opencode completion fish emits bash/zsh script, not fish syntax

Description

opencode completion fish does not emit fish syntax. It prints the yargs bash template, or the zsh template when $SHELL contains "zsh". Neither is valid fish, so sourcing the output fails immediately:

$ opencode completion fish | source
(line 12): Unsupported use of '='. In fish, please use 'set cur_word "${COMP_WORDS[COMP_CWORD]}"'.

With $SHELL pointing at zsh, the failure is the zsh variant:

(line 14): Unsupported use of '='. In fish, please use 'set reply ($(COMP_CWORD="$((CURRENT-1))" ... opencode --get-yargs-completions ...))'.

The shell positional argument is ignored when picking the template. opencode completion zsh also prints the bash template unless $SHELL happens to contain "zsh".

This is easy to hit by accident. fish ships a built-in completion for oc (the OpenShift CLI name) that runs oc completion fish | source. Alias oc to opencode and every oc at the prompt triggers the parse error.

Root cause is upstream yargs: this repo pins yargs@18.0.0, which only has bash and zsh templates in lib/completion-templates.ts. generateCompletionScript() has no fish branch. yargs main has since added completionFishTemplate, but no release contains it yet. Earlier fish completion PRs (#23263, #26193, #30168, #34197) were all closed without merging, and #26193 never got a maintainer review.

Expected output, matching the yargs main template:

complete -f -c opencode -a '(opencode --get-yargs-completions (commandline -o)[2..-1])'

Plugins

N/A

OpenCode version

1.18.15

Steps to reproduce

1. Run opencode completion fish and look at the output. It is a bash script (COMP_WORDS, mapfile, compgen), not fish.
2. In fish, run opencode completion fish | source.
3. fish reports "Unsupported use of '='".

Screenshot and/or share link

N/A, if default shell is zsh, and use fish, you could reproduce it stably.

Operating System

macOS 26 (arm64)

Terminal

Ghostty with fish 4.8.1