diff --git a/mobile/src/mobile-web/mobile-web-markdown-operations.ts b/mobile/src/mobile-web/mobile-web-markdown-operations.ts
index 464ab323d31..c44d48c4207 100644
--- a/mobile/src/mobile-web/mobile-web-markdown-operations.ts
+++ b/mobile/src/mobile-web/mobile-web-markdown-operations.ts
@@ -20,7 +20,10 @@ import type { RpcClient } from '../transport/rpc-client'
 import type { RpcFailure } from '../transport/types'
 import { MobileWebBrokerError } from './mobile-web-broker-error'
 import type { MobileWebNativeCapabilityAuthority } from './mobile-web-native-capability-authority'
-import type { MobileWebWorkspaceAuthority } from './mobile-web-workspace-authority'
+import type {
+  MobileWebHostWorkspaceId,
+  MobileWebWorkspaceAuthority
+} from './mobile-web-workspace-authority'
 
 type MarkdownOperationArgs = {
   operation: string
@@ -102,7 +105,7 @@ export async function executeMobileWebMarkdownOperation(
 async function verifyMarkdownTarget(
   args: MarkdownOperationArgs,
   target: MarkdownTarget
-): Promise<string> {
+): Promise<MobileWebHostWorkspaceId> {
   const hostWorkspaceId = args.workspaceAuthority.hostWorkspaceId(target.workspaceId)
   const response = await args.client.sendRequest('session.tabs.list', {
     worktree: `id:${hostWorkspaceId}`
diff --git a/mobile/src/mobile-web/mobile-web-native-chat-authority.ts b/mobile/src/mobile-web/mobile-web-native-chat-authority.ts
index 42c825519e8..061719c816e 100644
--- a/mobile/src/mobile-web/mobile-web-native-chat-authority.ts
+++ b/mobile/src/mobile-web/mobile-web-native-chat-authority.ts
@@ -1,8 +1,9 @@
+import type { MobileWebHostWorkspaceId } from './mobile-web-workspace-authority'
 import { MobileWebBrokerError } from './mobile-web-broker-error'
 import { MOBILE_WEB_NATIVE_CHAT_IMAGE_LIMIT } from '../../../src/shared/mobile-web/native-chat-operation-contract'
 
 export type MobileWebHostNativeChatBinding = {
-  hostWorkspaceId: string
+  hostWorkspaceId: MobileWebHostWorkspaceId
   hostTabId: string
   hostTerminalId: string | null
   agent: string
diff --git a/mobile/src/mobile-web/mobile-web-session-quick-command-operations.ts b/mobile/src/mobile-web/mobile-web-session-quick-command-operations.ts
index 647288642a4..e5591ab1be8 100644
--- a/mobile/src/mobile-web/mobile-web-session-quick-command-operations.ts
+++ b/mobile/src/mobile-web/mobile-web-session-quick-command-operations.ts
@@ -21,7 +21,10 @@ import { getRepoIdFromMobileWorktreeId } from '../session/mobile-session-route-h
 import { loadMobileNewTabAgentOptions } from '../session/mobile-new-tab-agent-loader'
 import type { RpcClient } from '../transport/rpc-client'
 import { MobileWebBrokerError } from './mobile-web-broker-error'
-import type { MobileWebWorkspaceAuthority } from './mobile-web-workspace-authority'
+import type {
+  MobileWebHostWorkspaceId,
+  MobileWebWorkspaceAuthority
+} from './mobile-web-workspace-authority'
 
 export async function executeMobileWebSessionQuickCommandOperation(args: {
   operation: string
@@ -99,7 +102,7 @@ async function readQuickCommands(client: RpcClient): Promise<TerminalQuickComman
 
 function projectQuickCommands(
   result: unknown,
-  hostWorkspaceId: string,
+  hostWorkspaceId: MobileWebHostWorkspaceId,
   pageWorkspaceId: string
 ): MobileWebQuickCommandSnapshotResult {
   const commands = parseNormalizedTerminalQuickCommands(
@@ -127,7 +130,7 @@ function projectQuickCommands(
 function hostMutation(
   mutation: MobileWebQuickCommandMutationPayload['mutation'],
   commands: TerminalQuickCommand[],
-  hostWorkspaceId: string,
+  hostWorkspaceId: MobileWebHostWorkspaceId,
   pageWorkspaceId: string
 ): TerminalQuickCommandMutation {
   const repoId = workspaceRepoId(hostWorkspaceId)
@@ -161,7 +164,7 @@ function hostMutation(
 async function launchQuickCommand(args: {
   client: RpcClient
   command: TerminalQuickCommand
-  hostWorkspaceId: string
+  hostWorkspaceId: MobileWebHostWorkspaceId
   pageWorkspaceId: string
   requestId: string
   workspaceAuthority: MobileWebWorkspaceAuthority
diff --git a/mobile/src/mobile-web/mobile-web-session-snapshot.ts b/mobile/src/mobile-web/mobile-web-session-snapshot.ts
index 7cba13a5cc6..46f066afc4f 100644
--- a/mobile/src/mobile-web/mobile-web-session-snapshot.ts
+++ b/mobile/src/mobile-web/mobile-web-session-snapshot.ts
@@ -1,3 +1,4 @@
+import type { MobileWebHostWorkspaceId } from './mobile-web-workspace-authority'
 import {
   AGENT_STATUS_ASSISTANT_MESSAGE_MAX_LENGTH,
   AGENT_STATUS_INTERACTIVE_PROMPT_MAX_LENGTH,
@@ -23,7 +24,7 @@ const TAB_TYPES = ['terminal', 'markdown', 'file', 'browser'] as const
 
 export function mobileWebSessionSnapshot(
   result: unknown,
-  hostWorkspaceId: string,
+  hostWorkspaceId: MobileWebHostWorkspaceId,
   pageWorkspaceId: string,
   browserAuthority: MobileWebBrowserAuthority,
   nativeChatAuthority: MobileWebNativeChatAuthority
@@ -101,7 +102,7 @@ function isActiveSessionTab(value: unknown): boolean {
 
 function mobileWebSessionTab(
   value: unknown,
-  hostWorkspaceId: string,
+  hostWorkspaceId: MobileWebHostWorkspaceId,
   browserAuthority: MobileWebBrowserAuthority,
   nativeChatAuthority: MobileWebNativeChatAuthority
 ): MobileWebSessionTab | null {
@@ -236,7 +237,7 @@ function safeNonnegativeInteger(value: unknown): number | undefined {
 
 function mobileWebNativeChatBinding(
   value: unknown,
-  hostWorkspaceId: string
+  hostWorkspaceId: MobileWebHostWorkspaceId
 ): MobileWebHostNativeChatBinding | null {
   if (
     !isRecord(value) ||
diff --git a/mobile/src/mobile-web/mobile-web-session-subscriptions.ts b/mobile/src/mobile-web/mobile-web-session-subscriptions.ts
index 67ece862cfc..fd3250e8c78 100644
--- a/mobile/src/mobile-web/mobile-web-session-subscriptions.ts
+++ b/mobile/src/mobile-web/mobile-web-session-subscriptions.ts
@@ -1,3 +1,4 @@
+import type { MobileWebHostWorkspaceId } from './mobile-web-workspace-authority'
 import { MOBILE_WEB_BRIDGE_MAX_SUBSCRIPTIONS } from '../../../src/shared/mobile-web/bridge-contract'
 import {
   MOBILE_WEB_SESSION_EVENT_MAX_BYTES,
@@ -37,7 +38,7 @@ export class MobileWebSessionSubscriptions {
     requestId: string
     subscriptionId: string
     pageWorkspaceId: string
-    hostWorkspaceId: string
+    hostWorkspaceId: MobileWebHostWorkspaceId
     client: RpcClient
   }): void {
     if (this.records.has(args.subscriptionId)) {
@@ -121,7 +122,7 @@ export class MobileWebSessionSubscriptions {
   private receive(
     subscriptionId: string,
     record: SubscriptionRecord,
-    hostWorkspaceId: string,
+    hostWorkspaceId: MobileWebHostWorkspaceId,
     pageWorkspaceId: string,
     event: unknown
   ): void {
diff --git a/mobile/src/mobile-web/mobile-web-workspace-authority-test-fixture.ts b/mobile/src/mobile-web/mobile-web-workspace-authority-test-fixture.ts
index ac71962aa35..d3c2a7cf1f9 100644
--- a/mobile/src/mobile-web/mobile-web-workspace-authority-test-fixture.ts
+++ b/mobile/src/mobile-web/mobile-web-workspace-authority-test-fixture.ts
@@ -1,4 +1,7 @@
-import { MobileWebWorkspaceAuthority } from './mobile-web-workspace-authority'
+import {
+  mobileWebHostWorkspaceIdFromHost,
+  MobileWebWorkspaceAuthority
+} from './mobile-web-workspace-authority'
 
 export function createMobileWebWorkspaceAuthorityFixture(
   pageWorkspaceId = 'workspace-1',
@@ -12,7 +15,7 @@ export function createMobileWebWorkspaceAuthorityFixture(
       if (candidate !== pageWorkspaceId) {
         throw new Error('not_found')
       }
-      return hostWorkspaceId
+      return mobileWebHostWorkspaceIdFromHost(hostWorkspaceId)
     }
   }
   return authority
diff --git a/mobile/src/mobile-web/mobile-web-workspace-authority.ts b/mobile/src/mobile-web/mobile-web-workspace-authority.ts
index ab41f0cc321..7beb16747ee 100644
--- a/mobile/src/mobile-web/mobile-web-workspace-authority.ts
+++ b/mobile/src/mobile-web/mobile-web-workspace-authority.ts
@@ -7,6 +7,24 @@ import {
 import { getProjectIdentityKey } from '../../../src/shared/project-host-setup-projection'
 import type { NewWorkspaceRepository } from '../worktree/host-workspace-creation-operations'
 
+declare const hostWorkspaceIdBrand: unique symbol
+declare const hostRepoIdBrand: unique symbol
+
+/** Host-side identifiers the authority mints from an opaque page handle. Branding keeps a page
+ * handle from being passed back in as if it were already resolved. */
+export type MobileWebHostWorkspaceId = string & { readonly [hostWorkspaceIdBrand]: true }
+export type MobileWebHostRepoId = string & { readonly [hostRepoIdBrand]: true }
+
+/** A value the host itself reported as a host identifier, so it never came from the page. Every
+ * use is a boundary annotation, not a conversion of a page handle. */
+export function mobileWebHostWorkspaceIdFromHost(value: string): MobileWebHostWorkspaceId {
+  return value as MobileWebHostWorkspaceId
+}
+
+export function mobileWebHostRepoIdFromHost(value: string): MobileWebHostRepoId {
+  return value as MobileWebHostRepoId
+}
+
 export type MobileWebHostWorkspaceBinding = {
   workspaceId: string
   repoId: string
@@ -63,15 +81,15 @@ export class MobileWebWorkspaceAuthority {
     return pageRepoId
   }
 
-  hostRepoId(pageRepoId: string): string {
+  hostRepoId(pageRepoId: string): MobileWebHostRepoId {
     const hostRepoId = this.hostRepoIdByPageId.get(pageRepoId)
     if (!hostRepoId) {
       throw new MobileWebBrokerError('not_found')
     }
-    return hostRepoId
+    return hostRepoId as MobileWebHostRepoId
   }
 
-  assertHostRepoBinding(pageRepoId: string, expectedHostRepoId: string): void {
+  assertHostRepoBinding(pageRepoId: string, expectedHostRepoId: MobileWebHostRepoId): void {
     if (this.hostRepoId(pageRepoId) !== expectedHostRepoId) {
       throw new MobileWebBrokerError('conflict')
     }
@@ -119,15 +137,18 @@ export class MobileWebWorkspaceAuthority {
     return connectionId
   }
 
-  hostWorkspaceId(pageWorkspaceId: string): string {
+  hostWorkspaceId(pageWorkspaceId: string): MobileWebHostWorkspaceId {
     const hostWorkspaceId = this.hostWorkspaceIdByPageId.get(pageWorkspaceId)
     if (!hostWorkspaceId) {
       throw new MobileWebBrokerError('not_found')
     }
-    return hostWorkspaceId
+    return hostWorkspaceId as MobileWebHostWorkspaceId
   }
 
-  assertHostWorkspaceBinding(pageWorkspaceId: string, expectedHostWorkspaceId: string): void {
+  assertHostWorkspaceBinding(
+    pageWorkspaceId: string,
+    expectedHostWorkspaceId: MobileWebHostWorkspaceId
+  ): void {
     if (this.hostWorkspaceId(pageWorkspaceId) !== expectedHostWorkspaceId) {
       throw new MobileWebBrokerError('conflict')
     }
diff --git a/mobile/src/mobile-web/mobile-web-workspace-creation-create-operations.ts b/mobile/src/mobile-web/mobile-web-workspace-creation-create-operations.ts
index 217047d4960..85f3d76cd85 100644
--- a/mobile/src/mobile-web/mobile-web-workspace-creation-create-operations.ts
+++ b/mobile/src/mobile-web/mobile-web-workspace-creation-create-operations.ts
@@ -13,7 +13,10 @@ import { normalizeWorkspaceAgent } from '../tasks/workspace-agent-selection'
 import { nativeHostWorkspaceCreationOperations } from '../worktree/native-host-workspace-creation-operations'
 import { MobileWebBrokerError } from './mobile-web-broker-error'
 import { requireRecentUserGesture } from './mobile-web-user-gesture-requirement'
-import type { MobileWebWorkspaceAuthority } from './mobile-web-workspace-authority'
+import {
+  mobileWebHostRepoIdFromHost,
+  type MobileWebWorkspaceAuthority
+} from './mobile-web-workspace-authority'
 
 export async function executeMobileWebWorkspaceCreationCreateOperation(args: {
   operation: string
@@ -73,7 +76,10 @@ function assertSelectionRepoBinding(
     ) {
       throw new MobileWebBrokerError('conflict')
     }
-    authority.assertHostRepoBinding(pageSelection.item.repoId, hostSelection.item.repoId)
+    authority.assertHostRepoBinding(
+      pageSelection.item.repoId,
+      mobileWebHostRepoIdFromHost(hostSelection.item.repoId)
+    )
   }
 }