Lightning-AI/pytorch-lightning · error · ValueError

f"`Trainer(barebones=True, fast_dev_run={fast_dev_run!r})` w

Error message

f"`Trainer(barebones=True, fast_dev_run={fast_dev_run!r})` was passed." " Development run is not meant for raw speed evaluation so it is disabled in barebones mode."

What it means

Trainer was created with barebones=True while fast_dev_run was enabled. A development run (1 batch, 1 epoch) is not meant for raw speed evaluation, so barebones mode rejects the combination.

Source

Thrown at src/lightning/pytorch/trainer/trainer.py:375

                    f"`Trainer(barebones=True, log_every_n_steps={log_every_n_steps!r})` was passed."
                    " Logging can impact raw speed so it is disabled in barebones mode."
                )
            log_every_n_steps = 0
            if enable_model_summary:
                raise ValueError(
                    f"`Trainer(barebones=True, enable_model_summary={enable_model_summary!r})` was passed."
                    " Model summary can impact raw speed so it is disabled in barebones mode."
                )
            enable_model_summary = False
            if num_sanity_val_steps is not None and num_sanity_val_steps != 0:
                raise ValueError(
                    f"`Trainer(barebones=True, num_sanity_val_steps={num_sanity_val_steps!r})` was passed."
                    " Sanity checking can impact raw speed so it is disabled in barebones mode."
                )
            num_sanity_val_steps = 0
            # opt-ins
            if fast_dev_run is not False and fast_dev_run != 0:
                raise ValueError(
                    f"`Trainer(barebones=True, fast_dev_run={fast_dev_run!r})` was passed."
                    " Development run is not meant for raw speed evaluation so it is disabled in barebones mode."
                )
            if detect_anomaly:
                raise ValueError(
                    f"`Trainer(barebones=True, detect_anomaly={detect_anomaly!r})` was passed."
                    " Anomaly detection can impact raw speed so it is disabled in barebones mode."
                )
            if profiler is not None:
                raise ValueError(
                    f"`Trainer(barebones=True, profiler={profiler!r})` was passed."
                    " Profiling can impact raw speed so it is disabled in barebones mode."
                )
            deactivated = (
                " - Checkpointing: `Trainer(enable_checkpointing=True)`",
                " - Progress bar: `Trainer(enable_progress_bar=True)`",
                " - Model summary: `Trainer(enable_model_summary=True)`",
                " - Logging: `Trainer(logger=True)`, `Trainer(log_every_n_steps>0)`,"

View on GitHub (pinned to 9fed5c27d2)

Solutions

Remove fast_dev_run when using barebones=True
Set fast_dev_run=False
Remove barebones=True if you actually want a dev run

Example fix

// before
trainer = Trainer(barebones=True, fast_dev_run=True)
// after
trainer = Trainer(barebones=True)

Defensive patterns

Strategy: validation

Validate before calling

if cfg.get("barebones") and cfg.get("fast_dev_run") not in (False, 0, None):
    cfg["fast_dev_run"] = False
trainer = Trainer(**cfg)

Prevention

Never combine debugging flags (fast_dev_run, detect_anomaly) with benchmarking flags (barebones)
Use fast_dev_run only in smoke tests, not benchmark scripts

When it happens

Trigger: Trainer(barebones=True, fast_dev_run=True) or fast_dev_run=<int> (anything other than False/0).

Common situations: Leaving fast_dev_run=True from a debugging session while also enabling barebones=True for benchmarking.

Related errors

AI-assisted analysis of Lightning-AI/pytorch-lightning@9fed5c27d2 (2026-08-28). Data as JSON: /api/errors/89c1bcc4828cefd4. Report an issue: GitHub.