Joern MCP Server

Instructions

Implementation Reference

• server_tools.py:87-96 (handler)

  Handler function for 'get_method_code_by_id' tool. Decorated with @joern_mcp.tool() for automatic registration in FastMCP. Queries Joern server remotely using joern_remote and parses response with extract_value to return method source code by ID.
  @joern_mcp.tool() def get_method_code_by_id(method_id:str) -> str: """Get the code of a method by its class full name and method name @param class_full_name: The fully qualified name of the class @param method_name: The name of the method @return: List of full name, name, signature and id of methods in the class """ response = joern_remote(f'get_method_code_by_id("{method_id}")') return extract_value(response)
• server.py:38-70 (helper)

  Core helper function joern_remote that sends HTTP POST requests to the Joern server endpoint with the CPG query, authenticates, handles timeouts, cleans ANSI sequences from stdout response.
  def joern_remote(query): """ Execute remote query and return results Parameters: query -- The query string to execute Returns: Returns the server response stdout content on success Returns None on failure, error message will be output to stderr """ data = {"query": query} headers = {'Content-Type': 'application/json'} try: response = requests.post( f'http://{server_endpoint}/query-sync', data=json.dumps(data), headers=headers, auth=basic_auth, timeout=timeout ) response.raise_for_status() result = response.json() return remove_ansi_escape_sequences(result.get('stdout', '')) except requests.exceptions.RequestException as e: sys.stderr.write(f"Request Error: {str(e)}\n") except json.JSONDecodeError: sys.stderr.write("Error: Invalid JSON response\n") return None
• common_tools.py:111-133 (helper)

  Utility function extract_value that parses Joern query responses to extract the relevant value (code, ID, name, etc.) based on patterns like triple quotes for code blocks or quoted strings.
  def extract_value(input_str: str) -> str: """Extract value from a string based on its pattern. This function automatically selects the appropriate extraction method based on the input string format: * If contains 'Long =', uses extract_long_value * If contains triple quotes, uses extract_code_between_triple_quotes * If contains single quotes, uses extract_quoted_string Args: input_str (str): Input string containing a value to extract Returns: str: Extracted value based on the detected pattern """ if 'Long =' in input_str: return extract_long_value(input_str) elif 'String = """' in input_str: return extract_code_between_triple_quotes(input_str) elif 'String = "' in input_str: return extract_quoted_string(input_str) else: return input_str
• server.py:94-106 (registration)

  Dynamic registration mechanism: server.py executes the code in server_tools.py via exec(), which runs all @joern_mcp.tool() decorators to register tools including 'get_method_code_by_id' to the FastMCP instance.
  SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__)) GENERATED_PY = os.path.join(SCRIPT_DIR, "server_tools.py") def generate(): """Generate and execute additional server tools from server_tools.py file. This function reads the content of server_tools.py and executes it to add more functionality to the server. """ with open(GENERATED_PY, "r") as f: code = f.read() exec(compile(code, GENERATED_PY, "exec")) generate()