Fusion
A main agent that plans and reviews but never edits. A cheaper sidekick that does the editing.
Frontier-level quality at roughly 35–41% lower cost.
npx skills add mihneaptu/opencode-fusion --skill fusion-setup -g -a opencode -y
The main agent turns your task into a plan, delegates code exploration and editing to the sidekick, reviews the returned diff, requests fixes, and verifies the final code.
Strong models are expensive. Weak models cut corners.
Running a top-tier model for every edit and every token gets costly fast. Running a cheaper model on its own tends to drop quality on real, multi-step work.
opencode Fusion splits the roles. You pay the premium only where judgment matters, and let a cheaper model handle the high-volume editing.
How it works
The main agent plans and reviews but cannot edit files. The sidekick does all the editing. That asymmetry is the whole idea.
-
You
Describe the task
Hand the main agent a goal in plain language.
-
Main agent
Plan and spec
It explores, decides, and writes a precise spec. It directs the work but cannot edit files.
cannot edit -
Sidekick
Execute the edits
A cheaper agent applies the changes from that spec across the codebase.
edits files -
Main agent
Verify and iterate
It runs tests and lint, reviews the diff, and loops until the work is right.
reviews diff
Built for cost and quality
Every part of the setup earns its place. Nothing is there for decoration.
-
Cost discipline
The expensive model spends its tokens on judgment: plans, specs, and reviews. The cheaper model handles the high-volume editing, so you pay the premium only where it changes the outcome.
-
Cross-vendor review
When main and sidekick use different model families, each tends to catch the other’s blind spots. OpenCode’s multi-provider setup makes this easy — pick main and sidekick from different families.
-
Precise spec hand-off
Work crosses to the sidekick as an explicit spec: objective, files, interfaces, constraints, and how to verify. Not a vague goal.
-
Parallel sidekicks
Independent edits fan out to several sidekick runs at once, so unrelated work does not queue behind a single thread.
-
Read-only exploration
Searching and reading the codebase is delegated to cheap read-only agents, keeping the main agent's context lean.
-
Least-privilege permissions
Edits and freeform shell are denied for the main agent at the OpenCode permission layer. Safety is harness wiring first, prompts second.
Where the savings come from
Illustrative — not a measured benchmark
Cognition has reported the Fusion sidekick pattern reaching roughly 35–41% lower cost on their own evaluation. The bars above only illustrate that idea.
Get started
One command, one prompt, one restart. No clone required.
-
# 1 · install the skill — global, no clone
npx skills add mihneaptu/opencode-fusion --skill fusion-setup -g -a opencode -y -
# 2 · open OpenCode — skills are picked up on demand
opencode -
# 3 · inside OpenCode — or run /fusion-setup
set up fusion
fusion configured — restart, then main plans, sidekick edits