diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/BetaMessageParam.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/BetaMessageParam.kt
index 06cfe2950..d7d0eaf2c 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/BetaMessageParam.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/BetaMessageParam.kt
@@ -80,6 +80,18 @@ private constructor(
     fun _additionalProperties(): Map<String, JsonValue> =
         Collections.unmodifiableMap(additionalProperties)
 
+    @JvmSynthetic
+    internal fun isEmptyAssistant(): Boolean =
+        _role().asString().orElse(null) == "assistant" &&
+            _content()
+                .asKnown()
+                .map {
+                    it.string().map(String::isEmpty).orElseGet {
+                        it.betaContentBlockParams().map(List<*>::isEmpty).orElse(false)
+                    }
+                }
+                .orElse(false)
+
     fun toBuilder() = Builder().from(this)
 
     companion object {
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCountTokensParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCountTokensParams.kt
index d46ba1b25..a119d38ea 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCountTokensParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCountTokensParams.kt
@@ -1847,7 +1847,13 @@ private constructor(
             fun addMessage(message: BetaMessageParam) = apply {
                 messages =
                     (messages ?: JsonField.of(mutableListOf())).also {
-                        checkKnown("messages", it).add(message)
+                        checkKnown("messages", it).apply {
+                            // Empty assistant content is only valid as the final message.
+                            if (lastOrNull()?.isEmptyAssistant() == true) {
+                                removeLast()
+                            }
+                            add(message)
+                        }
                     }
             }
 
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCreateParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCreateParams.kt
index a0b29866c..6ff5ecccd 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCreateParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/MessageCreateParams.kt
@@ -2809,7 +2809,13 @@ private constructor(
             fun addMessage(message: BetaMessageParam) = apply {
                 messages =
                     (messages ?: JsonField.of(mutableListOf())).also {
-                        checkKnown("messages", it).add(message)
+                        checkKnown("messages", it).apply {
+                            // Empty assistant content is only valid as the final message.
+                            if (lastOrNull()?.isEmptyAssistant() == true) {
+                                removeLast()
+                            }
+                            add(message)
+                        }
                     }
             }
 
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/batches/BatchCreateParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/batches/BatchCreateParams.kt
index 48dfb43be..10298efdb 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/batches/BatchCreateParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/beta/messages/batches/BatchCreateParams.kt
@@ -1720,7 +1720,13 @@ private constructor(
                 fun addMessage(message: BetaMessageParam) = apply {
                     messages =
                         (messages ?: JsonField.of(mutableListOf())).also {
-                            checkKnown("messages", it).add(message)
+                            checkKnown("messages", it).apply {
+                                // Empty assistant content is only valid as the final message.
+                                if (lastOrNull()?.isEmptyAssistant() == true) {
+                                    removeLast()
+                                }
+                                add(message)
+                            }
                         }
                 }
 
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCountTokensParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCountTokensParams.kt
index 819a07d79..ab89f239f 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCountTokensParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCountTokensParams.kt
@@ -1478,7 +1478,13 @@ private constructor(
             fun addMessage(message: MessageParam) = apply {
                 messages =
                     (messages ?: JsonField.of(mutableListOf())).also {
-                        checkKnown("messages", it).add(message)
+                        checkKnown("messages", it).apply {
+                            // Empty assistant content is only valid as the final message.
+                            if (lastOrNull()?.isEmptyAssistant() == true) {
+                                removeLast()
+                            }
+                            add(message)
+                        }
                     }
             }
 
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCreateParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCreateParams.kt
index 059142222..cade594fe 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCreateParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageCreateParams.kt
@@ -2173,7 +2173,13 @@ private constructor(
             fun addMessage(message: MessageParam) = apply {
                 messages =
                     (messages ?: JsonField.of(mutableListOf())).also {
-                        checkKnown("messages", it).add(message)
+                        checkKnown("messages", it).apply {
+                            // Empty assistant content is only valid as the final message.
+                            if (lastOrNull()?.isEmptyAssistant() == true) {
+                                removeLast()
+                            }
+                            add(message)
+                        }
                     }
             }
 
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageParam.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageParam.kt
index b75d5ad9f..3237d220c 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageParam.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/MessageParam.kt
@@ -80,6 +80,18 @@ private constructor(
     fun _additionalProperties(): Map<String, JsonValue> =
         Collections.unmodifiableMap(additionalProperties)
 
+    @JvmSynthetic
+    internal fun isEmptyAssistant(): Boolean =
+        _role().asString().orElse(null) == "assistant" &&
+            _content()
+                .asKnown()
+                .map {
+                    it.string().map(String::isEmpty).orElseGet {
+                        it.blockParams().map(List<*>::isEmpty).orElse(false)
+                    }
+                }
+                .orElse(false)
+
     fun toBuilder() = Builder().from(this)
 
     companion object {
diff --git a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/batches/BatchCreateParams.kt b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/batches/BatchCreateParams.kt
index 3c268455c..17a302825 100644
--- a/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/batches/BatchCreateParams.kt
+++ b/anthropic-java-core/src/main/kotlin/com/anthropic/models/messages/batches/BatchCreateParams.kt
@@ -1504,7 +1504,13 @@ private constructor(
                 fun addMessage(message: MessageParam) = apply {
                     messages =
                         (messages ?: JsonField.of(mutableListOf())).also {
-                            checkKnown("messages", it).add(message)
+                            checkKnown("messages", it).apply {
+                                // Empty assistant content is only valid as the final message.
+                                if (lastOrNull()?.isEmptyAssistant() == true) {
+                                    removeLast()
+                                }
+                                add(message)
+                            }
                         }
                 }
 
diff --git a/anthropic-java-core/src/test/kotlin/com/anthropic/models/beta/messages/Issue260RoundTripTest.kt b/anthropic-java-core/src/test/kotlin/com/anthropic/models/beta/messages/Issue260RoundTripTest.kt
new file mode 100644
index 000000000..53e003c9f
--- /dev/null
+++ b/anthropic-java-core/src/test/kotlin/com/anthropic/models/beta/messages/Issue260RoundTripTest.kt
@@ -0,0 +1,46 @@
+package com.anthropic.models.beta.messages
+
+import com.anthropic.core.jsonMapper
+import com.anthropic.models.messages.Model
+import com.fasterxml.jackson.module.kotlin.jacksonTypeRef
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+internal class Issue260RoundTripTest {
+
+    private val jsonMapper = jsonMapper()
+
+    @Test
+    fun `empty assistant response is omitted before a subsequent request message`() {
+        val message =
+            jsonMapper.readValue(
+                """{"role":"assistant","content":[]}""",
+                jacksonTypeRef<BetaMessage>(),
+            )
+
+        val paramsBuilder =
+            MessageCreateParams.builder()
+                .model(Model.CLAUDE_SONNET_4_20250514)
+                .maxTokens(64)
+                .addUserMessage("first question")
+                .addMessage(message)
+
+        val finalAssistantJson =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(
+                paramsBuilder.build()._body()
+            )
+
+        assertThat(finalAssistantJson.at("/messages/1/role").textValue()).isEqualTo("assistant")
+        assertThat(finalAssistantJson.at("/messages/1/content")).isEmpty()
+
+        val followUpJson =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(
+                paramsBuilder.addUserMessage("follow-up question").build()._body()
+            )
+
+        assertThat(followUpJson.at("/messages").size()).isEqualTo(2)
+        assertThat(followUpJson.at("/messages/0/content").textValue()).isEqualTo("first question")
+        assertThat(followUpJson.at("/messages/1/content").textValue())
+            .isEqualTo("follow-up question")
+    }
+}
diff --git a/anthropic-java-core/src/test/kotlin/com/anthropic/models/messages/Issue260RoundTripTest.kt b/anthropic-java-core/src/test/kotlin/com/anthropic/models/messages/Issue260RoundTripTest.kt
new file mode 100644
index 000000000..0e48a5f09
--- /dev/null
+++ b/anthropic-java-core/src/test/kotlin/com/anthropic/models/messages/Issue260RoundTripTest.kt
@@ -0,0 +1,95 @@
+package com.anthropic.models.messages
+
+import com.anthropic.core.jsonMapper
+import com.fasterxml.jackson.module.kotlin.jacksonTypeRef
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+internal class Issue260RoundTripTest {
+
+    private val jsonMapper = jsonMapper()
+
+    @Test
+    fun `web search response can be converted back into request params`() {
+        val message =
+            readMessage(
+                """
+                {
+                  "role": "assistant",
+                  "content": [{
+                    "type": "web_search_tool_result",
+                    "tool_use_id": "srvtoolu_123",
+                    "caller": {"type": "direct"},
+                    "content": [{
+                      "type": "web_search_result",
+                      "url": "https://example.com",
+                      "title": "Example",
+                      "encrypted_content": "encrypted"
+                    }]
+                  }]
+                }
+                """
+            )
+
+        val json =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(message.toParam())
+
+        assertThat(json.at("/content/0/content/0/encrypted_content").textValue())
+            .isEqualTo("encrypted")
+    }
+
+    @Test
+    fun `missing web search content no longer throws while converting to params`() {
+        val message =
+            readMessage(
+                """
+                {
+                  "role": "assistant",
+                  "content": [{
+                    "type": "web_search_tool_result",
+                    "tool_use_id": "srvtoolu_123",
+                    "caller": {"type": "direct"}
+                  }]
+                }
+                """
+            )
+
+        val json =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(message.toParam())
+
+        assertThat(json.at("/content/0").has("content")).isFalse()
+    }
+
+    @Test
+    fun `empty assistant response is omitted before a subsequent request message`() {
+        val message = readMessage("""{"role":"assistant","content":[]}""")
+
+        val paramsBuilder =
+            MessageCreateParams.builder()
+                .model(Model.CLAUDE_SONNET_4_20250514)
+                .maxTokens(64)
+                .addUserMessage("first question")
+                .addMessage(message)
+
+        val finalAssistantJson =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(
+                paramsBuilder.build()._body()
+            )
+
+        assertThat(finalAssistantJson.at("/messages/1/role").textValue()).isEqualTo("assistant")
+        assertThat(finalAssistantJson.at("/messages/1/content")).isEmpty()
+
+        val followUpJson =
+            jsonMapper.valueToTree<com.fasterxml.jackson.databind.JsonNode>(
+                paramsBuilder.addUserMessage("follow-up question").build()._body()
+            )
+
+        assertThat(followUpJson.at("/messages").size()).isEqualTo(2)
+        assertThat(followUpJson.at("/messages/0/content").textValue()).isEqualTo("first question")
+        assertThat(followUpJson.at("/messages/1/content").textValue())
+            .isEqualTo("follow-up question")
+    }
+
+    private fun readMessage(json: String): Message =
+        jsonMapper.readValue(json.trimIndent(), jacksonTypeRef<Message>())
+}