diff --git a/apps/desktop/src/lib/chat-messages.test.ts b/apps/desktop/src/lib/chat-messages.test.ts
index f32310adfcf..44a6915fed7 100644
--- a/apps/desktop/src/lib/chat-messages.test.ts
+++ b/apps/desktop/src/lib/chat-messages.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, it } from 'vitest'
 import type { ChatMessage, ChatMessagePart } from './chat-messages'
 import {
   appendAssistantTextPart,
+  appendReasoningPart,
   chatMessageText,
   preserveLocalAssistantErrors,
   renderMediaTags,
@@ -175,6 +176,52 @@ describe('renderMediaTags', () => {
   })
 })
 
+describe('interleaved reasoning/text coalescing', () => {
+  it('keeps narration contiguous when reasoning interrupts mid-sentence', () => {
+    // Models that interleave reasoning_content + content deltas emit
+    // text → reasoning → text within one tool-bounded segment. The two text
+    // fragments are really one sentence and must not be split by the
+    // "Thinking" block between them.
+    let parts: ChatMessagePart[] = appendAssistantTextPart([], 'Let me ')
+    parts = appendReasoningPart(parts, 'checking the file...')
+    parts = appendAssistantTextPart(parts, 'verify the full file is correct:')
+
+    expect(parts.map(p => p.type)).toEqual(['text', 'reasoning'])
+    expect((parts[0] as { text: string }).text).toBe('Let me verify the full file is correct:')
+    expect((parts[1] as { text: string }).text).toBe('checking the file...')
+  })
+
+  it('merges reasoning bursts that straddle a narration fragment', () => {
+    let parts: ChatMessagePart[] = appendReasoningPart([], 'first thought ')
+    parts = appendAssistantTextPart(parts, 'Working on it.')
+    parts = appendReasoningPart(parts, 'second thought')
+
+    expect(parts.map(p => p.type)).toEqual(['reasoning', 'text'])
+    expect((parts[0] as { text: string }).text).toBe('first thought second thought')
+    expect((parts[1] as { text: string }).text).toBe('Working on it.')
+  })
+
+  it('starts a fresh text part after a tool call (segment boundary)', () => {
+    let parts: ChatMessagePart[] = appendAssistantTextPart([], 'Let me check.')
+    parts = upsertToolPart(parts, { name: 'read_file', tool_id: 'tc-1' }, 'running')
+    parts = appendAssistantTextPart(parts, 'Now editing.')
+
+    expect(parts.map(p => p.type)).toEqual(['text', 'tool-call', 'text'])
+    expect((parts[0] as { text: string }).text).toBe('Let me check.')
+    expect((parts[2] as { text: string }).text).toBe('Now editing.')
+  })
+
+  it('does not merge reasoning across a tool call', () => {
+    let parts: ChatMessagePart[] = appendReasoningPart([], 'before tool')
+    parts = upsertToolPart(parts, { name: 'read_file', tool_id: 'tc-1' }, 'running')
+    parts = appendReasoningPart(parts, 'after tool')
+
+    expect(parts.map(p => p.type)).toEqual(['reasoning', 'tool-call', 'reasoning'])
+    expect((parts[0] as { text: string }).text).toBe('before tool')
+    expect((parts[2] as { text: string }).text).toBe('after tool')
+  })
+})
+
 describe('preserveLocalAssistantErrors', () => {
   it('preserves a local user+error pair when hydration omits the failed turn', () => {
     const nextMessages: ChatMessage[] = [
diff --git a/apps/desktop/src/lib/chat-messages.ts b/apps/desktop/src/lib/chat-messages.ts
index e4b1f2fb162..ef4eef6662d 100644
--- a/apps/desktop/src/lib/chat-messages.ts
+++ b/apps/desktop/src/lib/chat-messages.ts
@@ -178,50 +178,70 @@ function displayContentForMessage(role: SessionMessage['role'], content: unknown
   return [refs.join('\n'), visibleText].filter(Boolean).join('\n\n') || visibleText
 }
 
-export function appendTextPart(parts: ChatMessagePart[], delta: string): ChatMessagePart[] {
-  const next = [...parts]
-  const last = next.at(-1)
-
-  if (last?.type === 'text') {
-    next[next.length - 1] = { ...last, text: `${last.text}${delta}` }
-
-    return next
-  }
-
-  next.push(textPart(delta))
-
-  return next
+const STREAM_PART: Record<'reasoning' | 'text', (text: string) => ChatMessagePart> = {
+  reasoning: reasoningPart,
+  text: textPart
 }
 
-export function appendAssistantTextPart(parts: ChatMessagePart[], delta: string): ChatMessagePart[] {
-  const next = appendTextPart(parts, delta)
-  const last = next.at(-1)
+// Coalesce a streaming delta into the most recent same-type part within the
+// current segment, where a segment is bounded by any non-streaming part (a
+// tool call, image, …). The opposite streaming channel (text <-> reasoning) is
+// transparent, so a reasoning burst between two content deltas can't shred one
+// sentence into text / Thinking / text — the fragmentation models that
+// interleave reasoning_content + content otherwise produce. Tool calls still
+// open a fresh part, preserving narration order across steps.
+function appendStreamPart(
+  parts: ChatMessagePart[],
+  type: 'reasoning' | 'text',
+  delta: string
+): { index: number; parts: ChatMessagePart[] } {
+  const next = [...parts]
 
-  if (last?.type === 'text') {
-    const current = last.text
+  for (let i = next.length - 1; i >= 0; i--) {
+    const part = next[i]
 
-    const deltaMayContainMedia =
-      delta.includes('MEDIA:') || delta.includes('DIA:') || delta.includes('EDIA:') || delta.includes('IA:')
+    if (part.type === type) {
+      next[i] = { ...part, text: `${(part as { text: string }).text}${delta}` } as ChatMessagePart
 
-    const needsMediaPass = deltaMayContainMedia || current.includes('MEDIA:')
-    const nextText = needsMediaPass ? renderMediaTags(current) : current
-    next[next.length - 1] = nextText === current ? last : { ...last, text: nextText }
+      return { index: i, parts: next }
+    }
+
+    if (part.type !== 'text' && part.type !== 'reasoning') {
+      break
+    }
   }
 
-  return next
+  next.push(STREAM_PART[type](delta))
+
+  return { index: next.length - 1, parts: next }
+}
+
+export function appendTextPart(parts: ChatMessagePart[], delta: string): ChatMessagePart[] {
+  return appendStreamPart(parts, 'text', delta).parts
 }
 
 export function appendReasoningPart(parts: ChatMessagePart[], delta: string): ChatMessagePart[] {
-  const next = [...parts]
-  const last = next.at(-1)
+  return appendStreamPart(parts, 'reasoning', delta).parts
+}
 
-  if (last?.type === 'reasoning') {
-    next[next.length - 1] = { ...last, text: `${last.text}${delta}` }
+export function appendAssistantTextPart(parts: ChatMessagePart[], delta: string): ChatMessagePart[] {
+  const { index, parts: next } = appendStreamPart(parts, 'text', delta)
+  const part = next[index]
 
+  if (part?.type !== 'text') {
     return next
   }
 
-  next.push(reasoningPart(delta))
+  const mayContainMedia =
+    delta.includes('MEDIA:') || delta.includes('DIA:') || delta.includes('EDIA:') || delta.includes('IA:')
+
+  if (mayContainMedia || part.text.includes('MEDIA:')) {
+    const rendered = renderMediaTags(part.text)
+
+    if (rendered !== part.text) {
+      next[index] = { ...part, text: rendered }
+    }
+  }
 
   return next
 }