fmsg-mcp

Add fmsg recipients

add_recipients

DestructiveIdempotent

Grants new recipients access to an already sent message, including its attachments and thread. This cannot be undone and fails on terminal messages.

Instructions

Add recipients to a message that was already sent (one you sent or received as a primary recipient). They receive the message and its attachments, can read it and become participants of its thread. This cannot be undone. Fails on terminal messages.

Input Schema

TableJSON Schema

Name	Required	Description
`id`	Yes	the sent message to add recipients to
`add_to`	No	deprecated alias of recipients; pass one or the other
`recipients`	No	addresses or short names to add

Output Schema

TableJSON Schema

Name	Required	Description
`id`	Yes
`added`	Yes
`add_to`	Yes	deprecated copy of recipients
`resolved`	No	short names and how they resolved, when any were given; confirm with the user if an address looks wrong
`recipients`	No	the resolved addresses that were added

Schema Changelog

Changes observed during successful MCP inspections.

Changed9 schema fields changedv1.1.0Oct 15, 2026

addedInput schema / additionalProperties
```
Added value: +false
```

changedInput schema / properties / add_to / description

Previous value: -"addresses or short names to add"New value: +"deprecated alias of recipients; pass one or the other"

changedInput schema / properties / id / description

Previous value: -"fmsg message id"New value: +"the sent message to add recipients to"

addedInput schema / properties / recipients

Added value: +{
+  "description": "addresses or short names to add",
+  "items": {
+    "type": "string"
+  },
+  "minItems": 1,
+  "type": "array"
+}

changedInput schema / required

Previous value: -[
-  "id",
-  "add_to"
-]New value: +[
+  "id"
+]

changedOutput schema / additionalProperties
```
Previous value: -falseNew value: +{}
```
addedOutput schema / properties / add_to / description
```
Added value: +"deprecated copy of recipients"
```

addedOutput schema / properties / recipients

Added value: +{
+  "description": "the resolved addresses that were added",
+  "items": {
+    "type": "string"
+  },
+  "type": "array"
+}

addedOutput schema / properties / resolved

Added value: +{
+  "description": "short names and how they resolved, when any were given; confirm with the user if an address looks wrong",
+  "items": {
+    "additionalProperties": {},
+    "properties": {
+      "address": {
+        "type": "string"
+      },
+      "input": {
+        "type": "string"
+      }
+    },
+    "type": "object"
+  },
+  "type": "array"
+}

First observedv0.1.3Sep 5, 2026

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already declare destructiveHint=true and idempotentHint=true; the description adds real value on top by spelling out the consequences ('This cannot be undone', recipients receive the message and its attachments, can read it, and become thread participants). 'Cannot be undone' is consistent with destructiveHint and with idempotency (re-adding the same addresses is a no-op). Permission details beyond the eligibility rule are not covered.

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 tight sentences with zero filler. The core action and its scope come first, the side effects second, and the irreversibility/failure caveats last, which is the right front-loading for a destructive operation.

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?

An output schema exists, so return values need not be described. Together with annotations, the description covers action, eligibility, side effects, irreversibility, and failure mode. It is largely complete, though it says nothing about how already-participating recipients are handled.

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 documents id, recipients, and the deprecated add_to alias, including the 'pass one or the other' constraint. The description only echoes the sent-message targeting implied by id; baseline 3 applies.

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 ('Add recipients to a message that was already sent') and scopes it to messages the caller sent or received as a primary recipient. Clearly distinguishable from send_message, reply, and create_draft, which deal with unsent or new messages.

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?

Gives an explicit eligibility precondition (message already sent, caller was sender or primary recipient) and a failure condition ('Fails on terminal messages'), which tells the agent when not to bother. It does not name a sibling alternative (e.g., use reply instead when composing a new response), so it falls just short of full routing guidance.

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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