What
Some MCP servers, separate programs that give Claude Code extra tools, ask you to sign in through OAuth, a standard way of granting access through a web page. That page ends by sending you to a callback link, which can be pasted back so sign-in can finish.
When sign-in fails at that point, Claude Code now writes the full error to its log. What it sends back to the program that asked, such as an app built on the Claude Agent SDK, now depends on a check:
- When the check applies, the reply is only "OAuth authentication failed".
- Otherwise, the reply is the full error text.
Before, the error always went through a step that removed sensitive parts before it was sent.
Why
Programs that drive Claude Code may now see either a short generic message or the full error, and the full detail is always in the log.