diff --git a/packages/api/src/router/allocation-conflict-procedures.ts b/packages/api/src/router/allocation-conflict-procedures.ts
new file mode 100644
index 0000000..2829e77
--- /dev/null
+++ b/packages/api/src/router/allocation-conflict-procedures.ts
@@ -0,0 +1,125 @@
+import { validateAvailability } from "@capakraken/engine";
+import {
+  type AllocationConflictCheckResult,
+  type WeekdayAvailability,
+} from "@capakraken/shared";
+import { TRPCError } from "@trpc/server";
+import { z } from "zod";
+import { managerProcedure } from "../trpc.js";
+import { toIsoDate } from "./allocation-shared.js";
+
+const CheckConflictsInputSchema = z.object({
+  resourceId: z.string(),
+  startDate: z.coerce.date(),
+  endDate: z.coerce.date(),
+  hoursPerDay: z.number().min(0).max(24),
+  /** Pass the current assignment id when editing so it is excluded from existing load. */
+  excludeAssignmentId: z.string().optional(),
+});
+
+export const allocationConflictProcedures = {
+  /**
+   * Pre-flight conflict check for the allocation form.
+   * Returns overbooking details (per-day breakdown) and vacation overlaps.
+   * Read-only — no mutations.
+   */
+  checkConflicts: managerProcedure
+    .input(CheckConflictsInputSchema)
+    .query(async ({ ctx, input }): Promise<AllocationConflictCheckResult> => {
+      const resource = await ctx.db.resource.findUnique({
+        where: { id: input.resourceId },
+        select: {
+          availability: true,
+          fte: true,
+          country: { select: { dailyWorkingHours: true } },
+        },
+      });
+
+      if (!resource) {
+        throw new TRPCError({ code: "NOT_FOUND", message: "Resource not found" });
+      }
+
+      const fallbackDailyHours =
+        (resource.country?.dailyWorkingHours ?? 8) * (resource.fte ?? 1);
+      const availability = (resource.availability as WeekdayAvailability | null) ?? {
+        monday: fallbackDailyHours,
+        tuesday: fallbackDailyHours,
+        wednesday: fallbackDailyHours,
+        thursday: fallbackDailyHours,
+        friday: fallbackDailyHours,
+        saturday: 0,
+        sunday: 0,
+      };
+
+      // Load existing active assignments for the resource in the period
+      const existingAssignments = await ctx.db.assignment.findMany({
+        where: {
+          resourceId: input.resourceId,
+          status: { not: "CANCELLED" },
+          startDate: { lte: input.endDate },
+          endDate: { gte: input.startDate },
+          ...(input.excludeAssignmentId ? { id: { not: input.excludeAssignmentId } } : {}),
+        },
+        select: { startDate: true, endDate: true, hoursPerDay: true, status: true },
+      });
+
+      // Load approved vacations that overlap the requested period
+      const vacations = await ctx.db.vacation.findMany({
+        where: {
+          resourceId: input.resourceId,
+          status: "APPROVED",
+          startDate: { lte: input.endDate },
+          endDate: { gte: input.startDate },
+        },
+        select: { startDate: true, endDate: true, type: true, isHalfDay: true },
+        orderBy: { startDate: "asc" },
+      });
+
+      // Run pure availability check (no DB access).
+      // Cast status to the shared enum — Prisma and shared enums are string-compatible.
+      const availabilityResult = validateAvailability(
+        input.startDate,
+        input.endDate,
+        input.hoursPerDay,
+        availability,
+        existingAssignments as { startDate: Date; endDate: Date; hoursPerDay: number; status: import("@capakraken/shared").AllocationStatus }[],
+      );
+
+      // Compute max overbook percentage for the worst day
+      let maxOverbookPercent = 0;
+      for (const conflict of availabilityResult.conflicts) {
+        const totalBooked = conflict.existingHours + conflict.requestedHours;
+        const overbookPct =
+          conflict.availableHours > 0
+            ? ((totalBooked / conflict.availableHours) - 1) * 100
+            : 100;
+        if (overbookPct > maxOverbookPercent) maxOverbookPercent = overbookPct;
+      }
+
+      const isOverbooking = availabilityResult.totalConflictDays > 0;
+
+      return {
+        isOverbooking,
+        overbooking: isOverbooking
+          ? {
+              conflictDays: availabilityResult.conflicts.map((c) => ({
+                date: toIsoDate(c.date),
+                availableHours: c.availableHours,
+                existingHours: c.existingHours,
+                requestedHours: c.requestedHours,
+                overageHours: c.overageHours,
+              })),
+              totalConflictDays: availabilityResult.totalConflictDays,
+              maxOverbookPercent: Math.round(maxOverbookPercent),
+            }
+          : null,
+        vacationOverlap: vacations.map((v) => ({
+          startDate: toIsoDate(v.startDate),
+          endDate: toIsoDate(v.endDate),
+          type: v.type,
+          isHalfDay: v.isHalfDay,
+        })),
+        hasVacationOverlap: vacations.length > 0,
+      };
+    }),
+};
diff --git a/packages/api/src/router/allocation.ts b/packages/api/src/router/allocation.ts
index d3f361b..cd24a7b 100644
--- a/packages/api/src/router/allocation.ts
+++ b/packages/api/src/router/allocation.ts
@@ -1,4 +1,5 @@
 import { allocationAssignmentProcedures } from "./allocation-assignment-procedures.js";
+import { allocationConflictProcedures } from "./allocation-conflict-procedures.js";
 import { allocationDemandProcedures } from "./allocation-demand-procedures.js";
 import { allocationReadProcedures } from "./allocation-read-procedures.js";
 import { createTRPCRouter } from "../trpc.js";
@@ -7,4 +8,5 @@ export const allocationRouter = createTRPCRouter({
   ...allocationReadProcedures,
   ...allocationDemandProcedures,
   ...allocationAssignmentProcedures,
+  ...allocationConflictProcedures,
 });
diff --git a/packages/application/src/use-cases/allocation/create-assignment.ts b/packages/application/src/use-cases/allocation/create-assignment.ts
index b42f377..0af0ed9 100644
--- a/packages/application/src/use-cases/allocation/create-assignment.ts
+++ b/packages/application/src/use-cases/allocation/create-assignment.ts
@@ -143,10 +143,10 @@ export async function createAssignment(
     availabilityWindows,
   );
 
-  if (!availabilityResult.valid && availabilityResult.totalConflictDays > 5) {
+  if (!availabilityResult.valid && !input.allowOverbooking) {
     throw new TRPCError({
-      code: "BAD_REQUEST",
-      message: `Resource has availability conflicts on ${availabilityResult.totalConflictDays} days`,
+      code: "CONFLICT",
+      message: `Resource is overbooked on ${availabilityResult.totalConflictDays} day(s). Pass allowOverbooking: true to proceed.`,
     });
   }
 
@@ -179,6 +179,7 @@ export async function createAssignment(
       dailyCostCents: input.dailyCostCents ?? calculation.dailyCostCents,
       status: input.status,
       metadata: input.metadata as unknown as Prisma.InputJsonValue,
+      overbookingAcknowledged: input.allowOverbooking === true && !availabilityResult.valid,
     },
     include: ASSIGNMENT_RELATIONS_INCLUDE,
   });
@@ -194,3 +195,55 @@ export async function createAssignment(
 
   return assignment;
 }
+
+/**
+ * Creates an assignment record without running availability or duplicate checks.
+ * Use ONLY for fragment operations (splitting/carving an existing allocation)
+ * where the resource's total load does not change.
+ */
+export async function createAssignmentFragment(
+  db: DbClient,
+  input: {
+    demandRequirementId?: string | null | undefined;
+    resourceId: string;
+    projectId: string;
+    startDate: Date;
+    endDate: Date;
+    hoursPerDay: number;
+    percentage: number;
+    role?: string | null | undefined;
+    roleId?: string | null | undefined;
+    dailyCostCents: number;
+    status: string;
+    metadata: unknown;
+  },
+): Promise<AssignmentWithRelations> {
+  const assignment = await db.assignment.create({
+    data: {
+      demandRequirementId: input.demandRequirementId ?? null,
+      resourceId: input.resourceId,
+      projectId: input.projectId,
+      startDate: input.startDate,
+      endDate: input.endDate,
+      hoursPerDay: input.hoursPerDay,
+      percentage: input.percentage,
+      role: input.role ?? null,
+      roleId: input.roleId ?? null,
+      dailyCostCents: input.dailyCostCents,
+      status: input.status as import("@capakraken/db").AllocationStatus,
+      metadata: input.metadata as import("@capakraken/db").Prisma.InputJsonValue,
+    },
+    include: ASSIGNMENT_RELATIONS_INCLUDE,
+  });
+
+  await db.auditLog.create({
+    data: {
+      entityType: "Assignment",
+      entityId: assignment.id,
+      action: "CREATE",
+      changes: { after: assignment } as unknown as import("@capakraken/db").Prisma.InputJsonValue,
+    },
+  });
+
+  return assignment;
+}
diff --git a/packages/shared/src/schemas/allocation.schema.ts b/packages/shared/src/schemas/allocation.schema.ts
index 140ba5b..ea649f9 100644
--- a/packages/shared/src/schemas/allocation.schema.ts
+++ b/packages/shared/src/schemas/allocation.schema.ts
@@ -44,7 +44,7 @@ export const CreateAssignmentBaseSchema = z.object({
   status: z.nativeEnum(AllocationStatus).default(AllocationStatus.PROPOSED),
   metadata: z.record(z.string(), z.unknown()).default({}),
   /** When true the caller acknowledges the resource will be overbooked. */
-  allowOverbooking: z.boolean().optional().default(false),
+  allowOverbooking: z.boolean().optional(),
 });
 
 /**