diff --git a/src/llm/core/types.py b/src/llm/core/types.py
index 84eb7cf8c..8ba80a258 100644
--- a/src/llm/core/types.py
+++ b/src/llm/core/types.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 from dataclasses import dataclass, field
 from enum import Enum
 from typing import Any
@@ -44,6 +45,22 @@ class Message:
             ]
         return result
 
+    def to_openai_dict(self) -> dict[str, Any]:
+        result = self.to_dict()
+        if self.tool_calls:
+            result["tool_calls"] = [
+                {
+                    "id": tc.id,
+                    "type": "function",
+                    "function": {
+                        "name": tc.name,
+                        "arguments": json.dumps(tc.arguments),
+                    },
+                }
+                for tc in self.tool_calls
+            ]
+        return result
+
 
 @dataclass(frozen=True)
 class ToolDefinition:
diff --git a/src/llm/providers/astraflow.py b/src/llm/providers/astraflow.py
index 1f7cceab9..229e373d5 100644
--- a/src/llm/providers/astraflow.py
+++ b/src/llm/providers/astraflow.py
@@ -70,7 +70,7 @@ class _AstraflowBaseProvider(LLMProvider):
         try:
             params: dict[str, Any] = {
                 "model": llm_input.model or self.default_model,
-                "messages": [msg.to_dict() for msg in llm_input.messages],
+                "messages": [msg.to_openai_dict() for msg in llm_input.messages],
             }
             if llm_input.temperature != 1.0:
                 params["temperature"] = llm_input.temperature
diff --git a/src/llm/providers/atlas.py b/src/llm/providers/atlas.py
index b266bb614..6b34c74f6 100644
--- a/src/llm/providers/atlas.py
+++ b/src/llm/providers/atlas.py
@@ -83,7 +83,7 @@ class AtlasProvider(LLMProvider):
         try:
             params: dict[str, Any] = {
                 "model": llm_input.model or self.default_model,
-                "messages": [msg.to_dict() for msg in llm_input.messages],
+                "messages": [msg.to_openai_dict() for msg in llm_input.messages],
             }
             if llm_input.temperature != 1.0:
                 params["temperature"] = llm_input.temperature
diff --git a/src/llm/providers/openai.py b/src/llm/providers/openai.py
index 0bf84a33d..4759df84b 100644
--- a/src/llm/providers/openai.py
+++ b/src/llm/providers/openai.py
@@ -72,7 +72,7 @@ class OpenAIProvider(LLMProvider):
         try:
             params: dict[str, Any] = {
                 "model": input.model or "gpt-4o-mini",
-                "messages": [msg.to_dict() for msg in input.messages],
+                "messages": [msg.to_openai_dict() for msg in input.messages],
                 "temperature": input.temperature,
             }
             if input.max_tokens:
diff --git a/tests/test_provider_tools.py b/tests/test_provider_tools.py
index 4c9c76f92..e315bb35a 100644
--- a/tests/test_provider_tools.py
+++ b/tests/test_provider_tools.py
@@ -2,7 +2,9 @@ from types import SimpleNamespace
 
 import pytest
 
-from llm.core.types import LLMInput, Message, Role, ToolDefinition
+from llm.core.types import LLMInput, Message, Role, ToolCall, ToolDefinition
+from llm.providers.astraflow import AstraflowProvider
+from llm.providers.atlas import AtlasProvider
 from llm.providers.claude import ClaudeProvider
 from llm.providers.constants import EMPTY_FILTERED_RESPONSE_ERROR
 from llm.providers.openai import OpenAIProvider
@@ -84,6 +86,55 @@ def test_openai_provider_serializes_tools_for_chat_completions():
     ]
 
 
+@pytest.mark.parametrize(
+    "provider_type",
+    [OpenAIProvider, AstraflowProvider, AtlasProvider],
+)
+def test_openai_compatible_provider_serializes_assistant_tool_calls(provider_type):
+    provider = provider_type(api_key="test")
+    client = _OpenAIClient()
+    provider.client = client
+
+    provider.generate(
+        LLMInput(
+            messages=[
+                Message(role=Role.USER, content="Find ECC"),
+                Message(
+                    role=Role.ASSISTANT,
+                    content="",
+                    tool_calls=[
+                        ToolCall(
+                            id="call_1",
+                            name="search",
+                            arguments={"query": "ecc"},
+                        )
+                    ],
+                ),
+                Message(
+                    role=Role.TOOL,
+                    content="result",
+                    tool_call_id="call_1",
+                ),
+            ]
+        )
+    )
+
+    assert client.completions.params["messages"][1] == {
+        "role": "assistant",
+        "content": "",
+        "tool_calls": [
+            {
+                "id": "call_1",
+                "type": "function",
+                "function": {
+                    "name": "search",
+                    "arguments": '{"query": "ecc"}',
+                },
+            }
+        ],
+    }
+
+
 def test_openai_provider_can_be_constructed_without_credentials(monkeypatch):
     monkeypatch.delenv("OPENAI_API_KEY", raising=False)