diff --git a/src/openjarvis/core/types.py b/src/openjarvis/core/types.py
index 62cdc9f4..add45385 100644
--- a/src/openjarvis/core/types.py
+++ b/src/openjarvis/core/types.py
@@ -63,7 +63,7 @@ class Message:
     """A single chat message (OpenAI-compatible structure)."""
 
     role: Role
-    content: str = ""
+    content: str | None = ""
     name: Optional[str] = None
     tool_calls: Optional[List[ToolCall]] = None
     tool_call_id: Optional[str] = None
@@ -73,6 +73,11 @@ class Message:
     # empty for text-only messages (the common case).
     images: Optional[List[str]] = None
 
+    @property
+    def text(self) -> str:
+        """Return message content as text, treating ``None`` as empty."""
+        return self.content or ""
+
 
 @dataclass(slots=True)
 class Conversation:
diff --git a/src/openjarvis/engine/_base.py b/src/openjarvis/engine/_base.py
index 97f9a23e..6ecaf81a 100644
--- a/src/openjarvis/engine/_base.py
+++ b/src/openjarvis/engine/_base.py
@@ -13,6 +13,22 @@ class EngineConnectionError(Exception):
     """Raised when an engine is unreachable."""
 
 
+_REASONING_METADATA_KEYS = ("reasoning_content", "thinking")
+
+
+def _message_estimated_chars(message: Message) -> int:
+    parts = [message.text]
+    for key in _REASONING_METADATA_KEYS:
+        value = message.metadata.get(key)
+        if isinstance(value, str):
+            parts.append(value)
+    for tc in message.tool_calls or []:
+        parts.extend((tc.id, tc.name, tc.arguments))
+    if message.tool_call_id:
+        parts.append(message.tool_call_id)
+    return sum(len(part) for part in parts)
+
+
 def messages_to_dicts(messages: Sequence[Message]) -> List[Dict[str, Any]]:
     """Convert ``Message`` objects to OpenAI-format dicts."""
     out: List[Dict[str, Any]] = []
@@ -53,9 +69,11 @@ def estimate_prompt_tokens(messages: Sequence[Message]) -> int:
     provider would charge.
 
     Uses ~4 characters per token (standard BPE average for English) plus
-    a small per-message overhead for role markers and separators.
+    a small per-message overhead for role markers and separators. Counts
+    content, reasoning metadata, tool-call payloads, and tool result IDs
+    because all are replayed into later prompt turns when present.
     """
-    total_chars = sum(len(m.content or "") for m in messages)
+    total_chars = sum(_message_estimated_chars(m) for m in messages)
     # ~4 tokens overhead per message for role markers / separators
     overhead = len(messages) * 4
     return max(1, total_chars // 4 + overhead)
diff --git a/tests/core/test_types.py b/tests/core/test_types.py
index 2d80ed34..b91817f9 100644
--- a/tests/core/test_types.py
+++ b/tests/core/test_types.py
@@ -35,9 +35,15 @@ class TestMessage:
         msg = Message(role=Role.USER, content="hello")
         assert msg.role == Role.USER
         assert msg.content == "hello"
+        assert msg.text == "hello"
         assert msg.tool_calls is None
         assert msg.metadata == {}
 
+    def test_none_content_text_helper(self) -> None:
+        msg = Message(role=Role.ASSISTANT, content=None)
+        assert msg.content is None
+        assert msg.text == ""
+
     def test_tool_calls(self) -> None:
         tc = ToolCall(id="1", name="calc", arguments='{"x": 1}')
         msg = Message(role=Role.ASSISTANT, content="", tool_calls=[tc])
diff --git a/tests/engine/test_base.py b/tests/engine/test_base.py
index 397c18eb..f466b4da 100644
--- a/tests/engine/test_base.py
+++ b/tests/engine/test_base.py
@@ -9,9 +9,52 @@ def test_estimate_prompt_tokens_handles_none_content_tool_call_turn() -> None:
         Message(role=Role.USER, content="hi"),
         Message(
             role=Role.ASSISTANT,
-            content=None,  # type: ignore[arg-type]
+            content=None,
             tool_calls=[ToolCall(id="call_1", name="lookup", arguments="{}")],
         ),
     ]
 
-    assert estimate_prompt_tokens(messages) == 8
+    assert estimate_prompt_tokens(messages) == 12
+
+
+def test_estimate_prompt_tokens_counts_tool_call_arguments() -> None:
+    base = [
+        Message(role=Role.USER, content="hi"),
+        Message(role=Role.ASSISTANT, content=None),
+    ]
+    with_tool_call = [
+        Message(role=Role.USER, content="hi"),
+        Message(
+            role=Role.ASSISTANT,
+            content=None,
+            tool_calls=[ToolCall(id="", name="", arguments="abcdefgh")],
+        ),
+    ]
+
+    assert estimate_prompt_tokens(with_tool_call) - estimate_prompt_tokens(base) == 2
+
+
+def test_estimate_prompt_tokens_counts_reasoning_metadata() -> None:
+    base = [
+        Message(role=Role.USER, content="hi"),
+        Message(role=Role.ASSISTANT, content=None),
+    ]
+    with_reasoning = [
+        Message(role=Role.USER, content="hi"),
+        Message(
+            role=Role.ASSISTANT,
+            content=None,
+            metadata={"reasoning_content": "abcdefgh"},
+        ),
+    ]
+
+    assert estimate_prompt_tokens(with_reasoning) - estimate_prompt_tokens(base) == 2
+
+
+def test_estimate_prompt_tokens_counts_tool_result_ids() -> None:
+    messages = [
+        Message(role=Role.USER, content="hi"),
+        Message(role=Role.TOOL, content="ok", tool_call_id="abcdefgh"),
+    ]
+
+    assert estimate_prompt_tokens(messages) == 11