diff --git a/packages/start-client-core/src/client-rpc/createClientRpc.ts b/packages/start-client-core/src/client-rpc/createClientRpc.ts
index 0ab3c73e76..4176776bc4 100644
--- a/packages/start-client-core/src/client-rpc/createClientRpc.ts
+++ b/packages/start-client-core/src/client-rpc/createClientRpc.ts
@@ -4,7 +4,12 @@ import { serverFnFetcher } from './serverFnFetcher'
 import type { ClientFnMeta } from '../constants'
 
 export function createClientRpc(functionId: string) {
-  const url = process.env.TSS_SERVER_FN_BASE + functionId
+  const serverFnBase =
+    (typeof process !== 'undefined' && process.env?.TSS_SERVER_FN_BASE) ||
+    (typeof import.meta !== 'undefined' &&
+      (import.meta as any).env?.TSS_SERVER_FN_BASE) ||
+    '/_serverFn/'
+  const url = serverFnBase + functionId
   const serverFnMeta: ClientFnMeta = { id: functionId }
 
   const clientFn = (...args: Array<any>) => {
diff --git a/packages/start-client-core/tests/createClientRpc.test.ts b/packages/start-client-core/tests/createClientRpc.test.ts
new file mode 100644
index 0000000000..e61ddf0f4d
--- /dev/null
+++ b/packages/start-client-core/tests/createClientRpc.test.ts
@@ -0,0 +1,44 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+import { createClientRpc } from '../src/client-rpc/createClientRpc'
+import { TSS_SERVER_FUNCTION } from '../src/constants'
+
+describe('createClientRpc', () => {
+  const originalEnv = process.env.TSS_SERVER_FN_BASE
+
+  afterEach(() => {
+    if (originalEnv !== undefined) {
+      process.env.TSS_SERVER_FN_BASE = originalEnv
+    } else {
+      delete process.env.TSS_SERVER_FN_BASE
+    }
+  })
+
+  it('uses process.env.TSS_SERVER_FN_BASE when defined', () => {
+    process.env.TSS_SERVER_FN_BASE = '/custom-base/'
+    const rpc = createClientRpc('testFn')
+
+    expect(rpc.url).toBe('/custom-base/testFn')
+    expect(rpc.serverFnMeta).toEqual({ id: 'testFn' })
+    expect(rpc[TSS_SERVER_FUNCTION]).toBe(true)
+  })
+
+  it('falls back to default server function base when TSS_SERVER_FN_BASE is undefined', () => {
+    delete process.env.TSS_SERVER_FN_BASE
+    const rpc = createClientRpc('testFn')
+
+    expect(rpc.url).toBe('/_serverFn/testFn')
+  })
+
+  it('safely falls back without ReferenceError when global process is undefined', () => {
+    const originalProcess = globalThis.process
+    try {
+      // @ts-expect-error simulating browser environment where process is not defined
+      delete globalThis.process
+
+      const rpc = createClientRpc('testFn')
+      expect(rpc.url).toBe('/_serverFn/testFn')
+    } finally {
+      globalThis.process = originalProcess
+    }
+  })
+})