openlibrary-mcp-server

Get Cover URL

openlibrary_get_cover_url

Read-onlyIdempotent

Resolve a cover image URL for a book or author photo. Returns a direct HTTPS URL in the requested size (S/M/L). The Covers API always returns HTTP 200 — missing covers return a 1×1 placeholder GIF, not a 404 — so the identifier format is validated locally first: "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 before any request. URLs can be embedded in markdown as cover .

Input Schema

TableJSON 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

TableJSON Schema

Name	Required	Description
`url`	No	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.
`note`	No	Reminder that the URL always returns HTTP 200; a placeholder GIF is served when no cover exists.
`error`	No	Present when the call failed. Absent on success.

Schema Changelog

Changes observed during successful MCP inspections.

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

A4.6/5.0

Behavior5/5

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

Annotations already mark readOnly and idempotent, but the description adds critical non-obvious behavior: the Covers API always returns HTTP 200, missing covers return a placeholder GIF, identifiers are validated locally before any request, and author-by-ISBN lookups are rejected. This goes well beyond what annotations provide.

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 front-loaded with the core purpose and then layers behavioral and validation detail. It is longer than typical but every sentence conveys necessary information; the density is justified by the number of validation edge cases, though it could be broken into clearer subsections.

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?

The description covers output (HTTPS URL, size), behavioral quirks (placeholder, always-200), and validation rules, and an output schema exists. The only notable omission is what happens when validation rejects an identifier (the error path), and the exact URL format is not elaborated, though this may be in the output schema.

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

Parameters5/5

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

Even though schema coverage is 100%, the description enriches parameters materially: it defines local validation rules, ISBN-10 X check digit, valid OLID suffixes per target, and explicit rejection categories (path separators, control characters, author-by-ISBN). These are not present in the schema descriptions and are essential for correct invocation.

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 a specific verb ('Resolve') and resource ('cover image URL'), distinguishes book vs author, and immediately differentiates itself from sibling data-fetching and search tools. An agent can tell this is the only tool that produces cover URLs without inspecting schemas.

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 implies usage context by describing exactly what the tool returns (direct HTTPS URL in S/M/L) and sets expectations about validation. However, it does not explicitly compare with alternatives or state when not to use it, though no sibling tool competes for the same purpose.

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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