fix(ui): apply Runtime Host stream slices by offset, not size

apps/desktop/.storybook/main.ts

@@ -55,7 +55,6 @@ const config: StorybookConfig = {
           { find: 'node:crypto', replacement: STORYBOOK_NODE_CRYPTO_BOUNDARY },
           { find: '@maka/ui/icons', replacement: resolve(UI_SRC, 'icons.tsx') },
           { find: '@maka/ui/artifact-preview-registry', replacement: resolve(UI_SRC, 'artifact-preview-registry.ts') },
-          { find: '@maka/ui/assistant-stream', replacement: resolve(UI_SRC, 'assistant-stream.ts') },
           { find: '@maka/ui/maka-uri', replacement: resolve(UI_SRC, 'maka-uri.ts') },
           { find: /^@maka\/ui$/, replacement: resolve(UI_SRC, 'index.ts') },
         ],

apps/desktop/tsconfig.storybook.json

@@ -8,7 +8,6 @@
       "@maka/ui": ["../../packages/ui/src/index.ts"],
       "@maka/ui/icons": ["../../packages/ui/src/icons.tsx"],
       "@maka/ui/artifact-preview-registry": ["../../packages/ui/src/artifact-preview-registry.ts"],
-      "@maka/ui/assistant-stream": ["../../packages/ui/src/assistant-stream.ts"],
       "@maka/ui/maka-uri": ["../../packages/ui/src/maka-uri.ts"],
     }
   },

apps/desktop/vite.config.ts

@@ -58,7 +58,6 @@ export default defineConfig({
       { find: '@maka/ui/client-plugin', replacement: resolve(UI_SRC, 'client-plugin-slots.tsx') },
       { find: '@maka/ui/icons', replacement: resolve(UI_SRC, 'icons.tsx') },
       { find: '@maka/ui/artifact-preview-registry', replacement: resolve(UI_SRC, 'artifact-preview-registry.ts') },
-      { find: '@maka/ui/assistant-stream', replacement: resolve(UI_SRC, 'assistant-stream.ts') },
       { find: '@maka/ui/maka-uri', replacement: resolve(UI_SRC, 'maka-uri.ts') },
       { find: /^@maka\/ui$/, replacement: resolve(UI_SRC, 'index.ts') },
     ],

packages/cli/src/acp/session-event-mapper.ts

@@ -188,10 +188,7 @@ function deltaText(
   event: Extract<SessionEvent, { type: 'text_delta' | 'thinking_delta' }>,
   current = '',
 ): string {
-  return foldRuntimeHostAssistantDelta(current, {
-    startOffset: event.startOffset ?? current.length,
-    text: event.text,
-  }).text;
+  return foldRuntimeHostAssistantDelta(current, event).text;
 }
 
 function streamKey(kind: StreamKind, messageId: string): string {

packages/core/src/__tests__/events.test.ts

@@ -24,9 +24,27 @@ import {
   decodeToolStepProgress,
   encodeToolStepProgress,
   decodeMessageContent,
+  foldAssistantDelta,
   isQuoteRef,
 } from '../events.js';
 
+test('folds assistant deltas idempotently by source offset', () => {
+  assert.deepStrictEqual(foldAssistantDelta(0, { startOffset: 0, text: 'Hello' }), {
+    tail: 'Hello',
+    endOffset: 5,
+  });
+  assert.deepStrictEqual(foldAssistantDelta(5, { startOffset: 0, text: 'Hello world' }), {
+    tail: ' world',
+    endOffset: 11,
+  });
+  assert.deepStrictEqual(foldAssistantDelta(11, { startOffset: 0, text: 'Hello' }), {
+    tail: '',
+    endOffset: 11,
+  });
+  assert.deepStrictEqual(foldAssistantDelta(3, { text: 'abc' }), { tail: 'abc', endOffset: 6 });
+  assert.strictEqual(foldAssistantDelta(3, { startOffset: 4, text: 'gap' }), undefined);
+});
+
 test('aggregates inline references against the combined display text', () => {
   assert.deepStrictEqual(
     aggregateMessageContents([

packages/core/src/events.ts

@@ -680,6 +680,8 @@ export interface TextDeltaEvent extends BaseEvent {
 export interface TextCompleteEvent extends BaseEvent {
   type: 'text_complete';
   interrupted?: true;
+  /** `text` replaces the streamed deltas instead of extending them. */
+  replaced?: true;
   messageId: string;
   text: string;
   /** Provider-owned text metadata such as Responses URL citations. */
@@ -694,9 +696,27 @@ export interface ThinkingDeltaEvent extends BaseEvent {
   text: string;
 }
 
+/**
+ * Apply a text/thinking delta to a stream that has consumed `currentEnd`
+ * source characters. Overlap with consumed text is dropped, so replayed and
+ * reseeded deltas are idempotent. A delta that starts past `currentEnd` is a
+ * gap and returns `undefined`; one without `startOffset` appends.
+ */
+export function foldAssistantDelta(
+  currentEnd: number,
+  delta: { readonly startOffset?: number; readonly text: string },
+): { tail: string; endOffset: number } | undefined {
+  const startOffset = delta.startOffset ?? currentEnd;
+  if (startOffset > currentEnd) return undefined;
+  const tail = delta.text.slice(currentEnd - startOffset);
+  return { tail, endOffset: currentEnd + tail.length };
+}
+
 export interface ThinkingCompleteEvent extends BaseEvent {
   type: 'thinking_complete';
   interrupted?: true;
+  /** `text` replaces the streamed deltas instead of extending them. */
+  replaced?: true;
   messageId: string;
   text: string;
   /** Anthropic signed thinking — MUST be re-sent on replay. */

packages/runtime-host/src/__tests__/session-projector.test.ts

@@ -92,6 +92,7 @@ test('applies authoritative replacement once and does not complete it again at T
     deltaFrame(2, 5, '', { complete: true, interrupted: true }),
   ).events;
   assert.ok(completed[0]?.type === 'text_complete' && completed[0].interrupted === true);
+  assert.equal(completed[0].replaced, true);
   assert.deepEqual(
     completed.map((event) => [event.type, 'text' in event ? event.text : '']),
     [['text_complete', 'final']],