backend/API/Controllers/FilamentsController.cs

@@ -1,6 +1,7 @@
 using Extrudex.API.DTOs;
 using Extrudex.API.DTOs.Filaments;
 using Extrudex.Domain.Entities;
+using Extrudex.Domain.Interfaces;
 using Extrudex.Infrastructure.Data;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
@@ -17,16 +18,22 @@ namespace Extrudex.API.Controllers;
 public class FilamentsController : ControllerBase
 {
     private readonly ExtrudexDbContext _dbContext;
+    private readonly ILowStockDetector _lowStockDetector;
     private readonly ILogger<FilamentsController> _logger;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="FilamentsController"/> class.
     /// </summary>
     /// <param name="dbContext">The database context for data access.</param>
+    /// <param name="lowStockDetector">The low-stock detection service for filament alerts.</param>
     /// <param name="logger">The logger for diagnostic output.</param>
-    public FilamentsController(ExtrudexDbContext dbContext, ILogger<FilamentsController> logger)
+    public FilamentsController(
+        ExtrudexDbContext dbContext,
+        ILowStockDetector lowStockDetector,
+        ILogger<FilamentsController> logger)
     {
         _dbContext = dbContext;
+        _lowStockDetector = lowStockDetector;
         _logger = logger;
     }
 
@@ -95,7 +102,7 @@ public class FilamentsController : ControllerBase
             .OrderByDescending(s => s.CreatedAt)
             .Skip((pageNumber - 1) * pageSize)
             .Take(pageSize)
-            .Select(s => MapToFilamentResponse(s))
+            .Select(s => MapToFilamentResponse(s, _lowStockDetector))
             .ToListAsync();
 
         var response = new PagedResponse<FilamentResponse>
@@ -136,7 +143,7 @@ public class FilamentsController : ControllerBase
             return NotFound(new { error = $"Filament with ID '{id}' not found." });
         }
 
-        return Ok(MapToFilamentResponse(spool));
+        return Ok(MapToFilamentResponse(spool, _lowStockDetector));
     }
 
     /// <summary>
@@ -211,7 +218,7 @@ public class FilamentsController : ControllerBase
         if (entity.MaterialModifierId.HasValue)
             await _dbContext.Entry(entity).Reference(s => s.MaterialModifier).LoadAsync();
 
-        var response = MapToFilamentResponse(entity);
+        var response = MapToFilamentResponse(entity, _lowStockDetector);
         return CreatedAtAction(nameof(GetFilament), new { id = entity.Id }, response);
     }
 
@@ -292,7 +299,37 @@ public class FilamentsController : ControllerBase
         if (entity.MaterialModifierId.HasValue)
             await _dbContext.Entry(entity).Reference(s => s.MaterialModifier).LoadAsync();
 
-        return Ok(MapToFilamentResponse(entity));
+        return Ok(MapToFilamentResponse(entity, _lowStockDetector));
+    }
+
+    /// <summary>
+    /// Gets only the filament spools that are flagged as low stock.
+    /// A spool is considered low stock when its remaining weight percentage
+    /// is at or below the configured threshold.
+    /// </summary>
+    /// <returns>A list of low-stock filament spools with alert metadata.</returns>
+    /// <response code="200">Returns the list of low-stock filament spools.</response>
+    [HttpGet("low-stock")]
+    [ProducesResponseType(typeof(List<FilamentResponse>), StatusCodes.Status200OK)]
+    public async Task<ActionResult<List<FilamentResponse>>> GetLowStockFilaments()
+    {
+        _logger.LogDebug("Getting low-stock filaments (threshold: {Threshold}%)",
+            _lowStockDetector.LowStockThresholdPercent);
+
+        var spools = await _dbContext.Spools
+            .Include(s => s.MaterialBase)
+            .Include(s => s.MaterialFinish)
+            .Include(s => s.MaterialModifier)
+            .Where(s => s.IsActive)
+            .OrderByDescending(s => s.CreatedAt)
+            .ToListAsync();
+
+        var lowStockItems = spools
+            .Where(s => _lowStockDetector.IsLowStock(s.WeightRemainingGrams, s.WeightTotalGrams))
+            .Select(s => MapToFilamentResponse(s, _lowStockDetector))
+            .ToList();
+
+        return Ok(lowStockItems);
     }
 
     /// <summary>
@@ -361,10 +398,12 @@ public class FilamentsController : ControllerBase
     /// Maps a Spool domain entity to a FilamentResponse DTO.
     /// Denormalizes material names for display convenience.
     /// Populates the QrCodeUrl for easy frontend access to the spool's QR code.
+    /// Calculates low-stock status and remaining weight percentage.
     /// </summary>
     /// <param name="s">The spool entity to map.</param>
-    /// <returns>A FilamentResponse DTO with denormalized material names and QR code URL.</returns>
-    private static FilamentResponse MapToFilamentResponse(Spool s) => new()
+    /// <param name="lowStockDetector">The low-stock detection service for computing alert flags.</param>
+    /// <returns>A FilamentResponse DTO with denormalized material names, QR code URL, and low-stock metadata.</returns>
+    private static FilamentResponse MapToFilamentResponse(Spool s, ILowStockDetector lowStockDetector) => new()
     {
         Id = s.Id,
         MaterialBaseId = s.MaterialBaseId,
@@ -387,6 +426,8 @@ public class FilamentsController : ControllerBase
         StorageLocation = s.StorageLocation,
         CreatedAt = s.CreatedAt,
         UpdatedAt = s.UpdatedAt,
-        QrCodeUrl = $"/api/qr/spool/{s.Id}"
+        QrCodeUrl = $"/api/qr/spool/{s.Id}",
+        IsLowStock = lowStockDetector.IsLowStock(s.WeightRemainingGrams, s.WeightTotalGrams),
+        RemainingWeightPercent = lowStockDetector.GetRemainingWeightPercent(s.WeightRemainingGrams, s.WeightTotalGrams)
     };
 }

backend/API/DTOs/Filaments/FilamentDtos.cs

@@ -76,6 +76,19 @@ public class FilamentResponse
     /// Encodes a deep link to the spool's detail page.
     /// </summary>
     public string QrCodeUrl { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Whether this spool is flagged as low stock — remaining weight is at or
+    /// below the configured low-stock threshold percentage.
+    /// Useful for UI alerts and inventory dashboards.
+    /// </summary>
+    public bool IsLowStock { get; set; }
+
+    /// <summary>
+    /// Remaining filament weight as a percentage of total weight (0–100).
+    /// Rounded to one decimal place. Returns 0 if total weight is zero.
+    /// </summary>
+    public decimal RemainingWeightPercent { get; set; }
 }
 
 /// <summary>

backend/Domain/Interfaces/ILowStockDetector.cs

@@ -0,0 +1,39 @@
+namespace Extrudex.Domain.Interfaces;
+
+/// <summary>
+/// Detects low-stock filament spools based on configurable weight thresholds.
+/// Determines whether a spool's remaining filament falls below a critical level
+/// so that alerts and API flags can be surfaced to the user.
+/// </summary>
+public interface ILowStockDetector
+{
+    /// <summary>
+    /// Determines whether a spool is considered low stock based on its remaining
+    /// weight relative to its total weight and the configured threshold percentage.
+    /// </summary>
+    /// <param name="weightRemainingGrams">The current remaining weight in grams.</param>
+    /// <param name="weightTotalGrams">The total spool weight in grams when full.</param>
+    /// <returns>
+    /// <c>true</c> if the remaining weight percentage is at or below the configured
+    /// low-stock threshold; <c>false</c> otherwise. Returns <c>false</c> for spools
+    /// with zero total weight to avoid division-by-zero.
+    /// </returns>
+    bool IsLowStock(decimal weightRemainingGrams, decimal weightTotalGrams);
+
+    /// <summary>
+    /// Calculates the remaining weight as a percentage of total weight.
+    /// </summary>
+    /// <param name="weightRemainingGrams">The current remaining weight in grams.</param>
+    /// <param name="weightTotalGrams">The total spool weight in grams when full.</param>
+    /// <returns>
+    /// A value between 0 and 100 representing the percentage of filament remaining.
+    /// Returns 0 if total weight is zero to avoid division-by-zero.
+    /// </returns>
+    decimal GetRemainingWeightPercent(decimal weightRemainingGrams, decimal weightTotalGrams);
+
+    /// <summary>
+    /// Gets the currently configured low-stock threshold percentage.
+    /// Useful for API responses so clients know what threshold is in effect.
+    /// </summary>
+    decimal LowStockThresholdPercent { get; }
+}

backend/Infrastructure/Services/LowStockDetector.cs

@@ -0,0 +1,95 @@
+using Extrudex.Domain.Interfaces;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+
+namespace Extrudex.Infrastructure.Services;
+
+/// <summary>
+/// Detects low-stock filament spools by comparing the remaining weight percentage
+/// against a configurable threshold. The threshold can be set via:
+///   1. EXTRUDEX_LOW_STOCK_THRESHOLD env var (highest priority, e.g. "25")
+///   2. FilamentAlerts:LowStockThresholdPercent in appsettings.json
+///   3. Default: 20% (a standard spool is "low" when ≤20% remains)
+/// </summary>
+public class LowStockDetector : ILowStockDetector
+{
+    private readonly ILogger<LowStockDetector> _logger;
+
+    /// <summary>
+    /// The percentage threshold below which a spool is considered low stock.
+    /// For example, 20 means a spool is "low" when ≤20% of its filament remains.
+    /// </summary>
+    public decimal LowStockThresholdPercent { get; }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="LowStockDetector"/> class.
+    /// Reads the low-stock threshold from configuration with env var override support.
+    /// </summary>
+    /// <param name="configuration">Application configuration for threshold settings.</param>
+    /// <param name="logger">Logger for diagnostic output.</param>
+    public LowStockDetector(IConfiguration configuration, ILogger<LowStockDetector> logger)
+    {
+        _logger = logger;
+
+        // Priority: env var > appsettings > default (20%)
+        var envThreshold = Environment.GetEnvironmentVariable("EXTRUDEX_LOW_STOCK_THRESHOLD");
+        var configThreshold = configuration.GetValue<decimal?>("FilamentAlerts:LowStockThresholdPercent");
+
+        if (!string.IsNullOrEmpty(envThreshold) && decimal.TryParse(envThreshold, out var parsedEnv))
+        {
+            LowStockThresholdPercent = Math.Clamp(parsedEnv, 0m, 100m);
+            _logger.LogInformation(
+                "Low-stock threshold set from env var EXTRUDEX_LOW_STOCK_THRESHOLD: {Threshold}%",
+                LowStockThresholdPercent);
+        }
+        else if (configThreshold.HasValue)
+        {
+            LowStockThresholdPercent = Math.Clamp(configThreshold.Value, 0m, 100m);
+            _logger.LogInformation(
+                "Low-stock threshold set from config FilamentAlerts:LowStockThresholdPercent: {Threshold}%",
+                LowStockThresholdPercent);
+        }
+        else
+        {
+            LowStockThresholdPercent = 20m;
+            _logger.LogInformation(
+                "Low-stock threshold using default: {Threshold}%", LowStockThresholdPercent);
+        }
+    }
+
+    /// <inheritdoc />
+    public bool IsLowStock(decimal weightRemainingGrams, decimal weightTotalGrams)
+    {
+        if (weightTotalGrams <= 0m)
+        {
+            _logger.LogDebug(
+                "Spool with total weight {Total}g cannot be evaluated for low stock — treating as not low",
+                weightTotalGrams);
+            return false;
+        }
+
+        var remainingPercent = GetRemainingWeightPercent(weightRemainingGrams, weightTotalGrams);
+        var isLow = remainingPercent <= LowStockThresholdPercent;
+
+        if (isLow)
+        {
+            _logger.LogDebug(
+                "Spool is LOW STOCK: {Remaining}g / {Total}g = {Percent:F1}% (threshold: {Threshold}%)",
+                weightRemainingGrams, weightTotalGrams, remainingPercent, LowStockThresholdPercent);
+        }
+
+        return isLow;
+    }
+
+    /// <inheritdoc />
+    public decimal GetRemainingWeightPercent(decimal weightRemainingGrams, decimal weightTotalGrams)
+    {
+        if (weightTotalGrams <= 0m)
+            return 0m;
+
+        return Math.Round(
+            (weightRemainingGrams / weightTotalGrams) * 100m,
+            1,
+            MidpointRounding.AwayFromZero);
+    }
+}

backend/Program.cs

@@ -55,6 +55,9 @@ builder.Services.AddSingleton<IQrCodeService, QrCodeService>();
 // ── Cost Per Print Calculation ─────────────────────────────
 builder.Services.AddScoped<ICostPerPrintService, CostPerPrintService>();
 
+// ── Low Stock Detection ────────────────────────────────────
+builder.Services.AddSingleton<ILowStockDetector, LowStockDetector>();
+
 // ── FluentValidation ──────────────────────────────────────
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());

backend/appsettings.json

@@ -20,5 +20,8 @@
     "RequestTimeout": "00:00:15",
     "Enabled": true,
     "HistoryBatchSize": 25
+  },
+  "FilamentAlerts": {
+    "LowStockThresholdPercent": 20
   }
 }