diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index 8b338f1b571..479621c1dde 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -137,6 +137,7 @@ export namespace ACP {
     private eventStarted = false
     private bashSnapshots = new Map<string, string>()
     private toolStarts = new Set<string>()
+    private textSnapshots = new Map<string, string>()
     private permissionQueues = new Map<string, Promise<void>>()
     private permissionOptions: PermissionOption[] = [
       { optionId: "once", kind: "allow_once", name: "Allow once" },
@@ -446,6 +447,46 @@ export namespace ACP {
                 return
             }
           }
+
+          if (part.type === "text" && part.ignored !== true) {
+            const delta = this.textDelta(part.id, part.text)
+            if (!delta) return
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_message_chunk",
+                  content: {
+                    type: "text",
+                    text: delta,
+                  },
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send text update to ACP", { error })
+              })
+            return
+          }
+
+          if (part.type === "reasoning") {
+            const delta = this.textDelta(part.id, part.text)
+            if (!delta) return
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_thought_chunk",
+                  content: {
+                    type: "text",
+                    text: delta,
+                  },
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send reasoning update to ACP", { error })
+              })
+            return
+          }
           return
         }
 
@@ -476,6 +517,7 @@ export namespace ACP {
           if (!part) return
 
           if (part.type === "text" && props.field === "text" && part.ignored !== true) {
+            this.textSnapshots.set(part.id, part.text)
             await this.connection
               .sessionUpdate({
                 sessionId,
@@ -494,6 +536,7 @@ export namespace ACP {
           }
 
           if (part.type === "reasoning" && props.field === "text") {
+            this.textSnapshots.set(part.id, part.text)
             await this.connection
               .sessionUpdate({
                 sessionId,
@@ -1085,6 +1128,14 @@ export namespace ACP {
       return output
     }
 
+    private textDelta(id: string, next: string) {
+      const prev = this.textSnapshots.get(id) ?? ""
+      this.textSnapshots.set(id, next)
+      if (!next) return ""
+      if (next.startsWith(prev)) return next.slice(prev.length)
+      return next
+    }
+
     private async toolStart(sessionId: string, part: ToolPart) {
       if (this.toolStarts.has(part.callID)) return
       this.toolStarts.add(part.callID)
diff --git a/packages/opencode/test/acp/event-subscription.test.ts b/packages/opencode/test/acp/event-subscription.test.ts
index 1abf578281d..9ac40e9c196 100644
--- a/packages/opencode/test/acp/event-subscription.test.ts
+++ b/packages/opencode/test/acp/event-subscription.test.ts
@@ -345,6 +345,121 @@ describe("acp.agent event subscription", () => {
     })
   })
 
+
+  test("emits text chunks from message.part.updated when delta events are absent", async () => {
+    await using tmp = await tmpdir()
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const { agent, controller, chunks, stop } = createFakeAgent()
+        const cwd = "/tmp/opencode-acp-test"
+
+        const sessionId = await agent.newSession({ cwd, mcpServers: [] } as any).then((x) => x.sessionId)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.updated",
+            properties: {
+              part: {
+                id: "part_1",
+                sessionID: sessionId,
+                messageID: "msg_1",
+                type: "text",
+                text: "hello",
+              },
+            },
+          },
+        } as any)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.updated",
+            properties: {
+              part: {
+                id: "part_1",
+                sessionID: sessionId,
+                messageID: "msg_1",
+                type: "text",
+                text: "hello world",
+              },
+            },
+          },
+        } as any)
+
+        await new Promise((r) => setTimeout(r, 20))
+
+        expect(chunks.get(sessionId)).toBe("hello world")
+        stop()
+      },
+    })
+  })
+
+  test("does not duplicate chunks when message.part.delta is followed by message.part.updated", async () => {
+    await using tmp = await tmpdir()
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const { agent, controller, chunks, stop } = createFakeAgent()
+        const cwd = "/tmp/opencode-acp-test"
+
+        const sessionId = await agent.newSession({ cwd, mcpServers: [] } as any).then((x) => x.sessionId)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.delta",
+            properties: {
+              sessionID: sessionId,
+              messageID: "msg_1",
+              partID: "msg_1_part",
+              field: "text",
+              delta: "hello",
+            },
+          },
+        } as any)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.updated",
+            properties: {
+              part: {
+                id: "msg_1_part",
+                sessionID: sessionId,
+                messageID: "msg_1",
+                type: "text",
+                text: "hello",
+              },
+            },
+          },
+        } as any)
+
+        controller.push({
+          directory: cwd,
+          payload: {
+            type: "message.part.updated",
+            properties: {
+              part: {
+                id: "msg_1_part",
+                sessionID: sessionId,
+                messageID: "msg_1",
+                type: "text",
+                text: "hello world",
+              },
+            },
+          },
+        } as any)
+
+        await new Promise((r) => setTimeout(r, 20))
+
+        expect(chunks.get(sessionId)).toBe("hello world")
+        stop()
+      },
+    })
+  })
+
   test("does not create additional event subscriptions on repeated loadSession()", async () => {
     await using tmp = await tmpdir()
     await Instance.provide({