Scala Setup Guide for Serena

Scala Setup Guide for Serena#

This guide explains how to prepare a Scala project so that Serena can provide reliable code intelligence via Metals (Scala LSP) and how to run Scala tests manually.

Serena automatically bootstraps the Metals language server using Coursier when needed. Your project, however, must be importable by a build server (BSP) — typically via Bloop or sbt’s built‑in BSP — so that Metals can compile and index your code.

Prerequisites#

Install the following on your system and ensure they are available on PATH:

  • Java Development Kit (JDK). A modern LTS (e.g., 17 or 21) is recommended.

  • sbt

  • Coursier command (cs) or the legacy coursier launcher

    • Serena uses cs if available; if only coursier exists, it will attempt to install cs. If neither is present, install Coursier first.

Quick Start#

Start Serena in your project root. Metals asks whether to import a workspace it has not seen before, and Serena answers that prompt for it — the build is imported (for sbt, by running sbt bloopInstall), .bloop/ and .metals/ are created, and cross-file navigation works from there. The first run therefore takes as long as your build takes to load.

Set auto_import_build: false under ls_specific_settings.scala to decline instead; you then need to import the build yourself by one of the routes below, or cross-file queries will be served by the fallback presentation compiler and see only one file at a time.

Serena answers three of Metals’ prompts — “Import build”, “Import changes”, “Connect”. Anything else Metals asks is dismissed and logged, including “Multiple build definitions found. Which would you like to use?”, so a workspace holding more than one kind of build (say both an sbt and a Maven definition) still needs importing by one of the routes below.

Importing the build yourself (VS Code)#

  1. Open your Scala project in VS Code.

  2. When prompted by Metals, accept “Import build”. Wait until the import and initial compile/indexing finish.

  3. Run the “Connect to build server” command (id: build.connect).

  4. Once the import completes, start Serena in your project root and use it.

This flow ensures the .bloop/ and (if applicable) .metals/ directories are created and your build is known to the build server that Metals uses.

Importing the build yourself (No VS Code)#

Follow these steps if you prefer a manual setup or you are not using VS Code:

These instructions cover the setup for projects that use sbt as the build tool, with Bloop as the BSP server.

  1. Add Bloop to project/plugins.sbt in your Scala project:

    // project/plugins.sbt
addSbtPlugin("ch.epfl.scala" % "sbt-bloop" % "<version>")

    Replace <version> with an appropriate current version from the Metals documentation.

  2. Export Bloop configuration with sources:

    sbt -Dbloop.export-jar-classifiers=sources bloopInstall

    This creates a .bloop/ directory containing your project’s build metadata for the BSP server.

  3. Compile from sbt to verify the build:

    sbt compile

  4. Start Serena in your project root. Serena will bootstrap Metals (if not already present) and connect to the build server using the configuration exported above.

Using Serena with Scala#

  • Serena automatically detects Scala files (*.scala, *.sbt) and will start a Metals process per project when needed.

  • On first run, you may see messages like “Bootstrapping metals…” in the Serena logs — this is expected.

  • Optimal results require that your project compiles successfully via the build server (BSP). If compilation fails, fix build errors in sbt first.

Notes:

  • Ensure you completed the manual or auto‑import steps so that the build is compiled and indexed; otherwise, code navigation and references may be incomplete until the first successful compile.

Monorepos: builds below the repository root#

Metals serves one build per workspace folder, so what it needs is the build roots, not the repository root. Serena detects them: if the repository root is not itself a build root (no build.sbt, build.mill, pom.xml, .bsp/, …), it searches up to three levels below for directories that are, and passes those to Metals — one Metals service per build.

Override the detection where it guesses wrong:

# ~/.serena/serena_config.yml or .serena/project.yml
ls_specific_settings:
  scala:
    project_roots: ["backend", "tooling/plugin"]  # relative to the repository root
    project_root_scan_depth: 3                    # only applies when project_roots is unset

Waiting for Metals to be ready#

Metals needs its build imported, its index built and the project compiled before it can answer a cross-file question completely — references in particular come from SemanticDB, which the build server writes only as it compiles. Serena waits for all of that, tracking the work-done progress Metals reports, before the first such query of a session; a query made earlier would return a fraction of the true result and look no different from a complete one.

The first find_referencing_symbols of a session therefore takes as long as the project takes to compile. Subsequent queries do not wait. Where the default bound is wrong for a large build:

ls_specific_settings:
  scala:
    indexing_timeout: 180        # seconds to wait before giving up and answering anyway
    indexing_start_grace: 15     # seconds to wait for Metals to report anything at all
    indexing_quiet_period: 3     # seconds of silence that count as "finished"

Running Multiple Metals Instances#

Serena can run alongside other Metals instances (e.g., VS Code with Metals extension) on the same project. This is fully supported by Metals via H2 AUTO_SERVER mode.

How It Works#

Metals uses an H2 database (.metals/metals.mv.db) to cache semantic information. When multiple Metals instances run on the same project:

  • H2 AUTO_SERVER: The first instance becomes the TCP server; subsequent instances connect as clients

  • Bloop Build Server: All instances share a single Bloop process (port 8212)

  • Compilation Results: Shared via Bloop — no duplicate compilation

Stale Lock Detection#

If a Metals process crashes without proper cleanup, it may leave a stale lock file (.metals/metals.mv.db.lock.db). This can prevent proper AUTO_SERVER coordination, causing new instances to fall back to in-memory database mode (degraded experience).

Serena automatically detects and handles stale locks based on your configuration:

# ~/.serena/serena_config.yml or .serena/project.yml
ls_specific_settings:
  scala:
    on_stale_lock: "auto-clean"      # auto-clean | warn | fail
    log_multi_instance_notice: true  # Log info when another Metals detected

Stale Lock Modes#

Mode

Behavior

auto-clean

(Default, Recommended) Automatically removes stale lock files and proceeds normally.

warn

Logs a warning but proceeds. Metals may use in-memory database (slower).

fail

Raises an error and refuses to start. Useful for debugging lock issues.

Reference#