GPT-5.5 Medium fails with Provider Error: tools[0] missing field `function`

Support Bug Reports
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1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

Cursor Bug Report — GPT-5.5 tools[0] missing field function

Draft for Help → Report Issue / forum.cursor.com. Generated 2026-06-18.

Title

GPT-5.5 Medium fails with Provider Error: tools[0] missing field function

Description

Selecting GPT-5.5 Medium causes every chat request to fail immediately with a Provider Error. Other models (e.g. Composer, Claude) work in the same session.

Error message

Provider returned error: {"error":{"message":"Failed to deserialize the JSON body into the target type: tools[0]: missing field `function` at line 1 column 61842","type":"invalid_request_error","param":null,"code":"invalid_request_error"}}

Request ID

6de76c42-80b5-45e7-b322-7ac1ccda4f17

Steps to reproduce

  1. Open Cursor on Windows
  2. Select model GPT-5.5 Medium (Ask or Agent mode)
  3. Send any message in chat
  4. Request fails with Provider Error above

Expected behavior

GPT-5.5 should accept the request and respond like other models.

Actual behavior

Request is rejected before the model runs. Error indicates the tools array in the API payload is malformed — first tool entry is missing the required function field (OpenAI-compatible tools schema).

Environment

  • Cursor version: 3.7.42 (5702c9cfca656d8710fad58402fe37f14345e3a0, x64)
  • OS: Windows 10 (build 26200)
  • Mode: Ask (likely also Agent)
  • MCP servers (may be relevant): Notion, Figma, Datadog

Additional context

  • Error at column ~61842 suggests a large tools payload (built-in tools + MCP).
  • Likely a Cursor-side serialization bug when building the tools array for the GPT-5.5 provider, not a project/code issue.
  • Workaround: switch to another model; optionally disable MCP servers.

Attachments

  • Screenshot: Provider Error dialog (user-provided)
  • Optional: paste output from “Copy Request” on the error toast

Steps to Reproduce

  1. Open Cursor on Windows
  2. Select model GPT-5.5 Medium (Ask or Agent mode)
  3. Send any message in chat
  4. Request fails with Provider Error above

Operating System

Windows 10/11

Version Information

version :3.7.42

Does this stop you from using Cursor

Sometimes - I can sometimes use Cursor

5

Hey, thanks for the detailed report. This is a known bug on our side, and it’s not specific to GPT-5.5. The issue is how Cursor serializes tools for BYOK and OpenAI-compatible endpoints. The first tool is sent in the new format {"type":"custom",...}, but legacy OpenAI-compatible providers expect the function wrapper, so the request gets rejected before it reaches the model. This also affects gpt-5.4-nano, gpt-5.3-codex, and some others.

We’re already tracking the issue, but I can’t share an ETA for the fix yet.

Workaround for now:

  • Switch to a model hosted in Cursor, like Auto, Claude, or Composer. These don’t use your custom key, and they work fine.
  • Or temporarily disable your custom API key in settings until we ship a fix.

Once there’s an update on the fix, I’ll reply in the thread.