fix: set authenticatorAttachment to platform on iOS for passkey creation

[broody]

Problem

On Chrome iOS, clicking "Create Passkey" in the popup triggers the native Apple passkey overlay to show a "Sign in with passkey" dialog instead of creating a new passkey. Safari iOS works fine.

Root Cause

Chrome iOS (with Google Password Manager integration since Chrome 136) interprets authenticatorAttachment: undefined differently from Safari. When unset, Chrome iOS shows a sign-in dialog offering existing passkeys rather than directly creating a new one.

Fix

Explicitly set authenticatorAttachment: "platform" on iOS devices. This tells the browser we specifically want to create a new platform-bound passkey using Face ID/Touch ID, which triggers the correct "Create Passkey" flow on both Safari and Chrome iOS.

Note: The popup flow itself may no longer be necessary since Safari 15.5+ supports publickey-credentials-create in cross-origin iframes (and the iframe already has this permission set). Could be a follow-up to remove the popup entirely.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
controller-example-next	Ready	Preview	Feb 13, 2026 9:36pm
keychain	Ready	Preview	Feb 13, 2026 9:36pm
keychain-storybook	Ready	Preview	Feb 13, 2026 9:36pm

[claude]

Claude finished @broody's task —— View job

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I'll analyze the controller repository changes and update the documentation in the docs-repo directory accordingly.

Tasks:

• Read the changed controller files to understand the passkey fix
• Explore the docs-repo directory structure
• Identify relevant documentation that may need updates
• Update documentation if changes affect user-facing behavior
• Commit and push any documentation changes

Analysis:

The PR fixes a Chrome iOS specific issue where authenticatorAttachment: undefined was causing the browser to show a "Sign in with passkey" dialog instead of creating new passkeys. The fix explicitly sets authenticatorAttachment: "platform" on iOS devices (lines 45-49 in packages/keychain/src/hooks/account.ts).

Found relevant documentation files that mention passkeys/WebAuthn:

• headless-authentication.md - Contains WebAuthn authentication examples
• signer-management.md - Contains extensive passkey documentation and platform support details
• native/ios.md - iOS-specific integration docs

Conclusion:

No documentation updates needed. This fix is an internal implementation improvement that resolves a Chrome iOS-specific passkey creation issue. It doesn't change:

• User-facing API or behavior
• Authentication flow or requirements
• Platform support or compatibility
• Any documented features or usage patterns

The change maintains the existing passkey functionality while fixing a browser-specific bug, so current documentation remains accurate and complete.