diff --git a/.changeset/precondition-guard-default-on.md b/.changeset/precondition-guard-default-on.md
new file mode 100644
index 0000000000..c029a12c9d
--- /dev/null
+++ b/.changeset/precondition-guard-default-on.md
@@ -0,0 +1,8 @@
+---
+"workflow": minor
+"@workflow/core": minor
+"@workflow/world": patch
+"@workflow/errors": patch
+---
+
+The `WORKFLOW_PRECONDITION_GUARD` event-creation guard is now on by default; opt out with `WORKFLOW_PRECONDITION_GUARD=0`.
diff --git a/docs/content/docs/v5/configuration/runtime-tuning.mdx b/docs/content/docs/v5/configuration/runtime-tuning.mdx
index 581be18c07..7a31d0c401 100644
--- a/docs/content/docs/v5/configuration/runtime-tuning.mdx
+++ b/docs/content/docs/v5/configuration/runtime-tuning.mdx
@@ -40,10 +40,10 @@ For example, a workflow can run a 10-minute inline step even with `WORKFLOW_REPL
 
 ### `WORKFLOW_PRECONDITION_GUARD`
 
-- Default: disabled
-- Set `1` to enable an optimistic-concurrency guard for event creation: replay-context event creations send a `stateUpdatedAt` snapshot timestamp, and a backend that supports the guard rejects a creation with 412 ([`PreconditionFailedError`](/docs/api-reference/workflow-errors/precondition-failed-error)) when a newer out-of-band event (a received hook or a completed step) was recorded after that snapshot.
+- Default: enabled
+- An optimistic-concurrency guard for event creation: replay-context event creations send a `stateUpdatedAt` snapshot timestamp, and a backend that supports the guard rejects a creation with 412 ([`PreconditionFailedError`](/docs/api-reference/workflow-errors/precondition-failed-error)) when a newer out-of-band event (a received hook or a completed step) was recorded after that snapshot.
 - On rejection the runtime reloads the event log and retries, falling back to a queue re-invocation with a fresh replay if it cannot catch up.
-- Backends that do not support the guard ignore the snapshot.
+- Set `0` to disable. Backends that do not support the guard ignore the snapshot.
 
 ## Inline execution
 
diff --git a/packages/core/src/runtime.ts b/packages/core/src/runtime.ts
index b402b7a952..da295e773c 100644
--- a/packages/core/src/runtime.ts
+++ b/packages/core/src/runtime.ts
@@ -2446,6 +2446,17 @@ export function workflowEntrypoint(
                         // Serialize the original thrown value so its full
                         // type identity and custom properties round-trip
                         // through the event log.
+                        //
+                        // Precondition-guard asymmetry: unlike `run_completed`,
+                        // this terminal `run_failed` sends no `stateUpdatedAt`
+                        // snapshot, so it is never 412-rejected even if a hook
+                        // landed mid-replay and could have changed the path that
+                        // threw. This is intentional and fail-open: a spurious
+                        // failure is recoverable (the run can be re-run from the
+                        // dashboard), whereas a spurious *completion* commits a
+                        // wrong result. Guarding this write symmetrically would
+                        // also need the loaded event log, which is scoped to the
+                        // replay `try` above and not available in this catch.
                         try {
                           // Turbo: order the terminal write after the
                           // backgrounded run_started so the run exists.
diff --git a/packages/core/src/runtime/helpers.test.ts b/packages/core/src/runtime/helpers.test.ts
index 966f03840f..faad891b41 100644
--- a/packages/core/src/runtime/helpers.test.ts
+++ b/packages/core/src/runtime/helpers.test.ts
@@ -510,8 +510,8 @@ describe('withPreconditionRetry', () => {
     }
   });
 
-  it('passes no snapshot to op when the guard is not opted in', async () => {
-    delete process.env.WORKFLOW_PRECONDITION_GUARD;
+  it('passes no snapshot to op when the guard is explicitly disabled', async () => {
+    process.env.WORKFLOW_PRECONDITION_GUARD = '0';
     const log: MutableEventLog = {
       events: [makeUlidEvent(1_700_000_000_000)],
       cursor: 'c0',
@@ -528,6 +528,24 @@ describe('withPreconditionRetry', () => {
     expect(eventsListMock).not.toHaveBeenCalled();
   });
 
+  it('sends a snapshot by default when the guard variable is unset (on by default)', async () => {
+    delete process.env.WORKFLOW_PRECONDITION_GUARD;
+    const time = 1_700_000_000_000;
+    const log: MutableEventLog = {
+      events: [makeUlidEvent(time)],
+      cursor: 'c0',
+    };
+    const op = vi.fn(async (stateUpdatedAt?: number) => {
+      expect(stateUpdatedAt).toBe(time);
+      return 'ok';
+    });
+
+    await expect(withPreconditionRetry('wrun_test', log, op)).resolves.toBe(
+      'ok'
+    );
+    expect(op).toHaveBeenCalledTimes(1);
+  });
+
   it('passes the latest snapshot time to op and returns its result without reloading', async () => {
     const time = 1_700_000_000_000;
     const log: MutableEventLog = {
diff --git a/packages/core/src/runtime/helpers.ts b/packages/core/src/runtime/helpers.ts
index a4f7294c29..524c7c2846 100644
--- a/packages/core/src/runtime/helpers.ts
+++ b/packages/core/src/runtime/helpers.ts
@@ -589,13 +589,15 @@ export interface MutableEventLog {
 }
 
 /**
- * Whether the optimistic-concurrency guard for event creation is enabled
- * (`WORKFLOW_PRECONDITION_GUARD=1`, set where the runtime executes). Off by
- * default: replay-context creates only send a `stateUpdatedAt` snapshot (and
- * can therefore be rejected with 412 by the backend) when it is enabled.
+ * Whether the optimistic-concurrency guard for event creation is enabled.
+ * **On by default** where the runtime executes: replay-context creates send a
+ * `stateUpdatedAt` snapshot (and can be rejected with 412 by a supporting
+ * backend) unless `WORKFLOW_PRECONDITION_GUARD` is set to `0`. Backends without
+ * guard support ignore the snapshot, so enabling by default is
+ * backward-compatible.
  */
 export function isPreconditionGuardEnabled(): boolean {
-  return process.env.WORKFLOW_PRECONDITION_GUARD === '1';
+  return process.env.WORKFLOW_PRECONDITION_GUARD !== '0';
 }
 
 /**
@@ -620,7 +622,17 @@ export function latestEventStateUpdatedAt(events: Event[]): number | undefined {
   const eventId = last.eventId;
   const underscore = eventId.lastIndexOf('_');
   const rawUlid = underscore === -1 ? eventId : eventId.slice(underscore + 1);
-  return ulidToDate(rawUlid)?.getTime() ?? undefined;
+  const time = ulidToDate(rawUlid)?.getTime();
+  if (time === undefined) {
+    // Fail open: a non-decodable id disarms the guard for this create (no
+    // snapshot sent). Log so a fleet-wide silent disarm is diagnosable.
+    runtimeLogger.debug(
+      'Precondition guard: latest event id is not a decodable ULID; sending no snapshot',
+      { eventId }
+    );
+    return undefined;
+  }
+  return time;
 }
 
 /**
@@ -678,6 +690,13 @@ export async function withPreconditionRetry<T>(
         new Set(log.events.map((e) => e.eventId)),
         loaded.events
       );
+      // When several creates share one `log` (e.g. hook creations under
+      // `Promise.all` in `handleSuspension`), concurrent 412s can reload
+      // concurrently. The event merge above is safe — `appendUniqueEvents`
+      // builds its dedup set synchronously right before appending — but this
+      // cursor write is last-write-wins, so an interleaved older reload can
+      // briefly regress the cursor. The only consequence is refetching a few
+      // already-deduped events on a later load; correctness is unaffected.
       log.cursor = loaded.cursor ?? log.cursor;
     }
   }
diff --git a/packages/errors/src/index.ts b/packages/errors/src/index.ts
index 573e1d7218..3d7ab5a49f 100644
--- a/packages/errors/src/index.ts
+++ b/packages/errors/src/index.ts
@@ -769,6 +769,10 @@ export class ThrottleError extends WorkflowWorldError {
  * The workflow runtime handles this automatically: it reloads the event log
  * and retries, ultimately re-enqueueing the run if it cannot catch up. Users
  * interacting with world storage backends directly may encounter it.
+ *
+ * @property retryAfter - Delay in seconds before retrying. Accepted for
+ *   forward-compatibility; the runtime currently reloads and retries
+ *   immediately and does not read this field.
  */
 export class PreconditionFailedError extends WorkflowWorldError {
   constructor(message: string, options?: { retryAfter?: number }) {
diff --git a/packages/world/src/events.ts b/packages/world/src/events.ts
index cb5969ea23..f0c6f419aa 100644
--- a/packages/world/src/events.ts
+++ b/packages/world/src/events.ts
@@ -686,6 +686,16 @@ export interface CreateEventParams {
    * when a newer out-of-band event was recorded after this snapshot, enabling
    * an optimistic-concurrency guard. Omitted by callers without a loaded event
    * log.
+   *
+   * Backend contract (for World implementers who want to support the guard):
+   * maintain a per-run marker holding the ULID time of the most recent
+   * *externally-originated* event — a `hook_received` or `step_completed`
+   * created **without** a `stateUpdatedAt` (replay-origin events carry one and
+   * must not advance the marker). On a create that carries `stateUpdatedAt`,
+   * reject with 412 when `stateUpdatedAt < marker` (strictly older); an equal
+   * timestamp must pass (anti-livelock, so an up-to-date client is never
+   * rejected). A backend that ignores this field simply disables the guard —
+   * the client falls open and behaves as before.
    */
   stateUpdatedAt?: number;
   /**