inspect_expr · Fairchem MCP: Risk & Policy

WHAT IT DOES

What inspect_expr does on Fairchem

AI agents invoke inspect_expr to trigger actions in Fairchem. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why inspect_expr needs a policy

This tool evaluates arbitrary expressions in a live session namespace, which constitutes code execution. While it ostensibly only returns a repr (read-like behavior), evaluating arbitrary expressions can trigger side effects, modify state, or execute malicious code depending on what expression is passed. The 'execute' sibling tool and the session namespace context increase the blast radius significantly.

From the tool's definition "Evaluate an expression in the session namespace and return its repr"

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

inspect_expr Rate-limited

inspect_expr stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "inspect_expr": {
      "limits": [
        {
          "counter": "inspect_expr_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Fairchem, so you can run agents against it safely.

FAQ

Questions about inspect_expr

What does the inspect_expr tool do? +

Evaluate an expression in the session namespace and return its repr. It is categorised as a Execute tool in the Fairchem MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on inspect_expr? +

Register the Fairchem MCP server in PolicyLayer and add a rule for inspect_expr: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Fairchem. Nothing to install.

What risk level is inspect_expr? +

inspect_expr is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit inspect_expr? +

Yes. Add a rate_limit block to the inspect_expr rule in your PolicyLayer 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.

How do I block inspect_expr completely? +

Set action: deny in the PolicyLayer policy for inspect_expr. 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.

What MCP server provides inspect_expr? +

inspect_expr is provided by the Fairchem MCP server (jkitchin/fairchem-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.