[BUG] [Billing] Pro to Max upgrade fails before issuer authorisation (AU account)

What's wrong

I am an existing Claude Pro subscriber trying to upgrade to Claude Max 5x. Every attempt fails with:

The payment failed.

This looks like the same Pro to Max billing failure pattern reported in #55982 and #61386, where the upgrade PaymentIntent appears to be voided or otherwise fails server-side before issuer authorisation.

Key diagnostic

• My existing Claude Pro recurring subscription has billed normally for over a year.
• The failure only happens when attempting the Pro to Max upgrade.
• I tested multiple valid Visa cards with sufficient funds.
• I contacted one card issuer after multiple failed attempts. They confirmed they see zero failed, reversed, or pending authorisation attempts for the Anthropic/Stripe upgrade attempts. There was nothing for them to approve or decline.
• Stripe SMS verification completed successfully during one clean retry, so Stripe identity verification itself worked.
• The checkout still failed after that.

Troubleshooting already completed

• Tried the normal browser session.
• Fully quit/relaunched browser and retried in a clean private/incognito session.
• Retried after entering billing address exactly as shown by the card issuer.
• Tried with and without backup payment method enabled.
• Tried with and without ABN/tax ID details.
• Removed a secondary saved card to simplify to one card.
• Re-entered payment method details.
• No VPN during the clean retry.

Additional symptom

Payment method changes and billing address details do not appear to persist consistently. After saving/updating the payment method and returning to the checkout, the address is requested again or appears missing. A removed secondary card also later reappeared in the payment method UI.

Account / environment

• Region: Australia
• Existing plan: Claude Pro
• Attempted upgrade: Claude Max 5x
• Amount shown: AUD 143.88 today, then AUD 154.55/month + tax
• Browser: ARK on macOS

I am deliberately not posting account email, card last-four digits, billing address, screenshots, or CVV details publicly. I can provide private account identifiers directly to Anthropic support if needed.

Expected behaviour

The Pro to Max upgrade should generate a valid payment attempt, reach the issuer for authorisation if required, and update the subscription after successful payment.

Actual behaviour

The checkout returns "The payment failed". At least one issuer confirms no authorisation attempt reached them, suggesting the failure happens inside the Anthropic/Stripe checkout or subscription update flow before issuer authorisation.