feat(api): validate required openapi parameters

Co-Authored-By: Virgil <virgil@lethean.io>

client.go

@@ -50,8 +50,9 @@ type openAPIOperation struct {
 }
 
 type openAPIParameter struct {
-	name string
-	in   string
+	name     string
+	in       string
+	required bool
 }
 
 // OpenAPIClientOption configures a runtime OpenAPI client.
@@ -349,6 +350,11 @@ func (c *OpenAPIClient) buildURL(op openAPIOperation, params map[string]any) (st
 	} else {
 		pathValues = params
 	}
+
+	if err := validateRequiredParameters(op, params, pathKeys); err != nil {
+		return "", err
+	}
+
 	for _, key := range pathKeys {
 		if value, ok := pathValues[key]; ok {
 			placeholder := "{" + key + "}"
@@ -572,7 +578,8 @@ func parseOperationParameters(operation map[string]any) []openAPIParameter {
 		if name == "" || in == "" {
 			continue
 		}
-		params = append(params, openAPIParameter{name: name, in: in})
+		required, _ := param["required"].(bool)
+		params = append(params, openAPIParameter{name: name, in: in, required: required})
 	}
 
 	return params
@@ -587,6 +594,38 @@ func operationParameterLocation(op openAPIOperation, name string) string {
 	return ""
 }
 
+func validateRequiredParameters(op openAPIOperation, params map[string]any, pathKeys []string) error {
+	for _, param := range op.parameters {
+		if !param.required {
+			continue
+		}
+		if containsString(pathKeys, param.name) {
+			continue
+		}
+		if parameterProvided(params, param.name, param.in) {
+			continue
+		}
+		return coreerr.E("OpenAPIClient.buildURL", fmt.Sprintf("missing required %s parameter %q", param.in, param.name), nil)
+	}
+	return nil
+}
+
+func parameterProvided(params map[string]any, name, location string) bool {
+	if nested, ok := nestedMap(params, location); ok {
+		if _, exists := nested[name]; exists {
+			return true
+		}
+	}
+
+	if value, exists := params[name]; exists {
+		if value != nil {
+			return true
+		}
+	}
+
+	return false
+}
+
 func encodeJSONBody(v any) ([]byte, error) {
 	data, err := json.Marshal(v)
 	if err != nil {

client_test.go

@@ -384,6 +384,50 @@ paths:
 	}
 }
 
+func TestOpenAPIClient_Bad_MissingRequiredQueryParameter(t *testing.T) {
+	called := make(chan struct{}, 1)
+	mux := http.NewServeMux()
+	mux.HandleFunc("/submit", func(w http.ResponseWriter, r *http.Request) {
+		called <- struct{}{}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"success":true,"data":{"ok":true}}`))
+	})
+
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	specPath := writeTempSpec(t, `openapi: 3.1.0
+info:
+  title: Test API
+  version: 1.0.0
+paths:
+  /submit:
+    post:
+      operationId: submit_item
+      parameters:
+        - name: verbose
+          in: query
+          required: true
+`)
+
+	client := api.NewOpenAPIClient(
+		api.WithSpec(specPath),
+		api.WithBaseURL(srv.URL),
+	)
+
+	if _, err := client.Call("submit_item", map[string]any{
+		"name": "Ada",
+	}); err == nil {
+		t.Fatal("expected required query parameter validation error, got nil")
+	}
+
+	select {
+	case <-called:
+		t.Fatal("expected validation to fail before the HTTP call")
+	default:
+	}
+}
+
 func TestOpenAPIClient_Good_UsesHeaderAndCookieParameters(t *testing.T) {
 	errCh := make(chan error, 1)
 	mux := http.NewServeMux()