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ni-c

calibreweb-mcp

by ni-c
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TypeScript
Remote

Get a book cover

get_cover
Read-onlyIdempotent

Retrieve a book's cover image by its numeric ID. Returns the image directly; files over 1 MB are refused to protect context—use the coverUrl from the book entry for full-size covers.

Instructions

Fetches the cover of a book and returns it as an image. Book ids come from the other tools (books with id null have no cover). Calibre-Web serves the full-size cover; images over 1 MB are refused to protect the context window — use the coverUrl from the book entry instead.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
book_idYesNumeric book id from search_books, list_books or get_shelf_books

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
bytesYesSize of the decoded image.
bookIdYes
mimeTypeYesOf the image in the content block.

Schema Changelog

Changes observed during successful MCP inspections. Dates show when Glama detected each change.

  1. Changed2 schema fields changedv0.3.0
    • changedInput schema / $schema
      Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"
    • changedOutput schema / (root)
      Previous value: -nullNew value: +{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "additionalProperties": false,
+  "properties": {
+    "bookId": {
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "bytes": {
+      "description": "Size of the decoded image.",
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "mimeType": {
+      "description": "Of the image in the content block.",
+      "enum": [
+        "image/jpeg",
+        "image/png",
+        "image/gif",
+        "image/webp"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "bookId",
+    "mimeType",
+    "bytes"
+  ],
+  "type": "object"
+}
  2. First observedv0.1.2

TDQS

A4.9/5.0
Behavior5/5

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

Annotations already declare readOnlyHint and idempotentHint, so the safety profile is covered. The description adds meaningful behavioral details beyond annotations: the tool returns an image, refuses images over 1 MB to protect the context window, and points to a workaround. This is exactly the kind of contextual behavior an agent needs to know.

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 sentences, no fluff, and each sentence carries unique information: what it does, where ids come from, and the size limit with workaround. Critical details are front-loaded before edge-case warnings.

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 single-parameter tool with an output schema, annotations, and a clear description, nothing essential is missing. The description covers return type, id provenance, missing-cover case, size refusal behavior, and the alternative, so an agent can invoke it correctly without additional inference.

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?

The schema already documents the only parameter well, including provenance from specific sibling tools, giving a baseline of 3. The description adds extra value by noting that books with id null have no cover, which prevents a pointless call and clarifies a subtle edge case.

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 ('Fetches'), resource ('cover of a book'), and return type ('image'), which makes the tool's function immediately clear. It also distinguishes itself from sibling tools by referencing that book ids come from search/list/shelf tools, so there is no confusion about role.

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?

Provides explicit guidance on when to use the tool and when not to: ids come from other tools, null-id books have no cover, and large images are refused so the agent should use coverUrl instead. This gives both a precondition and a clear fallback alternative.

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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