diff --git a/packages/app/src/context/global-sync.tsx b/packages/app/src/context/global-sync.tsx
index 0facbdfff452..eb6c4d1d568e 100644
--- a/packages/app/src/context/global-sync.tsx
+++ b/packages/app/src/context/global-sync.tsx
@@ -782,21 +782,34 @@ function createGlobalSync() {
         break
       }
       case "message.updated": {
-        const messages = store.message[event.properties.info.sessionID]
+        const info = event.properties.info
+        const messages = store.message[info.sessionID]
         if (!messages) {
-          setStore("message", event.properties.info.sessionID, [event.properties.info])
+          setStore("message", info.sessionID, [info])
           break
         }
-        const result = Binary.search(messages, event.properties.info.id, (m) => m.id)
+
+        const clientID = info.role === "user" ? info.clientMessageID : undefined
+        if (clientID && clientID !== info.id) {
+          const optimistic = Binary.search(messages, clientID, (m) => m.id)
+          if (optimistic.found) {
+            setStore("message", info.sessionID, optimistic.index, reconcile(info))
+            setStore("part", info.id, store.part[clientID] ?? [])
+            setStore("part", clientID, undefined!)
+            break
+          }
+        }
+
+        const result = Binary.search(messages, info.id, (m) => m.id)
         if (result.found) {
-          setStore("message", event.properties.info.sessionID, result.index, reconcile(event.properties.info))
+          setStore("message", info.sessionID, result.index, reconcile(info))
           break
         }
         setStore(
           "message",
-          event.properties.info.sessionID,
+          info.sessionID,
           produce((draft) => {
-            draft.splice(result.index, 0, event.properties.info)
+            draft.splice(result.index, 0, info)
           }),
         )
         break
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 6358c6c5e9b0..b40e9e4e34c0 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -324,6 +324,7 @@ export namespace MessageV2 {
     system: z.string().optional(),
     tools: z.record(z.string(), z.boolean()).optional(),
     variant: z.string().optional(),
+    clientMessageID: z.string().optional(),
   }).meta({
     ref: "UserMessage",
   })
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index e0861c4df527..58092e618725 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -842,7 +842,7 @@ export namespace SessionPrompt {
         : undefined)
 
     const info: MessageV2.Info = {
-      id: input.messageID ?? Identifier.ascending("message"),
+      id: Identifier.ascending("message"),
       role: "user",
       sessionID: input.sessionID,
       time: {
@@ -853,6 +853,7 @@ export namespace SessionPrompt {
       model,
       system: input.system,
       variant,
+      clientMessageID: input.messageID,
     }
     using _ = defer(() => InstructionPrompt.clear(info.id))
 
@@ -1192,6 +1193,7 @@ export namespace SessionPrompt {
         agent: input.agent,
         model: input.model,
         messageID: input.messageID,
+        clientMessageID: input.messageID,
         variant: input.variant,
       },
       {
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 0cf70241ef6f..246e47a27cc2 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -121,6 +121,7 @@ export type UserMessage = {
     [key: string]: boolean
   }
   variant?: string
+  clientMessageID?: string
 }
 
 export type ProviderAuthError = {
diff --git a/packages/sdk/openapi.json b/packages/sdk/openapi.json
index d179ed8b8c4e..7143399448a1 100644
--- a/packages/sdk/openapi.json
+++ b/packages/sdk/openapi.json
@@ -6164,6 +6164,9 @@
           },
           "variant": {
             "type": "string"
+          },
+          "clientMessageID": {
+            "type": "string"
           }
         },
         "required": ["id", "sessionID", "role", "time", "agent", "model"]