diff --git a/src/bounties/bounty-state-machine.ts b/src/bounties/bounty-state-machine.ts
index b78b0714..f32edb08 100644
--- a/src/bounties/bounty-state-machine.ts
+++ b/src/bounties/bounty-state-machine.ts
@@ -40,9 +40,21 @@ export const BOUNTY_TRANSITIONS: Record<BountyStatus, BountyStatus[]> = {
   [BountyStatus.EXPIRED]: [BountyStatus.REFUNDED],
 };
 
-export class InvalidBountyTransitionError extends Error {
+import { HttpException, HttpStatus } from '@nestjs/common';
+
+/**
+ * A state-transition rejection is a CLIENT-facing error — the bounty's
+ * current state conflicts with the requested transition (e.g. claiming a
+ * bounty already claimed). Extending HttpException(409) surfaces it as a real
+ * HTTP status instead of an opaque 500 (#579) — a plain Error would hit the
+ * global filter's internal-error path and leak nothing useful.
+ */
+export class InvalidBountyTransitionError extends HttpException {
   constructor(from: BountyStatus, to: BountyStatus) {
-    super(`Cannot transition bounty from "${from}" to "${to}"`);
+    super(
+      `Cannot transition bounty from "${from}" to "${to}"`,
+      HttpStatus.CONFLICT,
+    );
     this.name = 'InvalidBountyTransitionError';
   }
 }
diff --git a/src/milestones/milestone-state-machine.ts b/src/milestones/milestone-state-machine.ts
index 05a7e287..b487881b 100644
--- a/src/milestones/milestone-state-machine.ts
+++ b/src/milestones/milestone-state-machine.ts
@@ -21,18 +21,30 @@ export const MILESTONE_TRANSITIONS: Record<MilestoneStatus, MilestoneStatus[]> =
     [MilestoneStatus.CLOSED]: [],
   };
 
-export class InvalidMilestoneTransitionError extends Error {
+import { HttpException, HttpStatus } from '@nestjs/common';
+
+/** A milestone transition-rejection is a client-facing 409, not a 500 (#579). */
+export class InvalidMilestoneTransitionError extends HttpException {
   constructor(from: MilestoneStatus, to: MilestoneStatus) {
-    super(`Cannot transition milestone from "${from}" to "${to}"`);
+    super(
+      `Cannot transition milestone from "${from}" to "${to}"`,
+      HttpStatus.CONFLICT,
+    );
     this.name = 'InvalidMilestoneTransitionError';
   }
 }
 
-export function canTransition(from: MilestoneStatus, to: MilestoneStatus): boolean {
+export function canTransition(
+  from: MilestoneStatus,
+  to: MilestoneStatus,
+): boolean {
   return MILESTONE_TRANSITIONS[from]?.includes(to) ?? false;
 }
 
-export function assertTransition(from: MilestoneStatus, to: MilestoneStatus): void {
+export function assertTransition(
+  from: MilestoneStatus,
+  to: MilestoneStatus,
+): void {
   if (!canTransition(from, to)) {
     throw new InvalidMilestoneTransitionError(from, to);
   }