feat(session): offer to open the conversation a fork continues

__tests__/unit/utils/inheritedHistory.test.ts

@@ -15,7 +15,12 @@ describe('inheritedHistorySeam', () => {
         forked_at: '2026-09-06T17:31:07.482Z',
         unavailable_reason: null,
       }),
-    ).toEqual({ kind: 'divider', beforeMessageIndex: 21, forkedAt: '2026-09-06T17:31:07.482Z' })
+    ).toEqual({
+      kind: 'divider',
+      beforeMessageIndex: 21,
+      forkedAt: '2026-09-06T17:31:07.482Z',
+      sourceId: '01a075cd-f290-7d63-9bd8-b37f70c2ef5f',
+    })
   })
 
   it('reports an unreadable parent regardless of the other fields', () => {
@@ -50,3 +55,30 @@ describe('inheritedHistorySeam', () => {
     ).toEqual({ kind: 'divider', beforeMessageIndex: 7, forkedAt: undefined })
   })
 })
+
+describe('inheritedHistorySeam — source id', () => {
+  const base: RawInheritedHistory = { through_message_index: 4, source_id: 'rollout-parent' }
+
+  it('carries source_id so the UI can offer to open the parent', () => {
+    const seam = inheritedHistorySeam(base)
+    expect(seam).toEqual(expect.objectContaining({ kind: 'divider', sourceId: 'rollout-parent' }))
+  })
+
+  it('omits it when the server does not send one', () => {
+    const seam = inheritedHistorySeam({ through_message_index: 4 })
+    expect(seam).toEqual(expect.objectContaining({ kind: 'divider' }))
+    expect((seam as { sourceId?: string }).sourceId).toBeUndefined()
+  })
+
+  it('rejects a non-string or empty source_id rather than passing it on', () => {
+    for (const bad of [42, '', null, {}]) {
+      const seam = inheritedHistorySeam({ ...base, source_id: bad as unknown as string })
+      expect((seam as { sourceId?: string }).sourceId).toBeUndefined()
+    }
+  })
+
+  it('has no source id on the unavailable seam, so no dead link is offered', () => {
+    const seam = inheritedHistorySeam({ ...base, unavailable_reason: 'source_missing' })
+    expect(seam).toEqual({ kind: 'unavailable' })
+  })
+})

app/session/[id].tsx

@@ -16,7 +16,7 @@ import { SafeAreaView } from 'react-native-safe-area-context'
 import { useLocalSearchParams, useRouter, useNavigation } from 'expo-router'
 import type { Href } from 'expo-router'
 import * as Clipboard from 'expo-clipboard'
-import { CopySimple, InfoIcon, PencilSimple, Sparkle, Star, StopCircle, GitDiff, Warning } from 'phosphor-react-native'
+import { ArrowUUpLeft, CopySimple, InfoIcon, PencilSimple, Sparkle, Star, StopCircle, GitDiff, Warning } from 'phosphor-react-native'
 import { SessionStatusBadge } from '@/components/sessions/SessionStatusBadge'
 import { deriveSessionPresentation, sessionOpensAsHistory } from '@/lib/sessionPresentation'
 import { useSessionDetail } from '@/hooks/useSession'
@@ -586,6 +586,16 @@ export default function SessionDetailScreen() {
     () => buildReviewFromMessages(reviewMessages).files.length > 0,
     [reviewMessages],
   )
+  // A `codex fork` session continues an earlier conversation, and the server
+  // names that parent in meta.inherited_history. Read off the query above rather
+  // than adding one: the field rides on the same conversation's meta. A non-fork
+  // or an older server simply omits it, which reads as "no parent to open", and
+  // the `unavailable` seam (source file gone) carries no id — so the entry stays
+  // disabled rather than offering a dead link.
+  const forkParentId =
+    reviewConversation?.inheritedHistory?.kind === 'divider'
+      ? reviewConversation.inheritedHistory.sourceId
+      : undefined
 
   // Leave-session policy lives in useSessionLeaveGuard.
   // Do not add a second guard here.
@@ -918,6 +928,14 @@ export default function SessionDetailScreen() {
             onPress: () => setInfoVisible(true),
             testID: 'session-info-button',
           },
+          {
+            key: 'fork-parent',
+            label: t('conversation:inheritedHistory.openSource'),
+            icon: ArrowUUpLeft,
+            onPress: () => router.push(`/conversation/${forkParentId}?server=${serverId}`),
+            disabled: !forkParentId,
+            testID: 'session-fork-parent-button',
+          },
           {
             key: 'diffs',
             label: t('conversation:review.open'),

locales/ar/conversation.json

@@ -25,7 +25,8 @@
   "inheritedHistory": {
     "forked": "تفرَّع إلى Threadbase",
     "forkedAt": "تفرَّع إلى Threadbase · {{time}}",
-    "unavailable": "لم يعد السجل الأقدم السابق للتفرُّع متاحًا."
+    "unavailable": "لم يعد السجل الأقدم السابق للتفرُّع متاحًا.",
+    "openSource": "الانتقال إلى الجلسة الأصلية"
   },
   "resume": {
     "start": "استئناف السيشن",

locales/en/conversation.json

@@ -25,7 +25,8 @@
   "inheritedHistory": {
     "forked": "Forked into Threadbase",
     "forkedAt": "Forked into Threadbase · {{time}}",
-    "unavailable": "Earlier history from before this fork isn't available."
+    "unavailable": "Earlier history from before this fork isn't available.",
+    "openSource": "Go to original session"
   },
   "resume": {
     "start": "Resume Session",

locales/he/conversation.json

@@ -25,7 +25,8 @@
   "inheritedHistory": {
     "forked": "פוצל אל Threadbase",
     "forkedAt": "פוצל אל Threadbase · {{time}}",
-    "unavailable": "ההיסטוריה שקדמה לפיצול אינה זמינה עוד."
+    "unavailable": "ההיסטוריה שקדמה לפיצול אינה זמינה עוד.",
+    "openSource": "מעבר לשיחת המקור"
   },
   "resume": {
     "start": "המשך סשן",

locales/ru/conversation.json

@@ -25,7 +25,8 @@
   "inheritedHistory": {
     "forked": "Форк в Threadbase",
     "forkedAt": "Форк в Threadbase · {{time}}",
-    "unavailable": "Более ранняя история до форка больше недоступна."
+    "unavailable": "Более ранняя история до форка больше недоступна.",
+    "openSource": "Перейти к исходной сессии"
   },
   "resume": {
     "start": "Возобновить сессию",

utils/inheritedHistory.ts

@@ -13,7 +13,7 @@ export interface RawInheritedHistory {
 
 /** What the message list should draw for the inherited/own boundary, if anything. */
 export type InheritedHistorySeam =
-  | { kind: 'divider'; beforeMessageIndex: number; forkedAt?: string }
+  | { kind: 'divider'; beforeMessageIndex: number; forkedAt?: string; sourceId?: string }
   | { kind: 'unavailable' }
 
 export function inheritedHistorySeam(
@@ -27,5 +27,8 @@ export function inheritedHistorySeam(
   if (typeof index !== 'number' || !Number.isInteger(index) || index <= 0) return undefined
 
   const forkedAt = typeof raw.forked_at === 'string' && raw.forked_at ? raw.forked_at : undefined
-  return { kind: 'divider', beforeMessageIndex: index, forkedAt }
+  // Carried so the UI can offer to open the parent. Absent on an older server,
+  // which reads as "no parent to navigate to" rather than an error.
+  const sourceId = typeof raw.source_id === 'string' && raw.source_id ? raw.source_id : undefined
+  return { kind: 'divider', beforeMessageIndex: index, forkedAt, sourceId }
 }