diff --git a/packages/ui/src/lib/sse-manager.ts b/packages/ui/src/lib/sse-manager.ts
index d777ef38..77a4fdb4 100644
--- a/packages/ui/src/lib/sse-manager.ts
+++ b/packages/ui/src/lib/sse-manager.ts
@@ -4,6 +4,7 @@ import {
   MessageRemovedEvent,
   MessagePartUpdatedEvent,
   MessagePartRemovedEvent,
+  MessagePartDeltaEvent,
 } from "../types/message"
 import type {
   EventLspUpdated,
@@ -58,6 +59,7 @@ type SSEEvent =
   | MessageRemovedEvent
   | MessagePartUpdatedEvent
   | MessagePartRemovedEvent
+  | MessagePartDeltaEvent
   | EventSessionUpdated
   | EventSessionCompacted
   | EventSessionDiff
@@ -118,6 +120,9 @@ class SSEManager {
       case "message.part.updated":
         this.onMessagePartUpdated?.(instanceId, event as MessagePartUpdatedEvent)
         break
+      case "message.part.delta":
+        this.onMessagePartDelta?.(instanceId, event as MessagePartDeltaEvent)
+        break
       case "message.removed":
         this.onMessageRemoved?.(instanceId, event as MessageRemovedEvent)
         break
@@ -184,6 +189,7 @@ class SSEManager {
   onMessageUpdate?: (instanceId: string, event: MessageUpdateEvent) => void
   onMessageRemoved?: (instanceId: string, event: MessageRemovedEvent) => void
   onMessagePartUpdated?: (instanceId: string, event: MessagePartUpdatedEvent) => void
+  onMessagePartDelta?: (instanceId: string, event: MessagePartDeltaEvent) => void
   onMessagePartRemoved?: (instanceId: string, event: MessagePartRemovedEvent) => void
   onSessionUpdate?: (instanceId: string, event: EventSessionUpdated) => void
   onSessionCompacted?: (instanceId: string, event: EventSessionCompacted) => void
diff --git a/packages/ui/src/stores/message-v2/bridge.ts b/packages/ui/src/stores/message-v2/bridge.ts
index 65e20bd8..af22d94f 100644
--- a/packages/ui/src/stores/message-v2/bridge.ts
+++ b/packages/ui/src/stores/message-v2/bridge.ts
@@ -104,6 +104,22 @@ export function applyPartUpdateV2(instanceId: string, part: ClientPart | null |
   })
 }
 
+export function applyPartDeltaV2(
+  instanceId: string,
+  input: { messageId: string; partId: string; field: string; delta: string },
+): void {
+  if (!input?.messageId || !input.partId || !input.field || typeof input.delta !== "string") {
+    return
+  }
+  const store = messageStoreBus.getOrCreate(instanceId)
+  store.applyPartDelta({
+    messageId: input.messageId,
+    partId: input.partId,
+    field: input.field,
+    delta: input.delta,
+  })
+}
+
 export function replaceMessageIdV2(instanceId: string, oldId: string, newId: string): void {
   if (!oldId || !newId || oldId === newId) return
   const store = messageStoreBus.getOrCreate(instanceId)
diff --git a/packages/ui/src/stores/message-v2/instance-store.ts b/packages/ui/src/stores/message-v2/instance-store.ts
index 0ebc3c8b..1bebb023 100644
--- a/packages/ui/src/stores/message-v2/instance-store.ts
+++ b/packages/ui/src/stores/message-v2/instance-store.ts
@@ -189,6 +189,7 @@ export interface InstanceMessageStore {
   hydrateMessages: (sessionId: string, inputs: MessageUpsertInput[], infos?: Iterable<MessageInfo>) => void
   upsertMessage: (input: MessageUpsertInput) => void
   applyPartUpdate: (input: PartUpdateInput) => void
+  applyPartDelta: (input: { messageId: string; partId: string; field: string; delta: string; bumpRevision?: boolean }) => void
   removeMessage: (messageId: string) => void
   removeMessagePart: (messageId: string, partId: string) => void
   bufferPendingPart: (entry: PendingPartEntry) => void
@@ -597,6 +598,45 @@ export function createInstanceMessageStore(instanceId: string, hooks?: MessageSt
     bumpSessionRevision(message.sessionId)
   }
 
+  function applyPartDelta(input: { messageId: string; partId: string; field: string; delta: string; bumpRevision?: boolean }) {
+    if (!input?.messageId || !input.partId || !input.field || typeof input.delta !== "string") {
+      return
+    }
+
+    const message = state.messages[input.messageId]
+    if (!message) {
+      // Best-effort: drop deltas for unknown messages.
+      return
+    }
+
+    let applied = false
+
+    setState(
+      "messages",
+      input.messageId,
+      produce((draft: MessageRecord) => {
+        const entry = draft.parts[input.partId]
+        if (!entry?.data) return
+        const part = entry.data as any
+        const currentValue = part?.[input.field]
+        if (typeof currentValue === "string" || currentValue === undefined || currentValue === null) {
+          part[input.field] = `${currentValue ?? ""}${input.delta}`
+          applied = true
+        }
+        if (!applied) return
+        entry.revision += 1
+        draft.updatedAt = Date.now()
+        if (input.bumpRevision ?? true) {
+          draft.revision += 1
+        }
+      }),
+    )
+
+    if (applied) {
+      bumpSessionRevision(message.sessionId)
+    }
+  }
+
   function removeMessage(messageId: string) {
     if (!messageId) return
 
@@ -1087,19 +1127,20 @@ export function createInstanceMessageStore(instanceId: string, hooks?: MessageSt
      setState(reconcile(createInitialState(instanceId)))
    }
  
-   return {
+    return {
 
      instanceId,
      state,
      setState,
      addOrUpdateSession,
-     hydrateMessages,
-     upsertMessage,
+      hydrateMessages,
+      upsertMessage,
       applyPartUpdate,
+      applyPartDelta,
       removeMessage,
       removeMessagePart,
       bufferPendingPart,
-     flushPendingParts,
+      flushPendingParts,
      replaceMessageId,
      setMessageInfo,
      getMessageInfo,
@@ -1125,4 +1166,3 @@ export function createInstanceMessageStore(instanceId: string, hooks?: MessageSt
     }
   }
 
-
diff --git a/packages/ui/src/stores/session-events.ts b/packages/ui/src/stores/session-events.ts
index f5516a8a..abe24e75 100644
--- a/packages/ui/src/stores/session-events.ts
+++ b/packages/ui/src/stores/session-events.ts
@@ -1,6 +1,7 @@
 import type {
   MessageInfo,
   MessagePartRemovedEvent,
+  MessagePartDeltaEvent,
   MessagePartUpdatedEvent,
   MessageRemovedEvent,
   MessageUpdateEvent,
@@ -48,6 +49,7 @@ import { loadMessages } from "./session-api"
 import { getOrCreateWorktreeClient, getRootClient, getWorktreeSlugForDirectory, getWorktreeSlugForSession } from "./worktrees"
 import {
   applyPartUpdateV2,
+  applyPartDeltaV2,
   replaceMessageIdV2,
   reconcilePendingQuestionsV2,
   upsertMessageInfoV2,
@@ -348,6 +350,14 @@ function handleMessageUpdate(instanceId: string, event: MessageUpdateEvent | Mes
   }
 }
 
+function handleMessagePartDelta(instanceId: string, event: MessagePartDeltaEvent): void {
+  const props = event.properties
+  if (!props) return
+  const { messageID, partID, field, delta } = props
+  if (!messageID || !partID || !field || typeof delta !== "string") return
+  applyPartDeltaV2(instanceId, { messageId: messageID, partId: partID, field, delta })
+}
+
 function handleSessionUpdate(instanceId: string, event: EventSessionUpdated): void {
   const info = event.properties?.info
 
@@ -625,6 +635,7 @@ function handleQuestionAnswered(
 export {
   handleMessagePartRemoved,
   handleMessageRemoved,
+  handleMessagePartDelta,
   handleMessageUpdate,
   handlePermissionReplied,
   handlePermissionUpdated,
diff --git a/packages/ui/src/stores/sessions.ts b/packages/ui/src/stores/sessions.ts
index ef056a0e..8b5e9a3e 100644
--- a/packages/ui/src/stores/sessions.ts
+++ b/packages/ui/src/stores/sessions.ts
@@ -58,6 +58,7 @@ import {
 import {
   handleMessagePartRemoved,
   handleMessageRemoved,
+  handleMessagePartDelta,
   handleMessageUpdate,
   handlePermissionReplied,
   handlePermissionUpdated,
@@ -74,6 +75,7 @@ import {
 
 sseManager.onMessageUpdate = handleMessageUpdate
 sseManager.onMessagePartUpdated = handleMessageUpdate
+sseManager.onMessagePartDelta = handleMessagePartDelta
 sseManager.onMessageRemoved = handleMessageRemoved
 sseManager.onMessagePartRemoved = handleMessagePartRemoved
 sseManager.onSessionUpdate = handleSessionUpdate
diff --git a/packages/ui/src/types/message.ts b/packages/ui/src/types/message.ts
index 712d3d78..db964d27 100644
--- a/packages/ui/src/types/message.ts
+++ b/packages/ui/src/types/message.ts
@@ -20,6 +20,19 @@ export type {
   SDKMessage
 }
 
+// Server streaming event: append-only delta updates.
+// Emitted over SSE by newer OpenCode builds.
+export interface MessagePartDeltaEvent {
+  type: "message.part.delta"
+  properties: {
+    sessionID: string
+    messageID: string
+    partID: string
+    field: string
+    delta: string
+  }
+}
+
 export interface RenderCache {
   text: string
   html: string