diff --git a/build.gradle.kts b/build.gradle.kts
index 8dfc1cc..39ce424 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -36,7 +36,11 @@ val coverageExclusions = listOf(
     "**/core/src/main/scala/de/nowchess/chess/registry/GameEntry.scala",
     "**/core/src/main/scala/de/nowchess/chess/registry/GameRegistryImpl.scala",
     // GameResource — REST integration layer with @Inject var fields; mocking dependencies for unit tests is infeasible with Quarkus DI; integration tests would require @QuarkusTest which Scoverage doesn't instrument
-    "**/core/src/main/scala/de/nowchess/chess/resource/GameResource.scala"
+    "**/core/src/main/scala/de/nowchess/chess/resource/GameResource.scala",
+    // IoResource — same rationale as GameResource; @QuarkusTest not instrumented by Scoverage
+    "**/io/src/main/scala/de/nowchess/io/service/resource/IoResource.scala",
+    // JacksonConfig — Quarkus lifecycle hook, no testable logic beyond ObjectMapper registration
+    "**/io/src/main/scala/de/nowchess/io/service/config/JacksonConfig.scala",
 )
 
 // Converts a Sonar-style glob to a scoverage regex (matched against full source path).
diff --git a/lint b/lint
old mode 100644
new mode 100755
diff --git a/modules/io/src/test/scala/de/nowchess/io/json/SquareKeyDeserializerTest.scala b/modules/io/src/test/scala/de/nowchess/io/json/SquareKeyDeserializerTest.scala
new file mode 100644
index 0000000..bef9cb0
--- /dev/null
+++ b/modules/io/src/test/scala/de/nowchess/io/json/SquareKeyDeserializerTest.scala
@@ -0,0 +1,60 @@
+package de.nowchess.io.json
+
+import com.fasterxml.jackson.core.`type`.TypeReference
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.module.SimpleModule
+import com.fasterxml.jackson.module.scala.DefaultScalaModule
+import de.nowchess.api.board.{File, Rank, Square}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class SquareKeyDeserializerTest extends AnyFunSuite with Matchers:
+
+  private def mapper: ObjectMapper =
+    val m   = new ObjectMapper()
+    val mod = new SimpleModule()
+    mod.addKeyDeserializer(classOf[Square], new SquareKeyDeserializer())
+    m.registerModule(DefaultScalaModule)
+    m.registerModule(mod)
+    m
+
+  private def readMap(json: String): Map[Square, Int] =
+    mapper.readValue(json, new TypeReference[Map[Square, Int]] {})
+
+  test("deserializes valid algebraic key") {
+    val result = readMap("""{"e4":1}""")
+    result(Square(File.E, Rank.R4)) shouldBe 1
+  }
+
+  test("deserializes a1 corner") {
+    val result = readMap("""{"a1":1}""")
+    result(Square(File.A, Rank.R1)) shouldBe 1
+  }
+
+  test("deserializes h8 corner") {
+    val result = readMap("""{"h8":1}""")
+    result(Square(File.H, Rank.R8)) shouldBe 1
+  }
+
+  test("deserializes multiple squares") {
+    val result = readMap("""{"a1":1,"h8":2,"e4":3}""")
+    result(Square(File.A, Rank.R1)) shouldBe 1
+    result(Square(File.H, Rank.R8)) shouldBe 2
+    result(Square(File.E, Rank.R4)) shouldBe 3
+  }
+
+  test("deserializeKey returns null for invalid square") {
+    new SquareKeyDeserializer().deserializeKey("invalid", null) shouldBe null
+  }
+
+  test("deserializeKey returns null for wrong-length key") {
+    new SquareKeyDeserializer().deserializeKey("e44", null) shouldBe null
+  }
+
+  test("deserializeKey returns null for bad file") {
+    new SquareKeyDeserializer().deserializeKey("z4", null) shouldBe null
+  }
+
+  test("deserializeKey returns null for bad rank") {
+    new SquareKeyDeserializer().deserializeKey("e9", null) shouldBe null
+  }
diff --git a/modules/io/src/test/scala/de/nowchess/io/json/SquareKeySerializerTest.scala b/modules/io/src/test/scala/de/nowchess/io/json/SquareKeySerializerTest.scala
new file mode 100644
index 0000000..bcc2f0f
--- /dev/null
+++ b/modules/io/src/test/scala/de/nowchess/io/json/SquareKeySerializerTest.scala
@@ -0,0 +1,50 @@
+package de.nowchess.io.json
+
+import com.fasterxml.jackson.core.`type`.TypeReference
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.module.SimpleModule
+import com.fasterxml.jackson.module.scala.DefaultScalaModule
+import de.nowchess.api.board.{File, Rank, Square}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class SquareKeySerializerTest extends AnyFunSuite with Matchers:
+
+  private def mapper: ObjectMapper =
+    val m   = new ObjectMapper()
+    val mod = new SimpleModule()
+    mod.addKeySerializer(classOf[Square], new SquareKeySerializer())
+    m.registerModule(DefaultScalaModule)
+    m.registerModule(mod)
+    m
+
+  test("serializes square as algebraic notation") {
+    val json = mapper.writeValueAsString(Map(Square(File.E, Rank.R4) -> 1))
+    json should include("\"e4\"")
+  }
+
+  test("serializes a1 corner") {
+    val json = mapper.writeValueAsString(Map(Square(File.A, Rank.R1) -> 1))
+    json should include("\"a1\"")
+  }
+
+  test("serializes h8 corner") {
+    val json = mapper.writeValueAsString(Map(Square(File.H, Rank.R8) -> 1))
+    json should include("\"h8\"")
+  }
+
+  test("round-trips with SquareKeyDeserializer") {
+    val rt = {
+      val m   = new ObjectMapper()
+      val mod = new SimpleModule()
+      mod.addKeySerializer(classOf[Square], new SquareKeySerializer())
+      mod.addKeyDeserializer(classOf[Square], new SquareKeyDeserializer())
+      m.registerModule(DefaultScalaModule)
+      m.registerModule(mod)
+      m
+    }
+    val original = Map(Square(File.D, Rank.R5) -> 99)
+    val json     = rt.writeValueAsString(original)
+    val result   = rt.readValue(json, new TypeReference[Map[Square, Int]] {})
+    result shouldBe original
+  }
diff --git a/modules/io/src/test/scala/de/nowchess/io/service/resource/IoResourceTest.scala b/modules/io/src/test/scala/de/nowchess/io/service/resource/IoResourceTest.scala
new file mode 100644
index 0000000..67b43a2
--- /dev/null
+++ b/modules/io/src/test/scala/de/nowchess/io/service/resource/IoResourceTest.scala
@@ -0,0 +1,78 @@
+package de.nowchess.io.service.resource
+
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.module.SimpleModule
+import com.fasterxml.jackson.module.scala.DefaultScalaModule
+import de.nowchess.api.board.Square
+import de.nowchess.api.game.GameContext
+import de.nowchess.io.json.{SquareKeyDeserializer, SquareKeySerializer}
+import io.quarkus.test.junit.QuarkusTest
+import io.restassured.RestAssured
+import io.restassured.http.ContentType
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+
+// scalafix:off
+@QuarkusTest
+class IoResourceTest:
+
+  private lazy val testMapper: ObjectMapper =
+    val m   = new ObjectMapper()
+    val mod = new SimpleModule()
+    mod.addKeySerializer(classOf[Square], new SquareKeySerializer())
+    mod.addKeyDeserializer(classOf[Square], new SquareKeyDeserializer())
+    m.registerModule(new DefaultScalaModule())
+    m.registerModule(mod)
+    m
+
+  private def contextJson(ctx: GameContext): String = testMapper.writeValueAsString(ctx)
+
+  @Test
+  def importFenReturns200(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body("""{"fen":"rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1"}""")
+      .post("/io/import/fen")
+    assertEquals(200, resp.statusCode())
+
+  @Test
+  def importFenInvalidReturns400(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body("""{"fen":"not-a-fen"}""")
+      .post("/io/import/fen")
+    assertEquals(400, resp.statusCode())
+
+  @Test
+  def importPgnReturns200(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body("""{"pgn":"1. e4 e5"}""")
+      .post("/io/import/pgn")
+    assertEquals(200, resp.statusCode())
+
+  @Test
+  def importPgnInvalidReturns400(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body("""{"pgn":"not valid pgn !!!###"}""")
+      .post("/io/import/pgn")
+    assertEquals(400, resp.statusCode())
+
+  @Test
+  def exportFenReturns200WithFen(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body(contextJson(GameContext.initial))
+      .post("/io/export/fen")
+    assertEquals(200, resp.statusCode())
+    assertTrue(resp.getBody.asString().contains("rnbqkbnr"))
+
+  @Test
+  def exportPgnReturns200(): Unit =
+    val resp = RestAssured.`given`()
+      .contentType(ContentType.JSON)
+      .body(contextJson(GameContext.initial))
+      .post("/io/export/pgn")
+    assertEquals(200, resp.statusCode())
+// scalafix:on