Python Server(port=0) reports an unusable port-zero URL

[promiseeuler]

Describe the bug
While using the released Python SDK, I found that rr.server.Server(host="127.0.0.1", port=0) successfully binds an OS-assigned ephemeral port, but server.url() reports rerun+http://127.0.0.1:0. The returned URL therefore cannot be used to connect to the running server.

To Reproduce

1. Install rerun-sdk==0.35.0.
2. Run:

import rerun as rr

with rr.server.Server(host="127.0.0.1", port=0) as server:
    print(server.url())

3. Observe:

rerun+http://127.0.0.1:0

The server is actually listening on a nonzero OS-assigned port.

Expected behavior
server.url() should contain the actual bound port so callers can connect to the server.

Screenshots
Not applicable.

Backtrace
Not applicable; this does not crash.

Desktop (please complete the following information):

• OS: macOS 26.4

Rerun version
rerun-sdk==0.35.0

Additional context
I searched the open issues for port=0, 127.0.0.1:0, and Python server ephemeral-port reports and did not find an exact duplicate. The current Python binding constructs the URL from the requested port before the server starts, rather than from the server handle’s bound address.

Agent

🤖 This issue was opened using an LLM-assisted workflow. I manually reproduced the behavior with the released SDK, checked for duplicates, and reviewed the current binding path.