diff --git a/packages/agent/src/utils/gateway.test.ts b/packages/agent/src/utils/gateway.test.ts
index 9a0b06e3e7..a7784ce067 100644
--- a/packages/agent/src/utils/gateway.test.ts
+++ b/packages/agent/src/utils/gateway.test.ts
@@ -124,15 +124,21 @@ describe("buildGatewayPropertyHeaders", () => {
     },
   );
 
-  it("strips characters an HTTP header value cannot carry", () => {
-    expect(buildGatewayPropertyHeaders({ task_title: "don’t🚀ship" })).toBe(
-      "x-posthog-property-task_title: dontship",
-    );
-  });
-
-  it("keeps latin1 characters such as accents", () => {
-    expect(buildGatewayPropertyHeaders({ task_title: "café" })).toBe(
-      "x-posthog-property-task_title: café",
+  it.each([
+    {
+      description: "smart quote and emoji",
+      title: "don’t🚀ship",
+      expected: "don%E2%80%99t%F0%9F%9A%80ship",
+    },
+    {
+      description: "Danish accented, CJK, and emoji characters",
+      title: "har vi nogle logs på café 東京 🚀",
+      expected:
+        "har vi nogle logs p%C3%A5 caf%C3%A9 %E6%9D%B1%E4%BA%AC %F0%9F%9A%80",
+    },
+  ])("encodes non-ASCII characters in $description", ({ title, expected }) => {
+    expect(buildGatewayPropertyHeaders({ task_title: title })).toBe(
+      `x-posthog-property-task_title: ${expected}`,
     );
   });
 });
diff --git a/packages/agent/src/utils/gateway.ts b/packages/agent/src/utils/gateway.ts
index ef39ec2bb5..4d80c2b0b3 100644
--- a/packages/agent/src/utils/gateway.ts
+++ b/packages/agent/src/utils/gateway.ts
@@ -28,14 +28,14 @@ export function resolveGatewayProduct({
 }
 
 /**
- * Make a value safe to embed in an HTTP header value. Collapses newlines to
- * spaces (the header block is newline-delimited) and drops characters outside
- * the valid header-byte range — control chars and code points above latin1
- * (emoji, smart quotes) — which an HTTP client (e.g. undici) would otherwise
- * reject before sending. ASCII is preserved.
+ *  Make a value safe to embed in an HTTP header value. Collapses newlines to
+ * spaces and percent-encodes non-ASCII characters so header values remain
+ * ASCII-safe. ASCII is preserved.
  */
 function sanitizeHeaderValue(value: string): string {
-  return value.replace(/[\r\n]+/g, " ").replace(/[^\x20-\x7e\x80-\xff]/g, "");
+  return value
+    .replace(/[\r\n]+/g, " ")
+    .replace(/[^\x20-\x7e]/gu, (char) => encodeURIComponent(char));
 }
 
 /**