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Thestral12

Postfleet MCP

by Thestral12

reply_email

Idempotent

Reply to an existing email using a unique idempotency key to prevent duplicate sending. If human approval is required, the reply is queued as a draft and returns a draft ID.

Instructions

Reply to an email you received. Call this when answering an existing message — recipient, subject, and conversation threading are derived automatically from the original. Pass a client_id (any unique string you make up) and reuse the SAME client_id if you retry after an error — that guarantees the reply is sent at most once. If the mailbox requires human approval, the reply is queued as a draft and returns {draft_id, status:"pending_approval"} instead of a message id — that is NOT a failure and must NOT be retried; it is sent once a human approves it (check with list_drafts).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
textYesPlain-text reply body, max 50,000 characters.
client_idYesREQUIRED idempotency key — any unique string you invent for this reply. Reuse the SAME value when retrying after an error and the reply goes out at most once; a new value sends a second copy. Cannot start with "draft:".
mailbox_idYesId of the mailbox the original message arrived in.
reply_to_message_idYesId of the message being replied to, from list_inbox or read_email. Recipient, subject and threading are derived from it.
Behavior5/5

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

Annotations declare idempotentHint=true and destructiveHint=false. The description explains BOTH: idempotency via client_id (how it works, retry behavior, and that a new client_id sends a second copy) and the non-destructive 'pending_approval' case (not a failure, check with list_drafts). This exceeds annotation-only info.

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 sentences, zero wasted words. Each sentence delivers critical payload: first sentence establishes purpose, second sentence covers idempotency mechanics, third sentence clarifies the draft-approval edge case. Perfect density.

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?

Given 4 required params with full schema coverage, no output schema, and the domain (email reply), the description covers all behavior: idempotency, threading derivation, pending-approval workflow, and sibling differentiation. Nothing essential is missing.

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% with good descriptions for each param. The description adds value by clarifying how reply_to_message_id drives recipient/subject/threading automatically, and giving extra context on client_id behavior (idempotency, reuse rule, 'draft:' prefix restriction) beyond the schema.

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 starts with 'Reply to an email you received' which is a specific verb-resource pair. It clearly distinguishes from siblings like send_email (new email) and create_draft (new draft) by stating it derives recipient/subject/threading from the original message.

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 says 'Call this when answering an existing message' and contrasts with alternatives by noting that send_email would be for new messages. Also provides guidance on when to retry and when not to retry (pending_approval case).

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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