fix(windows): refuse structured chat in a paired web client

Reverts the two review-loop commits (restoring a tree byte-identical to the
validated head) and closes the hole they were aiming at, without their cost.

A paired web client's `platform` describes the browser's machine, not the host
that will run the agent, so the Windows gate cannot be evaluated there. Before
this, a browser on macOS driving a Windows runtime read "not win32", skipped the
creation-time proof entirely and allowed structured chat — fail-OPEN, the
dangerous direction, bypassing the guarantee this lane is built on.

`isWebClient` is a required input like the other gate fields, so the compiler
enumerated all seven call sites. Refusal is synchronous and fail-closed: no
async round-trip, no null window, no cache to invalidate — unlike keying on an
asynchronously-fetched host platform, which would have made every desktop
launch wait on a round-trip to fix a paired-web-only hole.

Paired web therefore gets the legacy chat until the host publishes eligibility
itself; that is the proper fix and belongs in its own PR.

Ablation-proven: removing the guard reddens both refusal tests; the
desktop-unaffected test is a preservation check and passes either way.
Gates: tc 0, oxlint 0.

Known open: repos-onboarding-folder-startup.test.ts fails on this branch and
passes on plain main — under investigation, NOT caused by this commit.

src/renderer/src/components/sidebar/folder-workspace-composer-submit.ts

@@ -4,6 +4,7 @@ import {
   type LinkedWorkItemSummary
 } from '@/lib/new-workspace'
 import { seedNativeChatLaunchDraftForAgentTab } from '@/lib/agent-launch-prompt-delivery'
+import { isWebClientLocation } from '@/lib/web-client-location'
 import {
   pathUsesWslUnc,
   readWindowsProcessStartTimeGate
@@ -157,6 +158,7 @@ export async function submitFolderWorkspaceCreate({
         platform: CLIENT_PLATFORM,
         hostCapabilities: readLocalRuntimeCapabilities(),
         windowsProcessStartTime: readWindowsProcessStartTimeGate(),
+        isWebClient: isWebClientLocation(),
         // The workspace has no store entry yet, but it will be created under
         // the group's parent, so the parent decides WSL-ness before the click.
         worktreeUsesWslPath: pathUsesWslUnc(projectGroup.parentPath),

src/renderer/src/hooks/composer-state/full-creation-execution.ts

@@ -32,6 +32,7 @@ import { useCallback } from 'react'
 import type { PendingSmartGitHubSubmitResolution } from './source-selection-decisions'
 import { translate } from '@/i18n/i18n'
 import { settleComposerSubmit } from '@/lib/composer-submit-cancellation'
+import { isWebClientLocation } from '@/lib/web-client-location'
 import { readWindowsProcessStartTimeGate } from '@/lib/agent-launch-routing-windows-gate'
 import { toFolderWorkspaceLinkedTask } from '@/components/sidebar/folder-workspace-composer-helpers'
 import { CLIENT_PLATFORM, ensureAgentStartupInTerminal } from '@/lib/new-workspace'
@@ -144,6 +145,7 @@ export function useFullCreationExecution(input: FullCreationExecutionInput) {
         platform: CLIENT_PLATFORM,
         hostCapabilities: readLocalRuntimeCapabilities(),
         windowsProcessStartTime: readWindowsProcessStartTimeGate(),
+        isWebClient: isWebClientLocation(),
         // The workspace has no store entry until after this decision, so the
         // WSL-UNC check cannot run yet; it applies on the next launch.
         worktreeUsesWslPath: false,

src/renderer/src/hooks/composer-state/quick-creation-execution.ts

@@ -36,6 +36,7 @@ import { useCallback } from 'react'
 import type { Repo } from '../../../../shared/repo-types'
 import type { TuiAgent } from '../../../../shared/tui-agent'
 import type { WorktreeCreationRequest } from '@/lib/pending-worktree-creation'
+import { isWebClientLocation } from '@/lib/web-client-location'
 import { readWindowsProcessStartTimeGate } from '@/lib/agent-launch-routing-windows-gate'
 import { useAppStore } from '@/store'
 import { settleComposerSubmit } from '@/lib/composer-submit-cancellation'
@@ -210,6 +211,7 @@ export function useQuickCreationExecution(input: QuickCreationExecutionInput) {
             platform: CLIENT_PLATFORM,
             hostCapabilities: readLocalRuntimeCapabilities(),
             windowsProcessStartTime: readWindowsProcessStartTimeGate(),
+            isWebClient: isWebClientLocation(),
             // The workspace has no store entry until after this decision, so the
             // WSL-UNC check cannot run yet; it applies on the next launch.
             worktreeUsesWslPath: false,

src/renderer/src/lib/agent-launch-routing-windows-gate.ts

@@ -1,6 +1,7 @@
 import type { AppState } from '@/store/types'
 import { worktreeUsesWslPath } from '@/store/terminals/terminal-workspace-routing'
 import { isWslUncPath } from '../../../shared/wsl-paths'
+import { isWebClientLocation } from '@/lib/web-client-location'
 import {
   getCachedWindowsTerminalCapabilities,
   hasCachedWindowsTerminalCapabilities
@@ -47,9 +48,14 @@ export function pathUsesWslUnc(path: string | null | undefined): boolean {
 export function readWindowsStructuredGateInputs(
   state: Partial<Pick<AppState, 'folderWorkspaces' | 'worktreesByRepo'>>,
   worktreeId: string
-): { windowsProcessStartTime: WindowsProcessStartTimeGate; worktreeUsesWslPath: boolean } {
+): {
+  windowsProcessStartTime: WindowsProcessStartTimeGate
+  worktreeUsesWslPath: boolean
+  isWebClient: boolean
+} {
   return {
     windowsProcessStartTime: readWindowsProcessStartTimeGate(),
-    worktreeUsesWslPath: readWorktreeUsesWslPath(state, worktreeId)
+    worktreeUsesWslPath: readWorktreeUsesWslPath(state, worktreeId),
+    isWebClient: isWebClientLocation()
   }
 }

src/renderer/src/lib/agent-launch-routing.test.ts

@@ -21,6 +21,7 @@ function route(overrides: Partial<Parameters<typeof resolveAgentLaunchRoute>[0]>
     platform: 'darwin',
     hostCapabilities: [STRUCTURED_AGENT_SESSION_RUNTIME_CAPABILITY],
     windowsProcessStartTime: 'unavailable' as const,
+    isWebClient: false,
     worktreeUsesWslPath: false,
     workspaceKind: 'git-worktree',
     nativeChatTranscriptIsLocalReadable: true,
@@ -176,3 +177,27 @@ describe('resolveAgentLaunchRoute Windows structured gate', () => {
     ).toBe('legacy-native-chat')
   })
 })
+
+describe('resolveAgentLaunchRoute paired web client', () => {
+  // A web client's `platform` is the browser's machine, not the host that runs
+  // the agent, so the Windows gate cannot be evaluated. Refusing is the only
+  // safe answer until the host publishes eligibility itself.
+  it('refuses structured chat in a paired web client', () => {
+    expect(route({ isWebClient: true })).toBe('legacy-native-chat')
+  })
+
+  it('still refuses when the web client looks fully eligible on Windows', () => {
+    expect(
+      route({
+        isWebClient: true,
+        platform: 'win32',
+        windowsProcessStartTime: 'available',
+        worktreeUsesWslPath: false
+      })
+    ).toBe('legacy-native-chat')
+  })
+
+  it('leaves the desktop renderer unaffected', () => {
+    expect(route({ isWebClient: false })).toBe('structured-native-chat')
+  })
+})

src/renderer/src/lib/agent-launch-routing.ts

@@ -36,6 +36,11 @@ export type AgentLaunchRoutingInput = {
    *  the distro's runtime and must keep the legacy terminal. Required for the
    *  same reason. */
   worktreeUsesWslPath: boolean
+  /** True when this renderer is a paired web client. Its `platform` then
+   *  describes the browser's machine, not the host that will run the agent, so
+   *  the Windows gate below cannot be evaluated and must refuse rather than
+   *  guess. Lifting this needs host-published eligibility, not a client probe. */
+  isWebClient: boolean
   workspaceKind?: 'git-worktree' | 'folder' | 'floating'
   projectRuntime?: ProjectExecutionRuntimeResolution | null
   promptDelivery?: NativeChatLaunchPromptDelivery
@@ -106,6 +111,7 @@ export function resolveAgentLaunchRoute(input: AgentLaunchRoutingInput): AgentLa
     input.platform !== 'win32' ||
     (input.windowsProcessStartTime === 'available' && !input.worktreeUsesWslPath)
   const structuredSupported =
+    !input.isWebClient &&
     input.agent === 'codex' &&
     input.promptDelivery !== 'draft' &&
     input.workspaceKind !== 'floating' &&

src/renderer/src/lib/onboarding-folder-agent-startup.ts

@@ -1,4 +1,5 @@
 import { buildAgentStartupPlan } from '@/lib/tui-agent-startup'
+import { isWebClientLocation } from '@/lib/web-client-location'
 import { readWindowsProcessStartTimeGate } from '@/lib/agent-launch-routing-windows-gate'
 import { tuiAgentToAgentKind } from '@/lib/telemetry'
 import { isTuiAgentEnabled } from '../../../shared/tui-agent-selection'
@@ -139,6 +140,7 @@ export function resolveDismissedOnboardingFolderAgentLaunch(args: {
     platform: getClientPlatform(),
     hostCapabilities: readLocalRuntimeCapabilities(),
     windowsProcessStartTime: readWindowsProcessStartTimeGate(),
+    isWebClient: isWebClientLocation(),
     // The workspace has no store entry until after this decision, so the
     // WSL-UNC check cannot run yet; it applies on the next launch.
     worktreeUsesWslPath: false,