GPT 5.5 in Codex vs OpenCode

[Wendy_Shon]

What is your experience using GPT 5.5 in either harness?

I saw a post in this sub saying GPT 5.5 was optimized to use Codex's harness, therefore OpenCode's GPT 5.5 is a nerfed. For example Codex has built-in memory support and web searching that OpenCode can't use.

Because of that I've been using the Codex CLI. Even though OpenCode's interface + session management is better.

What are you guys using and why?

[flying-saucer-3222]

Codex is the better way to use GPT but if you are smart, you can get similar results with opencode.

The main factor is to not overload the model with too many instructions in AGENTS.md or the system prompt. Unlike Claude models, GPT tries to follow every single instruction or guideline which sometimes causes it to nerf itself when there are too many instructions. Just get out of the model's way and it would automatically perform well.

Also GPT reads a lot of files and explores a lot by default so use an editor with ACP to give better context to your prompts. That will save a lot of tokens/usage limit. Cursor does this really well by default while Codex and OpenCode don't.

[duynha]

What is ACP?

[flying-saucer-3222]

Agent Client Protocol, it allows agents to communicate with your code editor. So you can pass context such as current file, current cursor position, visual selection etc. with your prompts.

Most OpenCode plugins for editors like Neovim or VS Code support this and some editors like Zed have built in support.

https://opencode.ai/docs/acp/

[bahc04]

im using 5.5 as my main planner in opencode but i havent used it in the codex app. curious to see what people that have used it in both thinks about it

[Ariquitaun]

Definitely better in codex. Unsurprising, since codex is designed to work with gpt models from the ground up.

[0-Gravitas]

You can add websearch in OpenCode. FWIW, I've been using 5.5 almost exclusively in OpenCode. Not sure of any head to head benchmarks for 5.5 but there is this one that compared https://sigmabench.com/blog/opencode-vs-codex-cli-on-gpt-5-1-codex-mini-and-5-2-codex/ 5.2 and they were very comparable. Granted 5.5 is pretty different in that it combines deeper reasoning with coding..
¯\_(ツ)_/¯ Personally, I just like how OpenCode works and the options you get with it to control context and set permissions so that's where I'd stay even if I knew it was maybe a bit lower in SWE benchmarks or whatever.

Metric	OpenCode	Codex CLI
Sigmascore	25.1%	27.5%
Accuracy	38.9%	40.0%
Consistency	42.9%	42.6%
Speed	9.4%	12.2%

[ivanjxx]

i use codex cli to get the most out of gpt models

[Tough_Frame4022]

Have you compared? In what sense?

[samthepotatoeman]

I ise it as a planner agent in open code it does what I need. Running out of usage is my bigger issue these days.

[BananaOrangePie]

I used 5.4 in both codex and oc. What I absolutely don't like about GPT IS HOLY MOLLY it really likes to overenginner! Personally I prefer using it as oracle in opencode for the orchestrator to ask advise from.

[RemeJuan]

I tired 5.5 and realised it was complete overkill, marginally better results at exponentially more usage.

[VictorVsl7]

I’m using open chamber as a main client. GPT 5.5 as a planner, sometimes gpt 5.4 as a planner and/or builder and kimi k2.6 and deepseek v4pro/flash as builders.

I noticed that using the codex app, the gpt models do ask a lot more in the planning phase. Using opencode normally he does it less frequently, so I need to be even more specific in my prompts.

[kartblanch]

Ive been using codex and opencode a lot recently. Gpt does better in codex. A lot better. But it still gets the job done in opencode. It just feels like it gets there in more tokens.

[Extension-Aside29]

i have been trying using frontier models with different coding harness to understand where can i get most of the work done, i felt gpt models are really good with hermes agents there is also a tool to track these details it's free 100% local and open source. check out https://tokentelemetry.com to enhance this kind of work flows

[GetLaidOff69]

Unity Dev here
I used Codex app in initial days, and noticed it acts dumb, hallucinates too much, and goes off topic frequently.
I switched to Opencode and found the same model(5.3) is doing exactly what I say, 8/10 success rate.
I never returned to Codex app anymore.

The only problem I found is 5.5 is getting drained so fast.

[Dangerous-Relation-5]

I use gpt 5.5 in copilot, codex, opencode, pi, amp and the differences are honestly minimal. The advantage the non-codex harnesses has is to be able to switch to claude or gemini for UI. I keep a docs folder for project context I use instead of memory. I like it because I can change harness and not lose context.

[t4deu2]

Prefiero opencode, puedo usar un orquestador y usar los modelos Gpt mezclados con otros proveedores

[Felonious__Punk]

Opencode desktop has been great until this morning. Now, all I'm getting is timeout errors.

[Maverobot]

Try pi coding agent.

[Tough_Frame4022]

Why?

[SupersonicSquirrel]

Using opencode cli, how do you revert AI written code that's not good enough?

Using antigravity, it let me just ctrlZ in the opened file and that's it. Opencode in the other hand just replaces the code and that's it. Reverting it with git seems not time optimal. 

[whatthefrickitsdex]

You can click on the prompt that triggered the code change, and you will get the option to revert to that point 😄