diff --git a/apps/content/docs/error-handling.mdx b/apps/content/docs/error-handling.mdx
index 4c8b549a3..2f9e247dd 100644
--- a/apps/content/docs/error-handling.mdx
+++ b/apps/content/docs/error-handling.mdx
@@ -79,6 +79,10 @@ const exampleProcedure = os
   })
 ```
 
+:::warning
+For compatibility, do not use these error codes: `then`, `bind`, `call`, `apply`, `valueOf`, `toString`, `toJSON`.
+:::
+
 :::tip
 You can use typesafe errors across your entire project, but we recommend reserving them for application-specific cases. For common errors like `UNAUTHORIZED` or `RATE_LIMITED`, the client usually already understands the meaning. Skipping explicit schemas for those errors can also reduce type complexity and improve TypeScript performance.
 :::
diff --git a/packages/client/src/consts.ts b/packages/client/src/consts.ts
index 144e88199..4dc4665b2 100644
--- a/packages/client/src/consts.ts
+++ b/packages/client/src/consts.ts
@@ -7,7 +7,7 @@
  * proxy for them can cause unexpected behavior, compatibility issues, or
  * infinite proxy chains.
  */
-export const RECURSIVE_CLIENT_UNWRAP_KEYS = new Set([
+export const RECURSIVE_CLIENT_UNWRAP_KEYS = new Set<PropertyKey>([
   /**
    * Prevents the client from being treated as a thenable when users
    * accidentally write `await client`.
diff --git a/packages/contract/src/error-factory.test.ts b/packages/contract/src/error-factory.test.ts
index e951dac69..eef5149bd 100644
--- a/packages/contract/src/error-factory.test.ts
+++ b/packages/contract/src/error-factory.test.ts
@@ -223,17 +223,19 @@ describe('createORPCErrorConstructorMap', () => {
     expect('ANY_THING' in constructors).toBe(false)
   })
 
-  it('not proxy on symbol and then keys', async () => {
+  it('not proxy on symbol and RECURSIVE_CLIENT_UNWRAP_KEYS', async () => {
     const map = constructors as any
     expect(map[Symbol('something')]).toBeUndefined()
     expect(map.then).toBeUndefined()
     expect(await map).toBe(map)
+    expect(map.toJSON).toBeUndefined()
+    expect(String(map)).toBe('[object Object]')
   })
 
   it('does not resolve error codes through Object.prototype', () => {
-    const e = (constructors as any).toString()
+    const e = (constructors as any).constructor()
 
-    expect(e.code).toEqual('toString')
+    expect(e.code).toEqual('constructor')
     expect(e.defined).toEqual(false)
   })
 })
diff --git a/packages/contract/src/error-factory.ts b/packages/contract/src/error-factory.ts
index b0001c872..09c962f17 100644
--- a/packages/contract/src/error-factory.ts
+++ b/packages/contract/src/error-factory.ts
@@ -3,7 +3,7 @@ import type { MaybeOptionalOptions, Writable } from '@orpc/shared'
 import type { ErrorMap, ErrorMapItem } from './error'
 import type { AnySchema, InferSchemaInput, Schema } from './schema'
 
-import { ORPCError } from '@orpc/client'
+import { ORPCError, RECURSIVE_CLIENT_UNWRAP_KEYS } from '@orpc/client'
 import { getOwn, resolveMaybeOptionalOptions } from '@orpc/shared'
 import { ValidationError } from './error'
 import { type } from './schema-utils'
@@ -156,7 +156,7 @@ export type ORPCErrorConstructorMap<T extends ErrorMap> = {
 export function createORPCErrorConstructorMap<T extends ErrorMap>(errorMap: T): ORPCErrorConstructorMap<T> {
   const proxy = new Proxy(errorMap, {
     get(target, code) {
-      if (typeof code !== 'string' || code === 'then') {
+      if (typeof code !== 'string' || RECURSIVE_CLIENT_UNWRAP_KEYS.has(code)) {
         return Reflect.get(target, code)
       }
 
diff --git a/packages/pinia-colada/src/router-utils.test.ts b/packages/pinia-colada/src/router-utils.test.ts
index 477a58533..94b2487ba 100644
--- a/packages/pinia-colada/src/router-utils.test.ts
+++ b/packages/pinia-colada/src/router-utils.test.ts
@@ -100,6 +100,11 @@ describe('createRouterUtils', () => {
     expect(utils[Symbol.for('a')]).toBe(undefined)
   })
 
+  it('can be converted to primitive values', () => {
+    const utils = createRouterUtils(client) as any
+    expect(String(utils)).toBe('[object Object]')
+  })
+
   it('stops recursive on RECURSIVE_CLIENT_UNWRAP_KEYS', () => {
     client.then = vi.fn()
 
diff --git a/packages/pinia-colada/src/router-utils.ts b/packages/pinia-colada/src/router-utils.ts
index 830847a75..bb77a846a 100644
--- a/packages/pinia-colada/src/router-utils.ts
+++ b/packages/pinia-colada/src/router-utils.ts
@@ -125,9 +125,14 @@ function createRouterUtilsInternal<T extends AnyNestedClient>(
   const recursive = new Proxy(utils, {
     get(target, prop) {
       const value = Reflect.get(target, prop)
+
+      if (RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop)) {
+        return value ?? Reflect.get(Object.prototype, prop)
+      }
+
       const nextClient = Reflect.get(client, prop)
 
-      if (typeof prop !== 'string' || RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop) || !isTypescriptObject(nextClient)) {
+      if (typeof prop !== 'string' || !isTypescriptObject(nextClient)) {
         return value
       }
 
diff --git a/packages/swr/src/router-utils.test.ts b/packages/swr/src/router-utils.test.ts
index 564824fdc..3f440d660 100644
--- a/packages/swr/src/router-utils.test.ts
+++ b/packages/swr/src/router-utils.test.ts
@@ -91,6 +91,11 @@ describe('createRouterUtils', () => {
     expect(utils[Symbol.for('a')]).toBe(undefined)
   })
 
+  it('can be converted to primitive values', () => {
+    const utils = createRouterUtils(client) as any
+    expect(String(utils)).toBe('[object Object]')
+  })
+
   it('stops recursive on symbol and RECURSIVE_CLIENT_UNWRAP_KEYS of conflicting method proxies', () => {
     const utils = createRouterUtils(client) as any
     const keyUtils = utils.key // "key" is both a util method and a nested client
diff --git a/packages/swr/src/router-utils.ts b/packages/swr/src/router-utils.ts
index d94ecf701..83d4e1bb3 100644
--- a/packages/swr/src/router-utils.ts
+++ b/packages/swr/src/router-utils.ts
@@ -46,9 +46,14 @@ export function createRouterUtils<T extends AnyNestedClient>(
   const recursive = new Proxy(utils, {
     get(target, prop) {
       const value = Reflect.get(target, prop)
+
+      if (RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop)) {
+        return value ?? Reflect.get(Object.prototype, prop)
+      }
+
       const nextClient = Reflect.get(client, prop)
 
-      if (typeof prop !== 'string' || RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop) || !isTypescriptObject(nextClient)) {
+      if (typeof prop !== 'string' || !isTypescriptObject(nextClient)) {
         return value
       }
 
diff --git a/packages/tanstack-query/src/router-utils.test.ts b/packages/tanstack-query/src/router-utils.test.ts
index 529f65eae..38260cd1d 100644
--- a/packages/tanstack-query/src/router-utils.test.ts
+++ b/packages/tanstack-query/src/router-utils.test.ts
@@ -94,6 +94,11 @@ describe('createRouterUtils', () => {
     expect(utils[Symbol.for('a')]).toBe(undefined)
   })
 
+  it('can be converted to primitive values', () => {
+    const utils = createRouterUtils(client) as any
+    expect(String(utils)).toBe('[object Object]')
+  })
+
   it('supports scoped options', () => {
     const keyOptions = {
       queryOptions: {
diff --git a/packages/tanstack-query/src/router-utils.ts b/packages/tanstack-query/src/router-utils.ts
index 36b162cbb..fbeecf169 100644
--- a/packages/tanstack-query/src/router-utils.ts
+++ b/packages/tanstack-query/src/router-utils.ts
@@ -125,9 +125,14 @@ function createRouterUtilsInternal<T extends AnyNestedClient>(
   const recursive = new Proxy(utils, {
     get(target, prop) {
       const value = Reflect.get(target, prop)
+
+      if (RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop)) {
+        return value ?? Reflect.get(Object.prototype, prop)
+      }
+
       const nextClient = Reflect.get(client, prop)
 
-      if (typeof prop !== 'string' || RECURSIVE_CLIENT_UNWRAP_KEYS.has(prop) || !isTypescriptObject(nextClient)) {
+      if (typeof prop !== 'string' || !isTypescriptObject(nextClient)) {
         return value
       }