diff --git a/apps/desktop/src/components/assistant-ui/tool/fallback-model/index.ts b/apps/desktop/src/components/assistant-ui/tool/fallback-model/index.ts
index 6bcd06a63a9..5a7a5066c62 100644
--- a/apps/desktop/src/components/assistant-ui/tool/fallback-model/index.ts
+++ b/apps/desktop/src/components/assistant-ui/tool/fallback-model/index.ts
@@ -65,7 +65,7 @@ function fileEditPath(args: Record<string, unknown>, result: Record<string, unkn
   )
 }
 
-function fileEditBasename(path: string): string {
+export function fileEditBasename(path: string): string {
   const normalized = path.replace(/\\/g, '/').trim()
 
   return normalized.split('/').filter(Boolean).pop() || normalized
@@ -585,7 +585,7 @@ function summarizeBrowserSnapshot(snapshot: string): string {
   return labels.length ? `${stats}\nTop controls: ${labels.join(', ')}` : stats
 }
 
-function firstStringField(record: Record<string, unknown>, keys: readonly string[]): string {
+export function firstStringField(record: Record<string, unknown>, keys: readonly string[]): string {
   for (const key of keys) {
     const value = record[key]
 
diff --git a/apps/desktop/src/components/assistant-ui/tool/run-summary.test.ts b/apps/desktop/src/components/assistant-ui/tool/run-summary.test.ts
new file mode 100644
index 00000000000..ff36dcda30f
--- /dev/null
+++ b/apps/desktop/src/components/assistant-ui/tool/run-summary.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it } from 'vitest'
+
+import { summarizeToolRun, type ToolCallLike } from './run-summary'
+
+function tool(toolName: string, args: Record<string, unknown> = {}, result?: unknown): ToolCallLike {
+  return { args, result, toolCallId: `${toolName}-${Math.random()}`, toolName }
+}
+
+const edited = (path: string, diff = '') => tool('write_file', { path }, { path, inline_diff: diff })
+const read = (path: string) => tool('read_file', { path }, { content: '' })
+const ran = (command: string) => tool('terminal', { command }, { exit_code: 0 })
+
+describe('summarizeToolRun', () => {
+  it('names a lone edit and counts the rest', () => {
+    expect(
+      summarizeToolRun([edited('src/use-preview-routing.ts'), read('a.ts'), read('b.ts'), read('c.ts')]).text
+    ).toBe('Edited use-preview-routing.ts, explored 3 files')
+  })
+
+  it('orders clauses edit, explore, run regardless of call order', () => {
+    expect(
+      summarizeToolRun([ran('ls'), read('a.ts'), edited('src/attachments.tsx'), read('b.ts'), ran('pwd'), ran('id')])
+        .text
+    ).toBe('Edited attachments.tsx, explored 2 files, ran 3 commands')
+  })
+
+  it('counts commands rather than naming them once they have run', () => {
+    expect(summarizeToolRun([ran('git status')]).text).toBe('Ran 1 command')
+    expect(summarizeToolRun([read('status.ts'), ran('a'), ran('b'), ran('c'), ran('d'), ran('e')]).text).toBe(
+      'Explored status.ts, ran 5 commands'
+    )
+  })
+
+  it('counts a multi-file edit', () => {
+    expect(summarizeToolRun([edited('a.tsx'), edited('b.tsx'), read('c.ts')]).text).toBe(
+      'Edited 2 files, explored c.ts'
+    )
+  })
+
+  it('puts the running category in the present tense and leaves the rest past', () => {
+    const live = [edited('a.tsx'), tool('write_file', { path: 'b.tsx' }), ran('x'), ran('y')]
+
+    expect(summarizeToolRun(live).text).toBe('Editing 2 files, ran 2 commands')
+  })
+
+  it('names the command that is still running', () => {
+    expect(summarizeToolRun([tool('terminal', { command: 'npm run typecheck' })]).text).toMatch(/^Running /)
+  })
+
+  it('sums diff stats across the edits in the run', () => {
+    const summary = summarizeToolRun([
+      edited('a.tsx', '--- a\n+++ b\n+one\n+two\n-old'),
+      edited('b.tsx', '+three'),
+      ran('ls')
+    ])
+
+    expect(summary).toMatchObject({ added: 3, removed: 1 })
+  })
+
+  it('reports no diff stats for a run that changed nothing', () => {
+    expect(summarizeToolRun([read('a.ts'), ran('ls')])).toMatchObject({ added: 0, removed: 0 })
+  })
+})
diff --git a/apps/desktop/src/components/assistant-ui/tool/run-summary.ts b/apps/desktop/src/components/assistant-ui/tool/run-summary.ts
new file mode 100644
index 00000000000..7127a8f1efe
--- /dev/null
+++ b/apps/desktop/src/components/assistant-ui/tool/run-summary.ts
@@ -0,0 +1,168 @@
+import { summarizeShellCommand } from '@/lib/summarize-command'
+
+import {
+  countDiffLineStats,
+  fileEditBasename,
+  firstStringField,
+  inlineDiffFromResult,
+  isFileEditTool,
+  parseMaybeObject
+} from './fallback-model'
+
+/**
+ * The little a summary needs from a tool call, stated structurally so both
+ * shapes of tool part satisfy it — the stored `ChatMessagePart` and the live
+ * one assistant-ui hands to a renderer.
+ */
+export interface ToolCallLike {
+  args?: unknown
+  result?: unknown
+  toolCallId?: string
+  toolName: string
+}
+
+export function isToolCallPart<T extends { type: string }>(part: T): part is Extract<T, { type: 'tool-call' }> {
+  return part.type === 'tool-call'
+}
+
+type RunCategory = 'delegate' | 'edit' | 'explore' | 'other' | 'run'
+
+export interface RunSummary {
+  added: number
+  removed: number
+  text: string
+}
+
+// Clause order is fixed so the same run always reads the same way, whichever
+// category happens to be live.
+const CATEGORY_ORDER: readonly RunCategory[] = ['edit', 'explore', 'run', 'delegate', 'other']
+
+const CATEGORY_COPY: Record<RunCategory, { noun: [string, string]; past: string; present: string }> = {
+  delegate: { noun: ['task', 'tasks'], past: 'Delegated', present: 'Delegating' },
+  edit: { noun: ['file', 'files'], past: 'Edited', present: 'Editing' },
+  explore: { noun: ['file', 'files'], past: 'Explored', present: 'Exploring' },
+  other: { noun: ['tool', 'tools'], past: 'Used', present: 'Using' },
+  run: { noun: ['command', 'commands'], past: 'Ran', present: 'Running' }
+}
+
+const EXPLORE_TOOLS = new Set([
+  'list_files',
+  'read_file',
+  'search_files',
+  'session_search_recall',
+  'vision_analyze',
+  'web_extract',
+  'web_search'
+])
+
+function toolCategory(toolName: string): RunCategory {
+  if (isFileEditTool(toolName)) {
+    return 'edit'
+  }
+
+  if (toolName === 'terminal' || toolName === 'execute_code') {
+    return 'run'
+  }
+
+  if (toolName === 'delegate_task') {
+    return 'delegate'
+  }
+
+  if (EXPLORE_TOOLS.has(toolName) || toolName.startsWith('browser_')) {
+    return 'explore'
+  }
+
+  return 'other'
+}
+
+function isPending(tool: ToolCallLike): boolean {
+  return tool.result === undefined
+}
+
+/** The thing a tool acted on, as the header should name it. */
+function toolTarget(tool: ToolCallLike): string {
+  const args = parseMaybeObject(tool.args)
+
+  if (toolCategory(tool.toolName) === 'run') {
+    return summarizeShellCommand(firstStringField(args, ['command', 'code']))
+  }
+
+  const path = firstStringField(args, ['path', 'file', 'filepath'])
+
+  return path ? fileEditBasename(path) : firstStringField(args, ['query', 'url'])
+}
+
+function diffStats(tools: readonly ToolCallLike[]): { added: number; removed: number } {
+  let added = 0
+  let removed = 0
+
+  for (const tool of tools) {
+    if (!isFileEditTool(tool.toolName)) {
+      continue
+    }
+
+    const stats = countDiffLineStats(inlineDiffFromResult(tool.result))
+
+    added += stats.added
+    removed += stats.removed
+  }
+
+  return { added, removed }
+}
+
+/**
+ * One clause per category. A category holding a single thing says what it was
+ * ("Edited wiring.tsx"); anything else counts ("explored 3 files"). A settled
+ * command is the exception — "ran 5 commands" is the useful reading, and a
+ * command line only earns its space while it's the thing you're waiting on.
+ */
+function clause(category: RunCategory, tools: ToolCallLike[], live: boolean): string {
+  const copy = CATEGORY_COPY[category]
+  const verb = live ? copy.present : copy.past
+  const target = tools.length === 1 ? toolTarget(tools[0]) : ''
+
+  if (target && (live || category !== 'run')) {
+    return `${verb} ${target}`
+  }
+
+  return `${verb} ${tools.length} ${copy.noun[tools.length === 1 ? 0 : 1]}`
+}
+
+function lowerFirst(text: string): string {
+  return text.charAt(0).toLowerCase() + text.slice(1)
+}
+
+/**
+ * Collapse a run of tool calls into the single grey line that stands in for it
+ * — "Edited wiring.tsx, explored 3 files, ran 5 commands". The category holding
+ * the still-running tool speaks in the present tense so a live run reads as
+ * work in progress rather than work already done.
+ */
+export function summarizeToolRun(tools: readonly ToolCallLike[]): RunSummary {
+  const running = tools.find(isPending)
+  const liveCategory = running ? toolCategory(running.toolName) : null
+
+  const byCategory = new Map<RunCategory, ToolCallLike[]>()
+
+  for (const tool of tools) {
+    const category = toolCategory(tool.toolName)
+    const group = byCategory.get(category)
+
+    if (group) {
+      group.push(tool)
+    } else {
+      byCategory.set(category, [tool])
+    }
+  }
+
+  const clauses = CATEGORY_ORDER.flatMap(category => {
+    const group = byCategory.get(category)
+
+    return group ? [clause(category, group, category === liveCategory)] : []
+  })
+
+  return {
+    ...diffStats(tools),
+    text: clauses.map((text, index) => (index === 0 ? text : lowerFirst(text))).join(', ')
+  }
+}
diff --git a/apps/desktop/src/lib/tool-run-continuity.test.ts b/apps/desktop/src/lib/tool-run-continuity.test.ts
new file mode 100644
index 00000000000..a009053db3d
--- /dev/null
+++ b/apps/desktop/src/lib/tool-run-continuity.test.ts
@@ -0,0 +1,250 @@
+import { describe, expect, it } from 'vitest'
+
+import { isToolCallPart, type ToolCallLike } from '@/components/assistant-ui/tool/run-summary'
+import type { SessionMessage } from '@/types/hermes'
+
+import type { ChatMessage, ChatMessagePart } from './chat-messages'
+import {
+  appendAssistantTextPart,
+  appendReasoningPart,
+  assistantTextPart,
+  mergeFinalAssistantText,
+  toChatMessages,
+  upsertToolPart
+} from './chat-messages'
+import { coalesceToolOnlyAssistants, createToolMergeCache } from './chat-runtime'
+
+/**
+ * A turn described once, replayed two ways: as the gateway event stream the
+ * live view builds bubbles from, and as the persisted rows `toChatMessages`
+ * rehydrates on resume. Grouping is only stable if both produce the same runs.
+ */
+type TurnStep =
+  | { kind: 'interim'; text: string }
+  | { kind: 'final'; text: string }
+  | { kind: 'reasoning'; text: string }
+  | { kind: 'text'; text: string }
+  | { kind: 'tool'; id: string; name: string }
+
+// Mirrors use-message-stream: deltas and tool events accumulate on one pending
+// bubble; `message.interim` seals it in place and starts a fresh one; and
+// `message.complete` merges the final text onto whatever bubble is open.
+function replayLive(steps: TurnStep[]): ChatMessage[] {
+  const messages: ChatMessage[] = []
+  let streamIndex = 0
+  let open: ChatMessage | null = null
+
+  const openBubble = (): ChatMessage => {
+    if (open) {
+      return open
+    }
+
+    streamIndex += 1
+    open = { id: `assistant-stream-${streamIndex}`, role: 'assistant', parts: [], pending: true }
+    messages.push(open)
+
+    return open
+  }
+
+  const seal = (text: string, interim: boolean) => {
+    const bubble = open ?? openBubble()
+
+    bubble.parts = mergeFinalAssistantText(bubble.parts, text)
+    bubble.pending = false
+    bubble.interim = interim
+    open = null
+  }
+
+  for (const step of steps) {
+    switch (step.kind) {
+      case 'interim':
+        seal(step.text, true)
+
+        break
+
+      case 'final':
+        seal(step.text, false)
+
+        break
+      case 'reasoning': {
+        const bubble = openBubble()
+
+        bubble.parts = appendReasoningPart(bubble.parts, step.text)
+
+        break
+      }
+
+      case 'text': {
+        const bubble = openBubble()
+
+        bubble.parts = appendAssistantTextPart(bubble.parts, step.text)
+
+        break
+      }
+
+      case 'tool': {
+        const bubble = openBubble()
+
+        bubble.parts = upsertToolPart(bubble.parts, { tool_id: step.id, name: step.name }, 'complete')
+
+        break
+      }
+    }
+  }
+
+  return coalesceToolOnlyAssistants(messages, createToolMergeCache())
+}
+
+// The same turn as the gateway persists it: one row per agent iteration. A row
+// is a single API response, so its reasoning and content always precede its own
+// tool_calls — anything the agent says after a tool ran belongs to the next row.
+function replayStored(steps: TurnStep[]): ChatMessage[] {
+  const rows: SessionMessage[] = []
+  let timestamp = 0
+  let row: (SessionMessage & { tool_calls?: unknown[] }) | null = null
+
+  const openRow = (afterTools: boolean) => {
+    if (row && !(afterTools && row.tool_calls)) {
+      return row
+    }
+
+    timestamp += 1
+    row = { role: 'assistant', content: '', timestamp }
+    rows.push(row)
+
+    return row
+  }
+
+  for (const step of steps) {
+    switch (step.kind) {
+      case 'interim':
+
+      case 'final':
+      case 'text': {
+        const current = openRow(true)
+
+        current.content = `${current.content ?? ''}${step.text}`
+
+        if (step.kind !== 'text') {
+          row = null
+        }
+
+        break
+      }
+
+      case 'reasoning':
+        openRow(true).reasoning = step.text
+
+        break
+      case 'tool': {
+        const current = openRow(false)
+
+        current.tool_calls = [
+          ...(current.tool_calls ?? []),
+          { id: step.id, function: { name: step.name, arguments: '{}' } }
+        ]
+        timestamp += 1
+        rows.push({ role: 'tool', tool_call_id: step.id, tool_name: step.name, content: '{}', timestamp })
+
+        break
+      }
+    }
+  }
+
+  return coalesceToolOnlyAssistants(toChatMessages(rows), createToolMergeCache())
+}
+
+/**
+ * Maximal spans of back-to-back tool calls — the same rule assistant-ui applies
+ * when it hands `ToolGroupSlot` a range, restated here so these tests check our
+ * two part streams against each other rather than against the renderer.
+ */
+function toolRuns(parts: ChatMessagePart[]): ToolCallLike[][] {
+  const runs: ToolCallLike[][] = []
+  let previousWasTool = false
+
+  for (const part of parts) {
+    if (!isToolCallPart(part)) {
+      previousWasTool = false
+
+      continue
+    }
+
+    if (previousWasTool) {
+      runs[runs.length - 1].push(part)
+    } else {
+      runs.push([part])
+    }
+
+    previousWasTool = true
+  }
+
+  return runs
+}
+
+function runsAcross(messages: ChatMessage[]): string[][] {
+  return messages
+    .filter(message => message.role === 'assistant')
+    .flatMap(message => toolRuns(message.parts))
+    .map(run => run.map(tool => tool.toolCallId ?? ''))
+}
+
+const TURNS: Record<string, TurnStep[]> = {
+  'narration between two tool runs': [
+    { kind: 'interim', text: 'Let me check the config.' },
+    { kind: 'tool', id: 'a', name: 'read_file' },
+    { kind: 'tool', id: 'b', name: 'read_file' },
+    { kind: 'interim', text: 'Now let me edit it.' },
+    { kind: 'tool', id: 'c', name: 'write_file' },
+    { kind: 'final', text: 'Done.' }
+  ],
+  'reasoning between two tool runs': [
+    { kind: 'tool', id: 'a', name: 'terminal' },
+    { kind: 'reasoning', text: 'That failed, try the other path.' },
+    { kind: 'tool', id: 'b', name: 'terminal' },
+    { kind: 'final', text: 'Fixed.' }
+  ],
+  'unbroken run of tool calls': [
+    { kind: 'tool', id: 'a', name: 'read_file' },
+    { kind: 'tool', id: 'b', name: 'read_file' },
+    { kind: 'tool', id: 'c', name: 'search_files' },
+    { kind: 'final', text: 'Here is what I found.' }
+  ],
+  'reasoning then tools then final': [
+    { kind: 'reasoning', text: 'The user wants the lint config.' },
+    { kind: 'tool', id: 'a', name: 'search_files' },
+    { kind: 'tool', id: 'b', name: 'read_file' },
+    { kind: 'final', text: 'It lives in eslint.config.js.' }
+  ],
+  'tools with no narration at all': [
+    { kind: 'tool', id: 'a', name: 'terminal' },
+    { kind: 'final', text: 'Clean.' }
+  ]
+}
+
+describe('tool run segmentation survives rehydration', () => {
+  for (const [name, steps] of Object.entries(TURNS)) {
+    it(name, () => {
+      expect(runsAcross(replayLive(steps))).toEqual(runsAcross(replayStored(steps)))
+    })
+  }
+})
+
+describe('run identity', () => {
+  const tool = (id: string, name: string): ChatMessagePart =>
+    ({ args: {}, toolCallId: id, toolName: name, type: 'tool-call' }) as ChatMessagePart
+
+  it('keeps the first tool call at the head as the run grows', () => {
+    const [run] = toolRuns([tool('a', 'read_file'), tool('b', 'read_file')])
+    const [grown] = toolRuns([tool('a', 'read_file'), tool('b', 'read_file'), tool('c', 'terminal')])
+
+    expect(grown[0].toolCallId).toBe(run[0].toolCallId)
+    expect(grown).toHaveLength(3)
+  })
+
+  it('breaks a run on any non-tool part', () => {
+    const runs = toolRuns([tool('a', 'read_file'), assistantTextPart('Now editing.'), tool('b', 'write_file')])
+
+    expect(runs.map(run => run.map(t => t.toolCallId))).toEqual([['a'], ['b']])
+  })
+})