diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
index 660d3eedf4e..5f7bd493f64 100644
--- a/src/gateway/server-methods/chat.ts
+++ b/src/gateway/server-methods/chat.ts
@@ -21,7 +21,10 @@ import { normalizeInputProvenance, type InputProvenance } from "../../sessions/i
 import { resolveSendPolicy } from "../../sessions/send-policy.js";
 import { parseAgentSessionKey } from "../../sessions/session-key-utils.js";
 import { emitSessionTranscriptUpdate } from "../../sessions/transcript-events.js";
-import { resolveAssistantMessagePhase } from "../../shared/chat-message-content.js";
+import {
+  parseAssistantTextSignature,
+  resolveAssistantMessagePhase,
+} from "../../shared/chat-message-content.js";
 import {
   stripInlineDirectiveTagsForDisplay,
   stripInlineDirectiveTagsFromMessageForDisplay,
@@ -685,6 +688,38 @@ function sanitizeChatHistoryContentBlock(
   return { block: changed ? entry : block, changed };
 }
 
+function sanitizeAssistantPhasedContentBlocks(content: unknown[]): {
+  content: unknown[];
+  changed: boolean;
+} {
+  const hasExplicitPhasedText = content.some((block) => {
+    if (!block || typeof block !== "object") {
+      return false;
+    }
+    const entry = block as { type?: unknown; textSignature?: unknown };
+    return (
+      entry.type === "text" && Boolean(parseAssistantTextSignature(entry.textSignature)?.phase)
+    );
+  });
+  if (!hasExplicitPhasedText) {
+    return { content, changed: false };
+  }
+  const filtered = content.filter((block) => {
+    if (!block || typeof block !== "object") {
+      return true;
+    }
+    const entry = block as { type?: unknown; textSignature?: unknown };
+    if (entry.type !== "text") {
+      return true;
+    }
+    return parseAssistantTextSignature(entry.textSignature)?.phase === "final_answer";
+  });
+  return {
+    content: filtered,
+    changed: filtered.length !== content.length,
+  };
+}
+
 /**
  * Validate that a value is a finite number, returning undefined otherwise.
  */
@@ -823,6 +858,13 @@ function sanitizeChatHistoryMessage(
       entry.content = updated.map((item) => item.block);
       changed = true;
     }
+    if (entry.role === "assistant" && Array.isArray(entry.content)) {
+      const sanitizedPhases = sanitizeAssistantPhasedContentBlocks(entry.content);
+      if (sanitizedPhases.changed) {
+        entry.content = sanitizedPhases.content;
+        changed = true;
+      }
+    }
   }
 
   if (typeof entry.text === "string") {