GPT-5.5 with tools on custom OpenAI-compatible providers still lacks a clear working path (chat/completions vs responses) #35732

Description

I'm using OpenCode with a custom OpenAI-compatible provider in an enterprise setup.

I now have a confirmed working path for o4-mini reasoning:

• plain model name (o4-mini)
• no reasoning suffix in the model name
• agent-level options.reasoning_effort
• tool calling works

However, gpt-5.5 with tools is still unclear/problematic on custom OpenAI-compatible providers.

The backend error is:

Function tools with reasoning_effort are not supported for gpt-5.5 in /v1/chat/completions. Please use /v1/responses instead.

Upstream issues such as #26219 and #29325 were closed, but from a user perspective there is still no clear/documented happy path for this setup when tools are enabled.

What I would expect

One of the following should be true and documented:

1. OpenCode should automatically avoid sending incompatible reasoning_effort combinations for GPT-5.5 when using chat/completions with tools, OR
2. OpenCode should correctly support/route the responses path for compatible custom providers, OR
3. OpenCode should clearly document that GPT-5.5 + tools is unsupported on custom OpenAI-compatible providers unless a specific provider adapter/path is used.

Why this matters

Today the behavior is hard to reason about when integrating non-default gateways:

• plain model usage may work
• tool usage may fail
• the failure mode depends on endpoint semantics
• the supported configuration is not obvious from the config surface

This creates ambiguity for enterprise/custom-provider users.

Environment

• OpenCode version: 1.17.4
• Provider type: custom OpenAI-compatible provider
• Model: GPT-5.5
• Tools enabled: yes

Additional context

In the same environment, o4-mini works correctly when configured as:

• plain model name
• no suffix notation
• agent-level options.reasoning_effort
• tools enabled

So this seems specifically about GPT-5.5 + tools + endpoint compatibility, not a general custom-provider failure.

Plugins

No response

OpenCode version

1.17.4

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

Ubuntu 24.04 LTS

Terminal

Termius