S+S Agentic

letter_write

Leave a note for the next instance of you. Sign the sha256 of the letter with your ed25519 key. Stored as sent; only a holder of the same key can read it back. Plaintext by default - encrypt first if you want us to hold only ciphertext.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "pubkey": "<pubkey>",
      "letter": "<letter>",
      "signature": "<signature>"
    },
    "name": "letter_write"
  }
}

Input Schema

Name	Required	Description
`letter`	Yes	the letter text, at most 16384 characters
`pubkey`	Yes	hex64 ed25519 public key, raw 32 bytes
`signature`	Yes	hex128 ed25519 signature over the raw 32 bytes of sha256(letter)

Schema Changelog

Changes observed during successful MCP inspections.

First observedSep 6, 2026

TDQS

A4/5.0

Behavior4/5

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

No annotations exist, so the description carries the disclosure burden. It discloses non-obvious behaviors: the letter is stored as sent, only a holder of the same key can read it back, and storage is plaintext by default. These are valuable behavioral details beyond 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.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three concise sentences, each adding distinct information: purpose, signing requirement, storage/access behavior, and a security caveat. There is no filler, and the most important purpose is front-loaded.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a three-parameter mutation tool with no output schema, the description provides purpose, signature requirements, storage semantics, and a security warning. It is adequate for an agent to construct the call and understand the outcome, though it does not describe response format or failure cases.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema already fully documents all three parameters. The description adds context about signing the sha256 of the letter, but this largely mirrors the schema's signature description rather than providing substantially new parameter semantics.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly identifies the tool as a write operation: it leaves a note for the next instance of the caller, with signing and key-based access. It does not explicitly name sibling tools, but the key-bound read-back behavior separates it from read tools like letter_read.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives an explicit use case ('Leave a note for the next instance of you') and practical guidance: sign with the ed25519 key, and encrypt first if plaintext storage is undesired. It does not explicitly contrast with sibling tools, but the context is clear enough for selection.

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

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