core[patch]: Adds support for plain message objects as shorthand for messages

langchain-core/src/language_models/tests/chat_models.test.ts

@@ -5,6 +5,25 @@ import { z } from "zod";
 import { zodToJsonSchema } from "zod-to-json-schema";
 import { FakeChatModel, FakeListChatModel } from "../../utils/testing/index.js";
 
+test("Test ChatModel accepts array shorthand for messages", async () => {
+  const model = new FakeChatModel({});
+  const response = await model.invoke([["human", "Hello there!"]]);
+  expect(response.content).toEqual("Hello there!");
+});
+
+test("Test ChatModel accepts object shorthand for messages", async () => {
+  const model = new FakeChatModel({});
+  const response = await model.invoke([
+    {
+      type: "human",
+      content: "Hello there!",
+      additional_kwargs: {},
+      example: true,
+    },
+  ]);
+  expect(response.content).toEqual("Hello there!");
+});
+
 test("Test ChatModel uses callbacks", async () => {
   const model = new FakeChatModel({});
   let acc = "";

langchain-core/src/messages/base.ts

@@ -318,6 +318,10 @@ export abstract class BaseMessageChunk extends BaseMessage {
 
 export type BaseMessageLike =
   | BaseMessage
+  | ({
+      type: MessageType | "user" | "assistant" | "placeholder";
+    } & BaseMessageFields &
+      Record<string, unknown>)
   | [
       StringWithAutocomplete<
         MessageType | "user" | "assistant" | "placeholder"

langchain-core/src/messages/utils.ts

@@ -5,6 +5,7 @@ import {
   isBaseMessage,
   StoredMessage,
   StoredMessageV1,
+  BaseMessageFields,
 } from "./base.js";
 import {
   ChatMessage,
@@ -20,6 +21,23 @@ import { HumanMessage, HumanMessageChunk } from "./human.js";
 import { SystemMessage, SystemMessageChunk } from "./system.js";
 import { ToolMessage, ToolMessageFieldsWithToolCallId } from "./tool.js";
 
+function _constructMessageFromParams(
+  params: BaseMessageFields & { type: string }
+) {
+  const { type, ...rest } = params;
+  if (type === "human" || type === "user") {
+    return new HumanMessage(rest);
+  } else if (type === "ai" || type === "assistant") {
+    return new AIMessage(rest);
+  } else if (type === "system") {
+    return new SystemMessage(rest);
+  } else {
+    throw new Error(
+      `Unable to coerce message from array: only human, AI, or system message coercion is currently supported.`
+    );
+  }
+}
+
 export function coerceMessageLikeToMessage(
   messageLike: BaseMessageLike
 ): BaseMessage {
@@ -28,17 +46,11 @@ export function coerceMessageLikeToMessage(
   } else if (isBaseMessage(messageLike)) {
     return messageLike;
   }
-  const [type, content] = messageLike;
-  if (type === "human" || type === "user") {
-    return new HumanMessage({ content });
-  } else if (type === "ai" || type === "assistant") {
-    return new AIMessage({ content });
-  } else if (type === "system") {
-    return new SystemMessage({ content });
+  if (Array.isArray(messageLike)) {
+    const [type, content] = messageLike;
+    return _constructMessageFromParams({ type, content });
   } else {
-    throw new Error(
-      `Unable to coerce message from array: only human, AI, or system message coercion is currently supported.`
-    );
+    return _constructMessageFromParams(messageLike);
   }
 }
 

langchain-core/src/prompts/chat.ts

@@ -134,50 +134,43 @@ export class MessagesPlaceholder<
     return [this.variableName];
   }
 
-  validateInputOrThrow(
-    input: Array<unknown> | undefined,
-    variableName: Extract<keyof RunInput, string>
-  ): input is BaseMessage[] {
+  async formatMessages(
+    values: TypedPromptInputValues<RunInput>
+  ): Promise<BaseMessage[]> {
+    const input = values[this.variableName];
     if (this.optional && !input) {
-      return false;
+      return [];
     } else if (!input) {
       const error = new Error(
-        `Error: Field "${variableName}" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined`
+        `Field "${this.variableName}" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined`
       );
       error.name = "InputFormatError";
       throw error;
     }
 
-    let isInputBaseMessage = false;
-
-    if (Array.isArray(input)) {
-      isInputBaseMessage = input.every((message) =>
-        isBaseMessage(message as BaseMessage)
-      );
-    } else {
-      isInputBaseMessage = isBaseMessage(input as BaseMessage);
-    }
-
-    if (!isInputBaseMessage) {
+    let formattedMessages;
+    try {
+      if (Array.isArray(input)) {
+        formattedMessages = input.map(coerceMessageLikeToMessage);
+      } else {
+        formattedMessages = [coerceMessageLikeToMessage(input)];
+      }
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    } catch (e: any) {
       const readableInput =
         typeof input === "string" ? input : JSON.stringify(input, null, 2);
-
       const error = new Error(
-        `Error: Field "${variableName}" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: ${readableInput}`
+        [
+          `Field "${this.variableName}" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages or coerceable values as input.`,
+          `Received value: ${readableInput}`,
+          `Additional message: ${e.message}`,
+        ].join("\n\n")
       );
       error.name = "InputFormatError";
       throw error;
     }
 
-    return true;
-  }
-
-  async formatMessages(
-    values: TypedPromptInputValues<RunInput>
-  ): Promise<BaseMessage[]> {
-    this.validateInputOrThrow(values[this.variableName], this.variableName);
-
-    return values[this.variableName] ?? [];
+    return formattedMessages;
   }
 }
 

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

@@ -302,7 +302,7 @@ test("Test MessagesPlaceholder not optional", async () => {
   });
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   await expect(prompt.formatMessages({} as any)).rejects.toThrow(
-    'Error: Field "foo" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined'
+    'Field "foo" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined'
   );
 });
 
@@ -323,6 +323,60 @@ test("Test MessagesPlaceholder shorthand in a chat prompt template", async () =>
   ]);
 });
 
+test("Test MessagesPlaceholder shorthand in a chat prompt template with object format", async () => {
+  const prompt = ChatPromptTemplate.fromMessages([["placeholder", "{foo}"]]);
+  const messages = await prompt.formatMessages({
+    foo: [
+      {
+        type: "system",
+        content: "some initial content",
+      },
+      {
+        type: "human",
+        content: [
+          {
+            text: "page: 1\ndescription: One Purchase Flow\ntimestamp: '2024-06-04T14:46:46.062Z'\ntype: navigate\nscreenshot_present: true\n",
+            type: "text",
+          },
+          {
+            text: "page: 3\ndescription: intent_str=buy,mode_str=redirect,screenName_str=order-completed,\ntimestamp: '2024-06-04T14:46:58.846Z'\ntype: Screen View\nscreenshot_present: false\n",
+            type: "text",
+          },
+        ],
+      },
+      {
+        type: "assistant",
+        content: "some captivating response",
+      },
+    ],
+  });
+  expect(messages).toEqual([
+    new SystemMessage("some initial content"),
+    new HumanMessage({
+      content: [
+        {
+          text: "page: 1\ndescription: One Purchase Flow\ntimestamp: '2024-06-04T14:46:46.062Z'\ntype: navigate\nscreenshot_present: true\n",
+          type: "text",
+        },
+        {
+          text: "page: 3\ndescription: intent_str=buy,mode_str=redirect,screenName_str=order-completed,\ntimestamp: '2024-06-04T14:46:58.846Z'\ntype: Screen View\nscreenshot_present: false\n",
+          type: "text",
+        },
+      ],
+    }),
+    new AIMessage("some captivating response"),
+  ]);
+});
+
+test("Test MessagesPlaceholder with invalid shorthand should throw", async () => {
+  const prompt = ChatPromptTemplate.fromMessages([["placeholder", "{foo}"]]);
+  await expect(() =>
+    prompt.formatMessages({
+      foo: [{ badFormatting: true }],
+    })
+  ).rejects.toThrow();
+});
+
 test("Test using partial", async () => {
   const userPrompt = new PromptTemplate({
     template: "{foo}{bar}",