Adding interceptor support so trace propagation can be plugged

anthropic-java-client-okhttp/src/main/kotlin/com/anthropic/client/okhttp/OkHttpClient.kt

@@ -232,6 +232,7 @@ internal constructor(
         private var sslSocketFactory: SSLSocketFactory? = null
         private var trustManager: X509TrustManager? = null
         private var hostnameVerifier: HostnameVerifier? = null
+        private val interceptors: MutableList<okhttp3.Interceptor> = mutableListOf()
 
         fun timeout(timeout: Timeout) = apply { this.timeout = timeout }
 
@@ -279,6 +280,33 @@ internal constructor(
             this.hostnameVerifier = hostnameVerifier
         }
 
+        /**
+         * Adds a custom OkHttp [okhttp3.Interceptor] that will be applied to every request.
+         *
+         * This is useful for cross-cutting concerns such as trace context propagation. For example,
+         * to forward an `X-Trace-Id` header from an incoming request using a [ThreadLocal]:
+         *
+         * ```kotlin
+         * val traceIdHolder = ThreadLocal<String>()
+         *
+         * val httpClient = OkHttpClient.builder()
+         *     .backend(backend)
+         *     .addInterceptor { chain ->
+         *         val traceId = traceIdHolder.get()
+         *         val request = if (traceId != null) {
+         *             chain.request().newBuilder().addHeader("X-Trace-Id", traceId).build()
+         *         } else {
+         *             chain.request()
+         *         }
+         *         chain.proceed(request)
+         *     }
+         *     .build()
+         * ```
+         */
+        fun addInterceptor(interceptor: okhttp3.Interceptor) = apply {
+            interceptors.add(interceptor)
+        }
+
         fun build(): OkHttpClient =
             OkHttpClient(
                 okhttp3.OkHttpClient.Builder()
@@ -320,6 +348,8 @@ internal constructor(
                         }
 
                         hostnameVerifier?.let(::hostnameVerifier)
+
+                        interceptors.forEach { addInterceptor(it) }
                     }
                     .build()
                     .apply {

anthropic-java-client-okhttp/src/test/kotlin/com/anthropic/client/okhttp/OkHttpClientTest.kt

@@ -30,6 +30,43 @@ internal class OkHttpClientTest {
         httpClient = OkHttpClient.builder().backend(TestBackend(baseUrl)).build()
     }
 
+    @Test
+    fun addInterceptor_interceptorIsCalledAndCanAddHeaders() {
+        stubFor(post(urlPathEqualTo("/something")).willReturn(ok()))
+        val traceIdHolder = ThreadLocal<String>()
+        val clientWithInterceptor =
+            OkHttpClient.builder()
+                .backend(TestBackend(baseUrl))
+                .addInterceptor { chain ->
+                    val traceId = traceIdHolder.get()
+                    val request =
+                        if (traceId != null) {
+                            chain.request().newBuilder().addHeader("X-Trace-Id", traceId).build()
+                        } else {
+                            chain.request()
+                        }
+                    chain.proceed(request)
+                }
+                .build()
+
+        traceIdHolder.set("test-trace-123")
+        try {
+            clientWithInterceptor
+                .execute(
+                    HttpRequest.builder()
+                        .method(HttpMethod.POST)
+                        .baseUrl(baseUrl)
+                        .addPathSegment("something")
+                        .build()
+                )
+                .close()
+        } finally {
+            traceIdHolder.remove()
+        }
+
+        verify(postRequestedFor(urlPathEqualTo("/something")).withHeader("X-Trace-Id", equalTo("test-trace-123")))
+    }
+
     @Test
     fun executeAsync_whenFutureCancelled_cancelsUnderlyingCall() {
         stubFor(post(urlPathEqualTo("/something")).willReturn(ok()))