Preserve ILM-style conversation continuity during auto compaction

Auto compaction was keying off cumulative usage and re-summarizing from the front of the session, which made long chats shed continuity after the first compaction. The runtime now compacts against the current turn's prompt pressure and preserves prior compacted context as retained summary state instead of treating it like disposable history.

Constraint: Existing /compact behavior and saved-session resume flow had to keep working without schema changes
Rejected: Keep using cumulative input tokens | caused repeat compaction after every subsequent turn once the threshold was crossed
Rejected: Re-summarize prior compacted system messages as ordinary history | degraded continuity and could drop earlier context
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Preserve compacted-summary boundaries when extending compaction again; do not fold prior compacted context back into raw-message removal
Tested: cargo fmt --check; cargo clippy -p runtime -p commands --tests -- -D warnings; cargo test -p runtime; cargo test -p commands
Not-tested: End-to-end interactive CLI auto-compaction against a live Anthropic session

rust/crates/runtime/src/conversation.rs

@@ -234,6 +234,7 @@ where
         let mut assistant_messages = Vec::new();
         let mut tool_results = Vec::new();
         let mut iterations = 0;
+        let mut max_turn_input_tokens = 0;
 
         loop {
             iterations += 1;
@@ -250,6 +251,7 @@ where
             let events = self.api_client.stream(request)?;
             let (assistant_message, usage) = build_assistant_message(events)?;
             if let Some(usage) = usage {
+                max_turn_input_tokens = max_turn_input_tokens.max(usage.input_tokens);
                 self.usage_tracker.record(usage);
             }
             let pending_tool_uses = assistant_message
@@ -365,7 +367,7 @@ where
             }
         }
 
-        let auto_compaction = self.maybe_auto_compact();
+        let auto_compaction = self.maybe_auto_compact(max_turn_input_tokens);
 
         Ok(TurnSummary {
             assistant_messages,
@@ -426,17 +428,16 @@ where
         )
     }
 
-    fn maybe_auto_compact(&mut self) -> Option<AutoCompactionEvent> {
-        if self.usage_tracker.cumulative_usage().input_tokens
-            < self.auto_compaction_input_tokens_threshold
-        {
+    fn maybe_auto_compact(&mut self, turn_input_tokens: u32) -> Option<AutoCompactionEvent> {
+        if turn_input_tokens < self.auto_compaction_input_tokens_threshold {
             return None;
         }
 
         let result = compact_session(
             &self.session,
             CompactionConfig {
-                max_estimated_tokens: 0,
+                max_estimated_tokens: usize::try_from(self.auto_compaction_input_tokens_threshold)
+                    .unwrap_or(usize::MAX),
                 ..CompactionConfig::default()
             },
         );
@@ -1204,7 +1205,7 @@ mod tests {
     }
 
     #[test]
-    fn auto_compacts_when_cumulative_input_threshold_is_crossed() {
+    fn auto_compacts_when_turn_input_threshold_is_crossed() {
         struct SimpleApi;
         impl ApiClient for SimpleApi {
             fn stream(
@@ -1227,13 +1228,13 @@ mod tests {
         let session = Session {
             version: 1,
             messages: vec![
-                crate::session::ConversationMessage::user_text("one"),
+                crate::session::ConversationMessage::user_text("one ".repeat(30_000)),
                 crate::session::ConversationMessage::assistant(vec![ContentBlock::Text {
-                    text: "two".to_string(),
+                    text: "two ".repeat(30_000),
                 }]),
-                crate::session::ConversationMessage::user_text("three"),
+                crate::session::ConversationMessage::user_text("three ".repeat(30_000)),
                 crate::session::ConversationMessage::assistant(vec![ContentBlock::Text {
-                    text: "four".to_string(),
+                    text: "four ".repeat(30_000),
                 }]),
             ],
         };
@@ -1260,6 +1261,72 @@ mod tests {
         assert_eq!(runtime.session().messages[0].role, MessageRole::System);
     }
 
+    #[test]
+    fn auto_compaction_does_not_repeat_after_context_is_already_compacted() {
+        struct SequentialUsageApi {
+            call_count: usize,
+        }
+
+        impl ApiClient for SequentialUsageApi {
+            fn stream(
+                &mut self,
+                _request: ApiRequest,
+            ) -> Result<Vec<AssistantEvent>, RuntimeError> {
+                self.call_count += 1;
+                let input_tokens = if self.call_count == 1 { 120_000 } else { 64 };
+                Ok(vec![
+                    AssistantEvent::TextDelta("done".to_string()),
+                    AssistantEvent::Usage(TokenUsage {
+                        input_tokens,
+                        output_tokens: 4,
+                        cache_creation_input_tokens: 0,
+                        cache_read_input_tokens: 0,
+                    }),
+                    AssistantEvent::MessageStop,
+                ])
+            }
+        }
+
+        let session = Session {
+            version: 1,
+            messages: vec![
+                crate::session::ConversationMessage::user_text("one ".repeat(30_000)),
+                crate::session::ConversationMessage::assistant(vec![ContentBlock::Text {
+                    text: "two ".repeat(30_000),
+                }]),
+                crate::session::ConversationMessage::user_text("three ".repeat(30_000)),
+                crate::session::ConversationMessage::assistant(vec![ContentBlock::Text {
+                    text: "four ".repeat(30_000),
+                }]),
+            ],
+        };
+
+        let mut runtime = ConversationRuntime::new(
+            session,
+            SequentialUsageApi { call_count: 0 },
+            StaticToolExecutor::new(),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
+            vec!["system".to_string()],
+        )
+        .with_auto_compaction_input_tokens_threshold(100_000);
+
+        let first = runtime
+            .run_turn("trigger", None)
+            .expect("first turn should succeed");
+        assert_eq!(
+            first.auto_compaction,
+            Some(AutoCompactionEvent {
+                removed_message_count: 2,
+            })
+        );
+
+        let second = runtime
+            .run_turn("continue", None)
+            .expect("second turn should succeed");
+        assert_eq!(second.auto_compaction, None);
+        assert_eq!(runtime.session().messages[0].role, MessageRole::System);
+    }
+
     #[test]
     fn skips_auto_compaction_below_threshold() {
         struct SimpleApi;