Elixir Skills for Claude, Cursor, Codex

Nice Topic,

I’ve been building Giulia, an Elixir daemon that provides AST-level code intelligence via REST API. It parses your codebase with Sourceror, builds a dependency graph with libgraph, runs semantic search with Bumblebee/Nx (all-MiniLM-L6-v2, 80MB, on CPU), and stores everything in ETS. It runs on OTP, serves any client over HTTP, and responds in under 300ms.

The idea is simple: give AI coding agents (or any tool) structural understanding of Elixir codebases instead of letting them grep around like it’s 1985.

To test it on something I didn’t write, I cloned Commanded and ran a full analysis. Here’s what Giulia found.

The Headlines

• 66 modules, 438 functions, 500 graph vertices, 520 dependency edges

• Zero red-zone modules in the heatmap — speaks to Commanded’s maturity

• Zero behaviour fractures — every @behaviour contract is fully satisfied

• Zero orphan specs — every @spec matches its function

• Only 4 dead functions out of 438 (0.9%)

• 2 circular dependencies — one is a 10-module cycle through the entire command execution pipeline

• 7.3% spec coverage — only 32 specs for 438 functions across a public framework

The Insight That Static Analysis Misses

Most tools rank modules by complexity alone. Giulia combines complexity with topology — and the results tell a very different story.

Event.Handler has the highest complexity score in the project at 93. It has 36 functions across 1,549 lines and wears five hats: behaviour definition, macro, GenServer, event processor, and telemetry emitter.

Sounds dangerous, right? Except Giulia’s knowledge graph shows it has only 1 downstream dependent. It’s a leaf node. Refactoring it is low-risk.

Aggregates.Aggregate scores lower at 63 complexity. Only 716 lines, 28 functions, clean code with zero structural redundancy.

But it has degree 17 in the dependency graph — the highest in the project. Fan-in 6, fan-out 11, part of a 10-module circular dependency cycle. It’s the #1 change risk module (score 844) because modifying it affects the entire command execution pipeline from dispatch through persistence.

Half the code, lower complexity, but double the danger.

Any tool that ranks by one dimension misses this. Giulia gives both.

Three Core Modules, Side by Side

The three most important modules in the framework. Zero typespecs across all of them.

What Giulia Exposes via REST

Every endpoint responds in under 300ms, project-scoped with a ?path= parameter. Multi-project support — one daemon, multiple codebases indexed simultaneously.

Code Understanding: modules, functions, specs, types, callbacks, structs, module details (one call, full profile)

Knowledge Graph: impact maps (blast radius at depth N), dependents, dependencies, centrality, dependency path tracing, cycle detection

Health Metrics: change risk scores, god modules, fan-in/fan-out, coupling analysis, API surface ratios, heatmap (red/yellow/green zones), dead code detection, orphan specs, behaviour integrity

Semantic Search: two-stage retrieval — Bumblebee embeds module docs + function signatures, cosine similarity finds code by intent (“entity movement physics” finds set_velocity/2 without keyword match), surgical briefings combine semantic results with knowledge graph data

The Stack

All Elixir. All on the BEAM.

• Sourceror for AST parsing and code analysis

• libgraph for dependency topology, Dijkstra pathfinding, cycle detection

• Bumblebee + Nx (EXLA) for semantic embeddings (all-MiniLM-L6-v2)

• ETS for the artifact store (modules, functions, ASTs, vectors)

• Bandit + Plug for the REST API

• OTP supervision for the whole thing

No Python. No external vector database. No sidecar services. One supervised Elixir application.

I work with AI coding agents daily. They’re powerful but architecturally blind — they understand code as text, not as structure. They grep for function definitions instead of querying a graph. They can’t tell you the blast radius of a change or whether a module is a hub or a leaf.

Giulia gives them eyes. One API call returns what would take 10+ grep/read cycles and thousands of context window tokens. The AI agent I use reports ~80-90% token savings and calls the impact/centrality endpoints mandatory before planning any modification.

Giulia is a personal project, not open source yet. Happy to discuss the architecture, the analysis results, or the approach. Feedback welcome.

If someone want a full Analysis, just contact me for the document.

Best,

That is not at all my understand of skills and I have personally written several (although not the ones mentioned in my post above). Allow me to walk you through the evolution of providing direction to agents.

For a while now when you generate a new phoenix application it produces an AGENTS.md file. This is not an elixir specific thing - but it was an early attempt to provide agents with guidance. The two challenges were that it was generic (not focused on a specific topic or action) and it was loaded into the agents context. This meant that it was a nice suggestion although the agent was not really compelled to follow. If the context became cluttered it was very possible for this information to be completely removed from the context and it was nearly as if it did not exist. It was the best that was available until this past October when Claude Code introduced Skills.

Skills were different in two very important ways. First of all they were contextual - meaning that they were only loaded when the agent determined it needed the additional knowledge provided by the skill and secondly they were not part of the agent context. Think of them as a much stronger suggestion that is invoked just-in-time. They are a great improvement over the basic AGENTS file.

What sort of things go in Skills? The skills I have built are related to workflow for a task coordinator I use. I also use a brainstorming skill all of the time. The skills I mentioned above are invoked constantly during my development. For example, the ecto-database that is part of the collection largely contains a lot of the information that Phoenix places in the AGENTS.md file (with some additions) but it is in a more useful form - Skills over AGENTS file.

You might also notice that the plugin I mentioned also contains a few SubAgents. There is one for Testing. SubAgents combined with hooks add more rigour to the agents producing our code.

IMO agents by them selves do not produce exactly what we want. Sometimes that “general knowledge” we expect does not appear. Skills, SubAgents, and Hooks are there to help us direct the agent better.

I use these claude skills → GitHub - j-morgan6/elixir-phoenix-guide · GitHub