webmcp_invoke_tool

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full burden of behavioral disclosure. It explicitly states side effects may modify page state, trigger network requests, or perform other actions, and mentions prerequisites. It also describes the return as 'the output of the tool invocation'. This is honest and useful, though it remains generic because the actual behavior depends on the specific tool invoked. A 4 reflects thorough disclosure for a generic invoker.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is concise and well-structured, with clear sections for purpose, side effects, prerequisites, returns, and usage. Each sentence adds value, and the information is front-loaded with the core purpose. No redundancy or fluff.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a generic tool invoker, the description is complete: it covers what the tool does, side effects, prerequisites, return value, and usage guidance. It points to where to get valid frameId and toolName via the schema (referencing webmcp_list_tools), and the absence of an output schema is acceptable because the output varies per invoked tool. Nothing essential is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline is 3. The description does not add extra meaning beyond what the schema provides; all parameter details (input as JSON string, frameId must match a valid ID from webmcp_list_tools, toolName must match a registered name, instance_id optional) are already in the schema. No additional semantics are added, so the baseline 3 is maintained.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool invokes a WebMCP tool registered by the current webpage, with a specific verb ('invokes') and resource. It distinguishes itself from siblings like webmcp_list_tools (which lists) and webmcp_get_invocation (which retrieves a specific invocation) by focusing on direct invocation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides clear usage context: 'Use this to interact with page-provided tools' and lists prerequisites (WebMCP enabled, target frame exists, tool registered). It does not explicitly mention alternatives or when not to use, but the prerequisites implicitly guide the agent to check registrations first. Slightly stronger than a vague 'implied usage', so a 4 is appropriate.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.