diff --git a/src/main/codex-accounts/runtime-home-service.test.ts b/src/main/codex-accounts/runtime-home-service.test.ts
index 22ce038f45a..be90313c231 100644
--- a/src/main/codex-accounts/runtime-home-service.test.ts
+++ b/src/main/codex-accounts/runtime-home-service.test.ts
@@ -1893,6 +1893,30 @@ describe('CodexRuntimeHomeService', () => {
     )
   })
 
+  it('launches the system-default custom provider without requiring OAuth auth', async () => {
+    const systemCodexHome = getSystemCodexHomePath()
+    const canonicalConfigPath = join(systemCodexHome, 'config.toml')
+    const canonicalConfig = [
+      'model_provider = "codex-lb"',
+      '',
+      '[model_providers.codex-lb]',
+      'base_url = "https://codex-lb.example.test/v1"',
+      'env_key = "CODEX_LB_API_KEY"',
+      ''
+    ].join('\n')
+    writeFileSync(canonicalConfigPath, canonicalConfig, 'utf-8')
+    const store = createStore(createSettings({ codexSystemDefaultRealHomeEnabled: false }))
+    const { CodexRuntimeHomeService } = await import('./runtime-home-service')
+    const service = new CodexRuntimeHomeService(store as never)
+
+    expect(service.prepareForCodexLaunch()).toBe(getRuntimeCodexHomePath())
+    expect(readFileSync(join(getRuntimeCodexHomePath(), 'config.toml'), 'utf-8')).toBe(
+      canonicalConfig
+    )
+    expect(existsSync(getRuntimeCodexAuthPath())).toBe(false)
+    expect(readFileSync(canonicalConfigPath, 'utf-8')).toBe(canonicalConfig)
+  })
+
   it('links system Codex user resources into the managed runtime home before launch', async () => {
     const systemCodexHome = getSystemCodexHomePath()
     mkdirSync(join(systemCodexHome, 'skills', 'review'), { recursive: true })
diff --git a/src/main/codex-accounts/service.test.ts b/src/main/codex-accounts/service.test.ts
index 4b4511e3c8d..ba827190cfc 100644
--- a/src/main/codex-accounts/service.test.ts
+++ b/src/main/codex-accounts/service.test.ts
@@ -667,7 +667,8 @@ describe('CodexAccountService config sync', () => {
     vi.resetModules()
 
     const canonicalConfigPath = join(testState.fakeHomeDir, '.codex', 'config.toml')
-    const canonicalConfig = 'approval_policy = "never"\nsandbox_mode = "danger-full-access"\n'
+    const canonicalConfig =
+      'model_provider = "openai"\napproval_policy = "never"\nsandbox_mode = "danger-full-access"\n'
     writeFileSync(canonicalConfigPath, canonicalConfig, 'utf-8')
 
     const spawnMock = vi.fn(
@@ -788,6 +789,55 @@ describe('CodexAccountService config sync', () => {
     expect(spawnMock).toHaveBeenCalledTimes(1)
   })
 
+  it('rejects OAuth account add when canonical config pins a custom provider', async () => {
+    vi.resetModules()
+
+    const canonicalConfigPath = join(testState.fakeHomeDir, '.codex', 'config.toml')
+    const canonicalConfig = [
+      'model_provider = "codex-lb"',
+      'model = "gpt-5.2-codex"',
+      '',
+      '[model_providers.codex-lb]',
+      'name = "Codex load balancer"',
+      'base_url = "https://codex-lb.example.test/v1"',
+      'env_key = "CODEX_LB_API_KEY"',
+      ''
+    ].join('\n')
+    writeFileSync(canonicalConfigPath, canonicalConfig, 'utf-8')
+
+    const spawnMock = vi.fn()
+    vi.doMock('node:crypto', () => ({ randomUUID: () => 'account-id-for-test' }))
+    vi.doMock('node:child_process', () => ({ execFileSync: vi.fn(), spawn: spawnMock }))
+
+    try {
+      const settings = createSettings()
+      const store = createStore(settings)
+      const rateLimits = createRateLimits()
+      const runtimeHome = createRuntimeHome()
+      const { CodexAccountService } = await import('./service')
+      const service = new CodexAccountService(
+        store as never,
+        rateLimits as never,
+        runtimeHome as never
+      )
+
+      await expect(service.addAccount()).rejects.toThrow(
+        'Orca cannot add a Codex OAuth account while ~/.codex/config.toml pins the custom provider "codex-lb". Keep using the system-default account for this provider, or remove model_provider (or set it to "openai") before adding an OAuth account. Orca left your config unchanged.'
+      )
+
+      expect(spawnMock).not.toHaveBeenCalled()
+      expect(store.updateSettings).not.toHaveBeenCalled()
+      expect(runtimeHome.syncForCurrentSelection).not.toHaveBeenCalled()
+      expect(existsSync(join(testState.userDataDir, 'codex-accounts', 'account-id-for-test'))).toBe(
+        false
+      )
+      expect(readFileSync(canonicalConfigPath, 'utf-8')).toBe(canonicalConfig)
+    } finally {
+      vi.doUnmock('node:crypto')
+      vi.doUnmock('node:child_process')
+    }
+  })
+
   it('recreates the expected missing managed home before reauthenticating', async () => {
     vi.resetModules()
 
diff --git a/src/main/codex-accounts/service.ts b/src/main/codex-accounts/service.ts
index cab0554de1d..4abb6ad19bd 100644
--- a/src/main/codex-accounts/service.ts
+++ b/src/main/codex-accounts/service.ts
@@ -23,6 +23,7 @@ import { getCodexManagedHookInstallMaterial } from '../codex/hook-service'
 import { syncSystemConfigIntoManagedCodexHome } from '../codex/codex-config-mirror'
 import { getSystemCodexHomePath } from '../codex/codex-home-paths'
 import { MANAGED_HOOK_TIMEOUT_SECONDS } from '../agent-hooks/installer-utils'
+import { readCodexTopLevelModelProvider } from '../codex/codex-model-provider-config'
 import { resolveCodexCommand } from '../codex-cli/command'
 import type { Store } from '../persistence'
 import type { RateLimitService } from '../rate-limits/service'
@@ -139,7 +140,9 @@ export class CodexAccountService {
     const { managedHomePath } = managedHome
 
     try {
-      this.safeSyncCanonicalConfigIntoManagedHome(managedHomePath)
+      const canonicalConfig = this.readCanonicalConfigForManagedHome(managedHomePath)
+      this.assertOAuthAccountAddAllowed(canonicalConfig)
+      this.safeSyncCanonicalConfigIntoManagedHome(managedHomePath, canonicalConfig)
       await this.runCodexLogin(managedHomePath)
       const identity = this.readIdentityFromHome(managedHomePath)
       if (!identity.email) {
@@ -537,9 +540,12 @@ export class CodexAccountService {
     }
   }
 
-  private safeSyncCanonicalConfigIntoManagedHome(managedHomePath: string): void {
+  private safeSyncCanonicalConfigIntoManagedHome(
+    managedHomePath: string,
+    canonicalConfig?: CanonicalCodexConfig | null
+  ): void {
     try {
-      this.syncCanonicalConfigIntoManagedHome(managedHomePath)
+      this.syncCanonicalConfigIntoManagedHome(managedHomePath, canonicalConfig)
     } catch (error) {
       console.warn('[codex-accounts] Failed to seed managed config:', error)
     }
@@ -659,6 +665,21 @@ export class CodexAccountService {
     }
   }
 
+  private assertOAuthAccountAddAllowed(canonicalConfig: CanonicalCodexConfig | null): void {
+    const modelProvider = canonicalConfig
+      ? readCodexTopLevelModelProvider(canonicalConfig.contents)
+      : null
+    if (!modelProvider || modelProvider === 'openai') {
+      return
+    }
+
+    // Why: mirroring a custom-provider pin into an OAuth managed home makes
+    // the new OAuth credentials inert; fail before login and leave user config intact.
+    throw new Error(
+      `Orca cannot add a Codex OAuth account while ~/.codex/config.toml pins the custom provider ${JSON.stringify(modelProvider)}. Keep using the system-default account for this provider, or remove model_provider (or set it to "openai") before adding an OAuth account. Orca left your config unchanged.`
+    )
+  }
+
   private writeManagedConfig(managedHomePath: string, contents: string): void {
     const configPath = join(managedHomePath, 'config.toml')
     try {
diff --git a/src/main/codex/codex-config-path-reference-rewrite.ts b/src/main/codex/codex-config-path-reference-rewrite.ts
index 94e29db3d8f..542ce64a824 100644
--- a/src/main/codex/codex-config-path-reference-rewrite.ts
+++ b/src/main/codex/codex-config-path-reference-rewrite.ts
@@ -3,6 +3,7 @@ import {
   createTomlLineScanState,
   getTomlTableHeader,
   isTomlStructuralLine,
+  parseTomlSingleLineStringValue,
   updateTomlLineScanState
 } from './config-toml-line-scan'
 
@@ -22,12 +23,6 @@ const EXACT_PATH_CONFIG_KEYS = new Set([
   'sqlite_home'
 ])
 
-type ParsedTomlString = {
-  value: string
-  start: number
-  end: number
-}
-
 // Why: Orca mirrors config.toml into a managed CODEX_HOME, but Codex resolves
 // path-valued config settings from the file it read. Keep user-owned assets in
 // ~/.codex reachable after the mirror moves the TOML. Best-effort by design:
@@ -151,94 +146,6 @@ function quoteTomlBasicString(value: string): string {
   return `${quoted}"`
 }
 
-function parseTomlSingleLineStringValue(line: string, offset: number): ParsedTomlString | null {
-  let index = offset
-  while (line[index] === ' ' || line[index] === '\t') {
-    index += 1
-  }
-
-  if (line.startsWith('"""', index) || line.startsWith("'''", index)) {
-    return null
-  }
-
-  const quote = line[index]
-  if (quote !== '"' && quote !== "'") {
-    return null
-  }
-
-  const start = index
-  index += 1
-  let value = ''
-  while (index < line.length) {
-    const char = line[index]
-    if (char === quote) {
-      return { value, start, end: index + 1 }
-    }
-    if (quote === '"' && char === '\\') {
-      const escaped = parseTomlBasicStringEscape(line, index)
-      if (!escaped) {
-        return null
-      }
-      value += escaped.value
-      index = escaped.nextIndex
-      continue
-    }
-    value += char
-    index += 1
-  }
-  return null
-}
-
-function parseTomlBasicStringEscape(
-  line: string,
-  slashIndex: number
-): { value: string; nextIndex: number } | null {
-  const escaped = line[slashIndex + 1]
-  switch (escaped) {
-    case 'b':
-      return { value: '\b', nextIndex: slashIndex + 2 }
-    case 't':
-      return { value: '\t', nextIndex: slashIndex + 2 }
-    case 'n':
-      return { value: '\n', nextIndex: slashIndex + 2 }
-    case 'f':
-      return { value: '\f', nextIndex: slashIndex + 2 }
-    case 'r':
-      return { value: '\r', nextIndex: slashIndex + 2 }
-    case '"':
-    case '\\':
-      return { value: escaped, nextIndex: slashIndex + 2 }
-    case 'u':
-      return parseTomlUnicodeEscape(line, slashIndex + 2, 4)
-    case 'U':
-      return parseTomlUnicodeEscape(line, slashIndex + 2, 8)
-    default:
-      return null
-  }
-}
-
-function parseTomlUnicodeEscape(
-  line: string,
-  start: number,
-  length: number
-): { value: string; nextIndex: number } | null {
-  const raw = line.slice(start, start + length)
-  if (!new RegExp(`^[0-9a-fA-F]{${length}}$`).test(raw)) {
-    return null
-  }
-  const codePoint = Number.parseInt(raw, 16)
-  // Why: TOML escapes must be Unicode scalar values; String.fromCodePoint
-  // accepts lone surrogates, which would round-trip into invalid TOML.
-  if (codePoint >= 0xd800 && codePoint <= 0xdfff) {
-    return null
-  }
-  try {
-    return { value: String.fromCodePoint(codePoint), nextIndex: start + length }
-  } catch {
-    return null
-  }
-}
-
 function getTomlHeaderPath(header: string): string {
   const trimmed = header.trim()
   if (trimmed.startsWith('[[') && trimmed.endsWith(']]')) {
diff --git a/src/main/codex/codex-model-provider-config.test.ts b/src/main/codex/codex-model-provider-config.test.ts
new file mode 100644
index 00000000000..1304946768e
--- /dev/null
+++ b/src/main/codex/codex-model-provider-config.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it } from 'vitest'
+import { readCodexTopLevelModelProvider } from './codex-model-provider-config'
+
+describe('readCodexTopLevelModelProvider', () => {
+  it.each([
+    ['missing provider', 'model = "gpt-5"\n', null],
+    ['OpenAI provider', 'model_provider = "openai" # default\n', 'openai'],
+    ['literal custom provider', "model_provider = 'codex-lb'\n", 'codex-lb'],
+    ['quoted key', '"model_provider" = "codex-lb"\n', 'codex-lb'],
+    ['escaped quoted key', '"model\\u005Fprovider" = "codex-lb"\n', 'codex-lb'],
+    ['escaped provider', 'model_provider = "codex\\u002Dlb"\n', 'codex-lb'],
+    ['single-line multiline OpenAI', 'model_provider = """openai"""\n', 'openai'],
+    ['single-line multiline custom', "model_provider = '''codex-lb'''\n", 'codex-lb'],
+    ['CRLF custom provider', 'model = "gpt-5"\r\nmodel_provider = "codex-lb"\r\n', 'codex-lb'],
+    [
+      'continued multiline custom',
+      ['model_provider = """\\', '  codex-lb"""', ''].join('\n'),
+      'codex-lb'
+    ],
+    ['profile provider', ['[profiles.custom]', 'model_provider = "codex-lb"', ''].join('\n'), null],
+    [
+      'provider-like multiline data',
+      [
+        'developer_instructions = """',
+        '[profiles.custom]',
+        'model_provider = "not-a-setting"',
+        '"""',
+        'model_provider = "openai"',
+        ''
+      ].join('\n'),
+      'openai'
+    ],
+    [
+      'provider-like array data',
+      [
+        'notify = [',
+        '  "model_provider = \\"not-a-setting\\"",',
+        ']',
+        'model_provider = "openai"',
+        ''
+      ].join('\n'),
+      'openai'
+    ]
+  ])('reads %s', (_description, config, expected) => {
+    expect(readCodexTopLevelModelProvider(config)).toBe(expected)
+  })
+
+  it('uses the first duplicate assignment deterministically', () => {
+    expect(
+      readCodexTopLevelModelProvider(
+        ['model_provider = "openai"', 'model_provider = "codex-lb"', ''].join('\n')
+      )
+    ).toBe('openai')
+  })
+
+  it.each([
+    ['unquoted value', 'model_provider = codex-lb\n'],
+    ['unterminated value', 'model_provider = "codex-lb\n'],
+    ['assignment after a table', '[profiles.default]\nmodel_provider = "codex-lb"\n']
+  ])('ignores malformed or non-root %s', (_description, config) => {
+    expect(readCodexTopLevelModelProvider(config)).toBeNull()
+  })
+})
diff --git a/src/main/codex/codex-model-provider-config.ts b/src/main/codex/codex-model-provider-config.ts
new file mode 100644
index 00000000000..6a659804858
--- /dev/null
+++ b/src/main/codex/codex-model-provider-config.ts
@@ -0,0 +1,51 @@
+import {
+  createTomlLineScanState,
+  getTomlTableHeader,
+  isTomlStructuralLine,
+  parseTomlSingleLineStringValue,
+  parseTomlStringValue,
+  updateTomlLineScanState
+} from './config-toml-line-scan'
+
+// Why: only the root setting selects the provider for every OAuth login;
+// similarly named keys inside profiles or provider tables are not global pins.
+export function readCodexTopLevelModelProvider(config: string): string | null {
+  let state = createTomlLineScanState()
+  let lineOffset = 0
+  for (const line of config.split('\n')) {
+    if (isTomlStructuralLine(state)) {
+      if (getTomlTableHeader(line)) {
+        return null
+      }
+      const valueOffset = getModelProviderValueOffset(line)
+      if (valueOffset !== null) {
+        return parseTomlStringValue(config, lineOffset + valueOffset)?.value ?? null
+      }
+    }
+    state = updateTomlLineScanState(state, line)
+    lineOffset += line.length + 1
+  }
+  return null
+}
+
+function getModelProviderValueOffset(line: string): number | null {
+  let index = 0
+  while (line[index] === ' ' || line[index] === '\t') {
+    index += 1
+  }
+
+  if (line.startsWith('model_provider', index)) {
+    index += 'model_provider'.length
+  } else {
+    const parsedKey = parseTomlSingleLineStringValue(line, index)
+    if (parsedKey?.value !== 'model_provider') {
+      return null
+    }
+    index = parsedKey.end
+  }
+
+  while (line[index] === ' ' || line[index] === '\t') {
+    index += 1
+  }
+  return line[index] === '=' ? index + 1 : null
+}
diff --git a/src/main/codex/config-toml-line-scan.ts b/src/main/codex/config-toml-line-scan.ts
index c0136be9e32..48216621636 100644
--- a/src/main/codex/config-toml-line-scan.ts
+++ b/src/main/codex/config-toml-line-scan.ts
@@ -9,6 +9,12 @@ export type TomlLineScanState = {
   arrayDepth: number
 }
 
+export type ParsedTomlString = {
+  value: string
+  start: number
+  end: number
+}
+
 type TomlMultilineMode = 'basic' | 'literal' | null
 
 export function createTomlLineScanState(): TomlLineScanState {
@@ -93,6 +99,102 @@ export function getTomlTableHeader(line: string): string | null {
   return match?.[1] ?? null
 }
 
+export function parseTomlSingleLineStringValue(
+  line: string,
+  offset: number
+): ParsedTomlString | null {
+  let index = offset
+  while (line[index] === ' ' || line[index] === '\t') {
+    index += 1
+  }
+
+  if (line.startsWith('"""', index) || line.startsWith("'''", index)) {
+    return null
+  }
+
+  const quote = line[index]
+  if (quote !== '"' && quote !== "'") {
+    return null
+  }
+
+  const start = index
+  index += 1
+  let value = ''
+  while (index < line.length) {
+    const char = line[index]
+    if (char === '\n' || char === '\r') {
+      return null
+    }
+    if (char === quote) {
+      return { value, start, end: index + 1 }
+    }
+    if (quote === '"' && char === '\\') {
+      const escaped = parseTomlBasicStringEscape(line, index)
+      if (!escaped) {
+        return null
+      }
+      value += escaped.value
+      index = escaped.nextIndex
+      continue
+    }
+    value += char
+    index += 1
+  }
+  return null
+}
+
+// Why: serde accepts multiline TOML strings for String settings such as
+// model_provider, so safety classifiers cannot treat triple quotes as absent.
+export function parseTomlStringValue(source: string, offset: number): ParsedTomlString | null {
+  let index = offset
+  while (source[index] === ' ' || source[index] === '\t') {
+    index += 1
+  }
+
+  const delimiter = source.startsWith('"""', index)
+    ? '"""'
+    : source.startsWith("'''", index)
+      ? "'''"
+      : null
+  if (!delimiter) {
+    return parseTomlSingleLineStringValue(source, offset)
+  }
+
+  const start = index
+  index += delimiter.length
+  index = skipInitialTomlMultilineNewline(source, index)
+  let value = ''
+  while (index < source.length) {
+    if (source.startsWith(delimiter, index)) {
+      return { value, start, end: index + delimiter.length }
+    }
+
+    if (delimiter === '"""' && source[index] === '\\') {
+      const continuationEnd = getTomlMultilineContinuationEnd(source, index)
+      if (continuationEnd !== null) {
+        index = continuationEnd
+        continue
+      }
+      const escaped = parseTomlBasicStringEscape(source, index)
+      if (!escaped) {
+        return null
+      }
+      value += escaped.value
+      index = escaped.nextIndex
+      continue
+    }
+
+    if (source[index] === '\r') {
+      value += '\n'
+      index += source[index + 1] === '\n' ? 2 : 1
+      continue
+    }
+    value += source[index]
+    index += 1
+  }
+  return null
+}
+
 function skipTomlBasicString(line: string, startIndex: number): number {
   let index = startIndex
   while (index < line.length) {
@@ -113,3 +215,84 @@ function skipTomlLiteralString(line: string, startIndex: number): number {
   const endIndex = line.indexOf("'", startIndex)
   return endIndex === -1 ? line.length : endIndex + 1
 }
+
+function skipInitialTomlMultilineNewline(source: string, index: number): number {
+  if (source[index] === '\r' && source[index + 1] === '\n') {
+    return index + 2
+  }
+  return source[index] === '\n' ? index + 1 : index
+}
+
+function getTomlMultilineContinuationEnd(source: string, slashIndex: number): number | null {
+  let index = slashIndex + 1
+  while (source[index] === ' ' || source[index] === '\t') {
+    index += 1
+  }
+  if (source[index] === '\r' && source[index + 1] === '\n') {
+    index += 2
+  } else if (source[index] === '\n') {
+    index += 1
+  } else {
+    return null
+  }
+
+  while (
+    source[index] === ' ' ||
+    source[index] === '\t' ||
+    source[index] === '\n' ||
+    source[index] === '\r'
+  ) {
+    index += 1
+  }
+  return index
+}
+
+function parseTomlBasicStringEscape(
+  line: string,
+  slashIndex: number
+): { value: string; nextIndex: number } | null {
+  const escaped = line[slashIndex + 1]
+  switch (escaped) {
+    case 'b':
+      return { value: '\b', nextIndex: slashIndex + 2 }
+    case 't':
+      return { value: '\t', nextIndex: slashIndex + 2 }
+    case 'n':
+      return { value: '\n', nextIndex: slashIndex + 2 }
+    case 'f':
+      return { value: '\f', nextIndex: slashIndex + 2 }
+    case 'r':
+      return { value: '\r', nextIndex: slashIndex + 2 }
+    case '"':
+    case '\\':
+      return { value: escaped, nextIndex: slashIndex + 2 }
+    case 'u':
+      return parseTomlUnicodeEscape(line, slashIndex + 2, 4)
+    case 'U':
+      return parseTomlUnicodeEscape(line, slashIndex + 2, 8)
+    default:
+      return null
+  }
+}
+
+function parseTomlUnicodeEscape(
+  line: string,
+  start: number,
+  length: number
+): { value: string; nextIndex: number } | null {
+  const raw = line.slice(start, start + length)
+  if (!new RegExp(`^[0-9a-fA-F]{${length}}$`).test(raw)) {
+    return null
+  }
+  const codePoint = Number.parseInt(raw, 16)
+  // Why: TOML escapes must be Unicode scalar values; String.fromCodePoint
+  // accepts lone surrogates, which would round-trip into invalid TOML.
+  if (codePoint >= 0xd800 && codePoint <= 0xdfff) {
+    return null
+  }
+  try {
+    return { value: String.fromCodePoint(codePoint), nextIndex: start + length }
+  } catch {
+    return null
+  }
+}