diff --git a/packages/client/ui-slots/src/deferred.ts b/packages/client/ui-slots/src/deferred.ts
index 9b68c4d206..a5233812e6 100644
--- a/packages/client/ui-slots/src/deferred.ts
+++ b/packages/client/ui-slots/src/deferred.ts
@@ -36,6 +36,12 @@ export interface DeferredRegistration {
  * @param name - target slot name.
  * @param component - the component whose ledger presence marks "registered".
  * @param register - performs the actual registration; returns its disposer.
+ * @param onFailure - owns a registration failure that fires from a LATER
+ * ledger flush (a declaration landing after two providers deferred, say):
+ * the deferral first removes its own subscription, then hands the error
+ * over instead of throwing through the flush — the callback's chance to
+ * roll back sibling deferrals and surface the conflict on a loud channel.
+ * Absent, a late failure rethrows out of the flush.
  * @returns the deferral handle (dispose in the owning effect's disposer).
  * @throws the immediate registration's failure, after removing the
  * just-installed subscription — a throwing construction leaves nothing live.
@@ -45,6 +51,7 @@ export function deferRegistration(
   name: string,
   component: unknown,
   register: () => () => void,
+  onFailure?: (error: unknown) => void,
 ): DeferredRegistration {
   let dispose: (() => void) | undefined
   const tryRegister = (): void => {
@@ -52,7 +59,15 @@ export function deferRegistration(
     if (registry.entries(name).some(e => e.component === component)) return
     dispose = register()
   }
-  const unsubscribe = registry.subscribe(name, () => { tryRegister() })
+  const unsubscribe = registry.subscribe(name, () => {
+    try {
+      tryRegister()
+    } catch (error) {
+      unsubscribe()
+      if (onFailure === undefined) throw error
+      onFailure(error)
+    }
+  })
   try {
     tryRegister()
   } catch (error) {
diff --git a/packages/client/ui-slots/tests/deferred.spec.ts b/packages/client/ui-slots/tests/deferred.spec.ts
index 2153e52181..ba4d9b91ee 100644
--- a/packages/client/ui-slots/tests/deferred.spec.ts
+++ b/packages/client/ui-slots/tests/deferred.spec.ts
@@ -24,6 +24,27 @@ describe('deferRegistration', () => {
     expect(core.entries(HOLE)).toHaveLength(0)
   })
 
+  it('hands a late registration failure to onFailure after unsubscribing itself', async () => {
+    const core = new SlotCore()
+    const component = (): null => null
+    const foreign = (): null => null
+    const failures: unknown[] = []
+    // Nothing is declared yet: the deferral just subscribes and waits.
+    const register = vi.fn(() => core.register({ name: HOLE } as never, component as never))
+    deferRegistration(core, HOLE, component, register, (error) => { failures.push(error) })
+    // The declaration lands with a foreign occupant racing in first: the
+    // deferral's flush-time attempt fails, unsubscribes itself, and reports
+    // through onFailure instead of throwing out of the flush.
+    core.register({ name: 'root', children: { [HOLE]: { kind: 'single', scope: 'root' } } } as never, (() => null) as never)
+    const disposeForeign = core.register({ name: HOLE } as never, foreign as never)
+    await Promise.resolve()
+    expect(failures.map(String).join('')).toContain('already has a registration')
+    // Unsubscribed: freeing the hole must not resurrect the loser.
+    disposeForeign()
+    await Promise.resolve()
+    expect(core.entries(HOLE)).toHaveLength(0)
+  })
+
   it('drops its subscription when the immediate registration throws', async () => {
     const core = declared()
     const foreign = (): null => null
diff --git a/packages/host/directory-picker-native/src/client/index.ts b/packages/host/directory-picker-native/src/client/index.ts
index f58bcbc056..aa2936e1da 100644
--- a/packages/host/directory-picker-native/src/client/index.ts
+++ b/packages/host/directory-picker-native/src/client/index.ts
@@ -29,13 +29,21 @@ export function apply(ctx: ClientContext): void {
   ctx.effect(() => {
     // Constructing the pair can throw halfway (a declared hole already
     // occupied registers synchronously): roll the earlier deferral back so
-    // no live subscription outlives the failed fiber.
+    // no live subscription outlives the failed fiber. A conflict surfacing
+    // from a LATER ledger flush (holes declared after two flow providers
+    // activated) rolls the whole pair back the same way and re-raises on
+    // the global channel the boot's fail-loud handler owns — never a throw
+    // through the slot flush, never partial occupancy from this package.
     const deferred: ReturnType<typeof deferRegistration>[] = []
+    const lateConflict = (error: unknown): void => {
+      for (const entry of deferred) entry.dispose()
+      queueMicrotask(() => { throw error instanceof Error ? error : new Error(String(error)) })
+    }
     try {
       deferred.push(deferRegistration(ctx.slots, 'conversation.hero.workspace.directoryFlow', NativeDirectoryFlow, () =>
-        ctx.slots.register({ name: 'conversation.hero.workspace.directoryFlow', inject: injected }, NativeDirectoryFlow)))
+        ctx.slots.register({ name: 'conversation.hero.workspace.directoryFlow', inject: injected }, NativeDirectoryFlow), lateConflict))
       deferred.push(deferRegistration(ctx.slots, 'sidebar.workspaces.directoryFlow', NativeDirectoryFlow, () =>
-        ctx.slots.register({ name: 'sidebar.workspaces.directoryFlow', inject: injected }, NativeDirectoryFlow)))
+        ctx.slots.register({ name: 'sidebar.workspaces.directoryFlow', inject: injected }, NativeDirectoryFlow), lateConflict))
     } catch (error) {
       for (const entry of deferred) entry.dispose()
       throw error
diff --git a/packages/host/directory-picker-native/tests/client-flow.spec.tsx b/packages/host/directory-picker-native/tests/client-flow.spec.tsx
index a91732fab4..ecddf84eb4 100644
--- a/packages/host/directory-picker-native/tests/client-flow.spec.tsx
+++ b/packages/host/directory-picker-native/tests/client-flow.spec.tsx
@@ -56,6 +56,47 @@ describe('directory-picker-native client half', () => {
     for (const hole of HOLES) expect(after.slots.entries(hole)).toHaveLength(1)
   })
 
+  it('rolls back wholesale and reports loudly when a rival provider wins after deferred activation', async () => {
+    const b = await bench()
+    const rejections: unknown[] = []
+    const onUnhandled = (reason: unknown): void => { rejections.push(reason) }
+    // queueMicrotask throws surface as uncaughtException, not a rejection.
+    process.on('unhandledRejection', onUnhandled)
+    process.on('uncaughtException', onUnhandled)
+    try {
+      // This provider activates BEFORE any hole exists: both deferrals wait.
+      // (Duplicate rows of the SAME package converge silently — the deferral
+      // skips a hole its own component already occupies; the conflict needs
+      // a rival provider.)
+      await b.ctx.plugin({ inject: [...inject], apply }).await()
+      b.declare()
+      // A rival occupies both holes ahead of the pending microtask flush.
+      b.slots.register({ name: HOLES[0] } as never, () => null)
+      b.slots.register({ name: HOLES[1] } as never, () => null)
+      await new Promise(resolve => setTimeout(resolve, 20))
+      // The rival keeps both holes; this provider rolled back wholesale and
+      // surfaced the conflict on the fail-loud channel — no partial mix.
+      for (const hole of HOLES) expect(b.slots.entries(hole)).toHaveLength(1)
+      expect(rejections.map(String).join('\n')).toContain('already has a registration')
+
+      // Non-Error conflicts wrap before the loud rethrow (same channel).
+      const c = await bench()
+      await c.ctx.plugin({ inject: [...inject], apply }).await()
+      const original = c.slots.register.bind(c.slots)
+      const slotsAny = c.slots as { register: typeof original }
+      slotsAny.register = ((options: never, component: never) => {
+        if ((options as { name?: string }).name === HOLES[0]) throw 'string conflict'
+        return original(options, component)
+      }) as typeof original
+      c.declare()
+      await new Promise(resolve => setTimeout(resolve, 20))
+      expect(rejections.map(String).join('\n')).toContain('string conflict')
+    } finally {
+      process.off('unhandledRejection', onUnhandled)
+      process.off('uncaughtException', onUnhandled)
+    }
+  })
+
   it('rolls back the first deferral when the second hole is already occupied', async () => {
     const b = await bench()
     b.declare()