A harness for OpenCode that admits when it costs more

Phạm Cảnh — Mon, 09 Nov 2026 07:16:14 +0000

A coding harness is model-specific even when the tool is not. Context reserve, effort levels, temperature and privacy terms change with the provider. Re-answer those in a prompt and the model reads them once. Hard-code them and you fork the project.

opencode-harness-kit takes a different route: the model is data.

The problem

OpenCode lets you point the CLI at almost any provider. The hard part starts after that. A free route, a paid route and a local route each have different context limits, different reasoning controls and different terms about what happens to prompts. If you switch routes often, you end up maintaining the same model-specific answers in several places.

The design

oc-harness use <provider>/<model> reads the live models.dev record and writes a profile to ~/.config/opencode/profiles/<id>.json. The profile is compiled into opencode.jsonc as ten lanes:

plan
build
reviewer
locate
verify
vision
doc
schema
sweep
longform

Each lane gets its own model, output cap and fallback chain. The plugin adds a pruner for old tool output, a guard and per-lane telemetry. oc-harness verify checks that the config, prompts, plugin and skills still match the profile.

There is no model name in src/. The kit ships no model list and no default model, because which model you can reach is a fact about your account, not about the package.

The result that matters

The README publishes the A/B where the kit loses. Same task, same model, ten reps per arm:

Task	Arm	Uncached input	Wall time	Correct
One read	Without kit	10,540	21s	55/60
One read	With kit	56,205	43s	54/60
Twelve files	With kit vs without	-69%	-60%	no regression

On a task that fits in one read, the harness is a cost: more input tokens and more wall time for the same correctness. On a long task, the pruner earns its place. Both numbers are in the repo, not just the good one.

Install

npx -y opencode-harness-kit@latest init

The initializer asks once for a provider/model, writes the profile, and prints the route it selected. You can re-run it when a free model disappears or a provider changes its limits.

What it does not do

No mid-session failover. A plugin hook cannot change the model of a running session.
No hosted service.
No model list and no default model.
No telemetry upload.

Node 22. OpenCode V1 and V2. MIT.

Repo: https://github.com/pham-canh/opencode-harness-kit

I'm the author of opencode-harness-kit. The benchmark table and the losing arm are in the repo, so you can check the numbers rather than trust the summary.