diff --git a/pkg/tools/registry.go b/pkg/tools/registry.go
index e51dff71a..e49c4f215 100644
--- a/pkg/tools/registry.go
+++ b/pkg/tools/registry.go
@@ -352,6 +352,31 @@ func (r *ToolRegistry) ToProviderDefs() []providers.ToolDefinition {
 		name, _ := fn["name"].(string)
 		desc, _ := fn["description"].(string)
 		params, _ := fn["parameters"].(map[string]any)
+		// Normalize params so they include a properties field for object schemas.
+		// While JSON Schema does not require properties, some MCP servers omit it
+		// and strict OpenAI-compatible API validators (e.g., LM Studio) then fail.
+		if params == nil {
+			params = map[string]any{
+				"type":       "object",
+				"properties": map[string]any{},
+			}
+		} else {
+			// Ensure properties is a non-nil map[string]any. Some tools may provide
+			// properties with the wrong type or as a typed-nil map, which strict
+			// validators may reject.
+			propsVal, hasProps := params["properties"]
+			propsMap, okPropsMap := propsVal.(map[string]any)
+			if !hasProps || !okPropsMap || propsMap == nil {
+				// Make a defensive copy to avoid mutating the original tool schema,
+				// which could cause concurrent map writes if called from multiple goroutines.
+				paramsCopy := make(map[string]any, len(params)+1)
+				for k, v := range params {
+					paramsCopy[k] = v
+				}
+				paramsCopy["properties"] = map[string]any{}
+				params = paramsCopy
+			}
+		}
 
 		definitions = append(definitions, providers.ToolDefinition{
 			Type: "function",
diff --git a/pkg/tools/registry_test.go b/pkg/tools/registry_test.go
index 16bd30928..708807459 100644
--- a/pkg/tools/registry_test.go
+++ b/pkg/tools/registry_test.go
@@ -300,6 +300,118 @@ func TestToolRegistry_ToProviderDefs(t *testing.T) {
 	}
 }
 
+func TestToolRegistry_ToProviderDefs_NilParams(t *testing.T) {
+	r := NewToolRegistry()
+	r.Register(&mockRegistryTool{
+		name:   "nil-params",
+		desc:   "tool with nil params",
+		params: nil,
+		result: SilentResult("ok"),
+	})
+
+	defs := r.ToProviderDefs()
+	if len(defs) != 1 {
+		t.Fatalf("expected 1 provider def, got %d", len(defs))
+	}
+
+	params := defs[0].Function.Parameters
+	if params == nil {
+		t.Fatal("expected non-nil parameters")
+	}
+	if params["type"] != "object" {
+		t.Errorf("expected type 'object', got %v", params["type"])
+	}
+	propsMap, ok := params["properties"].(map[string]any)
+	if !ok || propsMap == nil {
+		t.Errorf("expected properties to be non-nil map[string]any, got %T", params["properties"])
+	}
+}
+
+func TestToolRegistry_ToProviderDefs_MissingProperties(t *testing.T) {
+	r := NewToolRegistry()
+	originalParams := map[string]any{"type": "object"}
+	r.Register(&mockRegistryTool{
+		name:   "missing-props",
+		desc:   "tool with params missing properties",
+		params: originalParams,
+		result: SilentResult("ok"),
+	})
+
+	defs := r.ToProviderDefs()
+	if len(defs) != 1 {
+		t.Fatalf("expected 1 provider def, got %d", len(defs))
+	}
+
+	params := defs[0].Function.Parameters
+	propsMap, ok := params["properties"].(map[string]any)
+	if !ok || propsMap == nil {
+		t.Errorf("expected properties to be non-nil map[string]any, got %T", params["properties"])
+	}
+
+	// Verify original params was not mutated (defensive copy)
+	if _, ok := originalParams["properties"]; ok {
+		t.Error("original params should not be mutated")
+	}
+}
+
+func TestToolRegistry_ToProviderDefs_WrongTypeProperties(t *testing.T) {
+	r := NewToolRegistry()
+	// properties is a string instead of map[string]any
+	originalParams := map[string]any{"type": "object", "properties": "invalid"}
+	r.Register(&mockRegistryTool{
+		name:   "wrong-type-props",
+		desc:   "tool with wrong type properties",
+		params: originalParams,
+		result: SilentResult("ok"),
+	})
+
+	defs := r.ToProviderDefs()
+	if len(defs) != 1 {
+		t.Fatalf("expected 1 provider def, got %d", len(defs))
+	}
+
+	params := defs[0].Function.Parameters
+	propsMap, ok := params["properties"].(map[string]any)
+	if !ok || propsMap == nil {
+		t.Errorf("expected properties to be non-nil map[string]any, got %T", params["properties"])
+	}
+
+	// Verify original params was not mutated (defensive copy)
+	if originalParams["properties"] != "invalid" {
+		t.Error("original params should not be mutated")
+	}
+}
+
+func TestToolRegistry_ToProviderDefs_TypedNilProperties(t *testing.T) {
+	r := NewToolRegistry()
+	// properties is a typed-nil map
+	var typedNil map[string]any
+	originalParams := map[string]any{"type": "object", "properties": typedNil}
+	r.Register(&mockRegistryTool{
+		name:   "typed-nil-props",
+		desc:   "tool with typed-nil properties",
+		params: originalParams,
+		result: SilentResult("ok"),
+	})
+
+	defs := r.ToProviderDefs()
+	if len(defs) != 1 {
+		t.Fatalf("expected 1 provider def, got %d", len(defs))
+	}
+
+	params := defs[0].Function.Parameters
+	propsMap, ok := params["properties"].(map[string]any)
+	if !ok || propsMap == nil {
+		t.Errorf("expected properties to be non-nil map[string]any, got %T", params["properties"])
+	}
+
+	// Verify original params was not mutated (defensive copy)
+	// The original should still have the typed-nil value
+	if origProps, ok := originalParams["properties"].(map[string]any); !ok || origProps != nil {
+		t.Error("original params should not be mutated")
+	}
+}
+
 func TestToolRegistry_List(t *testing.T) {
 	r := NewToolRegistry()
 	r.Register(newMockTool("x", ""))