diff --git a/packages/server/src/server/agent/providers/claude-agent.ts b/packages/server/src/server/agent/providers/claude-agent.ts
index 8d7ebf500..c27355636 100644
--- a/packages/server/src/server/agent/providers/claude-agent.ts
+++ b/packages/server/src/server/agent/providers/claude-agent.ts
@@ -770,6 +770,7 @@ type TimelineMessageState = {
 
 class TimelineAssembler {
   private readonly messages = new Map<string, TimelineMessageState>();
+  private readonly finalizedMessageIds = new Set<string>();
   private readonly activeMessageByRun = new Map<string, string>();
   private syntheticMessageCounter = 0;
 
@@ -799,6 +800,9 @@ class TimelineAssembler {
     if (!messageId) {
       return [];
     }
+    if (this.finalizedMessageIds.has(messageId)) {
+      return [];
+    }
     const state = this.ensureMessageState(messageId, runId);
     const fragments = this.extractFragments(message.message?.content);
     return this.applyAbsoluteFragments(state, fragments);
@@ -922,6 +926,8 @@ class TimelineAssembler {
     if (runId && this.activeMessageByRun.get(runId) === messageId) {
       this.activeMessageByRun.delete(runId);
     }
+    this.finalizedMessageIds.add(messageId);
+    this.messages.delete(messageId);
     return items;
   }