diff --git a/src/cli/doctor.ts b/src/cli/doctor.ts
index b55c020..ae3433b 100644
--- a/src/cli/doctor.ts
+++ b/src/cli/doctor.ts
@@ -1,12 +1,221 @@
 import type { Command } from 'commander';
+import type { Config } from '../config/index.js';
+import { getConfigPath, getDataDir, formatStatus } from './shared.js';
+import { existsSync, readFileSync, writeFileSync, unlinkSync } from 'fs';
+import { resolve, join } from 'path';
+import { parse } from 'yaml';
+import { configSchema } from '../config/schema.js';
+
+export interface CheckResult {
+  status: 'pass' | 'fail' | 'warn' | 'skip';
+  label: string;
+  detail?: string;
+}
+
+export interface DoctorContext {
+  configPath: string;
+  dataDir: string;
+  config?: Config;
+}
+
+type Check = (ctx: DoctorContext) => Promise<CheckResult>;
+
+const checkConfigExists: Check = async (ctx) => {
+  if (existsSync(ctx.configPath)) {
+    return { status: 'pass', label: 'Config file exists', detail: `(${ctx.configPath})` };
+  }
+  return { status: 'fail', label: 'Config file exists', detail: `not found at ${ctx.configPath}` };
+};
+
+const checkConfigParses: Check = async (ctx) => {
+  if (!existsSync(ctx.configPath)) {
+    return { status: 'skip', label: 'Config parses', detail: '(no config file)' };
+  }
+  try {
+    const raw = readFileSync(ctx.configPath, 'utf-8');
+    parse(raw);
+    return { status: 'pass', label: 'Config parses', detail: '(valid YAML)' };
+  } catch (err) {
+    return { status: 'fail', label: 'Config parses', detail: err instanceof Error ? err.message : String(err) };
+  }
+};
+
+const checkConfigValidates: Check = async (ctx) => {
+  if (!existsSync(ctx.configPath)) {
+    return { status: 'skip', label: 'Config validates', detail: '(no config file)' };
+  }
+  try {
+    const raw = readFileSync(ctx.configPath, 'utf-8');
+    const parsed = parse(raw);
+    if (!parsed || typeof parsed !== 'object') {
+      return { status: 'fail', label: 'Config validates', detail: 'YAML did not produce an object' };
+    }
+    const config = configSchema.parse(parsed);
+    ctx.config = config;
+    return { status: 'pass', label: 'Config validates', detail: '(schema valid)' };
+  } catch (err) {
+    return { status: 'fail', label: 'Config validates', detail: err instanceof Error ? err.message : String(err) };
+  }
+};
+
+const checkEnvVars: Check = async (ctx) => {
+  if (!existsSync(ctx.configPath)) {
+    return { status: 'skip', label: 'Env vars resolved', detail: '(no config file)' };
+  }
+  try {
+    const raw = readFileSync(ctx.configPath, 'utf-8');
+    const envVarPattern = /\$\{([^}]+)\}/g;
+    const unresolved: string[] = [];
+    let match;
+    while ((match = envVarPattern.exec(raw)) !== null) {
+      if (!process.env[match[1]]) {
+        unresolved.push(match[1]);
+      }
+    }
+    if (unresolved.length > 0) {
+      return { status: 'fail', label: 'Env vars resolved', detail: `unset: ${unresolved.join(', ')}` };
+    }
+    return { status: 'pass', label: 'Env vars resolved' };
+  } catch {
+    return { status: 'skip', label: 'Env vars resolved', detail: '(could not read config)' };
+  }
+};
+
+const checkDataDir: Check = async (ctx) => {
+  try {
+    const { mkdirSync } = await import('fs');
+    mkdirSync(ctx.dataDir, { recursive: true });
+
+    // Test write access
+    const testFile = join(ctx.dataDir, '.doctor-check');
+    writeFileSync(testFile, 'ok');
+    unlinkSync(testFile);
+    return { status: 'pass', label: 'Data directory writable', detail: `(${ctx.dataDir})` };
+  } catch {
+    return { status: 'fail', label: 'Data directory writable', detail: `cannot write to ${ctx.dataDir}` };
+  }
+};
+
+const checkSessionDb: Check = async (ctx) => {
+  try {
+    const { mkdirSync } = await import('fs');
+    mkdirSync(ctx.dataDir, { recursive: true });
+
+    const dbPath = resolve(ctx.dataDir, 'sessions.db');
+    const { SessionStore } = await import('../session/index.js');
+    const store = new SessionStore(dbPath);
+    store.listSessions(); // Quick query to verify DB works
+    store.close();
+    return { status: 'pass', label: 'Session DB accessible', detail: '(sessions.db)' };
+  } catch (err) {
+    return { status: 'fail', label: 'Session DB accessible', detail: err instanceof Error ? err.message : String(err) };
+  }
+};
+
+const checkModelConnectivity: Check = async (ctx) => {
+  if (!ctx.config) {
+    return { status: 'skip', label: 'Model connectivity', detail: '(config invalid)' };
+  }
+  // Skip actual API call in doctor — just verify config looks complete
+  const model = ctx.config.models.default;
+  if (!model.model) {
+    return { status: 'fail', label: 'Model connectivity', detail: 'no default model configured' };
+  }
+  return { status: 'pass', label: 'Model connectivity', detail: `(${model.provider}: ${model.model})` };
+};
+
+const checkTelegram: Check = async (ctx) => {
+  if (!ctx.config) {
+    return { status: 'skip', label: 'Telegram bot configured', detail: '(config invalid)' };
+  }
+  if (!ctx.config.telegram.bot_token || ctx.config.telegram.bot_token.length < 10) {
+    return { status: 'warn', label: 'Telegram bot configured', detail: 'token looks too short' };
+  }
+  return { status: 'pass', label: 'Telegram bot configured', detail: `(${ctx.config.telegram.allowed_chat_ids.length} allowed chat(s))` };
+};
+
+const checkMcpServers: Check = async (ctx) => {
+  if (!ctx.config) {
+    return { status: 'skip', label: 'MCP servers configured', detail: '(config invalid)' };
+  }
+  const servers = ctx.config.mcp.servers;
+  if (servers.length === 0) {
+    return { status: 'skip', label: 'MCP servers configured', detail: '(none configured)' };
+  }
+  return { status: 'pass', label: 'MCP servers configured', detail: `(${servers.length} server(s))` };
+};
+
+const checkSkills: Check = async (ctx) => {
+  if (!ctx.config) {
+    return { status: 'skip', label: 'Skills loaded', detail: '(config invalid)' };
+  }
+  try {
+    const { loadAllSkills } = await import('../skills/index.js');
+    const skills = loadAllSkills({
+      bundledDir: ctx.config.skills.bundled_dir,
+      managedDir: ctx.config.skills.managed_dir,
+      workspaceDir: ctx.config.skills.workspace_dir,
+    });
+    return { status: 'pass', label: 'Skills loaded', detail: `(${skills.length} skill(s))` };
+  } catch (err) {
+    return { status: 'fail', label: 'Skills loaded', detail: err instanceof Error ? err.message : String(err) };
+  }
+};
+
+const allChecks: Check[] = [
+  checkConfigExists,
+  checkConfigParses,
+  checkConfigValidates,
+  checkEnvVars,
+  checkDataDir,
+  checkSessionDb,
+  checkModelConnectivity,
+  checkTelegram,
+  checkMcpServers,
+  checkSkills,
+];
+
+/** Run all doctor checks in order. Exported for testing. */
+export async function runChecks(ctx: DoctorContext): Promise<CheckResult[]> {
+  const results: CheckResult[] = [];
+  for (const check of allChecks) {
+    const result = await check(ctx);
+    results.push(result);
+  }
+  return results;
+}
 
 export function registerDoctorCommand(program: Command): void {
   program
     .command('doctor')
     .description('Validate configuration and check system health')
     .option('-c, --config <path>', 'Config file path')
-    .action(async (_opts: { config?: string }) => {
-      console.error('Not yet implemented');
-      process.exit(1);
+    .action(async (opts: { config?: string }) => {
+      const configPath = opts.config ?? getConfigPath();
+      const dataDir = getDataDir();
+
+      console.log('Flynn Doctor');
+      console.log('============');
+      console.log('');
+
+      const ctx: DoctorContext = { configPath, dataDir };
+      const results = await runChecks(ctx);
+
+      for (const result of results) {
+        console.log(formatStatus(result.status, result.label, result.detail));
+      }
+
+      console.log('');
+
+      const counts = {
+        pass: results.filter(r => r.status === 'pass').length,
+        fail: results.filter(r => r.status === 'fail').length,
+        warn: results.filter(r => r.status === 'warn').length,
+        skip: results.filter(r => r.status === 'skip').length,
+      };
+
+      console.log(`Results: ${counts.pass} passed, ${counts.fail} failed, ${counts.warn} warnings, ${counts.skip} skipped`);
+
+      process.exit(counts.fail > 0 ? 1 : 0);
     });
 }