diff --git a/open-sse/translator/helpers/geminiHelper.ts b/open-sse/translator/helpers/geminiHelper.ts
index d784dbf888..b5bf0780b8 100644
--- a/open-sse/translator/helpers/geminiHelper.ts
+++ b/open-sse/translator/helpers/geminiHelper.ts
@@ -1,5 +1,7 @@
 // Gemini helper functions for translator
 
+import { safeParseJSON } from "./jsonUtil.ts";
+
 type JsonRecord = Record<string, unknown>;
 
 // Unsupported JSON Schema constraints that should be removed for Antigravity.
@@ -242,14 +244,9 @@ export function extractTextContent(content: unknown): string {
   return "";
 }
 
-// Try parse JSON safely
+// Try parse JSON safely (null fallback on parse error; re-export keeps legacy API).
 export function tryParseJSON(str: unknown): unknown {
-  if (typeof str !== "string") return str;
-  try {
-    return JSON.parse(str);
-  } catch {
-    return null;
-  }
+  return safeParseJSON(str, null);
 }
 
 // Generate request ID
diff --git a/open-sse/translator/helpers/jsonUtil.ts b/open-sse/translator/helpers/jsonUtil.ts
new file mode 100644
index 0000000000..0643c53336
--- /dev/null
+++ b/open-sse/translator/helpers/jsonUtil.ts
@@ -0,0 +1,18 @@
+// Safe JSON.parse helper shared by the translator layer.
+//
+// Behavior contract (must be preserved by all callers):
+//   - non-string input is returned unchanged (passthrough)
+//   - a valid JSON string is parsed and returned
+//   - on parse error the caller-chosen `fallback` is returned
+//
+// The `fallback` is explicit so the two historical `tryParseJSON` variants
+// can keep their distinct semantics: geminiHelper returns `null` on error,
+// while openai-to-claude returns the raw input string (passthrough).
+export function safeParseJSON<TFallback>(str: unknown, fallback: TFallback): unknown {
+  if (typeof str !== "string") return str;
+  try {
+    return JSON.parse(str);
+  } catch {
+    return fallback;
+  }
+}
diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index 55a1f28433..6e8d9b0058 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -4,6 +4,7 @@ import { FORMATS } from "../formats.ts";
 import { supportsClaudeMaxEffort, supportsXHighEffort } from "../../config/providerModels.ts";
 import { adjustMaxTokens } from "../helpers/maxTokensHelper.ts";
 import { sanitizeToolId } from "../helpers/schemaCoercion.ts";
+import { safeParseJSON } from "../helpers/jsonUtil.ts";
 import { DEFAULT_THINKING_CLAUDE_SIGNATURE } from "../../config/defaultThinkingSignature.ts";
 import { capMaxOutputTokens } from "../../../src/lib/modelCapabilities.ts";
 import { isAdaptiveThinkingOnly } from "../../../src/shared/constants/modelSpecs.ts";
@@ -701,14 +702,9 @@ function extractTextContent(content) {
   return "";
 }
 
-// Try parse JSON
-function tryParseJSON(str) {
-  if (typeof str !== "string") return str;
-  try {
-    return JSON.parse(str);
-  } catch {
-    return str;
-  }
+// Try parse JSON (passthrough fallback: return the raw input string on parse error).
+function tryParseJSON(str: unknown): unknown {
+  return safeParseJSON(str, str);
 }
 
 function stripCacheControl(value: unknown): unknown {
diff --git a/tests/unit/translator-json-util.test.ts b/tests/unit/translator-json-util.test.ts
new file mode 100644
index 0000000000..0824f52f9c
--- /dev/null
+++ b/tests/unit/translator-json-util.test.ts
@@ -0,0 +1,45 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// Unit coverage for the extracted safeParseJSON util and the two distinct
+// fallbacks its callers rely on:
+//   - geminiHelper.tryParseJSON  -> null fallback on parse error
+//   - openai-to-claude tryParseJSON -> raw-string passthrough on parse error
+const { safeParseJSON } = await import("../../open-sse/translator/helpers/jsonUtil.ts");
+const gemini = await import("../../open-sse/translator/helpers/geminiHelper.ts");
+
+test("safeParseJSON: non-string input is returned unchanged (passthrough)", () => {
+  assert.equal(safeParseJSON(42, null), 42);
+  assert.equal(safeParseJSON(null, "fb"), null);
+  assert.equal(safeParseJSON(undefined, "fb"), undefined);
+  const obj = { a: 1 };
+  assert.equal(safeParseJSON(obj, null), obj);
+});
+
+test("safeParseJSON: valid JSON strings parse regardless of fallback", () => {
+  assert.deepEqual(safeParseJSON('{"a":1}', null), { a: 1 });
+  assert.deepEqual(safeParseJSON("[1,2,3]", "ignored"), [1, 2, 3]);
+  assert.equal(safeParseJSON("42", null), 42);
+  assert.equal(safeParseJSON("true", null), true);
+  assert.equal(safeParseJSON("null", "ignored"), null);
+});
+
+test("safeParseJSON: invalid JSON returns the caller-chosen fallback", () => {
+  // null fallback (geminiHelper semantics)
+  assert.equal(safeParseJSON("not json", null), null);
+  assert.equal(safeParseJSON("{broken}", null), null);
+  assert.equal(safeParseJSON("", null), null);
+
+  // raw-string passthrough fallback (openai-to-claude semantics)
+  assert.equal(safeParseJSON("not json", "not json"), "not json");
+  assert.equal(safeParseJSON("{broken", "{broken"), "{broken");
+});
+
+test("geminiHelper.tryParseJSON still returns null on parse error (re-export delegates)", () => {
+  assert.deepEqual(gemini.tryParseJSON('{"ok":true}'), { ok: true });
+  assert.equal(gemini.tryParseJSON("{broken"), null);
+  assert.equal(gemini.tryParseJSON("not json"), null);
+  assert.equal(gemini.tryParseJSON(""), null);
+  // non-string passthrough preserved
+  assert.equal(gemini.tryParseJSON(123), 123);
+});