diff --git a/packages/opencode/test/acp/event-subscription.test.ts b/packages/opencode/test/acp/event-subscription.test.ts
index 8e139ff597..1145a1357d 100644
--- a/packages/opencode/test/acp/event-subscription.test.ts
+++ b/packages/opencode/test/acp/event-subscription.test.ts
@@ -122,12 +122,20 @@ function createFakeAgent() {
       messages: async () => {
         return { data: [] }
       },
-      message: async () => {
+      message: async (params?: any) => {
+        // Return a message with parts that can be looked up by partID
         return {
           data: {
             info: {
               role: "assistant",
             },
+            parts: [
+              {
+                id: params?.messageID ? `${params.messageID}_part` : "part_1",
+                type: "text",
+                text: "",
+              },
+            ],
           },
         }
       },
@@ -193,7 +201,7 @@ function createFakeAgent() {
 }
 
 describe("acp.agent event subscription", () => {
-  test("routes message.part.updated by the event sessionID (no cross-session pollution)", async () => {
+  test("routes message.part.delta by the event sessionID (no cross-session pollution)", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({
       directory: tmp.path,
@@ -207,14 +215,12 @@ describe("acp.agent event subscription", () => {
         controller.push({
           directory: cwd,
           payload: {
-            type: "message.part.updated",
+            type: "message.part.delta",
             properties: {
-              part: {
-                sessionID: sessionB,
-                messageID: "msg_1",
-                type: "text",
-                synthetic: false,
-              },
+              sessionID: sessionB,
+              messageID: "msg_1",
+              partID: "msg_1_part",
+              field: "text",
               delta: "hello",
             },
           },
@@ -230,7 +236,7 @@ describe("acp.agent event subscription", () => {
     })
   })
 
-  test("keeps concurrent sessions isolated when message.part.updated events are interleaved", async () => {
+  test("keeps concurrent sessions isolated when message.part.delta events are interleaved", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({
       directory: tmp.path,
@@ -248,14 +254,12 @@ describe("acp.agent event subscription", () => {
           controller.push({
             directory: cwd,
             payload: {
-              type: "message.part.updated",
+              type: "message.part.delta",
               properties: {
-                part: {
-                  sessionID: sessionId,
-                  messageID,
-                  type: "text",
-                  synthetic: false,
-                },
+                sessionID: sessionId,
+                messageID,
+                partID: `${messageID}_part`,
+                field: "text",
                 delta,
               },
             },
@@ -402,14 +406,12 @@ describe("acp.agent event subscription", () => {
         controller.push({
           directory: cwd,
           payload: {
-            type: "message.part.updated",
+            type: "message.part.delta",
             properties: {
-              part: {
-                sessionID: sessionB,
-                messageID: "msg_b",
-                type: "text",
-                synthetic: false,
-              },
+              sessionID: sessionB,
+              messageID: "msg_b",
+              partID: "msg_b_part",
+              field: "text",
               delta: "session_b_message",
             },
           },