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Get Mailbox Session

mailbox_get_session

Read-onlyIdempotent

Retrieve mailbox session metadata to discover supported capabilities, limits, and endpoint details before complex mailbox operations. Avoid errors by confirming what the mailbox supports.

Instructions

Use this before complex mailbox work to discover supported mailbox capabilities, limits, and endpoint metadata. Do not use it for message search.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`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.

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": {
+            "additionalProperties": false,
+            "properties": {
+              "capabilities": {
+                "additionalProperties": false,
+                "properties": {
+                  "attachments": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "download": {
+                        "type": "boolean"
+                      },
+                      "metadata": {
+                        "type": "boolean"
+                      },
+                      "parsing": {
+                        "type": "boolean"
+                      },
+                      "range_download": {
+                        "type": "boolean"
+                      },
+                      "send_with_uploaded_blob": {
+                        "type": "boolean"
+                      },
+                      "upload": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "upload",
+                      "download",
+                      "range_download",
+                      "metadata",
+                      "parsing",
+                      "send_with_uploaded_blob"
+                    ],
+                    "type": "object"
+                  },
+                  "folders": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "create": {
+                        "type": "boolean"
+                      },
+                      "delete": {
+                        "type": "boolean"
+                      },
+                      "detail": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "query_changes": {
+                        "type": "boolean"
+                      },
+                      "update": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "detail",
+                      "create",
+                      "update",
+                      "delete",
+                      "changes",
+                      "query_changes"
+                    ],
+                    "type": "object"
+                  },
+                  "identities": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "create": {
+                        "type": "boolean"
+                      },
+                      "delete": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "update": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "changes",
+                      "create",
+                      "update",
+                      "delete"
+                    ],
+                    "type": "object"
+                  },
+                  "messages": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "batch_get": {
+                        "type": "boolean"
+                      },
+                      "batch_update": {
+                        "type": "boolean"
+                      },
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "clean_content": {
+                        "type": "boolean"
+                      },
+                      "count": {
+                        "type": "boolean"
+                      },
+                      "delete": {
+                        "type": "boolean"
+                      },
+                      "detail": {
+                        "type": "boolean"
+                      },
+                      "keywords": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "permanent_delete": {
+                        "type": "boolean"
+                      },
+                      "query_changes": {
+                        "type": "boolean"
+                      },
+                      "raw_body": {
+                        "type": "boolean"
+                      },
+                      "search_snippets": {
+                        "type": "boolean"
+                      },
+                      "update_fields": {
+                        "items": {
+                          "enum": [
+                            "seen",
+                            "flagged",
+                            "keywords"
+                          ],
+                          "type": "string"
+                        },
+                        "type": "array"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "detail",
+                      "raw_body",
+                      "clean_content",
+                      "batch_get",
+                      "count",
+                      "search_snippets",
+                      "changes",
+                      "query_changes",
+                      "update_fields",
+                      "keywords",
+                      "batch_update",
+                      "delete",
+                      "permanent_delete"
+                    ],
+                    "type": "object"
+                  },
+                  "quotas": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "update": {
+                        "type": "boolean"
+                      },
+                      "usage": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "usage",
+                      "changes",
+                      "update"
+                    ],
+                    "type": "object"
+                  },
+                  "submissions": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "detail": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "send": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "detail",
+                      "changes",
+                      "send"
+                    ],
+                    "type": "object"
+                  },
+                  "sync": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "query_changes": {
+                        "items": {
+                          "enum": [
+                            "messages",
+                            "folders"
+                          ],
+                          "type": "string"
+                        },
+                        "type": "array"
+                      },
+                      "typed_changes": {
+                        "type": "boolean"
+                      },
+                      "types": {
+                        "items": {
+                          "enum": [
+                            "messages",
+                            "folders",
+                            "threads",
+                            "submissions",
+                            "identities",
+                            "quotas"
+                          ],
+                          "type": "string"
+                        },
+                        "type": "array"
+                      }
+                    },
+                    "required": [
+                      "typed_changes",
+                      "types",
+                      "query_changes"
+                    ],
+                    "type": "object"
+                  },
+                  "threads": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "changes": {
+                        "type": "boolean"
+                      },
+                      "clean_content": {
+                        "type": "boolean"
+                      },
+                      "detail": {
+                        "type": "boolean"
+                      },
+                      "list": {
+                        "type": "boolean"
+                      },
+                      "messages": {
+                        "type": "boolean"
+                      }
+                    },
+                    "required": [
+                      "list",
+                      "detail",
+                      "messages",
+                      "clean_content",
+                      "changes"
+                    ],
+                    "type": "object"
+                  }
+                },
+                "required": [
+                  "messages",
+                  "threads",
+                  "folders",
+                  "submissions",
+                  "identities",
+                  "quotas",
+                  "attachments",
+                  "sync"
+                ],
+                "type": "object"
+              },
+              "endpoints": {
+                "additionalProperties": false,
+                "properties": {
+                  "folders": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "mailbox": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "messages": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "submissions": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "sync": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "threads": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "usage": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  }
+                },
+                "required": [
+                  "mailbox",
+                  "messages",
+                  "threads",
+                  "folders",
+                  "submissions",
+                  "usage",
+                  "sync"
+                ],
+                "type": "object"
+              },
+              "gated": {
+                "additionalProperties": false,
+                "description": "Flags are true when a feature is intentionally unavailable through the mailbox API.",
+                "properties": {
+                  "attachment_parsing": {
+                    "type": "boolean"
+                  },
+                  "blob_copy": {
+                    "type": "boolean"
+                  },
+                  "blob_lookup": {
+                    "type": "boolean"
+                  },
+                  "events": {
+                    "type": "boolean"
+                  },
+                  "filter_script_writes": {
+                    "type": "boolean"
+                  },
+                  "identity_mutation": {
+                    "type": "boolean"
+                  },
+                  "message_copy": {
+                    "type": "boolean"
+                  },
+                  "message_import": {
+                    "type": "boolean"
+                  },
+                  "message_parse": {
+                    "type": "boolean"
+                  },
+                  "quota_mutation": {
+                    "type": "boolean"
+                  },
+                  "raw_protocol": {
+                    "type": "boolean"
+                  },
+                  "vacation_response": {
+                    "type": "boolean"
+                  }
+                },
+                "required": [
+                  "identity_mutation",
+                  "quota_mutation",
+                  "vacation_response",
+                  "filter_script_writes",
+                  "events",
+                  "raw_protocol",
+                  "attachment_parsing",
+                  "message_copy",
+                  "message_import",
+                  "message_parse",
+                  "blob_lookup",
+                  "blob_copy"
+                ],
+                "type": "object"
+              },
+              "limits": {
+                "additionalProperties": false,
+                "properties": {
+                  "attachment_upload_bytes_max": {
+                    "type": "integer"
+                  },
+                  "batch_ids_max": {
+                    "type": "integer"
+                  },
+                  "body_chars_max": {
+                    "type": "integer"
+                  },
+                  "changes_limit_max": {
+                    "type": "integer"
+                  },
+                  "inline_attachment_bytes_max": {
+                    "type": "integer"
+                  },
+                  "keywords_per_update_max": {
+                    "type": "integer"
+                  },
+                  "list_limit_max": {
+                    "type": "integer"
+                  },
+                  "message_body_chars_default": {
+                    "type": "integer"
+                  },
+                  "outbound_raw_message_bytes_max": {
+                    "type": "integer"
+                  },
+                  "thread_body_chars_default": {
+                    "type": "integer"
+                  }
+                },
+                "required": [
+                  "list_limit_max",
+                  "changes_limit_max",
+                  "batch_ids_max",
+                  "message_body_chars_default",
+                  "thread_body_chars_default",
+                  "body_chars_max",
+                  "inline_attachment_bytes_max",
+                  "attachment_upload_bytes_max",
+                  "outbound_raw_message_bytes_max",
+                  "keywords_per_update_max"
+                ],
+                "type": "object"
+              },
+              "mailbox": {
+                "additionalProperties": false,
+                "properties": {
+                  "display_name": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "email": {
+                    "example": "agent@example.com",
+                    "format": "email",
+                    "type": "string"
+                  },
+                  "id": {
+                    "description": "Mailbox public ID",
+                    "example": "mbx_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                    "type": "string"
+                  }
+                },
+                "required": [
+                  "id",
+                  "email",
+                  "display_name"
+                ],
+                "type": "object"
+              },
+              "states": {
+                "additionalProperties": false,
+                "properties": {
+                  "folders": {
+                    "type": "string"
+                  },
+                  "identities": {
+                    "type": "string"
+                  },
+                  "messages": {
+                    "type": "string"
+                  },
+                  "quotas": {
+                    "type": "string"
+                  },
+                  "submissions": {
+                    "type": "string"
+                  },
+                  "threads": {
+                    "type": "string"
+                  }
+                },
+                "required": [
+                  "messages",
+                  "folders",
+                  "threads",
+                  "submissions",
+                  "identities",
+                  "quotas"
+                ],
+                "type": "object"
+              }
+            },
+            "required": [
+              "mailbox",
+              "states",
+              "capabilities",
+              "limits",
+              "gated",
+              "endpoints"
+            ],
+            "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": "MailboxSessionResponse"
+  }
+}

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

First observedv1.0.0Jul 13, 2026

TDQS

A3.9/5.0

Behavior3/5

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

Annotations already establish read-only, idempotent, open-world, and non-destructive behavior, so the description does not need to restate safety. The description adds some context by specifying what the session discovery is for, but it does not add rich behavioral detail such as caching behavior beyond what the schema already documents. With annotations carrying the safety profile, a 3 is appropriate.

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 front-loaded sentences, with the primary use case stated first and the exclusion second. No filler or redundant restatement of the name or schema.

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 no required parameters, a complete output schema, and rich annotations, the description covers what an agent needs to select and invoke the tool correctly. Its only weakness is sibling differentiation, already accounted for in purpose and usage dimensions, so it is contextually complete for its complexity.

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 both mailbox_id and If-None-Match are already documented in the schema; the description does not need to repeat them. The description contributes no additional parameter-level meaning, leaving the baseline of 3 appropriate.

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

Purpose4/5

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

The description uses a specific verb ('discover') and resource scope ('supported mailbox capabilities, limits, and endpoint metadata'), making the core purpose clear. It also disambiguates from message-search tools by saying 'do not use it for message search.' However, it does not differentiate itself from the closely named sibling mailbox_get_connection, so it falls short of full sibling distinction.

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

Usage Guidelines4/5

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

It gives a clear trigger: 'Use this before complex mailbox work' and an explicit negative: 'Do not use it for message search.' It does not name an alternative tool for message search or for connection-level checks, so usage guidance is clear but not fully complete.

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