Context Usage

An OpenCode TUI plugin that adds a /context command: the exact breakdown behind the built-in Context panel, plus the session-wide usage and cost totals OpenCode tracks internally — including delegated subagent sessions.

Example output

The examples below use illustrative numbers.

Simple session

Context usage — Refactoring the auth middleware
provider/model · build

Context window (last request)
  Input (no cache)                       83
  Output                              1,012
  Reasoning                             342
  Cache read (hits)                 118,528
  Total                   119,965 / 1M (12%)

Session total — 29 turns · 4 prompts
  Input (no cache)                  156,297
  Output                             31,389
  Reasoning                          28,405
  Cache read (hits)               1,808,128
  Total                           2,024,219
  Cache hit rate                     92.0%
  Cost                           $0.043687

Session with delegated subagents

When the agent delegates work with task, each subagent runs in its own child session. The plugin walks that tree and adds two extra blocks:

Context usage — Fixing the checkout flow
provider/model · build

Context window (last request)
  Input (no cache)                       83
  Output                              1,012
  Reasoning                             342
  Cache read (hits)                 118,528
  Total                   119,965 / 1M (12%)

Session total — 29 turns · 4 prompts
  Input (no cache)                  156,297
  Output                             31,389
  Reasoning                          28,405
  Cache read (hits)               1,808,128
  Total                           2,024,219
  Cache hit rate                     92.0%
  Cost                           $0.043687

Subagents — 1 session
  Input (no cache)                   41,002
  Output                             18,544
  Reasoning                          12,301
  Cache read (hits)               1,092,999
  Total                           1,164,846
  Cache hit rate                     96.4%
  Cost                           $0.034800

Combined total — session + subagents
  Total                           3,189,065
  Cost                           $0.078487

Why

OpenCode's sidebar Context panel mixes two different scopes: tokens and % used are a snapshot of the last request, while spent is the session total. There is no built-in way to see the accumulated traffic of a session either.

This plugin makes both scopes explicit and shows the arithmetic behind each number, so you can answer questions like:

• How full is my context window right now?
• How many tokens have actually passed through the API in this session?
• How much of the prompt is served from cache, and how much of the cost is cache vs fresh tokens?
• How much did the subagents I delegated work to consume? (OpenCode tracks each subagent — task calls — in its own child session, excluded from the parent's totals. The plugin walks the delegation tree and shows the combined total.)

It reads the already-synced TUI state (context.data.session), so it is instant, never calls an LLM and has zero runtime dependencies.

Install

Add it to your global OpenCode 2 CLI config (~/.config/opencode/cli.json):

{
  "$schema": "https://opencode.ai/v2/cli.json",
  "plugins": ["opencode-context-usage"]
}

Or let the CLI do it:

opencode plugin opencode-context-usage

CLI plugin configuration is global (there is no project-local cli.json), and valid edits reload while the TUI is running.

Usage

Open a session and run:

/context

Alias: /ctx. Close the dialog with Esc or Ctrl+C.

Command name

The command registers as /context, with /ctx as an alias. You can change the name explicitly:

{
  "$schema": "https://opencode.ai/v2/cli.json",
  "plugins": [{ "package": "opencode-context-usage", "options": { "slash": "usage" } }]
}

How the numbers are computed

OpenCode normalizes provider usage in Session.getUsage: input excludes cache tokens and output excludes reasoning tokens.

Metric	Meaning
Input (no cache)	Prompt tokens not served from cache
Output	Generated tokens excluding reasoning (includes tool calls)
Reasoning	Reasoning / thinking tokens
Cache read (hits)	Prompt tokens served from cache
Cache write	Prompt tokens written to cache (row hidden when 0)
Total (window)	input + output + reasoning + cache read + cache write of the last completed request — the same number the built-in panel shows
Cache hit rate	cache read / (input + cache read + cache write)
Cost	Sum of the per-turn costs OpenCode records for the session (session.cost)
Subagents	Aggregated usage of every child session delegated from this one (recursively)
Combined total	Session total + subagents total (tokens and cost)

Session totals come from the authoritative session aggregate that OpenCode stores (the same source the built-in panel uses), not from re-summing the messages loaded in the TUI. On long sessions the TUI keeps only a window of recent messages, so the turn and prompt counts may show a + (for example 95+ turns) to indicate there are more messages than the loaded window. Subagent totals use the same session aggregates for each child session, so they are exact.

Compatibility

• OpenCode >= 2.0.0 (CLI plugins via cli.json, using @opencode/plugin/tui).
• Session and subagent totals come from the synced session state (context.data.session); the plugin never calls the server directly and never calls an LLM.
• The CLI plugin API is still evolving during the OpenCode 2 beta. If something breaks, please open an issue with your OpenCode version.

Development

npm install
npm run typecheck
npm test        # builds dist/ and runs the unit tests
npm run build

To try a local checkout in OpenCode 2, use the plugin discovery directory. Create <global-config>/plugins/context-usage/tui.ts (for example ~/.config/opencode/plugins/context-usage/tui.ts) that re-exports the built plugin:

export { default } from "/absolute/path/to/opencode-context-usage/dist/tui.js"

OpenCode discovers it automatically at startup — no cli.json entry is needed. Run opencode plugin list to confirm it is loaded. (Listing the package path directly in cli.json plugins is also documented, but an npm-installable package name is the reliable form once published.)

License

MIT