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Anthropic tool protection has two separate stages:
  • Tool exposure: remove blocked tools before Anthropic sees the request.
  • Tool-call check: evaluate returned tool_use blocks before your app executes local code.
Keep those separate. Exposure controls what the model can plan with. Tool-call checks control what your app is allowed to execute.

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Supported Tools

The current wrapper supports user-defined client tools with a stable name. The tool type may be omitted, custom, or function. 
const tools = [
  {
    name: "search_docs",
    description: "Search internal support documentation.",
    input_schema: {
      type: "object",
      properties: {
        query: { type: "string" },
      },
      required: ["query"],
      additionalProperties: false,
    },
  },
  {
    name: "send_email",
    description: "Send an email to a customer.",
    input_schema: {
      type: "object",
      properties: {
        to: { type: "string" },
        subject: { type: "string" },
        body: { type: "string" },
      },
      required: ["to", "subject", "body"],
      additionalProperties: false,
    },
  },
];
If policy blocks send_email, the wrapper forwards only the remaining tools.

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Tool Choice

When tools are filtered, the wrapper also sanitizes tool_choice. If the request forces a specific tool with tool_choice: { type: "tool", name: "send_email" } and Averta blocks send_email, the wrapper removes that forced choice before forwarding the request. Otherwise the request would ask Anthropic to call a tool it can no longer see.

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Tool-call Checks

After Anthropic responds, the wrapper reads each returned tool_use block: 
const toolUses = response.content.filter((block) => block.type === "tool_use");
For each block, Averta checks:
  • the tool call id from tool_use.id
  • the tool name from tool_use.name
  • the tool arguments from tool_use.input
  • bounded conversation context from the current Messages request
  • the request run_id created by the request checkpoint
If a tool-call decision is block, the wrapper throws AvertaSdkError and does not return that response to your application. Your app should not execute the tool.

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Decision Callback

client = wrapAnthropic(client, {
  onDecision(event) {
    if (event.checkpointType === "request") {
      console.log("blocked tools:", event.decision.blockedTools ?? []);
      console.log("forwarded tools:", event.forwardedTools.length);
    }

    if (event.checkpointType === "tool_call") {
      console.log(event.tool.name, event.decision.decision);
    }
  },
});

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Unsupported Tool Shapes

Unsupported Anthropic tool shapes fail closed before provider execution. This is deliberate. A guardrail wrapper that silently forwards unsupported tools would create false confidence.

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Debugging

SymptomCheck
A tool disappearedInspect request decision blockedTools.
Forced tool choice was ignoredThe forced tool may have been blocked and tool_choice sanitized.
messages.create(...) throws before Anthropic is calledCheck for unsupported tool shapes or streaming input.
Tool execution never startsThe returned tool_use may have been blocked by the tool-call checkpoint.