fix(inference): guard agent sid header against unconnected room

[adrian-cowham]

Summary

Fixes a crash in get_inference_headers() introduced by #5937 and carried through #5943. STT/TTS/LLM inference connections crash with Exception: cannot access local participant before connecting when their websocket opens before ctx.connect() finishes.

Root cause

#5937 added ctx.agent.sid to the header builder. ctx.agent resolves to room.local_participant, which raises a bare Exception (not RuntimeError) while the room is unconnected. The surrounding handler only caught RuntimeError (the no-job-context case), so the exception escaped and killed the STT pump:

File ".../inference/_utils.py", line 59, in get_inference_headers
    if isinstance(agent_sid := ctx.agent.sid, str) and agent_sid:
File ".../job.py", line 425, in agent
    return self._room.local_participant
File ".../rtc/room.py", line 210, in local_participant
    raise Exception("cannot access local participant before connecting")

Before #5937 the function only read ctx.job.room.sid and ctx.job.id (both from job info, available pre-connect), so it never touched local_participant.

Fix

Guard the access with room.isconnected() — the codebase-standard readiness check (utils/participant.py, job.py):

if ctx.room.isconnected() and isinstance(agent_sid := ctx.agent.sid, str) and agent_sid:
    headers[HEADER_AGENT_ID] = agent_sid

rtc.Room._local_participant is assigned once on connect and never cleared (not on disconnect or reconnect), so local_participant raises only before the first connect — exactly when isconnected() is False. The guard is therefore reliable, not a leaky proxy. MockRoom already stubs isconnected() -> True, so console/IPC mode is unaffected.

The agent id is best-effort attribution (per #5937); room and job ids stay available pre-connect, so attribution falls back to room/job level rather than crashing.

Testing

New tests/test_inference_utils.py (5 explicit cases): unconnected-room (the regression), connected, non-str sid (#5943 guard), empty sid, and no-job-context. The regression test fails on the pre-fix code with the exact reported error and passes with the fix.

• ruff format --check + ruff check: clean
• mypy -p livekit.agents: no new errors in inference/
• Inference suite: 65 passed

Note

For a websocket opened before ctx.connect(), the agent id stays omitted for that connection's lifetime (headers are fixed at open time). That's by design for a best-effort attribution header, not a defect.

🤖 Generated with Claude Code

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.