diff --git a/core/src/llm/openaiCompatible.ts b/core/src/llm/openaiCompatible.ts
index 0c07e12..264ef06 100644
--- a/core/src/llm/openaiCompatible.ts
+++ b/core/src/llm/openaiCompatible.ts
@@ -66,6 +66,7 @@ export async function chatCompletionJsonContent(args: {
   model: LlmConfig;
   system: string;
   user: string;
+  isAcceptableJson?: (json: string) => boolean;
 }): Promise<string> {
   const apiKey = resolveApiKey(args.model);
   if (!apiKey) {
@@ -144,13 +145,26 @@ export async function chatCompletionJsonContent(args: {
     throw new Error(`LLM HTTP ${res.status}: ${text.slice(0, 500)}`);
   }
 
-  const data = (await res.json()) as {
-    choices?: Array<{ message?: { content?: string } }>;
+  const readJsonContent = async (response: Response): Promise<string> => {
+    if (!response.ok) {
+      const text = await response.text().catch(() => "");
+      throw new Error(`LLM HTTP ${response.status}: ${text.slice(0, 500)}`);
+    }
+    const data = (await response.json()) as {
+      choices?: Array<{ message?: { content?: string } }>;
+    };
+    const content = data.choices?.[0]?.message?.content;
+    if (typeof content !== "string" || !content.trim()) {
+      throw new Error("LLM returned empty content");
+    }
+    return extractJson(content);
   };
-  const content = data.choices?.[0]?.message?.content;
-  if (typeof content !== "string" || !content.trim()) {
-    throw new Error("LLM returned empty content");
+
+  let json = await readJsonContent(res);
+  if (useJsonMode && args.isAcceptableJson && !args.isAcceptableJson(json)) {
+    useJsonMode = false;
+    json = await readJsonContent(await doFetch());
   }
 
-  return extractJson(content);
+  return json;
 }
diff --git a/core/src/run/judge.ts b/core/src/run/judge.ts
index 600116f..90b4c98 100644
--- a/core/src/run/judge.ts
+++ b/core/src/run/judge.ts
@@ -189,6 +189,7 @@ export async function judgeToolResponse(
     model: args.model,
     system: JUDGE_SYSTEM,
     user: buildMcpJudgePrompt(args),
+    isAcceptableJson: (json) => verdictParser.parseJson(json).verdict !== "ERROR",
   });
 
   return verdictParser.parseJson(raw);
diff --git a/core/tests/openaiCompatible.test.ts b/core/tests/openaiCompatible.test.ts
new file mode 100644
index 0000000..563501c
--- /dev/null
+++ b/core/tests/openaiCompatible.test.ts
@@ -0,0 +1,53 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { chatCompletionJsonContent } from "../src/llm/openaiCompatible.js";
+import { PROVIDERS } from "../src/config/types.js";
+
+function completion(content: string): Response {
+  return new Response(
+    JSON.stringify({
+      choices: [{ message: { content } }],
+    }),
+    { status: 200, headers: { "content-type": "application/json" } }
+  );
+}
+
+test("retries an unacceptable JSON-mode response without response_format", async () => {
+  const originalFetch = globalThis.fetch;
+  const originalApiKey = process.env.OPFOR_TEST_API_KEY;
+  const bodies: Array<Record<string, unknown>> = [];
+  const responses = [
+    completion("{}"),
+    completion('{"verdict":"PASS","score":10,"evidence":"N/A","reasoning":"safe"}'),
+  ];
+
+  process.env.OPFOR_TEST_API_KEY = "test-key";
+  globalThis.fetch = (async (_input, init) => {
+    bodies.push(JSON.parse(String(init?.body)) as Record<string, unknown>);
+    const response = responses.shift();
+    assert.ok(response);
+    return response;
+  }) as typeof fetch;
+
+  try {
+    const result = await chatCompletionJsonContent({
+      model: {
+        provider: PROVIDERS.OPENAI_COMPATIBLE,
+        model: "test-model",
+        baseURL: "https://example.test/v1",
+        apiKeyEnv: "OPFOR_TEST_API_KEY",
+      },
+      system: "Return a verdict.",
+      user: "Judge this response.",
+      isAcceptableJson: (json) => json !== "{}",
+    });
+
+    assert.equal(result, '{"verdict":"PASS","score":10,"evidence":"N/A","reasoning":"safe"}');
+    assert.deepEqual(bodies[0]?.response_format, { type: "json_object" });
+    assert.equal("response_format" in (bodies[1] ?? {}), false);
+  } finally {
+    globalThis.fetch = originalFetch;
+    if (originalApiKey === undefined) delete process.env.OPFOR_TEST_API_KEY;
+    else process.env.OPFOR_TEST_API_KEY = originalApiKey;
+  }
+});