openlibrary-mcp-server

Get Cover URL

openlibrary_get_cover_url

Read-onlyIdempotent

Construct a cover image URL for a book or author photo without fetching the image. Returns a direct HTTPS URL in the requested size (S/M/L). The identifier format is validated locally: "id" must be numeric, "isbn" 10 or 13 digits (an ISBN-10 may end in an X check digit), "olid" an edition OLID (OL…M) for target "book" and an author OLID (OL…A) for target "author". Identifiers with path separators or control characters, and author-by-ISBN lookups, are rejected. A returned URL does not verify that a cover exists.

Input Schema

Name	Required	Description	Default
`size`	No	Image size. S = small (~45px tall), M = medium (~150px tall), L = large (~400px tall).	M
`target`	No	"book" returns a book cover from covers.openlibrary.org/b/. "author" returns an author photo from covers.openlibrary.org/a/ — use with id_type "id" (photo_id) or "olid" (author OLID).	book
`id_type`	Yes	"id" is the numeric cover_i / cover ID from search or work results. "isbn" and "olid" look up the cover from those identifiers.
`identifier`	Yes	The identifier value, validated against id_type before the URL is built. For "id": a numeric cover or photo ID from work/edition/author data. For "isbn": 10 or 13 digits, hyphens optional — an ISBN-10 may end in an X check digit (e.g., 080442957X). For "olid": an edition OLID (OL…M) for target "book", an author OLID (OL…A) for target "author".

Output Schema

Name	Required	Description
`url`	No	Direct HTTPS URL to the cover image; not fetched or verified by this tool.
`note`	No	Reminder that the image is unverified and a missing cover may return a placeholder.
`error`	No	Present when the call failed. Absent on success.

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 26, 2026

changedOutput schema / properties / note / description

Previous value: -"Reminder that the URL always returns HTTP 200; a placeholder GIF is served when no cover exists."New value: +"Reminder that the image is unverified and a missing cover may return a placeholder."

changedOutput schema / properties / url / description

Previous value: -"Direct HTTPS URL to the cover image. The Covers API returns HTTP 200 for all requests — a 1×1 placeholder GIF is returned when no cover exists for the identifier."New value: +"Direct HTTPS URL to the cover image; not fetched or verified by this tool."

Changed1 schema field changedSep 24, 2026

changedInput schema / properties / identifier / description

Previous value: -"The identifier value, validated against id_type before the URL is built. For \"id\": a numeric cover or photo ID from work/edition/author data. For \"isbn\": 10 or 13 digits, hyphens optional. For \"olid\": an edition OLID (OL…M) for target \"book\", an author OLID (OL…A) for target \"author\"."New value: +"The identifier value, validated against id_type before the URL is built. For \"id\": a numeric cover or photo ID from work/edition/author data. For \"isbn\": 10 or 13 digits, hyphens optional — an ISBN-10 may end in an X check digit (e.g., 080442957X). For \"olid\": an edition OLID (OL…M) for target \"book\", an author OLID (OL…A) for target \"author\"."

Changed6 schema fields changedAug 25, 2026

changedInput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedInput schema / additionalProperties
```
Added value: +false
```

changedOutput schema / $schema

Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"

addedOutput schema / anyOf

Added value: +[
+  {
+    "not": {
+      "required": [
+        "error"
+      ]
+    },
+    "required": [
+      "url",
+      "note"
+    ]
+  },
+  {
+    "required": [
+      "error"
+    ]
+  }
+]

addedOutput schema / properties / error

Added value: +{
+  "additionalProperties": {},
+  "description": "Present when the call failed. Absent on success.",
+  "properties": {
+    "code": {
+      "description": "JSON-RPC error code for this failure.",
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "data": {
+      "additionalProperties": {},
+      "properties": {
+        "reason": {
+          "description": "Machine-readable failure mode. Declared by this tool: `invalid_identifier`: The identifier contains path separators, \"..\", or control characters, or does not match the format its id_type expects. `invalid_target`: The id_type is not valid for the target — an author photo cannot be resolved by ISBN. Other values are possible when a failure originates below the handler.",
+          "examples": [
+            "invalid_identifier",
+            "invalid_target"
+          ],
+          "type": "string"
+        },
+        "recovery": {
+          "additionalProperties": {},
+          "description": "Actionable next step for the caller.",
+          "properties": {
+            "hint": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "hint"
+          ],
+          "type": "object"
+        },
+        "retryable": {
+          "description": "Whether retrying may succeed.",
+          "type": "boolean"
+        }
+      },
+      "type": "object"
+    },
+    "message": {
+      "description": "Human-readable description of what went wrong.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "code",
+    "message"
+  ],
+  "type": "object"
+}

removedOutput schema / required

Removed value: -[
-  "url",
-  "note"
-]

Changed1 schema field changedJul 26, 2026

changedInput schema / properties / identifier / description

Previous value: -"The identifier value. For \"id\": numeric cover ID from work/edition data. For \"isbn\": 10 or 13 digits, hyphens stripped. For \"olid\": edition OLID (OL…M) or author OLID (OL…A) when target is \"author\"."New value: +"The identifier value, validated against id_type before the URL is built. For \"id\": a numeric cover or photo ID from work/edition/author data. For \"isbn\": 10 or 13 digits, hyphens optional. For \"olid\": an edition OLID (OL…M) for target \"book\", an author OLID (OL…A) for target \"author\"."

First observedMay 25, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Annotations already establish read-only/idempotent, and the description goes well beyond them: validation happens locally, no network request is made, a returned URL does not verify a cover exists, and author-by-ISBN is refused. Those caveats are exactly the behavioral facts an agent needs to avoid misinterpreting a synthesized URL as confirmed content.

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?

Front-loaded with the core action and the no-fetch property, then the constraints. The identifier-format sentence partially restates schema enums/descriptions, which costs a little density, but every sentence carries either scope or a validation rule.

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?

An output schema exists so return shape need not be explained, and the description covers validation, non-fetching behavior, and the non-verification caveat. The only residual gap is that it never routes the agent to a sibling when the identifier cannot be used.

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?

Schema coverage is 100%, so the baseline is 3; the description still adds value by explaining per-id_type validation (numeric id, 10/13-digit ISBN with optional X check digit, OL…M vs OL…A OLID depending on target) and the cross-parameter constraint that author plus ISBN is rejected. That constraint and validation behavior are not visible in the schema.

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 and resource ("Construct a cover image URL") and immediately scopes it with "without fetching the image," which separates it from every sibling that retrieves author/work/edition data. An agent can distinguish this URL-builder from the fetch-style tools without opening any schema.

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 rejection rules (path separators, control characters, author-by-ISBN) effectively define when the tool will not work, and the no-fetch framing implies when to prefer it over a data-fetching sibling. It stops short of naming a specific alternative tool to use instead when the call is invalid.

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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