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google-services-mcp

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Reply to an email

google_gmail_reply

Send a Gmail reply to an existing message within its thread, preserving threading headers so it stays in the correct conversation.

Instructions

Reply to an existing message inside its thread, preserving threading headers.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
bodyYesReply body.
fromNoSend-as alias or address to set as the From header.
accountNoAccount nickname to use.
replyToNoAddress to set as the Reply-To header.
bodyTypeNoBody format (default text).
threadIdYesThread ID of the conversation.
messageIdYesID of the message being replied to.
attachmentsNoLocal files to attach to the reply.
driveFileIdsNoDrive file IDs to attach by reference (downloaded under the Gmail size limit).

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv0.1.60
    • addedInput schema / properties / attachments / items / properties / cid
      Added value: +{
+  "description": "Content-ID for inline parts so HTML can reference them as cid:<cid>.",
+  "type": "string"
+}
    • addedInput schema / properties / attachments / items / properties / disposition
      Added value: +{
+  "description": "How the part is presented (default attachment; inline embeds it in the body).",
+  "enum": [
+    "attachment",
+    "inline"
+  ],
+  "type": "string"
+}
    • addedInput schema / properties / driveFileIds
      Added value: +{
+  "description": "Drive file IDs to attach by reference (downloaded under the Gmail size limit).",
+  "items": {
+    "type": "string"
+  },
+  "type": "array"
+}
    • addedInput schema / properties / from
      Added value: +{
+  "description": "Send-as alias or address to set as the From header.",
+  "type": "string"
+}
    • addedInput schema / properties / replyTo
      Added value: +{
+  "description": "Address to set as the Reply-To header.",
+  "type": "string"
+}
  2. Changed2 schema fields changedv0.1.30
    • removedInput schema / additionalProperties
      Removed value: -false
    • addedInput schema / properties / attachments
      Added value: +{
+  "description": "Local files to attach to the reply.",
+  "items": {
+    "properties": {
+      "filename": {
+        "description": "Attachment filename shown to recipients (defaults to the basename of path).",
+        "type": "string"
+      },
+      "mimeType": {
+        "description": "MIME type override (defaults to a guess from the filename).",
+        "type": "string"
+      },
+      "path": {
+        "description": "Local filesystem path of the file to attach.",
+        "type": "string"
+      }
+    },
+    "required": [
+      "path"
+    ],
+    "type": "object"
+  },
+  "type": "array"
+}
  3. First observedv0.1.0

TDQS

A3.6/5.0
Behavior3/5

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

No annotations are provided, so the description must carry behavioral burden. It contributes the key detail that threading headers are preserved, which is important behavior for a reply. However, it omits critical behavioral aspects such as whether sending is restricted to the account that owns the thread, whether any fields are mutable, or what happens if the messageId is not in the thread. This is a moderate gap for a mutation tool with zero annotation coverage.

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?

A single, efficient sentence that front-loads the action and resource and includes a crucial scope constraint. No wasted words.

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

Completeness2/5

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

Given that this is a mutation tool with no annotations and no output schema, the description is too sparse. It does not cover permissions, side effects, error conditions, or the fact that a new message is created in the thread. An agent would need to infer too much, making the definition incomplete for safe correct invocation.

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 the schema fully documents all parameters. The description adds little parameter-level meaning beyond what the schema provides; it mentions 'threading headers' but not parameter specifics. Baseline 3 is appropriate.

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?

The description states a specific verb and resource ('Reply to an existing message inside its thread') and clearly distinguishes this tool from the send sibling by emphasizing threading and preservation of threading headers.

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?

Usage is implied: to reply within a thread you need threadId and messageId, which differentiates it from google_gmail_send. However, the description does not explicitly say when to use this instead of google_gmail_send or other alternatives, nor does it mention prerequisites.

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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