diff --git a/gns3server/api/routes/controller/links.py b/gns3server/api/routes/controller/links.py
index 99ff3379c..1bddff2d1 100644
--- a/gns3server/api/routes/controller/links.py
+++ b/gns3server/api/routes/controller/links.py
@@ -32,7 +32,7 @@ from uuid import UUID, uuid4
 from gns3server.controller import Controller
 from gns3server.controller.controller_error import ControllerError
 from gns3server.db.repositories.rbac import RbacRepository
-from gns3server.controller.link import Link
+from gns3server.controller.link import Link, _UNSET
 from gns3server.utils.http_client import HTTPClient
 from gns3server.utils.port_allocator import link_id_to_port
 from gns3server.utils.websocket_to_websocket import websocket_proxy
@@ -510,7 +510,7 @@ async def update_marker(
         name=marker_name,
         bpf=marker_data.bpf if marker_data.bpf else None,
         tag=marker_data.tag,
-        direction=marker_data.direction,
+        direction=marker_data.direction if "direction" in marker_data.model_fields_set else _UNSET,
         color=marker_data.color,
         enabled=marker_data.enabled,
         highlight_duration=marker_data.highlight_duration,
diff --git a/gns3server/api/routes/controller/projects.py b/gns3server/api/routes/controller/projects.py
index de2e293a1..33ad90902 100644
--- a/gns3server/api/routes/controller/projects.py
+++ b/gns3server/api/routes/controller/projects.py
@@ -40,6 +40,7 @@ from uuid import UUID
 from gns3server import schemas
 from gns3server.controller import Controller
 from gns3server.controller.project import Project
+from gns3server.controller.link import _UNSET
 from gns3server.controller.controller_error import ControllerError, ControllerBadRequestError
 from gns3server.controller.import_project import import_project as import_controller_project
 from gns3server.controller.export_project import export_project as export_controller_project
@@ -293,7 +294,7 @@ async def update_marker_definition(
         name=def_name,
         bpf=def_data.bpf if def_data.bpf else None,
         tag=def_data.tag,
-        direction=def_data.direction,
+        direction=def_data.direction if "direction" in def_data.model_fields_set else _UNSET,
         color=def_data.color,
         highlight_duration=def_data.highlight_duration,
     )
diff --git a/gns3server/api/routes/mcp/__init__.py b/gns3server/api/routes/mcp/__init__.py
index 4cf389c13..3f2425b00 100644
--- a/gns3server/api/routes/mcp/__init__.py
+++ b/gns3server/api/routes/mcp/__init__.py
@@ -1015,7 +1015,7 @@ async def link_marker(
     name: Annotated[str | None, Field(description="Custom marker name for create action (auto-generated if omitted)")] = None,
     tag: Annotated[int | None, Field(description="Numeric tag for packet correlation")] = None,
     enabled: Annotated[bool | None, Field(description="Enable or disable the marker (for update action)")] = None,
-    direction: Annotated[str | None, Field(description="Direction filter: 'tx' for capture node sending only, 'rx' for capture node receiving only (omit for both)")] = 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,
     capture_node_id: Annotated[str | None, Field(description="UUID of the endpoint whose uBridge hosts the marker (the observer; tx/rx are from its perspective). Must be a link endpoint and marker-capable. Omit to auto-pick.")] = 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,
@@ -1026,7 +1026,7 @@ async def link_marker(
     Set action='create' to add a marker, 'update' to modify it, 'delete' to remove.
 
     Create requires: project_id, link_id, action='create', bpf
-    Update requires: project_id, link_id, action='update', marker_name, and at least one of (bpf, tag, enabled, color, highlight_duration)
+    Update requires: project_id, link_id, action='update', marker_name, and at least one of (bpf, tag, enabled, direction, color, highlight_duration)
     Delete requires: project_id, link_id, action='delete', marker_name
 
     To read current markers, use link_get — the response includes a 'markers' dict.
@@ -1052,7 +1052,7 @@ 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' for capture node sending only, 'rx' for capture node receiving only (omit for both)")] = 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.
 
@@ -1061,7 +1061,7 @@ 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, color, highlight_duration)
+    Update requires: project_id, action='update', def_name, and at least one of (bpf, tag, direction, color, highlight_duration)
     Delete requires: project_id, action='delete', def_name
     List requires:  project_id, action='list'
 
diff --git a/gns3server/api/routes/mcp/links.py b/gns3server/api/routes/mcp/links.py
index 2b854b07f..9ddf2b377 100644
--- a/gns3server/api/routes/mcp/links.py
+++ b/gns3server/api/routes/mcp/links.py
@@ -395,9 +395,12 @@ def link_marker_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         if not bpf:
             return {"error": "bpf is required for create action"}
         body: dict[str, Any] = {"bpf": bpf}
-        for opt in ("name", "tag", "direction", "capture_node_id", "color", "highlight_duration"):
+        for opt in ("name", "tag", "capture_node_id", "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"]
         return conn.http_call("post", base, json_data=body).json()
 
     marker_name = params.get("marker_name")
@@ -408,11 +411,17 @@ def link_marker_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
 
     if action == "update":
         body = {}
-        for opt in ("bpf", "tag", "direction", "enabled", "color", "highlight_duration"):
+        for opt in ("bpf", "tag", "enabled", "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, enabled, color, highlight_duration)"}
+            return {"error": "At least one update field is required (bpf, tag, enabled, direction, color, highlight_duration)"}
         return conn.http_call("put", url, json_data=body).json()
 
     # action == "delete"
@@ -448,9 +457,12 @@ def marker_definition_handler(params: dict[str, Any], gns3_ctx: dict[str, Any])
         if not bpf:
             return {"error": "bpf is required for create action"}
         body: dict[str, Any] = {"bpf": bpf}
-        for opt in ("name", "tag", "direction", "color", "highlight_duration"):
+        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"]
         return conn.http_call("post", base, json_data=body).json()
 
     def_name = params.get("def_name")
@@ -461,11 +473,17 @@ def marker_definition_handler(params: dict[str, Any], gns3_ctx: dict[str, Any])
 
     if action == "update":
         body = {}
-        for opt in ("bpf", "tag", "direction", "color", "highlight_duration"):
+        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, color, highlight_duration)"}
+            return {"error": "At least one update field is required (bpf, tag, direction, color, highlight_duration)"}
         return conn.http_call("put", url, json_data=body).json()
 
     # action == "delete"
diff --git a/gns3server/controller/link.py b/gns3server/controller/link.py
index c75a8c10d..743abc676 100644
--- a/gns3server/controller/link.py
+++ b/gns3server/controller/link.py
@@ -30,6 +30,13 @@ import logging
 log = logging.getLogger(__name__)
 
 
+# Sentinel for "argument not passed". Distinct from None so marker/definition
+# updaters can tell "caller omitted direction" (keep current value) from
+# "caller passed direction=None" (clear it back to both directions). See
+# UDPLink.update_marker and Project.update_marker_definition.
+_UNSET = object()
+
+
 FILTERS = [
     {
         "type": "frequency_drop",
@@ -346,7 +353,7 @@ class Link:
         """
         raise NotImplementedError
 
-    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=None):
+    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=_UNSET):
         """
         Update an existing marker's BPF, tag, or enabled flag.
 
diff --git a/gns3server/controller/project.py b/gns3server/controller/project.py
index 10b94b620..aa1cbd82d 100644
--- a/gns3server/controller/project.py
+++ b/gns3server/controller/project.py
@@ -37,6 +37,7 @@ from .snapshot import Snapshot
 from .drawing import Drawing
 from .topology import project_to_topology, load_topology
 from .udp_link import UDPLink
+from .link import _UNSET
 from ..config import Config
 from ..utils.path import check_path_allowed, get_default_project_directory
 from ..utils.application_id import get_next_application_id
@@ -947,7 +948,7 @@ class Project:
         self.dump()
         self.emit_notification("project.updated", self.asdict())
 
-    async def update_marker_definition(self, name, bpf=None, tag=None, direction=None, color=None, highlight_duration=None):
+    async def update_marker_definition(self, name, bpf=None, tag=None, direction=_UNSET, color=None, highlight_duration=None):
         """
         Update a marker definition and sync every inherited copy on every link.
         """
@@ -966,8 +967,8 @@ class Project:
             d["color"] = color
         if highlight_duration is not None:
             d["highlight_duration"] = highlight_duration
-        if direction is not None:
-            d["direction"] = direction
+        if direction is not _UNSET:
+            d["direction"] = direction  # None = clear back to both directions
 
         # Sync: update every inherited copy across all links.
         for link in list(self._links.values()):
diff --git a/gns3server/controller/udp_link.py b/gns3server/controller/udp_link.py
index 5e72831a5..e3764e317 100644
--- a/gns3server/controller/udp_link.py
+++ b/gns3server/controller/udp_link.py
@@ -17,7 +17,7 @@
 
 
 from .controller_error import ControllerError, ControllerNotFoundError
-from .link import Link
+from .link import Link, _UNSET
 from .node_types import BUILTIN_NODE_TYPES
 from gns3server.utils.packet_filter_validation import validate_bpf_syntax, FilterValidationError
 
@@ -430,7 +430,7 @@ class UDPLink(Link):
         self._project.emit_notification("link.updated", self.asdict())
         self._project.dump()
 
-    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=None, color=None, highlight_duration=None, inherited=False):
+    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=_UNSET, color=None, highlight_duration=None, inherited=False):
         """
         Update an existing marker's BPF/tag/enabled/color. Any change pushes via
         ``self.update()``; uBridge picks up the new params on the next NIO
@@ -470,8 +470,8 @@ class UDPLink(Link):
             marker_info["color"] = color
         if highlight_duration is not None:
             marker_info["highlight_duration"] = highlight_duration
-        if direction is not None:
-            marker_info["direction"] = direction
+        if direction is not _UNSET:
+            marker_info["direction"] = direction  # None = clear back to both directions
 
         if self._created:
             await self.update()