diff --git a/writer/pom.xml b/writer/pom.xml
index c82126f5..f80bf6c9 100644
--- a/writer/pom.xml
+++ b/writer/pom.xml
@@ -79,6 +79,8 @@
 						<configuration>
 							<targetClasses>
 								<param>io.github.dfa1.vortex.writer.ChunkImpl</param>
+								<param>io.github.dfa1.vortex.writer.WriteRegistry</param>
+								<param>io.github.dfa1.vortex.writer.WriteRegistry$Builder</param>
 							</targetClasses>
 						</configuration>
 					</plugin>
diff --git a/writer/src/test/java/io/github/dfa1/vortex/writer/WriteRegistryTest.java b/writer/src/test/java/io/github/dfa1/vortex/writer/WriteRegistryTest.java
new file mode 100644
index 00000000..dbcb41ba
--- /dev/null
+++ b/writer/src/test/java/io/github/dfa1/vortex/writer/WriteRegistryTest.java
@@ -0,0 +1,128 @@
+package io.github.dfa1.vortex.writer;
+
+import io.github.dfa1.vortex.core.VortexException;
+import io.github.dfa1.vortex.encoding.EncodingId;
+import io.github.dfa1.vortex.extension.ExtensionId;
+import io.github.dfa1.vortex.writer.encode.EncodingEncoder;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+
+/// Unit tests for [WriteRegistry] and its [WriteRegistry.Builder] — encoder/extension
+/// registration, duplicate rejection, lookup, and the fluent builder return.
+class WriteRegistryTest {
+
+    private static EncodingEncoder encoder(EncodingId id) {
+        EncodingEncoder enc = mock(EncodingEncoder.class);
+        given(enc.encodingId()).willReturn(id);
+        return enc;
+    }
+
+    private static ExtensionEncoder extension(ExtensionId id) {
+        ExtensionEncoder ext = mock(ExtensionEncoder.class);
+        given(ext.extensionId()).willReturn(id);
+        return ext;
+    }
+
+    @Test
+    void register_thenEncoderMap_containsEncoder() {
+        // Given
+        EncodingEncoder enc = encoder(EncodingId.VORTEX_PRIMITIVE);
+
+        // When
+        WriteRegistry result = WriteRegistry.builder().register(enc).build();
+
+        // Then
+        assertThat(result.encoderMap()).containsEntry(EncodingId.VORTEX_PRIMITIVE, enc);
+    }
+
+    @Test
+    void register_returnsSameBuilderForChaining() {
+        // Given
+        WriteRegistry.Builder sut = WriteRegistry.builder();
+
+        // When
+        WriteRegistry.Builder result = sut.register(encoder(EncodingId.VORTEX_PRIMITIVE));
+
+        // Then — the fluent return must be the same builder, not null
+        assertThat(result).isSameAs(sut);
+    }
+
+    @Test
+    void register_duplicateEncoderId_throws() {
+        // Given — two encoders advertising the same id
+        WriteRegistry.Builder sut = WriteRegistry.builder()
+                .register(encoder(EncodingId.VORTEX_PRIMITIVE));
+
+        // When / Then
+        assertThatThrownBy(() -> sut.register(encoder(EncodingId.VORTEX_PRIMITIVE)))
+                .isInstanceOf(VortexException.class)
+                .hasMessageContaining("already registered");
+    }
+
+    @Test
+    void registerExtension_thenLookup_returnsIt() {
+        // Given
+        ExtensionEncoder ext = extension(ExtensionId.VORTEX_DATE);
+
+        // When
+        WriteRegistry result = WriteRegistry.builder().register(ext).build();
+
+        // Then
+        assertThat(result.lookup(ExtensionId.VORTEX_DATE)).isSameAs(ext);
+    }
+
+    @Test
+    void registerExtension_returnsSameBuilderForChaining() {
+        // Given
+        WriteRegistry.Builder sut = WriteRegistry.builder();
+
+        // When
+        WriteRegistry.Builder result = sut.register(extension(ExtensionId.VORTEX_DATE));
+
+        // Then
+        assertThat(result).isSameAs(sut);
+    }
+
+    @Test
+    void registerExtension_duplicateId_throws() {
+        // Given
+        WriteRegistry.Builder sut = WriteRegistry.builder()
+                .register(extension(ExtensionId.VORTEX_DATE));
+
+        // When / Then
+        assertThatThrownBy(() -> sut.register(extension(ExtensionId.VORTEX_DATE)))
+                .isInstanceOf(VortexException.class)
+                .hasMessageContaining("already registered");
+    }
+
+    @Test
+    void lookup_unregisteredExtension_returnsNull() {
+        // Given — empty registry
+
+        // When / Then
+        assertThat(WriteRegistry.empty().lookup(ExtensionId.VORTEX_UUID)).isNull();
+    }
+
+    @Test
+    void empty_hasNoEncodersAndNullLookup() {
+        // Given / When
+        WriteRegistry sut = WriteRegistry.empty();
+
+        // Then
+        assertThat(sut.encoderMap()).isEmpty();
+        assertThat(sut.lookup(ExtensionId.VORTEX_DATE)).isNull();
+    }
+
+    @Test
+    void loadAll_discoversServiceLoadedEncoders() {
+        // Given / When — the module ships encoders via META-INF/services
+        WriteRegistry sut = WriteRegistry.loadAll();
+
+        // Then
+        assertThat(sut.encoderMap()).isNotEmpty();
+    }
+}