diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index feb6dc93..8547d514 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -25,8 +25,8 @@ below protects it.
 
 ```mermaid
 flowchart TB
-    Y[YAML file] -->|"parse + validate<br/>(schema.py, validation.py)"| MS[MathSchema]
-    MS -->|"expand macros: / expressions: (expansion.py)<br/>expand piecewise: blocks (piecewise.py)<br/>resolve names to typed nodes (resolution.py)<br/>check dim sets (dimensions.py)<br/>— backends never see any of them"| AST["core AST<br/>= the only contract between layers<br/>fully typed: names resolved, dims checked"]
+    Y[YAML file] -->|"parse + validate<br/>(language/schema.py, language/validation.py)"| MS[MathSchema]
+    MS -->|"expand macros: / expressions: (language/expansion.py)<br/>expand piecewise: blocks (piecewise.py)<br/>resolve names to typed nodes (language/resolution.py)<br/>check dim sets (language/dimensions.py)<br/>— backends never see any of them"| AST["core AST<br/>= the only contract between layers<br/>fully typed: names resolved, dims checked"]
     AST -->|"api.py: check / build / solve / write"| LOWER
     AST -.->|"lpspec.linopy<br/>(opt-in shim: build / extend)"| BUILD
     LOWER -->|"outside the language:<br/>LanguageError naming the construct"| ERR["load error<br/>(no fallback)"]
@@ -171,7 +171,12 @@ that made an implementation choice load-bearing in the language's rulebook.
    and the plan/query/xarray are backend-private. The AST crossing that seam is **fully
    resolved** — names are typed `Variable`/`Parameter`/`Dimension` nodes — so a backend cannot
    hold its own opinion about what a name refers to. Resolving independently is
-   how the two lanes silently disagreed about scoping before.
+   how the two lanes silently disagreed about scoping before. The waist is
+   closed from the front too: nothing under `src/lpspec/language/` imports
+   `lowering`, `piecewise`, `sources`, `api` or any consuming subpackage, so
+   what a model *means* cannot depend on what is done with it. That is the
+   mirror of rule 2 and it is enforced the same way, off the path
+   (`LANGUAGE_MAY_IMPORT`).
 2. **The engine knows nothing about linopy, xarray or YAML.**
    `src/lpspec/relational/` goes polars → highspy → solver, with linopy's
    semantics as a spec to match rather than code to share; it never sees the
@@ -271,21 +276,21 @@ than discovered at solve time.
 
 | Module | Role |
 |---|---|
-| `_yaml.py` | the only place a file is read: YAML 1.2 booleans, duplicate keys refused |
-| `schema.py` | pydantic schema incl. `expressions:` / `macros:` / `piecewise:` |
-| `expression_parser.py`, `where_parser.py` | text → core AST; grammar only, dependency-free |
-| `expansion.py` | named-expression / macro substitution (pre-dispatch) |
-| `resolution.py` | one flat namespace; `NameNode` → typed `Variable`/`Parameter`/`Dimension` nodes |
-| `dimensions.py` | static dim-set checking over the resolved AST |
-| `validation.py` | load-time: parse, expand, resolve, check everything |
+| `language/_yaml.py` | the only place a file is read: YAML 1.2 booleans, duplicate keys refused |
+| `language/schema.py` | pydantic schema incl. `expressions:` / `macros:` / `piecewise:` |
+| `language/expression_parser.py`, `language/where_parser.py` | text → core AST; grammar only, dependency-free |
+| `language/expansion.py` | named-expression / macro substitution (pre-dispatch) |
+| `language/resolution.py` | one flat namespace; `NameNode` → typed `Variable`/`Parameter`/`Dimension` nodes |
+| `language/dimensions.py` | static dim-set checking over the resolved AST |
+| `language/helpers.py` | the closed set of built-in operators: their *names* and *call shapes* — no registry |
+| `language/validation.py` | load-time: parse, expand, resolve, check everything |
 | `piecewise.py` | `piecewise:` → λ-formulation declarations + curvature guard |
 | `api.py` | native entry point: `check` / `build` / `solve` / `write`, linopy-free |
 | `typeset/` | **spike** — resolved AST → LaTeX / Typst / Markdown. A reader, not a lane: no model, no data, no plan ([README](https://github.com/FBumann/lpspec/blob/main/src/lpspec/typeset/README.md)) |
 | `__main__.py` | `python -m lpspec <format>` — a shell front for the verbs that bind no data |
 | `sources.py` | bind runtime data (parquet paths / in-memory tables) to a validated schema |
 | `lowering.py` | core AST → logical plan (defines the relational subset) |
-| `helpers.py` | the closed set of built-in operators: their *names* and *call shapes* — no registry |
-| `errors.py` | the exception hierarchy; the one module the engine may import |
+| `errors.py` | the exception hierarchy; the one module either fenced side may import |
 | `relational/plan.py` | frozen logical-plan dataclasses |
 | `relational/frames.py` | the boundary — caller tables in, via the Arrow PyCapsule protocol |
 | `relational/compiler.py` | plan → lazy frames; pure, reads nothing |
@@ -300,13 +305,23 @@ than discovered at solve time.
 | `linopy/builder.py` | eager backend: core AST → `linopy.Model` |
 | `linopy/semantics.py` | where this lane answers linopy's v1 arithmetic convention — one home, as linopy's own `semantics.py` is |
 
-Two subpackages, and the directory *is* the rule in both cases. Everything
-under `relational/` is the engine and imports nothing else from the package;
-everything under `linopy/` is the opt-in eager lane and is the only code
-allowed to import linopy or xarray; everything under `typeset/` reads the AST
-and writes text, and reaches neither the plan nor any data. `tests/test_architecture.py` reads
-membership off the path, so neither fence can be stepped over by naming a
-file differently.
+**Four subpackages, and the directory *is* the rule in every case.** Everything
+under `language/` produces the AST and may not reach a consumer of it;
+everything under `relational/` is the engine and imports nothing else from the
+package; everything under `linopy/` is the opt-in eager lane and is the only
+code allowed to import linopy or xarray; everything under `typeset/` reads the
+AST and writes text, and reaches neither the plan nor any data.
+`tests/test_architecture.py` reads membership off the path, so no fence can be
+stepped over by naming a file differently.
+
+`language/` and `relational/` are the two halves of the waist and their fences
+point the same way — outward, at `errors.py`, the one leaf both may import. The
+modules left at the top level are the ones that are legitimately *both*:
+`lowering.py` reads the AST and writes the plan, `piecewise.py` emits
+declarations but consults the subset test to do it, `sources.py` binds data to
+a validated schema, and `api.py` runs the lot. A module that belongs to neither
+side is a module that has to sit on the line, and the flat namespace is where
+it sits.
 
 ### Naming across the layers
 
@@ -316,7 +331,7 @@ suffix**, which is what keeps the three vocabularies from colliding:
 
 | Layer | Suffix | Example |
 |---|---|---|
-| YAML block (`schema.py`) | `Block` | `VariableBlock`, `PiecewiseBlock` |
+| YAML block (`language/schema.py`) | `Block` | `VariableBlock`, `PiecewiseBlock` |
 | Core AST (`*_parser.py`) | `Node` | `VariableNode`, `DimensionComparisonNode` |
 | Logical plan (`relational/plan.py`) | none / `Declaration` | `Variable`, `VariableDeclaration` |
 
@@ -364,7 +379,7 @@ lowering case → differential test on both sinks → SPEC §5/§7, and this fil
 structural.
 
 Two things are deliberately *not* per-primitive work, because they are one
-implementation each: a primitive's dim rule lives only in `dimensions.py` —
+implementation each: a primitive's dim rule lives only in `language/dimensions.py` —
 both its dim *set* and its verdict on an operand that lacks the dim being
 reduced along, which lowering asks for rather than deciding again — and the
 dense-label assignment that gives a coordinate its solver index lives only in
diff --git a/examples/walkthrough.py b/examples/walkthrough.py
index 0c4bca24..a27af396 100644
--- a/examples/walkthrough.py
+++ b/examples/walkthrough.py
@@ -27,8 +27,8 @@
 import polars as pl
 
 import lpspec as lps
-from lpspec.expansion import parse_and_expand
-from lpspec.expression_parser import parse_expression
+from lpspec.language.expansion import parse_and_expand
+from lpspec.language.expression_parser import parse_expression
 from lpspec.lowering import lower_program
 from lpspec.relational.executor import PolarsExecutor
 from lpspec.sources import tidy_sources
diff --git a/pyproject.toml b/pyproject.toml
index 92feae51..88e1bbfe 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -165,7 +165,7 @@ extend-fixable = ["B", "SIM", "RUF", "C4", "UP"]
 "bench/**/*.py" = ["T20"]  # a harness reports by printing; stdout is its interface
 "tools/**/*.py" = ["T20"]  # same: a generator says what it wrote
 "tests/golden/__main__.py" = ["T20"]  # likewise a generator, and only reachable as a command
-"src/lpspec/where_parser.py" = ["N806"]  # grammar tokens (NOT/AND/OR) are named after keywords
+"src/lpspec/language/where_parser.py" = ["N806"]  # grammar tokens (NOT/AND/OR) are named after keywords
 
 [tool.ruff.format]
 quote-style = "single"
diff --git a/src/lpspec/__init__.py b/src/lpspec/__init__.py
index 29b4d54a..7a67ec52 100644
--- a/src/lpspec/__init__.py
+++ b/src/lpspec/__init__.py
@@ -27,7 +27,7 @@
     PiecewiseExpansionError,
     SchemaError,
 )
-from lpspec.schema import MathSchema
+from lpspec.language.schema import MathSchema
 from lpspec.typeset import SymbolTable, to_latex, to_markdown, to_typst
 
 __all__ = [
diff --git a/src/lpspec/api.py b/src/lpspec/api.py
index cea35907..70e8e3eb 100644
--- a/src/lpspec/api.py
+++ b/src/lpspec/api.py
@@ -31,13 +31,13 @@
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
-from lpspec._yaml import read_yaml
+from lpspec.language._yaml import read_yaml
+from lpspec.language.schema import MathSchema
+from lpspec.language.validation import validate_expressions
 from lpspec.lowering import lower_program
 from lpspec.piecewise import expand_piecewise
 from lpspec.relational.executor import PolarsExecutor, Result
-from lpspec.schema import MathSchema
 from lpspec.sources import tidy_sources
-from lpspec.validation import validate_expressions
 
 if TYPE_CHECKING:
     from collections.abc import Mapping
diff --git a/src/lpspec/language/__init__.py b/src/lpspec/language/__init__.py
new file mode 100644
index 00000000..e6c068db
--- /dev/null
+++ b/src/lpspec/language/__init__.py
@@ -0,0 +1,21 @@
+"""The language: what a YAML file may say, and what it means.
+
+Everything from the bytes on disk to a fully typed, dim-checked core AST —
+the file reader, the schema, the two grammars, expansion, resolution, the dim
+rules, and the load-time pass that runs them all. The AST this package
+produces is the narrow waist of docs/ARCHITECTURE.md: everything downstream
+reads it, and nothing downstream is visible from here.
+
+**The directory is the rule, in the direction the engine's is not.** Hard rule
+2 says the engine never sees the schema or the AST; this is its mirror —
+nothing under ``language/`` may import ``lowering``, ``piecewise``,
+``sources``, ``api``, or any of the three consuming subpackages. What a model
+*means* cannot depend on what any consumer does with it, which is what makes
+``lps.check()`` a pass with no data and no plan, and a second consumer cheap.
+``errors.py`` stays outside deliberately: it is the dependency-free leaf both
+this package and the engine may import (``ENGINE_MAY_IMPORT``), and moving it
+in would put the language's path on the engine's import list.
+
+``tests/test_architecture.py`` reads membership off the path, so a new
+front-end module cannot land outside the fence by being spelled differently.
+"""
diff --git a/src/lpspec/_yaml.py b/src/lpspec/language/_yaml.py
similarity index 100%
rename from src/lpspec/_yaml.py
rename to src/lpspec/language/_yaml.py
diff --git a/src/lpspec/dimensions.py b/src/lpspec/language/dimensions.py
similarity index 98%
rename from src/lpspec/dimensions.py
rename to src/lpspec/language/dimensions.py
index 47213f99..a0f8d8f8 100644
--- a/src/lpspec/dimensions.py
+++ b/src/lpspec/language/dimensions.py
@@ -40,7 +40,7 @@
 from typing import TYPE_CHECKING, assert_never
 
 from lpspec.errors import DimensionError
-from lpspec.expression_parser import (
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -55,8 +55,8 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.resolution import Namespace, expression_of, where_of
-from lpspec.where_parser import (
+from lpspec.language.resolution import Namespace, expression_of, where_of
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     DimensionComparisonNode,
@@ -73,7 +73,7 @@
 if TYPE_CHECKING:
     from collections.abc import Mapping, Sequence
 
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 def dims_of(
diff --git a/src/lpspec/expansion.py b/src/lpspec/language/expansion.py
similarity index 98%
rename from src/lpspec/expansion.py
rename to src/lpspec/language/expansion.py
index c1ca6293..76878ce1 100644
--- a/src/lpspec/expansion.py
+++ b/src/lpspec/language/expansion.py
@@ -40,7 +40,7 @@
 from typing import TYPE_CHECKING, assert_never, overload
 
 from lpspec.errors import SchemaError
-from lpspec.expression_parser import (
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -60,7 +60,7 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
 
-    from lpspec.schema import MacroBlock, MathSchema
+    from lpspec.language.schema import MacroBlock, MathSchema
 
 #: Backstop against pathological nesting the cycle check cannot see.
 _MAX_DEPTH = 50
diff --git a/src/lpspec/expression_parser.py b/src/lpspec/language/expression_parser.py
similarity index 100%
rename from src/lpspec/expression_parser.py
rename to src/lpspec/language/expression_parser.py
diff --git a/src/lpspec/helpers.py b/src/lpspec/language/helpers.py
similarity index 100%
rename from src/lpspec/helpers.py
rename to src/lpspec/language/helpers.py
diff --git a/src/lpspec/resolution.py b/src/lpspec/language/resolution.py
similarity index 98%
rename from src/lpspec/resolution.py
rename to src/lpspec/language/resolution.py
index ae1e9992..292949d0 100644
--- a/src/lpspec/resolution.py
+++ b/src/lpspec/language/resolution.py
@@ -20,8 +20,8 @@
 from typing import TYPE_CHECKING, assert_never
 
 from lpspec.errors import LanguageError
-from lpspec.expansion import parse_and_expand
-from lpspec.expression_parser import (
+from lpspec.language.expansion import parse_and_expand
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -36,8 +36,8 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.helpers import BUILTINS, EDGE_WRAP, call_shape_error, edge_error, unknown_helper_message
-from lpspec.where_parser import (
+from lpspec.language.helpers import BUILTINS, EDGE_WRAP, call_shape_error, edge_error, unknown_helper_message
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     DimensionComparisonNode,
@@ -55,7 +55,7 @@
 if TYPE_CHECKING:
     from collections.abc import Iterable, Mapping
 
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 class Namespace:
diff --git a/src/lpspec/schema.py b/src/lpspec/language/schema.py
similarity index 99%
rename from src/lpspec/schema.py
rename to src/lpspec/language/schema.py
index b4aa641d..8f6a1a79 100644
--- a/src/lpspec/schema.py
+++ b/src/lpspec/language/schema.py
@@ -7,7 +7,7 @@
 
 from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
 
-from lpspec.helpers import BUILTIN_NAMES
+from lpspec.language.helpers import BUILTIN_NAMES
 
 if TYPE_CHECKING:
     from collections.abc import Iterable
diff --git a/src/lpspec/validation.py b/src/lpspec/language/validation.py
similarity index 96%
rename from src/lpspec/validation.py
rename to src/lpspec/language/validation.py
index 8576801d..8a707a47 100644
--- a/src/lpspec/validation.py
+++ b/src/lpspec/language/validation.py
@@ -23,10 +23,10 @@
 from types import MappingProxyType
 from typing import TYPE_CHECKING, assert_never
 
-from lpspec.dimensions import check_schema
 from lpspec.errors import SchemaError
-from lpspec.expansion import expand, parse_and_expand, parse_template
-from lpspec.expression_parser import (
+from lpspec.language.dimensions import check_schema
+from lpspec.language.expansion import expand, parse_and_expand, parse_template
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -40,14 +40,14 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.helpers import BUILTINS, unknown_helper_message
-from lpspec.resolution import Namespace, resolve_expression, resolve_where
-from lpspec.where_parser import parse_where
+from lpspec.language.helpers import BUILTINS, unknown_helper_message
+from lpspec.language.resolution import Namespace, resolve_expression, resolve_where
+from lpspec.language.where_parser import parse_where
 
 if TYPE_CHECKING:
     from collections.abc import Mapping, Sequence
 
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 def validate_expressions(
diff --git a/src/lpspec/where_parser.py b/src/lpspec/language/where_parser.py
similarity index 100%
rename from src/lpspec/where_parser.py
rename to src/lpspec/language/where_parser.py
diff --git a/src/lpspec/linopy/__init__.py b/src/lpspec/linopy/__init__.py
index b2cbbe86..dcf3e603 100644
--- a/src/lpspec/linopy/__init__.py
+++ b/src/lpspec/linopy/__init__.py
@@ -47,8 +47,10 @@
 
 
 from lpspec._notes import note
-from lpspec._yaml import read_yaml
 from lpspec.errors import LanguageError
+from lpspec.language._yaml import read_yaml
+from lpspec.language.schema import MathSchema
+from lpspec.language.validation import validate_expressions
 from lpspec.linopy.builder import build_model
 from lpspec.linopy.loader import (
     build_dim_coords,
@@ -57,8 +59,6 @@
     load_parameters,
 )
 from lpspec.piecewise import expand_piecewise, validate_piecewise_data
-from lpspec.schema import MathSchema
-from lpspec.validation import validate_expressions
 
 # **This lane speaks v1, and the option is global, so importing sets it.**
 #
diff --git a/src/lpspec/linopy/builder.py b/src/lpspec/linopy/builder.py
index 62d5471a..56f65ba0 100644
--- a/src/lpspec/linopy/builder.py
+++ b/src/lpspec/linopy/builder.py
@@ -17,7 +17,7 @@
 
 from lpspec._notes import note
 from lpspec.errors import DataError, LanguageError, null_bounds_message
-from lpspec.expression_parser import (
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -31,11 +31,9 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.helpers import EDGE_WRAP, unknown_helper_message
-from lpspec.linopy import semantics
-from lpspec.linopy.loader import check_divisors_cover
-from lpspec.resolution import Namespace, expression_of, where_of
-from lpspec.where_parser import (
+from lpspec.language.helpers import EDGE_WRAP, unknown_helper_message
+from lpspec.language.resolution import Namespace, expression_of, where_of
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     DimensionComparisonNode,
@@ -48,6 +46,8 @@
     VariableDefinedNode,
     WhereNode,
 )
+from lpspec.linopy import semantics
+from lpspec.linopy.loader import check_divisors_cover
 
 if TYPE_CHECKING:
     from collections.abc import Callable, Hashable, Mapping
@@ -55,7 +55,7 @@
     import linopy
     import pandas as pd
 
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 # Mapping from YAML comparison operators to linopy sign strings
 _SIGN_MAP = {'==': '=', '<=': '<=', '>=': '>='}
diff --git a/src/lpspec/linopy/loader.py b/src/lpspec/linopy/loader.py
index b63d439a..34ab7f7f 100644
--- a/src/lpspec/linopy/loader.py
+++ b/src/lpspec/linopy/loader.py
@@ -9,7 +9,7 @@
 import xarray as xr
 
 from lpspec.errors import DataError, duplicate_coordinate_message, sparse_divisor_message
-from lpspec.expression_parser import (
+from lpspec.language.expression_parser import (
     BinaryOperatorNode,
     ComparisonNode,
     FunctionCallNode,
@@ -20,7 +20,7 @@
 )
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 def build_master_coords(
diff --git a/src/lpspec/lowering.py b/src/lpspec/lowering.py
index 17ba5ddb..881d6625 100644
--- a/src/lpspec/lowering.py
+++ b/src/lpspec/lowering.py
@@ -29,9 +29,9 @@
 
 from typing import TYPE_CHECKING, assert_never
 
-from lpspec.dimensions import dims_of
 from lpspec.errors import LanguageError
-from lpspec.expression_parser import (
+from lpspec.language.dimensions import dims_of
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -45,10 +45,9 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.helpers import BUILTIN_NAMES, call_shape_error, edge_error
-from lpspec.relational import plan
-from lpspec.resolution import Namespace, expression_of, where_of
-from lpspec.where_parser import (
+from lpspec.language.helpers import BUILTIN_NAMES, call_shape_error, edge_error
+from lpspec.language.resolution import Namespace, expression_of, where_of
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     DimensionComparisonNode,
@@ -61,9 +60,10 @@
     VariableDefinedNode,
     WhereNode,
 )
+from lpspec.relational import plan
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 _SENSES = {'==', '<=', '>='}
 
diff --git a/src/lpspec/piecewise.py b/src/lpspec/piecewise.py
index 7180bcde..8a2a8ea2 100644
--- a/src/lpspec/piecewise.py
+++ b/src/lpspec/piecewise.py
@@ -42,13 +42,13 @@
 
 from typing import TYPE_CHECKING, Any
 
-from lpspec.dimensions import dims_of
 from lpspec.errors import LanguageError, PiecewiseExpansionError
-from lpspec.expression_parser import ComparisonNode, parse_expression
+from lpspec.language.dimensions import dims_of
+from lpspec.language.expression_parser import ComparisonNode, parse_expression
+from lpspec.language.resolution import Namespace, resolve_expression
+from lpspec.language.schema import MathSchema, PiecewiseBlock
 from lpspec.lowering import check_core_subset
 from lpspec.relational.frames import as_frame
-from lpspec.resolution import Namespace, resolve_expression
-from lpspec.schema import MathSchema, PiecewiseBlock
 
 if TYPE_CHECKING:
     from collections.abc import Mapping
diff --git a/src/lpspec/sources.py b/src/lpspec/sources.py
index d2be39e9..5e90c7e3 100644
--- a/src/lpspec/sources.py
+++ b/src/lpspec/sources.py
@@ -24,7 +24,7 @@
 from lpspec.relational.frames import as_frame, labels_frame
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 def tidy_sources(
diff --git a/src/lpspec/typeset/__init__.py b/src/lpspec/typeset/__init__.py
index c07ff9cf..327d0cba 100644
--- a/src/lpspec/typeset/__init__.py
+++ b/src/lpspec/typeset/__init__.py
@@ -42,8 +42,8 @@
 from typing import TYPE_CHECKING, Any
 
 from lpspec.api import load_schema
+from lpspec.language.resolution import Namespace
 from lpspec.piecewise import expand_piecewise
-from lpspec.resolution import Namespace
 from lpspec.typeset.latex import LatexFormat
 from lpspec.typeset.markdown import MarkdownFormat
 from lpspec.typeset.symbols import Symbols, SymbolTable
@@ -54,7 +54,7 @@
     from collections.abc import Mapping
     from pathlib import Path
 
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
     from lpspec.typeset.format import Format
 
 __all__ = ['FORMATS', 'SymbolTable', 'to_latex', 'to_markdown', 'to_typst', 'typeset']
diff --git a/src/lpspec/typeset/symbols.py b/src/lpspec/typeset/symbols.py
index c00cce5f..b2e51756 100644
--- a/src/lpspec/typeset/symbols.py
+++ b/src/lpspec/typeset/symbols.py
@@ -19,11 +19,11 @@
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
-from lpspec._yaml import read_yaml
 from lpspec.errors import SchemaError
+from lpspec.language._yaml import read_yaml
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
     from lpspec.typeset.format import Format
 
 __all__ = ['SymbolTable', 'Symbols']
diff --git a/src/lpspec/typeset/walk.py b/src/lpspec/typeset/walk.py
index 2f7fa486..c099ae3a 100644
--- a/src/lpspec/typeset/walk.py
+++ b/src/lpspec/typeset/walk.py
@@ -17,8 +17,8 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, assert_never
 
-from lpspec.dimensions import dims_of
-from lpspec.expression_parser import (
+from lpspec.language.dimensions import dims_of
+from lpspec.language.expression_parser import (
     ArithmeticNode,
     BinaryOperatorNode,
     ComparisonNode,
@@ -32,9 +32,8 @@
     UnaryOperatorNode,
     VariableNode,
 )
-from lpspec.resolution import expression_of, where_of
-from lpspec.typeset.format import Entry, Line
-from lpspec.where_parser import (
+from lpspec.language.resolution import expression_of, where_of
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     DimensionComparisonNode,
@@ -47,10 +46,11 @@
     VariableDefinedNode,
     WhereNode,
 )
+from lpspec.typeset.format import Entry, Line
 
 if TYPE_CHECKING:
-    from lpspec.resolution import Namespace
-    from lpspec.schema import MathSchema
+    from lpspec.language.resolution import Namespace
+    from lpspec.language.schema import MathSchema
     from lpspec.typeset.format import Format
     from lpspec.typeset.symbols import Symbols
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 092a15da..1d37fd7f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -27,7 +27,7 @@
 import pytest
 import yaml as pyyaml
 
-from lpspec.schema import MathSchema
+from lpspec.language.schema import MathSchema
 
 EXAMPLES_DIR = Path(__file__).parent.parent / 'examples'
 
@@ -126,14 +126,14 @@ def resolved(text, schema):
     this: a raw `parse_expression` result still holds NameNodes, and both
     backends now assert those never reach them (resolution.py).
     """
-    from lpspec.resolution import Namespace, expression_of
+    from lpspec.language.resolution import Namespace, expression_of
 
     return expression_of(text, schema, Namespace.of(schema), 't')
 
 
 def resolved_where(text, schema):
     """Parse + resolve a where string."""
-    from lpspec.resolution import Namespace, where_of
+    from lpspec.language.resolution import Namespace, where_of
 
     return where_of(text, Namespace.of(schema), 't')
 
diff --git a/tests/differential.py b/tests/differential.py
index c82bdd6b..0817806d 100644
--- a/tests/differential.py
+++ b/tests/differential.py
@@ -45,8 +45,8 @@
 if TYPE_CHECKING:
     from collections.abc import Iterator, Mapping
 
+    from lpspec.language.schema import MathSchema
     from lpspec.relational.executor import Result
-    from lpspec.schema import MathSchema
 
 #: Both lanes hand the same numbers to the same solver, so they must agree to
 #: solver precision, not to a fudge factor. One tolerance, one place.
diff --git a/tests/test_api.py b/tests/test_api.py
index a291a543..d8a6e8d2 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -291,7 +291,7 @@ def test_no_helper_registry_anywhere():
     makes the differential tests an oracle rather than a comparison of
     dialects (docs/ARCHITECTURE.md, "The expressive ceiling").
     """
-    import lpspec.helpers as helpers
+    import lpspec.language.helpers as helpers
 
     assert not hasattr(lps, 'register')
     assert not hasattr(helpers, 'register')
diff --git a/tests/test_architecture.py b/tests/test_architecture.py
index 3ef51d02..2214eff2 100644
--- a/tests/test_architecture.py
+++ b/tests/test_architecture.py
@@ -148,10 +148,56 @@ def test_engine_is_isolated():
     assert not offenders, f'engine reaches outside its subpackage: {offenders}'
 
 
+#: What ``language/`` may reach: itself, and the same dependency-free leaves the
+#: engine may reach. Both fences point at ``errors.py`` for the same reason —
+#: one exception hierarchy, owned by neither side. Widening this is a decision,
+#: exactly as widening ``ENGINE_MAY_IMPORT`` is.
+LANGUAGE_MAY_IMPORT = ENGINE_MAY_IMPORT
+
+
+def test_language_never_reaches_a_consumer():
+    """Hard rule 1, the other direction: the waist is closed from the front.
+
+    Hard rule 2 keeps the engine from seeing the schema or the AST. This is its
+    mirror: what a model *means* may not depend on what any consumer does with
+    it, so nothing under ``language/`` imports ``lowering``, ``piecewise``,
+    ``sources``, ``api``, or the relational / linopy / typeset subpackages.
+
+    That is what makes ``lps.check()`` a pass with no data and no plan, and a
+    second consumer cheap rather than a second opinion. Membership is read off
+    the path, so a new front-end module cannot land outside the fence by being
+    spelled differently.
+    """
+    offenders = {}
+    for path in (PKG / 'language').rglob('*.py'):
+        if '__pycache__' in path.parts:
+            continue
+        bad = []
+        for node in ast.walk(ast.parse(path.read_text())):  # lazy imports included — the rule is total
+            names = (
+                [a.name for a in node.names]
+                if isinstance(node, ast.Import)
+                else [node.module]
+                if isinstance(node, ast.ImportFrom) and node.module
+                else []
+            )
+            bad += [
+                n
+                for n in names
+                if n.startswith('lpspec') and not n.startswith('lpspec.language') and n not in LANGUAGE_MAY_IMPORT
+            ]
+        if bad:
+            offenders[str(path.relative_to(PKG))] = sorted(set(bad))
+    assert not offenders, (
+        f'the language reaches forward to a consumer: {offenders} — a front-end module '
+        f'may not depend on what is done with the AST it produces'
+    )
+
+
 def test_expansion_has_no_mutable_module_state():
     """Hard rule 5: YAML files are self-contained — nothing importable may
     accumulate state that changes what a file means."""
-    tree = ast.parse((PKG / 'expansion.py').read_text())
+    tree = ast.parse((PKG / 'language' / 'expansion.py').read_text())
     mutable = []
     for node in tree.body:
         if isinstance(node, (ast.Assign, ast.AnnAssign)):
@@ -193,7 +239,7 @@ def test_both_lanes_implement_exactly_the_closed_helper_set():
     Read statically: ``linopy/builder.py`` imports xarray at module level (it
     is linopy lane), and this check must still run on a bare install.
     """
-    from lpspec.helpers import BUILTIN_NAMES
+    from lpspec.language.helpers import BUILTIN_NAMES
 
     tree = ast.parse((PKG / 'linopy' / 'builder.py').read_text())
     table = next(
@@ -244,7 +290,7 @@ def test_every_schema_model_is_strict():
     """A schema model that inherits BaseModel directly silently drops unknown
     keys, which turns a typo into a different model. Strictness lives on
     ``_StrictBlock``, so the check is that nothing bypasses it."""
-    tree = ast.parse((PKG / 'schema.py').read_text())
+    tree = ast.parse((PKG / 'language' / 'schema.py').read_text())
     loose = [
         node.name
         for node in tree.body
diff --git a/tests/test_dimensions.py b/tests/test_dimensions.py
index 0f77c151..b3967e27 100644
--- a/tests/test_dimensions.py
+++ b/tests/test_dimensions.py
@@ -10,13 +10,13 @@
 
 import pytest
 
-from lpspec.dimensions import DimensionError, check_schema, dims_of
-from lpspec.resolution import Namespace, expression_of
+from lpspec.language.dimensions import DimensionError, check_schema, dims_of
+from lpspec.language.resolution import Namespace, expression_of
 from tests.conftest import override, schema_of
 from tools import constructs
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 #: A *network* dispatch model: `conftest.DISPATCH_MODEL` plus buses, so
 #: `group_sum` and per-bus loads are in scope. The dim rules are mostly about
diff --git a/tests/test_doc_examples.py b/tests/test_doc_examples.py
index e2d2b512..2a082956 100644
--- a/tests/test_doc_examples.py
+++ b/tests/test_doc_examples.py
@@ -50,8 +50,8 @@
 import yaml
 
 import lpspec as lps
+from lpspec.language.schema import MathSchema
 from lpspec.relational.executor import PolarsExecutor, Result
-from lpspec.schema import MathSchema
 
 try:
     from lpspec import linopy as linopy_lane
diff --git a/tests/test_expansion.py b/tests/test_expansion.py
index 657364ce..b589d5ee 100644
--- a/tests/test_expansion.py
+++ b/tests/test_expansion.py
@@ -9,10 +9,10 @@
 import numpy as np
 import pytest
 
-from lpspec.expansion import parse_and_expand
-from lpspec.expression_parser import parse_expression
-from lpspec.schema import MathSchema
-from lpspec.validation import validate_expressions
+from lpspec.language.expansion import parse_and_expand
+from lpspec.language.expression_parser import parse_expression
+from lpspec.language.schema import MathSchema
+from lpspec.language.validation import validate_expressions
 from tests.differential import differential
 from tests.oracle import pd
 
diff --git a/tests/test_linopy_lane.py b/tests/test_linopy_lane.py
index 096bb8e2..cbff4ab8 100644
--- a/tests/test_linopy_lane.py
+++ b/tests/test_linopy_lane.py
@@ -19,7 +19,7 @@
 import pytest
 
 from lpspec.errors import DataError, LanguageError
-from lpspec.schema import MathSchema
+from lpspec.language.schema import MathSchema
 from tests.oracle import builder, linopy, loader, lpspec_linopy, pd, xr
 
 
@@ -282,7 +282,7 @@ def gens():
 
 def _resolved(text, parameters=('p_max',), dimensions=('g',)):
     """Resolve then evaluate — the evaluator no longer takes strings."""
-    from lpspec.resolution import Namespace, where_of
+    from lpspec.language.resolution import Namespace, where_of
 
     return where_of(text, Namespace((), parameters, dimensions), 'test')
 
diff --git a/tests/test_lowering.py b/tests/test_lowering.py
index 44908d44..72047f56 100644
--- a/tests/test_lowering.py
+++ b/tests/test_lowering.py
@@ -14,6 +14,8 @@
 import pytest
 
 from lpspec.errors import DataError, DimensionError, LanguageError
+from lpspec.language.resolution import Namespace
+from lpspec.language.schema import MathSchema
 from lpspec.lowering import _lower_expr, _lower_where, lower_program
 from lpspec.relational.plan import (
     DimensionComparison,
@@ -23,8 +25,6 @@
     Sum,
     Variable,
 )
-from lpspec.resolution import Namespace
-from lpspec.schema import MathSchema
 from lpspec.sources import tidy_sources
 from tests.conftest import resolved, schema_of
 from tests.differential import differential
diff --git a/tests/test_parser.py b/tests/test_parser.py
index ba3f2a3c..44d8ee74 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -8,7 +8,7 @@
 
 import pytest
 
-from lpspec.expression_parser import (
+from lpspec.language.expression_parser import (
     BinaryOperatorNode,
     ComparisonNode,
     FunctionCallNode,
@@ -17,7 +17,7 @@
     UnaryOperatorNode,
     parse_expression,
 )
-from lpspec.where_parser import (
+from lpspec.language.where_parser import (
     AndNode,
     BooleanLiteralNode,
     NotNode,
diff --git a/tests/test_relational.py b/tests/test_relational.py
index 92170986..c6e48567 100644
--- a/tests/test_relational.py
+++ b/tests/test_relational.py
@@ -16,6 +16,7 @@
 
 import lpspec as lps
 from lpspec.errors import DataError, LanguageError
+from lpspec.language.schema import MathSchema
 from lpspec.lowering import lower_program
 from lpspec.relational import (
     PolarsExecutor,
@@ -35,7 +36,6 @@
     Variable,
     VariableDeclaration,
 )
-from lpspec.schema import MathSchema
 from tests.conftest import solve_lp_file
 from tests.differential import RTOL, differential
 from tests.oracle import linopy, pd, transport_eager_objective, xr
diff --git a/tests/test_resolution.py b/tests/test_resolution.py
index f4903534..2347994c 100644
--- a/tests/test_resolution.py
+++ b/tests/test_resolution.py
@@ -11,13 +11,13 @@
 import pytest
 
 from lpspec.errors import LanguageError
+from lpspec.language.resolution import Namespace, expression_of, where_of
+from lpspec.language.validation import validate_expressions
 from lpspec.lowering import lower_program
-from lpspec.resolution import Namespace, expression_of, where_of
-from lpspec.validation import validate_expressions
 from tests.conftest import DISPATCH_MODEL, schema_of
 
 if TYPE_CHECKING:
-    from lpspec.schema import MathSchema
+    from lpspec.language.schema import MathSchema
 
 
 def _schema(**overrides) -> MathSchema:
@@ -30,7 +30,7 @@ def _schema(**overrides) -> MathSchema:
 
 
 def test_no_unresolved_name_survives_the_pass():
-    from lpspec.expression_parser import NameNode
+    from lpspec.language.expression_parser import NameNode
 
     schema = _schema()
     ast = expression_of('sum(p * cost, over=generator) == load', schema, Namespace.of(schema), 't')
@@ -49,7 +49,7 @@ def walk(node):
 
 
 def test_names_are_typed_by_kind():
-    from lpspec.expression_parser import DimensionNode, ParameterNode, VariableNode
+    from lpspec.language.expression_parser import DimensionNode, ParameterNode, VariableNode
 
     schema = _schema()
     ast = expression_of('sum(p * cost, over=generator)', schema, Namespace.of(schema), 't')
diff --git a/tests/test_resolution_parity.py b/tests/test_resolution_parity.py
index bb5e43e8..2a0c52f3 100644
--- a/tests/test_resolution_parity.py
+++ b/tests/test_resolution_parity.py
@@ -121,8 +121,8 @@ def test_every_resolved_predicate_is_parity_tested():
     """
     from typing import get_args
 
-    from lpspec.resolution import Namespace, where_of
-    from lpspec.where_parser import UnresolvedComparisonNode, UnresolvedNameNode, WhereNode
+    from lpspec.language.resolution import Namespace, where_of
+    from lpspec.language.where_parser import UnresolvedComparisonNode, UnresolvedNameNode, WhereNode
 
     unresolved = {UnresolvedNameNode, UnresolvedComparisonNode}  # rewritten by resolution, never evaluated
     expected = set(get_args(WhereNode)) - unresolved
diff --git a/tests/test_schema.py b/tests/test_schema.py
index a158e270..7e79d976 100644
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -10,7 +10,7 @@
 import pytest
 from pydantic import ValidationError
 
-from lpspec.schema import MathSchema
+from lpspec.language.schema import MathSchema
 
 
 def test_empty_schema():
diff --git a/tests/test_validation.py b/tests/test_validation.py
index ab68d2ae..ea36cc63 100644
--- a/tests/test_validation.py
+++ b/tests/test_validation.py
@@ -6,8 +6,8 @@
 
 import pytest
 
-from lpspec.schema import MathSchema
-from lpspec.validation import validate_expressions
+from lpspec.language.schema import MathSchema
+from lpspec.language.validation import validate_expressions
 from tests.oracle import linopy, lpspec_linopy, pd
 
 
diff --git a/tests/test_yaml_loading.py b/tests/test_yaml_loading.py
index 7549da76..051e3e45 100644
--- a/tests/test_yaml_loading.py
+++ b/tests/test_yaml_loading.py
@@ -5,8 +5,8 @@
 import pytest
 
 import lpspec as lps
-from lpspec._yaml import read_yaml
-from lpspec.schema import MathSchema
+from lpspec.language._yaml import read_yaml
+from lpspec.language.schema import MathSchema
 
 MODEL = """dimensions:
   snapshot: {dtype: int, values: [0, 1]}
diff --git a/tools/gallery_math.py b/tools/gallery_math.py
index 1e30c4ce..3fdcc344 100644
--- a/tools/gallery_math.py
+++ b/tools/gallery_math.py
@@ -58,7 +58,7 @@
 import sys
 from pathlib import Path
 
-from lpspec._yaml import read_yaml
+from lpspec.language._yaml import read_yaml
 from lpspec.typeset import to_latex, to_markdown
 from tools.constructs import models