Kotlin & Java Library Sources

kotlin-lib-mcp

Give your AI agent the real sources of any Maven-published Kotlin/Java library.

An MCP server that, on request, downloads the sources of a library (e.g. io.ktor:ktor-client-core:3.5.1), parses them with the Kotlin Analysis API (standalone K2/FIR mode), and exposes structured information — public API surface, KDoc, dependencies/metadata, raw source + search — to MCP clients: Claude Code, Claude Desktop, IntelliJ IDEA (AI Assistant / Junie), VS Code and GitHub Copilot. An optional Compose Desktop dashboard runs the same server in-process.

[Twelve tools](#tools) — fetch_library · list_packages · list_declarations · get_api_signature · get_kdoc · get_source · search_source · diff_versions · diff_api · get_dependencies · list_versions · get_latest_version — plus MCP resources and a prompt.

Those install the Docker image. For the Claude Code plugin (which runs on your JDK), IntelliJ IDEA, or to run the release zip without Docker, see Quick start.

Compose Desktop dashboard

Why this and not a docs-lookup server?

Most documentation MCP servers scrape rendered doc sites or feed the model pre-digested summaries. This one works from the published sources jar — the ground truth:

• Resolved signatures, not regex guesses. Declarations are analyzed with the same Analysis API that powers the Kotlin IDE, so get_api_signature returns real, type-resolved signatures (with graceful best-effort fallback when transitive dependencies are missing).
• KMP-aware. Kotlin Multiplatform libraries publish per-target sources jars; these are resolved properly via .module Gradle metadata, and every symbol is tagged with its targets.
• KDoc as data. Summaries, descriptions and tags are extracted per declaration — not whole HTML pages.
• Exact version you asked for, offline after the first fetch. Everything is cached on disk keyed by group/artifact/version; no re-downloads, no drift between the docs and the version you actually depend on.
• Raw source when you need it. get_source and bounded search_source let the agent read the actual implementation, not just the API.

Quick start (no build required)

Option 1 — Claude Code plugin. The server plus skills that make Claude reach for it, two commands (/kotlin-lib:api, /kotlin-lib:migrate) and a setup helper. Needs a JDK 25+ — the one on your PATH, or any other set by path in the plugin's java option; no Docker. It downloads the release zip on first use:

/plugin marketplace add aoreshkov/kotlin-lib-mcp

/plugin install kotlin-lib@kotlin-lib-mcp

See plugin/README.md for what it bundles.

Option 2 — release zip. Download the latest release, unzip (needs a Java 25+ runtime), then:

claude mcp add kotlin-lib -- /path/to/kotlin-lib-mcp-server-<version>/bin/server --transport stdio

Option 3 — Docker.

claude mcp add kotlin-lib -- docker run -i --rm -v kotlin-lib-mcp-cache:/home/mcp/.cache ghcr.io/aoreshkov/kotlin-lib-mcp

Option 4 — IntelliJ IDEA / Android Studio. JetBrains IDEs are MCP clients too — which is where most Kotlin gets written. Open Settings | Tools | AI Assistant | Model Context Protocol (MCP), click Add, pick the stdio transport and paste:

{

"mcpServers": { "kotlin-lib": { "command": "docker", "args": ["run", "-i", "--rm", "-v", "kotlin-lib-mcp-cache:/home/mcp/.cache", "ghcr.io/aoreshkov/kotlin-lib-mcp"] } } }

Choose global or project level, Apply, and the tools appear in the AI Assistant chat. Junie takes the same JSON in its own MCP settings. Swap command/args for the release-zip launcher (bin/server --transport stdio) if you'd rather not use Docker.

Option 5 — MCP Registry. The server is published to the official MCP registry as io.github.aoreshkov/kotlin-lib-mcp, and listed in the GitHub MCP Registry; registry-aware clients can install it from there. The entry declares two optional arguments a client can ask you for when it installs: --repo, for each extra Maven repository (a private one, or https://maven.google.com for AndroidX), and --max-concurrent-fetches.

Or in .mcp.json / Claude Desktop config:

{

"mcpServers": { "kotlin-lib": { "command": "C:/path/to/kotlin-lib-mcp-server- /bin/server.bat", "args": ["--transport", "stdio"] } } }

Streamable HTTP. Run --transport http --port 3000 and point the client at http://127.0.0.1:3000/mcp. Out of the box it is reachable from this machine only: the server binds to loopback, and DNS-rebinding protection rejects any Host (or Origin) header other than localhost, 127.0.0.1 or [::1]. There is no built-in authentication, so to serve other machines put an authenticating reverse proxy in front, then bind wider with --host (e.g. 0.0.0.0) and name the public host with --allowed-host (plus --allowed-origin for browser clients).

In Docker, loopback is the container's own, which a published port cannot reach. Bind every interface inside the container and publish the port on the host's loopback instead — the endpoint is still http://127.0.0.1:3000/mcp, and still unreachable from other machines:

docker run --rm -p 127.0.0.1:3000:3000 -v kotlin-lib-mcp-cache:/home/mcp/.cache \

ghcr.io/aoreshkov/kotlin-lib-mcp --transport http --host 0.0.0.0 --port 3000

CLI flags — server --help lists them all with their defaults, the OTEL_* variables and examples: --transport stdio|http; --port, --host, --allowed-host and --allowed-origin, which apply to the http transport only, so passing one with stdio is a usage error; --cache-dir; --repo (repeatable, searched after Maven Central) and --no-maven-central (only the --repo repositories, for a mirror that proxies Central); --max-concurrent-fetches; and the opt-ins --forward-logs-to-client (stdio only), --otel (see Telemetry) and --tasks (see Tasks). --version prints the server's version. A command-line mistake exits 2 and an internal error 70 (sysexits.h). SIGTERM or Ctrl-C stops the server cleanly within 5 seconds: task records are saved and buffered traces flushed before the process ends.

Tools

All tools take a Maven coordinate (group:artifact:version). Call `fetch_library` first — it downloads, extracts and analyzes the sources once; every other tool answers from the cached index. fetch_library, list_versions and get_latest_version also accept group:artifact, and fetch_library accepts group:artifact:latest to resolve the latest stable release.

Every tool ships the metadata the MCP spec encourages clients to use: a display title, behavior annotations (readOnlyHint: true everywhere except fetch_library, which is additive-only — destructiveHint: false, idempotentHint: true; tools that reach Maven repositories set openWorldHint: true, cache-only tools false), a typed `outputSchema` derived from the response DTO's serializer, and an icon. Results carry both compact JSON text and the matching structuredContent object, so structured-output clients and plain-text clients see the same payload.

fetch_library also reports progress notifications (download → analyze → cache) when the client sends a progressToken. Logs go to stderr by default (which the spec blesses for all stdio logging); the deprecated MCP logging capability — mirroring logs to clients as notifications/message (respecting logging/setLevel) — is opt-in via --forward-logs-to-client, for stdio clients that surface MCP log messages but drop stderr. It is refused with --transport http, where every session would receive every other session's logs.

Elicitation

When fetch_library is called without a version (io.ktor:ktor-client-core, or …:latest) it has to guess. If the client advertised the `elicitation` capability, it asks instead: an elicitation/create form-mode request carrying a single-select version picker — the titled oneOf shape from SEP-1330, with the latest stable release pre-selected as the schema default.

| declines | fetches the latest stable release, as it