diff --git a/packages/opencode/src/provider/error.ts b/packages/opencode/src/provider/error.ts
index 21149a2cf389..b13a3375a483 100644
--- a/packages/opencode/src/provider/error.ts
+++ b/packages/opencode/src/provider/error.ts
@@ -99,21 +99,27 @@ export type ParsedStreamError =
       responseBody: string
     }
 
+// OpenRouter relays upstream overflow as a typeless `{ code: 502, message }` chunk.
+function isOverflow(body: { error?: { code?: string; message?: string }; message?: string }) {
+  return body.error?.code === "context_length_exceeded" || isContextOverflow(body.error?.message ?? body.message ?? "")
+}
+
 export function parseStreamError(input: unknown): ParsedStreamError | undefined {
   const raw = json(input)
   const body = typeof raw?.message === "string" ? (json(raw.message) ?? raw) : raw
   if (!body) return
 
   const responseBody = JSON.stringify(body)
+  if (isOverflow(body)) {
+    return {
+      type: "context_overflow",
+      message: "Input exceeds context window of this model",
+      responseBody,
+    }
+  }
   if (body.type !== "error") return
 
   switch (body?.error?.code) {
-    case "context_length_exceeded":
-      return {
-        type: "context_overflow",
-        message: "Input exceeds context window of this model",
-        responseBody,
-      }
     case "insufficient_quota":
       return {
         type: "api_error",
diff --git a/packages/opencode/test/session/message-v2.test.ts b/packages/opencode/test/session/message-v2.test.ts
index 1de84c9dd95b..4dbddcc09b43 100644
--- a/packages/opencode/test/session/message-v2.test.ts
+++ b/packages/opencode/test/session/message-v2.test.ts
@@ -1382,6 +1382,23 @@ describe("session.message-v2.fromError", () => {
     })
   })
 
+  test("serializes OpenRouter typeless overflow chunks as ContextOverflowError", () => {
+    const input = {
+      code: 502,
+      message: "Your input exceeds the context window of this model. Please adjust your input and try again.",
+      metadata: { error_type: "provider_unavailable" },
+    }
+    const result = MessageV2.fromError(input, { providerID })
+
+    expect(result).toStrictEqual({
+      name: "ContextOverflowError",
+      data: {
+        message: "Input exceeds context window of this model",
+        responseBody: JSON.stringify(input),
+      },
+    })
+  })
+
   test("serializes response error codes", () => {
     const cases = [
       {