diff --git a/pantheon/__main__.py b/pantheon/__main__.py
index e1cdc6c5..d5d772cd 100644
--- a/pantheon/__main__.py
+++ b/pantheon/__main__.py
@@ -115,9 +115,15 @@ def update_templates():
     console.print(f"\n[green]Updated {len(selected)} template(s).[/green]")
 
 
+def doctor():
+    """Check environment: Python version, API keys, core packages, optional runtimes."""
+    from pantheon.doctor import run_doctor
+    sys.exit(run_doctor())
+
+
 def main():
-    # Skip auto-setup if user explicitly requested "pantheon setup"
-    if len(sys.argv) < 2 or sys.argv[1] != "setup":
+    # Skip auto-setup if user explicitly requested "pantheon setup" or "pantheon doctor"
+    if len(sys.argv) < 2 or sys.argv[1] not in ("setup", "doctor"):
         from pantheon.repl.setup_wizard import check_and_run_setup
 
         check_and_run_setup()
@@ -141,6 +147,7 @@ def main():
             "setup": setup,
             "update-templates": update_templates,
             "store": StoreCLI,
+            "doctor": doctor,
         },
         name="pantheon",
     )
diff --git a/pantheon/doctor.py b/pantheon/doctor.py
new file mode 100644
index 00000000..1ee26476
--- /dev/null
+++ b/pantheon/doctor.py
@@ -0,0 +1,217 @@
+"""
+pantheon/doctor.py — Environment diagnostics command.
+
+Run with: pantheon doctor
+
+Surfaces common environment misconfigurations before execution,
+covering contracts already declared in package metadata and
+the current startup setup check.
+"""
+
+from __future__ import annotations
+
+import importlib
+import os
+import shutil
+import sys
+from dataclasses import dataclass, field
+from typing import List, Optional
+
+
+# ── Result model ────────────────────────────────────────────────────────────
+
+
+@dataclass
+class CheckResult:
+    name: str
+    status: str          # "PASS" | "WARN" | "FAIL"
+    message: str
+    fix_hints: List[str] = field(default_factory=list)
+
+
+# ── Individual checks ────────────────────────────────────────────────────────
+
+
+def _check_python_version() -> CheckResult:
+    """Python version satisfies requires-python >= 3.10."""
+    major, minor = sys.version_info.major, sys.version_info.minor
+    version_str = f"{major}.{minor}.{sys.version_info.micro}"
+    if (major, minor) >= (3, 10):
+        return CheckResult(
+            "Python version", "PASS",
+            f"Python {version_str}  (requires >=3.10)",
+        )
+    return CheckResult(
+        "Python version", "FAIL",
+        f"Python {version_str} is below the required >=3.10",
+        fix_hints=["Install Python 3.10+: https://python.org/downloads"],
+    )
+
+
+def _check_api_keys() -> CheckResult:
+    """Mirror check_and_run_setup() exactly, in the same order.
+
+    Replicates the four-step detection from check_and_run_setup() so that
+    doctor and the startup wizard always agree on whether the environment
+    is configured:
+      1. SKIP_SETUP_WIZARD env var
+      2. Standard PROVIDER_API_KEYS
+      3. Custom endpoint keys (CUSTOM_ENDPOINT_ENVS)
+      4. Legacy LLM_API_KEY
+    """
+    from pantheon.utils.model_selector import CUSTOM_ENDPOINT_ENVS, PROVIDER_API_KEYS
+
+    # 1. Explicit skip flag
+    if os.environ.get("SKIP_SETUP_WIZARD", "").lower() in ("1", "true", "yes"):
+        return CheckResult("LLM API key", "PASS", "SKIP_SETUP_WIZARD is set")
+
+    # 2. Standard provider keys
+    configured = [
+        name for name, env_var in PROVIDER_API_KEYS.items()
+        if os.environ.get(env_var, "")
+    ]
+    if configured:
+        return CheckResult(
+            "LLM API key", "PASS",
+            f"Provider key configured: {', '.join(configured)}",
+        )
+
+    # 3. Custom endpoint keys
+    custom_configured = [
+        cfg.display_name for cfg in CUSTOM_ENDPOINT_ENVS.values()
+        if os.environ.get(cfg.api_key_env, "")
+    ]
+    if custom_configured:
+        return CheckResult(
+            "LLM API key", "PASS",
+            f"Custom endpoint key configured: {', '.join(custom_configured)}",
+        )
+
+    # 4. Legacy LLM_API_KEY
+    if os.environ.get("LLM_API_KEY", ""):
+        return CheckResult("LLM API key", "PASS", "LLM_API_KEY is set")
+
+    return CheckResult(
+        "LLM API key", "WARN",
+        "No LLM provider key found — Pantheon will prompt the setup wizard on start",
+        fix_hints=["Run: pantheon setup"],
+    )
+
+
+def _check_core_imports() -> CheckResult:
+    """Core notebook packages are importable (declared as required deps)."""
+    missing = []
+    for pkg in ("ipykernel", "jupyter_client", "nbformat"):
+        try:
+            importlib.import_module(pkg)
+        except ImportError:
+            missing.append(pkg.replace("_", "-"))
+    if not missing:
+        return CheckResult(
+            "Core packages", "PASS",
+            "ipykernel, jupyter-client, nbformat are importable",
+        )
+    return CheckResult(
+        "Core packages", "FAIL",
+        f"Required packages not importable: {', '.join(missing)}",
+        fix_hints=[f"pip install {' '.join(missing)}"],
+    )
+
+
+def _check_r_runtime() -> Optional[CheckResult]:
+    """R runtime is available when the r extra (rpy2) is installed.
+
+    The r optional-dependency comment in pyproject.toml explicitly states
+    'requires R installed on system', so a missing R binary is a WARN.
+    """
+    try:
+        importlib.import_module("rpy2")
+    except ImportError:
+        return None  # r extra not installed — skip silently
+
+    if shutil.which("Rscript") or shutil.which("R"):
+        return CheckResult("R runtime", "PASS", "R is available on PATH")
+    return CheckResult(
+        "R runtime", "WARN",
+        "rpy2 is installed but R runtime not found on PATH",
+        fix_hints=["Install R: https://www.r-project.org/"],
+    )
+
+
+# ── Runner ───────────────────────────────────────────────────────────────────
+
+
+def _collect_checks() -> List[CheckResult]:
+    results: List[CheckResult] = []
+    results.append(_check_python_version())
+    results.append(_check_api_keys())
+    results.append(_check_core_imports())
+    r_check = _check_r_runtime()
+    if r_check is not None:
+        results.append(r_check)
+    return results
+
+
+def run_doctor() -> int:
+    """Run all environment checks and print a structured report.
+
+    Returns:
+        0  — all checks pass or warn only
+        1  — at least one FAIL
+    """
+    from rich import box
+    from rich.console import Console
+    from rich.table import Table
+
+    console = Console()
+    console.print()
+    console.print(
+        "[bold cyan]Pantheon Doctor[/bold cyan]  —  Environment Diagnostics"
+    )
+    console.print()
+
+    results = _collect_checks()
+
+    table = Table(box=box.SIMPLE, show_header=False, padding=(0, 1))
+    table.add_column("status", width=6, no_wrap=True)
+    table.add_column("check", style="bold", no_wrap=True)
+    table.add_column("detail")
+
+    _STATUS_STYLE = {
+        "PASS": "[green]PASS[/green]",
+        "WARN": "[yellow]WARN[/yellow]",
+        "FAIL": "[red]FAIL[/red]",
+    }
+
+    for r in results:
+        table.add_row(_STATUS_STYLE[r.status], r.name, r.message)
+
+    console.print(table)
+
+    # Fix hints for non-passing checks
+    actionable = [r for r in results if r.status in ("WARN", "FAIL") and r.fix_hints]
+    if actionable:
+        console.print()
+        for r in actionable:
+            console.print(f"  [bold]{r.name}[/bold]")
+            for hint in r.fix_hints:
+                console.print(f"    {hint}", style="dim")
+        console.print()
+
+    fails = sum(1 for r in results if r.status == "FAIL")
+    warns = sum(1 for r in results if r.status == "WARN")
+
+    if fails:
+        console.print(
+            f"[red]✗  {fails} failure(s).[/red]  "
+            "Fix the issues above before running Pantheon."
+        )
+        return 1
+    if warns:
+        console.print(
+            f"[yellow]⚠  {warns} warning(s).[/yellow]  "
+            "Pantheon can run; some capabilities may be unavailable."
+        )
+        return 0
+    console.print("[green]✓  All checks passed.[/green]")
+    return 0