diff --git a/amplifier_foundation/bundle/_dataclass.py b/amplifier_foundation/bundle/_dataclass.py
index e4de9b2..adc291d 100644
--- a/amplifier_foundation/bundle/_dataclass.py
+++ b/amplifier_foundation/bundle/_dataclass.py
@@ -56,6 +56,9 @@ class Bundle:
             When absent (the default), the YAML file's own directory is used.  Only
             single-level relative paths are supported; 3+ level nesting is out of scope.
         session: Session config (orchestrator, context).
+        routing: Optional routing configuration (e.g. ``{'matrix': 'openai'}``).
+            Opaque to foundation; interpreted by the host application. Acts as
+            a DEFAULT -- user/project settings override it.
         providers: List of provider configs.
         tools: List of tool configs.
         hooks: List of hook configs.
@@ -98,6 +101,7 @@ class Bundle:
     spawn: dict[str, Any] = field(
         default_factory=dict
     )  # Spawn config (exclude_tools, etc.)
+    routing: dict[str, Any] = field(default_factory=dict)
 
     # Resources
     agents: dict[str, dict[str, Any]] = field(default_factory=dict)
@@ -132,6 +136,8 @@ def __post_init__(self) -> None:
             self._pending_context = {}
         if self.origins is None:
             self.origins = {}
+        if self.routing is None:
+            self.routing = {}
 
     def compose(self, *others: Bundle) -> Bundle:
         """Compose this bundle with others (later overrides earlier).
@@ -186,6 +192,7 @@ def compose(self, *others: Bundle) -> Bundle:
             tools=list(self.tools),
             hooks=list(self.hooks),
             spawn=dict(self.spawn),
+            routing=dict(self.routing),
             agents=dict(self.agents),
             context=initial_context,
             _pending_context=initial_pending_context,
@@ -235,6 +242,10 @@ def compose(self, *others: Bundle) -> Bundle:
             # Spawn config: deep merge (later overrides)
             result.spawn = deep_merge(result.spawn, other.spawn)
 
+            # Routing: deep merge (later overrides). Opaque passthrough --
+            # foundation does not interpret matrix/overrides keys.
+            result.routing = deep_merge(result.routing, other.routing)
+
             # Module lists: merge by module ID
             result.providers = merge_module_lists(result.providers, other.providers)
             result.tools = merge_module_lists(result.tools, other.tools)
@@ -298,6 +309,13 @@ def to_mount_plan(self) -> dict[str, Any]:
         if self.spawn:
             mount_plan["spawn"] = dict(self.spawn)
 
+        # NOTE: self.routing is deliberately NOT included in the mount plan.
+        # The mount plan is the kernel-facing surface; routing is app-layer
+        # policy (which provider/model a role maps to). Adding it here would
+        # leak policy toward the kernel. Host apps read bundle.routing
+        # directly off the Bundle/PreparedBundle to apply their own routing
+        # policy on top of it -- do not "fix" this by adding it back.
+
         return mount_plan
 
     async def prepare(
@@ -770,6 +788,13 @@ def from_dict(cls, data: dict[str, Any], base_path: Path | None = None) -> Bundl
             data.get("context", {}), base_path
         )
 
+        # Routing is opaque passthrough: foundation stores and merges the dict,
+        # it does not validate or interpret matrix/overrides keys. Non-dict
+        # values (e.g. a bare string) coerce to an empty dict rather than
+        # raising, since foundation has no opinion on the schema.
+        _routing = data.get("routing") or {}
+        routing = _routing if isinstance(_routing, dict) else {}
+
         return cls(
             name=bundle_name,
             version=bundle_meta.get("version", "1.0.0"),
@@ -781,6 +806,7 @@ def from_dict(cls, data: dict[str, Any], base_path: Path | None = None) -> Bundl
             tools=tools,
             hooks=hooks,
             spawn=data.get("spawn", {}),
+            routing=routing,
             agents=_parse_agents(data.get("agents", {}), base_path),
             context=resolved_context,
             _pending_context=pending_context,
diff --git a/docs/BUNDLE_GUIDE.md b/docs/BUNDLE_GUIDE.md
index b5ba5d2..290bed2 100644
--- a/docs/BUNDLE_GUIDE.md
+++ b/docs/BUNDLE_GUIDE.md
@@ -1237,6 +1237,16 @@ spawn:
   # OR use explicit list:
   # tools: [tool-a, tool-b]         # Agents get ONLY these tools
 
+# Optional default routing matrix. Opaque to foundation -- it stores and
+# deep-merges this dict but does not interpret matrix/overrides. This is a
+# DEFAULT: user/project settings' own `routing:` block always wins. Omit
+# this key entirely and behavior is unchanged from today.
+routing:
+  matrix: openai
+  overrides:
+    coding:
+      model: gpt-5
+
 # Declare which agents this bundle PROVIDES (a mapping value).
 # Not to be confused with the same key in an *agent's* frontmatter, where a
 # string or list value declares which agents that agent may DELEGATE TO --
@@ -1292,6 +1302,8 @@ includes:
 - Later bundles override earlier ones
 - `session`: deep-merged (nested dicts merged recursively, later wins for scalars)
 - `spawn`: deep-merged (later overrides earlier)
+- `routing`: deep-merged (later overrides earlier); this is a DEFAULT -- any
+  `routing:` set in user/project settings overrides whatever a bundle declares
 - `providers`, `tools`, `hooks`: merged by module ID (configs for same module are deep-merged)
 - `agents`: merged by agent name (later wins)
 - `context`: accumulates with namespace prefix (each bundle contributes without collision)
diff --git a/tests/test_bundle_routing_field.py b/tests/test_bundle_routing_field.py
new file mode 100644
index 0000000..8eb139a
--- /dev/null
+++ b/tests/test_bundle_routing_field.py
@@ -0,0 +1,91 @@
+"""Tests for the Bundle.routing field (opaque passthrough for bundle-declared
+default routing matrix configuration).
+
+Foundation stores and merges this dict; it does not interpret or validate
+its contents (e.g. `matrix`, `overrides`). A separate app-cli PR consumes it.
+"""
+
+from amplifier_foundation.bundle import Bundle
+
+
+class TestBundleRoutingFromDict:
+    """Tests for Bundle.from_dict's handling of the routing field."""
+
+    def test_from_dict_no_routing_key_yields_empty_dict(self) -> None:
+        """A bundle dict with no 'routing' key produces bundle.routing == {}.
+
+        This is the single most important property: existing bundles with
+        no routing: key must behave byte-identically to today.
+        """
+        data = {"bundle": {"name": "test"}}
+        bundle = Bundle.from_dict(data)
+        assert bundle.routing == {}
+
+    def test_from_dict_reads_routing_matrix_and_overrides(self) -> None:
+        """routing: {matrix, overrides} is read through unchanged (opaque)."""
+        data = {
+            "bundle": {"name": "test"},
+            "routing": {
+                "matrix": "openai",
+                "overrides": {"coding": {"model": "gpt-5"}},
+            },
+        }
+        bundle = Bundle.from_dict(data)
+        assert bundle.routing == {
+            "matrix": "openai",
+            "overrides": {"coding": {"model": "gpt-5"}},
+        }
+
+    def test_from_dict_non_dict_routing_coerces_to_empty(self) -> None:
+        """A malformed 'routing' value (bare string) coerces to {} instead
+        of raising -- foundation does not validate routing semantics."""
+        data = {"bundle": {"name": "test"}, "routing": "openai"}
+        bundle = Bundle.from_dict(data)
+        assert bundle.routing == {}
+
+
+class TestBundleRoutingCompose:
+    """Tests for Bundle.compose's handling of the routing field."""
+
+    def test_compose_overlay_routing_wins_over_base(self) -> None:
+        """Later bundle's routing scalar values win over the base's."""
+        base = Bundle(name="base", routing={"matrix": "anthropic"})
+        overlay = Bundle(name="overlay", routing={"matrix": "openai"})
+        result = base.compose(overlay)
+        assert result.routing["matrix"] == "openai"
+
+    def test_compose_deep_merge_preserves_base_matrix_when_overlay_sets_only_overrides(
+        self,
+    ) -> None:
+        """An overlay declaring only 'overrides' keeps the base's 'matrix'
+        (deep merge, not replace)."""
+        base = Bundle(
+            name="base",
+            routing={"matrix": "anthropic", "overrides": {"coding": {"x": 1}}},
+        )
+        overlay = Bundle(
+            name="overlay",
+            routing={"overrides": {"coding": {"y": 2}}},
+        )
+        result = base.compose(overlay)
+        assert result.routing["matrix"] == "anthropic"
+        assert result.routing["overrides"] == {"coding": {"x": 1, "y": 2}}
+
+    def test_compose_base_routing_survives_overlay_without_routing(self) -> None:
+        """An overlay bundle with no routing at all does not clobber the
+        base's routing config."""
+        base = Bundle(name="base", routing={"matrix": "anthropic"})
+        overlay = Bundle(name="overlay")
+        result = base.compose(overlay)
+        assert result.routing == {"matrix": "anthropic"}
+
+
+class TestBundleRoutingMountPlan:
+    """Tests for Bundle.to_mount_plan's handling of the routing field."""
+
+    def test_to_mount_plan_omits_routing(self) -> None:
+        """routing is deliberately absent from the mount plan -- it is
+        app-layer policy, not a kernel-facing concern."""
+        bundle = Bundle(name="test", routing={"matrix": "anthropic"})
+        plan = bundle.to_mount_plan()
+        assert "routing" not in plan