Create a regex pattern matcher for client variable references

Oct 13, 2025 by pinkandtrash.com 59 views

Overview

In this article, we will explore the process of modifying the MCP service to automatically replace client variable placeholders with their actual values before executing MCP tool calls. This enhancement allows tools to receive the correct context without explicit handling by the LLM, making the overall system more efficient and user-friendly.

Description

The MCP service is a crucial component of our system, responsible for executing various tools and providing a seamless experience for users. However, the current implementation requires explicit handling of client variable placeholders, which can lead to errors and inconsistencies. To address this issue, we will update the callMcpFunction method to detect variable placeholders and implement a replacement function that substitutes variables with values.

Technical Details

Update the `callMcpFunction` method

The first step is to update the callMcpFunction method to detect variable placeholders. We will use a regular expression pattern matcher to identify client variable references in the parameter object. This will enable us to automatically replace the placeholders with their actual values before executing the MCP tool call.

const regex = /{client_(\w+)}/g;
const match = parameterObject.match(regex);
if (match) {
  // Replace variable placeholders with actual values
  parameterObject = replaceVariables(parameterObject, match);
}

Create a regex pattern matcher for client variable references

To create a regex pattern matcher, we need to define a regular expression that matches client variable references. The pattern should match the {client_ prefix followed by one or more word characters (letters, numbers, or underscores).

const regex = /{client_(\w+)}/g;

Implement a replacement function that substitutes variables with values

The replacement function will take the parameter object and the matched variable references as input and return the modified parameter object with the variable placeholders replaced with their actual values.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue);
}

Add support for different variable types (string, boolean, numeric)

To support different variable types, we need to modify the replacement function to handle string, boolean, and numeric variables correctly.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  if (variableValue === null || variableValue === undefined) {
    throw new Error(`Missing variable: ${variableName}`);
  }
  if (typeof variableValue === 'string') {
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue);
  } else if (typeof variableValue === 'boolean') {
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue.toString());
  } else if (typeof variableValue === 'number') {
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue.toString());
  } else {
    throw new Error(`Unsupported variable type: ${typeof variableValue}`);
  }
}

Handle deep object traversal for nested parameters

To handle deep object traversal, we need to modify the replacement function to recursively traverse the parameter object and replace variable placeholders in nested objects.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  if (variableValue === null || variableValue === undefined) {
    throw new Error(`Missing variable: ${variableName}`);
  }
  if (typeof variableValue === 'object') {
    return replaceVariablesInObject(parameterObject, variableValue, variableName);
  } else {
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue);
  }
}

function replaceVariablesInObject(parameterObject, object, variableName) {
  for (const key in object) {
    if (Object.prototype.hasOwnProperty.call(object, key)) {
      const value = object[key];
      if (typeof value === 'object') {
        replaceVariablesInObject(parameterObject, value, variableName);
      } else {
        parameterObject = parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), value);
      }
    }
  }
  return parameterObject;
}

Add validation to ensure all required variables exist

To ensure all required variables exist, we need to add validation to the replacement function to check if the variable value is not null or undefined.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  if (variableValue === null || variableValue === undefined) {
    throw new Error(`Missing variable: ${variableName}`);
  }
  // ...
}

Create appropriate error handling for missing variables

To create appropriate error handling for missing variables, we need to throw an error with a meaningful message when a variable is missing.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  if (variableValue === null || variableValue === undefined) {
    throw new Error(`Missing variable: ${variableName}`);
  }
  // ...
}

Add support for both absolute and relative paths for CWD

To add support for both absolute and relative paths for CWD, we need to modify the replacement function to handle both cases correctly.

function replaceVariables(parameterObject, match) {
  const variableName = match[1];
  const variableValue = getClientVariableValue(variableName);
  if (variableValue === null || variableValue === undefined) {
    throw new Error(`Missing variable: ${variableName}`);
  }
  if (variableValue.startsWith('/')) {
    // Absolute path
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), variableValue);
  } else {
    // Relative path
    return parameterObject.replace(new RegExp(`{client_${variableName}}`, 'g'), `${process.cwd()}/${variableValue}`);
  }
}

Complexity

The complexity of this task is medium, as it involves modifying the MCP service to automatically replace client variable placeholders with their actual values. This requires a good understanding of regular expressions, object traversal, and error handling.

Dependencies

This task depends on the following tasks:

CV-01: Define Client Variables Schema
CV-02: Create Client Variable Service

Acceptance Criteria

The acceptance criteria for this task are:

Client variable placeholders are automatically replaced before MCP calls
The replacement works in both top-level and nested parameters
Different variable types are handled correctly
Missing variables produce appropriate error messages
The implementation is performant, even with complex parameter objects
The original parameter object is not modified (immutability)
Both absolute and relative paths are handled correctly for CWD
[CV-03] Add Variable Injection to MCP Calls: Q&A =====================================================

Q: What is the purpose of adding variable injection to MCP calls?

A: The purpose of adding variable injection to MCP calls is to automatically replace client variable placeholders with their actual values before executing MCP tool calls. This allows tools to receive the correct context without explicit handling by the LLM, making the overall system more efficient and user-friendly.

Q: How does the variable injection process work?

A: The variable injection process involves the following steps:

Detecting variable placeholders in the parameter object using a regular expression pattern matcher.
Replacing the variable placeholders with their actual values using a replacement function.
Handling deep object traversal for nested parameters.
Adding validation to ensure all required variables exist.
Creating appropriate error handling for missing variables.

Q: What are the benefits of using variable injection in MCP calls?

A: The benefits of using variable injection in MCP calls include:

Improved efficiency: Tools receive the correct context without explicit handling by the LLM.
Enhanced user experience: Users do not need to worry about handling variable placeholders.
Reduced errors: Variable injection reduces the likelihood of errors caused by missing or incorrect variable values.

Q: How does the variable injection process handle different variable types?

A: The variable injection process handles different variable types (string, boolean, numeric) by using a replacement function that substitutes variables with values based on their type.

Q: What happens if a variable is missing or has an incorrect value?

A: If a variable is missing or has an incorrect value, the variable injection process throws an error with a meaningful message.

Q: How does the variable injection process handle absolute and relative paths for CWD?

A: The variable injection process handles absolute and relative paths for CWD by using a replacement function that substitutes variables with values based on their type and path.

Q: What are the dependencies for this task?

A: The dependencies for this task are:

CV-01: Define Client Variables Schema
CV-02: Create Client Variable Service

Q: What is the complexity of this task?

A: The complexity of this task is medium, as it involves modifying the MCP service to automatically replace client variable placeholders with their actual values.

Q: What are the acceptance criteria for this task?

A: The acceptance criteria for this task are:

Client variable placeholders are automatically replaced before MCP calls
The replacement works in both top-level and nested parameters
Different variable types are handled correctly
Missing variables produce appropriate error messages
The implementation is performant, even with complex parameter objects
The original parameter object is not modified (immutability)
Both absolute and relative paths are handled correctly for CWD

Q: How can I implement variable injection in my MCP calls?

A: To implement variable injection in your MCP calls, follow these steps:

Update the callMcpFunction method to detect variable placeholders using a regular expression pattern matcher.
Implement a replacement function that substitutes variables with values based on their type and path.
Handle deep object traversal for nested parameters.
Add validation to ensure all required variables exist.
Create appropriate error handling for missing variables.

Note: This is a general guide and may require modifications based on your specific implementation and requirements.