diff --git a/internal/llm/llm.go b/internal/llm/llm.go
index 6d965be..582edd8 100644
--- a/internal/llm/llm.go
+++ b/internal/llm/llm.go
@@ -532,6 +532,19 @@ func (m *Message) Validate() error {
 			"%q message with tool calls", m.Role,
 		)
 	}
+	if m.ToolCallID != "" && m.Role != RoleTool {
+		return fmt.Errorf("%q message with tool call id", m.Role)
+	}
+	seen := map[string]bool{}
+	for _, call := range m.ToolCalls {
+		if err := call.Validate(); err != nil {
+			return fmt.Errorf("invalid tool call: %w", err)
+		}
+		if seen[call.ID] {
+			return fmt.Errorf("duplicate tool call id %q", call.ID)
+		}
+		seen[call.ID] = true
+	}
 	return nil
 }
 
@@ -698,6 +711,20 @@ type ToolCall struct {
 	Type ToolType `json:"type"`
 }
 
+// Validate checks conformance to the OpenAI API specification.
+func (t *ToolCall) Validate() error {
+	if t.ID == "" {
+		return fmt.Errorf("missing id")
+	}
+	if t.Type != ToolTypeFunction {
+		return fmt.Errorf("invalid type: %q", t.Type)
+	}
+	if t.Function.Name == "" {
+		return fmt.Errorf("missing function name")
+	}
+	return nil
+}
+
 // ToolType identifies the kind of tool in a declaration or tool call.
 type ToolType string