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

The description adds substantial context beyond the annotations: it returns UNSIGNED transaction data for the owner's wallet, simulates first and refuses on revert, never signs or sends, the server never holds the key, and replacing a session restarts its mint budget. These are exactly the operational facts an agent needs and that readOnlyHint/idempotentHint alone cannot convey.

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?

Four short sentences, front-loaded with the core action, then the caveats (budget reset, key custody, unsigned/simulated output, no signing). No filler and nothing repeated from the title or annotations.

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?

There is no output schema, but the description explains the return well ('unsigned transaction data for the owner's own wallet; simulated first and refused if it would revert'). For a prepare-style, five-parameter tool whose schema is fully documented, this covers everything needed to call it correctly.

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 account, markets, sessionKey, validForSeconds, and the optional actions list are already documented in the schema. The description only echoes 'listed markets' and 'until it expires' without adding format, defaults, or interaction rules beyond what the schema states, so the baseline of 3 applies.

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 first sentence names the exact verb and resource ('let a session key act for the account') plus scope ('inside its limits, on the listed markets until it expires'). That is clearly distinguishable from sibling grant-lifecycle tools such as prepare_mandate_set_limits (caps) and prepare_mandate_revoke_session (teardown).

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?

Usage context is implied well: you call this to authorize a session key, and 'inside its limits' signals that limits must already exist, which the schema makes explicit by pointing at prepare_mandate_set_limits. The description doesn't state exclusions or name an alternative path (e.g., when to prefer revoking and re-granting), so it stops short of a 5.

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