diff --git a/packages/ui/src/components/chat/message/parts/DOCUMENTATION.md b/packages/ui/src/components/chat/message/parts/DOCUMENTATION.md
index 821d462e..6ca59ba8 100644
--- a/packages/ui/src/components/chat/message/parts/DOCUMENTATION.md
+++ b/packages/ui/src/components/chat/message/parts/DOCUMENTATION.md
@@ -89,6 +89,7 @@ Use this doc when you ask an agent to change tool/header/description behavior.
 - The rich tool diff preview lives in `ToolPartDiffPreview.tsx` and is lazy-loaded from `ToolPart`. It is the only tool-card piece that imports the `@pierre/diffs` + Shiki rendering stack, keeping that stack out of the eager chat startup graph. While its chunk loads (first rendered diff only) the plain-text patch from `PlainDiffFallback.tsx` renders as the Suspense fallback, mirroring the preview's error fallback. `ToolPart` itself must not statically import `@pierre/diffs` runtime modules or `@/lib/shiki/appThemeRegistry`.
 - Running bash output falls back to `state.metadata.output` until canonical `state.output` arrives. Its output viewport grows with the content up to `46vh`, then scrolls and follows new output until the user scrolls up; following resumes when the user returns to the bottom. Live output appends or replaces rewritten snapshots as plain text without worker highlighting; finalized output normalizes ANSI terminal controls with a bounded synthetic-cell budget, bypasses the throttle, and receives the normal one-time highlighted rendering.
 - Thinking/Justification duration is hidden in `sorted` mode (handled in `ReasoningPart.tsx` + `JustificationBlock.tsx`).
+- Reasoning streaming presentation derives from the live stream phase (`streaming`/`cooldown`), never from missing persisted timing: a cached part without `time.end` is not live, and a part whose `time.end` is set never streams (issue #2020).
 
 ## "I want to change description for Perplexity" (example recipe)
 
diff --git a/packages/ui/src/components/chat/message/parts/ReasoningPart.test.tsx b/packages/ui/src/components/chat/message/parts/ReasoningPart.test.tsx
index 90691c57..ac55bf68 100644
--- a/packages/ui/src/components/chat/message/parts/ReasoningPart.test.tsx
+++ b/packages/ui/src/components/chat/message/parts/ReasoningPart.test.tsx
@@ -1,9 +1,11 @@
 import React from 'react';
 import { describe, expect, test } from 'bun:test';
 import { renderToStaticMarkup } from 'react-dom/server';
+import type { Part } from '@opencode-ai/sdk/v2';
 
 import { I18nProvider } from '@/lib/i18n';
-import { ReasoningTimelineBlock } from './ReasoningPart';
+import ReasoningPart, { ReasoningTimelineBlock } from './ReasoningPart';
+import type { StreamPhase } from '../types';
 
 // A reasoning text whose summary (first 120 chars) fits in the header but
 // whose expanded body content should only appear when the disclosure is open.
@@ -113,3 +115,80 @@ describe('ReasoningTimelineBlock', () => {
     expect(markup).not.toContain('<!-- -->');
   });
 });
+
+// Regression tests for issue #2020: a persisted reasoning part must not be
+// presented as live streaming just because cached data lacks `time.end` or a
+// stream phase. Live activity derives from the live stream phase only.
+describe('ReasoningPart streaming gating (issue #2020)', () => {
+  // Short enough (< 80 chars) that the collapsed header summary contains the
+  // complete text, letting us assert full content on first paint.
+  const SHORT_REASONING = 'Persisted reasoning text that is already fully available.';
+
+  const BUSY_INDICATOR = 'animate-busy-pulse';
+
+  const makeReasoningPart = (time?: { start?: number; end?: number }): Part =>
+    ({
+      id: 'prt_reasoning_2020',
+      sessionID: 'ses_2020',
+      messageID: 'msg_2020',
+      type: 'reasoning',
+      text: SHORT_REASONING,
+      time,
+    }) as unknown as Part;
+
+  // Server rendering reads the UI store's initial state, which is
+  // chatRenderMode 'live' — the mode in which the streaming presentation is
+  // reachable and the issue reproduces.
+  const renderPart = (part: Part, streamPhase?: StreamPhase): string =>
+    renderToStaticMarkup(
+      <I18nProvider>
+        <ReasoningPart part={part} messageId="msg_2020" streamPhase={streamPhase} />
+      </I18nProvider>,
+    );
+
+  test('reasoning without time.end and without a live stream phase renders complete, not streaming', () => {
+    // Freshly opened completed session: cached part never received `time.end`
+    // and no message-level stream phase is available. The full text is already
+    // local, so the block must render as finished content on first paint.
+    const markup = renderPart(makeReasoningPart({ start: 1_000 }), undefined);
+
+    expect(markup).not.toContain(BUSY_INDICATOR);
+    expect(markup).toContain('aria-expanded="false"');
+    expect(markup).toContain(SHORT_REASONING);
+  });
+
+  test('reasoning without time.end in a completed message renders complete, not streaming', () => {
+    const markup = renderPart(makeReasoningPart({ start: 1_000 }), 'completed');
+
+    expect(markup).not.toContain(BUSY_INDICATOR);
+    expect(markup).toContain('aria-expanded="false"');
+    expect(markup).toContain(SHORT_REASONING);
+  });
+
+  test('reasoning with time.end is never treated as streaming, even when the phase claims streaming', () => {
+    const markup = renderPart(makeReasoningPart({ start: 1_000, end: 2_000 }), 'streaming');
+
+    expect(markup).not.toContain(BUSY_INDICATOR);
+    expect(markup).toContain('aria-expanded="false"');
+    expect(markup).toContain(SHORT_REASONING);
+  });
+
+  test('live in-progress reasoning still renders as streaming', () => {
+    // Genuinely live: the message-level stream phase reports streaming and the
+    // part has not ended. The block auto-expands and shows the busy indicator.
+    const markup = renderPart(makeReasoningPart({ start: 1_000 }), 'streaming');
+
+    expect(markup).toContain(BUSY_INDICATOR);
+    expect(markup).toContain('aria-expanded="true"');
+  });
+
+  test('remounting a completed reasoning part does not re-trigger the streaming presentation', () => {
+    const part = makeReasoningPart({ start: 1_000 });
+    const first = renderPart(part, undefined);
+    const second = renderPart(part, undefined);
+
+    expect(second).toBe(first);
+    expect(second).not.toContain(BUSY_INDICATOR);
+    expect(second).toContain(SHORT_REASONING);
+  });
+});
diff --git a/packages/ui/src/components/chat/message/parts/ReasoningPart.tsx b/packages/ui/src/components/chat/message/parts/ReasoningPart.tsx
index 1922100c..169c0eb7 100644
--- a/packages/ui/src/components/chat/message/parts/ReasoningPart.tsx
+++ b/packages/ui/src/components/chat/message/parts/ReasoningPart.tsx
@@ -261,7 +261,11 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
         };
     }, []);
 
-    if (!text || text.trim().length === 0) {
+    // While genuinely streaming, the busy header must appear as soon as
+    // reasoning starts even before the block-level reveal (commitStreamedText)
+    // has committed a first complete line — otherwise "Thinking…" never shows
+    // for the first moments of a short, single-paragraph response.
+    if (!isStreaming && (!text || text.trim().length === 0)) {
         return null;
     }
 
@@ -430,8 +434,12 @@ const ReasoningPart = React.memo(({
     const rawText = partWithText.text || partWithText.content || '';
     const textContent = React.useMemo(() => cleanReasoningText(rawText), [rawText]);
     const time = partWithText.time;
-    const canBeStreaming = streamPhase === undefined || streamPhase !== 'completed';
-    const isStreaming = chatRenderMode === 'live' && canBeStreaming && typeof time?.end !== 'number';
+    // Live activity derives from the live stream phase, never from the absence
+    // of persisted timing data: cached parts may lack `time.end` even though
+    // the message finished long ago (issue #2020). A part that has ended is
+    // never streaming, even while the rest of the message still streams.
+    const isLiveStreamPhase = streamPhase === 'streaming' || streamPhase === 'cooldown';
+    const isStreaming = chatRenderMode === 'live' && isLiveStreamPhase && typeof time?.end !== 'number';
     const throttledTextRaw = useStreamingTextThrottle({
         text: textContent,
         isStreaming,
@@ -441,9 +449,11 @@ const ReasoningPart = React.memo(({
     // never mutates in place.
     const throttledText = isStreaming ? commitStreamedText(throttledTextRaw) : throttledTextRaw;
 
-    // Show reasoning even if time.end isn't set yet (during streaming)
-    // Only hide if there's no text content
-    if (!throttledText || throttledText.trim().length === 0) {
+    // Show reasoning even if time.end isn't set yet (during streaming).
+    // While genuinely streaming, keep the block mounted even before the
+    // block-level reveal commits a first line, so the busy header appears
+    // immediately instead of waiting on committed text.
+    if (!isStreaming && (!throttledText || throttledText.trim().length === 0)) {
         return null;
     }