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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 v2.1.229 12 Aug 2026

MCP sign-in callback now uses 127.0.0.1

You'll notice

Useful4 Signal2

MCP

MCP browser sign-in now redirects to 127.0.0.1, fixing failures on IPv6-only localhost setups.

What

When an MCP server signs you in through your browser, the local address it redirects back to is now http://127.0.0.1 rather than http://localhost. This fixes sign-in on machines where localhost resolves to the IPv6 address ::1 and nothing is listening there.

Details

Servers you have already signed into keep working: the check that reuses a previously registered redirect address now accepts either origin, where it used to demand http://localhost exactly.
The port is still chosen at runtime and filled into the address.

Evidence

t === "http://127.0.0.1" || t === "http://localhost"

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

See this entry in the whole of v2.1.229 →