diff --git a/mobile/src/session/MobileNativeChatMessage.test.ts b/mobile/src/session/MobileNativeChatMessage.test.ts
index 8a0a2582bea..e01aa2b23da 100644
--- a/mobile/src/session/MobileNativeChatMessage.test.ts
+++ b/mobile/src/session/MobileNativeChatMessage.test.ts
@@ -1,5 +1,5 @@
 import { createElement } from 'react'
-import { act, create, type ReactTestRenderer } from 'react-test-renderer'
+import { act, create, type ReactTestInstance, type ReactTestRenderer } from 'react-test-renderer'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import type { NativeChatMessage } from '../../../src/shared/native-chat-types'
 import { MAX_TOOL_RESULT_CHARS } from './mobile-native-chat-message-styles'
@@ -110,4 +110,32 @@ describe('MobileNativeChatMessage', () => {
     expect(detail).toHaveLength(MAX_TOOL_RESULT_CHARS + 1)
     expect(detail?.endsWith('…')).toBe(true)
   })
+
+  it('expands formatted detail for a pending JSON-string tool input', () => {
+    const tree = render({
+      id: 'a1',
+      role: 'assistant',
+      blocks: [
+        {
+          type: 'tool-call',
+          name: 'CustomTool',
+          input: '{"cmd":"git status","description":"Inspect changes"}'
+        }
+      ],
+      timestamp: null,
+      source: 'transcript'
+    })
+    const textIn = (node: ReactTestInstance): string[] =>
+      node.findAllByType('Text' as never).map((text) => String(text.children.join('')))
+    const pressableWith = (label: string) =>
+      tree.root.findAllByType('Pressable' as never).find((node) => textIn(node).includes(label))!
+
+    act(() => pressableWith('1×').props.onPress())
+    expect(textIn(tree.root).some((text) => text.startsWith('{\n'))).toBe(false)
+
+    act(() => pressableWith('CustomTool').props.onPress())
+    expect(textIn(tree.root)).toContain(
+      '{\n  "cmd": "git status",\n  "description": "Inspect changes"\n}'
+    )
+  })
 })
diff --git a/mobile/src/session/MobileNativeChatMessage.tsx b/mobile/src/session/MobileNativeChatMessage.tsx
index f2a47e6a56c..395f5aca01d 100644
--- a/mobile/src/session/MobileNativeChatMessage.tsx
+++ b/mobile/src/session/MobileNativeChatMessage.tsx
@@ -19,6 +19,7 @@ import { nativeChatMessageText } from './mobile-native-chat-message-text'
 import {
   describeToolInput,
   formatToolInput,
+  isStructuredToolInput,
   summarizeToolRun,
   toolFilePath
 } from './mobile-native-chat-tool-summary'
@@ -109,7 +110,7 @@ function ToolLine({
     callDiff !== null ||
     result !== undefined ||
     preview.length > 40 ||
-    (call !== undefined && call.input !== null && typeof call.input === 'object')
+    (call !== undefined && isStructuredToolInput(call.input))
   // A tool that targets a file (Read/Edit/Write…) renders its preview as a
   // tappable link that opens the file, independent of the line's expand tap.
   const filePath = call ? toolFilePath(call.input) : null
diff --git a/mobile/src/session/mobile-native-chat-tool-summary.ts b/mobile/src/session/mobile-native-chat-tool-summary.ts
index 8f30e41fd97..375b6312bcb 100644
--- a/mobile/src/session/mobile-native-chat-tool-summary.ts
+++ b/mobile/src/session/mobile-native-chat-tool-summary.ts
@@ -3,6 +3,7 @@ export {
   countToolCalls,
   describeToolInput,
   formatToolInput,
+  isStructuredToolInput,
   summarizeToolInput,
   summarizeToolRun,
   toolFilePath
diff --git a/src/shared/native-chat-tool-summary.ts b/src/shared/native-chat-tool-summary.ts
index e4f34152941..6e2fbb04bc1 100644
--- a/src/shared/native-chat-tool-summary.ts
+++ b/src/shared/native-chat-tool-summary.ts
@@ -57,6 +57,11 @@ export function formatToolInput(input: unknown): string {
   }
 }
 
+export function isStructuredToolInput(input: unknown): boolean {
+  const normalized = normalizeToolInput(input)
+  return normalized !== null && typeof normalized === 'object'
+}
+
 export function toolFilePath(input: unknown): string | null {
   const normalized = normalizeToolInput(input)
   if (!normalized || typeof normalized !== 'object') {