diff --git a/backend/src/markets/market-settlement-state.util.spec.ts b/backend/src/markets/market-settlement-state.util.spec.ts
new file mode 100644
index 00000000..abbc0b28
--- /dev/null
+++ b/backend/src/markets/market-settlement-state.util.spec.ts
@@ -0,0 +1,83 @@
+import { MarketSettlementState } from './entities/market.entity';
+import {
+  ALLOWED_SETTLEMENT_TRANSITIONS,
+  canTransition,
+  describeIllegalTransition,
+} from './market-settlement-state.util';
+
+const ALL_STATES = Object.values(MarketSettlementState);
+
+/** Every transition the lifecycle is meant to permit, listed out by hand so
+ *  the test states the rules rather than re-deriving them from the table. */
+const VALID: ReadonlyArray<[MarketSettlementState, MarketSettlementState]> = [
+  [MarketSettlementState.PENDING, MarketSettlementState.PROPOSED],
+  [MarketSettlementState.PROPOSED, MarketSettlementState.SETTLING],
+  [MarketSettlementState.PROPOSED, MarketSettlementState.CHALLENGED],
+  [MarketSettlementState.SETTLING, MarketSettlementState.SETTLING],
+  [MarketSettlementState.SETTLING, MarketSettlementState.SETTLED],
+  [MarketSettlementState.CHALLENGED, MarketSettlementState.SETTLED],
+];
+
+const isValid = (from: MarketSettlementState, to: MarketSettlementState) =>
+  VALID.some(([f, t]) => f === from && t === to);
+
+describe('market settlement transitions', () => {
+  it.each(VALID)('allows %s -> %s', (from, to) => {
+    expect(canTransition(from, to)).toBe(true);
+  });
+
+  const INVALID = ALL_STATES.flatMap((from) =>
+    ALL_STATES.filter((to) => !isValid(from, to)).map(
+      (to) => [from, to] as const,
+    ),
+  );
+
+  it.each(INVALID)('rejects %s -> %s', (from, to) => {
+    expect(canTransition(from, to)).toBe(false);
+  });
+
+  it('covers every pair of states exactly once', () => {
+    expect(VALID.length + INVALID.length).toBe(
+      ALL_STATES.length * ALL_STATES.length,
+    );
+  });
+
+  it('treats settled as terminal', () => {
+    expect(ALLOWED_SETTLEMENT_TRANSITIONS[MarketSettlementState.SETTLED]).toEqual(
+      [],
+    );
+    for (const to of ALL_STATES) {
+      expect(canTransition(MarketSettlementState.SETTLED, to)).toBe(false);
+    }
+  });
+
+  it('has a table entry for every state, so a new state cannot be forgotten', () => {
+    for (const state of ALL_STATES) {
+      expect(ALLOWED_SETTLEMENT_TRANSITIONS[state]).toBeDefined();
+    }
+    expect(Object.keys(ALLOWED_SETTLEMENT_TRANSITIONS).sort()).toEqual(
+      [...ALL_STATES].sort(),
+    );
+  });
+
+  describe('describeIllegalTransition', () => {
+    it('names both states and what would have been allowed instead', () => {
+      const message = describeIllegalTransition(
+        MarketSettlementState.PENDING,
+        MarketSettlementState.SETTLED,
+      );
+      expect(message).toContain('"pending"');
+      expect(message).toContain('"settled"');
+      expect(message).toContain('proposed');
+    });
+
+    it('says so plainly when the source state is terminal', () => {
+      expect(
+        describeIllegalTransition(
+          MarketSettlementState.SETTLED,
+          MarketSettlementState.PROPOSED,
+        ),
+      ).toContain('terminal state');
+    });
+  });
+});
diff --git a/backend/src/markets/market-settlement-state.util.ts b/backend/src/markets/market-settlement-state.util.ts
new file mode 100644
index 00000000..4f6b675e
--- /dev/null
+++ b/backend/src/markets/market-settlement-state.util.ts
@@ -0,0 +1,61 @@
+import { MarketSettlementState } from './entities/market.entity';
+
+/**
+ * The single source of truth for which settlement transitions are legal.
+ *
+ * Until now these rules were spelled out three times as ad-hoc `if` checks in
+ * `markets.service.ts` and once more as an eligibility predicate in
+ * `market-settlement.scheduler.ts`. Each copy was correct, but nothing tied
+ * them together, so a new state or a changed rule had to be found in four
+ * places by hand.
+ *
+ * The lifecycle, as implemented today:
+ *
+ *   PENDING ──propose──▶ PROPOSED ──grace window expires──▶ SETTLING ──▶ SETTLED
+ *                            │
+ *                            └──challenge──▶ CHALLENGED ──admin adjudicates──▶ SETTLED
+ *
+ * SETTLED is terminal. Cancellation is deliberately absent: it is carried by
+ * the separate `is_cancelled` flag rather than by this enum, and folding it in
+ * here would change behaviour rather than describe it.
+ */
+export const ALLOWED_SETTLEMENT_TRANSITIONS: Readonly<
+  Record<MarketSettlementState, readonly MarketSettlementState[]>
+> = Object.freeze({
+  [MarketSettlementState.PENDING]: [MarketSettlementState.PROPOSED],
+  [MarketSettlementState.PROPOSED]: [
+    MarketSettlementState.SETTLING,
+    MarketSettlementState.CHALLENGED,
+  ],
+  // Re-entrant: the scheduler may re-claim a market it already marked
+  // SETTLING when a previous attempt crashed between claiming and settling.
+  [MarketSettlementState.SETTLING]: [
+    MarketSettlementState.SETTLING,
+    MarketSettlementState.SETTLED,
+  ],
+  [MarketSettlementState.CHALLENGED]: [MarketSettlementState.SETTLED],
+  [MarketSettlementState.SETTLED]: [],
+});
+
+export function canTransition(
+  from: MarketSettlementState,
+  to: MarketSettlementState,
+): boolean {
+  return ALLOWED_SETTLEMENT_TRANSITIONS[from].includes(to);
+}
+
+/**
+ * Message for a rejected transition. Names both states and what would have
+ * been allowed instead, so the caller is told what to do rather than only
+ * that they were wrong.
+ */
+export function describeIllegalTransition(
+  from: MarketSettlementState,
+  to: MarketSettlementState,
+): string {
+  const allowed = ALLOWED_SETTLEMENT_TRANSITIONS[from];
+  const suffix = allowed.length
+    ? `allowed from "${from}": ${allowed.join(', ')}`
+    : `"${from}" is a terminal state`;
+  return `Cannot move market settlement from "${from}" to "${to}" (${suffix})`;
+}
diff --git a/backend/src/markets/market-settlement.scheduler.ts b/backend/src/markets/market-settlement.scheduler.ts
index 01ec679c..38c0e96c 100644
--- a/backend/src/markets/market-settlement.scheduler.ts
+++ b/backend/src/markets/market-settlement.scheduler.ts
@@ -3,6 +3,7 @@ import { Cron, CronExpression } from '@nestjs/schedule';
 import { InjectRepository } from '@nestjs/typeorm';
 import { DataSource, QueryRunner, Repository } from 'typeorm';
 import { Market, MarketSettlementState } from './entities/market.entity';
+import { canTransition } from './market-settlement-state.util';
 import {
   SettlementAttempt,
   SettlementAttemptStatus,
@@ -229,10 +230,11 @@ export class MarketSettlementScheduler {
       const fresh = await queryRunner.manager.findOne(Market, {
         where: { id: market.id },
       });
+      // Asks the transition table rather than re-listing the states here, so
+      // this predicate cannot drift from the rules the service enforces.
       const stillEligible =
         fresh &&
-        (fresh.settlement_state === MarketSettlementState.PROPOSED ||
-          fresh.settlement_state === MarketSettlementState.SETTLING) &&
+        canTransition(fresh.settlement_state, MarketSettlementState.SETTLING) &&
         !!fresh.proposed_outcome;
 
       if (!stillEligible) {
diff --git a/backend/src/markets/markets.service.ts b/backend/src/markets/markets.service.ts
index 6b1dbbf0..9eb4ef49 100644
--- a/backend/src/markets/markets.service.ts
+++ b/backend/src/markets/markets.service.ts
@@ -42,6 +42,10 @@ import { Comment } from './entities/comment.entity';
 import { moderateCommentContent } from '../common/comment-moderation.util';
 import { MarketTemplate } from './entities/market-template.entity';
 import { Market, MarketSettlementState } from './entities/market.entity';
+import {
+  canTransition,
+  describeIllegalTransition,
+} from './market-settlement-state.util';
 import { UserBookmark } from './entities/user-bookmark.entity';
 import { MarketPriceSnapshot } from './entities/market-price-snapshot.entity';
 import { Prediction } from '../predictions/entities/prediction.entity';
@@ -564,9 +568,14 @@ export class MarketsService {
       throw new ConflictException('Market is already resolved');
     }
 
-    if (market.settlement_state !== MarketSettlementState.PENDING) {
+    if (
+      !canTransition(market.settlement_state, MarketSettlementState.PROPOSED)
+    ) {
       throw new ConflictException(
-        `Cannot propose a resolution while market is in "${market.settlement_state}" state`,
+        describeIllegalTransition(
+          market.settlement_state,
+          MarketSettlementState.PROPOSED,
+        ),
       );
     }
 
@@ -616,9 +625,15 @@ export class MarketsService {
   ): Promise<Market> {
     const market = await this.findByIdOrOnChainId(id);
 
-    if (market.settlement_state !== MarketSettlementState.PROPOSED) {
+    if (
+      !canTransition(market.settlement_state, MarketSettlementState.CHALLENGED)
+    ) {
       throw new BadRequestException(
-        'Market does not have a resolution pending challenge',
+        'Market does not have a resolution pending challenge: ' +
+          describeIllegalTransition(
+            market.settlement_state,
+            MarketSettlementState.CHALLENGED,
+          ),
       );
     }
 
@@ -661,7 +676,11 @@ export class MarketsService {
     const market = await this.findByIdOrOnChainId(id);
 
     if (market.settlement_state !== MarketSettlementState.CHALLENGED) {
-      throw new BadRequestException('Market does not have an active challenge');
+      // Narrower than the transition table on purpose: SETTLING may legally
+      // reach SETTLED, but only via the scheduler, never via adjudication.
+      throw new BadRequestException(
+        `Market does not have an active challenge (state: "${market.settlement_state}")`,
+      );
     }
 
     if (!market.outcome_options.includes(dto.outcome)) {