mcp-open-library

Get book cover URL

get_book_cover

Read-only

Retrieve a book cover image URL using an ISBN, OCLC, LCCN, OLID, or Open Library ID, with optional size. Reports when no cover exists instead of returning a blank placeholder URL.

Instructions

Get the URL for a book's cover image using a key (ISBN, OCLC, LCCN, OLID, ID) and value. Reports when no cover exists rather than returning a URL to a blank placeholder.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "key": "<key>",
      "value": "<value>"
    },
    "name": "get_book_cover"
  }
}

Input Schema

Name	Required	Description	Default
`key`	Yes	The type of identifier used (ISBN, OCLC, LCCN, OLID, ID). ID is Open Library's internal cover ID.
`size`	No	The desired size of the cover (S, M, or L). Defaults to L.	L
`value`	Yes	The value of the identifier.

Schema Changelog

Changes observed during successful MCP inspections.

Changed4 schema fields changedv1.2.1Aug 19, 2026

changedInput schema / properties / key / description

Previous value: -"The type of identifier used (ISBN, OCLC, LCCN, OLID, ID)."New value: +"The type of identifier used (ISBN, OCLC, LCCN, OLID, ID). ID is Open Library's internal cover ID."

addedInput schema / properties / size / default
```
Added value: +"L"
```

changedInput schema / properties / size / description

Previous value: -"The desired size of the cover (S, M, or L)."New value: +"The desired size of the cover (S, M, or L). Defaults to L."

addedInput schema / properties / value / minLength
```
Added value: +1
```

First observedv1.0.0Aug 14, 2025

TDQS

A4.2/5.0

Behavior4/5

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

The description adds behavior beyond the readOnlyHint and openWorldHint annotations by stating it 'reports when no cover exists rather than returning a URL to a blank placeholder.' This is a valuable edge-case disclosure not captured elsewhere, though it doesn't cover other potential behaviors like rate limits or authentication.

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?

The description is two sentences, front-loads the action and input types, and contains no redundant phrases. Every word contributes to understanding the tool's function and behavior.

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?

Given the tool's simplicity (3 parameters, no output schema, read-only annotations), the description adequately covers the purpose, identifier types, and a key edge-case (no cover). It could mention the size parameter or URL format, but those are already in the schema, so it feels complete enough.

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?

The input schema already provides 100% coverage with detailed descriptions for all three parameters, including enums and defaults. The description only repeats the key types (already in schema) and adds no additional semantic meaning beyond what is structured, so the baseline of 3 is appropriate.

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 retrieves a cover image URL using a specified key type and value. It specifies the exact identifier types (ISBN, OCLC, etc.), and the verb 'Get' with resource 'URL for a book's cover image' distinguishes it from sibling tools like get_book_by_id or search_books.

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 description provides context on when to use the tool by listing the required identifier types, but it does not explicitly contrast it with alternatives or state when not to use it. The purpose is clear enough that an agent would infer usage for cover retrieval, but explicit exclusions are missing.

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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