diff --git a/src/doctor.rs b/src/doctor.rs
new file mode 100644
index 0000000..50fd2a0
--- /dev/null
+++ b/src/doctor.rs
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: PMPL-1.0-or-later
+// Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+//
+// `verisimiser doctor` — environment-level diagnostics. Mirrors `just
+// doctor` with CLI semantics + `--json`. Closes #53.
+//
+// Distinction from `verisimiser validate`:
+//
+// - `validate` is *manifest-centric* — does this `verisimiser.toml` make
+//   sense? Per-check failure surfaces TOML, schema, sidecar issues.
+// - `doctor` is *environment-centric* — is this host fit to run
+//   verisimiser at all? Reports on toolchain, PATH, working directory.
+//   When a manifest path is supplied, also runs the manifest checks.
+
+use std::process::Command;
+
+use crate::manifest::{ValidationCheck, ValidationReport, validate_manifest};
+
+/// Run all doctor checks. If `manifest_path` is `Some(_)`, the manifest
+/// validation checks (from [`validate_manifest`]) are appended to the
+/// environment checks.
+pub fn run_doctor(manifest_path: Option<&str>) -> ValidationReport {
+    let mut checks: Vec<ValidationCheck> = Vec::new();
+
+    checks.push(check_command_in_path("cargo", "Rust toolchain (cargo)"));
+    checks.push(check_command_in_path("git", "git in PATH"));
+    checks.push(check_cwd_writable());
+
+    let manifest_label = manifest_path.unwrap_or("<none>").to_string();
+    if let Some(path) = manifest_path {
+        let report = validate_manifest(path);
+        checks.extend(report.checks);
+    }
+
+    let passed = checks.iter().all(|c| c.passed);
+    ValidationReport {
+        manifest: manifest_label,
+        passed,
+        checks,
+    }
+}
+
+/// Check whether a CLI tool resolves on PATH. Runs `<cmd> --version` with
+/// a short timeout-free invocation; we only care about exit status.
+fn check_command_in_path(cmd: &str, description: &str) -> ValidationCheck {
+    let name = format!("path-{}", cmd);
+    let status = Command::new(cmd).arg("--version").output();
+    match status {
+        Ok(out) if out.status.success() => ValidationCheck {
+            name,
+            description: description.to_string(),
+            passed: true,
+            detail: None,
+        },
+        Ok(out) => ValidationCheck {
+            name,
+            description: description.to_string(),
+            passed: false,
+            detail: Some(format!(
+                "`{} --version` exited with status {:?}",
+                cmd, out.status.code()
+            )),
+        },
+        Err(e) => ValidationCheck {
+            name,
+            description: description.to_string(),
+            passed: false,
+            detail: Some(format!("`{}` not found on PATH: {}", cmd, e)),
+        },
+    }
+}
+
+/// Check whether the current working directory is writable. Verisimiser
+/// writes manifests, sidecar databases, and generated DDL — a read-only
+/// cwd will fail with permission errors at runtime.
+fn check_cwd_writable() -> ValidationCheck {
+    let cwd_meta = std::env::current_dir().and_then(std::fs::metadata);
+    match cwd_meta {
+        Ok(md) if !md.permissions().readonly() => ValidationCheck {
+            name: "cwd-writable".to_string(),
+            description: "Current working directory is writable".to_string(),
+            passed: true,
+            detail: None,
+        },
+        Ok(_) => ValidationCheck {
+            name: "cwd-writable".to_string(),
+            description: "Current working directory is writable".to_string(),
+            passed: false,
+            detail: Some("cwd is read-only".to_string()),
+        },
+        Err(e) => ValidationCheck {
+            name: "cwd-writable".to_string(),
+            description: "Current working directory is writable".to_string(),
+            passed: false,
+            detail: Some(format!("cannot stat cwd: {}", e)),
+        },
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::run_doctor;
+
+    /// Doctor without a manifest path runs only environment checks.
+    #[test]
+    fn doctor_without_manifest_runs_env_checks_only() {
+        let report = run_doctor(None);
+        let names: Vec<&str> = report.checks.iter().map(|c| c.name.as_str()).collect();
+        assert!(names.contains(&"path-cargo"));
+        assert!(names.contains(&"path-git"));
+        assert!(names.contains(&"cwd-writable"));
+        // No manifest-* checks present.
+        assert!(!names.iter().any(|n| n.starts_with("manifest-")));
+    }
+
+    /// Doctor with a manifest path runs env checks AND manifest checks.
+    #[test]
+    fn doctor_with_manifest_runs_both_sets() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("verisimiser.toml");
+        let sidecar_path = dir.path().join("sidecar.db");
+        let body = format!(
+            "[project]\n\
+             name = \"test\"\n\
+             [database]\n\
+             backend = \"sqlite\"\n\
+             [sidecar]\n\
+             storage = \"sqlite\"\n\
+             path = \"{}\"\n",
+            sidecar_path.display().to_string().replace('\\', "/")
+        );
+        std::fs::write(&path, body).expect("write");
+
+        let report = run_doctor(Some(path.to_str().unwrap()));
+        let names: Vec<&str> = report.checks.iter().map(|c| c.name.as_str()).collect();
+        // Env checks still present.
+        assert!(names.contains(&"path-cargo"));
+        // Manifest-loads check appended.
+        assert!(names.contains(&"manifest-loads"));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 38eba2c..cad426a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -9,6 +9,7 @@
 
 pub mod abi;
 pub mod codegen;
+pub mod doctor;
 pub mod intercept;
 pub mod manifest;
 pub mod tier1;
diff --git a/src/main.rs b/src/main.rs
index 4e29e8a..92741e9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -17,7 +17,7 @@
 
 use anyhow::Result;
 use clap::{Parser, Subcommand};
-use verisimiser::{abi, codegen, manifest};
+use verisimiser::{abi, codegen, doctor, manifest};
 
 /// Long version string: `<crate-version> (<git-describe>, built <date>)`.
 const LONG_VERSION: &str = concat!(
@@ -115,6 +115,16 @@ enum Commands {
         #[arg(long)]
         json: bool,
     },
+    /// Environment-level diagnostics (toolchain, PATH, cwd). Optionally
+    /// also runs the manifest checks from `validate`.
+    Doctor {
+        /// If supplied, also run `validate` checks against this manifest.
+        #[arg(short, long)]
+        manifest: Option<String>,
+        /// Emit the structured ValidationReport as JSON instead of text.
+        #[arg(long)]
+        json: bool,
+    },
 }
 
 fn main() -> Result<()> {
@@ -236,32 +246,12 @@ fn main() -> Result<()> {
 
         Commands::Validate { manifest, json } => {
             let report = manifest::validate_manifest(&manifest);
-            if json {
-                println!("{}", serde_json::to_string_pretty(&report)?);
-            } else {
-                println!("Validating {} ...", report.manifest);
-                for check in &report.checks {
-                    let mark = if check.passed { "ok " } else { "FAIL" };
-                    println!("  [{}] {} — {}", mark, check.name, check.description);
-                    if let Some(detail) = &check.detail {
-                        println!("        {}", detail);
-                    }
-                }
-                if report.passed {
-                    println!("All {} checks passed.", report.checks.len());
-                } else {
-                    println!(
-                        "{}/{} checks failed.",
-                        report.failed_count(),
-                        report.checks.len()
-                    );
-                }
-            }
-            if report.passed {
-                Ok(())
-            } else {
-                anyhow::bail!("manifest validation failed");
-            }
+            emit_report(&report, json, "manifest validation")
+        }
+
+        Commands::Doctor { manifest, json } => {
+            let report = doctor::run_doctor(manifest.as_deref());
+            emit_report(&report, json, "doctor")
         }
 
         Commands::Version { json } => {
@@ -281,6 +271,42 @@ fn main() -> Result<()> {
     }
 }
 
+/// Render a `ValidationReport` (from `validate` or `doctor`) and exit
+/// non-zero if any check failed. Plain-text by default; JSON when
+/// `json == true`.
+fn emit_report(
+    report: &manifest::ValidationReport,
+    json: bool,
+    kind: &str,
+) -> Result<()> {
+    if json {
+        println!("{}", serde_json::to_string_pretty(report)?);
+    } else {
+        println!("Running {} for {} ...", kind, report.manifest);
+        for check in &report.checks {
+            let mark = if check.passed { "ok " } else { "FAIL" };
+            println!("  [{}] {} — {}", mark, check.name, check.description);
+            if let Some(detail) = &check.detail {
+                println!("        {}", detail);
+            }
+        }
+        if report.passed {
+            println!("All {} checks passed.", report.checks.len());
+        } else {
+            println!(
+                "{}/{} checks failed.",
+                report.failed_count(),
+                report.checks.len()
+            );
+        }
+    }
+    if report.passed {
+        Ok(())
+    } else {
+        anyhow::bail!("{} failed", kind);
+    }
+}
+
 /// Print the 8 octad dimensions with descriptions.
 fn print_octad() {
     println!("=== VeriSimDB Octad: Eight Dimensions ===");