IANA says Session Director, but on a developer's machine this is almost always the Karma browser test runner.
The registration is sd, "Session Director", from Van Jacobson's MBone-era session-announcement work, which you will effectively never meet today. What you will meet is Karma: its default port config value is 9876, and it runs a web server that browsers connect to in order to execute a project's unit tests. Karma was deprecated by its maintainers in favour of Web Test Runner, Jest, and Vitest, but it is still present in many Angular and older JavaScript projects, so a listener on 9876 usually means a test run in progress or a CI container that did not shut down.
Do not expose
Karma exists to load arbitrary project JavaScript into a connected browser. Exposing it hands strangers a code-execution surface in whatever browser reaches it; keep it bound to localhost.
$ curl -sI http://localhost:9876/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.