diff --git a/packages/http-transport/src/server.ts b/packages/http-transport/src/server.ts
index eec67317..89842295 100644
--- a/packages/http-transport/src/server.ts
+++ b/packages/http-transport/src/server.ts
@@ -438,13 +438,17 @@ export class HttpTransportServer implements TransportWorker<
           this.#corsOptions.origin === true
             ? { ...DEFAULT_CORS_PARAMS }
             : { ...EXPLICIT_ORIGIN_CORS_PARAMS }
-        // Iterating base params also drops allowCredentials for origin: true
         for (const key in params) {
           const value = this.#corsOptions[key]
           if (value !== undefined) {
             params[key] = value
           }
         }
+        // This explicit opt-in restores credentialed origin reflection without
+        // weakening the safe `cors: true` default.
+        if (this.#corsOptions.allowCredentials !== undefined) {
+          params.allowCredentials = this.#corsOptions.allowCredentials
+        }
       }
     } else if (typeof this.#corsOptions === 'function') {
       const result = this.#corsOptions(origin, request)
@@ -466,6 +470,9 @@ export class HttpTransportServer implements TransportWorker<
               params[key] = value
             }
           }
+          if (result.allowCredentials !== undefined) {
+            params.allowCredentials = result.allowCredentials
+          }
         }
       }
     }
diff --git a/packages/http-transport/src/types.ts b/packages/http-transport/src/types.ts
index 7c9aaef5..ef309a7c 100644
--- a/packages/http-transport/src/types.ts
+++ b/packages/http-transport/src/types.ts
@@ -22,31 +22,23 @@ export type HttpTransportOptions<
 }
 
 export type HttpTransportCorsCustomParams = {
+  /**
+   * `true` reflects any request origin, an array is an explicit allowlist.
+   * Credentials default on only for allowlisted origins.
+   */
+  origin: true | string[]
   allowMethods?: string[]
   allowHeaders?: string[]
+  /**
+   * Explicitly enables credentials when reflecting request origins. Only use
+   * this when the API should accept credentialed requests from any website.
+   */
+  allowCredentials?: string
   maxAge?: string
   exposeHeaders?: string[]
   requestHeaders?: string[]
   requestMethod?: string
-} & (
-  | {
-      /**
-       * `true` reflects any request origin, an array is an explicit
-       * allowlist. Credentials default on for allowlisted origins only.
-       */
-      origin: true | string[]
-      allowCredentials?: never
-    }
-  | {
-      /**
-       * Explicit `allowCredentials` requires an origin allowlist: combining
-       * credentials with a reflected origin (`origin: true`) would let any
-       * website make credentialed (cookie-authed) requests.
-       */
-      origin: string[]
-      allowCredentials?: string
-    }
-)
+}
 
 export type HttpTransportCorsOptions =
   | true
diff --git a/packages/http-transport/tests/cors.spec.ts b/packages/http-transport/tests/cors.spec.ts
index 7ab27e22..e022fbdf 100644
--- a/packages/http-transport/tests/cors.spec.ts
+++ b/packages/http-transport/tests/cors.spec.ts
@@ -46,13 +46,23 @@ describe('CORS credentials defaults', () => {
     expect(response.headers.get(ALLOW_CREDENTIALS)).toBeNull()
   })
 
-  it('omits credentials for custom params with origin: true', async () => {
+  it('omits credentials by default for custom params with origin: true', async () => {
     const response = await preflight({ origin: true })
 
     expect(response.headers.get(ALLOW_ORIGIN)).toBe(ORIGIN)
     expect(response.headers.get(ALLOW_CREDENTIALS)).toBeNull()
   })
 
+  it('allows explicit credentials with reflected origins', async () => {
+    const response = await preflight({
+      origin: true,
+      allowCredentials: 'true',
+    })
+
+    expect(response.headers.get(ALLOW_ORIGIN)).toBe(ORIGIN)
+    expect(response.headers.get(ALLOW_CREDENTIALS)).toBe('true')
+  })
+
   it('enables credentials for custom params with explicit origins', async () => {
     const response = await preflight({ origin: [ORIGIN] })
 
@@ -139,7 +149,7 @@ describe('CORS Vary header', () => {
 })
 
 describe('CORS config type surface', () => {
-  it('accepts safe configs and rejects credentials with reflected origin', () => {
+  it('accepts serializable explicit credentials with reflected origins', () => {
     const allowAll = Math.random() >= 0
     const allowedOrigins = [ORIGIN]
 
@@ -151,14 +161,13 @@ describe('CORS config type surface', () => {
       origin: allowedOrigins,
       allowCredentials: 'true',
     }
-    // @ts-expect-error credentials cannot be combined with origin reflection
-    const unsafe: HttpTransportCorsOptions = {
+    const reflectedCredentials: HttpTransportCorsOptions = {
       origin: true,
       allowCredentials: 'true',
     }
 
     expect(composed).toBeDefined()
     expect(credentialed).toBeDefined()
-    expect(unsafe).toBeDefined()
+    expect(reflectedCredentials).toBeDefined()
   })
 })