diff --git a/backend/agent_dispatch/_composed.py b/backend/agent_dispatch/_composed.py
new file mode 100644
index 0000000..e87159a
--- /dev/null
+++ b/backend/agent_dispatch/_composed.py
@@ -0,0 +1,66 @@
+"""What every backend/agent_dispatch mixin relies on the composed
+AgentDispatcher to provide.
+
+The nine `*DispatchOps` classes are mixins, not standalone types: each is
+composed exactly once, by
+`class AgentDispatcher(DispatcherCoreOps, BuilderDispatchOps, ...)` in
+backend/agents.py. They use `self._runs`, `self._settings_manager` and a few
+of DispatcherCoreOps' own methods across the composition - correct at
+runtime, invisible to a type checker reading one mixin on its own.
+
+Same shape, and same fix, as settings_store/_composed.py's
+SettingsManagerParts and backend/domain/_composed.py's SceneDocumentParts.
+See the first of those for the full reasoning.
+
+EVERYTHING HERE IS TYPE_CHECKING-ONLY: at runtime the class is empty, so
+inheriting it adds no attributes, no methods and no `__init__`.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from backend.events import SessionBus
+    from graphlink_settings_store import SettingsManager
+
+
+class DispatcherParts:
+    """Type-only declaration of the composed AgentDispatcher's shared surface.
+
+    DispatcherCoreOps inherits it too: it supplies most of what is declared
+    here and consumes the rest, the same way PersistenceOps does in
+    settings_store.
+    """
+
+    if TYPE_CHECKING:
+        # Established by DispatcherCoreOps.__init__.
+        _runs: Any                      # backend.run_lifecycle.RunRegistry
+        _settings_manager: SettingsManager
+        _provider_runtime: Any
+        _diagnostics: Any
+
+        def _runtime_kwargs(self) -> dict: ...
+
+        def _cancel_with_pending_approval_denied(self, request_id: str, kind: str) -> bool: ...
+
+        def builder_tool_registry(self, document: Any) -> object: ...
+
+        # The run engine every start_* surface funnels through, and the
+        # plain-blocking-action skeleton the gitlink/code-review surfaces
+        # share. Both live on DispatcherCoreOps.
+        async def _dispatch(self, *args: Any, **kwargs: Any) -> Any: ...
+
+        async def _run_node_blocking_action(
+            self,
+            *,
+            bus: SessionBus,
+            notifications_state: Any,
+            node: Any,
+            action: Any,
+            timeout: float,
+            timeout_message: str,
+            error_log_message: str,
+            error_notify_prefix: str,
+            default: Any = None,
+        ) -> Any: ...
diff --git a/backend/agent_dispatch/builder.py b/backend/agent_dispatch/builder.py
index 50824ff..2022372 100644
--- a/backend/agent_dispatch/builder.py
+++ b/backend/agent_dispatch/builder.py
@@ -24,8 +24,10 @@
 import asyncio
 import threading
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class BuilderDispatchOps:
+
+class BuilderDispatchOps(DispatcherParts):
     """The Builder agent loop and its tool-registry assembly (mixin - see module docstring)."""
 
     def builder_tool_registry(self, document) -> "object":
diff --git a/backend/agent_dispatch/chat.py b/backend/agent_dispatch/chat.py
index 3a8a1d5..23309ad 100644
--- a/backend/agent_dispatch/chat.py
+++ b/backend/agent_dispatch/chat.py
@@ -30,8 +30,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class ChatDispatchOps:
+
+class ChatDispatchOps(DispatcherParts):
     """Chat, conversation, and image reply surfaces (mixin - see module docstring)."""
 
     async def start_chat_reply(
diff --git a/backend/agent_dispatch/code_review.py b/backend/agent_dispatch/code_review.py
index 45ad1d6..77718f7 100644
--- a/backend/agent_dispatch/code_review.py
+++ b/backend/agent_dispatch/code_review.py
@@ -34,8 +34,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class CodeReviewDispatchOps:
+
+class CodeReviewDispatchOps(DispatcherParts):
     """Review Lens dispatch: PR-diff fetch plus the Review and Ask surfaces (mixin - see module docstring)."""
 
     async def fetch_code_review_diff(self, *, bus: SessionBus, notifications_state, node, pr_url: str):
diff --git a/backend/agent_dispatch/code_sandbox.py b/backend/agent_dispatch/code_sandbox.py
index 87afc10..0b21098 100644
--- a/backend/agent_dispatch/code_sandbox.py
+++ b/backend/agent_dispatch/code_sandbox.py
@@ -29,8 +29,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class CodeSandboxDispatchOps:
+
+class CodeSandboxDispatchOps(DispatcherParts):
     """The Execution Sandbox run surface and its plumbing (mixin - see module docstring)."""
 
     async def remove_code_sandbox_scratch_dir(self, sandbox_id: str) -> None:
diff --git a/backend/agent_dispatch/content.py b/backend/agent_dispatch/content.py
index 09a0f2e..6150922 100644
--- a/backend/agent_dispatch/content.py
+++ b/backend/agent_dispatch/content.py
@@ -26,8 +26,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class ContentDispatchOps:
+
+class ContentDispatchOps(DispatcherParts):
     """Chart, note, branch-comparison, and branch-synthesis generation (mixin - see module docstring)."""
 
     async def start_chart_generation(
diff --git a/backend/agent_dispatch/core.py b/backend/agent_dispatch/core.py
index bb35d2f..d7f383c 100644
--- a/backend/agent_dispatch/core.py
+++ b/backend/agent_dispatch/core.py
@@ -34,8 +34,10 @@
     from backend.events import SessionBus
     from graphlink_settings_store import SettingsManager
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class DispatcherCoreOps:
+
+class DispatcherCoreOps(DispatcherParts):
     """The shared dispatcher core: construction, model resolution, cancellation, approvals, and `_dispatch` (mixin - see module docstring)."""
 
     def __init__(self, settings_manager: SettingsManager, provider_runtime=None, diagnostics=None):
diff --git a/backend/agent_dispatch/gitlink.py b/backend/agent_dispatch/gitlink.py
index 907cb03..97fa5a8 100644
--- a/backend/agent_dispatch/gitlink.py
+++ b/backend/agent_dispatch/gitlink.py
@@ -34,8 +34,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class GitlinkDispatchOps:
+
+class GitlinkDispatchOps(DispatcherParts):
     """Gitlink dispatch: repo plumbing plus the Run and Apply surfaces (mixin - see module docstring)."""
 
     async def fetch_gitlink_repositories(self, *, bus: SessionBus, notifications_state, node) -> list[str]:
diff --git a/backend/agent_dispatch/harness.py b/backend/agent_dispatch/harness.py
index 1b79e96..b00aa26 100644
--- a/backend/agent_dispatch/harness.py
+++ b/backend/agent_dispatch/harness.py
@@ -24,8 +24,10 @@
 import asyncio
 import threading
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class HarnessDispatchOps:
+
+class HarnessDispatchOps(DispatcherParts):
     """The Harness agent: grants, process bookkeeping, and the Harness run (mixin - see module docstring)."""
 
     def answer_harness_question(self, request_id: str, answer) -> bool:
diff --git a/backend/agent_dispatch/research.py b/backend/agent_dispatch/research.py
index 48209f3..7a86242 100644
--- a/backend/agent_dispatch/research.py
+++ b/backend/agent_dispatch/research.py
@@ -29,8 +29,10 @@
 if TYPE_CHECKING:
     from backend.events import SessionBus
 
+from backend.agent_dispatch._composed import DispatcherParts
 
-class ResearchDispatchOps:
+
+class ResearchDispatchOps(DispatcherParts):
     """Web Research and Artifact generation dispatch (mixin - see module docstring)."""
 
     async def start_web_research(
diff --git a/backend/domain/_composed.py b/backend/domain/_composed.py
new file mode 100644
index 0000000..345fd2f
--- /dev/null
+++ b/backend/domain/_composed.py
@@ -0,0 +1,79 @@
+"""What every backend/domain mixin relies on SceneDocument to provide.
+
+BranchOps, GroupOps, LayoutOps and CommandOps are mixins, not standalone
+types: each is composed exactly once, by
+`class SceneDocument(BranchOps, GroupOps, LayoutOps, CommandOps)` in
+backend/domain/graph.py. They freely use `self.nodes`, `self.edges`,
+`self.command_log` and a handful of SceneDocument's own methods - correct at
+runtime, and invisible to a type checker looking at one mixin in isolation.
+
+That invisibility is most of why `[tool.mypy].files` could not be widened to
+backend/domain/. Nothing declared the contract between a mixin and the class
+composing it, so the tree could not be checked at all - not because it was
+badly typed, but because it was untypeable.
+
+Same shape, and same fix, as settings_store/_composed.py's
+SettingsManagerParts. See that module's docstring for the full reasoning.
+
+EVERYTHING HERE IS TYPE_CHECKING-ONLY. At runtime this class is empty, so
+inheriting it adds no attributes, no methods and no `__init__` - the real
+implementations still come from SceneDocument's own body and from the
+mixins themselves. It declares what the composed object has; it is never a
+second source of it.
+"""
+
+from __future__ import annotations
+
+import itertools
+from collections import deque
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from backend.domain.model import SceneEdge, SceneNode
+
+
+class SceneDocumentParts:
+    """Type-only declaration of the composed SceneDocument's shared surface.
+
+    Every mixin in this package inherits it. CommandOps supplies
+    command_log/redo_stack itself and consumes the rest, exactly as
+    PersistenceOps does in settings_store - a mixin both providing and
+    consuming is normal here.
+    """
+
+    if TYPE_CHECKING:
+        # Core graph state, held on SceneDocument itself.
+        nodes: dict[str, SceneNode]
+        edges: dict[str, SceneEdge]
+        image_assets: dict[str, Any]
+        measured_sizes: dict[str, Any]
+        pins: Any
+        _counter: itertools.count
+
+        # The undo/redo command layer (CommandOps' own, consumed by siblings).
+        command_log: deque
+        redo_stack: list
+        _composite_depth: int
+        _composite_buffer: list
+
+        # SceneDocument methods the mixins call across the composition.
+        def connect(self, source: str, target: str) -> SceneEdge: ...
+
+        def remove_nodes(self, node_ids: list[str]) -> None: ...
+
+        def add_chat_node(
+            self, x: float, y: float, content: str, is_user: bool,
+            parent_id: str | None = None, *args: Any, **kwargs: Any,
+        ) -> SceneNode: ...
+
+        def place_root(self, kind: str) -> tuple[float, float]: ...
+
+        def place_child(
+            self, parent_id: str | None, kind: str, *, prefer: str = "below",
+        ) -> tuple[float, float]: ...
+
+        def _recompute_group_bounds(self, node_id: str) -> None: ...
+
+        def _reaches(self, start: str, goal: str) -> bool: ...
+
+        def _detach_node_from_membership(self, node_id: str) -> None: ...
diff --git a/backend/domain/branches.py b/backend/domain/branches.py
index 0c0bb5a..22313fa 100644
--- a/backend/domain/branches.py
+++ b/backend/domain/branches.py
@@ -31,8 +31,10 @@
 if TYPE_CHECKING:
     from graphlink_model_catalog import ModelRef
 
+from backend.domain._composed import SceneDocumentParts
 
-class BranchOps:
+
+class BranchOps(SceneDocumentParts):
 
     def mark_branch_comparison_note(self, node_id: str, source_node_ids: list[str]) -> None:
         """ADR-002 Workstream 1 ("Compare Branches"): stamps an already-created
diff --git a/backend/domain/commands.py b/backend/domain/commands.py
index eff8628..671567c 100644
--- a/backend/domain/commands.py
+++ b/backend/domain/commands.py
@@ -85,6 +85,8 @@
     from backend.domain.model import SceneEdge, SceneNode
     from graphlink_navigation_pins import NavigationPinRecord
 
+from backend.domain._composed import SceneDocumentParts
+
 T = TypeVar("T")
 
 # Asset bytes (image/chart PNGs) are the one thing in this module that IS
@@ -444,7 +446,7 @@ def _merge_commands(command_type, provenance, commands, run_id=None):
     return merged
 
 
-class CommandOps:
+class CommandOps(SceneDocumentParts):
     def record_command(
         self,
         command_type: str,
diff --git a/backend/domain/groups.py b/backend/domain/groups.py
index 38b584f..97a057f 100644
--- a/backend/domain/groups.py
+++ b/backend/domain/groups.py
@@ -31,8 +31,10 @@
 )
 from backend.domain.node_states import ContainerState, FrameState
 
+from backend.domain._composed import SceneDocumentParts
 
-class GroupOps:
+
+class GroupOps(SceneDocumentParts):
 
     def _member_footprint(self, member: SceneNode) -> tuple[float, float]:
         """One member's (width, height) for bbox purposes, best source first:
diff --git a/backend/domain/layout.py b/backend/domain/layout.py
index 45a1c87..00821ac 100644
--- a/backend/domain/layout.py
+++ b/backend/domain/layout.py
@@ -34,6 +34,8 @@
     SceneNode,
 )
 
+from backend.domain._composed import SceneDocumentParts
+
 # Clearance between neighbouring nodes. Horizontal is a little wider than
 # vertical so sibling fans read as distinct columns; both are large enough
 # that edge routing has room to breathe between cards.
@@ -91,7 +93,7 @@ def _rects_clear(
     )
 
 
-class LayoutOps:
+class LayoutOps(SceneDocumentParts):
     """Placement/layout mixin for SceneDocument (same pattern as GroupOps).
     Reads self.nodes / self.edges / self.measured_sizes."""
 
diff --git a/tests/test_mixin_declaration_bases.py b/tests/test_mixin_declaration_bases.py
new file mode 100644
index 0000000..d92d424
--- /dev/null
+++ b/tests/test_mixin_declaration_bases.py
@@ -0,0 +1,83 @@
+"""The type-only mixin declaration bases must stay type-only.
+
+Three of them now exist, one per mixin family:
+
+    settings_store/_composed.py       SettingsManagerParts
+    backend/domain/_composed.py       SceneDocumentParts
+    backend/agent_dispatch/_composed.py  DispatcherParts
+
+Each declares what a mixin's composing class provides, so a checker can read
+one mixin in isolation. Together they took `mypy backend` from 1,059 errors
+to 844 and eliminated the "*Ops has no attribute" class entirely.
+
+Each also sits LAST in its composed class's MRO, immediately before object,
+which makes it the perfect place for an accident: anything defined outside
+the `if TYPE_CHECKING:` block becomes a real attribute that shadows nothing
+today but would be silently picked up the moment a sibling mixin stopped
+defining its own. A fallback nobody asked for, in the one class written to
+have no behaviour at all.
+
+tests/test_settings_mixin_contract.py covers SettingsManagerParts in more
+depth (signature matching, name-by-name provisioning). This file holds the
+properties that must be true of ALL of them, so a fourth base added later is
+covered by construction rather than by remembering.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from backend.agent_dispatch._composed import DispatcherParts
+from backend.agents import AgentDispatcher
+from backend.domain._composed import SceneDocumentParts
+from backend.domain.graph import SceneDocument
+from graphlink_settings_store import SettingsManager
+from settings_store._composed import SettingsManagerParts
+
+# (declaration base, the class that composes it)
+BASES = [
+    pytest.param(SettingsManagerParts, SettingsManager, id="settings"),
+    pytest.param(SceneDocumentParts, SceneDocument, id="domain"),
+    pytest.param(DispatcherParts, AgentDispatcher, id="dispatch"),
+]
+
+
+@pytest.mark.parametrize("base, composed", BASES)
+def test_the_declaration_base_has_no_runtime_body(base, composed):
+    """Nothing but dunders. A method or attribute here would be a real
+    implementation in a class whose entire purpose is to have none."""
+    own = [name for name in vars(base) if not name.startswith("__")]
+    assert own == [], f"{base.__name__} gained a runtime member: {own}"
+
+
+@pytest.mark.parametrize("base, composed", BASES)
+def test_it_defines_no_initializer(base, composed):
+    """An __init__ here would land in the composed class's MRO after every
+    real mixin and quietly change construction."""
+    assert "__init__" not in vars(base)
+
+
+@pytest.mark.parametrize("base, composed", BASES)
+def test_the_base_sits_last_in_the_mro(base, composed):
+    """Immediately before object: it must never take precedence over a real
+    mixin's implementation of the same name."""
+    mro = composed.__mro__
+    assert mro[-1] is object
+    assert mro[-2] is base, [c.__name__ for c in mro]
+
+
+@pytest.mark.parametrize("base, composed", BASES)
+def test_every_mixin_in_the_family_inherits_it(base, composed):
+    """A mixin that does not inherit the base is invisible to the checker
+    again - the exact hole these bases were added to close. Catches a new
+    sibling landing without one."""
+    mixins = [
+        cls for cls in composed.__mro__
+        if cls not in (composed, base, object) and cls.__name__.endswith("Ops")
+    ]
+    assert mixins, f"no *Ops mixins found in {composed.__name__}'s MRO"
+    missing = [cls.__name__ for cls in mixins if not issubclass(cls, base)]
+    assert not missing, (
+        f"these {composed.__name__} mixins do not inherit {base.__name__}, so nothing "
+        f"declares what they use from the composition: {missing}"
+    )