Sendmux Email Inbox API + Sending

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mailbox_get_thread

Read-onlyIdempotent

Retrieve a summary of an email thread by providing its thread ID to get conversation context without loading individual messages.

Instructions

Use this to read one thread summary by thread_id. Use mailbox_list_thread_messages when the task needs the messages in the thread.

Input Schema

TableJSON Schema

Name	Required	Description
`thread_id`	Yes	Thread ID.
`mailbox_id`	No	Mailbox public ID to target when the credential grants access to more than one mailbox. Omit when the credential is scoped to exactly one mailbox.
`If-None-Match`	No	Weak ETag from a previous response. Returns 304 when unchanged.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv1.0.14Sep 23, 2026

changedOutput schema / properties / result / allOf

Previous value: -[
-  {
-    "properties": {
-      "meta": {
-        "additionalProperties": {},
-        "properties": {},
-        "type": "object"
-      },
-      "ok": {
-        "enum": [
-          true
-        ],
-        "type": "boolean"
-      }
-    },
-    "required": [
-      "ok",
-      "meta"
-    ],
-    "type": "object"
-  },
-  {
-    "properties": {
-      "data": {
-        "additionalProperties": false,
-        "properties": {
-          "folder_ids": {
-            "items": {
-              "type": "string"
-            },
-            "type": "array"
-          },
-          "has_attachments": {
-            "type": "boolean"
-          },
-          "id": {
-            "description": "Thread ID",
-            "type": "string"
-          },
-          "last_message": {
-            "allOf": [
-              {
-                "additionalProperties": false,
-                "properties": {
-                  "attachments": {
-                    "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "content_id": {
-                          "description": "Content ID for inline attachments, when present.",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        },
-                        "content_type": {
-                          "example": "application/pdf",
-                          "type": "string"
-                        },
-                        "disposition": {
-                          "example": "attachment",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        },
-                        "download_url": {
-                          "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
-                          "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
-                          "type": "string"
-                        },
-                        "filename": {
-                          "example": "invoice.pdf",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        },
-                        "id": {
-                          "description": "Attachment blob ID.",
-                          "type": "string"
-                        },
-                        "size_bytes": {
-                          "example": 12345,
-                          "type": [
-                            "integer",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "id",
-                        "filename",
-                        "content_type",
-                        "size_bytes",
-                        "disposition",
-                        "content_id"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "bcc": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "cc": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "flags": {
-                    "additionalProperties": false,
-                    "properties": {
-                      "answered": {
-                        "type": "boolean"
-                      },
-                      "draft": {
-                        "type": "boolean"
-                      },
-                      "flagged": {
-                        "type": "boolean"
-                      },
-                      "seen": {
-                        "type": "boolean"
-                      }
-                    },
-                    "required": [
-                      "seen",
-                      "flagged",
-                      "draft",
-                      "answered"
-                    ],
-                    "type": "object"
-                  },
-                  "folder_ids": {
-                    "items": {
-                      "type": "string"
-                    },
-                    "type": "array"
-                  },
-                  "from": {
-                    "allOf": [
-                      {
-                        "additionalProperties": false,
-                        "properties": {
-                          "email": {
-                            "example": "agent@example.com",
-                            "format": "email",
-                            "type": "string"
-                          },
-                          "name": {
-                            "example": "Support",
-                            "type": [
-                              "string",
-                              "null"
-                            ]
-                          }
-                        },
-                        "required": [
-                          "email",
-                          "name"
-                        ],
-                        "type": "object"
-                      },
-                      {
-                        "type": [
-                          "object",
-                          "null"
-                        ]
-                      }
-                    ]
-                  },
-                  "has_attachments": {
-                    "type": "boolean"
-                  },
-                  "id": {
-                    "description": "Message ID",
-                    "type": "string"
-                  },
-                  "keywords": {
-                    "description": "Active message keywords, including system flags and custom labels.",
-                    "example": [
-                      "$seen",
-                      "$flagged",
-                      "agent_triaged"
-                    ],
-                    "items": {
-                      "type": "string"
-                    },
-                    "type": "array"
-                  },
-                  "preview": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "received_at": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "sent_at": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "size_bytes": {
-                    "type": [
-                      "integer",
-                      "null"
-                    ]
-                  },
-                  "subject": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "thread_id": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "to": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  }
-                },
-                "required": [
-                  "id",
-                  "thread_id",
-                  "folder_ids",
-                  "subject",
-                  "preview",
-                  "from",
-                  "to",
-                  "cc",
-                  "bcc",
-                  "received_at",
-                  "sent_at",
-                  "size_bytes",
-                  "has_attachments",
-                  "keywords",
-                  "flags"
-                ],
-                "type": "object"
-              },
-              {
-                "type": [
-                  "object",
-                  "null"
-                ]
-              }
-            ]
-          },
-          "message_count": {
-            "type": "integer"
-          },
-          "message_ids": {
-            "items": {
-              "type": "string"
-            },
-            "type": "array"
-          },
-          "participants": {
-            "items": {
-              "additionalProperties": false,
-              "properties": {
-                "email": {
-                  "example": "agent@example.com",
-                  "format": "email",
-                  "type": "string"
-                },
-                "name": {
-                  "example": "Support",
-                  "type": [
-                    "string",
-                    "null"
-                  ]
-                }
-              },
-              "required": [
-                "email",
-                "name"
-              ],
-              "type": "object"
-            },
-            "type": "array"
-          },
-          "states": {
-            "additionalProperties": false,
-            "properties": {
-              "email_state": {
-                "type": [
-                  "string",
-                  "null"
-                ]
-              },
-              "thread_state": {
-                "type": [
-                  "string",
-                  "null"
-                ]
-              }
-            },
-            "required": [
-              "thread_state",
-              "email_state"
-            ],
-            "type": "object"
-          },
-          "subject": {
-            "type": [
-              "string",
-              "null"
-            ]
-          },
-          "unread_count": {
-            "type": "integer"
-          }
-        },
-        "required": [
-          "id",
-          "subject",
-          "message_count",
-          "unread_count",
-          "has_attachments",
-          "folder_ids",
-          "participants",
-          "last_message",
-          "states",
-          "message_ids"
-        ],
-        "type": "object"
-      },
-      "meta": {
-        "additionalProperties": false,
-        "properties": {
-          "request_id": {
-            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
-            "type": "string"
-          }
-        },
-        "required": [
-          "request_id"
-        ],
-        "type": "object"
-      }
-    },
-    "required": [
-      "data"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "properties": {
+      "meta": {
+        "additionalProperties": {},
+        "properties": {},
+        "type": "object"
+      },
+      "ok": {
+        "enum": [
+          true
+        ],
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "ok",
+      "meta"
+    ],
+    "type": "object"
+  },
+  {
+    "properties": {
+      "data": {
+        "additionalProperties": false,
+        "properties": {
+          "folder_ids": {
+            "items": {
+              "type": "string"
+            },
+            "type": "array"
+          },
+          "has_attachments": {
+            "type": "boolean"
+          },
+          "id": {
+            "description": "Thread ID",
+            "type": "string"
+          },
+          "last_message": {
+            "anyOf": [
+              {
+                "additionalProperties": false,
+                "properties": {
+                  "attachments": {
+                    "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "content_id": {
+                          "description": "Content ID for inline attachments, when present.",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "content_type": {
+                          "example": "application/pdf",
+                          "type": "string"
+                        },
+                        "disposition": {
+                          "example": "attachment",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "download_url": {
+                          "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
+                          "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
+                          "type": "string"
+                        },
+                        "filename": {
+                          "example": "invoice.pdf",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "id": {
+                          "description": "Attachment blob ID.",
+                          "type": "string"
+                        },
+                        "size_bytes": {
+                          "example": 12345,
+                          "type": [
+                            "integer",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "id",
+                        "filename",
+                        "content_type",
+                        "size_bytes",
+                        "disposition",
+                        "content_id"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "bcc": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "cc": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "flags": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "answered": {
+                        "type": "boolean"
+                      },
+                      "draft": {
+                        "type": "boolean"
+                      },
+                      "flagged": {
+                        "type": "boolean"
+                      },
+                      "seen": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "seen",
+                      "flagged",
+                      "draft",
+                      "answered"
+                    ],
+                    "type": "object"
+                  },
+                  "folder_ids": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "from": {
+                    "anyOf": [
+                      {
+                        "additionalProperties": false,
+                        "properties": {
+                          "email": {
+                            "example": "agent@example.com",
+                            "format": "email",
+                            "type": "string"
+                          },
+                          "name": {
+                            "example": "Support",
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          }
+                        },
+                        "required": [
+                          "email",
+                          "name"
+                        ],
+                        "type": "object"
+                      },
+                      {
+                        "type": "null"
+                      }
+                    ]
+                  },
+                  "has_attachments": {
+                    "type": "boolean"
+                  },
+                  "id": {
+                    "description": "Message ID",
+                    "type": "string"
+                  },
+                  "keywords": {
+                    "description": "Active message keywords, including system flags and custom labels.",
+                    "example": [
+                      "$seen",
+                      "$flagged",
+                      "agent_triaged"
+                    ],
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "preview": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "received_at": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "sent_at": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "size_bytes": {
+                    "type": [
+                      "integer",
+                      "null"
+                    ]
+                  },
+                  "subject": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "thread_id": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "to": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  }
+                },
+                "required": [
+                  "id",
+                  "thread_id",
+                  "folder_ids",
+                  "subject",
+                  "preview",
+                  "from",
+                  "to",
+                  "cc",
+                  "bcc",
+                  "received_at",
+                  "sent_at",
+                  "size_bytes",
+                  "has_attachments",
+                  "keywords",
+                  "flags"
+                ],
+                "type": "object"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "message_count": {
+            "type": "integer"
+          },
+          "message_ids": {
+            "items": {
+              "type": "string"
+            },
+            "type": "array"
+          },
+          "participants": {
+            "items": {
+              "additionalProperties": false,
+              "properties": {
+                "email": {
+                  "example": "agent@example.com",
+                  "format": "email",
+                  "type": "string"
+                },
+                "name": {
+                  "example": "Support",
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                }
+              },
+              "required": [
+                "email",
+                "name"
+              ],
+              "type": "object"
+            },
+            "type": "array"
+          },
+          "states": {
+            "additionalProperties": false,
+            "properties": {
+              "email_state": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "thread_state": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              }
+            },
+            "required": [
+              "thread_state",
+              "email_state"
+            ],
+            "type": "object"
+          },
+          "subject": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "unread_count": {
+            "type": "integer"
+          }
+        },
+        "required": [
+          "id",
+          "subject",
+          "message_count",
+          "unread_count",
+          "has_attachments",
+          "folder_ids",
+          "participants",
+          "last_message",
+          "states",
+          "message_ids"
+        ],
+        "type": "object"
+      },
+      "meta": {
+        "additionalProperties": false,
+        "properties": {
+          "request_id": {
+            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+            "type": "string"
+          }
+        },
+        "required": [
+          "request_id"
+        ],
+        "type": "object"
+      }
+    },
+    "required": [
+      "data"
+    ],
+    "type": "object"
+  }
+]

Changed1 schema field changedv1.0.12Sep 19, 2026

changedOutput schema / properties / result / allOf

Previous value: -[
-  {
-    "properties": {
-      "meta": {
-        "additionalProperties": {},
-        "properties": {},
-        "type": "object"
-      },
-      "ok": {
-        "enum": [
-          true
-        ],
-        "type": "boolean"
-      }
-    },
-    "required": [
-      "ok",
-      "meta"
-    ],
-    "type": "object"
-  },
-  {
-    "properties": {
-      "data": {
-        "allOf": [
-          {
-            "additionalProperties": false,
-            "properties": {
-              "folder_ids": {
-                "items": {
-                  "type": "string"
-                },
-                "type": "array"
-              },
-              "has_attachments": {
-                "type": "boolean"
-              },
-              "id": {
-                "description": "Thread ID",
-                "type": "string"
-              },
-              "last_message": {
-                "allOf": [
-                  {
-                    "additionalProperties": false,
-                    "properties": {
-                      "attachments": {
-                        "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
-                        "items": {
-                          "additionalProperties": false,
-                          "properties": {
-                            "content_id": {
-                              "description": "Content ID for inline attachments, when present.",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            },
-                            "content_type": {
-                              "example": "application/pdf",
-                              "type": "string"
-                            },
-                            "disposition": {
-                              "example": "attachment",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            },
-                            "download_url": {
-                              "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
-                              "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
-                              "type": "string"
-                            },
-                            "filename": {
-                              "example": "invoice.pdf",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            },
-                            "id": {
-                              "description": "Attachment blob ID.",
-                              "type": "string"
-                            },
-                            "size_bytes": {
-                              "example": 12345,
-                              "type": [
-                                "integer",
-                                "null"
-                              ]
-                            }
-                          },
-                          "required": [
-                            "id",
-                            "filename",
-                            "content_type",
-                            "size_bytes",
-                            "disposition",
-                            "content_id"
-                          ],
-                          "type": "object"
-                        },
-                        "type": "array"
-                      },
-                      "bcc": {
-                        "items": {
-                          "additionalProperties": false,
-                          "properties": {
-                            "email": {
-                              "example": "agent@example.com",
-                              "format": "email",
-                              "type": "string"
-                            },
-                            "name": {
-                              "example": "Support",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            }
-                          },
-                          "required": [
-                            "email",
-                            "name"
-                          ],
-                          "type": "object"
-                        },
-                        "type": "array"
-                      },
-                      "cc": {
-                        "items": {
-                          "additionalProperties": false,
-                          "properties": {
-                            "email": {
-                              "example": "agent@example.com",
-                              "format": "email",
-                              "type": "string"
-                            },
-                            "name": {
-                              "example": "Support",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            }
-                          },
-                          "required": [
-                            "email",
-                            "name"
-                          ],
-                          "type": "object"
-                        },
-                        "type": "array"
-                      },
-                      "flags": {
-                        "additionalProperties": false,
-                        "properties": {
-                          "answered": {
-                            "type": "boolean"
-                          },
-                          "draft": {
-                            "type": "boolean"
-                          },
-                          "flagged": {
-                            "type": "boolean"
-                          },
-                          "seen": {
-                            "type": "boolean"
-                          }
-                        },
-                        "required": [
-                          "seen",
-                          "flagged",
-                          "draft",
-                          "answered"
-                        ],
-                        "type": "object"
-                      },
-                      "folder_ids": {
-                        "items": {
-                          "type": "string"
-                        },
-                        "type": "array"
-                      },
-                      "from": {
-                        "allOf": [
-                          {
-                            "additionalProperties": false,
-                            "properties": {
-                              "email": {
-                                "example": "agent@example.com",
-                                "format": "email",
-                                "type": "string"
-                              },
-                              "name": {
-                                "example": "Support",
-                                "type": [
-                                  "string",
-                                  "null"
-                                ]
-                              }
-                            },
-                            "required": [
-                              "email",
-                              "name"
-                            ],
-                            "type": "object"
-                          },
-                          {
-                            "type": [
-                              "object",
-                              "null"
-                            ]
-                          }
-                        ]
-                      },
-                      "has_attachments": {
-                        "type": "boolean"
-                      },
-                      "id": {
-                        "description": "Message ID",
-                        "type": "string"
-                      },
-                      "keywords": {
-                        "description": "Active message keywords, including system flags and custom labels.",
-                        "example": [
-                          "$seen",
-                          "$flagged",
-                          "agent_triaged"
-                        ],
-                        "items": {
-                          "type": "string"
-                        },
-                        "type": "array"
-                      },
-                      "preview": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "received_at": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "sent_at": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "size_bytes": {
-                        "type": [
-                          "integer",
-                          "null"
-                        ]
-                      },
-                      "subject": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "thread_id": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "to": {
-                        "items": {
-                          "additionalProperties": false,
-                          "properties": {
-                            "email": {
-                              "example": "agent@example.com",
-                              "format": "email",
-                              "type": "string"
-                            },
-                            "name": {
-                              "example": "Support",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            }
-                          },
-                          "required": [
-                            "email",
-                            "name"
-                          ],
-                          "type": "object"
-                        },
-                        "type": "array"
-                      }
-                    },
-                    "required": [
-                      "id",
-                      "thread_id",
-                      "folder_ids",
-                      "subject",
-                      "preview",
-                      "from",
-                      "to",
-                      "cc",
-                      "bcc",
-                      "received_at",
-                      "sent_at",
-                      "size_bytes",
-                      "has_attachments",
-                      "keywords",
-                      "flags"
-                    ],
-                    "type": "object"
-                  },
-                  {
-                    "type": [
-                      "object",
-                      "null"
-                    ]
-                  }
-                ]
-              },
-              "message_count": {
-                "type": "integer"
-              },
-              "participants": {
-                "items": {
-                  "additionalProperties": false,
-                  "properties": {
-                    "email": {
-                      "example": "agent@example.com",
-                      "format": "email",
-                      "type": "string"
-                    },
-                    "name": {
-                      "example": "Support",
-                      "type": [
-                        "string",
-                        "null"
-                      ]
-                    }
-                  },
-                  "required": [
-                    "email",
-                    "name"
-                  ],
-                  "type": "object"
-                },
-                "type": "array"
-              },
-              "states": {
-                "additionalProperties": false,
-                "properties": {
-                  "email_state": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "thread_state": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  }
-                },
-                "required": [
-                  "thread_state",
-                  "email_state"
-                ],
-                "type": "object"
-              },
-              "subject": {
-                "type": [
-                  "string",
-                  "null"
-                ]
-              },
-              "unread_count": {
-                "type": "integer"
-              }
-            },
-            "required": [
-              "id",
-              "subject",
-              "message_count",
-              "unread_count",
-              "has_attachments",
-              "folder_ids",
-              "participants",
-              "last_message",
-              "states"
-            ],
-            "type": "object"
-          },
-          {
-            "additionalProperties": false,
-            "properties": {
-              "message_ids": {
-                "items": {
-                  "type": "string"
-                },
-                "type": "array"
-              }
-            },
-            "required": [
-              "message_ids"
-            ],
-            "type": "object"
-          }
-        ]
-      },
-      "meta": {
-        "additionalProperties": false,
-        "properties": {
-          "request_id": {
-            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
-            "type": "string"
-          }
-        },
-        "required": [
-          "request_id"
-        ],
-        "type": "object"
-      }
-    },
-    "required": [
-      "data"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "properties": {
+      "meta": {
+        "additionalProperties": {},
+        "properties": {},
+        "type": "object"
+      },
+      "ok": {
+        "enum": [
+          true
+        ],
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "ok",
+      "meta"
+    ],
+    "type": "object"
+  },
+  {
+    "properties": {
+      "data": {
+        "additionalProperties": false,
+        "properties": {
+          "folder_ids": {
+            "items": {
+              "type": "string"
+            },
+            "type": "array"
+          },
+          "has_attachments": {
+            "type": "boolean"
+          },
+          "id": {
+            "description": "Thread ID",
+            "type": "string"
+          },
+          "last_message": {
+            "allOf": [
+              {
+                "additionalProperties": false,
+                "properties": {
+                  "attachments": {
+                    "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "content_id": {
+                          "description": "Content ID for inline attachments, when present.",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "content_type": {
+                          "example": "application/pdf",
+                          "type": "string"
+                        },
+                        "disposition": {
+                          "example": "attachment",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "download_url": {
+                          "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
+                          "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
+                          "type": "string"
+                        },
+                        "filename": {
+                          "example": "invoice.pdf",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "id": {
+                          "description": "Attachment blob ID.",
+                          "type": "string"
+                        },
+                        "size_bytes": {
+                          "example": 12345,
+                          "type": [
+                            "integer",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "id",
+                        "filename",
+                        "content_type",
+                        "size_bytes",
+                        "disposition",
+                        "content_id"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "bcc": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "cc": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "flags": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "answered": {
+                        "type": "boolean"
+                      },
+                      "draft": {
+                        "type": "boolean"
+                      },
+                      "flagged": {
+                        "type": "boolean"
+                      },
+                      "seen": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "seen",
+                      "flagged",
+                      "draft",
+                      "answered"
+                    ],
+                    "type": "object"
+                  },
+                  "folder_ids": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "from": {
+                    "allOf": [
+                      {
+                        "additionalProperties": false,
+                        "properties": {
+                          "email": {
+                            "example": "agent@example.com",
+                            "format": "email",
+                            "type": "string"
+                          },
+                          "name": {
+                            "example": "Support",
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          }
+                        },
+                        "required": [
+                          "email",
+                          "name"
+                        ],
+                        "type": "object"
+                      },
+                      {
+                        "type": [
+                          "object",
+                          "null"
+                        ]
+                      }
+                    ]
+                  },
+                  "has_attachments": {
+                    "type": "boolean"
+                  },
+                  "id": {
+                    "description": "Message ID",
+                    "type": "string"
+                  },
+                  "keywords": {
+                    "description": "Active message keywords, including system flags and custom labels.",
+                    "example": [
+                      "$seen",
+                      "$flagged",
+                      "agent_triaged"
+                    ],
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "preview": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "received_at": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "sent_at": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "size_bytes": {
+                    "type": [
+                      "integer",
+                      "null"
+                    ]
+                  },
+                  "subject": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "thread_id": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "to": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  }
+                },
+                "required": [
+                  "id",
+                  "thread_id",
+                  "folder_ids",
+                  "subject",
+                  "preview",
+                  "from",
+                  "to",
+                  "cc",
+                  "bcc",
+                  "received_at",
+                  "sent_at",
+                  "size_bytes",
+                  "has_attachments",
+                  "keywords",
+                  "flags"
+                ],
+                "type": "object"
+              },
+              {
+                "type": [
+                  "object",
+                  "null"
+                ]
+              }
+            ]
+          },
+          "message_count": {
+            "type": "integer"
+          },
+          "message_ids": {
+            "items": {
+              "type": "string"
+            },
+            "type": "array"
+          },
+          "participants": {
+            "items": {
+              "additionalProperties": false,
+              "properties": {
+                "email": {
+                  "example": "agent@example.com",
+                  "format": "email",
+                  "type": "string"
+                },
+                "name": {
+                  "example": "Support",
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                }
+              },
+              "required": [
+                "email",
+                "name"
+              ],
+              "type": "object"
+            },
+            "type": "array"
+          },
+          "states": {
+            "additionalProperties": false,
+            "properties": {
+              "email_state": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "thread_state": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              }
+            },
+            "required": [
+              "thread_state",
+              "email_state"
+            ],
+            "type": "object"
+          },
+          "subject": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "unread_count": {
+            "type": "integer"
+          }
+        },
+        "required": [
+          "id",
+          "subject",
+          "message_count",
+          "unread_count",
+          "has_attachments",
+          "folder_ids",
+          "participants",
+          "last_message",
+          "states",
+          "message_ids"
+        ],
+        "type": "object"
+      },
+      "meta": {
+        "additionalProperties": false,
+        "properties": {
+          "request_id": {
+            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+            "type": "string"
+          }
+        },
+        "required": [
+          "request_id"
+        ],
+        "type": "object"
+      }
+    },
+    "required": [
+      "data"
+    ],
+    "type": "object"
+  }
+]

Changed4 schema fields changedv1.0.8Sep 15, 2026

addedOutput schema / $schema

Added value: +"https://json-schema.org/draft/2020-12/schema"

removedOutput schema / additionalProperties
```
Removed value: -true
```

addedOutput schema / properties

Added value: +{
+  "result": {
+    "allOf": [
+      {
+        "properties": {
+          "meta": {
+            "additionalProperties": {},
+            "properties": {},
+            "type": "object"
+          },
+          "ok": {
+            "enum": [
+              true
+            ],
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "ok",
+          "meta"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "data": {
+            "allOf": [
+              {
+                "additionalProperties": false,
+                "properties": {
+                  "folder_ids": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "has_attachments": {
+                    "type": "boolean"
+                  },
+                  "id": {
+                    "description": "Thread ID",
+                    "type": "string"
+                  },
+                  "last_message": {
+                    "allOf": [
+                      {
+                        "additionalProperties": false,
+                        "properties": {
+                          "attachments": {
+                            "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
+                            "items": {
+                              "additionalProperties": false,
+                              "properties": {
+                                "content_id": {
+                                  "description": "Content ID for inline attachments, when present.",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                },
+                                "content_type": {
+                                  "example": "application/pdf",
+                                  "type": "string"
+                                },
+                                "disposition": {
+                                  "example": "attachment",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                },
+                                "download_url": {
+                                  "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
+                                  "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
+                                  "type": "string"
+                                },
+                                "filename": {
+                                  "example": "invoice.pdf",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                },
+                                "id": {
+                                  "description": "Attachment blob ID.",
+                                  "type": "string"
+                                },
+                                "size_bytes": {
+                                  "example": 12345,
+                                  "type": [
+                                    "integer",
+                                    "null"
+                                  ]
+                                }
+                              },
+                              "required": [
+                                "id",
+                                "filename",
+                                "content_type",
+                                "size_bytes",
+                                "disposition",
+                                "content_id"
+                              ],
+                              "type": "object"
+                            },
+                            "type": "array"
+                          },
+                          "bcc": {
+                            "items": {
+                              "additionalProperties": false,
+                              "properties": {
+                                "email": {
+                                  "example": "agent@example.com",
+                                  "format": "email",
+                                  "type": "string"
+                                },
+                                "name": {
+                                  "example": "Support",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                }
+                              },
+                              "required": [
+                                "email",
+                                "name"
+                              ],
+                              "type": "object"
+                            },
+                            "type": "array"
+                          },
+                          "cc": {
+                            "items": {
+                              "additionalProperties": false,
+                              "properties": {
+                                "email": {
+                                  "example": "agent@example.com",
+                                  "format": "email",
+                                  "type": "string"
+                                },
+                                "name": {
+                                  "example": "Support",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                }
+                              },
+                              "required": [
+                                "email",
+                                "name"
+                              ],
+                              "type": "object"
+                            },
+                            "type": "array"
+                          },
+                          "flags": {
+                            "additionalProperties": false,
+                            "properties": {
+                              "answered": {
+                                "type": "boolean"
+                              },
+                              "draft": {
+                                "type": "boolean"
+                              },
+                              "flagged": {
+                                "type": "boolean"
+                              },
+                              "seen": {
+                                "type": "boolean"
+                              }
+                            },
+                            "required": [
+                              "seen",
+                              "flagged",
+                              "draft",
+                              "answered"
+                            ],
+                            "type": "object"
+                          },
+                          "folder_ids": {
+                            "items": {
+                              "type": "string"
+                            },
+                            "type": "array"
+                          },
+                          "from": {
+                            "allOf": [
+                              {
+                                "additionalProperties": false,
+                                "properties": {
+                                  "email": {
+                                    "example": "agent@example.com",
+                                    "format": "email",
+                                    "type": "string"
+                                  },
+                                  "name": {
+                                    "example": "Support",
+                                    "type": [
+                                      "string",
+                                      "null"
+                                    ]
+                                  }
+                                },
+                                "required": [
+                                  "email",
+                                  "name"
+                                ],
+                                "type": "object"
+                              },
+                              {
+                                "type": [
+                                  "object",
+                                  "null"
+                                ]
+                              }
+                            ]
+                          },
+                          "has_attachments": {
+                            "type": "boolean"
+                          },
+                          "id": {
+                            "description": "Message ID",
+                            "type": "string"
+                          },
+                          "keywords": {
+                            "description": "Active message keywords, including system flags and custom labels.",
+                            "example": [
+                              "$seen",
+                              "$flagged",
+                              "agent_triaged"
+                            ],
+                            "items": {
+                              "type": "string"
+                            },
+                            "type": "array"
+                          },
+                          "preview": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "received_at": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "sent_at": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "size_bytes": {
+                            "type": [
+                              "integer",
+                              "null"
+                            ]
+                          },
+                          "subject": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "thread_id": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "to": {
+                            "items": {
+                              "additionalProperties": false,
+                              "properties": {
+                                "email": {
+                                  "example": "agent@example.com",
+                                  "format": "email",
+                                  "type": "string"
+                                },
+                                "name": {
+                                  "example": "Support",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                }
+                              },
+                              "required": [
+                                "email",
+                                "name"
+                              ],
+                              "type": "object"
+                            },
+                            "type": "array"
+                          }
+                        },
+                        "required": [
+                          "id",
+                          "thread_id",
+                          "folder_ids",
+                          "subject",
+                          "preview",
+                          "from",
+                          "to",
+                          "cc",
+                          "bcc",
+                          "received_at",
+                          "sent_at",
+                          "size_bytes",
+                          "has_attachments",
+                          "keywords",
+                          "flags"
+                        ],
+                        "type": "object"
+                      },
+                      {
+                        "type": [
+                          "object",
+                          "null"
+                        ]
+                      }
+                    ]
+                  },
+                  "message_count": {
+                    "type": "integer"
+                  },
+                  "participants": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "states": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "email_state": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "thread_state": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      }
+                    },
+                    "required": [
+                      "thread_state",
+                      "email_state"
+                    ],
+                    "type": "object"
+                  },
+                  "subject": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "unread_count": {
+                    "type": "integer"
+                  }
+                },
+                "required": [
+                  "id",
+                  "subject",
+                  "message_count",
+                  "unread_count",
+                  "has_attachments",
+                  "folder_ids",
+                  "participants",
+                  "last_message",
+                  "states"
+                ],
+                "type": "object"
+              },
+              {
+                "additionalProperties": false,
+                "properties": {
+                  "message_ids": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  }
+                },
+                "required": [
+                  "message_ids"
+                ],
+                "type": "object"
+              }
+            ]
+          },
+          "meta": {
+            "additionalProperties": false,
+            "properties": {
+              "request_id": {
+                "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "MailboxThreadDetailResponse"
+  }
+}

addedOutput schema / required
```
Added value: +[
+  "result"
+]
```

First observedv1.0.0Jul 13, 2026

TDQS

A4.3/5.0

Behavior3/5

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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, covering the safety profile. The description adds that the tool returns a thread summary rather than the messages themselves, which is useful, but it does not provide richer behavioral context such as failure modes or pagination. This is consistent with the baseline for annotation-covered tools.

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?

Two tight sentences with no filler. The primary usage is front-loaded and the alternative is stated immediately, making the description easy to scan and act on.

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?

The tool is simple, has one required parameter, a full output schema, and annotations covering safety and idempotency. The description provides the essential purpose and routing guidance without omitting anything needed for 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 description coverage is 100%, so the parameters are already fully documented in the input schema. The description does not add additional parameter-level meaning beyond reinforcing that thread_id identifies the thread to summarize.

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 uses a specific verb ('read') and resource ('one thread summary by thread_id'), and clearly distinguishes it from the sibling tool mailbox_list_thread_messages by focusing on the summary rather than the messages.

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?

It explicitly states when to use this tool and names the alternative for when messages are needed: 'Use mailbox_list_thread_messages when the task needs the messages in the thread.' This removes ambiguity for the agent.

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