Why Merlin AI Code Review Is Written in Rust

When building Merlin AI Code Review, we chose Rust as the implementation language — not as a fashionable choice, but because Rust's characteristics directly solve problems that matter for a CI tool: startup latency, memory safety, single-binary distribution, and async concurrency for parallel AI API calls.

Startup time: the CI tax

Every CI job has an overhead cost: checkout, dependency installation, tool initialization. For Python or Node.js tools, starting a runtime and loading dependencies can add 5–15 seconds. For a tool that runs on every PR, this overhead compounds across thousands of runs per month.

Merlin AI Code Review starts in milliseconds. The binary contains everything it needs — no runtime to initialize, no package manager to consult, no dependency resolution. The CI step goes from "download" to "first AI call" in under a second.

Single binary distribution

Rust compiles to statically linked binaries. Merlin AI Code Review distributes as a single executable for Linux (x86_64 and ARM64), macOS (x86_64 and ARM64), and Windows. CI integration is a curl and chmod:

No Docker image required. No runtime installation step. No dependency conflicts with other tools in the CI environment. This simplicity is a direct result of Rust's compilation model.

Memory safety without a garbage collector

Merlin AI Code Review handles large diffs — sometimes thousands of lines across dozens of files. It parses unified diff format, chunks content, manages concurrent HTTP connections to VCS and AI APIs, and maintains conversation history for the agent mode. All of this with:

• No garbage collection pauses (which would add latency variance to review times)
• No memory leaks (Rust's ownership model prevents them at compile time)
• No null pointer exceptions or buffer overflows
• Deterministic resource cleanup

For a tool that runs in security-sensitive CI environments processing potentially sensitive code diffs, memory safety isn't optional.

Async concurrency for parallel AI calls

Reviewing a large PR involves making multiple AI API calls — one per diff chunk, plus additional calls for security scanning, describe generation, etc. Merlin AI Code Review uses Tokio (Rust's async runtime) to make these calls concurrently rather than sequentially. A PR with 10 diff chunks doesn't take 10x longer than a single-chunk PR — the AI calls overlap.

This concurrency is implemented safely in Rust — no data races, no shared mutable state bugs, all verified at compile time by the borrow checker.

A codebase worth reviewing with Merlin AI Code Review

There's a pleasing recursive quality to Merlin AI Code Review's architecture: a tool designed to review code rigorously is itself written in a language where the compiler enforces the most important correctness properties. Merlin AI Code Review's own development uses Merlin AI Code Review for PR review — catching issues in a Rust codebase with a Rust binary reviewed by an AI that understands Rust idioms.

The result is a tool that's fast, reliable, memory-safe, and easy to integrate — properties that matter directly to the engineers who deploy it in their CI pipelines.