diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 181ab37..8ca86b0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -39,4 +39,16 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Run Swift Macro Compatibility Check
-        uses: Matejkob/swift-macro-compatibility-check@v1
\ No newline at end of file
+        uses: Matejkob/swift-macro-compatibility-check@v1
+
+  swift6-mode:
+    name: Swift 6 Language Mode
+    runs-on: macos-15
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Select Xcode 16.4
+        run: sudo xcode-select -s /Applications/Xcode_16.4.app
+
+      - name: Build in Swift 6 language mode
+        run: swift build -Xswiftc -swift-version -Xswiftc 6
\ No newline at end of file
diff --git a/Sources/KarrotCodableKit/BetterCodable/DateValue/Strategy/ISO8601WithFractionalSecondsStrategy.swift b/Sources/KarrotCodableKit/BetterCodable/DateValue/Strategy/ISO8601WithFractionalSecondsStrategy.swift
index d243220..127788f 100644
--- a/Sources/KarrotCodableKit/BetterCodable/DateValue/Strategy/ISO8601WithFractionalSecondsStrategy.swift
+++ b/Sources/KarrotCodableKit/BetterCodable/DateValue/Strategy/ISO8601WithFractionalSecondsStrategy.swift
@@ -16,7 +16,9 @@ import Foundation
 /// representing 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC
 /// (Pacific Standard Time).
 public struct ISO8601WithFractionalSecondsStrategy: DateValueCodableStrategy {
-  private static let formatter: ISO8601DateFormatter = {
+  // `ISO8601DateFormatter` is configured once and never mutated, so concurrent reads are safe.
+  // `nonisolated(unsafe)` acknowledges this until the SDK annotates the type `Sendable`.
+  private nonisolated(unsafe) static let formatter: ISO8601DateFormatter = {
     let formatter = ISO8601DateFormatter()
     formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
     return formatter
diff --git a/Sources/KarrotCodableKit/BetterCodable/LosslessValue/LosslessValue.swift b/Sources/KarrotCodableKit/BetterCodable/LosslessValue/LosslessValue.swift
index 893917f..97187e3 100644
--- a/Sources/KarrotCodableKit/BetterCodable/LosslessValue/LosslessValue.swift
+++ b/Sources/KarrotCodableKit/BetterCodable/LosslessValue/LosslessValue.swift
@@ -101,7 +101,8 @@ extension LosslessValueCodable: Hashable where Strategy.Value: Hashable {
   }
 }
 
-extension LosslessValueCodable: Sendable where Strategy.Value: Sendable {}
+// `type` is an immutable metatype (no shared mutable state); the where-clause keeps `wrappedValue` safe.
+extension LosslessValueCodable: @unchecked Sendable where Strategy.Value: Sendable {}
 
 public struct LosslessDefaultStrategy<Value: LosslessStringCodable>: LosslessDecodingStrategy {
   public static var losslessDecodableTypes: [(Decoder) -> LosslessStringCodable?] {
diff --git a/Sources/KarrotCodableKit/BetterCodable/LosslessValue/OptionalLosslessValue.swift b/Sources/KarrotCodableKit/BetterCodable/LosslessValue/OptionalLosslessValue.swift
index 4bbff1f..6527515 100644
--- a/Sources/KarrotCodableKit/BetterCodable/LosslessValue/OptionalLosslessValue.swift
+++ b/Sources/KarrotCodableKit/BetterCodable/LosslessValue/OptionalLosslessValue.swift
@@ -120,7 +120,8 @@ extension OptionalLosslessValueCodable: Hashable where Strategy.Value: Hashable
   }
 }
 
-extension OptionalLosslessValueCodable: Sendable where Strategy.Value: Sendable {}
+// `type` is an immutable metatype (no shared mutable state); the where-clause keeps `wrappedValue` safe.
+extension OptionalLosslessValueCodable: @unchecked Sendable where Strategy.Value: Sendable {}
 
 /// Decodes Optional Codable values into their respective preferred types.
 ///
diff --git a/Sources/KarrotCodableKit/PolymorphicCodable/Error/PolymorphicCodableError.swift b/Sources/KarrotCodableKit/PolymorphicCodable/Error/PolymorphicCodableError.swift
index e93dfca..d65b77f 100644
--- a/Sources/KarrotCodableKit/PolymorphicCodable/Error/PolymorphicCodableError.swift
+++ b/Sources/KarrotCodableKit/PolymorphicCodable/Error/PolymorphicCodableError.swift
@@ -34,3 +34,6 @@ public enum PolymorphicCodableError: LocalizedError {
     }
   }
 }
+
+// Immutable error value; the `decoded` payload is only read for diagnostics.
+extension PolymorphicCodableError: @unchecked Sendable {}
diff --git a/Sources/KarrotCodableKit/Resilient/ArrayDecodingError.swift b/Sources/KarrotCodableKit/Resilient/ArrayDecodingError.swift
index 214cba0..96803fe 100644
--- a/Sources/KarrotCodableKit/Resilient/ArrayDecodingError.swift
+++ b/Sources/KarrotCodableKit/Resilient/ArrayDecodingError.swift
@@ -40,3 +40,8 @@ extension ResilientDecodingOutcome {
   }
 }
 #endif
+
+#if DEBUG
+// DEBUG-only immutable diagnostic holder; `results` is read-only after init.
+extension ResilientDecodingOutcome.ArrayDecodingError: @unchecked Sendable {}
+#endif
diff --git a/Sources/KarrotCodableKit/Resilient/DictionaryDecodingError.swift b/Sources/KarrotCodableKit/Resilient/DictionaryDecodingError.swift
index 3b9928f..48ef4ab 100644
--- a/Sources/KarrotCodableKit/Resilient/DictionaryDecodingError.swift
+++ b/Sources/KarrotCodableKit/Resilient/DictionaryDecodingError.swift
@@ -38,3 +38,8 @@ extension ResilientDecodingOutcome {
   }
 }
 #endif
+
+#if DEBUG
+// DEBUG-only immutable diagnostic holder; `results` is read-only after init.
+extension ResilientDecodingOutcome.DictionaryDecodingError: @unchecked Sendable {}
+#endif
diff --git a/Sources/KarrotCodableKit/Resilient/ErrorReporting.swift b/Sources/KarrotCodableKit/Resilient/ErrorReporting.swift
index 580decb..eb34ee7 100644
--- a/Sources/KarrotCodableKit/Resilient/ErrorReporting.swift
+++ b/Sources/KarrotCodableKit/Resilient/ErrorReporting.swift
@@ -236,3 +236,6 @@ public struct UnknownNovelValueError: Error {
     self.novelValue = novelValue
   }
 }
+
+// Immutable; `novelValue` is captured once and only read for diagnostics.
+extension UnknownNovelValueError: @unchecked Sendable {}
diff --git a/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicCodableMacro.swift b/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicCodableMacro.swift
index e05f2d3..3efa1f4 100644
--- a/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicCodableMacro.swift
+++ b/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicCodableMacro.swift
@@ -12,8 +12,8 @@ import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
 
 public enum UnnestedPolymorphicCodableMacro: MemberMacro, UnnestedPolymorphicMacroType {
-  public static let protocolType = PolymorphicExtensionFactory.PolymorphicProtocolType.codable
-  public static let macroType = UnnestedPolymorphicCodeGenerator.MacroType.codable
+  public nonisolated(unsafe) static let protocolType = PolymorphicExtensionFactory.PolymorphicProtocolType.codable
+  public nonisolated(unsafe) static let macroType = UnnestedPolymorphicCodeGenerator.MacroType.codable
   public static let macroName = "UnnestedPolymorphicCodable"
 
   public static func expansion(
diff --git a/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicDecodableMacro.swift b/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicDecodableMacro.swift
index edd0655..6df11a4 100644
--- a/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicDecodableMacro.swift
+++ b/Sources/KarrotCodableKitMacros/UnnestedPolymorphicCodableMacro/UnnestedPolymorphicDecodableMacro.swift
@@ -12,8 +12,8 @@ import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
 
 public enum UnnestedPolymorphicDecodableMacro: MemberMacro, UnnestedPolymorphicMacroType {
-  public static let protocolType = PolymorphicExtensionFactory.PolymorphicProtocolType.decodable
-  public static let macroType = UnnestedPolymorphicCodeGenerator.MacroType.decodable
+  public nonisolated(unsafe) static let protocolType = PolymorphicExtensionFactory.PolymorphicProtocolType.decodable
+  public nonisolated(unsafe) static let macroType = UnnestedPolymorphicCodeGenerator.MacroType.decodable
   public static let macroName = "UnnestedPolymorphicDecodable"
 
   public static func expansion(