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Occurs when a prompt template received missing or invalid input variables. One unexpected way this can occur is if you add a JSON object directly into a prompt template: 
import { PromptTemplate } from "@langchain/core/prompts";
import { ChatOpenAI } from "@langchain/openai";

const prompt = PromptTemplate.fromTemplate(`You are a helpful assistant.

Here is an example of how you should respond:

{
  "firstName": "John",
  "lastName": "Doe",
  "age": 21
}

Now, answer the following question:

{question}`);
You might think that the above prompt template should require a single input key named question, but the JSON object will be interpreted as an additional variable because the curly braces ({) are not escaped, and should be preceded by a second brace instead, like this: 
import { PromptTemplate } from "@langchain/core/prompts";
import { ChatOpenAI } from "@langchain/openai";

const prompt = PromptTemplate.fromTemplate(`You are a helpful assistant.

Here is an example of how you should respond:

{{
  "firstName": "John",
  "lastName": "Doe",
  "age": 21
}}

Now, answer the following question:

{question}`);

​
Troubleshooting

To resolve this error, you can:
  1. Review your prompt template for correctness. When using f-string formats, ensure proper escaping of curly braces:
    • Use {{ for single braces in f-strings
    • Use {{{{ for double braces in f-strings
  2. When using MessagesPlaceholder components, confirm you’re passing message arrays or message-like objects. If using shorthand tuples, wrap variable names in curly braces like ["placeholder", "{messages}"]
  3. Debug by examining actual inputs to your prompt template by using LangSmith or logging to verify they match expectations
  4. If sourcing prompts from LangChain Prompt Hub, isolate and test the prompt with sample inputs to ensure it functions as intended
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