diff --git a/packages/opencode/src/acp-next/event.ts b/packages/opencode/src/acp-next/event.ts
new file mode 100644
index 0000000000..cffdccf14a
--- /dev/null
+++ b/packages/opencode/src/acp-next/event.ts
@@ -0,0 +1,190 @@
+import type { AgentSideConnection } from "@agentclientprotocol/sdk"
+import * as Log from "@opencode-ai/core/util/log"
+import type {
+  Event,
+  EventMessagePartDelta,
+  EventMessagePartUpdated,
+  OpencodeClient,
+  Part,
+  SessionMessageResponse,
+} from "@opencode-ai/sdk/v2"
+import { Effect } from "effect"
+import { ACPNextSession } from "./session"
+
+const log = Log.create({ service: "acp-next-event" })
+
+type Connection = Pick<AgentSideConnection, "sessionUpdate">
+type GlobalEventEnvelope = {
+  payload?: Event
+}
+type GlobalEventStream = {
+  stream: AsyncIterable<GlobalEventEnvelope>
+}
+
+export function start(input: {
+  sdk: OpencodeClient
+  connection: Connection
+  session: ACPNextSession.Interface
+}) {
+  const subscription = new Subscription(input)
+  subscription.start()
+  return subscription
+}
+
+export class Subscription {
+  private readonly abort = new AbortController()
+  private started = false
+
+  constructor(
+    private readonly input: {
+      sdk: OpencodeClient
+      connection: Connection
+      session: ACPNextSession.Interface
+    },
+  ) {}
+
+  start() {
+    if (this.started) return
+    this.started = true
+    this.run().catch((error: unknown) => {
+      if (this.abort.signal.aborted) return
+      log.error("event subscription failed", { error })
+    })
+  }
+
+  stop() {
+    this.abort.abort()
+  }
+
+  async handle(event: Event) {
+    switch (event.type) {
+      case "message.part.updated":
+        return this.handlePartUpdated(event)
+      case "message.part.delta":
+        return this.handlePartDelta(event)
+    }
+  }
+
+  private async run() {
+    while (!this.abort.signal.aborted) {
+      const events = (await this.input.sdk.global.event({
+        signal: this.abort.signal,
+      })) as GlobalEventStream
+
+      for await (const event of events.stream) {
+        if (this.abort.signal.aborted) return
+        if (!event.payload) continue
+        await this.handle(event.payload).catch((error: unknown) => {
+          log.error("failed to handle event", { error, type: event.payload?.type })
+        })
+      }
+      if (!this.abort.signal.aborted) await new Promise((resolve) => setTimeout(resolve, 1000))
+    }
+  }
+
+  private async handlePartUpdated(event: EventMessagePartUpdated) {
+    const part = event.properties.part
+    const sessionId = part.sessionID || event.properties.sessionID
+    const session = await Effect.runPromise(this.input.session.tryGet(sessionId))
+    if (!session) return
+
+    await Effect.runPromise(
+      this.input.session.recordPartMetadata({
+        sessionId: session.id,
+        messageId: part.messageID,
+        partId: part.id,
+        partType: part.type,
+        role: part.type === "reasoning" ? "assistant" : undefined,
+        ignored: part.type === "text" ? part.ignored : undefined,
+        toolCallId: part.type === "tool" ? part.callID : undefined,
+        metadata: "metadata" in part ? part.metadata : undefined,
+      }),
+    )
+  }
+
+  private async handlePartDelta(event: EventMessagePartDelta) {
+    const props = event.properties
+    const session = await Effect.runPromise(this.input.session.tryGet(props.sessionID))
+    if (!session) return
+
+    const known = await Effect.runPromise(
+      this.input.session.tryGetPartMetadata({
+        sessionId: session.id,
+        messageId: props.messageID,
+        partId: props.partID,
+      }),
+    )
+    const metadata =
+      known?.role && known.partType
+        ? known
+        : await this.fetchPartMetadata(session.id, session.cwd, props.messageID, props.partID)
+    if (metadata?.role !== "assistant") return
+    if (metadata.partType === "text" && props.field === "text" && metadata.ignored !== true) {
+      await this.input.connection.sessionUpdate({
+        sessionId: session.id,
+        update: {
+          sessionUpdate: "agent_message_chunk",
+          messageId: props.messageID,
+          content: {
+            type: "text",
+            text: props.delta,
+          },
+        },
+      })
+      return
+    }
+
+    if (metadata.partType === "reasoning" && props.field === "text") {
+      await this.input.connection.sessionUpdate({
+        sessionId: session.id,
+        update: {
+          sessionUpdate: "agent_thought_chunk",
+          messageId: props.messageID,
+          content: {
+            type: "text",
+            text: props.delta,
+          },
+        },
+      })
+    }
+  }
+
+  private async fetchPartMetadata(sessionId: string, cwd: string, messageId: string, partId: string) {
+    const message = await this.input.sdk.session
+      .message(
+        {
+          sessionID: sessionId,
+          messageID: messageId,
+          directory: cwd,
+        },
+        { throwOnError: true },
+      )
+      .then((response) => response.data)
+      .catch((error: unknown) => {
+        log.error("unexpected error when fetching message for delta metadata", { error, messageId, partId })
+        return undefined
+      })
+    if (!message) return
+
+    const part = message.parts.find((item) => item.id === partId)
+    if (!part) return
+    return await this.recordFetchedPart(sessionId, message, part)
+  }
+
+  private async recordFetchedPart(sessionId: string, message: SessionMessageResponse, part: Part) {
+    return await Effect.runPromise(
+      this.input.session.recordPartMetadata({
+        sessionId,
+        messageId: part.messageID,
+        partId: part.id,
+        partType: part.type,
+        role: message.info.role,
+        ignored: part.type === "text" ? part.ignored : undefined,
+        toolCallId: part.type === "tool" ? part.callID : undefined,
+        metadata: "metadata" in part ? part.metadata : undefined,
+      }),
+    )
+  }
+}
+
+export * as ACPNextEvent from "./event"
diff --git a/packages/opencode/src/acp-next/service.ts b/packages/opencode/src/acp-next/service.ts
index 5ec9fd48b8..a465264d45 100644
--- a/packages/opencode/src/acp-next/service.ts
+++ b/packages/opencode/src/acp-next/service.ts
@@ -31,11 +31,12 @@ import {
 } from "@agentclientprotocol/sdk"
 import { InstallationVersion } from "@opencode-ai/core/installation/version"
 import * as Log from "@opencode-ai/core/util/log"
-import type { OpencodeClient } from "@opencode-ai/sdk/v2"
+import type { Message, OpencodeClient } from "@opencode-ai/sdk/v2"
 import { Context, Effect, Layer, ManagedRuntime } from "effect"
 import * as ACPNextError from "./error"
 import { buildConfigOptions, parseModelSelection } from "./config-option"
 import { Directory } from "./directory"
+import { ACPNextEvent } from "./event"
 import { ACPNextSession } from "./session"
 import { ModelID, ProviderID } from "@/provider/schema"
 import { Provider } from "@/provider/provider"
@@ -71,10 +72,15 @@ export function make(input: {
   connection?: Pick<AgentSideConnection, "sessionUpdate">
   directory?: Directory.Interface
   session?: ACPNextSession.Interface
+  eventSubscription?: (subscription: ACPNextEvent.Subscription) => void
 }): Interface {
   const session = input.session ?? makeSessionService()
   const directoryService = input.directory ?? makeDirectoryService(input.sdk)
   const registeredMcp = new Map<string, Set<string>>()
+  if (input.connection) {
+    const subscription = ACPNextEvent.start({ sdk: input.sdk, connection: input.connection, session })
+    input.eventSubscription?.(subscription)
+  }
 
   const initialize = Effect.fn("ACPNext.initialize")(function* (params: InitializeRequest) {
     const authMethod: AuthMethod = {
@@ -476,17 +482,13 @@ type SdkResponse<T> = {
 }
 
 type MessageInfo = {
-  readonly role?: string
-  readonly model?: {
-    readonly providerID?: string
-    readonly modelID?: string
-    readonly variant?: string
-  }
-  readonly providerID?: string
-  readonly modelID?: string
-  readonly variant?: string
-  readonly mode?: string
-  readonly agent?: string
+  readonly role?: Message["role"]
+  readonly model?: Extract<Message, { role: "user" }>["model"]
+  readonly providerID?: Extract<Message, { role: "assistant" }>["providerID"]
+  readonly modelID?: Extract<Message, { role: "assistant" }>["modelID"]
+  readonly variant?: Extract<Message, { role: "assistant" }>["variant"]
+  readonly mode?: Extract<Message, { role: "assistant" }>["mode"]
+  readonly agent?: Message["agent"]
 }
 
 function request<T>(fn: () => Promise<T | SdkResponse<T>>, service?: string) {
diff --git a/packages/opencode/src/acp-next/session.ts b/packages/opencode/src/acp-next/session.ts
index 7a969c867f..6ab61f5f0a 100644
--- a/packages/opencode/src/acp-next/session.ts
+++ b/packages/opencode/src/acp-next/session.ts
@@ -1,4 +1,5 @@
 import type { McpServer } from "@agentclientprotocol/sdk"
+import type { Message, Part } from "@opencode-ai/sdk/v2"
 import { Context, Effect, Layer, Ref } from "effect"
 import type { ModelID, ProviderID } from "../provider/schema"
 import * as ACPNextError from "./error"
@@ -11,6 +12,9 @@ export type SelectedModel = {
 export type KnownMessagePartMetadata = {
   messageId: string
   partId: string
+  partType?: Part["type"]
+  role?: Message["role"]
+  ignored?: boolean
   toolCallId?: string
   metadata?: unknown
 }
@@ -40,6 +44,9 @@ export type RecordPartMetadataInput = {
   sessionId: string
   messageId: string
   partId: string
+  partType?: Part["type"]
+  role?: Message["role"]
+  ignored?: boolean
   toolCallId?: string
   metadata?: unknown
 }
@@ -146,6 +153,9 @@ export const layer = Layer.effect(
       const metadata = {
         messageId: input.messageId,
         partId: input.partId,
+        partType: input.partType,
+        role: input.role,
+        ignored: input.ignored,
         toolCallId: input.toolCallId,
         metadata: input.metadata,
       }
diff --git a/packages/opencode/src/acp-next/usage.ts b/packages/opencode/src/acp-next/usage.ts
index a37f370c68..eabbb02af2 100644
--- a/packages/opencode/src/acp-next/usage.ts
+++ b/packages/opencode/src/acp-next/usage.ts
@@ -1,5 +1,6 @@
 import type { AgentSideConnection, Usage } from "@agentclientprotocol/sdk"
 import * as Log from "@opencode-ai/core/util/log"
+import type { AssistantMessage as OpenCodeAssistantMessage, Message } from "@opencode-ai/sdk/v2"
 import { InstanceRef } from "@/effect/instance-ref"
 import { InstanceStore } from "@/project/instance-store"
 import { ModelID, ProviderID } from "@/provider/schema"
@@ -8,27 +9,14 @@ import { Context, Effect, Layer, SynchronizedRef } from "effect"
 
 const log = Log.create({ service: "acp-next-usage" })
 
-export type AssistantTokenCost = {
-  readonly cost: number
-  readonly tokens: {
-    readonly input: number
-    readonly output: number
-    readonly reasoning: number
-    readonly cache: {
-      readonly read: number
-      readonly write: number
-    }
-  }
-}
+export type AssistantTokenCost = Pick<OpenCodeAssistantMessage, "cost" | "tokens">
 
-export type AssistantMessage = AssistantTokenCost & {
-  readonly role: "assistant"
-  readonly providerID?: string
-  readonly modelID?: string
-}
+export type AssistantMessage = AssistantTokenCost &
+  Pick<OpenCodeAssistantMessage, "role"> &
+  Partial<Pick<OpenCodeAssistantMessage, "providerID" | "modelID">>
 
 export type SessionMessage = {
-  readonly info: { readonly role: string } | AssistantMessage
+  readonly info: { readonly role: Message["role"] } | AssistantMessage
 }
 
 export type MessagesInput = {
diff --git a/packages/opencode/test/acp-next/event.test.ts b/packages/opencode/test/acp-next/event.test.ts
new file mode 100644
index 0000000000..8a4c88f234
--- /dev/null
+++ b/packages/opencode/test/acp-next/event.test.ts
@@ -0,0 +1,332 @@
+import { describe, expect, it } from "bun:test"
+import type { AgentSideConnection } from "@agentclientprotocol/sdk"
+import type { Event, Message, OpencodeClient, Part, SessionMessageResponse } from "@opencode-ai/sdk/v2"
+import { Effect, ManagedRuntime } from "effect"
+import { ACPNextEvent } from "@/acp-next/event"
+import * as ACPNextService from "@/acp-next/service"
+import { Directory } from "@/acp-next/directory"
+import { ACPNextSession } from "@/acp-next/session"
+
+type SessionUpdateParams = Parameters<AgentSideConnection["sessionUpdate"]>[0]
+type GlobalEventEnvelope = {
+  payload?: Event
+}
+type DeltaPartType = Extract<Part, { type: "text" | "reasoning" }>["type"]
+
+const pollUntil = async (
+  check: () => boolean | Promise<boolean>,
+  message: string,
+  opts?: { timeoutMs?: number; intervalMs?: number },
+) => {
+  const started = Date.now()
+  while (true) {
+    if (await check()) return
+    if (Date.now() - started > (opts?.timeoutMs ?? 2000)) throw new Error(message)
+    await new Promise((resolve) => setTimeout(resolve, opts?.intervalMs ?? 5))
+  }
+}
+
+function makeSessionService() {
+  return ManagedRuntime.make(ACPNextSession.defaultLayer).runSync(
+    ACPNextSession.Service.use((service) => Effect.succeed(service)),
+  )
+}
+
+function createEventStream() {
+  const queue: GlobalEventEnvelope[] = []
+  const waiters: Array<(value: GlobalEventEnvelope | undefined) => void> = []
+  const state = { closed: false }
+
+  const push = (event: GlobalEventEnvelope) => {
+    const waiter = waiters.shift()
+    if (waiter) {
+      waiter(event)
+      return
+    }
+    queue.push(event)
+  }
+
+  const close = () => {
+    state.closed = true
+    for (const waiter of waiters.splice(0)) {
+      waiter(undefined)
+    }
+  }
+
+  const stream = async function* (signal?: AbortSignal) {
+    while (true) {
+      if (signal?.aborted) return
+      const next = queue.shift()
+      if (next) {
+        yield next
+        continue
+      }
+      if (state.closed) return
+      const value = await new Promise<GlobalEventEnvelope | undefined>((resolve) => {
+        waiters.push(resolve)
+        signal?.addEventListener("abort", () => resolve(undefined), { once: true })
+      })
+      if (!value) return
+      yield value
+    }
+  }
+
+  return { push, close, stream }
+}
+
+function createHarness(messages: Record<string, SessionMessageResponse> = {}) {
+  const updates: SessionUpdateParams[] = []
+  const calls = {
+    eventSubscribe: 0,
+    message: 0,
+  }
+  const events = createEventStream()
+  const sdk = {
+    global: {
+      event: (options?: { signal?: AbortSignal }) => {
+        calls.eventSubscribe++
+        return Promise.resolve({ stream: events.stream(options?.signal) })
+      },
+    },
+    session: {
+      message: (input: { messageID: string }) => {
+        calls.message++
+        return Promise.resolve({ data: messages[input.messageID] })
+      },
+      get: () => Promise.resolve({ data: { id: "ses_loaded" } }),
+      messages: () => Promise.resolve({ data: [] }),
+    },
+  } as unknown as OpencodeClient
+  const connection = {
+    sessionUpdate: (params: SessionUpdateParams) => {
+      updates.push(params)
+      return Promise.resolve()
+    },
+  } satisfies Pick<AgentSideConnection, "sessionUpdate">
+  const session = makeSessionService()
+  const subscription = new ACPNextEvent.Subscription({ sdk, connection, session })
+
+  return { calls, connection, events, sdk, session, subscription, updates }
+}
+
+function textDelta(sessionID: string, messageID: string, partID: string, delta: string): Event {
+  return {
+    id: `evt_${sessionID}_${messageID}_${partID}_${delta}`,
+    type: "message.part.delta",
+    properties: {
+      sessionID,
+      messageID,
+      partID,
+      field: "text",
+      delta,
+    },
+  }
+}
+
+function partUpdated(sessionID: string, messageID: string, partID: string, type: DeltaPartType): Event {
+  return {
+    id: `evt_${sessionID}_${messageID}_${partID}`,
+    type: "message.part.updated",
+    properties: {
+      sessionID,
+      time: Date.now(),
+      part:
+        type === "text"
+          ? {
+              id: partID,
+              sessionID,
+              messageID,
+              type: "text",
+              text: "",
+            }
+          : {
+              id: partID,
+              sessionID,
+              messageID,
+              type: "reasoning",
+              text: "",
+              time: { start: Date.now() },
+            },
+    },
+  }
+}
+
+function assistantMessage(sessionID: string, messageID: string, partID: string, type: DeltaPartType) {
+  return {
+    info: {
+      id: messageID,
+      sessionID,
+      role: "assistant",
+      time: { created: Date.now() },
+      parentID: "msg_parent",
+      modelID: "model",
+      providerID: "provider",
+      mode: "build",
+      agent: "build",
+      path: { cwd: "/workspace", root: "/workspace" },
+      cost: 0,
+      tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+    },
+    parts: [
+      type === "text"
+        ? {
+            id: partID,
+            sessionID,
+            messageID,
+            type: "text",
+            text: "",
+          }
+        : {
+            id: partID,
+            sessionID,
+            messageID,
+            type: "reasoning",
+            text: "",
+            time: { start: Date.now() },
+          },
+    ],
+  } satisfies SessionMessageResponse
+}
+
+async function createKnownSession(
+  session: ACPNextSession.Interface,
+  sessionId: string,
+  part: { messageId: string; partId: string; partType: Part["type"]; role?: Message["role"] },
+) {
+  await Effect.runPromise(session.create({ id: sessionId, cwd: "/workspace" }))
+  await Effect.runPromise(
+    session.recordPartMetadata({
+      sessionId,
+      messageId: part.messageId,
+      partId: part.partId,
+      partType: part.partType,
+      role: part.role ?? "assistant",
+    }),
+  )
+}
+
+describe("acp-next event routing", () => {
+  it("routes message.part.delta by sessionID without cross-session pollution", async () => {
+    const harness = createHarness()
+    await createKnownSession(harness.session, "ses_a", { messageId: "msg_a", partId: "part_a", partType: "text" })
+    await createKnownSession(harness.session, "ses_b", { messageId: "msg_b", partId: "part_b", partType: "text" })
+
+    await harness.subscription.handle(textDelta("ses_b", "msg_b", "part_b", "hello"))
+
+    expect(harness.updates.map((update) => update.sessionId)).toEqual(["ses_b"])
+    expect(harness.updates[0]?.update.sessionUpdate).toBe("agent_message_chunk")
+  })
+
+  it("keeps interleaved sessions isolated for text and reasoning deltas", async () => {
+    const harness = createHarness()
+    await createKnownSession(harness.session, "ses_a", { messageId: "msg_a", partId: "part_a", partType: "text" })
+    await createKnownSession(harness.session, "ses_b", {
+      messageId: "msg_b",
+      partId: "part_b",
+      partType: "reasoning",
+    })
+
+    await harness.subscription.handle(textDelta("ses_a", "msg_a", "part_a", "A1"))
+    await harness.subscription.handle(textDelta("ses_b", "msg_b", "part_b", "B1"))
+    await harness.subscription.handle(textDelta("ses_a", "msg_a", "part_a", "A2"))
+    await harness.subscription.handle(textDelta("ses_b", "msg_b", "part_b", "B2"))
+
+    expect(
+      harness.updates
+        .filter((update) => update.sessionId === "ses_a")
+        .map((update) => update.update.sessionUpdate),
+    ).toEqual(["agent_message_chunk", "agent_message_chunk"])
+    expect(
+      harness.updates
+        .filter((update) => update.sessionId === "ses_b")
+        .map((update) => update.update.sessionUpdate),
+    ).toEqual(["agent_thought_chunk", "agent_thought_chunk"])
+  })
+
+  it("does not create extra subscriptions on repeated loadSession", async () => {
+    const harness = createHarness()
+    let subscription: ACPNextEvent.Subscription | undefined
+    const service = ACPNextService.make({
+      sdk: harness.sdk,
+      connection: harness.connection,
+      directory: {
+        get: () =>
+          Effect.succeed(
+            Directory.build({
+              directory: "/workspace",
+              providers: {},
+              modes: [],
+              defaultModeID: "build",
+              commands: [],
+            }),
+          ),
+        refresh: () =>
+          Effect.succeed(
+            Directory.build({
+              directory: "/workspace",
+              providers: {},
+              modes: [],
+              defaultModeID: "build",
+              commands: [],
+            }),
+          ),
+        variants: Directory.variants,
+      },
+      session: harness.session,
+      eventSubscription: (started) => {
+        subscription = started
+      },
+    })
+
+    await pollUntil(() => harness.calls.eventSubscribe === 1, "event subscription did not start")
+    await Effect.runPromise(service.loadSession({ cwd: "/workspace", sessionId: "ses_loaded", mcpServers: [] }))
+    await Effect.runPromise(service.loadSession({ cwd: "/workspace", sessionId: "ses_loaded", mcpServers: [] }))
+    await Effect.runPromise(service.loadSession({ cwd: "/workspace", sessionId: "ses_loaded", mcpServers: [] }))
+
+    expect(harness.calls.eventSubscribe).toBe(1)
+    subscription?.stop()
+    harness.events.close()
+  })
+
+  it("does not call sdk.session.message repeatedly when metadata is known", async () => {
+    const harness = createHarness()
+    await createKnownSession(harness.session, "ses_a", { messageId: "msg_a", partId: "part_a", partType: "text" })
+
+    for (const delta of ["a", "b", "c", "d", "e"]) {
+      await harness.subscription.handle(textDelta("ses_a", "msg_a", "part_a", delta))
+    }
+
+    expect(harness.calls.message).toBe(0)
+    expect(harness.updates).toHaveLength(5)
+  })
+
+  it("fetches unknown part metadata once and reuses it for later deltas", async () => {
+    const harness = createHarness({
+      msg_a: assistantMessage("ses_a", "msg_a", "part_a", "text"),
+    })
+    await Effect.runPromise(harness.session.create({ id: "ses_a", cwd: "/workspace" }))
+
+    await harness.subscription.handle(partUpdated("ses_a", "msg_a", "part_a", "text"))
+    await harness.subscription.handle(textDelta("ses_a", "msg_a", "part_a", "a"))
+    await harness.subscription.handle(textDelta("ses_a", "msg_a", "part_a", "b"))
+
+    expect(harness.calls.message).toBe(1)
+    expect(harness.updates).toHaveLength(2)
+  })
+
+  it("ignores unknown sessions and live user parts without user_message_chunk duplication", async () => {
+    const harness = createHarness()
+    await createKnownSession(harness.session, "ses_user", {
+      messageId: "msg_user",
+      partId: "part_user",
+      partType: "text",
+      role: "user",
+    })
+
+    await harness.subscription.handle(textDelta("ses_missing", "msg_missing", "part_missing", "ignored"))
+    await harness.subscription.handle(partUpdated("ses_user", "msg_user", "part_live", "text"))
+    await harness.subscription.handle(textDelta("ses_user", "msg_user", "part_user", "hello"))
+
+    expect(harness.updates).toHaveLength(0)
+  })
+})