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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.231

MCP OAuth callback now redirects to localhost rather than 127.0.0.1

You'll notice
Useful3 Signal2
MCP

MCP OAuth logins now send you back via localhost, so servers only allowing the IP form will reject you.

What

When Claude Code logs in to an MCP server over OAuth, the default address it hands the server to send you back to is now http://localhost:<port>/callback instead of http://127.0.0.1:<port>/callback. If a server has the old IP form registered as its only allowed redirect, that login will now be rejected until the localhost form is added.

Details
  • The port is taken from the server's oauth.callbackPort setting, falling back to Claude Code's own MCP OAuth callback port.
  • Configuring an explicit redirectUri for a server still overrides this entirely, and that path logs "(no localhost listener)" since Claude Code is not the one receiving the redirect.
  • This aligns the default with two other places in the same code that already built the localhost form, so all three now agree.
Evidence

/callback

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.231 →