diff --git a/gns3server/api/routes/mcp/__init__.py b/gns3server/api/routes/mcp/__init__.py
index 3f2425b00..2369fe658 100644
--- a/gns3server/api/routes/mcp/__init__.py
+++ b/gns3server/api/routes/mcp/__init__.py
@@ -1052,7 +1052,6 @@ async def marker_definition(
     tag: Annotated[int | None, Field(description="Numeric tag for packet correlation")] = None,
     color: Annotated[str | None, Field(description="Hex color for UI highlight, e.g. '#ff5722'")] = None,
     highlight_duration: Annotated[int | None, Field(description="UI highlight duration in milliseconds")] = None,
-    direction: Annotated[str | None, Field(description="Direction filter: 'tx' (capture node sending only), 'rx' (receiving only), or 'both' (no filter — on update this clears a previously set direction). Omit to leave unchanged on update.")] = None,
 ) -> list[dict[str, Any]]:
     """Manage project-level marker definitions — traffic-insight rules that apply to ALL links.
 
@@ -1061,14 +1060,20 @@ async def marker_definition(
     On delete, 'global-{name}' is removed from every link.
 
     Create requires: project_id, action='create', bpf
-    Update requires: project_id, action='update', def_name, and at least one of (bpf, tag, direction, color, highlight_duration)
+    Update requires: project_id, action='update', def_name, and at least one of (bpf, tag, color, highlight_duration)
     Delete requires: project_id, action='delete', def_name
     List requires:  project_id, action='list'
 
+    A definition has NO direction (tx/rx): it fans out to every link and auto-selects
+    its capture node on each, so a fixed direction has no consistent meaning. Encode
+    the direction you want in the BPF instead (e.g. 'icmp and icmp[icmptype]==8' for
+    echo requests only). For a capture-node-relative direction on a single link, use
+    the per-link `link_marker` tool.
+
     Common BPF examples: 'arp', 'icmp', 'ospf', 'tcp port 22', 'udp port 53'
     """
     params = {"project_id": project_id, "action": action}
-    for opt in ("bpf", "def_name", "name", "tag", "direction", "color", "highlight_duration"):
+    for opt in ("bpf", "def_name", "name", "tag", "color", "highlight_duration"):
         val = locals().get(opt)
         if val is not None:
             params[opt] = val
diff --git a/gns3server/api/routes/mcp/links.py b/gns3server/api/routes/mcp/links.py
index 9ddf2b377..7e1afc6a2 100644
--- a/gns3server/api/routes/mcp/links.py
+++ b/gns3server/api/routes/mcp/links.py
@@ -460,9 +460,9 @@ def marker_definition_handler(params: dict[str, Any], gns3_ctx: dict[str, Any])
         for opt in ("name", "tag", "color", "highlight_duration"):
             if params.get(opt) is not None:
                 body[opt] = params[opt]
-        # direction: "tx"/"rx" set a one-way filter; "both"/omitted = no filter.
-        if params.get("direction") in ("tx", "rx"):
-            body["direction"] = params["direction"]
+        # No direction: a definition fans out to every link and auto-selects its
+        # capture node on each, so tx/rx (which is relative to that node) has no
+        # consistent meaning. Encode direction in the BPF instead.
         return conn.http_call("post", base, json_data=body).json()
 
     def_name = params.get("def_name")
@@ -476,14 +476,8 @@ def marker_definition_handler(params: dict[str, Any], gns3_ctx: dict[str, Any])
         for opt in ("bpf", "tag", "color", "highlight_duration"):
             if params.get(opt) is not None:
                 body[opt] = params[opt]
-        # direction tri-state: omitted=preserve, "tx"/"rx"=set, "both"=clear (→ null).
-        direction = params.get("direction")
-        if direction == "both":
-            body["direction"] = None
-        elif direction in ("tx", "rx"):
-            body["direction"] = direction
         if not body:
-            return {"error": "At least one update field is required (bpf, tag, direction, color, highlight_duration)"}
+            return {"error": "At least one update field is required (bpf, tag, color, highlight_duration)"}
         return conn.http_call("put", url, json_data=body).json()
 
     # action == "delete"
diff --git a/tests/api/routes/mcp/test_handlers.py b/tests/api/routes/mcp/test_handlers.py
index 5cd8a8bc0..fc8cd00d2 100644
--- a/tests/api/routes/mcp/test_handlers.py
+++ b/tests/api/routes/mcp/test_handlers.py
@@ -441,39 +441,44 @@ class TestLinkMarker:
 
 
 class TestMarkerDefinition:
-    """marker_definition_handler direction tri-state (same semantics as link markers)."""
+    """marker_definition_handler build create/update bodies.
+
+    A definition has NO direction: it fans out to every link and auto-selects its
+    capture node on each, so tx/rx (relative to that node) has no consistent
+    meaning — any direction passed is ignored, never reaching the request body.
+    """
 
     mod = "links"
 
-    def test_update_direction_both_clears(self, ctx):
+    def test_create_builds_body(self, ctx):
         from gns3server.api.routes.mcp.links import marker_definition_handler
         with patch(f"{BASE}.{self.mod}._get_connector") as m:
             conn = _mock_conn({"name": "arp"})
             m.return_value = conn
             marker_definition_handler(
-                {"project_id": "p", "action": "update",
-                 "def_name": "arp", "direction": "both"}, ctx,
+                {"project_id": "p", "action": "create",
+                 "bpf": "arp", "tag": 1, "color": "#fff"}, ctx,
             )
             conn.http_call.assert_called_with(
-                "put", "http://192.168.1.3:3080/v3/projects/p/marker-definitions/arp",
-                json_data={"direction": None},
+                "post", "http://192.168.1.3:3080/v3/projects/p/marker-definitions",
+                json_data={"bpf": "arp", "tag": 1, "color": "#fff"},
             )
 
-    def test_update_direction_tx_sets(self, ctx):
+    def test_create_ignores_direction(self, ctx):
         from gns3server.api.routes.mcp.links import marker_definition_handler
         with patch(f"{BASE}.{self.mod}._get_connector") as m:
             conn = _mock_conn({"name": "arp"})
             m.return_value = conn
             marker_definition_handler(
-                {"project_id": "p", "action": "update",
-                 "def_name": "arp", "direction": "tx"}, ctx,
+                {"project_id": "p", "action": "create",
+                 "bpf": "arp", "direction": "tx"}, ctx,
             )
             conn.http_call.assert_called_with(
-                "put", "http://192.168.1.3:3080/v3/projects/p/marker-definitions/arp",
-                json_data={"direction": "tx"},
+                "post", "http://192.168.1.3:3080/v3/projects/p/marker-definitions",
+                json_data={"bpf": "arp"},
             )
 
-    def test_update_direction_omitted_preserved(self, ctx):
+    def test_update_builds_body(self, ctx):
         from gns3server.api.routes.mcp.links import marker_definition_handler
         with patch(f"{BASE}.{self.mod}._get_connector") as m:
             conn = _mock_conn({"name": "arp"})
@@ -487,30 +492,24 @@ class TestMarkerDefinition:
                 json_data={"tag": 1},
             )
 
-    def test_create_direction_both_omitted(self, ctx):
+    def test_update_ignores_direction(self, ctx):
         from gns3server.api.routes.mcp.links import marker_definition_handler
         with patch(f"{BASE}.{self.mod}._get_connector") as m:
             conn = _mock_conn({"name": "arp"})
             m.return_value = conn
             marker_definition_handler(
-                {"project_id": "p", "action": "create",
-                 "bpf": "arp", "direction": "both"}, ctx,
+                {"project_id": "p", "action": "update",
+                 "def_name": "arp", "tag": 1, "direction": "rx"}, ctx,
             )
             conn.http_call.assert_called_with(
-                "post", "http://192.168.1.3:3080/v3/projects/p/marker-definitions",
-                json_data={"bpf": "arp"},
+                "put", "http://192.168.1.3:3080/v3/projects/p/marker-definitions/arp",
+                json_data={"tag": 1},
             )
 
-    def test_create_direction_tx(self, ctx):
+    def test_update_requires_a_field(self, ctx):
         from gns3server.api.routes.mcp.links import marker_definition_handler
-        with patch(f"{BASE}.{self.mod}._get_connector") as m:
-            conn = _mock_conn({"name": "arp"})
-            m.return_value = conn
-            marker_definition_handler(
-                {"project_id": "p", "action": "create",
-                 "bpf": "arp", "direction": "tx"}, ctx,
-            )
-            conn.http_call.assert_called_with(
-                "post", "http://192.168.1.3:3080/v3/projects/p/marker-definitions",
-                json_data={"bpf": "arp", "direction": "tx"},
+        with patch(f"{BASE}.{self.mod}._get_connector"):
+            result = marker_definition_handler(
+                {"project_id": "p", "action": "update", "def_name": "arp"}, ctx,
             )
+        assert "error" in result