diff --git a/packages/categorize/configure/src/defaults.js b/packages/categorize/configure/src/defaults.js
index 6fccb47ce4..1d3605df2c 100644
--- a/packages/categorize/configure/src/defaults.js
+++ b/packages/categorize/configure/src/defaults.js
@@ -1,10 +1,7 @@
-import { multiplePlacements } from './utils';
-
 export default {
   model: {
     allowAlternateEnabled: true,
     allowMaxChoicesPerCategory: false,
-    allowMultiplePlacementsEnabled: multiplePlacements.disabled,
     alternates: [],
     categories: [],
     categoriesPerRow: 2,
diff --git a/packages/categorize/configure/src/design/index.jsx b/packages/categorize/configure/src/design/index.jsx
index 89226d18e4..4d83692edd 100644
--- a/packages/categorize/configure/src/design/index.jsx
+++ b/packages/categorize/configure/src/design/index.jsx
@@ -538,10 +538,19 @@ export class Design extends React.Component {
       model.correctResponse || [],
     );
 
+    const resolvedAllowMultiplePlacements = (() => {
+      if (model.allowMultiplePlacementsEnabled != null) {
+        return model.allowMultiplePlacementsEnabled;
+      }
+      const allChoiceIds = (model.correctResponse || []).flatMap((cr) => cr.choices || []);
+      const isExclusive = allChoiceIds.length === new Set(allChoiceIds).size;
+      return isExclusive ? multiplePlacements.disabled : multiplePlacements.enabled;
+    })();
+
     const choices = model.choices.map((c) => {
       c.correctResponseCount = this.countChoiceInCorrectResponse(c);
       // ensure categoryCount is set even though updatedModel hasn't been called
-      c.categoryCount = this.checkAllowMultiplePlacements(model.allowMultiplePlacementsEnabled, c);
+      c.categoryCount = this.checkAllowMultiplePlacements(resolvedAllowMultiplePlacements, c);
       return c;
     });
 
@@ -612,7 +621,7 @@ export class Design extends React.Component {
             hideSettings={settingsPanelDisabled}
             settings={
               <Panel
-                model={model}
+                model={{ ...model, allowMultiplePlacementsEnabled: resolvedAllowMultiplePlacements }}
                 onChangeModel={this.updateModel}
                 configuration={configuration}
                 onChangeConfiguration={onConfigurationChanged}
@@ -692,7 +701,7 @@ export class Design extends React.Component {
               imageSupport={imageSupport}
               uploadSoundSupport={uploadSoundSupport}
               choices={choices}
-              model={model}
+              model={{ ...model, allowMultiplePlacementsEnabled: resolvedAllowMultiplePlacements }}
               onModelChanged={this.updateModel}
               toolbarOpts={toolbarOpts}
               spellCheck={spellCheckEnabled}
diff --git a/packages/categorize/controller/src/__tests__/index.test.js b/packages/categorize/controller/src/__tests__/index.test.js
index 5b7a38495b..dfc1825331 100644
--- a/packages/categorize/controller/src/__tests__/index.test.js
+++ b/packages/categorize/controller/src/__tests__/index.test.js
@@ -161,6 +161,73 @@ describe('controller', () => {
     });
   });
 
+  describe('allowMultiplePlacementsEnabled / categoryCount', () => {
+    const exclusiveCorrectResponse = [
+      { category: '1', choices: ['1', '2'] },
+      { category: '2', choices: ['3', '4'] },
+    ];
+    const nonExclusiveCorrectResponse = [
+      { category: '1', choices: ['1', '2'] },
+      { category: '2', choices: ['2', '3'] }, // '2' appears in both → reuse required
+    ];
+
+    it('explicit "Yes" → categoryCount 0 for all choices', async () => {
+      const q = makeQuestion({ allowMultiplePlacementsEnabled: 'Yes', correctResponse: exclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.choices.every((c) => c.categoryCount === 0)).toBe(true);
+    });
+
+    it('explicit "No" → categoryCount 1 for all choices', async () => {
+      const q = makeQuestion({ allowMultiplePlacementsEnabled: 'No', correctResponse: nonExclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.choices.every((c) => c.categoryCount === 1)).toBe(true);
+    });
+
+    it('no property + exclusive correct response → derived "No" → categoryCount 1', async () => {
+      const q = makeQuestion({ correctResponse: exclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.choices.every((c) => c.categoryCount === 1)).toBe(true);
+    });
+
+    it('no property + non-exclusive correct response → derived "Yes" → categoryCount 0', async () => {
+      const q = makeQuestion({ correctResponse: nonExclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.choices.every((c) => c.categoryCount === 0)).toBe(true);
+    });
+
+    it('no property + exclusive → sets allowMultiplePlacementsEnabled on out', async () => {
+      const q = makeQuestion({ correctResponse: exclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.allowMultiplePlacementsEnabled).toBe('No');
+    });
+
+    it('no property + non-exclusive → sets allowMultiplePlacementsEnabled on out', async () => {
+      const q = makeQuestion({ correctResponse: nonExclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.allowMultiplePlacementsEnabled).toBe('Yes');
+    });
+
+    it('explicit property → does not set allowMultiplePlacementsEnabled on out', async () => {
+      const q = makeQuestion({ allowMultiplePlacementsEnabled: 'No', correctResponse: exclusiveCorrectResponse });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.allowMultiplePlacementsEnabled).toBeUndefined();
+    });
+
+    it('perChoice → uses categoryCount from each choice', async () => {
+      const q = makeQuestion({
+        allowMultiplePlacementsEnabled: 'Set Per Choice',
+        choices: [
+          { id: '1', content: 'Foo', categoryCount: 0 },
+          { id: '2', content: 'Bar', categoryCount: 1 },
+        ],
+        correctResponse: exclusiveCorrectResponse,
+      });
+      const result = await model(q, {}, { mode: 'gather' }, jest.fn());
+      expect(result.choices.find((c) => c.id === '1').categoryCount).toBe(0);
+      expect(result.choices.find((c) => c.id === '2').categoryCount).toBe(1);
+    });
+  });
+
   describe('correct response', () => {
     it('returns correct response if env is correct', async () => {
       const sess = await createCorrectResponseSession(question, {
diff --git a/packages/categorize/controller/src/defaults.js b/packages/categorize/controller/src/defaults.js
index 852b9172d1..c6b4388762 100644
--- a/packages/categorize/controller/src/defaults.js
+++ b/packages/categorize/controller/src/defaults.js
@@ -1,7 +1,4 @@
-import { multiplePlacements } from './utils';
-
 export default {
-  allowMultiplePlacementsEnabled: multiplePlacements.disabled,
   alternates: [],
   categories: [],
   categoriesPerRow: 2,
diff --git a/packages/categorize/controller/src/index.js b/packages/categorize/controller/src/index.js
index 8cac3f3a07..87ff961add 100644
--- a/packages/categorize/controller/src/index.js
+++ b/packages/categorize/controller/src/index.js
@@ -154,13 +154,24 @@ export const model = async (question, session, env, updateSession) => {
     choices = await getShuffledChoices(choices, session, updateSession, 'id');
   }
 
+  const resolvedAllowMultiple = (() => {
+    if (question.allowMultiplePlacementsEnabled != null) {
+      return question.allowMultiplePlacementsEnabled;
+    }
+    // Derive from correct response: if any choice id appears in more than one category, reuse is required
+    const allChoiceIds = (correctResponse || []).flatMap((cr) => cr.choices || []);
+    const isExclusive = allChoiceIds.length === new Set(allChoiceIds).size;
+    return isExclusive ? multiplePlacements.disabled : multiplePlacements.enabled;
+  })();
+
   choices = (choices || []).map((c) => {
     let categoryCount;
-    if (normalizedQuestion.allowMultiplePlacementsEnabled === multiplePlacements.enabled) {
+    if (resolvedAllowMultiple === multiplePlacements.enabled) {
       categoryCount = 0;
-    } else if (normalizedQuestion.allowMultiplePlacementsEnabled === multiplePlacements.disabled) {
+    } else if (resolvedAllowMultiple === multiplePlacements.disabled) {
       categoryCount = 1;
     } else {
+      // perChoice — use the value set on each choice individually
       categoryCount = c.categoryCount || 0;
     }
     return { ...c, categoryCount };
@@ -203,6 +214,7 @@ export const model = async (question, session, env, updateSession) => {
     possibleResponses,
     responseAreasToBeFilled,
     hasUnplacedChoices,
+    ...(question.allowMultiplePlacementsEnabled == null && { allowMultiplePlacementsEnabled: resolvedAllowMultiple }),
   };
 
   if (role === 'instructor' && (mode === 'view' || mode === 'evaluate')) {
@@ -276,7 +288,7 @@ export const getLogTrace = (model, session, env) => {
   }
 
   if (hasAlternates) {
-    traceLog.push(`Alternate response combinations are accepted for this question.`);
+    traceLog.push('Alternate response combinations are accepted for this question.');
   }
 
   if (hasAlternates) {