From 32fdd21c80fa8b69fe987adcc56d19c0d8aae792 Mon Sep 17 00:00:00 2001
From: Vincent Koc <vincentkoc@ieee.org>
Date: Sun, 22 Mar 2026 19:48:04 -0700
Subject: [PATCH] fix(acp): preserve hidden thought replay on session load

---
 src/acp/translator.session-rate-limit.test.ts | 17 +++-
 src/acp/translator.ts                         | 88 ++++++++++++-------
 2 files changed, 72 insertions(+), 33 deletions(-)

diff --git a/src/acp/translator.session-rate-limit.test.ts b/src/acp/translator.session-rate-limit.test.ts
index 566b61a5027..cbb19b6548c 100644
--- a/src/acp/translator.session-rate-limit.test.ts
+++ b/src/acp/translator.session-rate-limit.test.ts
@@ -242,7 +242,7 @@ describe("acp session UX bridge behavior", () => {
     sessionStore.clearAllSessionsForTest();
   });
 
-  it("replays user and assistant text history on loadSession and returns initial controls", async () => {
+  it("replays user text, assistant text, and hidden assistant thinking on loadSession", async () => {
     const sessionStore = createInMemorySessionStore();
     const connection = createAcpConnection();
     const sessionUpdate = connection.__sessionUpdateMock;
@@ -283,7 +283,13 @@ describe("acp session UX bridge behavior", () => {
         return {
           messages: [
             { role: "user", content: [{ type: "text", text: "Question" }] },
-            { role: "assistant", content: [{ type: "text", text: "Answer" }] },
+            {
+              role: "assistant",
+              content: [
+                { type: "thinking", thinking: "Internal loop about NO_REPLY" },
+                { type: "text", text: "Answer" },
+              ],
+            },
             { role: "system", content: [{ type: "text", text: "ignore me" }] },
             { role: "assistant", content: [{ type: "image", image: "skip" }] },
           ],
@@ -332,6 +338,13 @@ describe("acp session UX bridge behavior", () => {
         content: { type: "text", text: "Question" },
       },
     });
+    expect(sessionUpdate).toHaveBeenCalledWith({
+      sessionId: "agent:main:work",
+      update: {
+        sessionUpdate: "agent_thought_chunk",
+        content: { type: "text", text: "Internal loop about NO_REPLY" },
+      },
+    });
     expect(sessionUpdate).toHaveBeenCalledWith({
       sessionId: "agent:main:work",
       update: {
diff --git a/src/acp/translator.ts b/src/acp/translator.ts
index d275913800e..cc20ac38fc6 100644
--- a/src/acp/translator.ts
+++ b/src/acp/translator.ts
@@ -134,6 +134,11 @@ type GatewayChatContentBlock = {
   thinking?: string;
 };
 
+type ReplayChunk = {
+  sessionUpdate: "user_message_chunk" | "agent_message_chunk" | "agent_thought_chunk";
+  text: string;
+};
+
 const SESSION_CREATE_RATE_LIMIT_DEFAULT_MAX_REQUESTS = 120;
 const SESSION_CREATE_RATE_LIMIT_DEFAULT_WINDOW_MS = 10_000;
 
@@ -261,25 +266,51 @@ function buildSessionPresentation(params: {
   return { configOptions, modes };
 }
 
-function extractReplayText(content: unknown): string | undefined {
-  if (typeof content === "string") {
-    return content.length > 0 ? content : undefined;
+function extractReplayChunks(message: GatewayTranscriptMessage): ReplayChunk[] {
+  const role = typeof message.role === "string" ? message.role : "";
+  if (role !== "user" && role !== "assistant") {
+    return [];
   }
-  if (!Array.isArray(content)) {
-    return undefined;
+  if (typeof message.content === "string") {
+    return message.content.length > 0
+      ? [
+          {
+            sessionUpdate: role === "user" ? "user_message_chunk" : "agent_message_chunk",
+            text: message.content,
+          },
+        ]
+      : [];
   }
-  const text = content
-    .map((block) => {
-      if (!block || typeof block !== "object" || Array.isArray(block)) {
-        return "";
-      }
-      const typedBlock = block as { type?: unknown; text?: unknown };
-      return typedBlock.type === "text" && typeof typedBlock.text === "string"
-        ? typedBlock.text
-        : "";
-    })
-    .join("");
-  return text.length > 0 ? text : undefined;
+  if (!Array.isArray(message.content)) {
+    return [];
+  }
+
+  const replayChunks: ReplayChunk[] = [];
+  for (const block of message.content) {
+    if (!block || typeof block !== "object" || Array.isArray(block)) {
+      continue;
+    }
+    const typedBlock = block as GatewayChatContentBlock;
+    if (typedBlock.type === "text" && typeof typedBlock.text === "string" && typedBlock.text) {
+      replayChunks.push({
+        sessionUpdate: role === "user" ? "user_message_chunk" : "agent_message_chunk",
+        text: typedBlock.text,
+      });
+      continue;
+    }
+    if (
+      role === "assistant" &&
+      typedBlock.type === "thinking" &&
+      typeof typedBlock.thinking === "string" &&
+      typedBlock.thinking
+    ) {
+      replayChunks.push({
+        sessionUpdate: "agent_thought_chunk",
+        text: typedBlock.thinking,
+      });
+    }
+  }
+  return replayChunks;
 }
 
 function buildSessionMetadata(params: {
@@ -1045,21 +1076,16 @@ export class AcpGatewayAgent implements Agent {
     transcript: ReadonlyArray<GatewayTranscriptMessage>,
   ): Promise<void> {
     for (const message of transcript) {
-      const role = typeof message.role === "string" ? message.role : "";
-      if (role !== "user" && role !== "assistant") {
-        continue;
+      const replayChunks = extractReplayChunks(message);
+      for (const chunk of replayChunks) {
+        await this.connection.sessionUpdate({
+          sessionId,
+          update: {
+            sessionUpdate: chunk.sessionUpdate,
+            content: { type: "text", text: chunk.text },
+          },
+        });
       }
-      const text = extractReplayText(message.content);
-      if (!text) {
-        continue;
-      }
-      await this.connection.sessionUpdate({
-        sessionId,
-        update: {
-          sessionUpdate: role === "user" ? "user_message_chunk" : "agent_message_chunk",
-          content: { type: "text", text },
-        },
-      });
     }
   }