diff --git a/src/lib/strings/__tests__/errors.test.ts b/src/lib/strings/__tests__/errors.test.ts
new file mode 100644
index 0000000000..aa9538032f
--- /dev/null
+++ b/src/lib/strings/__tests__/errors.test.ts
@@ -0,0 +1,139 @@
+import {XRPCError} from '@atproto/api'
+import {
+  type Procedure,
+  type Query,
+  XrpcResponseError,
+} from '@atproto/lex-client'
+import {describe, expect, it} from '@jest/globals'
+
+import {
+  getErrorHeader,
+  getErrorName,
+  getErrorStatus,
+  isXrpcError,
+} from '#/lib/xrpc-error'
+import {isErrorMaybeAppPasswordPermissions, isNetworkError} from '../errors'
+
+/**
+ * Old-world fixture: `@atproto/api` XRPCError. `.headers` is a plain record,
+ * `.error` is the lexicon code string, `.status` a numeric ResponseType enum.
+ */
+function oldError(
+  status: number,
+  error: string,
+  headers?: Record<string, string>,
+) {
+  return new XRPCError(status, error, undefined, headers)
+}
+
+/**
+ * New-world fixture: lex `XrpcResponseError`. Built from a WHATWG `Response`
+ * (so `.headers` is a `Headers` object and `.status` is the numeric HTTP
+ * status) plus a JSON error payload carrying `.error`.
+ */
+function lexError(
+  status: number,
+  error: string,
+  headers?: Record<string, string>,
+) {
+  const response = new Response(null, {status, headers})
+  const method = {} as Procedure | Query
+  return new XrpcResponseError(method, response, {
+    encoding: 'application/json',
+    body: {error, message: `${error} message`},
+  })
+}
+
+describe('isXrpcError', () => {
+  it('matches both old XRPCError and lex XrpcResponseError', () => {
+    expect(isXrpcError(oldError(400, 'TokenInvalid'))).toBe(true)
+    expect(isXrpcError(lexError(400, 'TokenInvalid'))).toBe(true)
+  })
+
+  it('rejects non-XRPC values', () => {
+    expect(isXrpcError(new Error('boom'))).toBe(false)
+    expect(isXrpcError('TokenInvalid')).toBe(false)
+    expect(isXrpcError(undefined)).toBe(false)
+  })
+})
+
+describe('getErrorStatus', () => {
+  it('reads status from both worlds', () => {
+    expect(typeof getErrorStatus(oldError(400, 'TokenInvalid'))).toBe('number')
+    expect(getErrorStatus(lexError(429, 'RateLimitExceeded'))).toBe(429)
+  })
+
+  it('returns undefined for non-XRPC values', () => {
+    expect(getErrorStatus(new Error('boom'))).toBeUndefined()
+  })
+})
+
+describe('getErrorName', () => {
+  it('reads the lexicon error code from both worlds', () => {
+    expect(getErrorName(oldError(400, 'TokenInvalid'))).toBe('TokenInvalid')
+    expect(getErrorName(lexError(400, 'TokenInvalid'))).toBe('TokenInvalid')
+  })
+
+  it('returns undefined for non-XRPC values', () => {
+    expect(getErrorName(new Error('boom'))).toBeUndefined()
+  })
+})
+
+describe('getErrorHeader', () => {
+  it('reads a header from the old record shape (case-insensitive)', () => {
+    const e = oldError(429, 'RateLimitExceeded', {'ratelimit-reset': '123'})
+    expect(getErrorHeader(e, 'ratelimit-reset')).toBe('123')
+    expect(getErrorHeader(e, 'RateLimit-Reset')).toBe('123')
+  })
+
+  it('reads a header from the lex Headers object', () => {
+    const e = lexError(429, 'RateLimitExceeded', {'ratelimit-reset': '123'})
+    expect(getErrorHeader(e, 'ratelimit-reset')).toBe('123')
+    expect(getErrorHeader(e, 'RateLimit-Reset')).toBe('123')
+  })
+
+  it('returns undefined for a missing header or non-XRPC value', () => {
+    expect(getErrorHeader(lexError(400, 'X'), 'nope')).toBeUndefined()
+    expect(getErrorHeader(new Error('boom'), 'ratelimit-reset')).toBeUndefined()
+  })
+})
+
+describe('isErrorMaybeAppPasswordPermissions', () => {
+  it('matches a TokenInvalid error from both worlds', () => {
+    expect(
+      isErrorMaybeAppPasswordPermissions(oldError(400, 'TokenInvalid')),
+    ).toBe(true)
+    expect(
+      isErrorMaybeAppPasswordPermissions(lexError(400, 'TokenInvalid')),
+    ).toBe(true)
+  })
+
+  it('still matches the string-based bad-token signals', () => {
+    expect(
+      isErrorMaybeAppPasswordPermissions(new Error('Bad token scope')),
+    ).toBe(true)
+    expect(
+      isErrorMaybeAppPasswordPermissions(new Error('Bad token method')),
+    ).toBe(true)
+  })
+
+  it('does not match unrelated XRPC errors', () => {
+    expect(
+      isErrorMaybeAppPasswordPermissions(oldError(400, 'InvalidRequest')),
+    ).toBe(false)
+    expect(
+      isErrorMaybeAppPasswordPermissions(lexError(400, 'InvalidRequest')),
+    ).toBe(false)
+  })
+})
+
+describe('isNetworkError', () => {
+  it('matches known network failure strings', () => {
+    expect(isNetworkError(new Error('Network request failed'))).toBe(true)
+    expect(isNetworkError('Failed to fetch')).toBe(true)
+  })
+
+  it('does not match unrelated errors', () => {
+    expect(isNetworkError(new Error('TokenInvalid'))).toBe(false)
+  })
+})
diff --git a/src/lib/strings/errors.ts b/src/lib/strings/errors.ts
index 11e7c11362..c737be01b3 100644
--- a/src/lib/strings/errors.ts
+++ b/src/lib/strings/errors.ts
@@ -1,6 +1,7 @@
-import {XRPCError} from '@atproto/api'
 import {t} from '@lingui/core/macro'
 
+import {getErrorName, getErrorStatus, isXrpcError} from '#/lib/xrpc-error'
+
 export function cleanError(e: unknown): string {
   if (!e) {
     return ''
@@ -67,7 +68,7 @@ export function isNetworkError(e: unknown) {
 }
 
 export function isErrorMaybeAppPasswordPermissions(e: unknown) {
-  if (e instanceof XRPCError && e.error === 'TokenInvalid') {
+  if (isXrpcError(e) && getErrorName(e) === 'TokenInvalid') {
     return true
   }
   const str = String(e)
@@ -93,5 +94,6 @@ export function isRetryableHttpStatus(status: number) {
 }
 
 export function shouldRetryError(e: unknown) {
-  return e instanceof XRPCError && isRetryableHttpStatus(e.status)
+  const status = getErrorStatus(e)
+  return status !== undefined && isRetryableHttpStatus(status)
 }
diff --git a/src/lib/xrpc-error.ts b/src/lib/xrpc-error.ts
new file mode 100644
index 0000000000..bc23fda70a
--- /dev/null
+++ b/src/lib/xrpc-error.ts
@@ -0,0 +1,44 @@
+import {XRPCError} from '@atproto/api'
+import {XrpcError, XrpcResponseError} from '@atproto/lex-client'
+
+/**
+ * True for an XRPC error from either the old bridge agent (`@atproto/api`
+ * `XRPCError`) or a lex `Client` (`@atproto/lex-client` `XrpcError`, the
+ * abstract base of `XrpcResponseError`/`XrpcInvalidResponseError`/
+ * `XrpcInternalError`). During the migration both worlds can throw, so matchers
+ * must accept both.
+ */
+export function isXrpcError(e: unknown): e is XRPCError | XrpcError {
+  return e instanceof XRPCError || e instanceof XrpcError
+}
+
+/**
+ * HTTP status, or undefined if not an XRPC error / no response. Only lex
+ * `XrpcResponseError` (a server response) carries a status; the internal/fetch
+ * lex errors do not.
+ */
+export function getErrorStatus(e: unknown): number | undefined {
+  if (e instanceof XRPCError) return e.status
+  if (e instanceof XrpcResponseError) return e.status
+  return undefined
+}
+
+/** The lexicon error code (`err.error`), from either world. */
+export function getErrorName(e: unknown): string | undefined {
+  if (isXrpcError(e)) return (e as {error?: string}).error
+  return undefined
+}
+
+/**
+ * Read a response header off an XRPC error, normalizing the shape change:
+ * old XRPCError.headers is a plain record; lex XrpcResponseError.headers is a
+ * WHATWG Headers object.
+ */
+export function getErrorHeader(e: unknown, name: string): string | undefined {
+  if (e instanceof XrpcResponseError) return e.headers.get(name) ?? undefined
+  if (e instanceof XRPCError) {
+    const h = (e as {headers?: Record<string, string>}).headers
+    return h?.[name.toLowerCase()]
+  }
+  return undefined
+}
diff --git a/src/types/bsky/index.ts b/src/types/bsky/index.ts
index 5e168858cb..e5b292e3a2 100644
--- a/src/types/bsky/index.ts
+++ b/src/types/bsky/index.ts
@@ -16,6 +16,17 @@ export * as post from '#/types/bsky/post'
 export * as profile from '#/types/bsky/profile'
 export * as starterPack from '#/types/bsky/starterPack'
 
+/**
+ * Unsafe cast from an old `@atproto/api` view/record type to its `#/lexicons`
+ * equivalent. Only for mixed-world boundaries during the migration where a
+ * producer has not yet flipped. Structurally the shapes are identical modulo
+ * branded string types; this asserts the brand the compiler cannot prove.
+ * Every use is a migration debt marker - grep `toLex` at Phase 4 cleanup.
+ */
+export function toLex<T>(v: unknown): T {
+  return v as T
+}
+
 /**
  * Fast type checking without full schema validation, for use with data we
  * trust, or for non-critical path use cases. Why? Our SDK's `is*` identity