Make default checkpoint out_dir name reflect only non-default hyperparams

Previously the same fixed 7 fields (mode/hidden_dim/n_blocks/emb_dim/
conditioning/lr/batch_size) were always baked into the name, even for a
vanilla run, and router config wasn't represented at all. Now
default_out_dir_name only includes fields that differ from
DEFAULT_CONFIG, adds router/seed/epochs as candidates, and caps at 6
shown fields with a hashed overflow suffix for heavily-swept configs.

giant/cli.py

@@ -528,16 +528,16 @@ def train(
         # hyperparam baked into the name (e.g. --lr for a fine-tune).
         out_dir = resume.parent
     else:
-        out_dir = Path(
-            f"checkpoints/{datetime.now().strftime('%Y%m%d_%H%M%S')}"
-            f"_{t['mode']}"
-            f"_h{m['hidden_dim']}"
-            f"_b{m['n_blocks']}"
-            f"_e{m['emb_dim']}"
-            f"_c{m['conditioning']}"
-            f"_lr{t['lr']}"
-            f"_bs{t['batch_size']}"
-        )
+        # Name only encodes what's non-default (see default_out_dir_name), so
+        # two runs with identical hyperparams in the same to-the-minute
+        # timestamp would otherwise collide on this name — which also
+        # doubles as the W&B run id (giant.train) — hence the suffix loop.
+        base_name = gconfig.default_out_dir_name(cfg)
+        out_dir = Path("checkpoints") / base_name
+        suffix = 2
+        while out_dir.exists():
+            out_dir = Path("checkpoints") / f"{base_name}_{suffix}"
+            suffix += 1
 
     typer.echo(f"device:  {_device}")
     typer.echo(f"out_dir: {out_dir}")

giant/config.py

@@ -1,3 +1,4 @@
+import hashlib
 import random
 import subprocess
 import sys
@@ -284,6 +285,93 @@ def resolve_expert_dims(
     return expert_hidden_dim, expert_n_blocks
 
 
+_CONDITIONING_CODE = {"physical": "phys", "embedding": "emb"}
+
+
+# Priority-ordered candidate fields for default_out_dir_name: (label, getter,
+# formatter). `getter(train, model)` returns None when the field is at its
+# default (and so should be omitted); otherwise formatter(value) renders the
+# name token. The router is a single unit gated on `router.enabled` rather
+# than one candidate per router key, since its type/n_experts are meaningless
+# while disabled.
+def _mode_candidate(train, model):
+    return None if train["mode"] == DEFAULT_CONFIG["train"]["mode"] else train["mode"]
+
+
+def _router_candidate(train, model):
+    router = model["router"]
+    if router["enabled"] == DEFAULT_CONFIG["model"]["router"]["enabled"]:
+        return None
+    return f"r-{router['type']}{router['n_experts']}"
+
+
+def _conditioning_candidate(train, model):
+    if model["conditioning"] == DEFAULT_CONFIG["model"]["conditioning"]:
+        return None
+    code = _CONDITIONING_CODE.get(model["conditioning"], model["conditioning"])
+    return f"c{code}"
+
+
+def _default_field_candidate(section_key, field, prefix):
+    def _candidate(train, model):
+        section = train if section_key == "train" else model
+        value = section[field]
+        if value == DEFAULT_CONFIG[section_key][field]:
+            return None
+        return f"{prefix}{value}"
+
+    return _candidate
+
+
+_OUT_DIR_NAME_CANDIDATES = [
+    ("mode", _mode_candidate),
+    ("router", _router_candidate),
+    ("conditioning", _conditioning_candidate),
+    ("hidden_dim", _default_field_candidate("model", "hidden_dim", "h")),
+    ("n_blocks", _default_field_candidate("model", "n_blocks", "b")),
+    ("emb_dim", _default_field_candidate("model", "emb_dim", "e")),
+    ("lr", _default_field_candidate("train", "lr", "lr")),
+    ("batch_size", _default_field_candidate("train", "batch_size", "bs")),
+    ("seed", _default_field_candidate("train", "seed", "seed")),
+    ("epochs", _default_field_candidate("train", "epochs", "ep")),
+]
+
+_OUT_DIR_NAME_MAX_FIELDS = 6
+
+
+def default_out_dir_name(cfg: dict, now: datetime | None = None) -> str:
+    """Build a default checkpoint out_dir name from what's non-default in `cfg`.
+
+    Only fields that differ from DEFAULT_CONFIG are included, so a fully
+    default run's name is just its timestamp — see
+    `_OUT_DIR_NAME_CANDIDATES` for the fixed, priority-ordered field list.
+    Beyond `_OUT_DIR_NAME_MAX_FIELDS` non-default fields, the remainder
+    collapse into a short deterministic hash suffix rather than growing the
+    name unboundedly. This name doubles as the run's W&B id (see
+    giant.train), which is the reason a timestamp is always included.
+    """
+    now = now or datetime.now()
+    train, model = cfg["train"], cfg["model"]
+    tokens = []
+    overflow = []
+    for label, candidate in _OUT_DIR_NAME_CANDIDATES:
+        token = candidate(train, model)
+        if token is None:
+            continue
+        if len(tokens) < _OUT_DIR_NAME_MAX_FIELDS:
+            tokens.append(token)
+        else:
+            overflow.append(f"{label}={token}")
+
+    name = now.strftime("%Y%m%d_%H%M")
+    if tokens:
+        name += "_" + "_".join(tokens)
+    if overflow:
+        digest = hashlib.md5("|".join(sorted(overflow)).encode()).hexdigest()[:6]
+        name += f"_+{len(overflow)}more-{digest}"
+    return name
+
+
 def seed_everything(seed: int) -> None:
     random.seed(seed)
     np.random.seed(seed)

tests/test_config.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+
 from giant import config as gconfig
 
 
@@ -144,4 +146,101 @@ def test_resolve_expert_dims_explicit_override_wins():
 def test_resolve_expert_dims_partial_override_mixes_explicit_and_inherited():
     router_cfg = {"expert_hidden_dim": 128, "expert_n_blocks": 0}
     hidden_dim, n_blocks = gconfig.resolve_expert_dims(router_cfg, 512, 6)
-    assert (hidden_dim, n_blocks) == (128, 6)  # n_blocks inherited, hidden_dim not
+    assert (hidden_dim, n_blocks) == (128, 6)
+
+
+def _default_cfg(**overrides):
+    train_overrides = {
+        k: v for k, v in overrides.items() if k in gconfig.DEFAULT_CONFIG["train"]
+    }
+    model_overrides = {
+        k: v for k, v in overrides.items() if k in gconfig.DEFAULT_CONFIG["model"]
+    }
+    router_overrides = overrides.get("router")
+    if router_overrides:
+        model_overrides["router"] = router_overrides
+    return gconfig.merge_cli_overrides(
+        gconfig.DEFAULT_CONFIG, None, train_overrides, model_overrides
+    )
+
+
+_NOW = datetime(2026, 7, 29, 14, 30)
+
+
+def test_default_out_dir_name_all_defaults_is_just_the_timestamp():
+    cfg = _default_cfg()
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_single_non_default_field():
+    cfg = _default_cfg(hidden_dim=512)
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_h512"
+
+
+def test_default_out_dir_name_conditioning_embedding_shown_abbreviated():
+    cfg = _default_cfg(conditioning="embedding")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_cemb"
+
+
+def test_default_out_dir_name_conditioning_default_omitted():
+    cfg = _default_cfg(conditioning="physical")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_router_enabled_shown_as_unit():
+    cfg = _default_cfg(router={"enabled": True, "type": "energy", "n_experts": 8})
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_r-energy8"
+
+
+def test_default_out_dir_name_router_disabled_omitted_even_if_subfields_nondefault():
+    cfg = _default_cfg(router={"enabled": False, "type": "pdg", "n_experts": 8})
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_mode_shown_bare_no_prefix():
+    cfg = _default_cfg(mode="wgan")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_wgan"
+
+
+def test_default_out_dir_name_overflow_caps_and_hashes_remainder():
+    cfg = _default_cfg(
+        mode="wgan",
+        router={"enabled": True, "type": "energy", "n_experts": 8},
+        conditioning="embedding",
+        hidden_dim=512,
+        n_blocks=8,
+        emb_dim=32,
+        lr=1e-3,
+        batch_size=2048,
+        seed=3,
+        epochs=200,
+    )
+    name = gconfig.default_out_dir_name(cfg, now=_NOW)
+    # First 6 by priority: mode, router, conditioning, hidden_dim, n_blocks, emb_dim.
+    assert name.startswith("20260729_1430_wgan_r-energy8_cemb_h512_b8_e32_+4more-")
+    digest = name.split("-")[-1]
+    assert len(digest) == 6
+
+
+def test_default_out_dir_name_overflow_hash_is_deterministic_and_value_sensitive():
+    base = dict(
+        mode="wgan",
+        router={"enabled": True, "type": "energy", "n_experts": 8},
+        conditioning="embedding",
+        hidden_dim=512,
+        n_blocks=8,
+        emb_dim=32,
+        lr=1e-3,
+        batch_size=2048,
+        seed=3,
+        epochs=200,
+    )
+    name_a = gconfig.default_out_dir_name(_default_cfg(**base), now=_NOW)
+    name_b = gconfig.default_out_dir_name(_default_cfg(**base), now=_NOW)
+    assert name_a == name_b  # stable across calls with the same overflow set
+
+    changed = dict(base, epochs=999)
+    name_c = gconfig.default_out_dir_name(_default_cfg(**changed), now=_NOW)
+    assert (
+        name_c != name_a
+    )  # differs when an overflowed value changes  # n_blocks inherited, hidden_dim not