diff --git a/index.js b/index.js
index d55980c..10d1791 100644
--- a/index.js
+++ b/index.js
@@ -16,16 +16,23 @@ function corsHeaders(request) {
   const requestedMethod = request?.headers.get("access-control-request-method")
   const requestedPrivateNetwork = request?.headers.get("access-control-request-private-network")
   const requestOrigin = request?.headers.get("origin") ?? ""
-  const allowOrigin = configuredOrigin === "*" ? "*" : (requestOrigin === configuredOrigin ? requestOrigin : configuredOrigin)
+  // When a specific origin is configured, only echo it back when the request
+  // origin matches exactly. On a mismatch we omit the header entirely so that
+  // browsers block the cross-origin request and the configured origin is not
+  // disclosed to untrusted callers.
+  const allowOrigin = configuredOrigin === "*" ? "*" : (requestOrigin === configuredOrigin ? requestOrigin : null)
 
   const headers = {
     vary: "origin, access-control-request-method, access-control-request-headers",
-    "access-control-allow-origin": allowOrigin,
     "access-control-allow-headers": requestedHeaders ?? "authorization, content-type, x-opencode-provider",
     "access-control-allow-methods": requestedMethod ?? "GET, POST, OPTIONS",
     "access-control-max-age": "86400",
   }
 
+  if (allowOrigin !== null) {
+    headers["access-control-allow-origin"] = allowOrigin
+  }
+
   if (requestedPrivateNetwork === "true") {
     headers["access-control-allow-private-network"] = "true"
   }
diff --git a/index.test.js b/index.test.js
index 49d5130..0129ada 100644
--- a/index.test.js
+++ b/index.test.js
@@ -162,14 +162,14 @@ test("health response includes CORS headers", async () => {
   assert.deepEqual(body, { healthy: true, service: "opencode-openai-proxy" })
 })
 
-test("configured origin is returned for normal requests", async () => {
+test("configured origin is returned when request origin matches", async () => {
   process.env.OPENCODE_LLM_PROXY_CORS_ORIGIN = "https://console.example.com"
 
   try {
     const handler = createProxyFetchHandler(createClient())
     const request = new Request("http://127.0.0.1:4010/health", {
       headers: {
-        Origin: "https://app.example.com",
+        Origin: "https://console.example.com",
       },
     })
 
@@ -181,7 +181,7 @@ test("configured origin is returned for normal requests", async () => {
   }
 })
 
-test("disallowed origin does not receive its own origin back", async () => {
+test("disallowed origin does not receive access-control-allow-origin header", async () => {
   process.env.OPENCODE_LLM_PROXY_CORS_ORIGIN = "https://allowed.example.com"
 
   try {
@@ -192,9 +192,9 @@ test("disallowed origin does not receive its own origin back", async () => {
 
     const response = await handler(request)
 
-    // The header must be the configured origin, not the request's origin
-    assert.equal(response.headers.get("access-control-allow-origin"), "https://allowed.example.com")
-    assert.notEqual(response.headers.get("access-control-allow-origin"), "https://evil.example.com")
+    // On an origin mismatch the header must be absent so neither the
+    // configured origin is leaked nor the browser grants cross-origin access.
+    assert.equal(response.headers.get("access-control-allow-origin"), null)
   } finally {
     delete process.env.OPENCODE_LLM_PROXY_CORS_ORIGIN
   }
@@ -211,7 +211,7 @@ test("request with no Origin header is handled gracefully", async () => {
   assert.equal(response.headers.get("access-control-allow-origin"), "*")
 })
 
-test("OPTIONS preflight for disallowed origin returns configured origin, not request origin", async () => {
+test("OPTIONS preflight for disallowed origin omits access-control-allow-origin header", async () => {
   process.env.OPENCODE_LLM_PROXY_CORS_ORIGIN = "https://allowed.example.com"
 
   try {
@@ -227,8 +227,9 @@ test("OPTIONS preflight for disallowed origin returns configured origin, not req
     const response = await handler(request)
 
     assert.equal(response.status, 204)
-    assert.equal(response.headers.get("access-control-allow-origin"), "https://allowed.example.com")
-    assert.notEqual(response.headers.get("access-control-allow-origin"), "https://evil.example.com")
+    // The configured origin must not be disclosed on a mismatch; the header
+    // should be absent so the preflight is rejected by the browser.
+    assert.equal(response.headers.get("access-control-allow-origin"), null)
   } finally {
     delete process.env.OPENCODE_LLM_PROXY_CORS_ORIGIN
   }