---
name: coding-mojo
description: Develop and run Mojo code in Claude.ai containers. Handles installation, compilation, and execution. Use when writing Mojo code, benchmarking Mojo vs Python, or when user mentions Mojo, Modular, or MAX. Routes to Modular's official skills (mojo-syntax, mojo-python-interop, mojo-gpu-fundamentals) for language-specific correction layers.
metadata:
  version: 0.2.1
---

# Mojo Development in Claude.ai Containers

Mojo is a systems programming language from Modular that combines Python-like syntax with C-level performance. This skill handles container setup and execution. For **language syntax and semantics**, defer to Modular's official skills at `github.com/modular/skills` — they are authoritative correction layers for pretrained knowledge.

## Installation

Install once per session (~20s via uv, ~500MB). Skip if already installed.

```bash
if mojo --version 2>/dev/null; then
  echo "Mojo already installed"
else
  # Compiler binary without ML extras (~350MB saved)
  uv pip install --system --break-system-packages modular --no-deps 2>&1 | tail -5
  # Entry points + base deps (numpy, pyyaml, rich)
  uv pip install --system --break-system-packages mojo max 2>&1 | tail -5
  mojo --version
fi
```

Verify:
```bash
echo 'def main(): print("Mojo ready")' > /tmp/_verify.mojo && mojo /tmp/_verify.mojo
```

## Running Mojo Code

**Quick tests** (write to temp file):
```bash
cat > /tmp/test.mojo << 'EOF'
def main():
    print("hello")
EOF
mojo /tmp/test.mojo
```

**File execution** (JIT compile + run, ~1.4s overhead):
```bash
cat > /home/claude/example.mojo << 'EOF'
def main():
    print("Hello from Mojo")
EOF
mojo /home/claude/example.mojo
```

**Build binary** (for benchmarking — ~6s cold compile, but binary runs at native speed):
```bash
mojo build /home/claude/example.mojo -o /home/claude/example
/home/claude/example
```

Use `mojo build` for benchmarks — `mojo` (JIT) includes ~1.4s compilation overhead per run. There is no `mojo -e` flag; always write to a file.

## Critical Syntax Corrections (v26.2)

Pretrained models generate outdated Mojo. These corrections are current as of Mojo 26.2:

| Wrong (pretrained) | Correct (26.2) | Notes |
|---|---|---|
| `fn main():` | `def main():` | `fn` is deprecated; `def` is the only function keyword |
| `let x = 5` | `var x = 5` | `let` removed; `var` for all bindings |
| `inout self` | `mut self` / `out self` | `mut` for mutation, `out` for `__init__` |
| `@parameter for` | `comptime for` | Compile-time loops |
| `List[Int](1, 2, 3)` | `[1, 2, 3]` | Collection literals |
| `from math import sqrt` | `from std.math import sqrt` | `std.` prefix required for **all** stdlib modules |
| `from time import X` | `from std.time import X` | Includes `perf_counter_ns`, `sleep`, etc. |
| `__str__` / `Stringable` | `write_to` / `Writable` | String conversion protocol |
| `String(self.x)` for int→str | `String(self.x)` | This one is actually correct, but `str()` is not |
| `list.append(item)` | `list.append(item^)` | Non-copyable types require `^` transfer operator |
| `var x: Int = perf_counter_ns()` | `var x: UInt = perf_counter_ns()` | Time functions return `UInt`, not `Int` |
| Implicit copy of `List[T]` | `.copy()` or `^` transfer | `List` is not implicitly copyable; use explicit copy or move |

## Companion Skills (Modular Official)

These skills from `github.com/modular/skills` provide deep syntax correction layers. If they are installed in the user's skill set, read them before writing Mojo code:

- **mojo-syntax** — Comprehensive syntax corrections, type system, ownership model. **Always use when writing any Mojo code.**
- **mojo-python-interop** — Calling Python from Mojo, type conversion, extension modules. Use when mixing Mojo and Python.
- **mojo-gpu-fundamentals** — GPU programming (no CUDA syntax — Mojo has its own model). Reference only in Claude.ai containers (no GPU available).
- **new-modular-project** — Project scaffolding with Pixi or uv. Use when starting a new Mojo/MAX project locally.

If companion skills are not installed, the correction table above covers the most common pretrained errors. For deeper work, fetch the skill content directly:
```bash
curl -sL -H "Authorization: token $GH_TOKEN" \
  -H "Accept: application/vnd.github.v3.raw" \
  "https://api.github.com/repos/modular/skills/contents/mojo-syntax/SKILL.md?ref=main"
```

## Container Constraints

- **No GPU**: Claude.ai containers are CPU-only. GPU skills are reference material for generating code the user will run locally.
- **Session-ephemeral**: Mojo installation doesn't persist across conversations. Reinstall each session.
- **Build artifacts**: Store in `/home/claude/`. Copy final outputs to `/mnt/user-data/outputs/`.
- **Timeout**: Long compilations or benchmarks may hit the ~200s bash timeout. Break work into smaller units.

## Benchmarking Pattern

Compare Mojo vs Python on the same algorithm:

```bash
# Python baseline
python3 -c "
import time
def fib(n):
    a, b = 0, 1
    for _ in range(n):
        a, b = b, a + b
    return a
# Warmup + timed runs
fib(90)
times = []
for _ in range(100):
    start = time.perf_counter()
    fib(90)
    times.append((time.perf_counter() - start) * 1e6)
import statistics
print(f'Python: median={statistics.median(times):.1f} µs, min={min(times):.1f} µs')
"

# Mojo version
cat > /home/claude/fib.mojo << 'EOF'
from std.time import perf_counter_ns

def fib(n: Int) -> Int:
    var a = 0
    var b = 1
    for _ in range(n):
        var tmp = a
        a = b
        b = tmp + b
    return a

def main():
    # Warmup
    _ = fib(90)
    
    # Timed runs
    var total_ns: UInt = 0
    var min_ns: UInt = 999999999
    for _ in range(100):
        var start = perf_counter_ns()
        _ = fib(90)
        var elapsed = perf_counter_ns() - start
        total_ns += elapsed
        if elapsed < min_ns:
            min_ns = elapsed
    print("Mojo: mean =", total_ns // 100, "ns, min =", min_ns, "ns")
EOF
mojo build /home/claude/fib.mojo -o /home/claude/fib
/home/claude/fib
```

Expected: Mojo is ~50x faster than CPython on tight numeric loops. SIMD and parallelism widen the gap further but require mojo-syntax and mojo-gpu-fundamentals skills for correct usage.