diff --git a/src/main/app-controller.ts b/src/main/app-controller.ts
index 31f7364..1a28278 100644
--- a/src/main/app-controller.ts
+++ b/src/main/app-controller.ts
@@ -38,6 +38,7 @@ import { rotateDebugToken, startDebugServer, stopDebugServer } from "./debug-ser
 import { encryptBackup, decryptBackup } from "./backup-crypto";
 import { getAuditLogPath, initAuditLog, logAuditEvent, shutdownAuditLog } from "./audit-log";
 import { initAccountRotationLog, shutdownAccountRotationLog } from "./account-rotation-log";
+import { runStartupHealthCheck } from "./startup-health-check";
 import { getDebugSetupCheck } from "./debug-setup";
 import { buildLinkExportSelection, serializeLinkExportText } from "./link-export";
 import { getRenameLogPath, initRenameLog, shutdownRenameLog } from "./rename-log";
@@ -117,6 +118,38 @@ export class AppController {
       appVersion: APP_VERSION,
       runtimeDir: this.storagePaths.baseDir
     });
+    // Startup Health-Check: surface problematic state early (missing download
+    // dir, low disk space, no provider configured, corrupted state file).
+    // Never blocks startup — findings go into the normal log + audit log so
+    // the user can diagnose issues before hitting them mid-download.
+    try {
+      const report = runStartupHealthCheck(this.settings, this.storagePaths);
+      if (report.errorCount > 0 || report.warnCount > 0) {
+        logger.warn(`Health-Check: ${report.errorCount} Fehler, ${report.warnCount} Warnungen, ${report.infoCount} Info`);
+      } else {
+        logger.info(`Health-Check: alles OK (${report.infoCount} Info)`);
+      }
+      for (const finding of report.findings) {
+        const line = finding.hint
+          ? `Health-Check [${finding.code}]: ${finding.message} — ${finding.hint}`
+          : `Health-Check [${finding.code}]: ${finding.message}`;
+        if (finding.severity === "ERROR") {
+          logger.error(line);
+        } else if (finding.severity === "WARN") {
+          logger.warn(line);
+        } else {
+          logger.info(line);
+        }
+        if (finding.severity !== "INFO") {
+          logAuditEvent(finding.severity, `Health-Check: ${finding.code}`, {
+            message: finding.message,
+            hint: finding.hint || ""
+          });
+        }
+      }
+    } catch (err) {
+      logger.warn(`Health-Check uebersprungen (Fehler): ${String((err as Error).message || err)}`);
+    }
     startDebugServer(this.manager, this.storagePaths.baseDir);
     this.runtimeStatsTimer = setInterval(() => {
       this.manager.persistRuntimeStats();
diff --git a/src/main/startup-health-check.ts b/src/main/startup-health-check.ts
new file mode 100644
index 0000000..cadc8cd
--- /dev/null
+++ b/src/main/startup-health-check.ts
@@ -0,0 +1,220 @@
+import fs from "node:fs";
+import path from "node:path";
+import { AppSettings } from "../shared/types";
+import { parseDebridLinkApiKeys } from "../shared/debrid-link-keys";
+import { parseMegaDebridAccounts } from "../shared/mega-debrid-accounts";
+import { StoragePaths } from "./storage";
+
+/** Startup Health-Check: runs once at app boot and surfaces potential problem
+ *  states BEFORE the user hits them mid-download.
+ *
+ *  Goals:
+ *   - Warn on missing / unreachable download directory
+ *   - Warn on low disk space (< 5 GB free)
+ *   - Warn when no debrid provider is configured (app is effectively offline)
+ *   - Warn when state file is suspiciously large (>50 MB → pruning recommended)
+ *
+ *  Non-goals: blocking startup. The check only logs — the app continues. */
+
+export type HealthCheckSeverity = "INFO" | "WARN" | "ERROR";
+
+export interface HealthCheckFinding {
+  severity: HealthCheckSeverity;
+  code: string;
+  message: string;
+  hint?: string;
+}
+
+export interface HealthCheckReport {
+  findings: HealthCheckFinding[];
+  errorCount: number;
+  warnCount: number;
+  infoCount: number;
+}
+
+const LOW_DISK_SPACE_BYTES = 5 * 1024 * 1024 * 1024; // 5 GB
+const LARGE_STATE_FILE_BYTES = 50 * 1024 * 1024;     // 50 MB
+
+function safeExists(p: string): boolean {
+  try {
+    return fs.existsSync(p);
+  } catch {
+    return false;
+  }
+}
+
+function getFileSizeBytes(p: string): number {
+  try {
+    const stat = fs.statSync(p);
+    return stat.size;
+  } catch {
+    return 0;
+  }
+}
+
+/** Attempt a tiny write-probe in the given directory. Returns true on
+ *  success, false if the directory isn't writable. We write and immediately
+ *  delete a uniquely-named temp file so we never leave garbage behind. */
+function isWritable(dir: string): boolean {
+  const probe = path.join(dir, `.rddl-health-probe-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`);
+  try {
+    fs.writeFileSync(probe, "x", { encoding: "utf8" });
+    fs.rmSync(probe, { force: true });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/** Query free disk space for a given path. Returns null if unsupported or
+ *  the query fails — callers treat null as "unknown" and skip the check. */
+function getFreeDiskSpaceBytes(target: string): number | null {
+  try {
+    // fs.statfsSync is available on Node 18.15+; on Windows it still maps to
+    // the underlying volume so it works for download dirs on any drive.
+    const statfs = (fs as unknown as { statfsSync?: (p: string) => { bavail: bigint; bsize: bigint } }).statfsSync;
+    if (typeof statfs !== "function") {
+      return null;
+    }
+    const result = statfs(target);
+    const bavail = BigInt(result.bavail);
+    const bsize = BigInt(result.bsize);
+    const free = bavail * bsize;
+    if (free > BigInt(Number.MAX_SAFE_INTEGER)) {
+      return Number.MAX_SAFE_INTEGER;
+    }
+    return Number(free);
+  } catch {
+    return null;
+  }
+}
+
+function countConfiguredProviders(settings: AppSettings): { count: number; providers: string[] } {
+  const providers: string[] = [];
+  if (settings.token?.trim() || settings.realDebridUseWebLogin) {
+    providers.push("Real-Debrid");
+  }
+  if (settings.allDebridToken?.trim() || settings.allDebridUseWebLogin) {
+    providers.push("AllDebrid");
+  }
+  if (settings.bestToken?.trim() || settings.bestDebridUseWebLogin) {
+    providers.push("BestDebrid");
+  }
+  if (settings.oneFichierApiKey?.trim()) {
+    providers.push("1Fichier");
+  }
+  if (settings.ddownloadLogin?.trim() && settings.ddownloadPassword?.trim()) {
+    providers.push("DDownload");
+  }
+  if (settings.linkSnappyLogin?.trim() && settings.linkSnappyPassword?.trim()) {
+    providers.push("LinkSnappy");
+  }
+  const dlKeys = parseDebridLinkApiKeys(settings.debridLinkApiKeys || "");
+  if (dlKeys.length > 0) {
+    providers.push(`Debrid-Link (${dlKeys.length} Key${dlKeys.length === 1 ? "" : "s"})`);
+  }
+  const megaAccounts = parseMegaDebridAccounts(settings.megaCredentials || "");
+  const legacyMegaConfigured = Boolean(settings.megaLogin?.trim() && settings.megaPassword?.trim());
+  if (megaAccounts.length > 0) {
+    providers.push(`Mega-Debrid (${megaAccounts.length} Acc)`);
+  } else if (legacyMegaConfigured) {
+    providers.push("Mega-Debrid");
+  }
+  return { count: providers.length, providers };
+}
+
+/** Pure check function: takes inputs, returns findings. Kept side-effect-free
+ *  so it's trivial to unit-test — the caller handles logging / persistence. */
+export function runStartupHealthCheck(settings: AppSettings, storagePaths: StoragePaths): HealthCheckReport {
+  const findings: HealthCheckFinding[] = [];
+
+  // ── 1. Download directory ───────────────────────────────────────────────
+  const outputDir = String(settings.outputDir || "").trim();
+  if (!outputDir) {
+    findings.push({
+      severity: "WARN",
+      code: "outputDir_missing",
+      message: "Kein Download-Ziel-Verzeichnis konfiguriert",
+      hint: "In den Einstellungen unter 'Downloads' einen Ziel-Ordner setzen, sonst koennen keine Downloads starten."
+    });
+  } else if (!safeExists(outputDir)) {
+    findings.push({
+      severity: "WARN",
+      code: "outputDir_not_found",
+      message: `Download-Ziel-Ordner existiert nicht: ${outputDir}`,
+      hint: "Der Ordner wird beim ersten Download automatisch erstellt, sofern der Elternordner existiert und beschreibbar ist."
+    });
+  } else if (!isWritable(outputDir)) {
+    findings.push({
+      severity: "ERROR",
+      code: "outputDir_not_writable",
+      message: `Download-Ziel-Ordner ist NICHT beschreibbar: ${outputDir}`,
+      hint: "Rechte pruefen oder anderen Ordner waehlen. Downloads werden sonst direkt scheitern."
+    });
+  } else {
+    // Check available disk space only when the directory is actually usable
+    const freeBytes = getFreeDiskSpaceBytes(outputDir);
+    if (freeBytes !== null && freeBytes < LOW_DISK_SPACE_BYTES) {
+      const freeMb = Math.round(freeBytes / (1024 * 1024));
+      findings.push({
+        severity: "WARN",
+        code: "low_disk_space",
+        message: `Wenig freier Speicher im Download-Ordner: ~${freeMb} MB verfuegbar (Schwelle ${LOW_DISK_SPACE_BYTES / (1024 * 1024 * 1024)} GB)`,
+        hint: "Groessere Downloads koennen auf halbem Weg fehlschlagen. Vorher Platz schaffen oder anderen Ordner waehlen."
+      });
+    }
+  }
+
+  // ── 2. Provider-Credentials ─────────────────────────────────────────────
+  const { count, providers } = countConfiguredProviders(settings);
+  if (count === 0) {
+    findings.push({
+      severity: "WARN",
+      code: "no_provider_configured",
+      message: "Kein Debrid-Provider konfiguriert — Downloads werden nicht funktionieren",
+      hint: "In den Einstellungen mindestens einen Provider (Real-Debrid, Mega-Debrid, Debrid-Link, ...) einrichten."
+    });
+  } else {
+    findings.push({
+      severity: "INFO",
+      code: "providers_configured",
+      message: `Konfigurierte Provider: ${providers.join(", ")}`
+    });
+  }
+
+  // ── 3. State-File-Groesse ──────────────────────────────────────────────
+  if (safeExists(storagePaths.sessionFile)) {
+    const sizeBytes = getFileSizeBytes(storagePaths.sessionFile);
+    if (sizeBytes > LARGE_STATE_FILE_BYTES) {
+      const sizeMb = Math.round(sizeBytes / (1024 * 1024));
+      findings.push({
+        severity: "WARN",
+        code: "large_state_file",
+        message: `State-Datei ist sehr gross: ${sizeMb} MB (${path.basename(storagePaths.sessionFile)})`,
+        hint: "Alte abgeschlossene Pakete aus der Queue entfernen, damit Startup + Save schneller werden."
+      });
+    }
+  }
+
+  // ── 4. Storage-Basis-Verzeichnis muss beschreibbar sein (fuer Logs) ────
+  if (!safeExists(storagePaths.baseDir)) {
+    findings.push({
+      severity: "ERROR",
+      code: "baseDir_missing",
+      message: `Runtime-Verzeichnis existiert nicht: ${storagePaths.baseDir}`,
+      hint: "Ohne Runtime-Verzeichnis koennen weder Settings noch Session-State persistiert werden."
+    });
+  } else if (!isWritable(storagePaths.baseDir)) {
+    findings.push({
+      severity: "ERROR",
+      code: "baseDir_not_writable",
+      message: `Runtime-Verzeichnis ist NICHT beschreibbar: ${storagePaths.baseDir}`,
+      hint: "Rechte auf das Runtime-Verzeichnis pruefen (%APPDATA%/Real-Debrid-Downloader/runtime)."
+    });
+  }
+
+  const errorCount = findings.filter((f) => f.severity === "ERROR").length;
+  const warnCount = findings.filter((f) => f.severity === "WARN").length;
+  const infoCount = findings.filter((f) => f.severity === "INFO").length;
+  return { findings, errorCount, warnCount, infoCount };
+}
diff --git a/tests/startup-health-check.test.ts b/tests/startup-health-check.test.ts
new file mode 100644
index 0000000..482760f
--- /dev/null
+++ b/tests/startup-health-check.test.ts
@@ -0,0 +1,137 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+import { defaultSettings } from "../src/main/constants";
+import { createStoragePaths } from "../src/main/storage";
+import { runStartupHealthCheck } from "../src/main/startup-health-check";
+
+const tempDirs: string[] = [];
+
+afterEach(() => {
+  for (const dir of tempDirs.splice(0)) {
+    try {
+      fs.rmSync(dir, { recursive: true, force: true });
+    } catch {
+      // ignore cleanup errors
+    }
+  }
+});
+
+function makeTempBase(): { baseDir: string; outputDir: string; paths: ReturnType<typeof createStoragePaths> } {
+  const baseDir = fs.mkdtempSync(path.join(os.tmpdir(), "rd-health-"));
+  tempDirs.push(baseDir);
+  const outputDir = path.join(baseDir, "downloads");
+  fs.mkdirSync(outputDir, { recursive: true });
+  return {
+    baseDir: path.join(baseDir, "runtime"),
+    outputDir,
+    paths: createStoragePaths(path.join(baseDir, "runtime"))
+  };
+}
+
+describe("runStartupHealthCheck", () => {
+  it("flags missing download directory", () => {
+    const { outputDir, paths } = makeTempBase();
+    fs.mkdirSync(paths.baseDir, { recursive: true });
+
+    const settings = {
+      ...defaultSettings(),
+      token: "rd-token",
+      outputDir: path.join(outputDir, "does-not-exist-subdir")
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    const codes = report.findings.map((f) => f.code);
+    expect(codes).toContain("outputDir_not_found");
+  });
+
+  it("flags no-provider-configured when all credentials are empty", () => {
+    const { outputDir, paths } = makeTempBase();
+    fs.mkdirSync(paths.baseDir, { recursive: true });
+
+    const settings = {
+      ...defaultSettings(),
+      token: "",
+      megaLogin: "",
+      megaPassword: "",
+      megaCredentials: "",
+      allDebridToken: "",
+      bestToken: "",
+      oneFichierApiKey: "",
+      debridLinkApiKeys: "",
+      outputDir
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    const codes = report.findings.map((f) => f.code);
+    expect(codes).toContain("no_provider_configured");
+    expect(report.warnCount).toBeGreaterThanOrEqual(1);
+  });
+
+  it("reports configured providers when at least one credential is set", () => {
+    const { outputDir, paths } = makeTempBase();
+    fs.mkdirSync(paths.baseDir, { recursive: true });
+
+    const settings = {
+      ...defaultSettings(),
+      token: "rd-token-here",
+      debridLinkApiKeys: "dl-key-a\ndl-key-b",
+      outputDir
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    const providersFinding = report.findings.find((f) => f.code === "providers_configured");
+    expect(providersFinding).toBeDefined();
+    expect(providersFinding?.message).toContain("Real-Debrid");
+    expect(providersFinding?.message).toContain("Debrid-Link");
+    expect(providersFinding?.message).toContain("2 Keys");
+  });
+
+  it("flags large state files", () => {
+    const { outputDir, paths } = makeTempBase();
+    fs.mkdirSync(paths.baseDir, { recursive: true });
+    // 60 MB dummy state file, threshold is 50 MB
+    fs.writeFileSync(paths.sessionFile, Buffer.alloc(60 * 1024 * 1024, 0));
+
+    const settings = {
+      ...defaultSettings(),
+      token: "rd-token",
+      outputDir
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    const codes = report.findings.map((f) => f.code);
+    expect(codes).toContain("large_state_file");
+  });
+
+  it("flags missing base dir as ERROR", () => {
+    const { outputDir, paths } = makeTempBase();
+    // Intentionally DON'T create baseDir.
+
+    const settings = {
+      ...defaultSettings(),
+      token: "rd-token",
+      outputDir
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    const codes = report.findings.map((f) => f.code);
+    expect(codes).toContain("baseDir_missing");
+    expect(report.errorCount).toBeGreaterThanOrEqual(1);
+  });
+
+  it("passes cleanly when everything is healthy", () => {
+    const { outputDir, paths } = makeTempBase();
+    fs.mkdirSync(paths.baseDir, { recursive: true });
+
+    const settings = {
+      ...defaultSettings(),
+      token: "rd-token-here",
+      outputDir
+    };
+    const report = runStartupHealthCheck(settings, paths);
+    expect(report.errorCount).toBe(0);
+    const codes = report.findings.map((f) => f.code);
+    expect(codes).not.toContain("outputDir_not_found");
+    expect(codes).not.toContain("outputDir_not_writable");
+    expect(codes).not.toContain("no_provider_configured");
+    expect(codes).not.toContain("baseDir_missing");
+    expect(codes).not.toContain("baseDir_not_writable");
+  });
+});