diff --git a/rust/crates/runtime/src/conversation.rs b/rust/crates/runtime/src/conversation.rs
index 35c0f73f..ea50e3bd 100644
--- a/rust/crates/runtime/src/conversation.rs
+++ b/rust/crates/runtime/src/conversation.rs
@@ -342,6 +342,7 @@ where
         let mut tool_results = Vec::new();
         let mut prompt_cache_events = Vec::new();
         let mut iterations = 0;
+        let mut auto_compaction = None;
 
         loop {
             iterations += 1;
@@ -397,6 +398,12 @@ where
                 .map_err(|error| RuntimeError::new(error.to_string()))?;
             assistant_messages.push(assistant_message);
 
+            // Run auto-compaction check before next API call, including on the terminal
+            // (no-tool) iteration, to prevent unbounded session growth (#3106).
+            if let Some(compaction) = self.maybe_auto_compact() {
+                auto_compaction = Some(compaction);
+            }
+
             if pending_tool_uses.is_empty() {
                 break;
             }
@@ -503,8 +510,6 @@ where
             }
         }
 
-        let auto_compaction = self.maybe_auto_compact();
-
         let summary = TurnSummary {
             assistant_messages,
             tool_results,