Execute arbitrary JavaScript with access to scene, renderer, camera. Returns the result. Runtime only (lost on reload). If the code mutates scene state, ASK the user first: runtime preview or persistent code change? SHADER PATCHING RECIPE — when modifying materials via onBeforeCompile: 1. Reset ...
Accepts freeform code/query input (code)
Part of the Threejs Devtools MCP server. Enforce policies on this tool with Intercept, the open-source MCP proxy.
AI agents invoke run_js to trigger processes or run actions in Threejs Devtools. Execute operations can have side effects beyond the immediate call -- triggering builds, sending notifications, or starting workflows. Rate limits and argument validation are essential to prevent runaway execution.
run_js can trigger processes with real-world consequences. An uncontrolled agent might start dozens of builds, send mass notifications, or kick off expensive compute jobs. Intercept enforces rate limits and validates arguments to keep execution within safe bounds.
Execute tools trigger processes. Rate-limit and validate arguments to prevent unintended side effects.
tools:
run_js:
rules:
- action: allow
rate_limit:
max: 10
window: 60
validate:
required_args: trueSee the full Threejs Devtools policy for all 60 tools.
Agents calling execute-class tools like run_js have been implicated in these attack patterns. Read the full case and prevention policy for each:
Other tools in the Execute risk category across the catalogue. The same policy patterns (rate-limit, validate) apply to each.
run_js is one of the high-risk operations in Threejs Devtools. For the full severity-focused view — only the high-risk tools with their recommended policies — see the breakdown for this server, or browse all high-risk tools across every MCP server.
Execute arbitrary JavaScript with access to scene, renderer, camera. Returns the result. Runtime only (lost on reload). If the code mutates scene state, ASK the user first: runtime preview or persistent code change? SHADER PATCHING RECIPE — when modifying materials via onBeforeCompile: 1. Reset patch flags: mat._curvedWorldPatched = false; mat._windPatched = false; 2. Delete old shader refs: delete mat.userData.shakeShader; stop old animations via mesh.userData._stopShake?.(); 3. ONE onBeforeCompile pass (never chain) — include curved world + your effect together 4. Replace '#include <project_vertex>' with full transform: USE_BATCHING/USE_INSTANCING guards → effect in world space → curved world (worldPos.y -= dist²*0.005) → viewMatrix*worldPos → gl_Position 5. Declare uniforms before main(): shader.vertexShader.replace('void main() {', 'uniform float uTime;\nvoid main() {') 6. Set mat.customProgramCacheKey = () => 'effect-' + Date.now(); — forces recompilation 7. mat.needsUpdate = true; 8. Animate uniforms via requestAnimationFrame loop 9. Use transformed.y for height-based effects (local vertex Y), worldPos for spatial phase Debug only — page reload will reset. No code changes needed.. It is categorised as a Execute tool in the Threejs Devtools MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Add a rule in your Intercept YAML policy under the tools section for run_js. You can allow, deny, rate-limit, or validate arguments. Then run Intercept as a proxy in front of the Threejs Devtools MCP server.
run_js is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the run_js rule in your Intercept policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the Intercept policy for run_js. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
run_js is provided by the Threejs Devtools MCP server (threejs-devtools-mcp). Intercept sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Open source. One binary. Zero dependencies.
npx -y @policylayer/intercept