fix(opencode): preserve ACP text deltas (#1023)

cli/src/agent/backends/acp/AcpMessageHandler.test.ts

@@ -377,6 +377,25 @@ describe('AcpMessageHandler', () => {
         expect(messages).toEqual([{ type: 'text', text: 'hello world' }]);
     });
 
+    it('preserves overlapping text chunks in delta mode', () => {
+        const messages: AgentMessage[] = [];
+        const handler = new AcpMessageHandler(
+            (message) => messages.push(message),
+            { textChunkMode: 'delta' }
+        );
+
+        for (const text of ['|-----|', '-----|', '-----|\n']) {
+            handler.handleUpdate({
+                sessionUpdate: ACP_SESSION_UPDATE_TYPES.agentMessageChunk,
+                content: { type: 'text', text }
+            });
+        }
+
+        handler.flushText();
+
+        expect(messages).toEqual([{ type: 'text', text: '|-----|-----|-----|\n' }]);
+    });
+
     it('keeps existing tool name when update only has kind fallback', () => {
         const messages: AgentMessage[] = [];
         const handler = new AcpMessageHandler((message) => messages.push(message));

cli/src/agent/backends/acp/AcpMessageHandler.ts

@@ -358,6 +358,8 @@ function normalizePlanEntries(entries: unknown): PlanItem[] {
     return items;
 }
 
+export type AcpTextChunkMode = 'dedupe' | 'delta';
+
 function getSuffixPrefixOverlap(base: string, next: string): number {
     const maxOverlap = Math.min(base.length, next.length);
     for (let length = maxOverlap; length > 0; length -= 1) {
@@ -380,8 +382,14 @@ export class AcpMessageHandler {
     private lastReasoningSnapshotAt: number | null = null;
     private lastReasoningSnapshotText = '';
     private reasoningSnapshotEmitted = false;
+    private readonly textChunkMode: AcpTextChunkMode;
 
-    constructor(private readonly onMessage: (message: AgentMessage) => void) {}
+    constructor(
+        private readonly onMessage: (message: AgentMessage) => void,
+        options: { textChunkMode?: AcpTextChunkMode } = {}
+    ) {
+        this.textChunkMode = options.textChunkMode ?? 'dedupe';
+    }
 
     /**
      * Emits any buffered assistant text as a single message and clears the
@@ -444,6 +452,13 @@ export class AcpMessageHandler {
     }
 
     private appendTextChunk(text: string): void {
+        if (this.textChunkMode === 'delta') {
+            if (text) {
+                this.bufferedText += text;
+            }
+            return;
+        }
+
         if (!text) {
             return;
         }

cli/src/agent/backends/acp/AcpSdkBackend.ts

@@ -2,7 +2,7 @@ import type { AgentFlavor } from '@hapi/protocol';
 import type { AgentBackend, AgentMessage, AgentSessionConfig, PermissionRequest, PermissionResponse, PromptContent } from '@/agent/types';
 import { asString, isObject } from '@hapi/protocol';
 import { AcpStdioTransport, type AcpStderrError } from './AcpStdioTransport';
-import { AcpMessageHandler } from './AcpMessageHandler';
+import { AcpMessageHandler, type AcpTextChunkMode } from './AcpMessageHandler';
 import { ACP_SESSION_UPDATE_TYPES } from './constants';
 import { logger } from '@/ui/logger';
 import { withRetry } from '@/utils/time';
@@ -101,7 +101,12 @@ export class AcpSdkBackend implements AgentBackend {
     private static readonly LATE_FLUSH_QUIET_PERIOD_MS = 250;
     private static readonly LATE_FLUSH_WINDOW_MS = 6000;
 
-    constructor(private readonly options: { command: string; args?: string[]; env?: Record<string, string> }) {}
+    constructor(private readonly options: {
+        command: string;
+        args?: string[];
+        env?: Record<string, string>;
+        textChunkMode?: AcpTextChunkMode;
+    }) {}
 
     async initialize(): Promise<void> {
         if (this.transport) return;
@@ -401,7 +406,7 @@ export class AcpSdkBackend implements AgentBackend {
             AcpSdkBackend.PRE_PROMPT_UPDATE_DRAIN_TIMEOUT_MS
         );
         this.messageHandler?.drainBuffers();
-        this.messageHandler = new AcpMessageHandler(onUpdate);
+        this.messageHandler = new AcpMessageHandler(onUpdate, { textChunkMode: this.options.textChunkMode });
         this.isProcessingMessage = true;
         this.lastSessionUpdateAt = Date.now();
         this.latestUsageUpdate = null;

cli/src/opencode/utils/opencodeBackend.ts

@@ -21,6 +21,7 @@ export function createOpencodeBackend(opts: {
     return new AcpSdkBackend({
         command: 'opencode',
         args,
-        env: filterEnv(env)
+        env: filterEnv(env),
+        textChunkMode: 'delta'
     });
 }