diff --git a/internal/handlers/validate.go b/internal/handlers/validate.go
index 05393e8..96af4ae 100644
--- a/internal/handlers/validate.go
+++ b/internal/handlers/validate.go
@@ -32,10 +32,14 @@ func (vh *PubliccodeymlValidatorHandler) Query(ctx fiber.Ctx) error {
 	var normalized *string
 
 	valid := true
-	parser := vh.parser
 
-	if checks := fiber.Query[bool](ctx, "external-checks", false); checks {
-		parser = vh.parserExternalChecks
+	parser, invalidExternalChecks := vh.parserForExternalChecks(ctx)
+	if invalidExternalChecks {
+		return common.Error(
+			fiber.StatusBadRequest,
+			"invalid external-checks parameter",
+			"external-checks must be true or false",
+		)
 	}
 
 	if len(ctx.Body()) == 0 {
@@ -72,3 +76,19 @@ func (vh *PubliccodeymlValidatorHandler) Query(ctx fiber.Ctx) error {
 	//nolint:wrapcheck
 	return ctx.JSON(fiber.Map{"valid": valid, "results": results, "normalized": normalized})
 }
+
+func (vh *PubliccodeymlValidatorHandler) parserForExternalChecks(ctx fiber.Ctx) (*publiccodeParser.Parser, bool) {
+	args := ctx.Request().URI().QueryArgs()
+	if !args.Has("external-checks") {
+		return vh.parser, false
+	}
+
+	switch string(args.Peek("external-checks")) {
+	case "true":
+		return vh.parserExternalChecks, false
+	case "false":
+		return vh.parser, false
+	default:
+		return nil, true
+	}
+}
diff --git a/main_test.go b/main_test.go
index d1a8b68..6b3a914 100644
--- a/main_test.go
+++ b/main_test.go
@@ -84,7 +84,7 @@ func runTestCases(t *testing.T, tests []TestCase) {
 			if test.headers != nil {
 				req.Header = test.headers
 			}
-			req.URL.RawQuery = u.Query().Encode()
+			req.URL.RawQuery = u.RawQuery
 
 			res, err := app.Test(req, fiber.TestConfig{Timeout: 0, FailOnTimeout: false})
 			assert.Nil(t, err)
@@ -185,6 +185,17 @@ func TestValidateEndpoint(t *testing.T) {
 				assert.Empty(t, response["results"])
 			},
 		},
+		{
+			description:         "validate: external checks explicitly disabled",
+			query:               "QUERY /v1/validate?external-checks=false",
+			body:                externalChecksYml,
+			expectedCode:        200,
+			expectedContentType: "application/json; charset=utf-8",
+			validateFunc: func(t *testing.T, response map[string]any) {
+				assert.Equal(t, true, response["valid"])
+				assert.Empty(t, response["results"])
+			},
+		},
 		{
 			description:         "validate: external checks enabled",
 			query:               "QUERY /v1/validate?external-checks=true",
@@ -200,6 +211,29 @@ func TestValidateEndpoint(t *testing.T) {
 				assert.Nil(t, response["normalized"])
 			},
 		},
+		{
+			description:         "validate: repeated external checks use first value",
+			query:               "QUERY /v1/validate?external-checks=true&external-checks=false",
+			body:                externalChecksYml,
+			expectedCode:        200,
+			expectedContentType: "application/json; charset=utf-8",
+			validateFunc: func(t *testing.T, response map[string]any) {
+				assert.Equal(t, false, response["valid"])
+				results, ok := response["results"].([]any)
+				assert.True(t, ok)
+				assert.NotEmpty(t, results)
+				assert.Contains(t, fmt.Sprint(results), "non-public")
+				assert.Nil(t, response["normalized"])
+			},
+		},
+		{
+			description:         "validate: malformed external checks flag",
+			query:               "QUERY /v1/validate?external-checks=invalid",
+			body:                validYml,
+			expectedCode:        400,
+			expectedBody:        `{"title":"invalid external-checks parameter","detail":"external-checks must be true or false","status":400}`,
+			expectedContentType: "application/problem+json",
+		},
 		{
 			description:         "validate: invalid file",
 			query:               "QUERY /v1/validate",
diff --git a/publiccode-validator-api.oas.yaml b/publiccode-validator-api.oas.yaml
index 3904ee5..10b1515 100644
--- a/publiccode-validator-api.oas.yaml
+++ b/publiccode-validator-api.oas.yaml
@@ -96,15 +96,22 @@ paths:
                         description: "missing required field"
                     normalized: null
         "400":
-          description: Bad request (e.g. empty body)
+          description: Bad request (e.g. empty body or invalid external-checks value)
           content:
             application/problem+json:
               schema:
                 $ref: "#/components/schemas/ProblemJSON"
-              example:
-                title: empty body
-                detail: need a body to validate
-                status: 400
+              examples:
+                emptyBody:
+                  value:
+                    title: empty body
+                    detail: need a body to validate
+                    status: 400
+                invalidExternalChecks:
+                  value:
+                    title: invalid external-checks parameter
+                    detail: external-checks must be true or false
+                    status: 400
         "405":
           description: Method not allowed
           headers: