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Signal MCP

by googlarz
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Python
Local

create_group

Create a new Signal group with you as admin: provide the group name and E.164 phone numbers of members, plus optional description or avatar. Calling it twice makes two groups.

Instructions

Create a new Signal group with you as admin. name (required) is the group name shown to everyone; members (required) is a list of E.164 phone numbers to add; description (optional) is the group info text; avatar (optional) is a local image path, which must be inside the allowed send folders (SIGNAL_MCP_SEND_ROOTS). Contacts Signal's servers and notifies every member; not idempotent — calling twice creates two groups. Returns status, groupId (base64; use it as group_id elsewhere), timestamp and per-member send results. Use update_group to change an existing group and send_group_message to post in it.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
nameYesGroup name visible to all members
avatarNoOptional local image file path for the group avatar
membersYesPhone numbers (E.164) of initial members to invite
descriptionNoOptional group description shown in group info

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv1.40.1
    • addedInput schema / properties / avatar
      Added value: +{
+  "description": "Optional local image file path for the group avatar",
+  "type": "string"
+}
  2. Changed3 schema fields changedv0.1.2
    • changedInput schema / properties / description / description
      Previous value: -"Optional group description"New value: +"Optional group description shown in group info"
    • changedInput schema / properties / members / description
      Previous value: -"Phone numbers of initial members"New value: +"Phone numbers (E.164) of initial members to invite"
    • changedInput schema / properties / name / description
      Previous value: -"Group name"New value: +"Group name visible to all members"
  3. First observedv0.1.0

TDQS

A4.6/5.0
Behavior4/5

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

Annotations already declare readOnlyHint=false, idempotentHint=false and openWorldHint=true, but the description adds the concrete consequences: it contacts Signal's servers, notifies every member, creates a second group if called twice, and discloses the returned fields. It does not cover permission/admin-setup requirements or rate limits, so it stops short of 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?

Front-loaded with the core action, then parameters, side effects, returns and alternatives in a compact sequence. It is dense with clauses but each sentence carries distinct information; no filler.

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

Completeness5/5

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

With no output schema, the description compensates by enumerating return fields (status, groupId, timestamp, per-member results). Combined with side effects, path constraints and sibling routing, an agent has everything needed to call it correctly.

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 description coverage is 100%, so the baseline is 3, but the description earns more: it specifies members must be E.164 and that avatar must reside inside SIGNAL_MCP_SEND_ROOTS, a path constraint the schema does not state. It also clarifies how the returned groupId is reused as group_id.

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

Purpose5/5

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

States a specific verb and resource ('Create a new Signal group') plus the implicit role ('with you as admin'). It names two sibling tools it is not (update_group, send_group_message), so an agent can distinguish it without opening any schema.

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

Usage Guidelines5/5

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

Explicitly routes the agent: 'Use update_group to change an existing group and send_group_message to post in it.' The when-not condition is stated alongside the alternative, leaving nothing to inference.

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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