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Important tools

  1. Cellar

When you need the API of a JVM dependency, always use cellar. Use it before metals-mcp.

Project-aware commands (run from project root)

For querying the current project's code and dependencies (auto-detects build tool):

cellar get [--module <name>] <fqn>       # single symbol
cellar list [--module <name>] <package>  # explore a package
cellar search [--module <name>] <query>  # find by name
  • Mill/sbt projects: --module is required (e.g. --module lib, --module core)
  • scala-cli projects: --module is not supported (omit it)
  • --no-cache: skip classpath cache, re-extract from build tool
  • --java-home: override JRE classpath

External commands (query arbitrary Maven coordinates)

For querying any published artifact by explicit coordinate:

cellar get-external <coordinate> <fqn>       # single symbol
cellar list-external <coordinate> <package>  # explore a package
cellar search-external <coordinate> <query>  # find by name
cellar deps <coordinate>                     # dependency tree

Coordinates must be explicit: group:artifact_3:version

Development Principles

  1. Think Before Coding

Don't assume. Don't hide confusion. Surface tradeoffs.

Before implementing:

State your assumptions explicitly. If uncertain, ask.
If multiple interpretations exist, present them - don't pick silently.
If a simpler approach exists, say so. Push back when warranted.
If something is unclear, stop. Name what's confusing. Ask.
  1. Simplicity First

Minimum code that solves the problem. Nothing speculative.

No features beyond what was asked.
No abstractions for single-use code.
No "flexibility" or "configurability" that wasn't requested.
No error handling for impossible scenarios.
If you write 200 lines and it could be 50, rewrite it.

Ask yourself: "Would a senior engineer say this is overcomplicated?" If yes, simplify.

  1. Surgical Changes

Touch only what you must. Clean up only your own mess.

When editing existing code:

Don't "improve" adjacent code, comments, or formatting.
Don't refactor things that aren't broken.
Match existing style, even if you'd do it differently.
If you notice unrelated dead code, mention it - don't delete it.

When your changes create orphans:

Remove imports/variables/functions that YOUR changes made unused.
Don't remove pre-existing dead code unless asked.

The test: Every changed line should trace directly to the user's request.

  1. Goal-Driven Execution

Define success criteria. Loop until verified.

Transform tasks into verifiable goals:

"Add validation" → "Write tests for invalid inputs, then make them pass"
"Fix the bug" → "Write a test that reproduces it, then make it pass"
"Refactor X" → "Ensure tests pass before and after"

For multi-step tasks, state a brief plan:

  1. [Step] → verify: [check]
  2. [Step] → verify: [check]
  3. [Step] → verify: [check]

Strong success criteria let you loop independently. Weak criteria ("make it work") require constant clarification.

  1. Don't write tests that test the compiler

Examples are testing match exhaustivity, typesystem etc.

Code Conventions

  • Use fs2.io.file.Path for file references, not java.io.File or java.nio.file.Path
  • Coursier error handling: match coursierapi.error.CoursierError, call CoordinateCompleter.suggest to attach suggestions to CellarError.CoordinateNotFound

Documentation / Development history

All plans, decision and architecture designs are available in ai/ folder