diff --git a/packages/web/server/lib/tts/DOCUMENTATION.md b/packages/web/server/lib/tts/DOCUMENTATION.md
index e80dc746..d39caf69 100644
--- a/packages/web/server/lib/tts/DOCUMENTATION.md
+++ b/packages/web/server/lib/tts/DOCUMENTATION.md
@@ -5,10 +5,12 @@ This module provides server-side Text-to-Speech services using OpenAI's TTS API,
 
 ## Entrypoints and structure
 - `packages/web/server/lib/tts/index.js`: Public entrypoint imported by `packages/web/server/index.js`.
-- `packages/web/server/lib/tts/routes.js`: Express route registration for `/api/voice/*` and `/api/tts/*` endpoints.
+- `packages/web/server/lib/tts/routes.js`: Express route registration for `/api/voice/*`, `/api/tts/*`, and `/api/stt/*` endpoints.
 - `packages/web/server/lib/tts/capability-runtime.js`: runtime helper for probing local macOS `say` TTS voice capability.
 - `packages/web/server/lib/tts/service.js`: TTS service implementation with OpenAI integration.
 - `packages/web/server/lib/tts/summarization.js`: Text summarization and sanitization utilities using opencode.ai zen API.
+- `packages/web/server/lib/tts/stt.js`: STT proxy for OpenAI-compatible transcription endpoints.
+- `packages/web/server/lib/tts/base-url.js`: shared base URL validation and normalization for custom OpenAI-compatible endpoints.
 
 ## Public exports
 
diff --git a/packages/web/server/lib/tts/base-url.js b/packages/web/server/lib/tts/base-url.js
new file mode 100644
index 00000000..493d18e4
--- /dev/null
+++ b/packages/web/server/lib/tts/base-url.js
@@ -0,0 +1,62 @@
+const LOCAL_BASE_URL_HOSTS = new Set([
+  'localhost',
+  '127.0.0.1',
+  '::1',
+  'host.docker.internal',
+]);
+
+const isEnvFlagEnabled = (value) => {
+  if (value === true || value === 1) return true;
+  if (typeof value !== 'string') return false;
+  const normalized = value.trim().toLowerCase();
+  return normalized === '1' || normalized === 'true';
+};
+
+const normalizeHostname = (hostname) => {
+  if (typeof hostname !== 'string') return '';
+  const trimmed = hostname.trim().toLowerCase();
+  if (!trimmed) return '';
+  if (trimmed.startsWith('[') && trimmed.endsWith(']')) {
+    return trimmed.slice(1, -1);
+  }
+  return trimmed;
+};
+
+const isAllowedLocalHost = (hostname) => {
+  const normalized = normalizeHostname(hostname);
+  return LOCAL_BASE_URL_HOSTS.has(normalized);
+};
+
+export const normalizeCustomOpenAIBaseURL = (value) => {
+  if (typeof value !== 'string' || !value.trim()) {
+    return { value: undefined };
+  }
+
+  let parsed;
+  try {
+    parsed = new URL(value.trim());
+  } catch {
+    return { error: 'Custom server URL is invalid' };
+  }
+
+  if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
+    return { error: 'Custom server URL must use http or https' };
+  }
+
+  if (parsed.username || parsed.password) {
+    return { error: 'Custom server URL must not include credentials' };
+  }
+
+  const allowRemote = isEnvFlagEnabled(process.env.OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS);
+  if (!allowRemote && !isAllowedLocalHost(parsed.hostname)) {
+    return {
+      error: 'Remote custom server URLs are disabled. Set OPENCHAMBER_ALLOW_REMOTE_OPENAI_COMPAT_URLS=true to allow this host.',
+    };
+  }
+
+  parsed.hash = '';
+  parsed.search = '';
+  const pathname = parsed.pathname.replace(/\/+$/, '');
+  const normalizedPath = pathname.length > 0 ? pathname : '';
+  return { value: `${parsed.protocol}//${parsed.host}${normalizedPath}` };
+};
diff --git a/packages/web/server/lib/tts/routes.js b/packages/web/server/lib/tts/routes.js
index 217e0d96..e4235aca 100644
--- a/packages/web/server/lib/tts/routes.js
+++ b/packages/web/server/lib/tts/routes.js
@@ -234,9 +234,13 @@ export function registerTtsRoutes(app, { resolveZenModel, sayTTSCapability }) {
         const { transcribeAudio } = await import('./stt.js');
 
         const mimeType = (req.headers['content-type'] || 'audio/webm').split(',')[0].trim();
-        const baseURL = req.headers['x-base-url'];
-        const model = req.headers['x-model'] || 'deepdml/faster-whisper-large-v3-turbo-ct2';
-        const language = req.headers['x-language'] || undefined;
+        const baseURL = typeof req.headers['x-base-url'] === 'string' ? req.headers['x-base-url'].trim() : '';
+        const model = typeof req.headers['x-model'] === 'string' && req.headers['x-model'].trim().length > 0
+          ? req.headers['x-model'].trim()
+          : 'deepdml/faster-whisper-large-v3-turbo-ct2';
+        const language = typeof req.headers['x-language'] === 'string' && req.headers['x-language'].trim().length > 0
+          ? req.headers['x-language'].trim()
+          : undefined;
 
         if (!req.body || !Buffer.isBuffer(req.body) || req.body.length === 0) {
           return res.status(400).json({ error: 'Audio data is required' });
diff --git a/packages/web/server/lib/tts/stt.js b/packages/web/server/lib/tts/stt.js
index a947ae6a..1136c332 100644
--- a/packages/web/server/lib/tts/stt.js
+++ b/packages/web/server/lib/tts/stt.js
@@ -6,6 +6,7 @@
  */
 
 import OpenAI, { toFile } from 'openai';
+import { normalizeCustomOpenAIBaseURL } from './base-url.js';
 
 /**
  * Transcribe an audio buffer via an OpenAI-compatible /v1/audio/transcriptions endpoint.
@@ -19,12 +20,20 @@ import OpenAI, { toFile } from 'openai';
  * @returns {Promise<string>} Transcribed text
  */
 export async function transcribeAudio({ audioBuffer, mimeType, model, baseURL, language }) {
+  const normalizedBaseURLResult = normalizeCustomOpenAIBaseURL(baseURL);
+  if (normalizedBaseURLResult.error) {
+    throw new Error(normalizedBaseURLResult.error);
+  }
+
+  const normalizedBaseURL = normalizedBaseURLResult.value;
+  if (!normalizedBaseURL) {
+    throw new Error('Custom server URL is required');
+  }
+
   const clientOpts = {
     apiKey: process.env.OPENAI_API_KEY || 'not-required',
   };
-  if (baseURL) {
-    clientOpts.baseURL = baseURL;
-  }
+  clientOpts.baseURL = normalizedBaseURL;
 
   const client = new OpenAI(clientOpts);