core: split message part updates into delta events for smoother streaming

Streaming text and reasoning content now uses incremental delta events instead of sending full message parts on each update. This reduces bandwidth and improves real-time response smoothness in the TUI.

packages/app/src/context/global-sync.tsx

@@ -839,6 +839,23 @@ function createGlobalSync() {
         )
         break
       }
+      case "message.part.delta": {
+        const parts = store.part[event.properties.messageID]
+        if (!parts) break
+        const result = Binary.search(parts, event.properties.partID, (p) => p.id)
+        if (!result.found) break
+        setStore(
+          "part",
+          event.properties.messageID,
+          produce((draft) => {
+            const part = draft[result.index]
+            const field = event.properties.field as keyof typeof part
+            const existing = part[field] as string | undefined
+            ;(part[field] as string) = (existing ?? "") + event.properties.delta
+          }),
+        )
+        break
+      }
       case "message.part.removed": {
         const messageID = event.properties.messageID
         const parts = store.part[messageID]

packages/opencode/src/acp/agent.ts

@@ -365,46 +365,68 @@ export namespace ACP {
                 return
             }
           }
+          return
+        }
 
-          if (part.type === "text") {
-            const delta = props.delta
-            if (delta && part.ignored !== true) {
-              await this.connection
-                .sessionUpdate({
-                  sessionId,
-                  update: {
-                    sessionUpdate: "agent_message_chunk",
-                    content: {
-                      type: "text",
-                      text: delta,
-                    },
+        case "message.part.delta": {
+          const props = event.properties
+          const session = this.sessionManager.tryGet(props.sessionID)
+          if (!session) return
+          const sessionId = session.id
+
+          const message = await this.sdk.session
+            .message(
+              {
+                sessionID: props.sessionID,
+                messageID: props.messageID,
+                directory: session.cwd,
+              },
+              { throwOnError: true },
+            )
+            .then((x) => x.data)
+            .catch((error) => {
+              log.error("unexpected error when fetching message", { error })
+              return undefined
+            })
+
+          if (!message || message.info.role !== "assistant") return
+
+          const part = message.parts.find((p) => p.id === props.partID)
+          if (!part) return
+
+          if (part.type === "text" && props.field === "text" && part.ignored !== true) {
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_message_chunk",
+                  content: {
+                    type: "text",
+                    text: props.delta,
                   },
-                })
-                .catch((error) => {
-                  log.error("failed to send text to ACP", { error })
-                })
-            }
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send text delta to ACP", { error })
+              })
             return
           }
 
-          if (part.type === "reasoning") {
-            const delta = props.delta
-            if (delta) {
-              await this.connection
-                .sessionUpdate({
-                  sessionId,
-                  update: {
-                    sessionUpdate: "agent_thought_chunk",
-                    content: {
-                      type: "text",
-                      text: delta,
-                    },
+          if (part.type === "reasoning" && props.field === "text") {
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_thought_chunk",
+                  content: {
+                    type: "text",
+                    text: props.delta,
                   },
-                })
-                .catch((error) => {
-                  log.error("failed to send reasoning to ACP", { error })
-                })
-            }
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send reasoning delta to ACP", { error })
+              })
           }
           return
         }

packages/opencode/src/cli/cmd/tui/context/sync.tsx

@@ -299,6 +299,24 @@ export const { use: useSync, provider: SyncProvider } = createSimpleContext({
           break
         }
 
+        case "message.part.delta": {
+          const parts = store.part[event.properties.messageID]
+          if (!parts) break
+          const result = Binary.search(parts, event.properties.partID, (p) => p.id)
+          if (!result.found) break
+          setStore(
+            "part",
+            event.properties.messageID,
+            produce((draft) => {
+              const part = draft[result.index]
+              const field = event.properties.field as keyof typeof part
+              const existing = part[field] as string | undefined
+              ;(part[field] as string) = (existing ?? "") + event.properties.delta
+            }),
+          )
+          break
+        }
+
         case "message.part.removed": {
           const parts = store.part[event.properties.messageID]
           const result = Binary.search(parts, event.properties.partID, (p) => p.id)

packages/opencode/src/session/index.ts

@@ -603,21 +603,9 @@ export namespace Session {
     },
   )
 
-  const UpdatePartInput = z.union([
-    MessageV2.Part,
-    z.object({
-      part: MessageV2.TextPart,
-      delta: z.string(),
-    }),
-    z.object({
-      part: MessageV2.ReasoningPart,
-      delta: z.string(),
-    }),
-  ])
+  const UpdatePartInput = MessageV2.Part
 
-  export const updatePart = fn(UpdatePartInput, async (input) => {
-    const part = "delta" in input ? input.part : input
-    const delta = "delta" in input ? input.delta : undefined
+  export const updatePart = fn(UpdatePartInput, async (part) => {
     const { id, messageID, sessionID, ...data } = part
     const time = Date.now()
     Database.use((db) => {
@@ -634,13 +622,25 @@ export namespace Session {
       Database.effect(() =>
         Bus.publish(MessageV2.Event.PartUpdated, {
           part,
-          delta,
         }),
       )
     })
     return part
   })
 
+  export const updatePartDelta = fn(
+    z.object({
+      sessionID: z.string(),
+      messageID: z.string(),
+      partID: z.string(),
+      field: z.string(),
+      delta: z.string(),
+    }),
+    async (input) => {
+      Bus.publish(MessageV2.Event.PartDelta, input)
+    },
+  )
+
   export const getUsage = fn(
     z.object({
       model: z.custom<Provider.Model>(),

packages/opencode/src/session/message-v2.ts

@@ -417,7 +417,16 @@ export namespace MessageV2 {
       "message.part.updated",
       z.object({
         part: Part,
-        delta: z.string().optional(),
+      }),
+    ),
+    PartDelta: BusEvent.define(
+      "message.part.delta",
+      z.object({
+        sessionID: z.string(),
+        messageID: z.string(),
+        partID: z.string(),
+        field: z.string(),
+        delta: z.string(),
       }),
     ),
     PartRemoved: BusEvent.define(

packages/opencode/src/session/processor.ts

@@ -63,17 +63,19 @@ export namespace SessionProcessor {
                   if (value.id in reasoningMap) {
                     continue
                   }
-                  reasoningMap[value.id] = {
+                  const reasoningPart = {
                     id: Identifier.ascending("part"),
                     messageID: input.assistantMessage.id,
                     sessionID: input.assistantMessage.sessionID,
-                    type: "reasoning",
+                    type: "reasoning" as const,
                     text: "",
                     time: {
                       start: Date.now(),
                     },
                     metadata: value.providerMetadata,
                   }
+                  reasoningMap[value.id] = reasoningPart
+                  await Session.updatePart(reasoningPart)
                   break
 
                 case "reasoning-delta":
@@ -81,7 +83,13 @@ export namespace SessionProcessor {
                     const part = reasoningMap[value.id]
                     part.text += value.text
                     if (value.providerMetadata) part.metadata = value.providerMetadata
-                    if (part.text) await Session.updatePart({ part, delta: value.text })
+                    await Session.updatePartDelta({
+                      sessionID: part.sessionID,
+                      messageID: part.messageID,
+                      partID: part.id,
+                      field: "text",
+                      delta: value.text,
+                    })
                   }
                   break
 
@@ -288,17 +296,20 @@ export namespace SessionProcessor {
                     },
                     metadata: value.providerMetadata,
                   }
+                  await Session.updatePart(currentText)
                   break
 
                 case "text-delta":
                   if (currentText) {
                     currentText.text += value.text
                     if (value.providerMetadata) currentText.metadata = value.providerMetadata
-                    if (currentText.text)
-                      await Session.updatePart({
-                        part: currentText,
-                        delta: value.text,
-                      })
+                    await Session.updatePartDelta({
+                      sessionID: currentText.sessionID,
+                      messageID: currentText.messageID,
+                      partID: currentText.id,
+                      field: "text",
+                      delta: value.text,
+                    })
                   }
                   break
 

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -480,7 +480,17 @@ export type EventMessagePartUpdated = {
   type: "message.part.updated"
   properties: {
     part: Part
-    delta?: string
+  }
+}
+
+export type EventMessagePartDelta = {
+  type: "message.part.delta"
+  properties: {
+    sessionID: string
+    messageID: string
+    partID: string
+    field: string
+    delta: string
   }
 }
 
@@ -650,10 +660,6 @@ export type Todo = {
    * Priority level of the task: high, medium, low
    */
   priority: string
-  /**
-   * Unique identifier for the todo item
-   */
-  id: string
 }
 
 export type EventTodoUpdated = {
@@ -896,6 +902,7 @@ export type Event =
   | EventMessageUpdated
   | EventMessageRemoved
   | EventMessagePartUpdated
+  | EventMessagePartDelta
   | EventMessagePartRemoved
   | EventPermissionAsked
   | EventPermissionReplied