diff --git a/src/OpenAC.MosswartMassacre/Backend/FrameworkNumberConverter.cs b/src/OpenAC.MosswartMassacre/Backend/FrameworkNumberConverter.cs
new file mode 100644
index 0000000..68a4aff
--- /dev/null
+++ b/src/OpenAC.MosswartMassacre/Backend/FrameworkNumberConverter.cs
@@ -0,0 +1,82 @@
+using System.Globalization;
+using Newtonsoft.Json;
+
+namespace OpenAC.MosswartMassacre.Backend;
+
+/// <summary>
+/// Writes doubles and floats with the digits the original plugin wrote.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Newtonsoft.Json writes a double as <c>value.ToString("R")</c> plus <c>.0</c>
+/// when that has no decimal point. The original ran on .NET Framework, where
+/// "R" meant: fifteen significant digits if they read back as the same
+/// double, otherwise seventeen (a float: seven, otherwise nine). Modern .NET
+/// prints the shortest text that reads back instead, so a value such as
+/// -0.066666670143604279 would come out as -0.06666667014360428: the same
+/// number, different bytes. This converter repeats the Framework rule.
+/// </para>
+/// <para>
+/// Non-finite values keep Newtonsoft's own handling.
+/// </para>
+/// </remarks>
+internal sealed class FrameworkNumberConverter : JsonConverter
+{
+    public static FrameworkNumberConverter Instance { get; } = new();
+
+    public override bool CanRead => false;
+
+    public override bool CanConvert(Type objectType) =>
+        objectType == typeof(double) || objectType == typeof(double?)
+        || objectType == typeof(float) || objectType == typeof(float?);
+
+    public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
+    {
+        switch (value)
+        {
+            case null:
+                writer.WriteNull();
+                break;
+            case double d when double.IsFinite(d):
+                writer.WriteRawValue(EnsureDecimalPlace(FrameworkRoundTrip(d)));
+                break;
+            case float f when float.IsFinite(f):
+                writer.WriteRawValue(EnsureDecimalPlace(FrameworkRoundTrip(f)));
+                break;
+            case double d:
+                writer.WriteValue(d);
+                break;
+            case float f:
+                writer.WriteValue(f);
+                break;
+            default:
+                writer.WriteValue(value);
+                break;
+        }
+    }
+
+    public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer) =>
+        throw new NotSupportedException();
+
+    /// <summary>.NET Framework's "R" for a double: G15 if it reads back exactly, else G17.</summary>
+    internal static string FrameworkRoundTrip(double value)
+    {
+        string text = value.ToString("G15", CultureInfo.InvariantCulture);
+        if (double.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture) == value)
+            return text;
+        return value.ToString("G17", CultureInfo.InvariantCulture);
+    }
+
+    /// <summary>.NET Framework's "R" for a float: G7 if it reads back exactly, else G9.</summary>
+    internal static string FrameworkRoundTrip(float value)
+    {
+        string text = value.ToString("G7", CultureInfo.InvariantCulture);
+        if (float.Parse(text, NumberStyles.Float, CultureInfo.InvariantCulture) == value)
+            return text;
+        return value.ToString("G9", CultureInfo.InvariantCulture);
+    }
+
+    /// <summary>Newtonsoft's rule: a number with no point and no exponent gets ".0".</summary>
+    private static string EnsureDecimalPlace(string text) =>
+        text.Contains('.') || text.Contains('E') || text.Contains('e') ? text : text + ".0";
+}
diff --git a/src/OpenAC.MosswartMassacre/Backend/Wire.cs b/src/OpenAC.MosswartMassacre/Backend/Wire.cs
index 69061cb..1a56319 100644
--- a/src/OpenAC.MosswartMassacre/Backend/Wire.cs
+++ b/src/OpenAC.MosswartMassacre/Backend/Wire.cs
@@ -16,7 +16,14 @@ internal static class Wire
     // A private serializer rather than JsonConvert.SerializeObject: that one
     // picks up JsonConvert.DefaultSettings, which any code in this load
     // context could change.
-    private static readonly JsonSerializer Serializer = JsonSerializer.Create();
+    private static readonly JsonSerializer Serializer = CreateSerializer();
+
+    private static JsonSerializer CreateSerializer()
+    {
+        JsonSerializer serializer = JsonSerializer.Create();
+        serializer.Converters.Add(FrameworkNumberConverter.Instance);
+        return serializer;
+    }
 
     public static string Serialize(object? payload)
     {
diff --git a/tests/OpenAC.MosswartMassacre.Tests/Backend/WireNumberTests.cs b/tests/OpenAC.MosswartMassacre.Tests/Backend/WireNumberTests.cs
new file mode 100644
index 0000000..9928835
--- /dev/null
+++ b/tests/OpenAC.MosswartMassacre.Tests/Backend/WireNumberTests.cs
@@ -0,0 +1,52 @@
+using OpenAC.MosswartMassacre.Backend;
+
+namespace OpenAC.MosswartMassacre.Tests.Backend;
+
+/// <summary>
+/// Numbers written the way the original plugin (Newtonsoft.Json on .NET
+/// Framework) wrote them.
+/// </summary>
+public sealed class WireNumberTests
+{
+    [Theory]
+    // 15 digits read back: the short form.
+    [InlineData(0.1, "0.1")]
+    [InlineData(75.0, "75.0")]
+    [InlineData(42.5, "42.5")]
+    [InlineData(-12.0, "-12.0")]
+    [InlineData(1e-05, "1E-05")]
+    // 15 digits do not read back: seventeen, not the shortest round trip.
+    [InlineData(-0.066666670143604279, "-0.066666670143604279")]
+    [InlineData((double)33.65f, "33.650001525878906")]
+    [InlineData(0.30000000000000004, "0.30000000000000004")]
+    public void Doubles_use_the_frameworks_round_trip_digits(double value, string expected)
+    {
+        Assert.Equal(expected, Wire.Serialize(value));
+        Assert.Equal("{\"v\":" + expected + "}", Wire.Serialize(new { v = value }));
+        Assert.Equal("{\"v\":" + expected + "}", Wire.Serialize(new { v = (double?)value }));
+        Assert.Equal("{\"1\":" + expected + "}", Wire.Serialize(new Dictionary<int, double> { [1] = value }));
+    }
+
+    [Fact]
+    public void The_shortest_form_and_the_framework_form_can_differ()
+    {
+        // The case the converter exists for.
+        Assert.Equal("-0.06666667014360428", (-0.066666670143604279).ToString("R", System.Globalization.CultureInfo.InvariantCulture));
+        Assert.Equal("-0.066666670143604279", Wire.Serialize(-0.066666670143604279));
+    }
+
+    [Theory]
+    [InlineData(2.7115397f, "2.71153975")]
+    [InlineData(0.1f, "0.1")]
+    [InlineData(3f, "3.0")]
+    [InlineData(1.0000001f, "1.00000012")]
+    public void Floats_use_the_frameworks_round_trip_digits(float value, string expected) =>
+        Assert.Equal("{\"v\":" + expected + "}", Wire.Serialize(new { v = value }));
+
+    [Fact]
+    public void Nulls_and_non_finite_values_keep_newtonsofts_handling()
+    {
+        Assert.Equal("{\"v\":null}", Wire.Serialize(new { v = (double?)null }));
+        Assert.Equal("{\"v\":\"NaN\"}", Wire.Serialize(new { v = double.NaN }));
+    }
+}
diff --git a/tests/OpenAC.MosswartMassacre.Tests/Inventory/InventoryGoldenTests.cs b/tests/OpenAC.MosswartMassacre.Tests/Inventory/InventoryGoldenTests.cs
index 263fffc..4e37554 100644
--- a/tests/OpenAC.MosswartMassacre.Tests/Inventory/InventoryGoldenTests.cs
+++ b/tests/OpenAC.MosswartMassacre.Tests/Inventory/InventoryGoldenTests.cs
@@ -36,7 +36,7 @@ public sealed class InventoryGoldenTests
     // 600 - 200 (no material) - (240 active Incantation of Impenetrability - 20 Minor
     // Impenetrability bonus) = 180.
     private const string BreastplateRecord =
-        """{"HasIdData":true,"Id":-2147482877,"LastIdTime":""" + IdStamp + ""","ObjectClass":2,"Icon":4660,"ContainerId":1342177281,"BoolValues":{"69":true},"DoubleValues":{"167772160":1.2000000476837158,"167772161":1.0,"167772162":0.800000011920929,"167772163":0.20000000298023224,"167772164":1.5,"167772165":0.4000000059604645,"167772166":0.6000000238418579},"IntValues":{"5":800,"19":5000,"28":600,"265":5,"218103808":6044,"218103809":4660,"218103810":1342177281,"218103821":1024,"218103822":512,"218103824":1,"218103834":2,"218103835":18,"218103838":2,"218103843":1,"218103848":2},"StringValues":{"1":"Celdon Breastplate"},"ActiveSpells":[4407,1483],"Spells":[1486,2604],"Material":null,"Name":"Celdon Breastplate","EquipSkill":null,"Mastery":null,"ItemSet":"Noble Relic Set","ArmorLevel":600,"Imbue":null,"Tinks":-1,"MaxDamage":-1,"ElementalDmgBonus":-1,"Variance":-1.0,"DamageBonus":-1.0,"ElementalDamageVersusMonsters":-1.0,"AttackBonus":-1.0,"MeleeDefenseBonus":-1.0,"MagicDBonus":-1.0,"MissileDBonus":-1.0,"ManaCBonus":-1.0,"WieldLevel":-1,"SkillLevel":-1,"LoreRequirement":-1,"SalvageWorkmanship":-1.0,"Workmanship":-1,"Value":5000,"Burden":800,"DamRating":-1,"DamResistRating":-1,"CritRating":-1,"CritResistRating":-1,"CritDamRating":-1,"CritDamResistRating":-1,"HealBoostRating":-1,"VitalityRating":-1,"TotalRating":-1,"CalcedStartingArmorLevel":180,"CalcedBuffedTinkedDoT":-1.0,"CalcedBuffedMissileDamage":-1.0,"BuffedElementalDamageVersusMonsters":-1.0,"BuffedAttackBonus":-1.0,"BuffedMeleeDefenseBonus":-1.0,"BuffedManaCBonus":-1.0}""";
+        """{"HasIdData":true,"Id":-2147482877,"LastIdTime":""" + IdStamp + ""","ObjectClass":2,"Icon":4660,"ContainerId":1342177281,"BoolValues":{"69":true},"DoubleValues":{"167772160":1.2000000476837158,"167772161":1.0,"167772162":0.800000011920929,"167772163":0.20000000298023224,"167772164":1.5,"167772165":0.40000000596046448,"167772166":0.60000002384185791},"IntValues":{"5":800,"19":5000,"28":600,"265":5,"218103808":6044,"218103809":4660,"218103810":1342177281,"218103821":1024,"218103822":512,"218103824":1,"218103834":2,"218103835":18,"218103838":2,"218103843":1,"218103848":2},"StringValues":{"1":"Celdon Breastplate"},"ActiveSpells":[4407,1483],"Spells":[1486,2604],"Material":null,"Name":"Celdon Breastplate","EquipSkill":null,"Mastery":null,"ItemSet":"Noble Relic Set","ArmorLevel":600,"Imbue":null,"Tinks":-1,"MaxDamage":-1,"ElementalDmgBonus":-1,"Variance":-1.0,"DamageBonus":-1.0,"ElementalDamageVersusMonsters":-1.0,"AttackBonus":-1.0,"MeleeDefenseBonus":-1.0,"MagicDBonus":-1.0,"MissileDBonus":-1.0,"ManaCBonus":-1.0,"WieldLevel":-1,"SkillLevel":-1,"LoreRequirement":-1,"SalvageWorkmanship":-1.0,"Workmanship":-1,"Value":5000,"Burden":800,"DamRating":-1,"DamResistRating":-1,"CritRating":-1,"CritResistRating":-1,"CritDamRating":-1,"CritDamResistRating":-1,"HealBoostRating":-1,"VitalityRating":-1,"TotalRating":-1,"CalcedStartingArmorLevel":180,"CalcedBuffedTinkedDoT":-1.0,"CalcedBuffedMissileDamage":-1.0,"BuffedElementalDamageVersusMonsters":-1.0,"BuffedAttackBonus":-1.0,"BuffedMeleeDefenseBonus":-1.0,"BuffedManaCBonus":-1.0}""";
 
     // Wielded: the container key and ContainerId are the wielder. The second header
     // (public flags 0x04000000) brings the underlay key, here 0.
diff --git a/tests/OpenAC.MosswartMassacre.Tests/Radar/RadarTests.cs b/tests/OpenAC.MosswartMassacre.Tests/Radar/RadarTests.cs
index dda6d6f..a9eaa65 100644
--- a/tests/OpenAC.MosswartMassacre.Tests/Radar/RadarTests.cs
+++ b/tests/OpenAC.MosswartMassacre.Tests/Radar/RadarTests.cs
@@ -11,7 +11,7 @@ public sealed class RadarTests
 {
     private const string Stamp = "2026-09-25T08:30:15.1230000Z";
 
-    private static string D(double value) => value.ToString("R", CultureInfo.InvariantCulture);
+    private static string D(double value) => OpenAC.MosswartMassacre.Backend.FrameworkNumberConverter.FrameworkRoundTrip(value);
 
     private static PluginWorldObject Landscape(uint id, string name, PluginObjectClass cls, PluginNavigationPosition position, uint wielder = 0) =>
         new(id, 1u, name, cls, 0u, 0u, wielder) { IsLandscape = true, HasPosition = true, Position = position };
diff --git a/tests/OpenAC.MosswartMassacre.Tests/Sharing/VitalSharingTests.cs b/tests/OpenAC.MosswartMassacre.Tests/Sharing/VitalSharingTests.cs
index 2785e4d..10ac642 100644
--- a/tests/OpenAC.MosswartMassacre.Tests/Sharing/VitalSharingTests.cs
+++ b/tests/OpenAC.MosswartMassacre.Tests/Sharing/VitalSharingTests.cs
@@ -83,8 +83,8 @@ public sealed class VitalSharingTests
 
         Feature(harness).OnTick();
 
-        string ew = Math.Round((double)33.65f, 7).ToString("R", CultureInfo.InvariantCulture);
-        string ns = Math.Round((double)42.55f, 7).ToString("R", CultureInfo.InvariantCulture);
+        string ew = OpenAC.MosswartMassacre.Backend.FrameworkNumberConverter.FrameworkRoundTrip(Math.Round((double)33.65f, 7));
+        string ns = OpenAC.MosswartMassacre.Backend.FrameworkNumberConverter.FrameworkRoundTrip(Math.Round((double)42.55f, 7));
         Assert.Equal(
             [
                 "{\"type\":\"share_vital_update\",\"timestamp\":\"" + Stamp + "\",\"character_name\":\"Mossy\","
diff --git a/tests/OpenAC.MosswartMassacre.Tests/Streams/StreamGoldenTests.cs b/tests/OpenAC.MosswartMassacre.Tests/Streams/StreamGoldenTests.cs
index 68fe572..10dbbc0 100644
--- a/tests/OpenAC.MosswartMassacre.Tests/Streams/StreamGoldenTests.cs
+++ b/tests/OpenAC.MosswartMassacre.Tests/Streams/StreamGoldenTests.cs
@@ -36,7 +36,7 @@ public sealed class StreamGoldenTests
             true);
     }
 
-    private static string D(float value) => ((double)value).ToString("R", CultureInfo.InvariantCulture);
+    private static string D(float value) => OpenAC.MosswartMassacre.Backend.FrameworkNumberConverter.FrameworkRoundTrip((double)value);
 
     private static PluginHarness LoggedIn(Action<PluginHarness>? arrange = null)
     {
@@ -130,7 +130,7 @@ public sealed class StreamGoldenTests
         harness.Context.Settings.Current.CharTag = "tank";
         harness.Clock.Advance(TimeSpan.FromMinutes(90));
 
-        string json = Newtonsoft.Json.JsonConvert.SerializeObject(harness.Feature<TelemetryStream>().Build());
+        string json = OpenAC.MosswartMassacre.Backend.Wire.Serialize(harness.Feature<TelemetryStream>().Build());
         var frame = Newtonsoft.Json.Linq.JObject.Parse(json);
 
         string[] expectedOrder =