[FEATURE] Allow Claude Pro/Max subscription auth to be used from third-party apps (not just Claude Code / claude.ai)

Problem Statement

Anthropic's Consumer ToS currently restricts OAuth tokens from Free/Pro/Max subscriptions to first-party surfaces only (Claude Code and claude.ai). Related prior requests on this were closed:

• #18340 (closed as duplicate)
• #1454 (closed as Not Planned)

I want to raise this again with a concrete, current product scenario rather than an abstract "please open this up" ask.

Use Case Example

Heptabase (a note-taking app) recently shipped an in-app AI chat feature where the message composer lets you choose which "engine" pays for the message: Heptabase's own credits, your own API key, or — notably — your own ChatGPT Codex plan subscription. When you pick the Codex option, the conversation runs against your personal ChatGPT/Codex quota (not billed per-token), and keeps its own memory/session state inside Heptabase.

When asked whether the same could be done with a Claude subscription, Heptabase's founder confirmed this isn't possible today, specifically because Claude's Pro/Max OAuth credential is restricted to Claude Code and claude.ai per Anthropic's terms — unlike OpenAI, which apparently permits this kind of bring-your-own-subscription usage from third-party apps for Codex.

Proposed Solution

Some option that lets a subscriber authorize a specific, user-approved third-party app to spend against their own Pro/Max quota (with clear rate limits matching their tier), rather than forcing pay-per-token API billing for any non-Anthropic surface. This could be a narrower, explicitly-consented variant of OAuth (e.g. a scoped token type distinct from the current Claude Code/claude.ai one) rather than reopening the same broad OAuth token for all third parties.

Alternative Solutions

Continuing to require API-key billing for any third-party integration is the current state — this issue is asking Anthropic to reconsider that boundary given real user demand (as seen in #18340, #1454, and this Heptabase example), not to bypass it via workarounds.

Priority

Medium - Would be very helpful

Additional Context

This is a policy/product question (Anthropic's own ToS boundary), not a technical limitation — claude setup-token shows the infrastructure to mint subscription-scoped tokens already exists.