@rynfar/meridian-plugin-opencode-scrub

A Meridian plugin that strips OpenCode's identifying fingerprints from the system prompt before it reaches Claude.

Why

Vanilla OpenCode's system prompt (loaded from packages/opencode/src/session/prompt/anthropic.txt when the model id matches claude) opens with "You are OpenCode, the best coding agent on the planet." and carries a feedback block pointing at github.com/anomalyco/opencode, a docs paragraph referencing opencode.ai/docs, and a "Professional objectivity" sentence that names OpenCode directly.

OpenCode's runtime environment() builder also prepends a line like "You are powered by the model named claude-opus-4-6. The exact model ID is anthropic/claude-opus-4-6" — a phrasing Claude Code never emits, which makes it a strong third-party tell.

When Meridian routes OpenCode → Claude Max, those identity sections are:

1. Redundant — Claude Code's preset already owns identity, tone, safety, and tool-use guidance.
2. A detection fingerprint — they give Anthropic's detection a clear "this isn't Claude Code" signal that can trigger third-party extra-usage errors or rate-limit flags.

This plugin surgically removes those lines and replaces the identity paragraph with a neutral, generic coding-assistant framing. It removes the duplicate environment preamble and redundant fields while keeping a bare <env> block with the client Working directory line. Tone rules, task management, tool usage policy, code references, and user CLAUDE.md content remain intact.

The scrub also handles OhMyOpenCode-style custom personas (Sisyphus et al.): the You are "Sisyphus" ... from OhMyOpenCode. identity line and the <omo-env>...</omo-env> block are stripped, while the persona's orchestration rules (Phase 0 intent gate, explore/librarian delegation, Oracle consultation, tone guidelines) are preserved.

The scrub is idempotent — running it twice on the same string is a no-op.

Install

Option 1: npm (recommended)

mkdir -p ~/.config/meridian && cd ~/.config/meridian
npm init -y >/dev/null
npm install @rynfar/meridian-plugin-opencode-scrub

Then point Meridian's plugin config at the installed file:

cat > ~/.config/meridian/plugins.json <<'JSON'
{
  "plugins": [
    {
      "path": "/Users/YOU/.config/meridian/node_modules/@rynfar/meridian-plugin-opencode-scrub/dist/index.js",
      "enabled": true
    }
  ]
}
JSON

Restart Meridian (or curl -X POST http://localhost:3456/plugins/reload).

Option 2: Local clone (for development)

git clone https://github.com/rynfar/meridian-plugin-opencode-scrub.git ~/repos/meridian-plugin-opencode-scrub
cd ~/repos/meridian-plugin-opencode-scrub
npm install
npm run build

Point ~/.config/meridian/plugins.json at dist/index.js in the clone.

Verify at http://localhost:3456/plugins — you should see opencode-scrub listed as active.

Behavior

Input	Output
No system prompt	unchanged
System prompt without OpenCode identity markers	unchanged (idempotent)
Vanilla OpenCode (anthropic.txt)	identity line swapped for generic, feedback block removed, docs paragraph removed, "OpenCode honestly applies" neutralized
OhMyOpenCode/Sisyphus prompt	OMO identity line removed, <omo-env> block removed, "You are powered by..." line removed; persona rules preserved
OpenCode prompt + user CLAUDE.md additions	identity stripped, all user content preserved
OpenCode environment preamble + <env>	duplicate preamble and redundant fields removed; bare Working directory retained for Meridian's cwd extraction

The plugin runs for the opencode adapter and for passthrough requests that still carry an OpenCode-specific identity or runtime marker. This covers OpenCode routed through LiteLLM when its client headers are removed. Other passthrough prompts, including genuine Claude Code prompts with an <env> block, remain byte-for-byte unchanged.

Rules

The scrub applies 8 independent regex replacements, each idempotent and each a no-op when its pattern is absent:

1. Vanilla identity line — You are OpenCode, the best coding agent on the planet. → generic
2. Feedback block — If the user asks for help or wants to give feedback...github.com/anomalyco/opencode
3. Docs paragraph — When the user directly asks about OpenCode...opencode.ai/docs
4. Objectivity brand — It is best for the user if OpenCode honestly applies → ...the assistant honestly applies
5. OMO identity line — You are "Sisyphus" ... from OhMyOpenCode.
6. OMO env block — <omo-env>...</omo-env>
7. Powered-by line — You are powered by the model named ...
8. Residual brand tokens — bare OpenCode → the assistant

When scrubOpencodeFingerprints is called inside an OpenCode client hook, the bare working-directory line keeps the client's project path available to Meridian. Server-side use remains compatible: Meridian extracts the raw path before applying its plugin transform.

Development

npm install
npm run build

The built plugin is a single ES module at dist/index.js with dist/index.d.ts for types.

License

MIT