Sendmux Email Inbox API + Sending

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Get Attachment Metadata

sending_get_attachment

Read-onlyIdempotent

Retrieve metadata for a temporary attachment ID before sending an email. Verify attachment details without downloading file bytes.

Instructions

Use this to inspect a temporary Sending attachment_id before sending. It returns metadata only, not file bytes.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`attachment_id`	Yes	Temporary attachment ID returned by an upload endpoint.

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": false,
+            "properties": {
+              "request_id": {
+                "description": "Unique request identifier",
+                "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id"
+            ],
+            "type": "object"
+          },
+          "ok": {
+            "enum": [
+              true
+            ],
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "ok",
+          "meta"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "data": {
+            "additionalProperties": false,
+            "properties": {
+              "attachment_id": {
+                "description": "Temporary attachment ID to use in email attachments[].",
+                "example": "att_tz4a98xxat96iws9zmbrgj3a",
+                "pattern": "^att_[a-z0-9]{24}$",
+                "type": "string"
+              },
+              "content_type": {
+                "description": "Stored attachment MIME type",
+                "example": "application/pdf",
+                "type": "string"
+              },
+              "expires_at": {
+                "description": "ISO timestamp when this temporary attachment reference expires.",
+                "example": "2026-07-07T08:15:00.000Z",
+                "format": "date-time",
+                "type": "string"
+              },
+              "filename": {
+                "description": "Stored attachment filename",
+                "example": "analysis.pdf",
+                "type": "string"
+              },
+              "size_bytes": {
+                "description": "Stored attachment byte size",
+                "example": 20480,
+                "type": "integer"
+              }
+            },
+            "required": [
+              "attachment_id",
+              "filename",
+              "content_type",
+              "size_bytes",
+              "expires_at"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "AttachmentUploadResponse"
+  }
+}

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

First observedv1.0.0Jul 13, 2026

TDQS

A4.3/5.0

Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context beyond annotations: it returns metadata only, not file bytes, and operates on a temporary attachment, which helps set agent expectations.

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 sentences with no filler. It front-loads the usage directive and immediately states the return behavior, making the tool's core purpose and limitation immediately visible.

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?

For a one-parameter tool with an output schema and full annotation coverage, the description provides all necessary context: what it does, when to use it, and what it returns. Nothing an agent needs to call it correctly is missing.

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 coverage is 100%, so the schema already fully describes attachment_id as a temporary upload ID. The description reinforces 'temporary' and 'Sending' context, but adds little semantic value beyond what the schema provides, staying at the baseline for high schema coverage.

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 clearly states the action ('inspect a temporary Sending attachment_id'), the resource, and the timing ('before sending'). It also explicitly distinguishes the output as 'metadata only, not file bytes', differentiating it from attachment-reading tools.

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?

The phrase 'before sending' provides clear contextual timing for when this tool is appropriate, implying use after upload and before send. It does not explicitly name sibling alternatives or exclusions, but the context is unambiguous enough for simple tool selection.

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