diff --git a/src/config.test.ts b/src/config.test.ts
index 7b3eeeb6..465771f8 100644
--- a/src/config.test.ts
+++ b/src/config.test.ts
@@ -1,8 +1,9 @@
 import assert from "node:assert/strict";
-import { mkdtempSync, writeFileSync } from "node:fs";
+import { mkdtempSync, readFileSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { loadConfig } from "./config.js";
+import { writeDevspaceConfig } from "./user-config.js";
 
 const emptyConfigDir = mkdtempSync(join(tmpdir(), "devspace-empty-config-test-"));
 const baseEnv = {
@@ -148,11 +149,42 @@ assert.deepEqual(
   loadConfig({ ...baseEnv, DEVSPACE_PUBLIC_BASE_URL: "https://abc.trycloudflare.com/" }).allowedHosts,
   ["localhost", "127.0.0.1", "::1", "abc.trycloudflare.com"],
 );
+assert.throws(
+  () => loadConfig({ ...baseEnv, DEVSPACE_PUBLIC_BASE_URL: "ftp://example.com/devspace" }),
+  /publicBaseUrl must use http or https/,
+);
 assert.deepEqual(
   loadConfig({ ...baseEnv, DEVSPACE_ALLOWED_HOSTS: "*" }).allowedHosts,
   ["*"],
 );
 
+const invalidConfigDir = mkdtempSync(join(tmpdir(), "devspace-invalid-public-url-test-"));
+writeFileSync(
+  join(invalidConfigDir, "config.json"),
+  JSON.stringify({
+    allowedRoots: [process.cwd()],
+    publicBaseUrl: "ftp://example.com/devspace",
+  }),
+);
+writeFileSync(
+  join(invalidConfigDir, "auth.json"),
+  JSON.stringify({ ownerToken: "persisted-owner-token-long-enough" }),
+);
+assert.throws(
+  () => loadConfig({ DEVSPACE_CONFIG_DIR: invalidConfigDir }),
+  /publicBaseUrl must use http or https/,
+);
+
+const writeConfigDir = mkdtempSync(join(tmpdir(), "devspace-public-url-write-test-"));
+const writeEnv = { DEVSPACE_CONFIG_DIR: writeConfigDir };
+writeDevspaceConfig({ publicBaseUrl: "https://devspace.example.com/" }, writeEnv);
+const persistedBeforeInvalidWrite = readFileSync(join(writeConfigDir, "config.json"), "utf8");
+assert.throws(
+  () => writeDevspaceConfig({ publicBaseUrl: "ftp://example.com/devspace" }, writeEnv),
+  /publicBaseUrl must use http or https/,
+);
+assert.equal(readFileSync(join(writeConfigDir, "config.json"), "utf8"), persistedBeforeInvalidWrite);
+
 const configDir = mkdtempSync(join(tmpdir(), "devspace-config-test-"));
 writeFileSync(
   join(configDir, "config.json"),
diff --git a/src/config.ts b/src/config.ts
index 54a131c9..5c16dae5 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -3,7 +3,12 @@ import { join, resolve } from "node:path";
 import { expandHomePath } from "./roots.js";
 import type { LoggingConfig, LogFormat, LogLevel } from "./logger.js";
 import type { OAuthConfig } from "./oauth-provider.js";
-import { devspaceAgentsDir, devspaceSkillsDir, loadDevspaceFiles } from "./user-config.js";
+import {
+  devspaceAgentsDir,
+  devspaceSkillsDir,
+  loadDevspaceFiles,
+  normalizePublicBaseUrl,
+} from "./user-config.js";
 import { resolveSubagentsConfig, type SubagentsConfig } from "./local-agent-config.js";
 
 export type ToolMode = "minimal" | "full" | "codex";
@@ -212,7 +217,7 @@ export function loadConfig(env: NodeJS.ProcessEnv = process.env): ServerConfig {
   const files = loadDevspaceFiles(env);
   const host = env.HOST ?? files.config.host ?? "127.0.0.1";
   const port = parsePort(env.PORT ?? files.config.port);
-  const publicBaseUrl = parsePublicBaseUrl(
+  const publicBaseUrl = normalizePublicBaseUrl(
     env.DEVSPACE_PUBLIC_BASE_URL ?? files.config.publicBaseUrl ?? localPublicBaseUrl(host, port),
   );
   const derivedAllowedHosts = [
@@ -258,14 +263,6 @@ function numberConfigValue(value: number | undefined): string | undefined {
   return value === undefined ? undefined : String(value);
 }
 
-function parsePublicBaseUrl(value: string): string {
-  const parsed = new URL(value);
-  parsed.hash = "";
-  parsed.search = "";
-  parsed.pathname = parsed.pathname.replace(/\/+$/, "");
-  return parsed.toString().replace(/\/$/, "");
-}
-
 function localPublicBaseUrl(host: string, port: number): string {
   const publicHost = host === "0.0.0.0" || host === "::" ? "127.0.0.1" : host;
   const formattedHost = publicHost.includes(":") && !publicHost.startsWith("[")
diff --git a/src/user-config.ts b/src/user-config.ts
index 98d05ac6..6643a64a 100644
--- a/src/user-config.ts
+++ b/src/user-config.ts
@@ -76,13 +76,27 @@ export function loadDevspaceFiles(env: NodeJS.ProcessEnv = process.env): Devspac
   };
 }
 
+export function normalizePublicBaseUrl(value: string): string {
+  const parsed = new URL(value);
+  if (parsed.protocol !== "http:" && parsed.protocol !== "https:") {
+    throw new Error("publicBaseUrl must use http or https.");
+  }
+  parsed.hash = "";
+  parsed.search = "";
+  parsed.pathname = parsed.pathname.replace(/\/+$/, "");
+  return parsed.toString().replace(/\/$/, "");
+}
+
 export function writeDevspaceConfig(
   config: DevspaceUserConfig,
   env: NodeJS.ProcessEnv = process.env,
 ): string {
   const filePath = devspaceConfigPath(env);
+  const normalizedConfig = config.publicBaseUrl == null
+    ? config
+    : { ...config, publicBaseUrl: normalizePublicBaseUrl(config.publicBaseUrl) };
   mkdirSync(devspaceConfigDir(env), { recursive: true });
-  writeJsonFile(filePath, config, 0o600);
+  writeJsonFile(filePath, normalizedConfig, 0o600);
   return filePath;
 }