diff --git a/.changeset/fix-openrouter-referer-header.md b/.changeset/fix-openrouter-referer-header.md
new file mode 100644
index 00000000000..cd0d110d446
--- /dev/null
+++ b/.changeset/fix-openrouter-referer-header.md
@@ -0,0 +1,5 @@
+---
+"@effect/ai-openrouter": patch
+---
+
+Fix typo in HTTP header name: `HTTP-Referrer` → `HTTP-Referer`. The HTTP spec spells it "Referer" (single r), and OpenRouter expects this exact header name for app attribution.
diff --git a/packages/ai/openrouter/src/OpenRouterClient.ts b/packages/ai/openrouter/src/OpenRouterClient.ts
index ecf1f56c649..9cd54a6c56d 100644
--- a/packages/ai/openrouter/src/OpenRouterClient.ts
+++ b/packages/ai/openrouter/src/OpenRouterClient.ts
@@ -102,7 +102,7 @@ export const make: (options: {
       request.pipe(
         HttpClientRequest.prependUrl(options.apiUrl ?? "https://openrouter.ai/api/v1"),
         options.apiKey ? HttpClientRequest.bearerToken(options.apiKey) : identity,
-        options.referrer ? HttpClientRequest.setHeader("HTTP-Referrer", options.referrer) : identity,
+        options.referrer ? HttpClientRequest.setHeader("HTTP-Referer", options.referrer) : identity,
         options.title ? HttpClientRequest.setHeader("X-Title", options.title) : identity,
         HttpClientRequest.acceptJson
       )