fix: reject prompt variables in the reserved underscore namespace

src/backend/tests/unit/components/prompts/test_validate_prompt_mustache.py

@@ -26,9 +26,13 @@ def test_multiple_simple_variables_accepted(self):
         assert sorted(result) == ["first_name", "last_name"]
 
     def test_underscore_variables_accepted(self):
-        """Test that variables with underscores are accepted."""
-        result = validate_prompt("{{user_name}} - {{_private}}", is_mustache=True)
-        assert sorted(result) == ["_private", "user_name"]
+        """Test that variables with underscores are accepted.
+
+        A leading underscore is rejected -- see TestValidatePromptReservedPrefix in
+        test_validate_prompt_reserved_prefix.py. Underscores anywhere else are fine.
+        """
+        result = validate_prompt("{{user_name}} - {{private_}}", is_mustache=True)
+        assert sorted(result) == ["private_", "user_name"]
 
     def test_numeric_suffix_variables_accepted(self):
         """Test that variables with numeric suffixes are accepted."""

src/backend/tests/unit/components/prompts/test_validate_prompt_reserved_prefix.py

@@ -0,0 +1,68 @@
+"""Tests for the reserved `_*` namespace in prompt variable names.
+
+Keys prefixed with an underscore are node-template metadata (`_type`,
+`_frontend_node_flow_id`, ...), not component fields. The frontend filters that namespace
+out of every render path, so a variable such as `{_x}` used to be accepted by
+`validate_prompt` and written into the template while never producing an input field or a
+handle -- it could not be given a value and resolved to an empty string at run time.
+
+Regression test for LE-2144. The case list is mirrored by the frontend unit test for
+`isReservedVariableName`; keep the two in sync.
+"""
+
+import pytest
+from lfx.base.prompts.api_utils import validate_prompt
+
+REJECTED = ["_x", "_", "__y", "_type", "_frontend_node_flow_id"]
+ACCEPTED = ["var", "a_b", "var_1", "private_", "x"]
+
+
+class TestValidatePromptReservedPrefix:
+    """Variable names starting with an underscore are rejected in both syntaxes."""
+
+    @pytest.mark.parametrize("name", REJECTED)
+    def test_leading_underscore_rejected_fstring(self, name):
+        with pytest.raises(ValueError, match="cannot start with `_`"):
+            validate_prompt(f"Hello {{{name}}}, how are you?")
+
+    @pytest.mark.parametrize("name", REJECTED)
+    def test_leading_underscore_rejected_mustache(self, name):
+        with pytest.raises(ValueError, match="cannot start with `_`"):
+            validate_prompt(f"Hello {{{{{name}}}}}, how are you?", is_mustache=True)
+
+    @pytest.mark.parametrize("name", ACCEPTED)
+    def test_regular_names_still_accepted_fstring(self, name):
+        assert validate_prompt(f"Hello {{{name}}}, how are you?") == [name]
+
+    @pytest.mark.parametrize("name", ACCEPTED)
+    def test_regular_names_still_accepted_mustache(self, name):
+        assert validate_prompt(f"Hello {{{{{name}}}}}, how are you?", is_mustache=True) == [name]
+
+    def test_error_names_only_the_offending_variables(self):
+        """A mixed template reports the rejected names, not every variable in it."""
+        with pytest.raises(ValueError, match=r"Invalid input variables: `_a`, `_b`\.") as exc_info:
+            validate_prompt("{name} {_a} {city} {_b}")
+        assert "name" not in str(exc_info.value).split(".")[0]
+
+    def test_names_are_backticked_so_markdown_keeps_the_underscores(self):
+        """The frontend renders this message with react-markdown.
+
+        Bare underscores pair up into emphasis markers there, so `_x` would reach the
+        user as `x` and the rule itself would read "cannot start with ''".
+        """
+        with pytest.raises(ValueError, match="Invalid input variables") as exc_info:
+            validate_prompt("{_a} {_b}")
+        message = str(exc_info.value)
+        assert "`_a`" in message
+        assert "`_b`" in message
+        assert "start with `_`" in message
+
+    def test_metadata_key_no_longer_reaches_the_template_writer(self):
+        """`{_type}` used to pass validation and then fail with an opaque HTTP 500.
+
+        `add_new_variables_to_template` read `template["_type"]["value"]` on the plain
+        string that holds the node type, raising `string indices must be integers`.
+        It is now rejected up front with an actionable message.
+        """
+        with pytest.raises(ValueError, match="cannot start with `_`"):
+            validate_prompt("Hello {_type}")

src/frontend/src/components/core/parameterRenderComponent/components/accordionPromptComponent/helpers/__tests__/prompt-highlight.test.ts

@@ -0,0 +1,49 @@
+import { getHighlightedHTML } from "../prompt-highlight";
+
+// This helper drives the prompt preview the user actually sees on the node whenever the
+// inspection panel is enabled, so the reserved-name marking has to reach it too.
+describe("getHighlightedHTML", () => {
+  describe("f-string", () => {
+    it("marks a name starting with an underscore as invalid", () => {
+      expect(getHighlightedHTML("Hello {_x}!", false)).toContain(
+        '<span class="chat-message-highlight-invalid">{_x}</span>',
+      );
+    });
+
+    it("keeps a regular name highlighted normally", () => {
+      expect(getHighlightedHTML("Hello {var}!", false)).toContain(
+        '<span class="chat-message-highlight">{var}</span>',
+      );
+    });
+
+    it("marks only the offending variable in a mixed template", () => {
+      const html = getHighlightedHTML("Hello {_x}, meet {var}.", false);
+      expect(html).toContain(
+        '<span class="chat-message-highlight-invalid">{_x}</span>',
+      );
+      expect(html).toContain(
+        '<span class="chat-message-highlight">{var}</span>',
+      );
+    });
+
+    it("leaves an underscore that is not the first character valid", () => {
+      expect(getHighlightedHTML("Hi {user_name}!", false)).toContain(
+        '<span class="chat-message-highlight">{user_name}</span>',
+      );
+    });
+  });
+
+  describe("double brackets", () => {
+    it("marks a name starting with an underscore as invalid", () => {
+      expect(getHighlightedHTML("Hello {{_x}}!", true)).toContain(
+        '<span class="chat-message-highlight-invalid">{{_x}}</span>',
+      );
+    });
+
+    it("keeps a regular name highlighted normally", () => {
+      expect(getHighlightedHTML("Hello {{var}}!", true)).toContain(
+        '<span class="chat-message-highlight">{{var}}</span>',
+      );
+    });
+  });
+});

src/frontend/src/components/core/parameterRenderComponent/components/accordionPromptComponent/helpers/prompt-highlight.ts

@@ -1,13 +1,14 @@
 import { regexHighlight } from "@/constants/constants";
+import { variableHighlightClass } from "@/utils/promptVariables";
 
 /** Apply variable highlighting to the prompt text. */
 export const getHighlightedHTML = (text: string, isDoubleBrackets: boolean) => {
   if (isDoubleBrackets) {
     return text
       .replace(/</g, "&lt;")
       .replace(/>/g, "&gt;")
-      .replace(/\{\{([a-zA-Z_][a-zA-Z0-9_]*)\}\}/g, (match) => {
-        return `<span class="chat-message-highlight">${match}</span>`;
+      .replace(/\{\{([a-zA-Z_][a-zA-Z0-9_]*)\}\}/g, (match, varName) => {
+        return `<span class="${variableHighlightClass(varName)}">${match}</span>`;
       });
   }
   return text
@@ -28,7 +29,7 @@ export const getHighlightedHTML = (text: string, isDoubleBrackets: boolean) => {
 
       return (
         `${outerLeft}` +
-        `<span class="chat-message-highlight">{${varName}}</span>` +
+        `<span class="${variableHighlightClass(varName)}">{${varName}}</span>` +
         `${outerRight}`
       );
     });

src/frontend/src/components/core/parameterRenderComponent/components/mustachePromptComponent/__tests__/mustachePromptComponent.test.tsx

@@ -255,17 +255,33 @@ describe("MustachePromptAreaComponent", () => {
       );
     });
 
-    it("should highlight variables starting with underscore", () => {
+    it("should mark variables starting with underscore as invalid", () => {
       render(
         <MustachePromptAreaComponent
           {...defaultProps}
           value="Value: {{_private}}"
         />,
       );
 
+      // The underscore namespace is reserved for template metadata, so the name will be
+      // rejected on Check & Save -- the preview says so instead of looking valid.
       const sanitizedHtml = screen.getByTestId("sanitized-html");
       expect(sanitizedHtml.innerHTML).toContain(
-        '<span class="chat-message-highlight">{{_private}}</span>',
+        '<span class="chat-message-highlight-invalid">{{_private}}</span>',
+      );
+    });
+
+    it("should keep an underscore that is not the first character valid", () => {
+      render(
+        <MustachePromptAreaComponent
+          {...defaultProps}
+          value="Value: {{user_name}}"
+        />,
+      );
+
+      const sanitizedHtml = screen.getByTestId("sanitized-html");
+      expect(sanitizedHtml.innerHTML).toContain(
+        '<span class="chat-message-highlight">{{user_name}}</span>',
       );
     });
 

src/frontend/src/components/core/parameterRenderComponent/components/mustachePromptComponent/index.tsx

@@ -1,6 +1,7 @@
 import ForwardedIconComponent from "@/components/common/genericIconComponent";
 import SanitizedHTMLWrapper from "@/components/common/sanitizedHTMLWrapper";
 import MustachePromptModal from "@/modals/mustachePromptModal";
+import { variableHighlightClass } from "@/utils/promptVariables";
 import { cn } from "../../../../../utils/utils";
 import { Button } from "../../../../ui/button";
 import { getNodeScopedDomId } from "../../helpers/get-node-scoped-dom-id";
@@ -32,8 +33,8 @@ export default function MustachePromptAreaComponent({
     .replace(/</g, "&lt;")
     .replace(/>/g, "&gt;")
     // highlight only simple mustache variables {{variable_name}} - no complex syntax
-    .replace(/\{\{([a-zA-Z_][a-zA-Z0-9_]*)\}\}/g, (match, varName) => {
-      return `<span class="chat-message-highlight">{{${varName}}}</span>`;
+    .replace(/\{\{([a-zA-Z_][a-zA-Z0-9_]*)\}\}/g, (_match, varName) => {
+      return `<span class="${variableHighlightClass(varName)}">{{${varName}}}</span>`;
     })
     // preserve new-lines
     .replace(/\n/g, "<br />");

src/frontend/src/components/core/parameterRenderComponent/components/promptComponent/__tests__/promptComponent.test.tsx

@@ -0,0 +1,94 @@
+import { render, screen } from "@testing-library/react";
+import PromptAreaComponent from "../index";
+
+jest.mock("@/modals/promptModal", () => {
+  return function MockPromptModal({ children }: { children: React.ReactNode }) {
+    return <div data-testid="mock-prompt-modal">{children}</div>;
+  };
+});
+
+jest.mock("@/components/common/genericIconComponent", () => ({
+  __esModule: true,
+  default: ({ name, className }: { name: string; className?: string }) => (
+    <span data-testid={`icon-${name}`} className={className}>
+      {name}
+    </span>
+  ),
+}));
+
+jest.mock("@/components/common/sanitizedHTMLWrapper", () => {
+  return function MockSanitizedHTMLWrapper({
+    content,
+    className,
+  }: {
+    content: string;
+    className?: string;
+  }) {
+    return (
+      <div
+        data-testid="sanitized-html"
+        className={className}
+        dangerouslySetInnerHTML={{ __html: content }}
+      />
+    );
+  };
+});
+
+describe("PromptAreaComponent variable highlighting", () => {
+  const defaultProps = {
+    field_name: "template",
+    nodeClass: undefined,
+    handleOnNewValue: jest.fn(),
+    handleNodeClass: jest.fn(),
+    value: "",
+    disabled: false,
+    editNode: false,
+    id: "prompt-template",
+    nodeId: "node-1",
+    readonly: false,
+  } as any;
+
+  it("marks a name starting with an underscore as invalid", () => {
+    render(<PromptAreaComponent {...defaultProps} value="Hello {_x}!" />);
+
+    expect(screen.getByTestId("sanitized-html").innerHTML).toContain(
+      '<span class="chat-message-highlight-invalid">{_x}</span>',
+    );
+  });
+
+  it("keeps a regular name highlighted normally", () => {
+    render(<PromptAreaComponent {...defaultProps} value="Hello {var}!" />);
+
+    expect(screen.getByTestId("sanitized-html").innerHTML).toContain(
+      '<span class="chat-message-highlight">{var}</span>',
+    );
+  });
+
+  it("keeps an underscore that is not the first character valid", () => {
+    render(<PromptAreaComponent {...defaultProps} value="Hi {user_name}!" />);
+
+    expect(screen.getByTestId("sanitized-html").innerHTML).toContain(
+      '<span class="chat-message-highlight">{user_name}</span>',
+    );
+  });
+
+  it("marks only the offending variable in a mixed template", () => {
+    render(
+      <PromptAreaComponent {...defaultProps} value="Hello {_x}, meet {var}." />,
+    );
+
+    const html = screen.getByTestId("sanitized-html").innerHTML;
+    expect(html).toContain(
+      '<span class="chat-message-highlight-invalid">{_x}</span>',
+    );
+    expect(html).toContain('<span class="chat-message-highlight">{var}</span>');
+  });
+
+  it("leaves double-brace escapes untouched", () => {
+    render(<PromptAreaComponent {...defaultProps} value="Literal {{_x}} here" />);
+
+    expect(screen.getByTestId("sanitized-html").innerHTML).not.toContain(
+      "chat-message-highlight-invalid",
+    );
+  });
+});

src/frontend/src/components/core/parameterRenderComponent/components/promptComponent/index.tsx

@@ -2,6 +2,7 @@ import ForwardedIconComponent from "@/components/common/genericIconComponent";
 import SanitizedHTMLWrapper from "@/components/common/sanitizedHTMLWrapper";
 import { regexHighlight } from "@/constants/constants";
 import PromptModal from "@/modals/promptModal";
+import { variableHighlightClass } from "@/utils/promptVariables";
 import { cn } from "../../../../../utils/utils";
 import { Button } from "../../../../ui/button";
 import { getNodeScopedDomId } from "../../helpers/get-node-scoped-dom-id";
@@ -51,7 +52,7 @@ export default function PromptAreaComponent({
 
       return (
         `${outerLeft}` +
-        `<span class="chat-message-highlight">{${varName}}</span>` +
+        `<span class="${variableHighlightClass(varName)}">{${varName}}</span>` +
         `${outerRight}`
       );
     })

src/frontend/src/locales/de.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "Eingabeaufforderungsvariablen:",
   "modal.prompt.title": "Eingabeaufforderung bearbeiten",
   "modal.prompt.variablesHint": "Prompt-Variablen können unter einem beliebigen Namen in geschweiften Klammern angelegt werden, z. B. ` {variable_name} `",
+  "modal.prompt.reservedPrefix": "Variablennamen dürfen nicht mit „_“ beginnen. Dieses Präfix ist internen Feldern vorbehalten.",
   "modal.restoreVersion": "Version wiederherstellen",
   "modal.saveButton": "Speichern",
   "modal.secretKey.createDescription": "Erstellen Sie einen geheimen API-Schlüssel, um die Langflow-API zu nutzen.",

src/frontend/src/locales/en.json

@@ -457,6 +457,7 @@
   "modal.prompt.title": "Edit Prompt",
   "modal.prompt.promptVariables": "Prompt Variables:",
   "modal.prompt.variablesHint": "Prompt variables can be created with any chosen name inside curly brackets, e.g. {variable_name}",
+  "modal.prompt.reservedPrefix": "Variable names can't start with \"_\". That prefix is reserved for internal fields.",
   "modal.prompt.checkAndSave": "Check & Save",
   "modal.api.title": "API access",
   "modal.api.description": "API access requires an API key. You can",

src/frontend/src/locales/es.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "Variables de la línea de comandos:",
   "modal.prompt.title": "Editar mensaje",
   "modal.prompt.variablesHint": "Las variables de línea de comandos se pueden crear con cualquier nombre que se desee entre llaves, por ejemplo: ` {variable_name} `",
+  "modal.prompt.reservedPrefix": "Los nombres de variables no pueden empezar por \"_\". Ese prefijo está reservado para campos internos.",
   "modal.restoreVersion": "Restaurar versión",
   "modal.saveButton": "Guardar",
   "modal.secretKey.createDescription": "Crea una clave API secreta para utilizar la API de Langflow.",

src/frontend/src/locales/fr.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "Variables d'invite :",
   "modal.prompt.title": "Modifier l'invite",
   "modal.prompt.variablesHint": "Les variables de ligne de commande peuvent être créées avec le nom de votre choix entre accolades, par exemple : {variable_name}",
+  "modal.prompt.reservedPrefix": "Les noms de variables ne peuvent pas commencer par « _ ». Ce préfixe est réservé aux champs internes.",
   "modal.restoreVersion": "Restaurer la version",
   "modal.saveButton": "Sauvegarder",
   "modal.secretKey.createDescription": "Créez une clé API secrète pour utiliser l'API Langflow.",

src/frontend/src/locales/ja.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "プロンプト変数：",
   "modal.prompt.title": "プロンプトを編集",
   "modal.prompt.variablesHint": "プロンプト変数は、中括弧（{}）の中に任意の名前を指定して作成できます。例： {variable_name}",
+  "modal.prompt.reservedPrefix": "変数名は「_」で始めることはできません。このプレフィックスは内部フィールド用に予約されています。",
   "modal.restoreVersion": "復元バージョン",
   "modal.saveButton": "保存",
   "modal.secretKey.createDescription": "Langflow API を使用するための秘密の API キーを作成してください。",

src/frontend/src/locales/pt.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "Variáveis de prompt:",
   "modal.prompt.title": "Editar prompt",
   "modal.prompt.variablesHint": "As variáveis de prompt podem ser criadas com qualquer nome escolhido entre chaves, por exemplo: ` {variable_name} `",
+  "modal.prompt.reservedPrefix": "Nomes de variáveis não podem começar com \"_\". Esse prefixo é reservado para campos internos.",
   "modal.restoreVersion": "Restaurar versão",
   "modal.saveButton": "Salvar",
   "modal.secretKey.createDescription": "Crie uma chave API secreta para usar a API do Langflow.",

src/frontend/src/locales/zh-Hans.json

@@ -1505,6 +1505,7 @@
   "modal.prompt.promptVariables": "提示变量：",
   "modal.prompt.title": "编辑提示",
   "modal.prompt.variablesHint": "可以在大括号内使用任意名称创建提示变量，例如： {variable_name}",
+  "modal.prompt.reservedPrefix": "变量名不能以“_”开头。该前缀保留给内部字段使用。",
   "modal.restoreVersion": "恢复版本",
   "modal.saveButton": "保存",
   "modal.secretKey.createDescription": "创建一个密钥以使用 Langflow API。",