fix(acp): backfill missing text chunks from part updates

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 +447,18 @@ export namespace ACP {
                 return
             }
           }
+
+          if (part.type === "text" && part.ignored !== true) {
+            const delta = this.textDelta(part.id, part.text)
+            if (delta) await this.sendTextChunk(sessionId, "agent_message_chunk", delta)
+            return
+          }
+
+          if (part.type === "reasoning") {
+            const delta = this.textDelta(part.id, part.text)
+            if (delta) await this.sendTextChunk(sessionId, "agent_thought_chunk", delta)
+            return
+          }
           return
         }
 
@@ -476,38 +489,14 @@ export namespace ACP {
           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 delta to ACP", { error })
-              })
+            this.textSnapshots.set(part.id, (this.textSnapshots.get(part.id) ?? "") + props.delta)
+            await this.sendTextChunk(sessionId, "agent_message_chunk", props.delta)
             return
           }
 
           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 delta to ACP", { error })
-              })
+            this.textSnapshots.set(part.id, (this.textSnapshots.get(part.id) ?? "") + props.delta)
+            await this.sendTextChunk(sessionId, "agent_thought_chunk", props.delta)
           }
           return
         }
@@ -1085,6 +1074,31 @@ 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)
+      // Fallback: if text was edited mid-stream (rare), send full text to ensure client gets complete content
+      return next
+    }
+
+    private async sendTextChunk(
+      sessionId: string,
+      update: "agent_message_chunk" | "agent_thought_chunk",
+      text: string,
+    ) {
+      await this.connection
+        .sessionUpdate({
+          sessionId,
+          update: {
+            sessionUpdate: update,
+            content: { type: "text", text },
+          },
+        })
+        .catch((error) => log.error(`failed to send ${update} to ACP`, { error }))
+    }
+
     private async toolStart(sessionId: string, part: ToolPart) {
       if (this.toolStarts.has(part.callID)) return
       this.toolStarts.add(part.callID)