diff --git a/pyproject.toml b/pyproject.toml
index 2d5fb2b..86d6e8a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -12,6 +12,7 @@ dependencies = [
     "marimo>=0.20.4",
     "bridgestan>=2.7.0",
     "torch>=2.10.0",
+    "click>=8.0",
 ]
 
 [project.optional-dependencies]
@@ -21,6 +22,9 @@ jax = ["jax>=0.4", "jaxlib>=0.4"]
 torch = ["torch>=2.0"]
 dl = ["jax>=0.4", "jaxlib>=0.4", "torch>=2.0"]
 
+[project.scripts]
+transpailer = "transpailer.cli:cli"
+
 [build-system]
 build-backend = "hatchling.build"
 requires = ["hatchling"]
diff --git a/transpailer/cli.py b/transpailer/cli.py
new file mode 100644
index 0000000..0121c9c
--- /dev/null
+++ b/transpailer/cli.py
@@ -0,0 +1,262 @@
+"""CLI for the Transpailer — AI-powered transpilation between computational frameworks."""
+
+from __future__ import annotations
+
+import os
+import re
+import sys
+from pathlib import Path
+
+import click
+
+
+_SKILLS_DIR = Path(__file__).parent / "skills"
+
+# Mapping from (source, target) pairs to relevant skill files
+_SKILL_MAP: dict[tuple[str, str], list[str]] = {
+    ("stan", "pymc"): ["stan_to_pymc.md"],
+    ("stan", "rust"): ["stan.md"],
+    ("pymc", "rust"): ["pymc_optimization.md"],
+    ("jax", "pytorch"): ["jax_to_pytorch.md"],
+    ("pytorch", "jax"): ["pytorch_to_jax.md"],
+    ("pytorch", "rust"): ["pytorch_to_rust.md"],
+}
+
+# Framework aliases
+_ALIASES: dict[str, str] = {
+    "pm": "pymc",
+    "numpyro": "numpyro",
+    "np": "numpyro",
+    "tf": "tensorflow",
+    "torch": "pytorch",
+    "pt": "pytorch",
+    "rs": "rust",
+    "turing": "turing.jl",
+    "brms": "brms",
+}
+
+# File extension → framework
+_EXT_MAP: dict[str, str] = {
+    ".stan": "stan",
+    ".bug": "bugs",
+    ".bugs": "bugs",
+    ".jl": "julia",
+    ".r": "r",
+    ".R": "r",
+}
+
+# Python import patterns → framework
+_IMPORT_PATTERNS: list[tuple[str, str]] = [
+    (r"import\s+pymc|from\s+pymc", "pymc"),
+    (r"import\s+numpyro|from\s+numpyro", "numpyro"),
+    (r"import\s+torch|from\s+torch", "pytorch"),
+    (r"import\s+jax|from\s+jax", "jax"),
+    (r"import\s+tensorflow|from\s+tensorflow|import\s+tf", "tensorflow"),
+    (r"import\s+pyro\b|from\s+pyro\b", "pyro"),
+    (r"import\s+stan|from\s+cmdstanpy|from\s+pystan", "stan"),
+]
+
+
+def _detect_framework(code: str, filename: str) -> str:
+    """Auto-detect source framework from file extension and content."""
+    ext = Path(filename).suffix.lower()
+    if ext in _EXT_MAP:
+        return _EXT_MAP[ext]
+
+    if ext in (".py", ""):
+        for pattern, framework in _IMPORT_PATTERNS:
+            if re.search(pattern, code):
+                return framework
+
+    raise click.UsageError(
+        f"Cannot auto-detect source framework for '{filename}'. "
+        "Use --from to specify it explicitly."
+    )
+
+
+def _normalize_framework(name: str) -> str:
+    """Normalize framework name (handle aliases)."""
+    return _ALIASES.get(name.lower(), name.lower())
+
+
+def _load_skills(source: str, target: str) -> str:
+    """Load relevant skill files for this transpilation pair."""
+    filenames = _SKILL_MAP.get((source, target), [])
+    parts = []
+    for fname in filenames:
+        skill_path = _SKILLS_DIR / fname
+        if skill_path.exists():
+            parts.append(skill_path.read_text())
+    return "\n\n".join(parts)
+
+
+_SYSTEM_PROMPT = """\
+You are the Transpailer — an expert AI that transpiles code between computational \
+frameworks. You produce clean, idiomatic code in the target framework that is \
+functionally equivalent to the input.
+
+Supported frameworks include (but are not limited to):
+- Probabilistic programming: Stan, PyMC, NumPyro, Turing.jl, BUGS, brms
+- Deep learning: PyTorch, JAX, TensorFlow
+- Systems: Rust, C++
+
+Rules:
+1. Output ONLY the transpiled code — no explanations, no markdown fences.
+2. Preserve the mathematical/computational semantics exactly.
+3. Use idiomatic patterns of the target framework.
+4. Include all necessary imports.
+5. Translate language-specific conventions (e.g. 1-based → 0-based indexing).
+6. Keep variable names as close to the original as possible.
+"""
+
+
+def _transpile(
+    code: str,
+    source: str,
+    target: str,
+    *,
+    model: str = "claude-sonnet-4-20250514",
+    verbose: bool = False,
+) -> str:
+    """Call Claude to transpile code from source to target framework."""
+    import anthropic
+
+    skills = _load_skills(source, target)
+
+    system = _SYSTEM_PROMPT
+    if skills:
+        system += f"\n\n# Domain knowledge\n\n{skills}"
+
+    user_msg = (
+        f"Transpile the following {source} code to {target}.\n\n"
+        f"```\n{code}\n```"
+    )
+
+    api_key = os.environ.get("ANTHROPIC_API_KEY")
+    if not api_key:
+        raise click.ClickException(
+            "ANTHROPIC_API_KEY environment variable is required. "
+            "Set it with: export ANTHROPIC_API_KEY=sk-..."
+        )
+
+    client = anthropic.Anthropic(api_key=api_key)
+
+    if verbose:
+        click.echo(f"Calling {model}...", err=True)
+
+    response = client.messages.create(
+        model=model,
+        max_tokens=16384,
+        system=system,
+        messages=[{"role": "user", "content": user_msg}],
+    )
+
+    result = response.content[0].text
+
+    # Strip markdown code fences if the model wrapped its output
+    result = re.sub(r"^```\w*\n", "", result)
+    result = re.sub(r"\n```\s*$", "", result)
+
+    if verbose:
+        usage = response.usage
+        click.echo(
+            f"Done. Tokens: {usage.input_tokens} in / {usage.output_tokens} out",
+            err=True,
+        )
+
+    return result
+
+
+# ── CLI ──────────────────────────────────────────────────────────────────────
+
+
+@click.group()
+@click.version_option(package_name="transpailer")
+def cli():
+    """Transpailer: AI-powered transpilation between computational frameworks."""
+
+
+@cli.command()
+@click.argument("input_file", type=click.Path(exists=True), required=False)
+@click.option(
+    "--to",
+    "target",
+    required=True,
+    help="Target framework (pymc, pytorch, jax, stan, numpyro, tensorflow, rust, ...)",
+)
+@click.option(
+    "--from",
+    "source",
+    default=None,
+    help="Source framework (auto-detected from file extension/content if omitted)",
+)
+@click.option(
+    "-o",
+    "--output",
+    type=click.Path(),
+    default=None,
+    help="Output file path (default: stdout)",
+)
+@click.option(
+    "--model",
+    default="claude-sonnet-4-20250514",
+    help="Claude model to use",
+    show_default=True,
+)
+@click.option("-v", "--verbose", is_flag=True, help="Verbose output on stderr")
+def convert(
+    input_file: str | None,
+    target: str,
+    source: str | None,
+    output: str | None,
+    model: str,
+    verbose: bool,
+) -> None:
+    """Transpile code from one framework to another.
+
+    \b
+    Examples:
+      transpailer convert model.stan --to pymc
+      transpailer convert train.py --to jax
+      transpailer convert model.py --to pytorch
+      cat model.stan | transpailer convert --to pymc
+    """
+    # Read input
+    if input_file:
+        code = Path(input_file).read_text()
+        filename = Path(input_file).name
+    elif not sys.stdin.isatty():
+        code = sys.stdin.read()
+        filename = "stdin"
+    else:
+        raise click.UsageError(
+            "No input file provided and no data on stdin. "
+            "Usage: transpailer convert <file> --to <framework>"
+        )
+
+    target = _normalize_framework(target)
+
+    # Detect source framework
+    if source:
+        source = _normalize_framework(source)
+    else:
+        source = _detect_framework(code, filename)
+        if verbose:
+            click.echo(f"Detected source: {source}", err=True)
+
+    if source == target:
+        raise click.UsageError(f"Source and target are the same: {source}")
+
+    if verbose:
+        click.echo(f"Transpiling {source} → {target}...", err=True)
+
+    result = _transpile(code, source, target, model=model, verbose=verbose)
+
+    # Write output
+    if output:
+        Path(output).parent.mkdir(parents=True, exist_ok=True)
+        Path(output).write_text(result)
+        if verbose:
+            click.echo(f"Written to {output}", err=True)
+    else:
+        click.echo(result)
diff --git a/uv.lock b/uv.lock
index 9235903..fdf62c6 100644
--- a/uv.lock
+++ b/uv.lock
@@ -2503,6 +2503,7 @@ source = { editable = "." }
 dependencies = [
     { name = "anthropic" },
     { name = "bridgestan" },
+    { name = "click" },
     { name = "marimo" },
     { name = "numpy" },
     { name = "nutpie" },
@@ -2541,6 +2542,7 @@ dev = [
 requires-dist = [
     { name = "anthropic", specifier = ">=0.52" },
     { name = "bridgestan", specifier = ">=2.7.0" },
+    { name = "click", specifier = ">=8.0" },
     { name = "jax", marker = "extra == 'dl'", specifier = ">=0.4" },
     { name = "jax", marker = "extra == 'jax'", specifier = ">=0.4" },
     { name = "jaxlib", marker = "extra == 'dl'", specifier = ">=0.4" },