Context

Context-window breakdown for opencode. /context opens a dialog showing where the window went — system prompt, tool definitions, thinking, tool inputs and outputs (split per tool), user and assistant messages — with a stacked bar and a measured used / limit header, in the style of delta.app's Context Window. A context_breakdown tool returns the same numbers as markdown so a model can ask for them itself.

Install

Add it to the plugins array in ~/.config/opencode/opencode.json and restart (or /reload) opencode:

{
  "plugins": ["@jeffreyjyz/opencode-context"]
}

For local development point at the checkout instead — an absolute path is treated as a local plugin:

{
  "plugins": ["/Users/you/dev/cmdcode-tools/opencode-context"]
}

A symlink under ~/.config/opencode/plugins/ is also picked up. After bun run build in the checkout, the root index.js / tui.js shims load dist/.

Use

• /context — opens the breakdown for the current session.
• context_breakdown — the model calls it and gets the breakdown as markdown.

How it works

Two halves of one package, because they run in different processes:

half	file	job
server	src/index.ts	measures the assembled system prompt and tool definitions on every request (their sizes only) and registers the tool
TUI	src/tui.tsx	the /context slash command and the dialog

The message store holds no system prompt or tool schemas, so the server half records their byte sizes in $XDG_CACHE_HOME/opencode-context/<session>.json (one small JSON per session; no prompt text) and the dialog reads them. The rest of the breakdown comes straight from opencode's live data — the replayable context (session.context, post-compaction), the model's context limit, and the agent instructions as a fallback.

Units: each row shows UTF-8 bytes (delta's size column) and the header shows the provider's measured tokens over the model limit. Per-row percentages are share of the byte total.

Development

bun install
bun test
bun run typecheck
bun run build

Notes

• The System Prompt and Tool Definitions rows need one request to have happened in the session (that is when they are measured); before that the System Prompt row falls back to the agent instructions and Tool Definitions is hidden.
• Only sizes are cached, never prompt or tool text.
• The install cache for a published version can lag npm; see the ecosystem notes in AGENTS.md before publishing.