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

Annotations already declare non-readOnly, non-destructive, closed-world behavior, so the bar is lower; the description still adds real value by stating the 4 MiB decoded-byte ceiling and that content is 'kept without parsing or executing it' — a security-relevant guarantee not in the schema. It does not mention permission requirements or what happens on idempotency-key reuse (that detail lives only in the schema).

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 dense, front-loaded sentences with the size ceiling and encoding stated up front. Slight redundancy between 'opaque file' and 'without parsing or executing it', but both clauses carry usable information.

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 an output schema present, return values need no explanation, and the description covers the crucial operational facts (size cap, encoding, idempotency, opaque storage). Remaining gaps — sha256 semantics and the private-Space access precondition — are minor but real for a 6-parameter mutation tool.

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 only 33%, but the description compensates for the most important gaps by specifying canonical base64 encoding and the 4 MiB decoded limit for data_base64, plus the stable-key nature of idempotency_key. It says nothing about sha256 (undocumented in the schema) or media_type defaults, so coverage remains partial.

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 ('Store an opaque file') plus the scope ('in a private Space') and a hard size limit, so the agent knows exactly what the call does. It does not explicitly distinguish itself from siblings like space_file_copy or space_file_get, leaving that inference to the agent.

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 is implied rather than stated: the phrase 'using canonical base64 and a stable idempotency key' hints at the required write pattern, but there is no explicit when-to-use/when-not or comparison to alternatives such as space_file_copy. The agent must infer routing from tool name alone.

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