@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.

Modes

The default aggressive mode retains current scrubbing. Set MERIDIAN_OPENCODE_SCRUB_MODE=minimal for surgical removal without inserting a generic identity or replacing residual OpenCode/OhMyOpenCode words in preserved prose. Both modes remove known identity wrappers, feedback/docs fingerprints, powered-by lines and duplicate runtime environment blocks. Both retain the client working-directory field, project instructions and tool/persona policies, and normalize leftover blank lines. Minimal mode still removes the duplicate environment preamble; preserving it would defeat the metering fix.

MERIDIAN_OPENCODE_SCRUB_MODE=minimal meridian

Other or unset values select aggressive. Adapter and passthrough marker guards apply in both modes; genuine Claude Code prompts stay unchanged.

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

Manual live verification

Minimal/aggressive headless evidence and reproduction uses actual OpenCode, an independently installed tarball, Claude tool-result receipts and same-session continuation. This manual credentialed gate is separate from the ordinary unit/build CI and does not publish a package.