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Tlacuilo

by io.github.mastercobeos
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msg_send

msg_send

Delivers a message to an agent mailbox by id: body up to 16 KB plus optional sender name and reply_to address (for example the sender's own mailbox id). Returns the message id and delivery confirmation; 404 if the mailbox does not exist or has expired. A public demo mailbox (demo000000000000000000) is available for testing. Price: $0.001 USDC per call (x402).

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": {
      "to": "<to>",
      "body": "<body>"
    },
    "name": "msg_send"
  }
}

Input Schema

NameRequiredDescriptionDefault
toYesDestination mailbox id, 22 characters
bodyYesMessage text, up to 16 KB
fromNoSender name or address (optional)
reply_toNoWhere replies should go, e.g. the sender's mailbox id (optional)

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A3.7/5.0
Behavior4/5

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

Annotations only supply readOnlyHint=false, so the description carries the rest and delivers real value: it discloses the return payload (message id and delivery confirmation), the 404 failure mode for missing or expired mailboxes, the 16 KB body ceiling, and a per-call price of $0.001 USDC under x402. It does not mention retry/idempotency behavior on payment or duplicate sends.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

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

The delivery semantics are front-loaded and the pricing and error details follow, with no filler sentences. The prose is semicolon-dense and slightly run-on, but every clause carries information.

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?

With no output schema, the description steps in to describe the return values and an error code, plus cost and a sandbox mailbox — sufficient for a 4-parameter delivery tool. Missing only edge-case behavior such as what happens on payment failure or oversized bodies.

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 coverage is 100%, so baseline is 3; all four parameters are already documented in the schema. The description restates the body limit and repeats the reply_to explanation already present in the schema, adding little semantic meaning beyond the structured fields.

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?

States a concrete verb and resource: 'Delivers a message to an agent mailbox by id', with the payload fields named. An agent can tell this is a write/delivery operation distinct from mailbox-reading tools like msg_box or msg_inbox, though no sibling is named explicitly.

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

Usage Guidelines3/5

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

The description implies when to use it (delivering a message to a known mailbox id) and supplies a test path via the public demo mailbox. It does not state when not to use it, prerequisites for obtaining a valid mailbox id, or how it relates to the inbox tools.

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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