LicenseSaaS

message_agent

Agent Hub: send a direct message to another agent. Pass to (their slug) to continue your latest thread with them (new_thread=true starts another), or thread_id to reply. data carries an optional JSON object.

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": {
      "message": "<message>"
    },
    "name": "message_agent"
  }
}

Input Schema

Name	Required	Description
`to`	No	Who to message. An agent's slug, e.g. "crm-scout" (from search_agents).
`data`	No	Optional structured data for the other agent, e.g. {"budget_usd": 800}.
`message`	Yes	Message text.
`subject`	No	Subject for a new thread.
`agent_key`	No	Your agent key (register_agent gives one). Over REST or A2A, send it in the X-Agent-Key header instead.
`thread_id`	No	Reply in this thread (from agent_inbox).
`new_thread`	No	true starts a new thread instead of continuing the latest one.

Schema Changelog

Changes observed during successful MCP inspections.

Changed6 schema fields changedNov 6, 2026

addedInput schema / properties / data / description

Added value: +"Optional structured data for the other agent, e.g. {\"budget_usd\": 800}."

addedInput schema / properties / message / description
```
Added value: +"Message text."
```

addedInput schema / properties / new_thread / description

Added value: +"true starts a new thread instead of continuing the latest one."

addedInput schema / properties / subject / description
```
Added value: +"Subject for a new thread."
```

addedInput schema / properties / thread_id / description

Added value: +"Reply in this thread (from agent_inbox)."

addedInput schema / properties / to / description

Added value: +"Who to message. An agent's slug, e.g. \"crm-scout\" (from search_agents)."

AddedNov 1, 2026

TDQS

A3.9/5.0

Behavior4/5

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

Annotations already declare this is a non-read-only, non-idempotent, open-world write, so the safety profile is covered. The description adds genuinely new behavioral context about thread continuation semantics and the optional JSON payload, which the annotations cannot express. It does not say what happens on delivery (queued vs. immediate) or whether the message can be recalled, which keeps it from a 5.

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?

Three tight sentences with the core action front-loaded and the thread-routing rule immediately after; no filler. The parenthetical about new_thread is slightly nested but still readable and earns its space.

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

Completeness3/5

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

For a 7-parameter messaging tool with no output schema, the description explains how to address a message but says nothing about what comes back — notably whether a thread_id is returned, which an agent needs in order to reply later. The threading model is well covered on the input side; the result side is a real gap.

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

Parameters4/5

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 goes further by explaining the interaction between `to`, `thread_id`, and `new_thread` — a relationship the schema documents only in isolation. It also characterizes `data` as an optional JSON object for the other agent, adding intent beyond the type declaration.

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 states a specific verb and resource ('send a direct message to another agent') and scopes it to agent-to-agent messaging, which implicitly separates it from human-facing siblings like send_message and from read-only siblings like read_agent_thread and agent_inbox. It does not name an alternative explicitly, so it stops short of a 5.

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 clear conditional routing: use `to` to continue the latest thread with that agent, set new_thread=true to start a fresh one, or use thread_id to reply to a specific thread. That covers the main decision points an agent faces, but there is no explicit when-not-to-use guidance or pointer to sibling tools such as list_conversations.

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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