facebookresearch/detectron2 · error · ValueError

Unknown LR scheduler: {}

Error message

Unknown LR scheduler: {}

What it means

build_lr_scheduler only supports a fixed set of scheduler names (WarmupMultiStepLR, WarmupCosineLR, WarmupPolyLR, and fvcore-based composite names). Any other cfg.SOLVER.LR_SCHEDULER_NAME string is rejected.

Source

Thrown at detectron2/solver/build.py:314

            )
        sched = MultiStepParamScheduler(
            values=[cfg.SOLVER.GAMMA**k for k in range(len(steps) + 1)],
            milestones=steps,
            num_updates=cfg.SOLVER.MAX_ITER,
        )
    elif name == "WarmupCosineLR":
        end_value = cfg.SOLVER.BASE_LR_END / cfg.SOLVER.BASE_LR
        assert end_value >= 0.0 and end_value <= 1.0, end_value
        sched = CosineParamScheduler(1, end_value)
    elif name == "WarmupStepWithFixedGammaLR":
        sched = StepWithFixedGammaParamScheduler(
            base_value=1.0,
            gamma=cfg.SOLVER.GAMMA,
            num_decays=cfg.SOLVER.NUM_DECAYS,
            num_updates=cfg.SOLVER.MAX_ITER,
        )
    else:
        raise ValueError("Unknown LR scheduler: {}".format(name))

    sched = WarmupParamScheduler(
        sched,
        cfg.SOLVER.WARMUP_FACTOR,
        min(cfg.SOLVER.WARMUP_ITERS / cfg.SOLVER.MAX_ITER, 1.0),
        cfg.SOLVER.WARMUP_METHOD,
        cfg.SOLVER.RESCALE_INTERVAL,
    )
    return LRMultiplier(optimizer, multiplier=sched, max_iter=cfg.SOLVER.MAX_ITER)

View on GitHub (pinned to a2f4a8771a)

Solutions

1. Use one of: 'WarmupMultiStepLR', 'WarmupCosineLR', 'WarmupPolyLR', or an fvcore composite like 'WarmupCosineLR' variants / 'MultiStepLR'
2. Check the spelling against detectron2/solver/build.py's if/elif chain
3. Implement a custom scheduler and register it in your own build path

Example fix

# before
cfg.SOLVER.LR_SCHEDULER_NAME = 'cosine'
# after
cfg.SOLVER.LR_SCHEDULER_NAME = 'WarmupCosineLR'

supported = {'WarmupMultiStepLR', 'WarmupCosineLR', 'WarmupPolyLR'}
assert cfg.SOLVER.LR_SCHEDULER_NAME in supported or '+' in cfg.SOLVER.LR_SCHEDULER_NAME

When it happens

Trigger: Setting cfg.SOLVER.LR_SCHEDULER_NAME to an unsupported/misspelled value such as 'cosine', 'step', or a custom name, then calling build_lr_scheduler(cfg, optimizer).

Common situations: Porting configs from other frameworks (mmdetection, torchvision) whose scheduler names differ; typos like 'WarmupCosine' without the LR suffix.

Related errors

• Unknown warmup method: {}
• Milestones should be a list of increasing integers. Got {}
• Class with @configurable must have a 'from_config' classmeth
• {name} must take 'cfg' as the first argument!
• total_batch_size and single_gpu_batch_size are mutually inco

AI-assisted analysis of facebookresearch/detectron2@a2f4a8771a (2026-08-27). Data as JSON: /api/errors/b46c6a2a88326182. Report an issue: GitHub.