diff --git a/bot_bottle/manifest_extends.py b/bot_bottle/manifest_extends.py
index 37176f3..a4a2920 100644
--- a/bot_bottle/manifest_extends.py
+++ b/bot_bottle/manifest_extends.py
@@ -6,6 +6,7 @@ from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
     from .manifest import ManifestBottle, ManifestGitEntry
+    from .manifest_egress import ManifestEgressConfig
 
 
 def resolve_bottles(raws: dict[str, dict[str, object]]) -> dict[str, ManifestBottle]:
@@ -99,9 +100,16 @@ def _merge_bottles(
     else:
         merged_git = parent.git
 
-    # Presence-driven full-replace for the remaining list-valued +
-    # scalar fields.
-    merged_egress = child.egress if "egress" in child_raw else parent.egress
+    # egress.routes: missing means inherit; otherwise parent and child
+    # route lists concatenate. Other egress scalar fields remain
+    # presence-driven overlays.
+    merged_egress = (
+        _merge_egress(parent.egress, child.egress, child_raw)
+        if "egress" in child_raw
+        else parent.egress
+    )
+
+    # Presence-driven full-replace for the remaining scalar fields.
     merged_agent_provider = (
         child.agent_provider
         if "agent_provider" in child_raw
@@ -140,3 +148,17 @@ def _merge_git_remotes(
     for entry in child:
         by_host[entry.UpstreamHost] = entry
     return tuple(by_host.values())
+
+
+def _merge_egress(
+    parent: ManifestEgressConfig,
+    child: ManifestEgressConfig,
+    child_raw: dict[str, object],
+) -> ManifestEgressConfig:
+    from .manifest_egress import ManifestEgressConfig
+    from .manifest_util import as_json_object
+
+    child_egress_raw = as_json_object(child_raw.get("egress"), "child egress")
+    routes = parent.routes + child.routes
+    log = child.Log if "log" in child_egress_raw else parent.Log
+    return ManifestEgressConfig(routes=routes, Log=log)
diff --git a/tests/unit/test_manifest_extends.py b/tests/unit/test_manifest_extends.py
index b1a20ad..85b9590 100644
--- a/tests/unit/test_manifest_extends.py
+++ b/tests/unit/test_manifest_extends.py
@@ -173,10 +173,10 @@ class TestExtendsGitMerge(unittest.TestCase):
         self.assertEqual("Child", m.bottles["child"].git_user.name)
 
 
-class TestExtendsListsFullReplace(unittest.TestCase):
-    """egress: remains full-replace when the child declares it."""
+class TestExtendsEgressMerge(unittest.TestCase):
+    """egress.routes merges; egress.log overlays only when declared."""
 
-    def test_child_egress_replaces_parent_entirely(self):
+    def test_child_egress_routes_merge_with_parent(self):
         m = _build(
             base={"egress": {"routes": [{"host": "a.example.com"}]}},
             child={
@@ -185,7 +185,7 @@ class TestExtendsListsFullReplace(unittest.TestCase):
             },
         )
         hosts = [r.Host for r in m.bottles["child"].egress.routes]
-        self.assertEqual(["b.example.com"], hosts)
+        self.assertEqual(["a.example.com", "b.example.com"], hosts)
 
     def test_child_omits_egress_inherits(self):
         m = _build(
@@ -195,6 +195,52 @@ class TestExtendsListsFullReplace(unittest.TestCase):
         hosts = [r.Host for r in m.bottles["child"].egress.routes]
         self.assertEqual(["a.example.com"], hosts)
 
+    def test_child_egress_log_inherits_parent_routes(self):
+        m = _build(
+            base={
+                "egress": {
+                    "routes": [{"host": "a.example.com"}],
+                    "log": 1,
+                },
+            },
+            child={"extends": "base", "egress": {"log": 2}},
+        )
+        child = m.bottles["child"].egress
+        self.assertEqual(["a.example.com"], [r.Host for r in child.routes])
+        self.assertEqual(2, child.Log)
+
+    def test_child_egress_routes_inherit_parent_log_when_omitted(self):
+        m = _build(
+            base={
+                "egress": {
+                    "routes": [{"host": "a.example.com"}],
+                    "log": 1,
+                },
+            },
+            child={
+                "extends": "base",
+                "egress": {"routes": [{"host": "b.example.com"}]},
+            },
+        )
+        child = m.bottles["child"].egress
+        self.assertEqual(
+            ["a.example.com", "b.example.com"],
+            [r.Host for r in child.routes],
+        )
+        self.assertEqual(1, child.Log)
+
+    def test_duplicate_host_across_parent_and_child_dies(self):
+        msg = _error_message(
+            _build,
+            base={"egress": {"routes": [{"host": "a.example.com"}]}},
+            child={
+                "extends": "base",
+                "egress": {"routes": [{"host": "A.EXAMPLE.COM"}]},
+            },
+        )
+        self.assertIn("duplicate host", msg)
+        self.assertIn("A.EXAMPLE.COM", msg)
+
 
 class TestExtendsGitUserOverlay(unittest.TestCase):
     """git-gate.user: per-field overlay. Each non-empty field on child