diff --git a/src/query.ts b/src/query.ts
index 106c88fa5..a53d238cb 100644
--- a/src/query.ts
+++ b/src/query.ts
@@ -479,6 +479,22 @@ async function* queryLoop(
 
     let messagesForQuery = getMessagesAfterCompactBoundary(messages)
 
+    // Release toolUseResult payloads from previous turns. By this point the
+    // UI has already rendered those results and the next API call only needs
+    // message.message.content (tool_result blocks), not the raw output object.
+    // This prevents unbounded memory growth in long sessions before compact
+    // triggers — a single FileRead of a 400KB file would otherwise stay in
+    // mutableMessages forever.
+    for (const msg of messagesForQuery) {
+      if (
+        msg.type === 'user' &&
+        'toolUseResult' in msg &&
+        msg.toolUseResult !== undefined
+      ) {
+        delete (msg as Message & { toolUseResult?: unknown }).toolUseResult
+      }
+    }
+
     let tracking = autoCompactTracking
 
     // Enforce per-message budget on aggregate tool result size. Runs BEFORE
diff --git a/src/services/compact/compact.ts b/src/services/compact/compact.ts
index cc34a1154..40bb76a04 100644
--- a/src/services/compact/compact.ts
+++ b/src/services/compact/compact.ts
@@ -336,12 +336,31 @@ export type RecompactionInfo = {
 export function buildPostCompactMessages(result: CompactionResult): Message[] {
   return ([result.boundaryMarker] as Message[]).concat(
     result.summaryMessages,
-    result.messagesToKeep ?? [],
+    stripToolUseResults(result.messagesToKeep),
     result.attachments,
     result.hookResults,
   )
 }
 
+/** Release large tool result payloads from kept messages after compaction.
+ *  toolUseResult is only used for UI rendering, not API calls. */
+function stripToolUseResults(messages: Message[] | undefined): Message[] {
+  if (!messages) return []
+  return messages.map(msg => {
+    if (
+      msg.type === 'user' &&
+      'toolUseResult' in msg &&
+      msg.toolUseResult !== undefined
+    ) {
+      const { toolUseResult, ...rest } = msg as Message & {
+        toolUseResult: unknown
+      }
+      return rest as Message
+    }
+    return msg
+  })
+}
+
 /**
  * Annotate a compact boundary with relink metadata for messagesToKeep.
  * Preserved messages keep their original parentUuids on disk (dedup-skipped);