diff --git a/mcp.go b/mcp.go
index 2b95887..f05573d 100644
--- a/mcp.go
+++ b/mcp.go
@@ -21,6 +21,13 @@ type mcpTool struct {
 }
 
 func (t *mcpTool) Tool() openai.Tool {
+	// A tool without arguments has no properties, and the unmarshal leaves
+	// the map nil. jsonschema.Definition encoded that as {}, but this
+	// struct would send null, which vLLM and others reject.
+	props := t.props
+	if props == nil {
+		props = map[string]jsonschema.Definition{}
+	}
 	return openai.Tool{
 		Type: openai.ToolTypeFunction,
 		Function: &openai.FunctionDefinition{
@@ -28,7 +35,7 @@ func (t *mcpTool) Tool() openai.Tool {
 			Description: t.description,
 			Parameters: mcpToolParameters{
 				Type:                 jsonschema.Object,
-				Properties:           t.props,
+				Properties:           props,
 				Required:             t.inputSchema.Required,
 				AdditionalProperties: t.inputSchema.AdditionalProperties,
 			},
diff --git a/mcp_params_test.go b/mcp_params_test.go
new file mode 100644
index 0000000..3f5ab13
--- /dev/null
+++ b/mcp_params_test.go
@@ -0,0 +1,34 @@
+package cogito
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// An MCP tool that takes no arguments has no "properties" in its input
+// schema, so the unmarshal leaves props nil. OpenAI-compatible servers
+// such as vLLM validate the schema and reject "properties": null, which
+// fails the whole request, not only this tool.
+func TestMCPToolWithoutArgumentsSendsEmptyProperties(t *testing.T) {
+	tool := &mcpTool{
+		name:        "list_things",
+		inputSchema: toolInputSchema{Type: "object"},
+	}
+
+	dat, err := json.Marshal(tool.Tool().Function.Parameters)
+	if err != nil {
+		t.Fatalf("marshal parameters: %v", err)
+	}
+
+	var params map[string]any
+	if err := json.Unmarshal(dat, &params); err != nil {
+		t.Fatalf("unmarshal parameters: %v", err)
+	}
+	props, ok := params["properties"].(map[string]any)
+	if !ok {
+		t.Fatalf("properties must be an object, got %s", dat)
+	}
+	if len(props) != 0 {
+		t.Fatalf("properties must be empty, got %s", dat)
+	}
+}