diff --git a/packages/api/src/__tests__/calculation-rule-support.test.ts b/packages/api/src/__tests__/calculation-rule-support.test.ts
new file mode 100644
index 0000000..1a4893b
--- /dev/null
+++ b/packages/api/src/__tests__/calculation-rule-support.test.ts
@@ -0,0 +1,77 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildCalculationRuleCreateData,
+  buildCalculationRuleUpdateData,
+} from "../router/calculation-rule-support.js";
+
+describe("calculation-rule-support", () => {
+  it("buildCalculationRuleCreateData omits undefined optional fields", () => {
+    const result = buildCalculationRuleCreateData({
+      name: "Sick Days",
+      triggerType: "SICK",
+      costEffect: "ZERO",
+      chargeabilityEffect: "SKIP",
+      priority: 0,
+      isActive: true,
+    });
+
+    expect(result).toEqual({
+      name: "Sick Days",
+      triggerType: "SICK",
+      costEffect: "ZERO",
+      chargeabilityEffect: "SKIP",
+      priority: 0,
+      isActive: true,
+    });
+    expect(result).not.toHaveProperty("description");
+    expect(result).not.toHaveProperty("projectId");
+    expect(result).not.toHaveProperty("orderType");
+    expect(result).not.toHaveProperty("costReductionPercent");
+  });
+
+  it("buildCalculationRuleCreateData keeps provided optional fields", () => {
+    const result = buildCalculationRuleCreateData({
+      name: "Vacation Discount",
+      description: "Reduce cost for vacation",
+      triggerType: "VACATION",
+      projectId: "project_1",
+      orderType: "CHARGEABLE",
+      costEffect: "REDUCE",
+      costReductionPercent: 25,
+      chargeabilityEffect: "COUNT",
+      priority: 10,
+      isActive: false,
+    });
+
+    expect(result).toEqual({
+      name: "Vacation Discount",
+      description: "Reduce cost for vacation",
+      triggerType: "VACATION",
+      projectId: "project_1",
+      orderType: "CHARGEABLE",
+      costEffect: "REDUCE",
+      costReductionPercent: 25,
+      chargeabilityEffect: "COUNT",
+      priority: 10,
+      isActive: false,
+    });
+  });
+
+  it("buildCalculationRuleUpdateData only includes provided fields and preserves falsy values", () => {
+    const result = buildCalculationRuleUpdateData({
+      description: "",
+      costReductionPercent: 0,
+      priority: 0,
+      isActive: false,
+    });
+
+    expect(result).toEqual({
+      description: "",
+      costReductionPercent: 0,
+      priority: 0,
+      isActive: false,
+    });
+    expect(result).not.toHaveProperty("name");
+    expect(result).not.toHaveProperty("projectId");
+  });
+});
diff --git a/packages/api/src/router/calculation-rule-support.ts b/packages/api/src/router/calculation-rule-support.ts
new file mode 100644
index 0000000..5791f60
--- /dev/null
+++ b/packages/api/src/router/calculation-rule-support.ts
@@ -0,0 +1,43 @@
+import type { Prisma } from "@capakraken/db";
+import {
+  CreateCalculationRuleSchema,
+  UpdateCalculationRuleSchema,
+} from "@capakraken/shared";
+import { z } from "zod";
+
+type CreateCalculationRuleInput = z.infer<typeof CreateCalculationRuleSchema>;
+type UpdateCalculationRuleInput = Omit<z.infer<typeof UpdateCalculationRuleSchema>, "id">;
+
+export function buildCalculationRuleCreateData(
+  input: CreateCalculationRuleInput,
+): Prisma.CalculationRuleUncheckedCreateInput {
+  return {
+    name: input.name,
+    triggerType: input.triggerType,
+    costEffect: input.costEffect,
+    chargeabilityEffect: input.chargeabilityEffect,
+    ...(input.description !== undefined ? { description: input.description } : {}),
+    ...(input.projectId !== undefined ? { projectId: input.projectId } : {}),
+    ...(input.orderType !== undefined ? { orderType: input.orderType as never } : {}),
+    ...(input.costReductionPercent !== undefined ? { costReductionPercent: input.costReductionPercent } : {}),
+    priority: input.priority,
+    isActive: input.isActive,
+  };
+}
+
+export function buildCalculationRuleUpdateData(
+  input: UpdateCalculationRuleInput,
+): Prisma.CalculationRuleUncheckedUpdateInput {
+  return {
+    ...(input.name !== undefined ? { name: input.name } : {}),
+    ...(input.description !== undefined ? { description: input.description } : {}),
+    ...(input.triggerType !== undefined ? { triggerType: input.triggerType } : {}),
+    ...(input.projectId !== undefined ? { projectId: input.projectId } : {}),
+    ...(input.orderType !== undefined ? { orderType: input.orderType as never } : {}),
+    ...(input.costEffect !== undefined ? { costEffect: input.costEffect } : {}),
+    ...(input.costReductionPercent !== undefined ? { costReductionPercent: input.costReductionPercent } : {}),
+    ...(input.chargeabilityEffect !== undefined ? { chargeabilityEffect: input.chargeabilityEffect } : {}),
+    ...(input.priority !== undefined ? { priority: input.priority } : {}),
+    ...(input.isActive !== undefined ? { isActive: input.isActive } : {}),
+  };
+}
diff --git a/packages/api/src/router/calculation-rules.ts b/packages/api/src/router/calculation-rules.ts
index 08baf31..ac62a16 100644
--- a/packages/api/src/router/calculation-rules.ts
+++ b/packages/api/src/router/calculation-rules.ts
@@ -7,6 +7,10 @@ import { findUniqueOrThrow } from "../db/helpers.js";
 import { PROJECT_BRIEF_SELECT } from "../db/selects.js";
 import { createTRPCRouter, controllerProcedure, managerProcedure } from "../trpc.js";
 import { createAuditEntry } from "../lib/audit.js";
+import {
+  buildCalculationRuleCreateData,
+  buildCalculationRuleUpdateData,
+} from "./calculation-rule-support.js";
 
 export const calculationRuleRouter = createTRPCRouter({
   list: controllerProcedure.query(async ({ ctx }) => {
@@ -40,18 +44,7 @@ export const calculationRuleRouter = createTRPCRouter({
     .input(CreateCalculationRuleSchema)
     .mutation(async ({ ctx, input }) => {
       const rule = await ctx.db.calculationRule.create({
-        data: {
-          name: input.name,
-          triggerType: input.triggerType,
-          costEffect: input.costEffect,
-          chargeabilityEffect: input.chargeabilityEffect,
-          ...(input.description !== undefined ? { description: input.description } : {}),
-          ...(input.projectId !== undefined ? { projectId: input.projectId } : {}),
-          ...(input.orderType !== undefined ? { orderType: input.orderType as never } : {}),
-          ...(input.costReductionPercent !== undefined ? { costReductionPercent: input.costReductionPercent } : {}),
-          priority: input.priority,
-          isActive: input.isActive,
-        },
+        data: buildCalculationRuleCreateData(input),
       });
 
       void createAuditEntry({
@@ -77,22 +70,9 @@ export const calculationRuleRouter = createTRPCRouter({
         "CalculationRule",
       );
 
-      // Build update data using exactOptionalPropertyTypes pattern
-      const updateData: Record<string, unknown> = {};
-      if (data.name !== undefined) updateData.name = data.name;
-      if (data.description !== undefined) updateData.description = data.description;
-      if (data.triggerType !== undefined) updateData.triggerType = data.triggerType;
-      if (data.projectId !== undefined) updateData.projectId = data.projectId;
-      if (data.orderType !== undefined) updateData.orderType = data.orderType;
-      if (data.costEffect !== undefined) updateData.costEffect = data.costEffect;
-      if (data.costReductionPercent !== undefined) updateData.costReductionPercent = data.costReductionPercent;
-      if (data.chargeabilityEffect !== undefined) updateData.chargeabilityEffect = data.chargeabilityEffect;
-      if (data.priority !== undefined) updateData.priority = data.priority;
-      if (data.isActive !== undefined) updateData.isActive = data.isActive;
-
       const updated = await ctx.db.calculationRule.update({
         where: { id },
-        data: updateData,
+        data: buildCalculationRuleUpdateData(data),
       });
 
       void createAuditEntry({