fix(langchain,core): Add shim for hub mustache templates with nested input variables

langchain-core/src/prompts/chat.ts

@@ -771,16 +771,30 @@ function _coerceMessagePromptTemplateLike<
   ) {
     const messageContent = messagePromptTemplateLike[1];
     if (
-      typeof messageContent !== "string" ||
-      messageContent[0] !== "{" ||
-      messageContent[messageContent.length - 1] !== "}"
+      extra?.templateFormat === "mustache" &&
+      typeof messageContent === "string" &&
+      messageContent.slice(0, 2) === "{{" &&
+      messageContent.slice(-2) === "}}"
     ) {
-      throw new Error(
-        `Invalid placeholder template: "${messagePromptTemplateLike[1]}". Expected a variable name surrounded by curly braces.`
-      );
+      const variableName = messageContent.slice(2, -2);
+      return new MessagesPlaceholder({ variableName, optional: true });
+    } else if (
+      typeof messageContent === "string" &&
+      messageContent[0] === "{" &&
+      messageContent[messageContent.length - 1] === "}"
+    ) {
+      const variableName = messageContent.slice(1, -1);
+      return new MessagesPlaceholder({ variableName, optional: true });
     }
-    const variableName = messageContent.slice(1, -1);
-    return new MessagesPlaceholder({ variableName, optional: true });
+    throw new Error(
+      `Invalid placeholder template for format ${
+        extra?.templateFormat ?? `"f-string"`
+      }: "${
+        messagePromptTemplateLike[1]
+      }". Expected a variable name surrounded by ${
+        extra?.templateFormat === "mustache" ? "double" : "single"
+      } curly braces.`
+    );
   }
   const message = coerceMessageLikeToMessage(messagePromptTemplateLike);
   let templateData:

langchain-core/src/prompts/tests/chat.mustache.test.ts

@@ -149,3 +149,34 @@ test("Mustache image template with nested URL and chat prompts HumanMessagePromp
 
   expect(template.inputVariables.sort()).toEqual(["image_url", "name"]);
 });
+
+test("Mustache image template with nested props", async () => {
+  const template = ChatPromptTemplate.fromMessages(
+    [
+      ["human", "{{agent.name}}"],
+      ["placeholder", "{{messages}}"],
+    ],
+    {
+      templateFormat: "mustache",
+    }
+  );
+
+  const messages = await template.formatMessages({
+    agent: { name: "testing" },
+    messages: [
+      {
+        role: "assistant",
+        content: "hi there!",
+      },
+    ],
+  });
+
+  expect(messages).toEqual([
+    new HumanMessage({
+      content: "testing",
+    }),
+    new AIMessage("hi there!"),
+  ]);
+
+  expect(template.inputVariables.sort()).toEqual(["agent", "messages"]);
+});

langchain/src/hub.ts

@@ -109,6 +109,29 @@
     };
   }
 
+  // Some nested mustache prompts have improperly parsed variables that include a dot.
+  if (promptObject.manifest.kwargs.template_format === "mustache") {
+    const stripDotNotation = (varName: string) => varName.split(".")[0];
+
+    const { input_variables } = promptObject.manifest.kwargs;
+    if (Array.isArray(input_variables)) {
+      promptObject.manifest.kwargs.input_variables =
+        input_variables.map(stripDotNotation);
+    }
+
+    const { messages } = promptObject.manifest.kwargs;
+    if (Array.isArray(messages)) {
+      promptObject.manifest.kwargs.messages = messages.map((message: any) => {
+        const nestedVars = message?.kwargs?.prompt?.kwargs?.input_variables;
+        if (Array.isArray(nestedVars)) {
+          message.kwargs.prompt.kwargs.input_variables =
+            nestedVars.map(stripDotNotation);
+        }
+        return message;
+      });
+    }
+  }
+
   try {
     const loadedPrompt = await load<T>(
       JSON.stringify(promptObject.manifest),

langchain/src/tests/hub.int.test.ts

@@ -1,6 +1,12 @@
 /* eslint-disable no-process-env */
 
 import { ChatPromptTemplate } from "@langchain/core/prompts";
+import {
+  AIMessage,
+  HumanMessage,
+  SystemMessage,
+} from "@langchain/core/messages";
+import { ChatPromptValue } from "@langchain/core/prompt_values";
 import * as hub from "../hub.js";
 
 test("Test LangChain Hub client pushing a new repo", async () => {
@@ -18,3 +24,18 @@ test("Test LangChain Hub client pushing a new repo", async () => {
     await pulledPrompt.invoke({ input: "testing" })
   );
 });
+
+test("Test LangChain Hub with a faulty mustache prompt", async () => {
+  const pulledPrompt = await hub.pull("jacob/lahzo-testing");
+  const res = await pulledPrompt.invoke({
+    agent: { name: "testing" },
+    messages: [new AIMessage("foo")],
+  });
+  expect(res).toEqual(
+    new ChatPromptValue([
+      new SystemMessage("You are a chatbot."),
+      new HumanMessage("testing"),
+      new AIMessage("foo"),
+    ])
+  );
+});