reject_write · Fhir Synthetic MCP: Risk & Policy

WHAT IT DOES

What reject_write does on Fhir Synthetic

AI agents use reject_write to create or update resources in Fhir Synthetic — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Fhir Synthetic environment.

WHY IT NEEDS A POLICY

Why reject_write needs a policy

Rejecting a staged write modifies the system by removing or marking a pending write as rejected—a Write operation. It is reversible (the user could re-propose), involves no financial impact, and does not delete underlying data.

From the tool's definition Tool name 'reject_write' and description 'Reject a staged write' indicate modification of system state: the tool cancels or removes a pending write operation that was previously proposed. This is a reversible state change.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

reject_write Rate-limited

reject_write stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "reject_write": {
      "limits": [
        {
          "counter": "reject_write_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about reject_write

What does the reject_write tool do? +

HUMAN-IN-THE-LOOP GATE. Reject a staged write. It is categorised as a Write tool in the Fhir Synthetic MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on reject_write? +

Register the Fhir Synthetic MCP server in PolicyLayer and add a rule for reject_write: 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 Fhir Synthetic. Nothing to install.

What risk level is reject_write? +

reject_write is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit reject_write? +

Yes. Add a rate_limit block to the reject_write 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 reject_write completely? +

Set action: deny in the PolicyLayer policy for reject_write. 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 reject_write? +

reject_write is provided by the Fhir Synthetic MCP server (krishnakakani-github/fhir-synthetic-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.