GPT-4o cannot properly call custom functions more than half the time

When GPT-4o tries to call a custom function, it doesn’t actually call any function. Instead, it prints the function call in the normal assistant response.

Sometimes, it works fine. Most times, it prints the function call and any arguments that go with it, right in the assistant response. It makes this model unusable when you need custom functions to work. Not to mention confuses the hell out of users.

What can I do? It doesn’t seem to matter how I word the function description. GPT-4-turbo, GPT-4, and GPT-3.5-turbo all work fine with customer functions in my code.

As you setting the function_call parameter to force it to use a given function for chat completions, or the equivalent tool_choice param for thread runs?

1 Like

Yes, using the function call parameter.

I should mention that I am using the Assistants API v2

Having a similar issue.

My assistants keep saying “Okay, I’m going to [use the function] now” and then they don’t. GPT-4-Turbo didn’t seem to have this problem as often.

FYI I’m not having this issue with Chat Completions.

I’m having the exact opposite issue. GPT-4o keeps trying to call a non existent function instead of just responding in a langgraph group chat. The agent that keeps failing only has one tool (that it is using correctly) but then calling non existent tool.

I’ve tried playing with prompts all over the place to resolve this but it consistently is messing up.

I’m using chat completion and it’s correctly returning a function call, but I’m definitely seeing worse performance than gpt-4. It’s generally making worse decisions, not obeying enums as well as previously, and often calling only one function where multiple are needed.

i am using gpt assistant and facing exact same issue , did you find that what wrong with it ?

whats the solution of this ?

I agree and find that function calling in GPt-4o is unusable when compared to GPT-4 Turbo. In some cases I am seeing the same function being called 2 or 3 times with parameters being correct for 1 and completely made up for the others. Also seeing function calls returned in the assistant response. When instructed in the system message to only call a single function, multiple functions are often called.

Same here. Instead of returning with the correct function calls, gpt-4o sometimes returns a content string in the form of

${actual content}

, as if it is calling a JS function.
My (very temporary) solution is to manually parse this string to retrieve function name and arguments. Here’s the code (node.js + typescript + langchain):

import type { AIMessage } from '@langchain/core/messages';
import jsonc from 'jsonc-parser';

 * Maunally parse and rectify a faulty tool calling response from gpt-4o. 
 * @param {AIMessage} msg 
 * @returns {void}
export function patchFautyFunctionCall(msg: AIMessage): void {
  if (!msg.content || typeof msg.content !== 'string') {
  const matchResult = msg.content.match(/functions\.(.*?)[\(\n]/);
  if (!matchResult) {

  // The string that comes after functions.${function_name}(
  // If the argument is an object, gpt-4o sometimes omits double quotes around attribute names, 
  // making it an invalid JSON string. quotifyJSONString adds the missing quotation marks. 
  const jsonString = quotifyJSONString(msg.content.substring((matchResult.index ?? 0) + matchResult[0].length));
  const [jsonObj, prefixLen] = parseJSONPrefix(jsonString);
  if (prefixLen === 0) {
  msg.content = jsonString.substring(prefixLen);
  if (msg.tool_calls === undefined) {
    msg.tool_calls = [];
    name: matchResult[1],
    args: jsonObj,

function quotifyJSONString(unquotedJson: string): string {
  const attributePattern = /([{,]\s*)([a-zA-Z_][a-zA-Z0-9_]*)(\s*:)/g;

  // Replace unquoted attribute names with quoted ones
  return unquotedJson.replace(attributePattern, '$1"$2"$3');

 * Try to parse the prefix of a JSON string into an object.
 * @param {string} str A string that might have a valid JSON prefix.
 * @returns {[any, number]} [The parsed object, size of the valid JSON prefix]
function parseJSONPrefix(str: string): [any, number] {
  const errors: jsonc.ParseError[] = [];
  const obj = jsonc.parse(str, errors);

  if (errors.length === 0) {
    return [obj, str.length];
  if (errors[0].offset === 0) {
    // No valid prefix
    return [undefined, 0];

  return [obj, errors[0].offset];

Hi @cbarber713, OAI staff here, and sorry for the late reply. I’d like to get more details on your use case. It sounds like you were using:

  • assistants API v2
  • with tools and tool_choice param

My questions is: what value did you pass to tool_choice? Is it auto, required or a specific function like {"type": "function", "function": "your_func_name"}

Any information that can help me reproduce this bug is highly appreciated.


@turbolucius, @voidptr_t,
if any of you can provide details that would help me reproduce this bug, it would help me figure out what’s going on faster. :pray: