diff --git a/doc/release_notes.rst b/doc/release_notes.rst
index fc0d88f8..9210ff80 100644
--- a/doc/release_notes.rst
+++ b/doc/release_notes.rst
@@ -40,6 +40,14 @@ Upcoming Version
 
 *Other*
 
+* ``Model.add_spec`` resolves the parameters it retains before it builds. A ``retain="all"`` build that could not read a parameter no declaration uses raised after the variables and constraints were already added, leaving a model that the "builds into an empty model" guard then refused to build into again.
+
+* ``repr(model.spec)`` no longer raises ``KeyError`` for a dimension the spec declares but nothing reaches, which needs no source and so has no coordinates; it is shown as ``unreached``.
+
+* A grouped sum through a lookup that maps no member at all now holds the empty sum, ``0``, on every declared group, as its documented rule says. It raised xarray's ``ValueError: <dim> must not be empty`` instead.
+
+* ``read_netcdf`` no longer rewrites the coordinates of a container that merely shares a dimension's *name* with a spec-built model's master coordinates. A hand-added variable on its own labels kept them; before, it was silently relabelled onto the master ones, or the read failed outright when the two lengths differed.
+
 * ``add_piecewise_formulation`` gained a ``mask`` parameter declaring which breakpoint slots hold a real breakpoint. It is needed for **ragged** curves — entities with different numbers of breakpoints — which are stored densely with the surplus slots left absent. Under v1 that absence must be declared (``mask=x_pts.notnull()``) rather than read off the NaN padding. (https://github.com/PyPSA/linopy/issues/884)
 
 *Internal*
diff --git a/linopy/io.py b/linopy/io.py
index 275cb2b7..22781750 100644
--- a/linopy/io.py
+++ b/linopy/io.py
@@ -1107,7 +1107,14 @@ def restore_dtypes(ds: xr.Dataset) -> xr.Dataset:
 
 
 def restamp_coords(m: Model, coords: Mapping[str, pd.Index]) -> None:
-    """Put *coords* on every container of *m* that carries one of those dimensions."""
+    """
+    Put *coords* on every container of *m* that was built on them.
+
+    Only on those: a container may carry a dimension of that name and its own
+    labels -- a hand-added variable beside a spec-built one -- and restamping
+    it would rewrite labels it never had, or fail outright over a length the
+    master coordinate does not share.
+    """
     from linopy.constraints import Constraint, CSRConstraint
     from linopy.csr import Grid
 
@@ -1122,7 +1129,7 @@ def restamp_coords(m: Model, coords: Mapping[str, pd.Index]) -> None:
         elif isinstance(constraint, CSRConstraint):
             constraint._grid = Grid(
                 {
-                    d: coords.get(d, index)
+                    d: _restamped(index, coords.get(str(d)))
                     for d, index in constraint._grid.indexes.items()
                 }
             )
@@ -1130,15 +1137,29 @@ def restamp_coords(m: Model, coords: Mapping[str, pd.Index]) -> None:
 
 def _stamped(data: xr.Dataset, coords: Mapping[str, pd.Index]) -> xr.Dataset:
     """*data* with *coords* in place of the ones a dtype narrowed."""
-    indexes = data.indexes
     stale = {
-        dim: index
-        for dim, index in coords.items()
-        if dim in indexes and indexes[dim].dtype != index.dtype
+        str(dim): restamped
+        for dim, index in data.indexes.items()
+        if (restamped := _restamped(index, coords.get(str(dim)))) is not index
     }
     return data.assign_coords(stale) if stale else data
 
 
+def _restamped(found: pd.Index, master: pd.Index | None) -> pd.Index:
+    """
+    *master* where *found* is it as a netcdf type gave it back, else *found* itself.
+
+    A narrowed int or a widened bool holds the same labels at another dtype
+    and is the one to replace -- which is what ``Index.equals`` asks, since it
+    compares labels and not dtypes. An index of another length, or of other
+    labels entirely, belongs to a container that was never built on *master*
+    and is left alone.
+    """
+    if master is None or found.dtype == master.dtype:
+        return found
+    return master if found.equals(master) else found
+
+
 def to_netcdf(m: Model, *args: Any, **kwargs: Any) -> None:
     """
     Write out the model to a netcdf file.
diff --git a/linopy/spec/accessor.py b/linopy/spec/accessor.py
index bb588a2a..b8cec579 100644
--- a/linopy/spec/accessor.py
+++ b/linopy/spec/accessor.py
@@ -84,8 +84,10 @@ def attach(
         )
     text, program = _source(spec)
     attached: Attached = attach_data(program, sources, retain=retain)
-    build(model, attached)
+    # Resolved before the build, so a parameter no declaration reads cannot fail
+    # halfway through one and leave a model too full to build into again.
     parameters = attached.retained().assign_coords(dict(attached.coords))
+    build(model, attached)
     return ModelSpec(model, program, text, parameters, attached)
 
 
@@ -116,6 +118,11 @@ def _source(spec: SpecLike) -> tuple[str, ms.Program]:
     return loaded.to_yaml(), to_program(loaded)
 
 
+def _dimension(dim: str, coords: Mapping[str, pd.Index]) -> str:
+    """A dimension and how many labels it holds; a declared one nothing reaches holds none."""
+    return f"{dim} ({len(coords[dim])})" if dim in coords else f"{dim} (unreached)"
+
+
 def _row(label: str, items: list[str], cap: int = 8) -> str:
     """One aligned summary line, capped with a ``(+N more)`` tail."""
     shown = items[:cap]
@@ -156,7 +163,7 @@ def __repr__(self) -> str:
         head = f"ModelSpec: {self.description}" if self.description else "ModelSpec"
         rows = [
             head,
-            _row("Dimensions", [f"{d} ({len(coords[d])})" for d in p.dimensions]),
+            _row("Dimensions", [_dimension(d, coords) for d in p.dimensions]),
             _row("Variables", list(p.variables)),
             _row("Constraints", list(p.constraints)),
         ]
diff --git a/linopy/spec/operators.py b/linopy/spec/operators.py
index 17fd2d46..7b86565a 100644
--- a/linopy/spec/operators.py
+++ b/linopy/spec/operators.py
@@ -81,6 +81,8 @@ def grouped_sum(
     mappings = _renamed(mappings, into)
     present = _present(mappings)
     dim = str(mappings[0].dims[0])
+    if not bool(present.any()):
+        return _empty_groups(array, dim, into=into, labels=labels)
     if not bool(present.all()):
         keep = present.to_numpy()
         mappings = tuple(m.isel({dim: keep}) for m in mappings)
@@ -92,6 +94,40 @@ def grouped_sum(
     return summed.reindex({d: labels[d] for d in into}).fillna(0.0)
 
 
+def _empty_groups(
+    array: Array,
+    dim: str,
+    *,
+    into: tuple[str, ...],
+    labels: Mapping[str, pd.Index],
+) -> Array:
+    """
+    The grouped sum of an operand no member of *dim* is mapped out of.
+
+    Every declared group holds the empty sum, which is 0. Grouping cannot say
+    so itself: filtering the operand down to its mapped members leaves nothing,
+    and an empty dimension is one xarray refuses to group over.
+    """
+    kept = [d for d in _coord_dims(array) if d != dim]
+    zeros = xr.DataArray(
+        np.zeros([array.sizes[d] for d in kept] + [len(labels[d]) for d in into]),
+        coords={
+            **{d: array.indexes[d] for d in kept},
+            **{d: labels[d] for d in into},
+        },
+        dims=kept + list(into),
+    )
+    if isinstance(array, xr.DataArray):
+        return zeros
+    return LinearExpression.from_constant(array.model, zeros)
+
+
+def _coord_dims(array: Array) -> list[str]:
+    """The dimensions the operand is labelled over, without a term's own ``_term``."""
+    dims = array.dims if isinstance(array, xr.DataArray) else array.coord_dims
+    return [str(d) for d in dims]
+
+
 @overload
 def at(
     array: xr.DataArray, mappings: tuple[xr.DataArray, ...], *, into: tuple[str, ...]
diff --git a/test/test_spec_accessor.py b/test/test_spec_accessor.py
index ed8457d5..91605cec 100644
--- a/test/test_spec_accessor.py
+++ b/test/test_spec_accessor.py
@@ -133,6 +133,30 @@ def test_the_spec_keeps_its_parameters_off_the_model() -> None:
     assert m.spec.parameters["cost"].dims == ("generator",)
 
 
+def test_a_build_that_cannot_retain_leaves_the_model_buildable() -> None:
+    """retain='all' reaches parameters no declaration does, and must not half-build on one."""
+    spec = with_(yaml_dict(), parameters={"spare": {"dims": ["generator"]}})
+    m = Model()
+    with pytest.raises(SpecDataError, match="no data provided for parameter 'spare'"):
+        m.add_spec(spec, DISPATCH_DATA, retain="all")
+    assert not len(m.variables) and not len(m.constraints)
+
+    spare = pd.Series([1.0, 2.0], index=GENERATOR)
+    m.add_spec(spec, {**DISPATCH_DATA, "spare": spare}, retain="all")
+    assert "spare" in m.spec.parameters
+
+
+def test_a_declared_dimension_with_no_source_still_reprs() -> None:
+    """A dimension nothing reaches needs no source, so the repr must do without its labels."""
+    spec = with_(
+        yaml_dict(), dimensions={"spare": {"dtype": "int", "description": "unreached"}}
+    )
+    m = Model.from_spec(spec, DISPATCH_DATA)
+
+    assert "spare (unreached)" in repr(m.spec)
+    assert "snapshot (3)" in repr(m.spec)
+
+
 def test_an_unknown_expression_is_a_key_error_with_a_hint() -> None:
     m = Model.from_spec(yaml_dict(), DISPATCH_DATA)
     with pytest.raises(KeyError, match="unknown named expression 'spent'.*spend"):
diff --git a/test/test_spec_io.py b/test/test_spec_io.py
index 30139a14..4dddf94e 100644
--- a/test/test_spec_io.py
+++ b/test/test_spec_io.py
@@ -190,6 +190,24 @@ def test_a_parameter_keeps_its_dtype(tmp_path: Path, engine: str, name: str) ->
     assert p.spec.parameters[name].dtype == m.spec.parameters[name].dtype
 
 
+@pytest.mark.parametrize("engine", ENGINES)
+@pytest.mark.parametrize(
+    "labels", [[10, 11, 12], [0, 1]], ids=["relabelled", "shorter"]
+)
+def test_a_hand_added_variable_keeps_its_own_labels(
+    tmp_path: Path, engine: str, labels: list[int]
+) -> None:
+    """A container sharing a master dimension's name but not its labels is left alone."""
+    own = pd.Index(labels, name="snapshot")
+    m = Model.from_spec(EXAMPLE_DISPATCH, DISPATCH_DATA, retain="all")
+    m.add_variables(coords=[own], name="side")
+    p = roundtrip(m, tmp_path, engine)
+
+    assert p.variables["side"].indexes["snapshot"].equals(own)
+    assert p.spec.coords["snapshot"].equals(m.spec.coords["snapshot"])
+    assert p.variables["p"].indexes["snapshot"].equals(m.spec.coords["snapshot"])
+
+
 @pytest.mark.parametrize("engine", ENGINES)
 @pytest.mark.parametrize("frozen", [False, True], ids=["dataset", "csr"])
 def test_every_container_shares_the_master_coordinate_dtypes(
diff --git a/test/test_spec_operators.py b/test/test_spec_operators.py
index 4e470eed..bd9a1b45 100644
--- a/test/test_spec_operators.py
+++ b/test/test_spec_operators.py
@@ -202,6 +202,29 @@ def test_an_empty_group_on_the_constant_side_is_a_zero_and_not_a_gap() -> None:
     assert float(m.solution["imports"].sel(bus="south")) == pytest.approx(0.0)
 
 
+def test_a_lookup_that_maps_nothing_leaves_every_group_at_the_empty_sum() -> None:
+    """Filtering to the mapped members leaves nothing, and nothing is what xarray will not group."""
+    spec = with_(
+        GROUPED_SPEC,
+        variables={
+            "out": {
+                "foreach": ["generator"],
+                "bounds": {"lower": 0, "upper": "capacity"},
+            }
+        },
+        expressions={"per_bus": "sum(out, by=gen_bus)"},
+    )
+    sources = grouped_sources(pd.Series([3.0, 4.0], index=GENS))
+    sources["gen_bus"] = pd.Series([], dtype=object)
+    m = solved(spec, sources)
+
+    assert m.objective.value == pytest.approx(0.0)
+    per_bus = m.spec.expressions["per_bus"]
+    assert per_bus.expression.nterm == 0
+    assert per_bus.solution.indexes["bus"].tolist() == ["north", "south"]
+    np.testing.assert_allclose(per_bus.solution.values, [0.0, 0.0])
+
+
 def test_a_member_with_no_value_is_still_refused_through_a_group() -> None:
     with pytest.raises(SpecDataError, match="parameter 'capacity' covers 1 fewer"):
         Model.from_spec(GROUPED_SPEC, grouped_sources(pd.Series([3.0], index=GENS[:1])))