AT Protocol MCP Server

reply_to_post

Reply to an existing post on AT Protocol to create a threaded reply with proper parent and root CID references. Use this when your content belongs inside an existing thread.

Instructions

Reply to an existing post on AT Protocol, creating a threaded reply with proper parent and root CID references. Requires authentication (app password). Creates a new post record linked into the thread; the action cannot be undone (use delete_post to remove it afterwards). Use this instead of create_post whenever the new content belongs inside an existing thread; use create_thread to start a multi-post thread from scratch. Subject to per-tool rate limiting.

Input Schema

TableJSON Schema

Name	Required	Description
`text`	Yes	Text content of the reply (1–300 graphemes / up to 3000 bytes). Mentions (@handle), links, and hashtags are automatically resolved into rich-text facets.
`root`	Yes	AT-URI of the top-level post that started the thread (at://did/app.bsky.feed.post/rkey). Must be the original root even when replying to a nested reply.
`parent`	Yes	AT-URI of the immediate post being replied to (at://did/app.bsky.feed.post/rkey). May equal root when replying directly to the thread starter.
`langs`	No	List of BCP-47 language tags indicating the language(s) of the reply text. Omit if unknown.

Output Schema

TableJSON Schema

Name	Required	Description
`uri`	Yes	AT-URI of the newly created reply post (at://did/app.bsky.feed.post/rkey).
`cid`	Yes	Content identifier (CID) of the newly created reply record.
`success`	Yes	True when the reply was created successfully.
`message`	Yes	Human-readable status message describing the outcome.
`replyTo`	Yes	The root and parent URIs that this reply is linked to.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv0.6.2Jun 23, 2026
- removedInput schema / additionalProperties
```
Removed value: -false
```

Changed7 schema fields changedv0.6.0Jun 11, 2026

addedInput schema / properties / langs / description

Added value: +"List of BCP-47 language tags indicating the language(s) of the reply text. Omit if unknown."

addedInput schema / properties / langs / items / description

Added value: +"A single BCP-47 language tag for the reply text (e.g. \"en\", \"en-US\", \"pt-BR\")."

addedInput schema / properties / parent / description

Added value: +"AT-URI of the immediate post being replied to (at://did/app.bsky.feed.post/rkey). May equal root when replying directly to the thread starter."

addedInput schema / properties / root / description

Added value: +"AT-URI of the top-level post that started the thread (at://did/app.bsky.feed.post/rkey). Must be the original root even when replying to a nested reply."

addedInput schema / properties / text / description

Added value: +"Text content of the reply (1–300 graphemes / up to 3000 bytes). Mentions (@handle), links, and hashtags are automatically resolved into rich-text facets."

changedInput schema / properties / text / maxLength
```
Previous value: -300New value: +3000
```

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "cid": {
+      "description": "Content identifier (CID) of the newly created reply record.",
+      "type": "string"
+    },
+    "message": {
+      "description": "Human-readable status message describing the outcome.",
+      "type": "string"
+    },
+    "replyTo": {
+      "description": "The root and parent URIs that this reply is linked to.",
+      "properties": {
+        "parent": {
+          "description": "AT-URI of the immediate post that was replied to.",
+          "type": "string"
+        },
+        "root": {
+          "description": "AT-URI of the top-level post that started the thread.",
+          "type": "string"
+        }
+      },
+      "required": [
+        "root",
+        "parent"
+      ],
+      "type": "object"
+    },
+    "success": {
+      "description": "True when the reply was created successfully.",
+      "type": "boolean"
+    },
+    "uri": {
+      "description": "AT-URI of the newly created reply post (at://did/app.bsky.feed.post/rkey).",
+      "type": "string"
+    }
+  },
+  "required": [
+    "uri",
+    "cid",
+    "success",
+    "message",
+    "replyTo"
+  ],
+  "type": "object"
+}

Changed4 schema fields changedv0.3.0Jun 9, 2026
- removedInput schema / $schema
```
Removed value: -"http://json-schema.org/draft-07/schema#"
```
- removedInput schema / properties / langs / items / maxLength
```
Removed value: -2
```
- removedInput schema / properties / langs / items / minLength
```
Removed value: -2
```
- addedInput schema / properties / langs / items / pattern
```
Added value: +"^[a-zA-Z]{2,3}(-[a-zA-Z0-9]{2,8})*$"
```
First observedv0.2.1Jun 2, 2026

TDQS

A4.9/5.0

Behavior5/5

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

Description adds value beyond annotations by noting it creates a new post record, cannot be undone (delete_post as fallback), requires authentication, and is subject to rate limiting. No contradiction with annotations (destructiveHint false, idempotentHint false, openWorldHint true).

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?

Four sentences that are front-loaded with the core action and then provide alternatives, behavioral notes, and constraints. No wasted words; every sentence adds value.

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 the tool's complexity (4 params, output schema), the description covers purpose, usage guidelines, behavioral traits, and parameter context adequately. Mentions rate limiting and undoability, leaving no critical gaps.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

While input schema covers all parameters with descriptions, the tool description adds semantic context for root and parent parameters (e.g., root must be original top-level post). Schema coverage is 100%, so baseline is 3; additional guidance justifies 4.

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 tool's purpose: replying to an existing post on AT Protocol with proper thread references. It uses specific verbs ('reply', 'creating') and distinguishes itself from siblings like create_post and create_thread.

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?

Explicitly states when to use this tool ('when the new content belongs inside an existing thread') and when to use alternatives ('use create_thread to start a multi-post thread from scratch'). Also mentions authentication requirements and that the action cannot be undone.

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