diff --git a/packages/opencode/src/provider/error.ts b/packages/opencode/src/provider/error.ts
index b13a3375a483..52d6f08ee62e 100644
--- a/packages/opencode/src/provider/error.ts
+++ b/packages/opencode/src/provider/error.ts
@@ -99,7 +99,7 @@ export type ParsedStreamError =
       responseBody: string
     }
 
-// OpenRouter relays upstream overflow as a typeless `{ code: 502, message }` chunk.
+// OpenRouter relays upstream failures as typeless `{ code: 5xx, message }` chunks.
 function isOverflow(body: { error?: { code?: string; message?: string }; message?: string }) {
   return body.error?.code === "context_length_exceeded" || isContextOverflow(body.error?.message ?? body.message ?? "")
 }
@@ -117,6 +117,14 @@ export function parseStreamError(input: unknown): ParsedStreamError | undefined
       responseBody,
     }
   }
+  if (typeof body.code === "number" && body.code >= 500) {
+    return {
+      type: "api_error",
+      message: typeof body.message === "string" ? body.message : "Server error.",
+      isRetryable: true,
+      responseBody,
+    }
+  }
   if (body.type !== "error") return
 
   switch (body?.error?.code) {
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index 4dbddcc09b43..cbc89d254b3f 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -1399,6 +1399,24 @@ describe("session.message-v2.fromError", () => {
     })
   })
 
+  test("serializes OpenRouter typeless 5xx chunks as retryable APIError", () => {
+    const input = {
+      code: 502,
+      message: "Stream ended before a terminal response event",
+      metadata: { error_type: "provider_unavailable" },
+    }
+    const result = MessageV2.fromError(input, { providerID })
+
+    expect(result).toStrictEqual({
+      name: "APIError",
+      data: {
+        message: input.message,
+        isRetryable: true,
+        responseBody: JSON.stringify(input),
+      },
+    })
+  })
+
   test("serializes response error codes", () => {
     const cases = [
       {