diff --git a/src/providers/anthropic.c b/src/providers/anthropic.c
index 2801427..1919c02 100644
--- a/src/providers/anthropic.c
+++ b/src/providers/anthropic.c
@@ -265,6 +265,111 @@ static int do_request(const char *body, provider_response_t *response)
 	return ret;
 }
 
+static int append_one_tool_result(cJSON *content_arr, const provider_message_t *msg)
+{
+	cJSON *tr = cJSON_CreateObject();
+	if (!tr)
+		return -1;
+	if (!cJSON_AddStringToObject(tr, "type", "tool_result") ||
+	    !cJSON_AddStringToObject(tr, "tool_use_id", msg->tool_use_id ? msg->tool_use_id : "") ||
+	    !cJSON_AddStringToObject(tr, "content", msg->content ? msg->content : "")) {
+		cJSON_Delete(tr);
+		return -1;
+	}
+	cJSON_AddItemToArray(content_arr, tr);
+	return 0;
+}
+
+/* One user message must carry every tool_result for that assistant turn. */
+static int append_tool_result_message(cJSON *msg_arr, const provider_message_t *messages,
+                                      size_t message_count, size_t *index)
+{
+	cJSON *msg = cJSON_CreateObject();
+	cJSON *content_arr = cJSON_CreateArray();
+	if (!msg || !content_arr) {
+		cJSON_Delete(msg);
+		cJSON_Delete(content_arr);
+		return -1;
+	}
+	cJSON_AddItemToObject(msg, "role", cJSON_CreateString("user"));
+	cJSON_AddItemToObject(msg, "content", content_arr);
+	cJSON_AddItemToArray(msg_arr, msg);
+	for (;;) {
+		if (append_one_tool_result(content_arr, &messages[*index]) != 0)
+			return -1;
+		if (*index + 1 >= message_count || !messages[*index + 1].tool_use_id)
+			break;
+		(*index)++;
+	}
+	return 0;
+}
+static int append_assistant_tool_content(cJSON *msg, const provider_message_t *message)
+{
+	cJSON *content_arr = cJSON_CreateArray();
+	size_t k;
+	if (!content_arr)
+		return -1;
+	if (message->content && message->content[0] != '\0') {
+		cJSON *text_block = cJSON_CreateObject();
+		if (!text_block) {
+			cJSON_Delete(content_arr);
+			return -1;
+		}
+		cJSON_AddItemToObject(text_block, "type", cJSON_CreateString("text"));
+		cJSON_AddItemToObject(text_block, "text", cJSON_CreateString(message->content));
+		cJSON_AddItemToArray(content_arr, text_block);
+	}
+	for (k = 0; k < message->tool_calls_count; k++) {
+		const provider_tool_call_t *tc = &message->tool_calls[k];
+		cJSON *tu = cJSON_CreateObject();
+		cJSON *input;
+		if (!tu) {
+			cJSON_Delete(content_arr);
+			return -1;
+		}
+		cJSON_AddItemToObject(tu, "type", cJSON_CreateString("tool_use"));
+		cJSON_AddItemToObject(tu, "id", cJSON_CreateString(tc->id ? tc->id : ""));
+		cJSON_AddItemToObject(tu, "name", cJSON_CreateString(tc->name ? tc->name : ""));
+		input = tc->arguments && tc->arguments[0] ? cJSON_Parse(tc->arguments) : cJSON_CreateObject();
+		if (input)
+			cJSON_AddItemToObject(tu, "input", input);
+		cJSON_AddItemToArray(content_arr, tu);
+	}
+	cJSON_AddItemToObject(msg, "content", content_arr);
+	return 0;
+}
+
+static int append_anthropic_messages(cJSON *msg_arr, const provider_message_t *messages,
+                                     size_t message_count)
+{
+	size_t i;
+	for (i = 0; i < message_count; i++) {
+		const char *role = messages[i].role ? messages[i].role : "user";
+		cJSON *msg;
+		if (strcmp(role, "system") == 0)
+			continue;
+		if (messages[i].tool_use_id) {
+			if (append_tool_result_message(msg_arr, messages, message_count, &i) != 0)
+				return -1;
+			continue;
+		}
+		msg = cJSON_CreateObject();
+		if (!msg)
+			return -1;
+		cJSON_AddItemToArray(msg_arr, msg);
+		cJSON_AddItemToObject(msg, "role", cJSON_CreateString(role));
+		if (messages[i].tool_calls && messages[i].tool_calls_count > 0 &&
+		    strcmp(role, "assistant") == 0) {
+			if (append_assistant_tool_content(msg, &messages[i]) != 0)
+				return -1;
+		} else {
+			cJSON_AddItemToObject(msg, "content",
+			                      cJSON_CreateString(messages[i].content ? messages[i].content : ""));
+		}
+	}
+	return 0;
+}
+
 static int build_and_send(const provider_message_t *messages, size_t message_count,
                           const provider_tool_def_t *tools, size_t tool_count,
                           provider_response_t *response)
@@ -286,59 +391,12 @@ static int build_and_send(const provider_message_t *messages, size_t message_cou
 	}
 	cJSON *msg_arr = cJSON_CreateArray();
 	if (!msg_arr) { cJSON_Delete(root); provider_set_error(response, "Out of memory"); return -1; }
-	for (size_t i = 0; i < message_count; i++) {
-		const char *role = messages[i].role ? messages[i].role : "user";
-		if (strcmp(role, "system") == 0) continue;
-		cJSON *msg = cJSON_CreateObject();
-		if (!msg) {
-			cJSON_Delete(root);
-			provider_set_error(response, "Out of memory");
-			return -1;
-		}
-		cJSON_AddItemToObject(msg, "role", cJSON_CreateString(role));
-		if (messages[i].tool_use_id) {
-			cJSON *content_arr = cJSON_CreateArray();
-			if (content_arr) {
-				cJSON *tr = cJSON_CreateObject();
-				if (tr) {
-					cJSON_AddItemToObject(tr, "type", cJSON_CreateString("tool_result"));
-					cJSON_AddItemToObject(tr, "tool_use_id", cJSON_CreateString(messages[i].tool_use_id));
-					cJSON_AddItemToObject(tr, "content", cJSON_CreateString(messages[i].content ? messages[i].content : ""));
-					cJSON_AddItemToArray(content_arr, tr);
-				}
-				cJSON_AddItemToObject(msg, "content", content_arr);
-			} else
-				cJSON_AddItemToObject(msg, "content", cJSON_CreateString(""));
-		} else if (messages[i].tool_calls && messages[i].tool_calls_count > 0 && strcmp(role, "assistant") == 0) {
-			cJSON *content_arr = cJSON_CreateArray();
-			if (content_arr) {
-				if (messages[i].content && messages[i].content[0] != '\0') {
-					cJSON *text_block = cJSON_CreateObject();
-					if (text_block) {
-						cJSON_AddItemToObject(text_block, "type", cJSON_CreateString("text"));
-						cJSON_AddItemToObject(text_block, "text", cJSON_CreateString(messages[i].content));
-						cJSON_AddItemToArray(content_arr, text_block);
-					}
-				}
-				for (size_t k = 0; k < messages[i].tool_calls_count; k++) {
-					const provider_tool_call_t *tc = &messages[i].tool_calls[k];
-					cJSON *tu = cJSON_CreateObject();
-					if (!tu) break;
-					cJSON_AddItemToObject(tu, "type", cJSON_CreateString("tool_use"));
-					cJSON_AddItemToObject(tu, "id", cJSON_CreateString(tc->id ? tc->id : ""));
-					cJSON_AddItemToObject(tu, "name", cJSON_CreateString(tc->name ? tc->name : ""));
-					cJSON *input = tc->arguments && tc->arguments[0] ? cJSON_Parse(tc->arguments) : cJSON_CreateObject();
-					if (input) cJSON_AddItemToObject(tu, "input", input);
-					cJSON_AddItemToArray(content_arr, tu);
-				}
-				cJSON_AddItemToObject(msg, "content", content_arr);
-			} else
-				cJSON_AddItemToObject(msg, "content", cJSON_CreateString(messages[i].content ? messages[i].content : ""));
-		} else
-			cJSON_AddItemToObject(msg, "content", cJSON_CreateString(messages[i].content ? messages[i].content : ""));
-		cJSON_AddItemToArray(msg_arr, msg);
-	}
 	cJSON_AddItemToObject(root, "messages", msg_arr);
+	if (append_anthropic_messages(msg_arr, messages, message_count) != 0) {
+		cJSON_Delete(root);
+		provider_set_error(response, "Out of memory");
+		return -1;
+	}
 	if (tool_count > 0 && tools) {
 		cJSON *tools_arr = cJSON_CreateArray();
 		if (tools_arr) {
@@ -420,4 +478,22 @@ int anthropic_parse_response_for_test(const char *json, provider_response_t *res
 	if (!json || !response) return -1;
 	return parse_response_body(json, response);
 }
+
+char *anthropic_messages_json_for_test(const provider_message_t *messages, size_t message_count)
+{
+	cJSON *arr;
+	char *printed;
+	if (!messages && message_count > 0)
+		return NULL;
+	arr = cJSON_CreateArray();
+	if (!arr)
+		return NULL;
+	if (append_anthropic_messages(arr, messages, message_count) != 0) {
+		cJSON_Delete(arr);
+		return NULL;
+	}
+	printed = cJSON_PrintUnformatted(arr);
+	cJSON_Delete(arr);
+	return printed;
+}
 #endif
diff --git a/tests/test_anthropic.c b/tests/test_anthropic.c
index a82b28f..248e560 100644
--- a/tests/test_anthropic.c
+++ b/tests/test_anthropic.c
@@ -6,6 +6,7 @@
 
 #include "core/config.h"
 #include "providers/provider.h"
+#include "cJSON.h"
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -15,6 +16,7 @@
 
 #ifdef SHELLCLAW_TEST
 extern int anthropic_parse_response_for_test(const char *json, provider_response_t *response);
+extern char *anthropic_messages_json_for_test(const provider_message_t *messages, size_t message_count);
 extern void anthropic_test_fail_next_reallocs(int n);
 #endif
 
@@ -233,6 +235,58 @@ static int test_parse_text_realloc_failure_is_error(void)
 	return 0;
 }
 
+static int test_parallel_tool_results_share_one_user_message(void)
+{
+	provider_tool_call_t calls[2] = {
+		{.id = "toolu_a", .name = "shell", .arguments = "{\"command\":\"pwd\"}"},
+		{.id = "toolu_b", .name = "read_file", .arguments = "{\"path\":\"a.txt\"}"},
+	};
+	provider_message_t messages[4];
+	char *json;
+	cJSON *arr;
+	cJSON *assistant;
+	cJSON *user;
+	cJSON *content;
+	cJSON *role;
+	cJSON *block;
+
+	memset(messages, 0, sizeof(messages));
+	messages[0].role = "user";
+	messages[0].content = "do both";
+	messages[1].role = "assistant";
+	messages[1].content = "";
+	messages[1].tool_calls = calls;
+	messages[1].tool_calls_count = 2;
+	messages[2].role = "user";
+	messages[2].content = "out-a";
+	messages[2].tool_use_id = "toolu_a";
+	messages[3].role = "user";
+	messages[3].content = "out-b";
+	messages[3].tool_use_id = "toolu_b";
+	json = anthropic_messages_json_for_test(messages, 4);
+	ASSERT(json != NULL);
+	arr = cJSON_Parse(json);
+	cJSON_free(json);
+	ASSERT(arr != NULL && cJSON_IsArray(arr));
+	ASSERT(cJSON_GetArraySize(arr) == 3);
+	assistant = cJSON_GetArrayItem(arr, 1);
+	content = assistant ? cJSON_GetObjectItem(assistant, "content") : NULL;
+	ASSERT(cJSON_IsArray(content) && cJSON_GetArraySize(content) == 2);
+	user = cJSON_GetArrayItem(arr, 2);
+	role = user ? cJSON_GetObjectItem(user, "role") : NULL;
+	ASSERT(cJSON_IsString(role) && strcmp(role->valuestring, "user") == 0);
+	content = user ? cJSON_GetObjectItem(user, "content") : NULL;
+	ASSERT(cJSON_IsArray(content) && cJSON_GetArraySize(content) == 2);
+	block = cJSON_GetArrayItem(content, 0);
+	role = block ? cJSON_GetObjectItem(block, "tool_use_id") : NULL;
+	ASSERT(cJSON_IsString(role) && strcmp(role->valuestring, "toolu_a") == 0);
+	block = cJSON_GetArrayItem(content, 1);
+	role = block ? cJSON_GetObjectItem(block, "tool_use_id") : NULL;
+	ASSERT(cJSON_IsString(role) && strcmp(role->valuestring, "toolu_b") == 0);
+	cJSON_Delete(arr);
+	return 0;
+}
+
 static int test_parse_tool_realloc_failure_is_error(void)
 {
 	char body[1024];
@@ -268,6 +322,7 @@ int main(void)
 	RUN(test_parse_null_json_returns_error());
 	RUN(test_parse_text_past_initial_cap());
 	RUN(test_parse_six_tool_use_blocks());
+	RUN(test_parallel_tool_results_share_one_user_message());
 	RUN(test_parse_text_realloc_failure_is_error());
 	RUN(test_parse_tool_realloc_failure_is_error());
 #endif