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Read-only

Renders what the post will look like on each network, without publishing. Use it when the question is how something will read rather than whether it is allowed; validate_post answers the second.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
draftNoKeep it in Mellow as a draft. Nothing is sent to any network.
mediaNoOrdered media. Each entry is a public https URL, or an object with url and an optional thumbnailUrl.
captionNoThe text of the post. Networks with shorter limits can be given their own with perChannel.
optionsNoPer-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). A TikTok that promotes the owner's own product or business takes discloseYourBrand: true (TikTok labels it Promotional content); one another brand paid for takes discloseBrandedContent: true. See mellow://guide/platforms for every field.
channelsYesChannel IDs from list_channels, each beginning spc_.
perChannelNoOverrides for one channel, keyed by channel ID. Accepts caption, media, and that network's options. This is how you give one channel a shorter caption without shortening the post everywhere.
scheduledAtNoISO 8601 time to publish at. Omit to publish immediately.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • changedInput schema / properties / options / description
      Previous value: -"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."New value: +"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). A TikTok that promotes the owner's own product or business takes discloseYourBrand: true (TikTok labels it Promotional content); one another brand paid for takes discloseBrandedContent: true. See mellow://guide/platforms for every field."
  2. Changed2 schema fields changed
    • changedInput schema / properties / media / items / anyOf
      Previous value: -[
-  {
-    "description": "A public https URL.",
-    "type": "string"
-  },
-  {
-    "properties": {
-      "thumbnailTimestampMs": {
-        "description": "Milliseconds into the video to take a poster frame from.",
-        "type": "number"
-      },
-      "thumbnailUrl": {
-        "description": "Poster frame, where the network accepts one.",
-        "type": "string"
-      },
-      "url": {
-        "description": "A public https URL.",
-        "type": "string"
-      }
-    },
-    "required": [
-      "url"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "description": "A public https URL.",
+    "type": "string"
+  },
+  {
+    "properties": {
+      "tags": {
+        "description": "Accounts marked on this item. Instagram and Facebook only, at most 20. Not the same as options.instagram.collaborators: a collaborator co-owns the post and it appears on their profile, a tag marks somebody on the picture. A bare string is an Instagram username.",
+        "items": {
+          "anyOf": [
+            {
+              "description": "An Instagram username.",
+              "type": "string"
+            },
+            {
+              "properties": {
+                "id": {
+                  "description": "Instagram username, Facebook user id, or Instagram product id.",
+                  "type": "string"
+                },
+                "kind": {
+                  "description": "Defaults to user. Products are Instagram only.",
+                  "enum": [
+                    "user",
+                    "product"
+                  ],
+                  "type": "string"
+                },
+                "platform": {
+                  "description": "Defaults to instagram.",
+                  "enum": [
+                    "instagram",
+                    "facebook"
+                  ],
+                  "type": "string"
+                },
+                "x": {
+                  "description": "Fraction from the left edge, 0 to 1. Send with y, or neither.",
+                  "type": "number"
+                },
+                "y": {
+                  "description": "Fraction from the top edge, 0 to 1. Videos and stories take no position.",
+                  "type": "number"
+                }
+              },
+              "required": [
+                "id"
+              ],
+              "type": "object"
+            }
+          ]
+        },
+        "maxItems": 20,
+        "type": "array"
+      },
+      "thumbnailTimestampMs": {
+        "description": "Milliseconds into the video to take a poster frame from.",
+        "type": "number"
+      },
+      "thumbnailUrl": {
+        "description": "Poster frame, where the network accepts one.",
+        "type": "string"
+      },
+      "url": {
+        "description": "A public https URL.",
+        "type": "string"
+      }
+    },
+    "required": [
+      "url"
+    ],
+    "type": "object"
+  }
+]
    • changedInput schema / properties / options / description
      Previous value: -"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest requires boardIds. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."New value: +"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."
  3. Changed2 schema fields changed
    • changedInput schema / properties / media / items / anyOf
      Previous value: -[
-  {
-    "description": "A public https URL.",
-    "type": "string"
-  },
-  {
-    "properties": {
-      "tags": {
-        "description": "Accounts marked on this item. Instagram and Facebook only, at most 20. Not the same as options.instagram.collaborators: a collaborator co-owns the post and it appears on their profile, a tag marks somebody on the picture. A bare string is an Instagram username.",
-        "items": {
-          "anyOf": [
-            {
-              "description": "An Instagram username.",
-              "type": "string"
-            },
-            {
-              "properties": {
-                "id": {
-                  "description": "Instagram username, Facebook user id, or Instagram product id.",
-                  "type": "string"
-                },
-                "kind": {
-                  "description": "Defaults to user. Products are Instagram only.",
-                  "enum": [
-                    "user",
-                    "product"
-                  ],
-                  "type": "string"
-                },
-                "platform": {
-                  "description": "Defaults to instagram.",
-                  "enum": [
-                    "instagram",
-                    "facebook"
-                  ],
-                  "type": "string"
-                },
-                "x": {
-                  "description": "Fraction from the left edge, 0 to 1. Send with y, or neither.",
-                  "type": "number"
-                },
-                "y": {
-                  "description": "Fraction from the top edge, 0 to 1. Videos and stories take no position.",
-                  "type": "number"
-                }
-              },
-              "required": [
-                "id"
-              ],
-              "type": "object"
-            }
-          ]
-        },
-        "maxItems": 20,
-        "type": "array"
-      },
-      "thumbnailTimestampMs": {
-        "description": "Milliseconds into the video to take a poster frame from.",
-        "type": "number"
-      },
-      "thumbnailUrl": {
-        "description": "Poster frame, where the network accepts one.",
-        "type": "string"
-      },
-      "url": {
-        "description": "A public https URL.",
-        "type": "string"
-      }
-    },
-    "required": [
-      "url"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "description": "A public https URL.",
+    "type": "string"
+  },
+  {
+    "properties": {
+      "thumbnailTimestampMs": {
+        "description": "Milliseconds into the video to take a poster frame from.",
+        "type": "number"
+      },
+      "thumbnailUrl": {
+        "description": "Poster frame, where the network accepts one.",
+        "type": "string"
+      },
+      "url": {
+        "description": "A public https URL.",
+        "type": "string"
+      }
+    },
+    "required": [
+      "url"
+    ],
+    "type": "object"
+  }
+]
    • changedInput schema / properties / options / description
      Previous value: -"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."New value: +"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest requires boardIds. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."
  4. Changed1 schema field changed
    • changedInput schema / properties / options / description
      Previous value: -"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest requires boardIds. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."New value: +"Per-network settings, keyed by platform: instagram, facebook, threads, tiktok, tiktok_business, youtube, x, linkedin, pinterest, bluesky. YouTube requires title. Pinterest pins to the account's first board unless boardIds names one, and a Pinterest video needs a cover: thumbnailUrl on the media item. Instagram and Facebook take placement (timeline, reels, stories). See mellow://guide/platforms for every field."
  5. First observed

TDQS

A4.4/5.0
Behavior4/5

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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is carried by structured data. The description adds the crucial reassurance that nothing is published, which matters because the schema's own fields (scheduledAt, media, caption) make this look like a mutating create_post call. It does not discuss rate limits, persistence of previews, or auth needs, so it stops short of full behavioral disclosure.

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, zero filler, and the non-publishing constraint is front-loaded before the sibling comparison. Every clause earns its place.

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

Completeness4/5

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

With no output schema, the description carries some burden for explaining what comes back, and 'renders what the post will look like on each network' reasonably implies a per-network preview payload. Given seven parameters and deep nesting, it leans heavily on the schema, but the routing and the no-publish guarantee are the pieces an agent most needs here.

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% and the nested options/media fields are richly documented in the schema itself, so the baseline of 3 applies. The description names no parameters and adds no syntax or format detail beyond what the schema already provides.

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?

States a specific verb (renders) and a specific resource (what the post will look like on each network), plus the key scoping constraint (without publishing). It explicitly names the sibling validate_post and what that tool covers instead, so an agent can separate the two without opening either schema.

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?

Gives an explicit selection rule: use this when the question is how something will read, use validate_post when the question is whether it is allowed. The alternative is named and the condition that selects it is stated, leaving nothing to inference.

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

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