Tool use error stops new commands from being registered

Claude was working fine and then hung on a simple command. I hit Esc and typed a new command, and got this message:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.204: Did not find 1 tool_result block(s) at the beginning of

this message. Messages following tool_use blocks must begin with a matching number of tool_result blocks."}}

Now it responds with this error no matter what I type. Running /doctor reported no issues.