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

With no annotations, the description fully discloses behavior: encryption and decryption are the same XOR operation, there is no padding, ciphertext is the same length as plaintext, input range is 1B–16MB, and CTR provides no authentication. It also specifies the key-length requirements and the IV semantics, so the agent understands side effects and security limitations.

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?

The description is dense but every sentence contributes: mode semantics, key constraints, security caveat, and return fields. The only minor blemish is the duplicated leading tag '[symmetric_cipher] [symmetric_cipher]', which adds no value.

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?

Despite lacking an output schema, it lists the expected return fields (output_data_in_hex, output_length, output_sha256, algorithm) and covers all operational constraints needed to call the tool correctly. The security caveat and alternative-tool routing make the context complete for an agent.

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?

The schema already covers 100% of parameters, so the baseline is 3. The description adds meaningful beyond-schema detail by specifying exact key byte lengths, implementation origins (cryptography vs GmSSL, same source as eet sm4-ctr), and clarifying that iv_in_hex is the 16-byte initial counter block.

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 immediately states it performs CTR-mode encryption/decryption for AES-256-CTR and SM4-CTR, making the operation and scope explicit. It also distinguishes itself from stream_cipher's AEAD modes by naming them as the alternative when tamper resistance is required.

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 explicit guidance: use stream_cipher's AEAD modes when tamper resistance is needed, and use CTR only when there is already an independent authentication layer or an interoperability scenario. This clearly tells an agent when to select this tool versus its siblings.

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