webmcp-tool-annotations

WebMCP Tool Annotations

Before writing code

Fetch live docs:

1. Fetch https://webmachinelearning.github.io/webmcp/ for the annotations specification
2. Web-search webmcp tool annotations readOnlyHint destructiveHint idempotentHint for annotation field details
3. Web-search site:developer.chrome.com webmcp annotations permissions for how Chrome uses annotations
4. Web-search site:github.com mcp-b annotations for polyfill support for annotations

Conceptual Architecture

What Annotations Are

Tool annotations are optional metadata attached to a tool definition that inform the browser and agent about the tool's behavior and safety characteristics. The browser uses annotations to decide whether to prompt the user for confirmation before allowing agent invocation.

Available Annotations

Annotation	Type	Default	Meaning
readOnlyHint	boolean	false	Tool only reads data, does not modify any state
destructiveHint	boolean	false	Tool performs irreversible or significant actions
idempotentHint	boolean	false	Multiple calls with the same input produce the same effect as one call

How the Browser Uses Annotations

• readOnlyHint: true — Browser may allow agent to invoke without user confirmation
• destructiveHint: true — Browser should require explicit user consent before invocation
• idempotentHint: true — Browser may allow retries without additional confirmation
• No annotations — Browser applies its default permission policy

Annotation Combinations

readOnly	destructive	idempotent	Example	Browser Behavior
true	false	true	searchProducts	Likely auto-approved
false	false	true	addToCart	May auto-approve or prompt once
false	false	false	updateProfile	Likely prompts user
false	true	false	placeOrder	Always prompts user
false	true	true	cancelSubscription	Always prompts user (destructive overrides)

Commerce Tool Annotation Guide

Tool	readOnly	destructive	idempotent
searchProducts	true	false	true
viewProductDetails	true	false	true
getCartContents	true	false	true
addToCart	false	false	true
removeFromCart	false	false	true
updateCartQuantity	false	false	true
applyCoupon	false	false	true
checkout	false	true	false
placeOrder	false	true	false
initiateReturn	false	true	false
cancelOrder	false	true	false
deleteAccount	false	true	false
updateShippingAddress	false	false	true
getOrderHistory	true	false	true

Implementation

navigator.modelContext.registerTool({
  name: "searchProducts",
  description: "Search the product catalog",
  inputSchema: { /* ... */ },
  annotations: {
    readOnlyHint: true,
    idempotentHint: true
  },
  async execute(input) { /* ... */ }
});

navigator.modelContext.registerTool({
  name: "placeOrder",
  description: "Complete the purchase and charge the payment method",
  inputSchema: { /* ... */ },
  annotations: {
    destructiveHint: true
  },
  async execute(input, client) { /* ... with requestUserInteraction */ }
});

Annotation Accuracy Matters

Incorrect annotations are a security risk:

• Marking a purchase tool as readOnlyHint: true could let agents bypass user confirmation
• Marking a search tool as destructiveHint: true creates unnecessary friction
• Annotations are hints, not guarantees — the tool must still implement proper checks

Best Practices

• Annotate every tool — even if using defaults, explicit annotations make intent clear
• Combine destructiveHint with requestUserInteraction() for defense-in-depth
• Review annotations in code review as carefully as you review access control
• Test annotation behavior with real agents to verify browser permission prompts appear when expected
• Document the annotation rationale for each tool

Fetch the specification for any new annotation types, exact field names, and browser interpretation rules before implementing.