fetch_url

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

With no annotations, the description carries the full burden and does well: it discloses SSRF protections (localhost/loopback/link-local/private IP blocking), the MCP_ALLOW_PRIVATE_NETWORK override, the response-size cap via MCP_MAX_RESPONSE_SIZE, and the timeout default source. It does not describe error/failure behavior or redirect handling, which keeps it from a 5.

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?

Front-loaded purpose sentence followed by two constraint sentences and a clean Args block. Every sentence earns its place; the env-var references are slightly boilerplate but informative.

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 2-param tool with no output schema and no annotations, the description covers the return shape, safety constraints, and both parameters. It is close to complete, only missing failure/redirect semantics and any response-format detail.

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 0%, so the description must compensate and largely does: it defines url as an http:// or https:// URL and timeout as an optional per-request timeout in seconds defaulting to MCP_HTTP_TIMEOUT, adding real meaning beyond the bare schema (whose default is only null).

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?

States a specific verb (fetch) and resource (public HTTP/HTTPS URL) and enumerates the return payload (status, headers, body). An agent can immediately distinguish this from siblings like validate_url, parse_url, check_connectivity, and resolve_dns.

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 word 'public' and the SSRF note imply this is for externally-reachable URLs only, and the SSRF/private-network exclusion gives partial when-not guidance. However, it never names an alternative sibling (e.g., validate_url to check a URL without fetching) or states explicit when-to-use conditions.

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