diff --git a/src/lib/guardrails/credentialMasker.ts b/src/lib/guardrails/credentialMasker.ts
index 6ac88f8fb3..16cbb4e702 100644
--- a/src/lib/guardrails/credentialMasker.ts
+++ b/src/lib/guardrails/credentialMasker.ts
@@ -54,6 +54,9 @@ function walkValue(
   seen = new WeakSet<object>()
 ): { modified: boolean; value: unknown } {
   if (typeof value === "string") {
+    // #13462: Skip redaction for base64-encoded image data URLs — applying
+    // credential regexes to binary transport data corrupts valid base64.
+    if (isImageDataUrl(value)) return { modified: false, value };
     const r = redactCredentials(value);
     if (r.detections.length) detections.push(...r.detections);
     return { modified: r.modified, value: r.text };
@@ -100,6 +103,16 @@ function walkValue(
   return { modified: false, value };
 }
 
+/**
+ * Check if a string is a base64-encoded image data URL that should not be
+ * subject to text-pattern redaction.  Applying credential regexes to binary
+ * transport data causes false-positive corruption of valid base64 content.
+ */
+function isImageDataUrl(value: string): boolean {
+  // Match data URLs with image MIME types (data:image/png;base64,... etc.)
+  return /^data:image\/[a-zA-Z0-9.+-]+;base64,/.test(value);
+}
+
 /** Walk request payloads without changing safe values. */
 function redactPayload(
   payload: unknown,
diff --git a/tests/unit/credential-masker-image-data-url-13462.test.ts b/tests/unit/credential-masker-image-data-url-13462.test.ts
new file mode 100644
index 0000000000..f029eec43b
--- /dev/null
+++ b/tests/unit/credential-masker-image-data-url-13462.test.ts
@@ -0,0 +1,53 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { CredentialMaskerGuardrail } from "../../src/lib/guardrails/credentialMasker.ts";
+
+// #13462: credential regexes applied to a base64 image data URL can match inside the
+// binary payload and "redact" part of it, corrupting the image sent upstream.
+
+async function withCredentialRedactionEnabled(fn: () => Promise<void>) {
+  const original = process.env.CREDENTIAL_REDACTION_ENABLED;
+  process.env.CREDENTIAL_REDACTION_ENABLED = "true";
+  try {
+    await fn();
+  } finally {
+    if (original === undefined) delete process.env.CREDENTIAL_REDACTION_ENABLED;
+    else process.env.CREDENTIAL_REDACTION_ENABLED = original;
+  }
+}
+
+// Base64 bytes that coincidentally spell a Google API key shape (AIza + 20+ chars), the
+// collision the issue observed in a real screenshot.
+const KEY_SHAPED = "AIza" + "B7qX".repeat(8);
+const IMAGE_URL = `data:image/png;base64,iVBORw0KGgoAAAANSUhEUg${KEY_SHAPED}AAAAAElFTkSuQmCC`;
+
+test("#13462 base64 image data URLs pass through the credential masker unchanged", async () => {
+  await withCredentialRedactionEnabled(async () => {
+    const guardrail = new CredentialMaskerGuardrail();
+    const payload = {
+      messages: [
+        {
+          role: "user",
+          content: [{ type: "image_url", image_url: { url: IMAGE_URL } }],
+        },
+      ],
+    };
+    const result = await guardrail.preCall(payload, {} as never);
+    const next = (result?.modifiedPayload ?? payload) as typeof payload;
+    assert.equal(next.messages[0].content[0].image_url.url, IMAGE_URL);
+  });
+});
+
+test("#13462 the same key-shaped text outside a data URL is still redacted", async () => {
+  await withCredentialRedactionEnabled(async () => {
+    const guardrail = new CredentialMaskerGuardrail();
+    const result = await guardrail.preCall(
+      { messages: [{ role: "user", content: `key=${KEY_SHAPED}` }] },
+      {} as never
+    );
+    const next = result?.modifiedPayload as { messages: Array<{ content: string }> };
+    assert.ok(next, "expected the plain-text token to be redacted");
+    assert.match(next.messages[0].content, /\[REDACTED:google\]/);
+  });
+});