GitHub Actions "The runner has not received any logs" (output flushing)

When a process buffers stdout/stderr heavily or the runner loses contact mid-step, the live log can appear empty even though work is happening. The job can fail or stall waiting on logs.

What this error means

A long step shows no live output and the run reports the runner has not received any logs, sometimes ending in a lost-communication failure.

github-actions

The runner has not received any logs from the job for some time and may have lost communication with the server.

Common causes

Heavily buffered output

A child process buffers stdout so nothing streams until it flushes or exits.

Transient runner/network blip

A momentary loss of contact between the runner and the service interrupts log delivery.

How to fix it

Flush output and harden the step

Disable buffering where possible (for example stdbuf -oL, PYTHONUNBUFFERED=1).
Emit periodic progress so the log stream stays active.
Split very long steps so progress is visible and recoverable.

.github/workflows/ci.yml

- run: |
    export PYTHONUNBUFFERED=1
    stdbuf -oL ./long_task.sh

How to prevent it

Stream output line-buffered for long-running commands.
On Latchkey managed runners, transient log-delivery and runner-communication blips are auto-retried and the job is re-served from a warm pool, so a momentary network glitch does not become a red build.