diff --git a/pkg/config/config.go b/pkg/config/config.go
index f3cbf43ad..20cdd74c2 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -352,6 +352,26 @@ type SessionConfig struct {
 	DmScope       string              `json:"dm_scope,omitempty"`
 }
 
+// ApplyDmScope translates the user-facing dm_scope value into the internal
+// dimensions array that the routing layer consumes. It is a no-op when
+// DmScope is empty or when Dimensions is already set (explicit Dimensions
+// take precedence over the derived value).
+func (s *SessionConfig) ApplyDmScope() {
+	if s.DmScope == "" || len(s.Dimensions) > 0 {
+		return
+	}
+	switch s.DmScope {
+	case "per-channel-peer":
+		s.Dimensions = []string{"chat", "sender"}
+	case "per-channel":
+		s.Dimensions = []string{"chat"}
+	case "per-peer":
+		s.Dimensions = []string{"sender"}
+	case "global":
+		s.Dimensions = nil
+	}
+}
+
 // RoutingConfig controls the intelligent model routing feature.
 // When enabled, each incoming message is scored against structural features
 // (message length, code blocks, tool call history, conversation depth, attachments).
@@ -1477,6 +1497,8 @@ func LoadConfig(path string) (*Config, error) {
 		cfg.Agents.Defaults.Workspace = filepath.Join(homePath, pkg.WorkspaceName)
 	}
 
+	cfg.Session.ApplyDmScope()
+
 	return cfg, nil
 }
 
diff --git a/pkg/config/config_test.go b/pkg/config/config_test.go
index ea90fafe4..12cd70734 100644
--- a/pkg/config/config_test.go
+++ b/pkg/config/config_test.go
@@ -1728,6 +1728,65 @@ func TestDefaultConfig_SessionDimensions(t *testing.T) {
 	}
 }
 
+func TestSessionConfig_ApplyDmScope(t *testing.T) {
+	tests := []struct {
+		name       string
+		dmScope    string
+		dimensions []string
+		want       []string
+	}{
+		{
+			name:    "per-channel-peer",
+			dmScope: "per-channel-peer",
+			want:    []string{"chat", "sender"},
+		},
+		{
+			name:    "per-channel",
+			dmScope: "per-channel",
+			want:    []string{"chat"},
+		},
+		{
+			name:    "per-peer",
+			dmScope: "per-peer",
+			want:    []string{"sender"},
+		},
+		{
+			name:    "global",
+			dmScope: "global",
+			want:    nil,
+		},
+		{
+			name:       "explicit dimensions take precedence",
+			dmScope:    "per-channel-peer",
+			dimensions: []string{"sender"},
+			want:       []string{"sender"},
+		},
+		{
+			name:    "empty dm_scope is no-op",
+			dmScope: "",
+			want:    nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := &SessionConfig{
+				DmScope:    tt.dmScope,
+				Dimensions: tt.dimensions,
+			}
+			s.ApplyDmScope()
+			if len(s.Dimensions) != len(tt.want) {
+				t.Fatalf("Dimensions = %v, want %v", s.Dimensions, tt.want)
+			}
+			for i, v := range tt.want {
+				if s.Dimensions[i] != v {
+					t.Errorf("Dimensions[%d] = %q, want %q", i, s.Dimensions[i], v)
+				}
+			}
+		})
+	}
+}
+
 func TestDefaultConfig_WorkspacePath_Default(t *testing.T) {
 	t.Setenv("PICOCLAW_HOME", "")
 
diff --git a/web/backend/api/config.go b/web/backend/api/config.go
index b37958040..50d9b0293 100644
--- a/web/backend/api/config.go
+++ b/web/backend/api/config.go
@@ -76,6 +76,7 @@ func (h *Handler) handleUpdateConfig(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, fmt.Sprintf("Invalid JSON: %v", err), http.StatusBadRequest)
 		return
 	}
+	cfg.Session.ApplyDmScope()
 	if execAllowRemoteOmitted(body) {
 		cfg.Tools.Exec.AllowRemote = config.DefaultConfig().Tools.Exec.AllowRemote
 	}
@@ -181,6 +182,7 @@ func (h *Handler) handlePatchConfig(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, fmt.Sprintf("Merged config is invalid: %v", err), http.StatusBadRequest)
 		return
 	}
+	newCfg.Session.ApplyDmScope()
 
 	// Restore security fields (tokens/keys) from the loaded config before validation,
 	// because private fields are lost during JSON round-trip.