diff --git a/flopy4/mf6/converter/ingress/structure.py b/flopy4/mf6/converter/ingress/structure.py
index f3708c0e..01cd70a8 100644
--- a/flopy4/mf6/converter/ingress/structure.py
+++ b/flopy4/mf6/converter/ingress/structure.py
@@ -18,6 +18,7 @@
 from flopy4.mf6.item import (
     Item,
     RowContext,
+    conditional_fields,
     dim_lookup,
     item_list_type,
     item_list_words,
@@ -55,6 +56,7 @@ def _parse_rows(
     context: LoadContext,
     sizes: "dict[str, int] | None" = None,
     boundnames: bool = False,
+    enabled: frozenset[str] = frozenset(),
     union_arm: str = "cellid",
     union_forms: Callable[[Mapping[str, Any]], Any] | None = None,
     load_child: Callable[[Any, str], Any] | None = None,
@@ -69,7 +71,9 @@ def _parse_rows(
     ncelldim = context.ncelldim(classes[0]) if len(classes) == 1 else None
     if ncelldim is None:
         ncelldim = ncelldim_from_dims(dict(context.dims))
-    row_context = RowContext(ncelldim=ncelldim, sizes=sizes or {}, boundnames=boundnames)
+    row_context = RowContext(
+        ncelldim=ncelldim, sizes=sizes or {}, boundnames=boundnames, enabled=enabled
+    )
     if isinstance(item_cls, tuple):
         return parse_union_items(
             rows, item_cls, words=words, context=row_context, load_child=load_child
@@ -1060,6 +1064,9 @@ def structure_component(
             if (n := dim_value(shape, lookup=lambda name: dim_lookup(kwargs, name))) is not None:
                 sizes[shape] = n
     boundnames = bool(kwargs.get("boundnames", False))
+    enabled = frozenset(
+        o for ic in item_types for o in conditional_fields(ic).values() if kwargs.get(o)
+    )
 
     # The grid's dims give the cellids' width (see LoadContext.ncelldim).
     # Same "self dims, since a DimensionProvider has none threaded to it
@@ -1091,6 +1098,7 @@ def load_child(f: Any, token: str) -> Any:
             item_cls,
             sizes=sizes,
             boundnames=boundnames,
+            enabled=enabled,
             context=rows_context,
             union_arm=union_arm,
             union_forms=union_forms,
@@ -1116,6 +1124,7 @@ def load_child(f: Any, token: str) -> Any:
                 item_cls,
                 sizes=sizes,
                 boundnames=boundnames,
+                enabled=enabled,
                 context=rows_context,
                 union_arm=union_arm,
                 union_forms=union_forms,
@@ -1152,6 +1161,7 @@ def load_child(f: Any, token: str) -> Any:
                     period_item_cls,
                     sizes=sizes,
                     boundnames=boundnames,
+                    enabled=enabled,
                     context=rows_context,
                     union_arm=union_arm,
                     union_forms=union_forms,
diff --git a/flopy4/mf6/gwf/evt.py b/flopy4/mf6/gwf/evt.py
index e9b8c89f..00dc53f9 100644
--- a/flopy4/mf6/gwf/evt.py
+++ b/flopy4/mf6/gwf/evt.py
@@ -36,7 +36,7 @@ class StressPeriodData(Item):
             default=(), shape=("nseg-1",), time_series=True, optional=True
         )
         petm0: Optional[Union[float, TimeSeriesRef]] = double(
-            default=None, time_series=True, optional=True
+            default=None, time_series=True, when="surf_rate_specified"
         )
         aux: tuple[Union[float, TimeSeriesRef], ...] = array(
             default=(), shape=("auxiliary",), time_series=True, optional=True
diff --git a/flopy4/mf6/item.py b/flopy4/mf6/item.py
index 346667c2..62978165 100644
--- a/flopy4/mf6/item.py
+++ b/flopy4/mf6/item.py
@@ -127,6 +127,23 @@ def _has_boundname_field(cls: type) -> bool:
     return any(f.name == "boundname" for f in cast(type[Record], cls).fields())
 
 
+def conditional_fields(item_cls: "type[Item] | tuple[type[Item], ...]") -> dict[str, str]:
+    """An item class's fields an option turns on (the DFN's ``when``), by
+    name, to the option."""
+    classes = item_cls if isinstance(item_cls, tuple) else (item_cls,)
+    return {
+        f.name: option
+        for c in classes
+        for f in cast(type[Record], c).fields()
+        if (option := f.metadata.get("when"))
+    }
+
+
+def _may_be_absent(f: attrs.Attribute) -> bool:
+    """Whether a field may be missing from a row."""
+    return bool(f.metadata.get("optional") or f.metadata.get("when"))
+
+
 @lru_cache(maxsize=None)
 def _union_arms(
     cls: type, type_str: str
@@ -316,7 +333,7 @@ def solve(i: int, pos: int, form: "tuple[str, ...] | None") -> "tuple[int, list]
         if pos == len(tokens) or (form is not None and i == len(form)):
             # trailing optional columns may be absent, but not a form's
             ends = pos == len(tokens) and (i == len(form) if form else True)
-            absent = ends and fields[i].metadata.get("optional")
+            absent = ends and _may_be_absent(fields[i])
             return (0, [None] * (len(fields) - i)) if absent else None
         best = None
         kinds = fields[i].metadata["union"] if form is None else (form[i],)
@@ -374,11 +391,13 @@ class RowContext:
     exchange's models may be on different grids); None if not known.
     ``sizes`` gives the counts of the inline arrays the package sizes, by
     shape expression (aux, by ``auxiliary``); one not given isn't known.
-    ``boundnames`` is the package's option."""
+    ``boundnames`` is the package's option. ``enabled`` are the set
+    options that turn fields on; None if not known."""
 
     ncelldim: int | Mapping[str, int] | None = None
     sizes: Mapping[str, int] = attrs.field(factory=dict)
     boundnames: bool = False
+    enabled: frozenset[str] | None = None
 
     def cellid_width(self, f: attrs.Attribute) -> int | None:
         if isinstance(self.ncelldim, Mapping):
@@ -482,8 +501,9 @@ def _layout(
     The boundname is there if the package's ``boundnames`` option is set
     (and a value is left for it). For Python values with it unset, one
     value more than the row's fields take is the boundname, and the
-    package then sets the option. An optional untagged field (EVT's petm0)
-    is there if the values make room for it. A width that isn't known (no
+    package then sets the option. A field an option turns on (EVT's petm0)
+    is there if the option is enabled, or, if that isn't known, if the
+    values make room for it. A width that isn't known (no
     grid yet) is the one the values make room for, if only one does;
     otherwise this raises, asking for the field as a tuple. Like MF6, a
     file row's extra tokens are ignored, if no layout uses them all.
@@ -491,6 +511,13 @@ def _layout(
     n = len(values)
     slots = _slots(cls, file)
     has_boundname = _has_boundname_field(cls)
+    gated = conditional_fields(cls)
+    if context.enabled is None:
+        off: frozenset[str] = frozenset()
+        on: frozenset[str] = frozenset()
+    else:
+        off = frozenset(name for name, o in gated.items() if o not in context.enabled)
+        on = frozenset(gated) - off
 
     def cellid_widths(f: attrs.Attribute, unknown: dict) -> list[tuple[int, dict]]:
         if (w := context.cellid_width(f)) is not None:
@@ -504,13 +531,19 @@ def flat(span: list) -> bool:
         """Whether values are a field's values, not whole fields."""
         return file or not any(_is_seq(x) or isinstance(x, Record) for x in span)
 
-    def options(slot: _Slot, pos: int, end: int, out: dict, unknown: dict):
+    def fits(slot: _Slot, pos: int, end: int, out: dict, unknown: dict, conditional: bool):
         """Each way the slot can take values from ``pos``: the position
         after it, what it took, and the unknown widths it assumed."""
         f = slot.field
         left = end - pos
         v = values[pos] if left else None
-        optional = f is not None and f.metadata.get("optional")
+        optional = f is not None and _may_be_absent(f)
+        if conditional and f is not None:
+            if f.name in off:
+                yield pos, None, unknown
+                return
+            if f.name in on:
+                optional = bool(f.metadata.get("optional"))
         if slot.kind == "keyword":
             if left and str(v).upper() == cast(type[Record], cls).keyword().upper():
                 yield pos + 1, None, unknown
@@ -527,7 +560,7 @@ def options(slot: _Slot, pos: int, end: int, out: dict, unknown: dict):
             if file:
                 yield end, ("unions", values[pos:end], slot.group), unknown
                 return
-            required = sum(1 for g in slot.group if not g.metadata.get("optional"))
+            required = sum(1 for g in slot.group if not _may_be_absent(g))
             for m in range(min(len(slot.group), left), required - 1, -1):
                 yield pos + m, ("unions", values[pos : pos + m], slot.group), unknown
             return
@@ -584,10 +617,12 @@ def options(slot: _Slot, pos: int, end: int, out: dict, unknown: dict):
     together = [
         s.field.name
         for s in slots
-        if cast(type[Item], cls)._columns and s.kind == "value" and s.field.metadata.get("optional")
+        if cast(type[Item], cls)._columns and s.kind == "value" and _may_be_absent(s.field)
     ]
 
-    def walk(k: int, pos: int, end: int, out: dict, unknown: dict, surplus: bool):
+    def walk(
+        k: int, pos: int, end: int, out: dict, unknown: dict, surplus: bool, conditional: bool
+    ):
         if k == len(slots):
             if len({name in out for name in together}) > 1:
                 return
@@ -595,16 +630,16 @@ def walk(k: int, pos: int, end: int, out: dict, unknown: dict, surplus: bool):
                 yield out, unknown
             return
         slot = slots[k]
-        for pos2, took, unknown2 in options(slot, pos, end, out, unknown):
+        for pos2, took, unknown2 in fits(slot, pos, end, out, unknown, conditional):
             out2 = out if took is None else {**out, slot.field.name: took}
-            yield from walk(k + 1, pos2, end, out2, unknown2, surplus)
+            yield from walk(k + 1, pos2, end, out2, unknown2, surplus, conditional)
 
-    def solve(boundname: list[bool], surplus: bool = False):
+    def solve(boundname: list[bool], surplus: bool = False, conditional: bool = True):
         for bn in boundname:
             end = n - bn
             if end < 0:
                 continue
-            for out, unknown in walk(0, 0, end, {}, {}, surplus):
+            for out, unknown in walk(0, 0, end, {}, {}, surplus, conditional):
                 yield ({**out, "boundname": ("bn", values[-1])} if bn else out), unknown
 
     fields = attrs.fields_dict(cls)  # type: ignore[arg-type]
@@ -612,9 +647,7 @@ def solve(boundname: list[bool], surplus: bool = False):
     def optionals(out: dict) -> int:
         """How many optional fields a layout has."""
         return sum(
-            1
-            for k in out
-            if k == "boundname" or (k in fields and fields[k].metadata.get("optional"))
+            1 for k in out if k == "boundname" or (k in fields and _may_be_absent(fields[k]))
         )
 
     def first(solutions) -> "dict | None":
@@ -636,8 +669,10 @@ def first(solutions) -> "dict | None":
 
     bn = [True, False] if has_boundname and context.boundnames else [False]
     out = first(solve(bn))
+    if out is None and not file and (off or on):
+        out = first(solve(bn, conditional=False))
     if out is None and not file and has_boundname and not context.boundnames:
-        out = first(solve([True]))
+        out = first(solve([True])) or first(solve([True], conditional=False))
     if out is None and file:
         out = first(solve(bn, surplus=True))
     if out is None:
@@ -784,7 +819,7 @@ def to_tokens(self, *, ncelldim: int | Mapping[str, int] | None = None) -> tuple
                     )
                 val = n
             if val is None and not (_is_one_cellid(f) and f.metadata.get("allow_zero")):
-                if _is_one_cellid(f) and not f.metadata.get("optional"):
+                if _is_one_cellid(f) and not _may_be_absent(f):
                     raise ValueError(f"{cls.__name__}.{f.name}: no cellid given")
                 continue
             if not keyword_emitted and not _is_row_key(f):
diff --git a/flopy4/mf6/package.py b/flopy4/mf6/package.py
index 584e285a..f6031d52 100644
--- a/flopy4/mf6/package.py
+++ b/flopy4/mf6/package.py
@@ -1,4 +1,6 @@
 import operator
+import re
+import warnings
 from abc import ABC
 from pathlib import Path
 from typing import Any, Optional
@@ -18,6 +20,7 @@
 from flopy4.mf6.item import (
     Item,
     RowContext,
+    conditional_fields,
     dim_lookup,
     inline_dim,
     item_list_type,
@@ -60,6 +63,15 @@
 }
 
 
+_NUMBER = re.compile(r"[+-]?(\d+\.?\d*|\.\d+)([ed][+-]?\d+)?", re.IGNORECASE)
+
+
+def _reads_as_number(text: str) -> bool:
+    """Whether MF6 could read a word as a number, with an E or a Fortran D
+    exponent."""
+    return _NUMBER.fullmatch(text) is not None
+
+
 def _check_rows(dim: str, declared: int, nrows: int, bound: str | None) -> None:
     if not _BOUND_OPS[bound](nrows, declared):
         raise ValueError(
@@ -365,7 +377,7 @@ def _row_context(self, item_cls: "type[Item] | tuple[type[Item], ...]") -> RowCo
         """What the layout of a row of ``item_cls`` given in Python depends
         on (see `Item.from_values`): the grid's ncelldim, if known (an
         exchange's two grids aren't, here), the sizes of the inline arrays
-        the package sizes, and the boundnames option. A size the rows can
+        the package sizes, and its enabled options. A size the rows can
         set (EVT's nseg, GNC's numalphaj) is solved from them if unset;
         one they can't (aux's, by the names in ``auxiliary``) is 0."""
         fields = attrs.fields_dict(type(self))  # type: ignore[arg-type]
@@ -388,8 +400,12 @@ def lookup(name: str) -> Any:
                 ncelldim = self.resolve_dims("ncelldim").get("ncelldim")
             except ValueError:
                 pass
+        enabled = frozenset(o for o in conditional_fields(item_cls).values() if values.get(o))
         return RowContext(
-            ncelldim=ncelldim, sizes=sizes, boundnames=bool(getattr(self, "boundnames", False))
+            ncelldim=ncelldim,
+            sizes=sizes,
+            boundnames=bool(getattr(self, "boundnames", False)),
+            enabled=enabled,
         )
 
     def _coerce_item_list(
@@ -415,8 +431,8 @@ def _coerce_item_list(
         takes. Ambiguous columnar dict-of-lists input isn't supported (no
         single arm to build columns from).
 
-        A row with a boundname sets the package's boundnames option, which
-        MF6 needs to read it.
+        A row with a boundname, or a field an option turns on, sets that
+        option, which MF6 needs to read it.
         """
         context = self._row_context(item_cls)
         if isinstance(item_cls, tuple):
@@ -439,9 +455,17 @@ def _coerce_item_list(
                     items.append(item_cls(**row))
                 else:
                     items.append(item_cls.from_values(row, context))
-        if not context.boundnames and "boundnames" in attrs.fields_dict(type(self)):  # type: ignore[arg-type]
-            if any(getattr(item, "boundname", None) for item in items):
-                object.__setattr__(self, "boundnames", True)
+        on = {
+            option
+            for name, option in conditional_fields(item_cls).items()
+            if any(getattr(item, name, None) is not None for item in items)
+        }
+        if any(getattr(item, "boundname", None) for item in items):
+            on.add("boundnames")
+        fields = attrs.fields_dict(type(self))  # type: ignore[arg-type]
+        for option in on:
+            if option in fields and not getattr(self, option):
+                object.__setattr__(self, option, True)
         return items
 
     def _normalize_griddata(self, fields, dims: dict) -> None:
@@ -510,6 +534,7 @@ def write(self, format: str = MF6, context: Optional[WriteContext] = None) -> No
         self._sync_dims()
         self._check_tas_refs()
         self._check_ts_refs()
+        self._check_obs_ids()
         self._name_file_children()
         super().write(format=format, context=context)
 
@@ -549,6 +574,30 @@ def _check_ts_refs(self) -> None:
                                 f"defines (defined: {sorted(defined) or 'none'})"
                             )
 
+    def _check_obs_ids(self) -> None:
+        """Warn if an observation names a boundname MF6 may read as a cell."""
+        boundnames = set()
+        for f in attrs.fields(type(self)):  # type: ignore[arg-type]
+            if item_list_type(f.type) is None:
+                continue
+            value = getattr(self, f.name)
+            for rows in (value if isinstance(value, dict) else {None: value}).values():
+                boundnames |= {b for row in rows or [] if (b := getattr(row, "boundname", None))}
+        numeric = {b.lower() for b in boundnames if _reads_as_number(b)}
+        if not numeric:
+            return
+        for obs in self._children_of("utl-obs"):
+            for block in obs.continuous or []:
+                for row in block.continuous:
+                    for id_ in (row.id_, row.id2):
+                        if isinstance(id_, str) and id_.lower() in numeric:
+                            warnings.warn(
+                                f"{type(self).__name__}: observation {row.obsname!r} names "
+                                f"boundname {id_!r}, which MF6 may read as a cell, not a "
+                                f"boundname; give the boundaries a name that isn't a number",
+                                stacklevel=3,
+                            )
+
     def _series_names(self, dfn_name: str) -> set[str]:
         """The series names, lowercase, the package's TS6 or TAS6 files
         (``utl-ts``, ``utl-tas``) define."""
diff --git a/flopy4/mf6/spec.py b/flopy4/mf6/spec.py
index 37ba9aaa..10c74f98 100644
--- a/flopy4/mf6/spec.py
+++ b/flopy4/mf6/spec.py
@@ -87,6 +87,7 @@ class FieldKwargs(TypedDict, total=False):
     optional: bool
     netcdf: bool
     tagged: bool
+    when: str | None
 
 
 def _pipe(*hooks):
@@ -99,7 +100,7 @@ def _field(type_: str, kwargs: FieldKwargs, **attributes) -> Any:
     common ones in ``kwargs``) in its metadata where set."""
     metadata = dict(kwargs.get("metadata") or {})
     metadata["type"] = type_
-    for key in ("block", "longname", "optional", "netcdf", "tagged"):
+    for key in ("block", "longname", "optional", "netcdf", "tagged", "when"):
         if kwargs.get(key):
             metadata[key] = kwargs[key]  # type: ignore[literal-required]
     metadata.update({k: v for k, v in attributes.items() if v not in (None, False, ())})
diff --git a/flopy4/mf6/utils/codegen/filters.py b/flopy4/mf6/utils/codegen/filters.py
index ed011ba4..55297994 100644
--- a/flopy4/mf6/utils/codegen/filters.py
+++ b/flopy4/mf6/utils/codegen/filters.py
@@ -871,24 +871,23 @@ def attr_column(
     *,
     optional: bool,
     time_series: bool = False,
+    when: str | None = None,
     default: str = "None",
     args: tuple[str, ...] = (),
     **kwargs,
 ) -> ItemColumn:
     """The generic rendering: ``name: T = fn(*args, meta)``, ``fn`` the field
-    function for the DFN type (see `flopy4.mf6.spec`), or, when optional,
-    ``Optional[T]`` with a ``None`` default (or ``T`` with ``default``)."""
+    function for the DFN type (see `flopy4.mf6.spec`), or, when optional
+    or ``when`` an option is set, ``Optional[T]`` with a ``None`` default
+    (or ``T`` with ``default``)."""
     meta = dict(meta)
     if time_series:
         meta["time_series"] = True
     if optional:
-        # Needed even for time_series fields: _n_fixed_tokens() (item.py)
-        # uses this to tell "always present" fixed columns apart from
-        # trailing columns that may be entirely absent from a given row
-        # (e.g. EVT's pxdp/petm/petm0, only written when
-        # surf_rate_specified) when inferring a variable-width cellid's
-        # element count from raw token counts.
         meta["optional"] = True
+    if when:
+        meta["when"] = when
+    optional = optional or bool(when)
     positional = [_dq(a) for a in args]
     if optional:
         annotation = py_type if default != "None" else f"Optional[{py_type}]"
@@ -920,6 +919,7 @@ def value_column(
     *,
     optional: bool = False,
     time_series: bool = False,
+    when: str | None = None,
 ) -> ItemColumn:
     """A plain scalar. A time series value may also be a series reference."""
     py_type = "Union[float, TimeSeriesRef]" if time_series else _DFN_PY.get(dfn_type, "float")
@@ -930,6 +930,7 @@ def value_column(
         {},
         optional=optional,
         time_series=time_series,
+        when=when,
     )
 
 
diff --git a/flopy4/mf6/utils/codegen/make.py b/flopy4/mf6/utils/codegen/make.py
index acdd5568..507a6268 100644
--- a/flopy4/mf6/utils/codegen/make.py
+++ b/flopy4/mf6/utils/codegen/make.py
@@ -243,6 +243,7 @@ def _item_columns(
         f = col.field
         name = col.name
         dfn_type = _dfn_type_str(f)
+        when = getattr(f, "when", None)
         optional = bool(f.optional) or name in counts
         ts = bool(getattr(f, "time_series", False))
         if isinstance(f, File):
@@ -308,7 +309,11 @@ def _item_columns(
                 dfn_type,
                 optional=optional,
                 time_series=ts,
+                when=when,
             )
+            when = None
+        if when and name != "boundname":
+            raise NotImplementedError(f"column {name!r}: when={when!r} on a non-scalar column")
         items.append(item)
     return items
 
diff --git a/pixi.lock b/pixi.lock
index 5a3e363d..b24acfdb 100644
--- a/pixi.lock
+++ b/pixi.lock
@@ -106,7 +106,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/noarch/python_abi-3.13-9_cp313.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2026c-h151e31d_0.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/07/70/746ab7140121dcf04bade9ab7d2e65b6d5eb5b23bb7dc04b7ad22742ca0f/xugrid-0.15.3-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/09/4e/23e0009b6d5d415c223ea38fcf1e38dc9b0fc6d3ff15b5c5090dcc1f6d88/sparse-0.19.2-py2.py3-none-any.whl
@@ -255,7 +255,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/osx-64/tk-8.6.13-ha6c374e_4.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-64/zstd-1.5.7-hbc1a06c_7.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
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       - pypi: https://files.pythonhosted.org/packages/05/4a/78c6285577c375e7cf27277ea8ee6961224327f1e1a0c44af5f17f23635c/scipy-1.18.1-cp312-cp312-macosx_12_0_arm64.whl
@@ -4609,7 +4609,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/win-64/vcomp14-14.51.36247-habf1de7_41.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/zstd-1.5.7-h534d264_7.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/8b/b83f7780dad22e0878e4c7bd9158ebd24ed12bc3d5e3a471cd0576f77ded/pyzmq-27.2.0-cp312-abi3-win_amd64.whl
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/3a/f0ee4d4dde9d3b69dedf1b5f74e7a40017046d55052d173e418c6a94f960/rpds_py-2026.6.3-cp312-cp312-win_amd64.whl
@@ -4863,7 +4863,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/noarch/python_abi-3.13-9_cp313.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2026c-h151e31d_0.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/4b/29cac41a4d98d144bf5f6d33995617b185d14b22401f75ca86f384e87ff1/h11-0.16.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/07/6c/aa3f2f849e01cb6a001cd8554a88d4c77c5c1a31c95bdf1cf9301e6d9ef4/defusedxml-0.7.1-py2.py3-none-any.whl
@@ -5107,7 +5107,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/osx-64/tk-8.6.13-ha6c374e_4.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-64/zstd-1.5.7-hbc1a06c_7.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/4b/29cac41a4d98d144bf5f6d33995617b185d14b22401f75ca86f384e87ff1/h11-0.16.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/07/06/b823a7e818c756d9a7123ba2cda7d07bc2dd32835648d1a7b7b7a05d848d/tomli-2.4.1-cp313-cp313-macosx_10_13_x86_64.whl
@@ -5352,7 +5352,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/tk-8.6.13-hbeba79b_4.conda
       - conda: https://conda.anaconda.org/conda-forge/osx-arm64/zstd-1.5.7-hf451053_7.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/4b/29cac41a4d98d144bf5f6d33995617b185d14b22401f75ca86f384e87ff1/h11-0.16.0-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/07/6c/aa3f2f849e01cb6a001cd8554a88d4c77c5c1a31c95bdf1cf9301e6d9ef4/defusedxml-0.7.1-py2.py3-none-any.whl
@@ -5591,7 +5591,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/win-64/vcomp14-14.51.36247-habf1de7_41.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/zstd-1.5.7-h534d264_7.conda
       - pypi: ./
-      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+      - pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
       - pypi: https://files.pythonhosted.org/packages/02/8b/b83f7780dad22e0878e4c7bd9158ebd24ed12bc3d5e3a471cd0576f77ded/pyzmq-27.2.0-cp312-abi3-win_amd64.whl
       - pypi: https://files.pythonhosted.org/packages/02/be/5d2d47b1fb58943194fb59dcf222f7c4e35122ec0ffe8c36e18b5d728f0b/tblib-3.2.2-py3-none-any.whl
       - pypi: https://files.pythonhosted.org/packages/04/4b/29cac41a4d98d144bf5f6d33995617b185d14b22401f75ca86f384e87ff1/h11-0.16.0-py3-none-any.whl
@@ -10852,7 +10852,7 @@ packages:
   - build ; extra == 'build'
   - twine ; extra == 'build'
   requires_python: '>=3.11,<3.14'
-- pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#873c5cac2b964841b43def94da867ce36bb9cd1b
+- pypi: git+https://github.com/MODFLOW-ORG/modflow-devtools.git#9a1662ef87c006346b5769f856f730bad35f218f
   name: modflow-devtools
   version: 1.10.0.dev1
   requires_dist:
diff --git a/test/mf6/test_mf6_codec.py b/test/mf6/test_mf6_codec.py
index 99d44b72..cb486cce 100644
--- a/test/mf6/test_mf6_codec.py
+++ b/test/mf6/test_mf6_codec.py
@@ -1857,16 +1857,76 @@ def test_row_boundname_by_option(boundnames):
     assert row.boundname == ("5" if boundnames else None)
 
 
-@pytest.mark.parametrize("boundnames", [True, False])
-def test_row_optional_column_or_boundname(boundnames):
-    """EVT's petm0 is there if the row makes room for it, after the
-    boundname, which the option places."""
+@pytest.mark.parametrize(
+    "boundnames, surf_rate, tokens, petm0, boundname",
+    [
+        # a row may leave out its boundname, but not petm0
+        (True, True, [0.5], 0.5, None),
+        (True, True, [0.5, "w"], 0.5, "w"),
+        (True, False, [0.5], None, "0.5"),
+        (False, True, [0.5, "w"], 0.5, None),  # MF6 ignores the extra token
+        (False, False, ["w"], None, None),
+    ],
+)
+def test_row_conditional_field_or_boundname(boundnames, surf_rate, tokens, petm0, boundname):
+    """EVT's petm0 is there iff SURF_RATE_SPECIFIED is set, and the
+    boundname if BOUNDNAMES is."""
     from flopy4.mf6.gwf import Evt
     from flopy4.mf6.item import RowContext
 
-    context = RowContext(ncelldim=3, sizes={"nseg-1": 0, "auxiliary": 0}, boundnames=boundnames)
-    row = Evt.StressPeriodData.from_tokens([1, 1, 1, 10.0, 0.001, 2.0, 0.5], context)
-    assert (row.petm0, row.boundname) == ((None, "0.5") if boundnames else (0.5, None))
+    context = RowContext(
+        ncelldim=3,
+        sizes={"nseg-1": 0, "auxiliary": 0},
+        boundnames=boundnames,
+        enabled=frozenset({"surf_rate_specified"} if surf_rate else ()),
+    )
+    row = Evt.StressPeriodData.from_tokens([1, 1, 1, 10.0, 0.001, 2.0, *tokens], context)
+    assert (row.petm0, row.boundname) == (petm0, boundname)
+
+
+def test_row_conditional_field_sets_option():
+    """A Python row with EVT's petm0 sets SURF_RATE_SPECIFIED, which MF6
+    needs to read it; then a row's last value is its boundname."""
+    from flopy4.mf6.gwf import Evt
+
+    evt = Evt(stress_period_data={0: [((0, 0, 0), 10.0, 0.001, 2.0, 0.5)]})
+    assert evt.stress_period_data[0][0].petm0 == 0.5
+    assert evt.surf_rate_specified
+    assert not Evt(stress_period_data={0: [((0, 0, 0), 10.0, 0.001, 2.0)]}).surf_rate_specified
+
+    evt = Evt(
+        boundnames=True,
+        surf_rate_specified=True,
+        stress_period_data={0: [((0, 0, 0), 10.0, 0.001, 2.0, 0.5, 7)]},
+    )
+    row = evt.stress_period_data[0][0]
+    assert (row.petm0, row.boundname) == (0.5, "7")
+
+
+def test_evt_load_petm0_without_boundname():
+    """With both options set, a file row leaving out its boundname keeps
+    its petm0."""
+    from flopy4.mf6.converter.ingress.structure import structure_component
+    from flopy4.mf6.gwf import Evt
+
+    text = """BEGIN OPTIONS
+  BOUNDNAMES
+  SURF_RATE_SPECIFIED
+END OPTIONS
+
+BEGIN DIMENSIONS
+  MAXBOUND 2
+  NSEG 1
+END DIMENSIONS
+
+BEGIN PERIOD 1
+  1 1 1 10.0 0.001 2.0 0.5
+  1 1 2 10.0 0.001 2.0 0.25 well
+END PERIOD 1
+"""
+    evt = structure_component(loads(text), Evt)
+    rows = evt.stress_period_data[0]
+    assert [(r.petm0, r.boundname) for r in rows] == [(0.5, None), (0.25, "well")]
 
 
 @pytest.mark.parametrize("boundname", [None, "b"])
diff --git a/test/mf6/test_mf6_io_roundtrip.py b/test/mf6/test_mf6_io_roundtrip.py
index ebb31621..bbe1d258 100644
--- a/test/mf6/test_mf6_io_roundtrip.py
+++ b/test/mf6/test_mf6_io_roundtrip.py
@@ -5,6 +5,7 @@
 """
 
 import math
+import warnings
 from collections.abc import Mapping
 from pathlib import Path
 
@@ -433,3 +434,43 @@ def test_quoted_multiword_boundname_and_obsname_round_trips(tmp_path):
     assert rghb.stress_period_data[0][0].boundname == "my bound"
     robs = rghb.obs.continuous[0].continuous[0]
     assert (robs.obsname, robs.id_) == ("my obs", "my bound")
+
+
+@pytest.mark.parametrize(
+    "boundname, warns",
+    [("5", True), ("1.50", True), ("1d0", True), ("b5", False), ("inf", False)],
+)
+def test_obs_id_naming_numeric_boundname_warns(tmp_path, boundname, warns):
+    """MF6 may read an observation's id that looks like a number as a
+    cell, not a boundname, so writing one warns."""
+    sim = Simulation(
+        tdis=Time(perlen=[1.0], nstp=[1], tsmult=[1.0], time_units="DAYS"),
+        workspace=tmp_path,
+        name="sim1",
+        solutiongroup=[Ims(filename="gwf.ims", models=["gwf1"])],
+    )
+    dis = Dis(nlay=1, nrow=1, ncol=2, delr=1.0, delc=1.0, top=1.0, botm=0.0)
+    gwf = Gwf(parent=sim, dis=dis, name="gwf1", save_flows=True)
+    Ic(parent=gwf, strt=0.0)
+    Npf(parent=gwf, icelltype=0, k=1.0)
+    ghb = Ghb(
+        parent=gwf,
+        boundnames=True,
+        stress_period_data={0: [[(0, 0, 0), 1.0, 10.0, boundname]]},
+        name="GHB-1",
+    )
+    Obs(
+        parent=ghb,
+        continuous=[
+            Obs.ContinuousBlock(
+                output=Obs.Output(obs_output_file_name="gwf1.ghb.obs.csv"),
+                continuous=[Obs.Continuous(obsname="o", obstype="GHB", id_=boundname)],
+            )
+        ],
+        name="obs",
+    )
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        sim.write()
+    messages = [str(w.message) for w in caught if "MF6 may read as a cell" in str(w.message)]
+    assert len(messages) == warns