[BUG] MCP OAuth redirect_uri hardcodes the `localhost` hostname — breaks IdPs that allowlist only `127.0.0.1`

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The redirect_uri Claude Code sends for MCP server OAuth is hardcoded to the localhost hostname. Only the port is configurable (MCP_OAUTH_CALLBACK_PORT) — there is no way to make it use 127.0.0.1.

This makes remote MCP servers unusable behind an IdP that allowlists loopback by IP. In my case the Atlassian MCP server (https://mcp.atlassian.com/v1/mcp/authv2): my org's Atlassian admin has domain restrictions enabled, localhost is not on the allowlist, and the browser is stopped with "Access to this domain is restricted — Your admin has blocked this domain http://localhost:3118/callback in your organization's settings" before the authorization code is ever handed back. The exact same Atlassian MCP server authenticates fine from GitHub Copilot, which uses http://127.0.0.1:<port>/callback.

Digging into the shipped bundle (v2.1.220), the MCP callback URL builder is:

function sBe(e = oTs) { return `http://localhost:${e}/callback` }   // oTs = 3118 (default port)
function ES_() { let e = Bd(process.env.MCP_OAUTH_CALLBACK_PORT); return e > 0 && e <= 65535 ? e : void 0 }

Notably the local callback listener already binds to the IP, not to the hostname:

n.listen(e, "127.0.0.1", () => { ... })

So only the string advertised as redirect_uri says localhost; the receiving socket is already 127.0.0.1. Switching the emitted URL to 127.0.0.1 should be a one-token change with no behavioral impact on the listener.

This is also what RFC 8252 §8.3 recommends — it explicitly says native apps should use the loopback IP literal rather than localhost, since localhost may resolve to a non-loopback interface and requires the host to be configured correctly.

Related prior reports, both auto-closed by the stale bot with "please open a new issue if this is still relevant": #56557 (same request, for the CIMD redirect_uris) and #63030 (same class of failure against Entra/Azure, where the app registration only has http://127.0.0.1/callback).

What Should Happen?

Claude Code should emit http://127.0.0.1:<port>/callback as the MCP OAuth redirect_uri — or, at minimum, expose an override (e.g. MCP_OAUTH_CALLBACK_HOST, mirroring the existing MCP_OAUTH_CALLBACK_PORT) so users behind an IdP that only allows the IP literal can authenticate.

Error Messages/Logs

# Shown by Atlassian in the browser, instead of redirecting back to Claude Code:
Access to this domain is restricted
Your admin has blocked this domain "http://localhost:3118/callback" in your organization's settings.
To request access, contact your organization admin or find out about domain restrictions.

Steps to Reproduce

1. Have an Atlassian (or Entra/Okta) org where the admin's domain allowlist does not include localhost but does include 127.0.0.1.
2. Add the remote MCP server:

``
claude mcp add --transport http atlassian-mcp https://mcp.atlassian.com/v1/mcp/authv2
``

1. Run /mcp and start authentication for atlassian-mcp.
2. Complete the Atlassian login in the browser.
3. Instead of redirecting back to the local callback listener, the browser lands on the "Access to this domain is restricted" page naming http://localhost:3118/callback, and the OAuth flow never completes.

Confirmation that the hostname is the only difference: the identical MCP server authenticates successfully from GitHub Copilot in the same browser/org, and its redirect_uri uses 127.0.0.1.

Workaround I'm currently using: byte-patching http://localhost:${e}/callback → http://127.0.0.1:${e}/callback in the installed binary (same byte length, single occurrence, does not touch the Anthropic account login flow which uses separate literals) and re-signing it ad-hoc. That obviously does not survive an auto-update.

Claude Model

Opus

Is this a regression?

No, this never worked

Claude Code Version

2.1.220 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

macOS 15 (Darwin 24.6.0), arm64, installed via npm (@anthropic-ai/claude-code).

The suggested fix touches one string literal. The MCP account-login flow for the Anthropic account itself (MANUAL_REDIRECT_URL / the console flow) uses different literals and would be unaffected.