diff --git a/src/main/runtime/rpc/methods/host-capabilities.test.ts b/src/main/runtime/rpc/methods/host-capabilities.test.ts
index 64ff48e69a2..8e2f15cd25c 100644
--- a/src/main/runtime/rpc/methods/host-capabilities.test.ts
+++ b/src/main/runtime/rpc/methods/host-capabilities.test.ts
@@ -3,15 +3,31 @@ import { RpcDispatcher } from '../dispatcher'
 import type { RpcRequest } from '../core'
 import type { OrcaRuntimeService } from '../../orca-runtime'
 
-const { isPwshAvailable, isWslAvailable, listWslDistros, isGitBashAvailable } = vi.hoisted(() => ({
+const {
+  isPwshAvailable,
+  isPwshAvailableAsync,
+  isWslAvailable,
+  isWslAvailableAsync,
+  listWslDistros,
+  listWslDistrosAsync,
+  isGitBashAvailable
+} = vi.hoisted(() => ({
   isPwshAvailable: vi.fn(),
+  isPwshAvailableAsync: vi.fn(),
   isWslAvailable: vi.fn(),
+  isWslAvailableAsync: vi.fn(),
   listWslDistros: vi.fn(),
+  listWslDistrosAsync: vi.fn(),
   isGitBashAvailable: vi.fn()
 }))
 
-vi.mock('../../../pwsh', () => ({ isPwshAvailable }))
-vi.mock('../../../wsl', () => ({ isWslAvailable, listWslDistros }))
+vi.mock('../../../pwsh', () => ({ isPwshAvailable, isPwshAvailableAsync }))
+vi.mock('../../../wsl', () => ({
+  isWslAvailable,
+  isWslAvailableAsync,
+  listWslDistros,
+  listWslDistrosAsync
+}))
 vi.mock('../../../git-bash', () => ({ isGitBashAvailable }))
 
 import { HOST_CAPABILITY_METHODS } from './host-capabilities'
@@ -23,15 +39,18 @@ function makeRequest(method: string, params?: unknown): RpcRequest {
 describe('host capability RPC methods', () => {
   beforeEach(() => {
     isPwshAvailable.mockReset()
+    isPwshAvailableAsync.mockReset()
     isWslAvailable.mockReset()
+    isWslAvailableAsync.mockReset()
     listWslDistros.mockReset()
+    listWslDistrosAsync.mockReset()
     isGitBashAvailable.mockReset()
   })
 
   it('reports Windows shell capability probes through explicit methods', async () => {
-    isPwshAvailable.mockReturnValue(true)
-    isWslAvailable.mockReturnValue(true)
-    listWslDistros.mockReturnValue(['Ubuntu'])
+    isPwshAvailableAsync.mockResolvedValue(true)
+    isWslAvailableAsync.mockResolvedValue(true)
+    listWslDistrosAsync.mockResolvedValue(['Ubuntu'])
     isGitBashAvailable.mockReturnValue(true)
     const runtime = { getRuntimeId: () => 'test-runtime' } as unknown as OrcaRuntimeService
     const dispatcher = new RpcDispatcher({ runtime, methods: HOST_CAPABILITY_METHODS })
@@ -55,4 +74,25 @@ describe('host capability RPC methods', () => {
       result: true
     })
   })
+
+  // Why: web/mobile clients reach these over the relay, so a sync probe would stall the
+  // desktop main event loop on execFileSync wsl.exe/pwsh.exe for up to 5s per call.
+  it('answers the Windows shell capability methods without a blocking spawn', async () => {
+    isPwshAvailableAsync.mockResolvedValue(false)
+    isWslAvailableAsync.mockResolvedValue(false)
+    listWslDistrosAsync.mockResolvedValue([])
+    const runtime = { getRuntimeId: () => 'test-runtime' } as unknown as OrcaRuntimeService
+    const dispatcher = new RpcDispatcher({ runtime, methods: HOST_CAPABILITY_METHODS })
+
+    await dispatcher.dispatch(makeRequest('host.wsl.isAvailable'))
+    await dispatcher.dispatch(makeRequest('host.wsl.listDistros'))
+    await dispatcher.dispatch(makeRequest('host.pwsh.isAvailable'))
+
+    expect(isWslAvailableAsync).toHaveBeenCalledTimes(1)
+    expect(listWslDistrosAsync).toHaveBeenCalledTimes(1)
+    expect(isPwshAvailableAsync).toHaveBeenCalledTimes(1)
+    expect(isWslAvailable).not.toHaveBeenCalled()
+    expect(listWslDistros).not.toHaveBeenCalled()
+    expect(isPwshAvailable).not.toHaveBeenCalled()
+  })
 })
diff --git a/src/main/runtime/rpc/methods/host-capabilities.ts b/src/main/runtime/rpc/methods/host-capabilities.ts
index be5e13dc2f0..afa1af88474 100644
--- a/src/main/runtime/rpc/methods/host-capabilities.ts
+++ b/src/main/runtime/rpc/methods/host-capabilities.ts
@@ -1,6 +1,6 @@
 import { defineMethod, type RpcMethod } from '../core'
-import { isPwshAvailable } from '../../../pwsh'
-import { isWslAvailable, listWslDistros } from '../../../wsl'
+import { isPwshAvailableAsync } from '../../../pwsh'
+import { isWslAvailableAsync, listWslDistrosAsync } from '../../../wsl'
 import { isGitBashAvailable } from '../../../git-bash'
 
 export const HOST_CAPABILITY_METHODS: RpcMethod[] = [
@@ -9,20 +9,22 @@ export const HOST_CAPABILITY_METHODS: RpcMethod[] = [
     params: null,
     handler: async () => ({ platform: process.platform })
   }),
+  // Why: paired web/mobile clients route capability reads here, so a sync probe would
+  // execFileSync wsl.exe/pwsh.exe on this host's main event loop for up to 5s per call.
   defineMethod({
     name: 'host.wsl.isAvailable',
     params: null,
-    handler: async () => isWslAvailable()
+    handler: async () => isWslAvailableAsync()
   }),
   defineMethod({
     name: 'host.wsl.listDistros',
     params: null,
-    handler: async () => listWslDistros()
+    handler: async () => listWslDistrosAsync()
   }),
   defineMethod({
     name: 'host.pwsh.isAvailable',
     params: null,
-    handler: async () => isPwshAvailable()
+    handler: async () => isPwshAvailableAsync()
   }),
   defineMethod({
     name: 'host.gitBash.isAvailable',
diff --git a/src/relay/preflight-handler.test.ts b/src/relay/preflight-handler.test.ts
index 80bc2ba490a..06886180624 100644
--- a/src/relay/preflight-handler.test.ts
+++ b/src/relay/preflight-handler.test.ts
@@ -5,13 +5,17 @@ const { execFileAsyncMock } = vi.hoisted(() => ({
   execFileAsyncMock: vi.fn()
 }))
 
-const { isPwshAvailableMock, isWslAvailableMock, listWslDistrosMock, isGitBashAvailableMock } =
-  vi.hoisted(() => ({
-    isPwshAvailableMock: vi.fn(),
-    isWslAvailableMock: vi.fn(),
-    listWslDistrosMock: vi.fn(),
-    isGitBashAvailableMock: vi.fn()
-  }))
+const {
+  isPwshAvailableAsyncMock,
+  isWslAvailableAsyncMock,
+  listWslDistrosAsyncMock,
+  isGitBashAvailableMock
+} = vi.hoisted(() => ({
+  isPwshAvailableAsyncMock: vi.fn(),
+  isWslAvailableAsyncMock: vi.fn(),
+  listWslDistrosAsyncMock: vi.fn(),
+  isGitBashAvailableMock: vi.fn()
+}))
 
 vi.mock('child_process', () => {
   const execFileWithPromisify = Object.assign(vi.fn(), {
@@ -20,10 +24,10 @@ vi.mock('child_process', () => {
   return { execFile: execFileWithPromisify }
 })
 
-vi.mock('../main/pwsh', () => ({ isPwshAvailable: isPwshAvailableMock }))
+vi.mock('../main/pwsh', () => ({ isPwshAvailableAsync: isPwshAvailableAsyncMock }))
 vi.mock('../main/wsl', () => ({
-  isWslAvailable: isWslAvailableMock,
-  listWslDistros: listWslDistrosMock
+  isWslAvailableAsync: isWslAvailableAsyncMock,
+  listWslDistrosAsync: listWslDistrosAsyncMock
 }))
 vi.mock('../main/git-bash', () => ({ isGitBashAvailable: isGitBashAvailableMock }))
 
@@ -61,9 +65,9 @@ function fishLookupArgs(command: string): string[] {
 
 beforeEach(() => {
   execFileAsyncMock.mockReset()
-  isPwshAvailableMock.mockReset()
-  isWslAvailableMock.mockReset()
-  listWslDistrosMock.mockReset()
+  isPwshAvailableAsyncMock.mockReset()
+  isWslAvailableAsyncMock.mockReset()
+  listWslDistrosAsyncMock.mockReset()
   isGitBashAvailableMock.mockReset()
 })
 
@@ -319,9 +323,9 @@ describe('PreflightHandler', () => {
       configurable: true,
       value: 'win32'
     })
-    isWslAvailableMock.mockReturnValue(true)
-    listWslDistrosMock.mockReturnValue(['Ubuntu'])
-    isPwshAvailableMock.mockReturnValue(true)
+    isWslAvailableAsyncMock.mockResolvedValue(true)
+    listWslDistrosAsyncMock.mockResolvedValue(['Ubuntu'])
+    isPwshAvailableAsyncMock.mockResolvedValue(true)
     isGitBashAvailableMock.mockReturnValue(true)
 
     const requestHandlers = new Map<string, (params: Record<string, unknown>) => Promise<unknown>>()
diff --git a/src/relay/preflight-handler.ts b/src/relay/preflight-handler.ts
index 101c796137c..a703b26847e 100644
--- a/src/relay/preflight-handler.ts
+++ b/src/relay/preflight-handler.ts
@@ -4,8 +4,8 @@ import { promisify } from 'node:util'
 import path, { win32 } from 'node:path'
 import type { RelayDispatcher } from './dispatcher'
 import { buildRelayCommandEnv } from './relay-command-env'
-import { isPwshAvailable } from '../main/pwsh'
-import { isWslAvailable, listWslDistros } from '../main/wsl'
+import { isPwshAvailableAsync } from '../main/pwsh'
+import { isWslAvailableAsync, listWslDistrosAsync } from '../main/wsl'
 import { isGitBashAvailable } from '../main/git-bash'
 import { buildPosixCommandPathLookupScript } from '../shared/posix-command-path-lookup'
 
@@ -101,11 +101,11 @@ export class PreflightHandler {
     hostPlatform: NodeJS.Platform | null
   }> {
     const [wslAvailable, pwshAvailable, gitBashAvailable] = await Promise.all([
-      Promise.resolve(isWslAvailable()).catch(() => false),
-      Promise.resolve(isPwshAvailable()).catch(() => false),
+      isWslAvailableAsync().catch(() => false),
+      isPwshAvailableAsync().catch(() => false),
       Promise.resolve(isGitBashAvailable()).catch(() => false)
     ])
-    const wslDistros = wslAvailable ? await Promise.resolve(listWslDistros()).catch(() => []) : []
+    const wslDistros = wslAvailable ? await listWslDistrosAsync().catch(() => []) : []
     return {
       wslAvailable,
       wslDistros,