opencode-slim-system

Replaces OpenCode's bundled system prompt and built-in tool descriptions with compact versions to reduce per-request token overhead.

Saves roughly ~3,160 tokens/request from tool descriptions and ~1,890 tokens/request from the system prompt — ~5,050 total (estimated at ~4:1 chars/token ratio, varies by tokenizer). Exact savings depend on your OpenCode version and model.

How It Works

Two plugin hooks plus a TUI sidebar:

tool.definition

Replaces each built-in tool's description with a slim version from ~/.config/opencode/slim-system/tool/{id}.txt. Covers 15 OpenCode built-in tools. Plugin and MCP tools are left untouched.

experimental.chat.system.transform

Replaces the stock system prompt with ~/.config/opencode/slim-system/prompt/default.txt. Detects stock prompts by marker strings (e.g. "best coding agent on the planet"). The environment block (model info, directory, date) is preserved. Custom agent prompts without markers are not affected.

Why hooks and not file overrides: OpenCode's built-in prompts and tool descriptions are compiled into the binary. There is no filesystem override. The plugin hook is the only way to swap them.

What's Included

File	Purpose
tool/*.txt	15 slim tool descriptions
prompt/default.txt	Slim system prompt (~240 tokens)
src/index.ts	Server plugin — hooks into tool.definition and experimental.chat.system.transform
tui/index.tsx	TUI sidebar — shows slim count, version, update indicator
src/defaults.ts	Embedded fallback defaults (auto-generated)
tests/core.test.ts	34 tests covering parsing, resolution, I/O, integrity

Sidebar Panel

Registered at sidebar order 899. Shows:

• Tools slimmed — count of covered built-in tools (15 on stock OpenCode)
• Token savings — estimated per-request (approximate, labeled ~)
• ⬆ Update available — when npm has a newer version
• plugin not loaded — shown briefly before the server plugin writes its status file (normal)
• Update dialog — on first TUI start per version, shows current vs latest + GitHub releases link

Status File

The server plugin writes /tmp/opencode-slim-system.json at startup. The TUI sidebar polls it every 5 seconds. A background npm check updates latest_version and update_available fields within 1-2 seconds.

Per-model customization

The plugin reads the current model from opencode.jsonc and extracts the model key (the short name after the last /):

Full model ID	Model key
opencode/deepseek-v4-flash-free	deepseek-v4-flash-free
opencode/claude-sonnet-4	claude-sonnet-4

Per-model tool descriptions: Create {toolID}.{modelKey}.txt files. bash.claude-sonnet-4.txt activates when opencode/claude-sonnet-4 is active. Falls back to bash.txt for all other models.

Per-model system prompts: Create prompt/{modelKey}.txt. Takes precedence over prompt/default.txt when running that model.

Configuration

{
  "plugin": [
    ["opencode-slim-system", {
      "exclude": ["websearch"],
      "toolsDir": "/home/user/.config/opencode/slim-tools/"
    }]
  ]
}

Options

Key	Type	Description
reset	boolean	Wipes config directory and reseeds from embedded defaults on next restart
exclude	string[]	Tool IDs to keep at original stock descriptions
tools	Record<string, string>	Inline description overrides for any tool ID
toolsDir	string	Custom path for {id}.txt files (default: ~/.config/opencode/slim-system/tool/)

Priority chain: options.tools[toolID] → toolsDir/{toolID}.{modelKey}.txt → toolsDir/{toolID}.txt → config dir → embedded defaults → stock.

Customization

Edit any .txt file in ~/.config/opencode/slim-system/tool/ or prompt/default.txt and restart OpenCode. Changes persist across npm updates. Placeholders (${os}, ${shell}, ${chaining}, etc.) are resolved at runtime.

To reset a file to default, delete it and restart — the plugin re-seeds from its embedded defaults.

Drift Detection

When OpenCode adds new built-in tools or changes tool IDs, the shipped tool/*.txt files may fall out of sync.

# From the repo clone
./slim-plugin-check          # check coverage
./slim-plugin-check --diff   # show add/remove commands

# Via npm
npx opencode-slim-check

A CI workflow (.github/workflows/drift-check.yml) runs weekly and opens an issue if drift is detected.

CLI Commands

Command	Description
opencode-slim-export	Export config dir as JSON. npx opencode-slim-export > backup.json
opencode-slim-import <file>	Import JSON blob into config dir. npx opencode-slim-import backup.json
opencode-slim-dump	Dump embedded defaults as JSON (ignores user edits).
opencode-slim-check	Drift check (same as ./slim-plugin-check)

Limitations

• Token savings are approximate. The 4:1 chars-to-token ratio varies by tokenizer. The STOCK_TOOL_CHARS baseline is measured from OpenCode v1.17.9 template files; newer versions may have different stock description lengths.
• slimmed count shows config dir files, not runtime coverage. The sidebar shows 15 files in the config dir. Actual tools slimmed depends on your experimental flags (lsp, plan_exit are conditional; tools without those flags enabled will have stock descriptions but still count toward the 15). The real number may be 13-14 depending on your config.
• Drift detection is a maintainer tool. npx opencode-slim-check is for repo maintainers, not end users. The plugin works fine without it.
• System prompt replacement uses marker heuristics. Custom prompts (agents with .md files) are not touched.
• plan_exit, lsp, question are conditional tools. They only exist in the tool registry when the corresponding experimental flags are enabled. Our tool descriptions for them cover all cases, but they're not always active.

Files on Disk

Path	Purpose
~/.config/opencode/slim-system/tool/	User-editable slim tool descriptions (auto-seeded)
~/.config/opencode/slim-system/prompt/default.txt	User-editable slim system prompt (auto-seeded)
/tmp/opencode-slim-system.json	Runtime status (ephemeral)
~/.local/state/opencode-slim-system/announced.json	Last announced update version

Installation

npm install -g opencode-slim-system

In ~/.config/opencode/opencode.jsonc:

{
  "plugin": ["opencode-slim-system"]
}

In ~/.opencode/tui.json and/or ~/.config/opencode/tui.json:

{
  "plugin": ["opencode-slim-system"]
}

Restart OpenCode. On first TUI load you'll see a toast confirming the plugin loaded.

License

MIT