diff --git a/pkg/gateway/mcp/missing_action_test.go b/pkg/gateway/mcp/missing_action_test.go
new file mode 100644
index 00000000..b76d4fb7
--- /dev/null
+++ b/pkg/gateway/mcp/missing_action_test.go
@@ -0,0 +1,75 @@
+package mcp
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/hookdeck/hookdeck-cli/pkg/hookdeck"
+)
+
+// Every tool split from one resource shares that resource's handler, and the
+// handler fills in an omitted action with a read -- list or get. So on a write,
+// pause or use tool, an omitted action ran a sibling tool's read and reported
+// success: hookdeck_projects_use {} returned the project list and switched
+// nothing, and gateway_connections_pause {} listed connections. An agent that
+// forgot the argument was told it had worked.
+//
+// The schema marks action as required on every tool. Read tools keep their
+// default; every other tool now enforces it.
+//
+// This walks every registered tool rather than a list, so a tool split in
+// future is covered without anyone remembering to add it.
+func TestNonReadToolsRequireAnAction(t *testing.T) {
+	var apiCalls int
+	session := mockAPIWithClientWriteEnabled(t, map[string]http.HandlerFunc{
+		hookdeck.APIPathPrefix + "/": func(w http.ResponseWriter, r *http.Request) {
+			apiCalls++
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{"models":[],"pagination":{}}`))
+		},
+	})
+
+	tools, err := session.ListTools(context.Background(), nil)
+	require.NoError(t, err)
+
+	checked := 0
+	for _, tool := range tools.Tools {
+		props, _ := tool.InputSchema.(map[string]any)["properties"].(map[string]any)
+		if _, takesAction := props["action"]; !takesAction {
+			continue // login, help: no action to omit
+		}
+		if strings.HasSuffix(tool.Name, "_read") {
+			continue // read tools may default to list or get
+		}
+		checked++
+
+		t.Run(tool.Name, func(t *testing.T) {
+			before := apiCalls
+			result := callTool(t, session, tool.Name, map[string]any{})
+
+			require.True(t, result.IsError, "%s with no action must be refused, not run a sibling's read", tool.Name)
+			body := textContent(t, result)
+			assert.Contains(t, body, "action is required")
+			assert.Contains(t, body, "This tool offers:")
+			assert.Equal(t, before, apiCalls, "nothing may reach the API")
+		})
+	}
+	require.GreaterOrEqual(t, checked, 9, "expected at least the nine split tools that share a list/get default")
+}
+
+// Read tools keep defaulting, so the change is confined to the tools where the
+// default ran something the caller could not have meant.
+func TestReadToolsStillDefaultWhenActionIsOmitted(t *testing.T) {
+	session := mockAPIWithClientWriteEnabled(t, map[string]http.HandlerFunc{
+		hookdeck.APIPathPrefix + "/": func(w http.ResponseWriter, r *http.Request) {
+			_, _ = w.Write([]byte(`{"models":[],"pagination":{}}`))
+		},
+	})
+	result := callTool(t, session, "gateway_connections_read", map[string]any{})
+	assert.False(t, result.IsError, textContent(t, result))
+}
diff --git a/pkg/mcpcore/toolspec.go b/pkg/mcpcore/toolspec.go
index d7c16317..6091d60e 100644
--- a/pkg/mcpcore/toolspec.go
+++ b/pkg/mcpcore/toolspec.go
@@ -631,6 +631,21 @@ func rejectUnknownArgs(srv *Server, spec ToolSpec, group string, visible map[str
 			return next(ctx, req)
 		}
 
+		// An omitted action is filled in by the handler, and that handler is
+		// shared by every tool split from the same resource. Its fallback is a
+		// read -- list or get -- so on a write, pause or use tool an omitted
+		// action ran a sibling tool's read and reported success:
+		// hookdeck_projects_use {} returned the project list and switched
+		// nothing, and gateway_connections_pause {} listed connections. Only a
+		// read tool may default; everywhere else the schema's "required" is
+		// enforced here, with the actions this tool actually offers.
+		if in.String(actionArgName) == "" && group != GroupRead {
+			return ErrorResult(fmt.Sprintf(
+				"%s is required. This tool offers: %s.",
+				actionArgName, strings.Join(spec.Actions.InGroup(group).Names(), ", "),
+			)), nil
+		}
+
 		// Only defer when the caller is reaching for an action this tool does
 		// not carry. On an action this tool does have, a hidden argument is
 		// just as ignorable as an invented one, and has to be rejected.