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karbassi

slack-mcp

by karbassi

chat_unfurl

Add custom unfurls to Slack messages by associating URLs with attachment or Block Kit objects. Control user authentication prompts and display custom content when links are shared.

Instructions

Provide custom unfurl behavior for URLs in messages.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
channelYesID of the channel containing the message with URLs to unfurl (e.g. ``C0123``).
tsYesTimestamp of the message containing the URLs (e.g. ``1700000000.000100``).
unfurlsYesMap of URLs to their unfurl attachment or Block Kit objects (e.g. ``{"https://example.com": {...}}``).
user_auth_blocksNoBlock Kit blocks shown to the user in an auth prompt when ``user_auth_required`` is True.
user_auth_messageNoPlain-text message shown to the user in an auth prompt when ``user_auth_required`` is True.
user_auth_requiredNoPrompt the user to authenticate before unfurling the URL.
user_auth_urlNoURL to redirect the user to for authentication when ``user_auth_required`` is True.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault

No arguments

Behavior2/5

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

With no annotations provided, the description must convey behavioral traits. It only says 'Provide custom unfurl behavior' without disclosing that unfurling is a mutation (removing existing previews), potential auth requirements, or side effects like rate limits. This omission burdens the agent.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, front-loaded sentence that efficiently conveys the core function. It is concise and easily parsed, though its brevity may sacrifice necessary detail.

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

Completeness2/5

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

Given the tool's complexity (7 params, nested objects, output schema) and a large set of siblings, the description is inadequate. It fails to explain the unfurling workflow, when to use auth-related parameters, or what the output contains. The output schema exists but the description doesn't leverage it.

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?

All 7 parameters are documented in the schema (100% coverage), so the description is not required to elaborate. The description adds no extra meaning beyond the schema; it does not mention any parameter purpose or constraints. Baseline of 3 is 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 clearly states the tool's purpose: providing custom unfurl behavior for URLs in messages. It identifies a specific verb ('Provide') and resource ('URLs in messages'), and while sibling tools like chat_post_message exist, none directly compete with unfurling, making it reasonably distinct.

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

Usage Guidelines2/5

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

The description offers no guidance on when to use this tool versus alternatives, nor does it specify prerequisites (e.g., app permissions, message ownership). It merely states what it does without contextualizing its usage relative to the many sibling chat tools.

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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