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

Adds substantial behavior beyond the annotations: it discloses that this read does not mark the thread as seen, that attachments are never inlined (typed placeholders instead), that the window verdict is derived only from the first page, and that message content is untrusted data wrapped in a labeled envelope. These are safety- and correctness-relevant traits an agent cannot get from readOnlyHint/destructiveHint alone.

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?

Core purpose and scope are front-loaded in the first sentence, and each subsequent sentence adds a distinct trait (window check, untrusted-content envelope, no read receipt). It is dense but slightly overpacked with parentheticals that could be tightened.

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?

With no output schema, the description carries the burden of describing the return shape and does so explicitly — fields, attachment placeholder behavior, and the window flag. An agent has everything needed to call it and interpret the response 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 coverage is 100%, so the baseline is 3, but the description adds meaning the schema does not: it explains that `after` continuation pages can only confirm the messaging window open, not closed, which changes how an agent should interpret the parameter's effect on the result. Remaining parameters are left entirely to the schema.

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 and resource — 'Read one Messenger thread newest-message-first' — and enumerates what comes back (sender, timestamp, direction, body, attachment placeholders). It is clearly distinguishable from the sibling facebook_list_conversations, which enumerates threads rather than reading one.

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?

Gives a concrete precondition use case: call this before facebook_send_message to learn whether the 24-hour window is open, and explains the cursor caveat for continuation pages. It does not state explicit when-not-to-use conditions beyond that, so it falls short of a full 5.

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