**TL;DR**: add support for OpenCode Go (flat-rate monthly subscription)
along the already-implemented OpenCode Zed (pay-as-you-go billing).

> [!WARNING]
> This code was written by LLMs, under the supervision of a so-called
developer that never wrote Rust profesionally and that spends more time
in Pages&Keynote than in an IDE.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Background

OpenCode offers a few different ways to access models:
- **free access to 3 models**, with feedback and data used to improve
the model. These models use the OpenCode Zen API endpoints, but have
different usage limits (200 requests per 5 hours) and have a different
privacy policy. Some people disable or block the free models, some
people are super-excited to have access to LLMs for free, and some
people like using the free models to test new LLMs (at launch MiMo-V2
had 2 free weeks of usage, for example).
- **pay-as-you-go access to 30 models** as part of the [OpenCode
Zen](https://opencode.ai/zen) subscription. These models use the same
OpenCode Zen API endpoints.
- **flat-rate monthly access to 7 models** as part of the [OpenCode
Go](https://opencode.ai/go) subscription. These models use the OpenCode
Zen API endpoints with an extra `/go` appended to the path. There are
5-hour, weekly, and monthly usage limits and, additionally, users can
toggle a switch in the OpenCode Console to use Zen models with their
pay-as-you-go billing after the Go limits are hit.

There's also a currently-paused [OpenCode
Black](https://opencode.ai/black) flat-rate subscription with way higher
usage limits and with access to more models, with $100 and $200 monthly
plans.

The whole thing is a bit messy, but it's great value and highly reliable
LLM access!

<br>

https://github.com/zed-industries/zed/pull/49589 added support for
OpenCode Zen by implementing a new `opencode` provider. OpenCode Go
[could be used by overriding the API
URL](https://github.com/zed-industries/zed/pull/49589#issuecomment-4130300454),
but that is a terrible user experience: some models have to be manually
added, the model list always shows the 30-something OpenCode Zen models,
and free models cannot be used at all.

I was annoyed by the experience of using OpenCode Go with Zed and this
past week I had to test a bunch of LLMs and providers and harnesses, so
I took this on as a test case 🙂

## Implementation

This PR makes the OpenCode provider more general (not just for Zen) and
adds an `OpenCodeModelSubscription` concept which is then used to
implement support for OpenCode Go. The free models are also broken out
into their own subscription for a prettier model list.

For a better user experience, the different subscriptions can be enabled
or disabled, both in the settings file and in the UX:
<img width="434" height="176" alt="Screenshot showing the OpenCode
provider configuration, with the newly added toggles"
src="https://github.com/user-attachments/assets/3520e114-00c8-4794-84bf-35cd72d9c57e"
/>


The code was written by LLMs, but I do understand it and I did a bunch
of "manual" iterations and "manual" tweaks. Still, my Rust experience is
non-existent so **I won't feel offended if y'all reject this PR**! I did
consider alternatives (adding a new `opencode-go` provider and renaming
this to `opencode-zen`, for example, or adding support for custom API
URLs in OpenCode custom models which would've been the smallest code
change but a terrible user experience, and so on) but all alternatives
would have been, in my opinion, a worse user experience.

**Tests I did**:
- confirmed OpenCode Go models work as expected
- confirmed OpenCode Zen Free models work as expected
- confirmed I get an error when trying to use OpenCode Zen models since
I don't have that subscription
- confirmed the subcription toggles work as expected (model are
shown/hidden, settings file is updated)

**Notes**:
- this PR is best reviewed commit-by-commit. I did not create a separate
PR for the model updates to minimize delays
- my exeprience with Rust is roughly zero, but I tried to strike a
balance between idiomatic Rust and easy-to-read code
- users of the OpenCode provider might have to do some re-configuration
after this PR is merged since the model identifiers now include the
subscription, eg `claude-haiku-4-5` is now `zen/claude-haiku-4-5`. Since
this is a relatively new provider and the impact is small, I preffered
that rather than adding complex migration/mapping logic.
- does changing the provider name from "OpenCode Zen" to "OpenCode"
break anything for y'all at Zed?
- does changing the telemetry id from `"opencode/<model-id>"` to
`"opencode/<subscription>/<model-id>"` break anything for y'all at Zed?

---

Release Notes:
- OpenCode provider: add support for OpenCode Go

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>