Open a reply or draft composer

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

Consistent with readOnlyHint/idempotentHint and adds substantial context beyond them: 'never sends, in any mode, and writes nothing itself,' the view's save path appends a new draft rather than editing in place, no draft is ever deleted, and errors are retryable with partial_reason/index_note and 'do not establish that the message or draft is absent.' That last point materially affects agent retry behavior and is not derivable from annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the routing sentence and the create_draft preference before the mechanical detail, which is the right order. It is dense and long, and a few clauses (size_note encoding, not_copied/Bcc) sit at the same level of prominence as core routing, but nearly every sentence carries operational information rather than filler.

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

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

For a 9-parameter, zero-required tool with no output schema, the description covers return contents ('the prefilled fields, the From address and warnings as text'), prefill behavior per mode, attachment handling, failure modes and the follow-up call needed without a view. An agent has everything required to invoke it and act on the result.

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

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and already documents the message_id/draft_id exclusivity and reply_all dependency, so much is restated. The description still adds real semantics the schema lacks: how To is derived (Reply-To else From, self-replies go to recipients), Cc suppression of own addresses, attachment paths relative to PROTON_MCP_ATTACHMENTS_DIR, and from_address needing an entry from list_from_addresses.

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

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

Opening line states a concrete verb+resource ('open a reply or new-draft composer') and immediately scopes it as 'the default for any request to reply, draft a reply, write back or compose,' which lets an agent distinguish it from create_draft, forward_message and delete_draft without opening a schema.

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

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

Explicit routing rule: 'Prefer it to create_draft, the no-view path,' plus per-mode guidance (message_id to reply, draft_id to reopen, neither for a new draft) and a fallback flow for hosts without a view ('show the returned fields and warnings to the user and call create_draft... only after they agree'). Refusal conditions (missing files, both ids, reply_all without message_id) are also spelled out.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.