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Validate a post

validate_post
Read-only

Checks a post against every channel it names and publishes NOTHING. Returns every problem at once, each naming the channel and the field. Call this before create_post every time — it costs nothing and it is the difference between fixing a post once and learning the rules by publishing badly.

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

A4.7/5.0
Behavior5/5

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

Beyond the readOnlyHint annotation, the description adds important behavioral context: it publishes nothing, returns every problem at once with each naming the channel and field, and carries no cost. This clarifies return shape and side-effect behavior where no output schema exists.

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?

Three tightly written sentences with zero waste. The core behavior and non-publishing guarantee are front-loaded, and the call-to-action is direct and memorable.

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 read-only validation tool with rich schema coverage and no output schema, the description supplies the essential missing context: what the return value contains (every problem, channel and field) and the safety guarantee (publishes nothing). Nothing needed to call it correctly is absent.

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 documents all seven parameters in detail. The description only mentions channels implicitly and adds no syntax or format guidance beyond what the schema provides, so the baseline 3 applies.

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 states a specific verb and resource: it checks a post against every named channel and explicitly does not publish. It also distinguishes this validation tool from the sibling create_post, making the purpose unmistakable.

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 instructs the agent to call this before create_post every time and explains why: it costs nothing and prevents learning rules by publishing badly. The alternative (skipping validation) is implicitly discouraged, and the trigger condition is clear.

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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