diff --git a/src/lib/ai/__tests__/json-dialect.test.ts b/src/lib/ai/__tests__/json-dialect.test.ts
new file mode 100644
index 000000000..427909c4d
--- /dev/null
+++ b/src/lib/ai/__tests__/json-dialect.test.ts
@@ -0,0 +1,37 @@
+import { describe, expect, it } from "vitest";
+
+import { isResponseFormatRejection } from "../json-dialect";
+
+describe("isResponseFormatRejection", () => {
+  it("reads a complaint about response_format as a JSON-mode rejection", () => {
+    expect(
+      isResponseFormatRejection(400, "Unsupported value: 'response_format'"),
+    ).toBe(true);
+    expect(isResponseFormatRejection(422, "unknown parameter")).toBe(true);
+  });
+
+  it("does not read a complaint about reasoning_effort as one", () => {
+    expect(
+      isResponseFormatRejection(
+        400,
+        "Unrecognized request argument supplied: reasoning_effort",
+      ),
+    ).toBe(false);
+    expect(
+      isResponseFormatRejection(400, "unknown field `reasoning effort`"),
+    ).toBe(false);
+  });
+
+  it("still flips when both fields are named", () => {
+    expect(
+      isResponseFormatRejection(
+        400,
+        "unknown parameters: response_format, reasoning_effort",
+      ),
+    ).toBe(true);
+  });
+
+  it("ignores server errors", () => {
+    expect(isResponseFormatRejection(500, "response_format")).toBe(false);
+  });
+});
diff --git a/src/lib/ai/json-dialect.ts b/src/lib/ai/json-dialect.ts
index f63a5a481..0c1859105 100644
--- a/src/lib/ai/json-dialect.ts
+++ b/src/lib/ai/json-dialect.ts
@@ -56,13 +56,21 @@ export function resetJsonModeDialectCache(): void {
 /**
  * True when a 4xx error body reads as "this endpoint rejects the
  * `response_format` field" — either it names the field outright or it
- * complains about an unknown/unexpected parameter.
+ * complains about an unknown/unexpected parameter. A complaint that names
+ * `reasoning_effort` and not `response_format` is about the reasoning field,
+ * so it must not switch the endpoint's JSON mode off.
  */
 export function isResponseFormatRejection(
   status: number,
   bodyExcerpt: string,
 ): boolean {
   if (status < 400 || status >= 500) return false;
+  if (
+    /reasoning[\s_-]*effort/i.test(bodyExcerpt) &&
+    !/response_format/i.test(bodyExcerpt)
+  ) {
+    return false;
+  }
   return (
     /response_format/i.test(bodyExcerpt) ||
     /(unknown|unexpected|unrecognized|unsupported|extra)[\s_-]*(parameter|field|argument|property|key)/i.test(