AGT OpenCode Plugin

This package is the production package surface for Agent Governance Toolkit on OpenCode.

It ships an OpenCode plugin that uses:

• OpenCode's in-process plugin hooks for deterministic session, prompt, tool, and output governance
• a bundled stdio MCP server (server/agt-mcp.mjs) for operator-facing AGT inspection tools
• the AGT TypeScript SDK for policy evaluation, prompt defense, and MCP threat scanning

Public Preview — APIs and policy schema may change.

What this package is

• a first-party OpenCode plugin package
• a parity layer for the existing Antigravity and Claude Code governance packages, adapted to OpenCode's richer in-process hook contract
• a publishable npm package (@microsoft/agent-governance-opencode) that can also be loaded locally from a workspace .opencode/plugins/ directory

What this package is not

• a Copilot-style extension
• a universal governance layer for every IDE surface
• a guarantee of full Copilot CLI feature parity

Why OpenCode benefits from in-process governance

Unlike Claude Code (subprocess hooks) and Antigravity (subprocess hooks), OpenCode loads plugins in-process as async TypeScript/JavaScript functions. That means this package can:

• enforce policy on tool.execute.before without an extra subprocess round trip
• redact secrets from tool.execute.after output before the model sees it (a parity win over Claude Code, which cannot rewrite tool output)
• expose custom tools like agt_policy_status directly to the model without needing a separate MCP server

The stdio MCP server is still shipped for operators who want to invoke governance tools from external workflows.

Current scope

This initial package enforces:

• session.start — injects AGT governance context into the session
• event (chat-style) — scans submitted prompts; throws to block
• tool.execute.before — allow / review / deny tool calls
• tool.execute.after — scans tool output and redacts known secret patterns (AWS, GitHub PAT, OpenAI, JWT, PEM private keys, Azure storage keys)
• tool.execute.error — records audit entry for failed tool calls

It also exposes two custom tools (in-process and via the stdio MCP server):

• agt_policy_status — return the active AGT policy snapshot
• agt_policy_check_text — inspect arbitrary text for prompt-injection and context-poisoning findings

Local development

Run these commands from the package directory:

cd agent-governance-opencode
npm install
npm run check

Loading the plugin in OpenCode

OpenCode loads plugins from:

1. opencode.json plugin entries (npm specifiers)
2. ~/.config/opencode/plugins/*.{ts,js,mjs} (user-global)
3. .opencode/plugins/*.{ts,js,mjs} (workspace-local)

Option A — workspace opencode.json

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["@microsoft/agent-governance-opencode"]
}

Option B — workspace plugin file (no install required)

Create .opencode/plugins/agt.mjs:

export { default } from "../../agent-governance-opencode/src/index.mjs";

Option C — install the bundled MCP server

In opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "agt-governance": {
      "type": "local",
      "command": [
        "node",
        "./node_modules/@microsoft/agent-governance-opencode/server/agt-mcp.mjs"
      ]
    }
  }
}

Configuration

The plugin loads policy from (in order):

1. AGT_OPENCODE_POLICY_PATH environment variable
2. ./.agt/policy.json in the working directory
3. ~/.config/opencode/agt/policy.json
4. The bundled config/default-policy.json (enforce mode, fail-closed)

Audit log path defaults to ~/.config/opencode/agt/audit.json and can be overridden via AGT_OPENCODE_AUDIT_PATH.

Important parity notes

• OpenCode's in-process plugin contract does not currently expose a server-side "ask the user" decision from inside tool.execute.before. When AGT decides review, this plugin marks the args with __agt_review_reason and lets OpenCode's normal permission flow run. Operators who want hard-deny behaviour on review should set toolPolicies.defaultEffect: "deny" in their policy.
• Output redaction is conservative: only well-known credential patterns are redacted. The audit entry records that a redaction occurred but never the redacted value.
• AGT fails closed by default. If the policy file is corrupt or evaluation throws, requests are denied. Set denyOnPolicyError: false in policy to opt into advisory mode.