diff --git a/src/internal/headers.ts b/src/internal/headers.ts
index c724a9d225..50e1e205a6 100644
--- a/src/internal/headers.ts
+++ b/src/internal/headers.ts
@@ -74,7 +74,14 @@ export const buildHeaders = (newHeaders: HeadersLike[]): NullableHeaders => {
   for (const headers of newHeaders) {
     const seenHeaders = new Set<string>();
     for (const [name, value] of iterateHeaders(headers)) {
-      const lowerName = name.toLowerCase();
+      // HTTP header names are ASCII per RFC 9110, but `String.prototype.toLowerCase()`
+      // applies the host locale to certain Unicode mappings. On Turkish locales the
+      // uppercase `I` in `OpenAI-Organization` becomes the dotless `ı`, producing
+      // `openaı-organization`, which fails Node's HTTP token validation
+      // (`TypeError: Header name must be a valid HTTP token`). Pin the locale to
+      // `en-US` so the casing rule stays ASCII-safe regardless of the runtime locale.
+      // https://github.com/openai/openai-node/issues/1928
+      const lowerName = name.toLocaleLowerCase('en-US');
       if (!seenHeaders.has(lowerName)) {
         targetHeaders.delete(name);
         seenHeaders.add(lowerName);
diff --git a/src/internal/utils/log.ts b/src/internal/utils/log.ts
index 94de74b5b7..a2bdbb226e 100644
--- a/src/internal/utils/log.ts
+++ b/src/internal/utils/log.ts
@@ -103,18 +103,25 @@ export const formatRequestDetails = (details: {
   if (details.headers) {
     details.headers = Object.fromEntries(
       (details.headers instanceof Headers ? [...details.headers] : Object.entries(details.headers)).map(
-        ([name, value]) => [
-          name,
-          (
-            name.toLowerCase() === 'authorization' ||
-            name.toLowerCase() === 'api-key' ||
-            name.toLowerCase() === 'x-api-key' ||
-            name.toLowerCase() === 'cookie' ||
-            name.toLowerCase() === 'set-cookie'
-          ) ?
-            '***'
-          : value,
-        ],
+        ([name, value]) => {
+          // Lowercase with an explicit `en-US` locale: on Turkish locales a plain
+          // `.toLowerCase()` maps `I` → `ı`, so `Authorization` becomes
+          // `authorızatıon` and the credential redaction below silently misses it.
+          // https://github.com/openai/openai-node/issues/1928
+          const lowerName = name.toLocaleLowerCase('en-US');
+          return [
+            name,
+            (
+              lowerName === 'authorization' ||
+              lowerName === 'api-key' ||
+              lowerName === 'x-api-key' ||
+              lowerName === 'cookie' ||
+              lowerName === 'set-cookie'
+            ) ?
+              '***'
+            : value,
+          ];
+        },
       ),
     );
   }
diff --git a/tests/buildHeaders.test.ts b/tests/buildHeaders.test.ts
index 3f8e4d28ec..f4a8356bdb 100644
--- a/tests/buildHeaders.test.ts
+++ b/tests/buildHeaders.test.ts
@@ -85,4 +85,24 @@ describe('buildHeaders', () => {
       expect(inspectNullableHeaders(buildHeaders(input))).toEqual(expected);
     });
   }
+
+  // Regression for https://github.com/openai/openai-node/issues/1928
+  test('lowercases header names with ASCII rules even under a Turkish process locale', () => {
+    const originalLowerCase = String.prototype.toLowerCase;
+    // Simulate the Turkish locale's casing rule that maps uppercase `I` to the
+    // dotless `ı`. This is what V8 actually does on Turkish Windows / Linux
+    // installs, but we monkey-patch it here so the test is locale-independent.
+    String.prototype.toLowerCase = function turkishToLowerCase(this: string): string {
+      return this.replace(/I/g, 'ı').replace(/[A-HJ-Z]/g, (c) => c.charCodeAt(0) === 73 ? c : String.fromCharCode(c.charCodeAt(0) | 0x20));
+    };
+    try {
+      const result = buildHeaders([{ 'OpenAI-Organization': 'org_test' }]);
+      // The canonical key must be ASCII-only — no dotless `ı`.
+      const keys = [...result.values.keys()];
+      expect(keys).toContain('openai-organization');
+      expect(keys.every((k) => /^[\x00-\x7F]*$/.test(k))).toBe(true);
+    } finally {
+      String.prototype.toLowerCase = originalLowerCase;
+    }
+  });
 });