diff --git a/lambdas/supplier-allocator/src/services/__tests__/supplier-config.test.ts b/lambdas/supplier-allocator/src/services/__tests__/supplier-config.test.ts
index 6663d120f..1f6807dba 100644
--- a/lambdas/supplier-allocator/src/services/__tests__/supplier-config.test.ts
+++ b/lambdas/supplier-allocator/src/services/__tests__/supplier-config.test.ts
@@ -483,7 +483,8 @@ describe("supplier-config service", () => {
 
       expect(result).toEqual(["spec1"]);
     });
-    it("throws when no eligible packs found for letter", async () => {
+
+    it("throws when no eligible packs found for letter based on sheets", async () => {
       const deps = makeDeps();
       deps.supplierConfigRepo.getPackSpecification = jest
         .fn()
@@ -505,11 +506,18 @@ describe("supplier-config service", () => {
         "No eligible pack specifications found for letter variant id undefined and pack specification ids spec1",
       );
       expect(deps.logger.info).toHaveBeenCalledWith({
-        description: "Pack specification filtered out based on constraints",
+        description:
+          "Pack specification filtered out based on pageCount constraints",
         packSpecId: "spec1",
         pageCount: 3,
-        constraintValue: 2,
-        constraintOperator: "LESS_THAN",
+        violatedConstraints: expect.arrayContaining([
+          expect.objectContaining({
+            actualValue: 3,
+            constraint: "sheets",
+            constraintValue: 2,
+            operator: "LESS_THAN",
+          }),
+        ]),
       });
       expect(deps.logger.error).toHaveBeenCalledWith(
         expect.objectContaining({
@@ -519,6 +527,160 @@ describe("supplier-config service", () => {
         }),
       );
     });
+
+    it("does not filter pack specification when duplex reduces sheets and constraint is LESS_THAN_OR_EQUAL", async () => {
+      const deps = makeDeps();
+      deps.supplierConfigRepo.getPackSpecification = jest
+        .fn()
+        .mockResolvedValue({
+          id: "spec1",
+          assembly: { duplex: true },
+          constraints: {
+            sheets: { operator: "LESS_THAN_OR_EQUAL", value: 2 },
+          },
+        } as any);
+
+      const letterEvent = {
+        data: {
+          pageCount: 3,
+          letterVariantId: "variant-1",
+        },
+      } as any;
+
+      const result = await filterPacksForLetter(letterEvent, ["spec1"], deps);
+
+      expect(result).toEqual(["spec1"]);
+      expect(deps.logger.info).not.toHaveBeenCalled();
+    });
+
+    it("filters pack specification when sides fail LESS_THAN_OR_EQUAL", async () => {
+      const deps = makeDeps();
+      deps.supplierConfigRepo.getPackSpecification = jest
+        .fn()
+        .mockResolvedValue({
+          id: "spec1",
+          constraints: {
+            sides: { operator: "LESS_THAN_OR_EQUAL", value: 2 },
+          },
+        } as any);
+
+      const letterEvent = {
+        data: {
+          pageCount: 3,
+          letterVariantId: "variant-1",
+        },
+      } as any;
+
+      await expect(
+        filterPacksForLetter(letterEvent, ["spec1"], deps),
+      ).rejects.toThrow(/No eligible pack specifications found/);
+
+      expect(deps.logger.info).toHaveBeenCalledWith({
+        description:
+          "Pack specification filtered out based on pageCount constraints",
+        packSpecId: "spec1",
+        pageCount: 3,
+        violatedConstraints: expect.arrayContaining([
+          expect.objectContaining({
+            constraint: "sides",
+            actualValue: 3,
+            constraintValue: 2,
+            operator: "LESS_THAN_OR_EQUAL",
+          }),
+        ]),
+      });
+    });
+
+    it("filters by sides using page count even when duplex is enabled", async () => {
+      const deps = makeDeps();
+      deps.supplierConfigRepo.getPackSpecification = jest
+        .fn()
+        .mockResolvedValue({
+          id: "spec1",
+          assembly: { duplex: true },
+          constraints: {
+            sheets: { operator: "LESS_THAN_OR_EQUAL", value: 2 },
+            sides: { operator: "LESS_THAN_OR_EQUAL", value: 2 },
+          },
+        } as any);
+
+      const letterEvent = {
+        data: {
+          pageCount: 3,
+          letterVariantId: "variant-1",
+        },
+      } as any;
+
+      await expect(
+        filterPacksForLetter(letterEvent, ["spec1"], deps),
+      ).rejects.toThrow(/No eligible pack specifications found/);
+
+      expect(deps.logger.info).toHaveBeenCalledWith({
+        description:
+          "Pack specification filtered out based on pageCount constraints",
+        packSpecId: "spec1",
+        pageCount: 3,
+        violatedConstraints: expect.arrayContaining([
+          expect.objectContaining({
+            constraint: "sides",
+            actualValue: 3,
+            constraintValue: 2,
+            operator: "LESS_THAN_OR_EQUAL",
+          }),
+        ]),
+      });
+    });
+
+    it("does not filter pack specification when sides is valid", async () => {
+      const deps = makeDeps();
+      deps.supplierConfigRepo.getPackSpecification = jest
+        .fn()
+        .mockResolvedValue({
+          id: "spec1",
+          assembly: { duplex: true },
+          constraints: {
+            sheets: { operator: "LESS_THAN_OR_EQUAL", value: 4 },
+          },
+        } as any);
+
+      const letterEvent = {
+        data: {
+          pageCount: 3,
+          letterVariantId: "variant-1",
+        },
+      } as any;
+
+      const result = await filterPacksForLetter(letterEvent, ["spec1"], deps);
+
+      expect(result).toEqual(["spec1"]);
+      expect(deps.logger.info).not.toHaveBeenCalled();
+    });
+
+    it("keeps pack specification when sides constraint passes", async () => {
+      const deps = makeDeps();
+      deps.supplierConfigRepo.getPackSpecification = jest
+        .fn()
+        .mockResolvedValue({
+          id: "spec1",
+          constraints: {
+            sides: { operator: "LESS_THAN_OR_EQUAL", value: 3 },
+          },
+        } as any);
+
+      const letterEvent = {
+        data: {
+          pageCount: 3,
+          letterVariantId: "variant-1",
+        },
+      } as any;
+
+      const result = await filterPacksForLetter(letterEvent, ["spec1"], deps);
+
+      expect(result).toEqual(["spec1"]);
+      expect(deps.logger.info).not.toHaveBeenCalled();
+      expect(deps.logger.error).not.toHaveBeenCalled();
+    });
+
     it("returns eligible packs for all constraint types", async () => {
       const deps = makeDeps();
       const constraints: { operator: string; value: number }[] = [
diff --git a/lambdas/supplier-allocator/src/services/supplier-config.ts b/lambdas/supplier-allocator/src/services/supplier-config.ts
index d5a3774cd..52ac1c434 100644
--- a/lambdas/supplier-allocator/src/services/supplier-config.ts
+++ b/lambdas/supplier-allocator/src/services/supplier-config.ts
@@ -212,41 +212,109 @@ function evaluateContraint(
 
 // This function is used to filter the pack specifications for a letter based on the letter data pages and pack specification constraints sheets
 
+type ConstraintName = "sheets" | "sides";
+
+type ViolatedConstraint = {
+  constraint: ConstraintName;
+  actualValue: number;
+  constraintValue: number;
+  operator: string;
+};
+
+function hasConstraint(constraint?: {
+  value?: number;
+  operator?: string;
+}): constraint is { value: number; operator: string } {
+  return constraint?.value !== undefined && constraint.operator !== undefined;
+}
+
+function getViolatedConstraints(
+  pageCount: number,
+  duplex: boolean | undefined,
+  constraints?: {
+    sheets?: { value?: number; operator?: string };
+    sides?: { value?: number; operator?: string };
+  },
+): ViolatedConstraint[] {
+  if (!constraints) {
+    return [];
+  }
+
+  const violations: ViolatedConstraint[] = [];
+
+  if (hasConstraint(constraints.sheets)) {
+    const sheetCount = duplex ? Math.ceil(pageCount / 2) : pageCount;
+    const passes = evaluateContraint(
+      sheetCount,
+      constraints.sheets.value,
+      constraints.sheets.operator,
+    );
+
+    if (!passes) {
+      violations.push({
+        constraint: "sheets",
+        actualValue: sheetCount,
+        constraintValue: constraints.sheets.value,
+        operator: constraints.sheets.operator,
+      });
+    }
+  }
+
+  if (hasConstraint(constraints.sides)) {
+    const passes = evaluateContraint(
+      pageCount,
+      constraints.sides.value,
+      constraints.sides.operator,
+    );
+
+    if (!passes) {
+      violations.push({
+        constraint: "sides",
+        actualValue: pageCount,
+        constraintValue: constraints.sides.value,
+        operator: constraints.sides.operator,
+      });
+    }
+  }
+
+  return violations;
+}
+
 export async function filterPacksForLetter(
   letterEvent: PreparedEvents,
   packSpecificationIds: string[],
   deps: Deps,
 ): Promise<string[]> {
-  const filteredPackIds: string[] = [];
-  for (const packSpecId of packSpecificationIds) {
-    const packSpec =
-      await deps.supplierConfigRepo.getPackSpecification(packSpecId);
-    if (
-      !packSpec.constraints ||
-      !packSpec.constraints.sheets ||
-      !packSpec.constraints.sheets.value ||
-      !packSpec.constraints.sheets.operator
-    ) {
-      filteredPackIds.push(packSpecId);
-    } else {
-      const isValid = evaluateContraint(
-        letterEvent.data.pageCount,
-        packSpec.constraints.sheets.value,
-        packSpec.constraints.sheets.operator,
+  const { pageCount } = letterEvent.data;
+
+  const evaluationResults = await Promise.all(
+    packSpecificationIds.map(async (packSpecId) => {
+      const packSpec =
+        await deps.supplierConfigRepo.getPackSpecification(packSpecId);
+
+      const violatedConstraints = getViolatedConstraints(
+        pageCount,
+        packSpec?.assembly?.duplex,
+        packSpec.constraints,
       );
-      if (isValid) {
-        filteredPackIds.push(packSpecId);
-      } else {
+
+      if (violatedConstraints.length > 0) {
         deps.logger.info({
-          description: "Pack specification filtered out based on constraints",
+          description: `Pack specification filtered out based on pageCount constraints`,
           packSpecId,
-          pageCount: letterEvent.data.pageCount,
-          constraintValue: packSpec.constraints.sheets.value,
-          constraintOperator: packSpec.constraints.sheets.operator,
+          pageCount,
+          violatedConstraints,
         });
       }
-    }
-  }
+
+      return { packSpecId, isValid: violatedConstraints.length === 0 };
+    }),
+  );
+
+  const filteredPackIds = evaluationResults
+    .filter((result) => result.isValid)
+    .map((result) => result.packSpecId);
+
   if (filteredPackIds.length === 0) {
     deps.logger.error({
       description: "No eligible pack specifications found for letter",
@@ -257,5 +325,6 @@ export async function filterPacksForLetter(
       `No eligible pack specifications found for letter variant id ${letterEvent.data.letterVariantId} and pack specification ids ${packSpecificationIds.join(", ")}`,
     );
   }
+
   return filteredPackIds;
 }
diff --git a/tests/component-tests/allocation-tests/letter-allocation-capacity.spec.ts b/tests/component-tests/allocation-tests/letter-allocation-capacity.spec.ts
index e2353c3d8..7a172a4a8 100644
--- a/tests/component-tests/allocation-tests/letter-allocation-capacity.spec.ts
+++ b/tests/component-tests/allocation-tests/letter-allocation-capacity.spec.ts
@@ -58,14 +58,14 @@ test.describe("Allocator Lambda Tests", () => {
     const preparedEvent = createPreparedV1Event({
       domainId,
       letterVariantId: letterVariant,
-      pageCount: 6, // pagecount that makes notify-c5 ineligible and notify-c4 eligible based on their pack configs
+      pageCount: 11, // pagecount that makes notify-c5 ineligible and notify-c4 eligible based on their pack configs (note packs are duplex)
     });
 
     const response = await sendSnsEvent(preparedEvent);
     expect(response.MessageId).toBeTruthy();
 
     const supplierAllocatorLog = await getAllocationLog(
-      "Pack specification filtered out based on constraints",
+      "Pack specification filtered out based on pageCount constraints",
     );
     const filteredPackSpecId = supplierAllocatorLog.packSpecId;
     logger.info(`Pack spec filtered out ${filteredPackSpecId}`);