diff --git a/src/agents/compaction-real-conversation.test.ts b/src/agents/compaction-real-conversation.test.ts
new file mode 100644
index 000000000000..16ade06ec31f
--- /dev/null
+++ b/src/agents/compaction-real-conversation.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "vitest";
+import {
+  hasMeaningfulConversationContent,
+  isRealConversationMessage,
+} from "./compaction-real-conversation.js";
+import type { AgentMessage } from "./runtime/index.js";
+
+type SummaryRole = "branchSummary" | "compactionSummary";
+
+function summaryMessage(role: SummaryRole, summary: string): AgentMessage {
+  return { role, summary, timestamp: 1 } as AgentMessage;
+}
+
+describe("compaction real conversation classification", () => {
+  it.each<SummaryRole>(["branchSummary", "compactionSummary"])(
+    "treats non-empty %s messages as conversation anchors",
+    (role) => {
+      const summary = summaryMessage(role, "The user asked for a repository audit.");
+      const toolResult = {
+        role: "toolResult",
+        toolCallId: "call-1",
+        toolName: "exec",
+        content: [{ type: "text", text: "audit output" }],
+      } as AgentMessage;
+      const messages = [summary, toolResult];
+
+      expect(hasMeaningfulConversationContent(summary)).toBe(true);
+      expect(isRealConversationMessage(summary, messages, 0)).toBe(true);
+      expect(isRealConversationMessage(toolResult, messages, 1)).toBe(true);
+    },
+  );
+
+  it.each<SummaryRole>(["branchSummary", "compactionSummary"])(
+    "rejects blank %s messages",
+    (role) => {
+      const summary = summaryMessage(role, "   ");
+
+      expect(hasMeaningfulConversationContent(summary)).toBe(false);
+      expect(isRealConversationMessage(summary, [summary], 0)).toBe(false);
+    },
+  );
+
+  it("rejects tool-call-only messages and orphan tool results", () => {
+    const toolCall = {
+      role: "assistant",
+      content: [{ type: "toolCall", id: "call-1", name: "exec", arguments: {} }],
+    } as AgentMessage;
+    const orphanToolResult = {
+      role: "toolResult",
+      toolCallId: "call-1",
+      toolName: "exec",
+      content: [{ type: "text", text: "audit output" }],
+    } as AgentMessage;
+
+    expect(isRealConversationMessage(toolCall, [toolCall], 0)).toBe(false);
+    expect(isRealConversationMessage(orphanToolResult, [orphanToolResult], 0)).toBe(false);
+  });
+});
diff --git a/src/agents/compaction-real-conversation.ts b/src/agents/compaction-real-conversation.ts
index fe12fa36cbe9..edadf01cb876 100644
--- a/src/agents/compaction-real-conversation.ts
+++ b/src/agents/compaction-real-conversation.ts
@@ -30,6 +30,10 @@ function hasMeaningfulText(text: string): boolean {
   return true;
 }
 
+function isSummaryRole(role: unknown): boolean {
+  return role === "branchSummary" || role === "compactionSummary";
+}
+
 /** Returns whether a message has content worth preserving as conversation. */
 export function hasMeaningfulConversationContent(message: AgentMessage): boolean {
   if ((message as { role?: unknown }).role === "custom") {
@@ -49,7 +53,7 @@ export function hasMeaningfulConversationContent(message: AgentMessage): boolean
     const output = typeof bash.output === "string" ? bash.output : "";
     return hasMeaningfulText(`${command}\n${output}`);
   }
-  if ((message as { role?: unknown }).role === "branchSummary") {
+  if (isSummaryRole((message as { role?: unknown }).role)) {
     const summary = (message as { summary?: unknown }).summary;
     return typeof summary === "string" && hasMeaningfulText(summary);
   }
@@ -93,10 +97,7 @@ function hasMeaningfulMessageContent(content: unknown): boolean {
 function isToolResultConversationAnchor(message: AgentMessage): boolean {
   const role = (message as { role?: unknown }).role;
   return (
-    (role === "user" ||
-      role === "custom" ||
-      role === "bashExecution" ||
-      role === "branchSummary") &&
+    (role === "user" || role === "custom" || role === "bashExecution" || isSummaryRole(role)) &&
     hasMeaningfulConversationContent(message)
   );
 }
@@ -112,7 +113,7 @@ export function isRealConversationMessage(
     message.role === "assistant" ||
     message.role === "custom" ||
     message.role === "bashExecution" ||
-    message.role === "branchSummary"
+    isSummaryRole(message.role)
   ) {
     return hasMeaningfulConversationContent(message);
   }
diff --git a/src/agents/embedded-agent-runner/compact.hooks.test.ts b/src/agents/embedded-agent-runner/compact.hooks.test.ts
index ce4111c31a4c..136bd253261d 100644
--- a/src/agents/embedded-agent-runner/compact.hooks.test.ts
+++ b/src/agents/embedded-agent-runner/compact.hooks.test.ts
@@ -2170,6 +2170,38 @@ describe("compactEmbeddedAgentSessionDirect hooks", () => {
     });
   });
 
+  it("compacts an overflow transcript anchored by a compaction summary", async () => {
+    sessionMessages.splice(
+      0,
+      sessionMessages.length,
+      {
+        role: "compactionSummary",
+        summary: "The user asked for a long-running repository audit.",
+        timestamp: 1,
+      },
+      {
+        role: "assistant",
+        content: [{ type: "toolCall", id: "call-1", name: "exec", arguments: {} }],
+        timestamp: 2,
+      },
+      {
+        role: "toolResult",
+        toolCallId: "call-1",
+        toolName: "exec",
+        content: [{ type: "text", text: "audit output" }],
+        isError: false,
+        timestamp: 3,
+      },
+    );
+
+    const result = await compactEmbeddedAgentSessionDirect(
+      wrappedCompactionArgs({ trigger: "overflow" }),
+    );
+
+    expect(result).toMatchObject({ ok: true, compacted: true });
+    expect(sessionCompactImpl).toHaveBeenCalledOnce();
+  });
+
   it("skips compaction when the transcript only contains boilerplate replies and tool output", () => {
     const messages = [
       { role: "user", content: "<b>HEARTBEAT_OK</b>", timestamp: 1 },