There are many of these available, alongside the provider ones.
Are there any benchmarks which help me decide which ones I should try over the standard codex/claude?
They are vendor offerings but you don’t have to take them.
and "agent" as the instance which is running... this is the more overloaded term because there are "agent" definitions, this is where a combinatorial exists, because the running "agent" is a combination of agent definitions, context, tool choice where you can modify each independently to create a new "agent"
I think Scion (Google's) does a decent job defining these concepts: https://googlecloudplatform.github.io/scion/concepts/
For example, consider something like SWEBench, where a test should pass. Harness or a skill can just tell the agent keep trying until the test pass. So same model can have a lot of behavior changes between the harnesses.
In my experience (i own a multi-agent IDE with 50+ supported harness) folks stick to first party, usually either openai or anthropic, grok, agy; if they are willing to pay for the subscription the value is unbeatable. And they don't risk trying different harnesses.
I do often see benchmarks comparing them and usually Pi comes out on top, but I think the benchmarks are generally testing one moderately sized clear task. Pi is minimal and great in that situation.
For more general usage, where tasks are not clear, you might interrupt at any moment to steer it, etc. benchmarks are harder I guess. I personally love pi because anytime there's something I don't like, I spin up a separate session and fix it.
E.g. I've found the "reply in simple technical English" hack amazing but I have to keep saying it over and over. So I had an agent write an extension in about ten minutes that appends a short hidden message saying all user and dev facing text must be in STE, apply that without talking about STE. Works amazingly well.
Here is an example I encountered recently: The Figma MCP Server documents usage with Claude (Code), Codex, Cursor, Grok and Antigravity [], but notably not OpenCode. More so, their oauth registration seems to use some kind of whitelist, meaning just adding the MCP by url does not work either. To make it work, my agent edited the config entry to use "Codex" as the oauth name, which worked.
That's just one example, but something you see quite frequently.
In other words, big industry supporting bigger industry. I don't think that's surprising or says much about the real landscape.
Pi is the most basic with the ability to add features.
opencode also supports more features and was build as an open alternative to claude.
From what I gathered, a simple harness like the one from Pi is sufficient.
Similarly, I would advise against using tons of skills and keep the ones you genuienly find useful (they are just .md files so you can copy them). The only skill I use is /strategic-compact from everything claude code (ecc) because I like to read the summary before a manual compaction.