fix: downgrade okhttp for compatibility

Author: bjarn

build.gradle

@@ -20,14 +20,14 @@ repositories {
 
 dependencies {
     // HTTP Client
-    api 'com.squareup.okhttp3:okhttp-jvm:5.3.2'
+    api 'com.squareup.okhttp3:okhttp:4.9.3'
 
     // JSON Processing
     implementation 'com.fasterxml.jackson.core:jackson-databind:2.21.1'
 
     // Testing
     testImplementation 'org.junit.jupiter:junit-jupiter:5.10.1'
-    testImplementation 'com.squareup.okhttp3:mockwebserver:5.3.2'
+    testImplementation 'com.squareup.okhttp3:mockwebserver:4.9.3'
 }
 
 test {

pom.xml

@@ -39,7 +39,7 @@
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <okhttp.version>5.3.2</okhttp.version>
+        <okhttp.version>4.9.3</okhttp.version>
         <jackson.version>2.21.0</jackson.version>
         <junit.version>5.10.1</junit.version>
     </properties>
@@ -48,7 +48,7 @@
         <!-- HTTP Client -->
         <dependency>
             <groupId>com.squareup.okhttp3</groupId>
-            <artifactId>okhttp-jvm</artifactId>
+            <artifactId>okhttp</artifactId>
             <version>${okhttp.version}</version>
         </dependency>
 

src/test/java/co/lettermint/OkHttpCompatibilityTest.java

@@ -0,0 +1,99 @@
+package co.lettermint;
+
+import co.lettermint.models.SendEmailResponse;
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+import okhttp3.mockwebserver.RecordedRequest;
+import okio.ByteString;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Verifies OkHttp/Okio compatibility after downgrade to 4.9.3.
+ * Exercises ByteString and request/response serialization paths
+ * that previously caused NoSuchFieldError with Okio 3.x.
+ */
+class OkHttpCompatibilityTest {
+
+    private MockWebServer server;
+    private Lettermint lettermint;
+
+    @BeforeEach
+    void setUp() throws IOException {
+        server = new MockWebServer();
+        server.start();
+        lettermint = new Lettermint("test-token", server.url("/v1").toString());
+    }
+
+    @AfterEach
+    void tearDown() throws IOException {
+        server.shutdown();
+    }
+
+    @Test
+    void okioByteStringIsAccessible() {
+        // This is the exact code path that fails with NoSuchFieldError
+        // when Okio 2.x and 3.x jars conflict on the classpath
+        ByteString bs = ByteString.encodeUtf8("hello");
+        assertEquals("hello", bs.utf8());
+    }
+
+    @Test
+    void fullRoundTripRequest() throws Exception {
+        server.enqueue(new MockResponse()
+                .setBody("{\"message_id\": \"msg_compat\", \"status\": \"queued\"}")
+                .setHeader("Content-Type", "application/json"));
+
+        SendEmailResponse response = lettermint.email()
+                .from("test@example.com")
+                .to("recipient@example.com")
+                .subject("OkHttp 4.9.3 compat test")
+                .text("Verifying request body serialization through Okio")
+                .html("<p>With HTML body too</p>")
+                .send();
+
+        assertEquals("msg_compat", response.getMessageId());
+        assertEquals("queued", response.getStatus());
+
+        RecordedRequest recorded = server.takeRequest();
+        assertEquals("POST", recorded.getMethod());
+
+        // readUtf8() goes through Okio's Buffer/ByteString internally
+        String body = recorded.getBody().readUtf8();
+        assertTrue(body.contains("\"subject\":\"OkHttp 4.9.3 compat test\""));
+        assertTrue(body.contains("\"text\":\"Verifying request body serialization through Okio\""));
+        assertTrue(body.contains("\"html\":\"<p>With HTML body too</p>\""));
+    }
+
+    @Test
+    void multipleSequentialRequests() throws Exception {
+        for (int i = 0; i < 3; i++) {
+            server.enqueue(new MockResponse()
+                    .setBody("{\"message_id\": \"msg_" + i + "\", \"status\": \"queued\"}")
+                    .setHeader("Content-Type", "application/json"));
+        }
+
+        for (int i = 0; i < 3; i++) {
+            SendEmailResponse response = lettermint.email()
+                    .from("test@example.com")
+                    .to("r@example.com")
+                    .subject("Msg " + i)
+                    .text("Body " + i)
+                    .send();
+
+            assertEquals("msg_" + i, response.getMessageId());
+        }
+
+        // Verify all 3 requests went through with correct bodies
+        for (int i = 0; i < 3; i++) {
+            RecordedRequest req = server.takeRequest();
+            String body = req.getBody().readUtf8();
+            assertTrue(body.contains("\"subject\":\"Msg " + i + "\""));
+        }
+    }
+}