric.name == "compact.post_request"
+                && record.metric.correlation_id.as_deref() == Some(pre_run_correlation)
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(pre_run_posts.len(), 1);
+    assert_eq!(
+        pre_run_posts[0].metric.dimensions["input_total_tokens"],
+        "80"
+    );
+    assert!(metrics.iter().all(|record| {
+        record.metric.name != "compact.post_request"
+            || record.metric.dimensions["input_total_tokens"] != "90"
+    }));
+
+    handle
+        .send(Method::Shutdown {
+            command: protocol::WorkerCommandEnvelope::new(3),
+        })
+        .await
+        .unwrap();
+    tokio::time::timeout(std::time::Duration::from_secs(2), shutdown_receiver)
+        .await
+        .expect("controller shutdown timeout")
+        .expect("shutdown confirmation");
+}
+
 #[tokio::test]
 async fn compacted_context_above_request_threshold_fails_before_provider_request() {
     let client = MockClient::new(vec![
diff --git a/crates/worker/tests/controller_test.rs b/crates/worker/tests/controller_test.rs
index d92c539ae..0f996122b 100644
--- a/crates/worker/tests/controller_test.rs
+++ b/crates/worker/tests/controller_test.rs
@@ -3,7 +3,9 @@ use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 
 use agen::Engine;
-use agen::llm_client::event::{ErrorEvent, Event as LlmEvent, ResponseStatus, StatusEvent};
+use agen::llm_client::event::{
+    ErrorEvent, Event as LlmEvent, ResponseStatus, StatusEvent, UsageEvent,
+};
 use agen::llm_client::types::Item;
 use agen::llm_client::{ClientError, LlmClient, Request};
 use agen::tool::{Tool, ToolDefinition, ToolError, ToolMeta, ToolOutput};
@@ -2580,18 +2582,32 @@ async fn drain_until<F: FnMut(&Event) -> bool>(
 /// the interrupted turn's history consistency.
 #[tokio::test]
 async fn pause_then_resume_preserves_notifications_and_history_consistency() {
-    // Response 1: hang after opening a text block (no stop / completed),
-    // so the Engine is parked inside the stream read and `cancel_rx`
-    // races it cleanly on Method::Pause.
+    // Response 1: report billable partial usage, then hang after opening a text
+    // block (no stop / completed). Pause must flush that usage without treating
+    // the provider request as normally completed.
     let hang = MockResponse::Hang(vec![
         LlmEvent::text_block_start(0),
         LlmEvent::text_delta(0, "partial..."),
+        LlmEvent::Usage(UsageEvent {
+            input_tokens: Some(90),
+            output_tokens: Some(2),
+            total_tokens: Some(92),
+            cache_read_input_tokens: Some(0),
+            cache_creation_input_tokens: Some(0),
+        }),
     ]);
     // Response 2: a clean assistant reply delivered on Resume.
     let ok = MockResponse::Complete(vec![
         LlmEvent::text_block_start(0),
         LlmEvent::text_delta(0, "resumed output"),
         LlmEvent::text_block_stop(0, None),
+        LlmEvent::Usage(UsageEvent {
+            input_tokens: Some(100),
+            output_tokens: Some(3),
+            total_tokens: Some(103),
+            cache_read_input_tokens: Some(0),
+            cache_creation_input_tokens: Some(0),
+        }),
         LlmEvent::Status(StatusEvent {
             status: ResponseStatus::Completed,
         }),
@@ -2645,6 +2661,23 @@ async fn pause_then_resume_preserves_notifications_and_history_consistency() {
 
     tokio::time::sleep(std::time::Duration::from_millis(50)).await;
     assert_eq!(handle.shared_state.catalog_status(), WorkerStatus::Paused);
+    let (paused_entries, _) = handle.sink.subscribe_with_snapshot();
+    let paused_usage = paused_entries
+        .iter()
+        .filter_map(|entry| match entry {
+            LogEntry::LlmUsage {
+                input_total_tokens,
+                output_tokens,
+                ..
+            } => Some((*input_total_tokens, *output_tokens)),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(
+        paused_usage,
+        vec![(90, 2)],
+        "Pause must durably account for usage already observed on the interrupted request"
+    );
 
     handle
         .send(Method::Notify {
@@ -2729,6 +2762,23 @@ async fn pause_then_resume_preserves_notifications_and_history_consistency() {
         })
         .unwrap_or_default();
     assert_eq!(assistant_text, "resumed output");
+    let (final_entries, _) = handle.sink.subscribe_with_snapshot();
+    let final_usage = final_entries
+        .iter()
+        .filter_map(|entry| match entry {
+            LogEntry::LlmUsage {
+                input_total_tokens,
+                output_tokens,
+                ..
+            } => Some((*input_total_tokens, *output_tokens)),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(
+        final_usage,
+        vec![(90, 2), (100, 3)],
+        "the paused request must remain a distinct billing record rather than merging into resume"
+    );
     let has_tool_call = history.iter().any(|i| i.is_tool_call());
     assert!(!has_tool_call, "no orphan tool_call in history");
 }