From 742c005ac8bc4a2fb47a8d6654edda8c8ff35272 Mon Sep 17 00:00:00 2001
From: Vincent Koc <vincentkoc@ieee.org>
Date: Sun, 22 Mar 2026 19:42:53 -0700
Subject: [PATCH] fix(acp): preserve hidden thought chunks from gateway chat

---
 src/acp/translator.cancel-scoping.test.ts | 43 +++++++++++++++++++++++
 src/acp/translator.ts                     | 38 +++++++++++++++++---
 2 files changed, 77 insertions(+), 4 deletions(-)

diff --git a/src/acp/translator.cancel-scoping.test.ts b/src/acp/translator.cancel-scoping.test.ts
index e862222f7a0..12f015008f2 100644
--- a/src/acp/translator.cancel-scoping.test.ts
+++ b/src/acp/translator.cancel-scoping.test.ts
@@ -194,6 +194,49 @@ describe("acp translator cancel and run scoping", () => {
     await expect(pending.promptPromise).resolves.toEqual({ stopReason: "end_turn" });
   });
 
+  it("projects gateway thinking blocks into hidden ACP thought chunks", async () => {
+    const sessionKey = "agent:main:shared";
+    const harness = createHarness([{ sessionId: "session-1", sessionKey }]);
+    const pending = await startPendingPrompt(harness, "session-1");
+    harness.sessionUpdateSpy.mockClear();
+
+    await harness.agent.handleGatewayEvent(
+      createChatEvent({
+        runId: pending.runId,
+        sessionKey,
+        seq: 1,
+        state: "delta",
+        message: {
+          content: [
+            { type: "thinking", thinking: "Internal loop about NO_REPLY" },
+            { type: "text", text: "Final visible reply" },
+          ],
+        },
+      }),
+    );
+
+    expect(harness.sessionUpdateSpy).toHaveBeenNthCalledWith(
+      1,
+      expect.objectContaining({
+        sessionId: "session-1",
+        update: expect.objectContaining({
+          sessionUpdate: "agent_thought_chunk",
+          content: { type: "text", text: "Internal loop about NO_REPLY" },
+        }),
+      }),
+    );
+    expect(harness.sessionUpdateSpy).toHaveBeenNthCalledWith(
+      2,
+      expect.objectContaining({
+        sessionId: "session-1",
+        update: expect.objectContaining({
+          sessionUpdate: "agent_message_chunk",
+          content: { type: "text", text: "Final visible reply" },
+        }),
+      }),
+    );
+  });
+
   it("drops tool events when runId does not match the active prompt", async () => {
     const sessionKey = "agent:main:shared";
     const harness = createHarness([{ sessionId: "session-1", sessionKey }]);
diff --git a/src/acp/translator.ts b/src/acp/translator.ts
index 8ab1f821fc8..d275913800e 100644
--- a/src/acp/translator.ts
+++ b/src/acp/translator.ts
@@ -68,6 +68,8 @@ type PendingPrompt = {
   reject: (err: Error) => void;
   sentTextLength?: number;
   sentText?: string;
+  sentThoughtLength?: number;
+  sentThought?: string;
   toolCalls?: Map<string, PendingToolCall>;
 };
 
@@ -126,6 +128,12 @@ type GatewayTranscriptMessage = {
   content?: unknown;
 };
 
+type GatewayChatContentBlock = {
+  type?: string;
+  text?: string;
+  thinking?: string;
+};
+
 const SESSION_CREATE_RATE_LIMIT_DEFAULT_MAX_REQUESTS = 120;
 const SESSION_CREATE_RATE_LIMIT_DEFAULT_WINDOW_MS = 10_000;
 
@@ -834,22 +842,44 @@ export class AcpGatewayAgent implements Agent {
     sessionId: string,
     messageData: Record<string, unknown>,
   ): Promise<void> {
-    const content = messageData.content as Array<{ type: string; text?: string }> | undefined;
-    const fullText = content?.find((c) => c.type === "text")?.text ?? "";
+    const content = messageData.content as GatewayChatContentBlock[] | undefined;
     const pending = this.pendingPrompts.get(sessionId);
     if (!pending) {
       return;
     }
 
+    const fullThought = content
+      ?.filter((block) => block?.type === "thinking")
+      .map((block) => block.thinking ?? "")
+      .join("\n")
+      .trimEnd();
+    const sentThoughtSoFar = pending.sentThoughtLength ?? 0;
+    if (fullThought && fullThought.length > sentThoughtSoFar) {
+      const newThought = fullThought.slice(sentThoughtSoFar);
+      pending.sentThoughtLength = fullThought.length;
+      pending.sentThought = fullThought;
+      await this.connection.sessionUpdate({
+        sessionId,
+        update: {
+          sessionUpdate: "agent_thought_chunk",
+          content: { type: "text", text: newThought },
+        },
+      });
+    }
+
+    const fullText = content
+      ?.filter((block) => block?.type === "text")
+      .map((block) => block.text ?? "")
+      .join("\n")
+      .trimEnd();
     const sentSoFar = pending.sentTextLength ?? 0;
-    if (fullText.length <= sentSoFar) {
+    if (!fullText || fullText.length <= sentSoFar) {
       return;
     }
 
     const newText = fullText.slice(sentSoFar);
     pending.sentTextLength = fullText.length;
     pending.sentText = fullText;
-
     await this.connection.sessionUpdate({
       sessionId,
       update: {