diff --git a/packages/cultnet-ts/src/idunn-runtime-authority.ts b/packages/cultnet-ts/src/idunn-runtime-authority.ts
index b675d5c3..d55c4dc4 100644
--- a/packages/cultnet-ts/src/idunn-runtime-authority.ts
+++ b/packages/cultnet-ts/src/idunn-runtime-authority.ts
@@ -9,6 +9,7 @@ import {
   IDUNN_RUNTIME_ACTIVATION_CREDENTIAL_NAME,
   runtimePresenceActivationSigningMessage,
   runtimePresenceProofPayload,
+  truncateRuntimePresenceDetail,
   runtimePresenceProviderSigningMessage,
   encodeRuntimePresenceHealth,
   type RuntimePresenceHealth,
@@ -241,7 +242,7 @@ export function createIdunnRuntimePresencePublisher(
     const signed = signIdunnRuntimePresence(options.authority, {
       capabilities: options.capabilities,
       state,
-      detail: detail.slice(0, 512),
+      detail: truncateRuntimePresenceDetail(detail),
       writeLeaseSha256,
       publisherSequence: sequence,
       observedAtUnixMillis: Date.now(),
diff --git a/packages/cultnet-ts/src/runtime-presence-health.ts b/packages/cultnet-ts/src/runtime-presence-health.ts
index 8d70da8e..4cdccd1e 100644
--- a/packages/cultnet-ts/src/runtime-presence-health.ts
+++ b/packages/cultnet-ts/src/runtime-presence-health.ts
@@ -171,6 +171,25 @@ function presenceTuple(
   return fields;
 }
 
+/** The Rust owner (`cultnet-rs`) bounds presence detail by UTF-8 bytes, not UTF-16 units. */
+export const RUNTIME_PRESENCE_DETAIL_MAX_BYTES = 512;
+
+/**
+ * Longest prefix of `detail` within the UTF-8 byte limit, cut on a code point
+ * boundary. The single rule for both signing and validation: a detail is valid
+ * exactly when this returns it unchanged.
+ */
+export function truncateRuntimePresenceDetail(detail: string): string {
+  let bytes = 0;
+  let end = 0;
+  for (const char of detail) {
+    bytes += Buffer.byteLength(char, "utf8");
+    if (bytes > RUNTIME_PRESENCE_DETAIL_MAX_BYTES) break;
+    end += char.length;
+  }
+  return detail.slice(0, end);
+}
+
 /**
  * Reject statements the contract forbids, before they are signed.
  *
@@ -185,7 +204,7 @@ export function validateRuntimePresence(presence: RuntimePresenceHealth): void {
   if (presence.observedAtUnixMillis <= 0 || !Number.isSafeInteger(presence.observedAtUnixMillis)) {
     throw new Error("runtime presence observation time must be a positive integer");
   }
-  if (presence.detail.length > 512 || /\p{Cc}/u.test(presence.detail)) {
+  if (truncateRuntimePresenceDetail(presence.detail) !== presence.detail || /\p{Cc}/u.test(presence.detail)) {
     throw new Error("runtime presence detail is too long or carries control characters");
   }
   if (presence.state === "warming" && presence.writeLeaseSha256 !== null) {
diff --git a/packages/cultnet-ts/test/runtime-presence-health.test.ts b/packages/cultnet-ts/test/runtime-presence-health.test.ts
index fa21aeaa..5a4de05f 100644
--- a/packages/cultnet-ts/test/runtime-presence-health.test.ts
+++ b/packages/cultnet-ts/test/runtime-presence-health.test.ts
@@ -18,6 +18,7 @@ import {
   RUNTIME_PRESENCE_FIELD_COUNT,
   encodeRuntimePresenceHealth,
   runtimePresenceProofPayload,
+  truncateRuntimePresenceDetail,
   validateRuntimePresence,
   type RuntimePresenceHealth,
 } from "../src/runtime-presence-health";
@@ -121,3 +122,32 @@ test("statements the contract forbids are refused before they are signed", () =>
   const partialLineage = { ...referencePresence(), stateContractSha256: null };
   assert.throws(() => validateRuntimePresence(partialLineage), /state lineage is partial/);
 });
+
+const utf8Bytes = (text: string) => Buffer.byteLength(text, "utf8");
+const isValidUtf8 = (text: string) => !Buffer.from(text, "utf8").toString("utf8").includes("\uFFFD") && !text.includes("\uFFFD");
+
+test("presence detail is truncated to 512 UTF-8 bytes, not UTF-16 units", () => {
+  const truncated = truncateRuntimePresenceDetail("é".repeat(300));
+  assert.ok(utf8Bytes(truncated) <= 512);
+  assert.equal(truncated, "é".repeat(256));
+  assert.ok(isValidUtf8(truncated));
+});
+
+test("presence detail drops a 4-byte character straddling the boundary whole", () => {
+  const truncated = truncateRuntimePresenceDetail("a".repeat(510) + "😀");
+  assert.equal(truncated, "a".repeat(510));
+  assert.ok(isValidUtf8(truncated));
+  assert.equal(truncateRuntimePresenceDetail("a".repeat(508) + "😀"), "a".repeat(508) + "😀");
+});
+
+test("presence detail of exactly 512 ASCII bytes passes unchanged and 513 is cut", () => {
+  const exact = "a".repeat(512);
+  assert.equal(truncateRuntimePresenceDetail(exact), exact);
+  assert.doesNotThrow(() => validateRuntimePresence({ ...referencePresence(), detail: exact }));
+  assert.equal(truncateRuntimePresenceDetail(exact + "a"), exact);
+  assert.throws(() => validateRuntimePresence({ ...referencePresence(), detail: exact + "a" }), /too long/);
+});
+
+test("validation refuses a detail that is short in UTF-16 units but over 512 UTF-8 bytes", () => {
+  assert.throws(() => validateRuntimePresence({ ...referencePresence(), detail: "é".repeat(300) }), /too long/);
+});