Proton MCP Server

Instructions

Implementation Reference

• src/tools/reply.ts:13-98 (handler)

  The `registerReplyEmailTool` function registers the `proton_reply_email` tool and contains the main logic handler which fetches the original email, determines recipients based on the `reply_all` parameter, and sends the reply using the `sendMail` service.

  export function registerReplyEmailTool(server: McpServer) {
    server.registerTool(
      'proton_reply_email',
      {
        title: 'Reply to Email',
        description: 'Send a reply to an existing email. Properly sets In-Reply-To and References headers for threading. Can reply to all or just the sender.',
        inputSchema: ReplyEmailSchema,
        annotations: {
          readOnlyHint: false,
          destructiveHint: false,
          idempotentHint: false,
          openWorldHint: false,
        },
      },
      async (params: z.infer<typeof ReplyEmailSchema>) => {
        try {
          // Fetch the original message to get Message-ID and thread info
          const original = await fetchMessage(params.folder, params.uid);
  
          // Build recipient list
          let replyTo: string[] = [];
  
          if (params.reply_all) {
            // Reply to all: sender + cc recipients (exclude self)
            if (original.from.email && original.from.email !== PROTON_USER) {
              replyTo.push(original.from.email);
            }
  
            if (original.cc && original.cc.length > 0) {
              for (const cc of original.cc) {
                if (cc.email && cc.email !== PROTON_USER) {
                  replyTo.push(cc.email);
                }
              }
            }
          } else {
            // Reply to just the sender
            if (original.from.email) {
              replyTo.push(original.from.email);
            }
          }
  
          if (replyTo.length === 0) {
            return {
              content: [
                {
                  type: 'text',
                  text: 'Error: No valid recipients found for reply.',
                },
              ],
            };
          }
  
          // Send reply with threading headers
          const messageId = await sendMail({
            from: PROTON_USER,
            to: replyTo,
            subject: original.subject.startsWith('Re: ')
              ? original.subject
              : `Re: ${original.subject}`,
            text: params.body,
            inReplyTo: original.messageId,
            references: original.references,
          });
  
          return {
            content: [
              {
                type: 'text',
                text: `Reply sent successfully!\nMessage ID: ${messageId}\nTo: ${replyTo.join(', ')}\nOriginal Subject: ${original.subject}`,
              },
            ],
          };
        } catch (error) {
          return {
            content: [
              {
                type: 'text',
                text: `Error sending reply: ${error instanceof Error ? error.message : String(error)}`,
              },
            ],
          };
        }
      }
    );
  }

• src/schemas/index.ts:44-49 (schema)

  The `ReplyEmailSchema` Zod object defines the expected input parameters for the `proton_reply_email` tool, including `folder`, `uid`, `body`, and `reply_all`.

  export const ReplyEmailSchema = z.object({
    folder: z.string().default('INBOX'),
    uid: z.number().int().positive(),
    body: z.string(),
    reply_all: z.boolean().default(false),
  });