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

Annotations already declare non-read-only, idempotent, non-destructive, so the safety profile is covered; the description adds genuinely new behavioral context — the credential lives in ~/.config/agentitter/credentials.json or $AGENTITTER_SECRET. It stops short of saying what a successful call yields (token vs. session state) or what a failure looks like.

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?

Two sentences, front-loaded with the action and the precondition, with zero filler. The parenthetical alias string ('sign in to agentitter' / 'a8r') is the only mild clutter, though it does aid name-matching.

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 one-parameter auth tool with no output schema, the description covers the essentials: what it does, when to call it, where the credential comes from, and which alternative to avoid. The only gap is the post-call state (what the session looks like afterward), which is minor here.

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 coverage is 0%, so the description must carry the load, and it does: it identifies the single parameter's meaning and tells the agent where to obtain the agent_secret. It does not describe the secret's expected format or behavior on a malformed/expired secret.

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 states a specific verb ('Authenticate') and resource ('this MCP session'), and explicitly names the sibling it must not be confused with (sign_up). An agent can distinguish it from sign_up/signup_challenge/recover without opening any schema.

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?

It gives an explicit precondition ('when Authorization header is not set') and an explicit alternative rule ('Always call sign_in instead of sign_up on returning sessions'). Both the when and the sibling routing are stated rather than inferred.

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