diff --git a/loop/agent.go b/loop/agent.go
index adbdf51..315ff89 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -613,7 +613,7 @@
 	if resp.StopReason == ant.StopReasonToolUse {
 		var toolCalls []ToolCall
 		for _, part := range resp.Content {
-			if part.Type == "tool_use" {
+			if part.Type == ant.ContentTypeToolUse {
 				toolCalls = append(toolCalls, ToolCall{
 					Name:       part.ToolName,
 					Input:      string(part.ToolInput),
@@ -997,13 +997,13 @@
 		case <-ctx.Done():
 			return m, ctx.Err()
 		case msg := <-a.inbox:
-			m = append(m, ant.Content{Type: "text", Text: msg})
+			m = append(m, ant.StringContent(msg))
 		}
 	}
 	for {
 		select {
 		case msg := <-a.inbox:
-			m = append(m, ant.Content{Type: "text", Text: msg})
+			m = append(m, ant.StringContent(msg))
 		default:
 			return m, nil
 		}
@@ -1087,7 +1087,7 @@
 	}
 
 	userMessage := ant.Message{
-		Role:    "user",
+		Role:    ant.MessageRoleUser,
 		Content: msgs,
 	}
 
@@ -1211,19 +1211,19 @@
 
 	// Inject any auto-generated messages from quality checks
 	for _, msg := range autoqualityMessages {
-		msgs = append(msgs, ant.Content{Type: "text", Text: msg})
+		msgs = append(msgs, ant.StringContent(msg))
 	}
 
 	// Handle cancellation by appending a message about it
 	if cancelled {
-		msgs = append(msgs, ant.Content{Type: "text", Text: cancelToolUseMessage})
+		msgs = append(msgs, ant.StringContent(cancelToolUseMessage))
 		// EndOfTurn is false here so that the client of this agent keeps processing
 		// further messages; the conversation is not over.
 		a.pushToOutbox(ctx, AgentMessage{Type: ErrorMessageType, Content: userCancelMessage, EndOfTurn: false})
 	} else if err := a.convo.OverBudget(); err != nil {
 		// Handle budget issues by appending a message about it
 		budgetMsg := "We've exceeded our budget. Please ask the user to confirm before continuing by ending the turn."
-		msgs = append(msgs, ant.Content{Type: "text", Text: budgetMsg})
+		msgs = append(msgs, ant.StringContent(budgetMsg))
 		a.pushToOutbox(ctx, budgetMessage(fmt.Errorf("warning: %w (ask to keep trying, if you'd like)", err)))
 	}
 
@@ -1233,7 +1233,7 @@
 	// Send the combined message to continue the conversation
 	a.stateMachine.Transition(ctx, StateSendingToolResults, "Sending tool results back to LLM")
 	resp, err := a.convo.SendMessage(ant.Message{
-		Role:    "user",
+		Role:    ant.MessageRoleUser,
 		Content: results,
 	})
 	if err != nil {
@@ -1268,7 +1268,7 @@
 	// Collect all text content
 	var allText strings.Builder
 	for _, content := range msg.Content {
-		if content.Type == "text" && content.Text != "" {
+		if content.Type == ant.ContentTypeText && content.Text != "" {
 			if allText.Len() > 0 {
 				allText.WriteString("\n\n")
 			}
@@ -1604,10 +1604,7 @@
 
 	Reply with ONLY the reprompt text.
 	`
-	userMessage := ant.Message{
-		Role:    "user",
-		Content: []ant.Content{{Type: "text", Text: msg}},
-	}
+	userMessage := ant.UserStringMessage(msg)
 	// By doing this in a subconversation, the agent doesn't call tools (because
 	// there aren't any), and there's not a concurrency risk with on-going other
 	// outstanding conversations.
diff --git a/loop/agent_test.go b/loop/agent_test.go
index f1d5b51..24c1c55 100644
--- a/loop/agent_test.go
+++ b/loop/agent_test.go
@@ -492,7 +492,7 @@
 }
 
 func (m *mockConvoInterface) SendUserTextMessage(s string, otherContents ...ant.Content) (*ant.MessageResponse, error) {
-	return m.SendMessage(ant.Message{Role: "user", Content: []ant.Content{{Type: "text", Text: s}}})
+	return m.SendMessage(ant.UserStringMessage(s))
 }
 
 func (m *mockConvoInterface) ToolResultContents(ctx context.Context, resp *ant.MessageResponse) ([]ant.Content, error) {
@@ -503,7 +503,7 @@
 }
 
 func (m *mockConvoInterface) ToolResultCancelContents(resp *ant.MessageResponse) ([]ant.Content, error) {
-	return []ant.Content{{Type: "text", Text: "Tool use cancelled"}}, nil
+	return []ant.Content{ant.StringContent("Tool use cancelled")}, nil
 }
 
 func (m *mockConvoInterface) CancelToolUse(toolUseID string, cause error) error {
@@ -544,7 +544,7 @@
 		return &ant.MessageResponse{
 			StopReason: ant.StopReasonEndTurn,
 			Content: []ant.Content{
-				{Type: "text", Text: "This is a test response"},
+				ant.StringContent("This is a test response"),
 			},
 		}, nil
 	}
@@ -619,8 +619,8 @@
 			return &ant.MessageResponse{
 				StopReason: ant.StopReasonToolUse,
 				Content: []ant.Content{
-					{Type: "text", Text: "I'll use a tool"},
-					{Type: "tool_use", ToolName: "test_tool", ToolInput: []byte("{}"), ID: "test_id"},
+					ant.StringContent("I'll use a tool"),
+					{Type: ant.ContentTypeToolUse, ToolName: "test_tool", ToolInput: []byte("{}"), ID: "test_id"},
 				},
 			}, nil
 		}
@@ -628,14 +628,14 @@
 		return &ant.MessageResponse{
 			StopReason: ant.StopReasonEndTurn,
 			Content: []ant.Content{
-				{Type: "text", Text: "Finished using the tool"},
+				ant.StringContent("Finished using the tool"),
 			},
 		}, nil
 	}
 
 	// Tool result content handler
 	mockConvo.ToolResultContentsFunc = func(ctx context.Context, resp *ant.MessageResponse) ([]ant.Content, error) {
-		return []ant.Content{{Type: "text", Text: "Tool executed successfully"}}, nil
+		return []ant.Content{ant.StringContent("Tool executed successfully")}, nil
 	}
 
 	// Track state transitions
diff --git a/loop/agent_user_cancel_test.go b/loop/agent_user_cancel_test.go
index f79e73e..e55ba17 100644
--- a/loop/agent_user_cancel_test.go
+++ b/loop/agent_user_cancel_test.go
@@ -21,12 +21,7 @@
 			inbox:  make(chan string, 1),
 			outbox: make(chan AgentMessage, 1),
 		}
-		userMsg := ant.Message{
-			Role: "user",
-			Content: []ant.Content{
-				{Type: "text", Text: "hi"},
-			},
-		}
+		userMsg := ant.UserStringMessage("hi")
 		userMsgResponse := &ant.MessageResponse{}
 		mockConvo.ExpectCall("SendMessage", userMsg).Return(userMsgResponse, nil)
 
@@ -55,9 +50,9 @@
 			outbox: make(chan AgentMessage, 1),
 		}
 		userMsg := ant.Message{
-			Role: "user",
+			Role: ant.MessageRoleUser,
 			Content: []ant.Content{
-				{Type: "text", Text: "hi"},
+				{Type: ant.ContentTypeText, Text: "hi"},
 			},
 		}
 		userMsgResponse := &ant.MessageResponse{
@@ -86,7 +81,7 @@
 			},
 		}
 		toolUseResultsMsg := ant.Message{
-			Role:    "user",
+			Role:    ant.MessageRoleUser,
 			Content: toolUseContents,
 		}
 		toolUseResponse := &ant.MessageResponse{
@@ -132,12 +127,7 @@
 			inbox:  make(chan string, 1),
 			outbox: make(chan AgentMessage, 10), // don't let anything block on outbox.
 		}
-		userMsg := ant.Message{
-			Role: "user",
-			Content: []ant.Content{
-				{Type: "text", Text: "hi"},
-			},
-		}
+		userMsg := ant.UserStringMessage("hi")
 		userMsgResponse := &ant.MessageResponse{
 			StopReason: ant.StopReasonToolUse,
 			Content: []ant.Content{
@@ -153,12 +143,7 @@
 				OutputTokens: 200,
 			},
 		}
-		toolUseResultsMsg := ant.Message{
-			Role: "user",
-			Content: []ant.Content{
-				{Type: "text", Text: cancelToolUseMessage},
-			},
-		}
+		toolUseResultsMsg := ant.UserStringMessage(cancelToolUseMessage)
 		toolUseResponse := &ant.MessageResponse{
 			StopReason: ant.StopReasonEndTurn,
 			Content: []ant.Content{
@@ -223,9 +208,9 @@
 			outbox: make(chan AgentMessage, 10), // don't let anything block on outbox.
 		}
 		userMsg := ant.Message{
-			Role: "user",
+			Role: ant.MessageRoleUser,
 			Content: []ant.Content{
-				{Type: "text", Text: "hi"},
+				{Type: ant.ContentTypeText, Text: "hi"},
 			},
 		}
 		userMsgResponse := &ant.MessageResponse{
@@ -244,9 +229,9 @@
 			},
 		}
 		toolUseResultsMsg := ant.Message{
-			Role: "user",
+			Role: ant.MessageRoleUser,
 			Content: []ant.Content{
-				{Type: "text", Text: cancelToolUseMessage},
+				{Type: ant.ContentTypeText, Text: cancelToolUseMessage},
 			},
 		}
 		toolUseResultResponse := &ant.MessageResponse{
@@ -263,9 +248,9 @@
 			},
 		}
 		userFollowUpMsg := ant.Message{
-			Role: "user",
+			Role: ant.MessageRoleUser,
 			Content: []ant.Content{
-				{Type: "text", Text: "that was the wrong thing to do"},
+				{Type: ant.ContentTypeText, Text: "that was the wrong thing to do"},
 			},
 		}
 		userFollowUpResponse := &ant.MessageResponse{
@@ -339,12 +324,7 @@
 
 		// Define test message
 		// This simulates something that would result in claude  responding with tool_use responses.
-		userMsg := ant.Message{
-			Role: "user",
-			Content: []ant.Content{
-				{Type: "text", Text: "use test_tool for something"},
-			},
-		}
+		userMsg := ant.UserStringMessage("use test_tool for something")
 		// Mock initial response with tool use
 		userMsgResponse := &ant.MessageResponse{
 			StopReason: ant.StopReasonToolUse,
@@ -370,11 +350,8 @@
 			},
 		}
 		canceledToolUseMsg := ant.Message{
-			Role: "user",
-			Content: append(canceledToolUseContents, ant.Content{
-				Type: ant.ContentTypeText,
-				Text: cancelToolUseMessage,
-			}),
+			Role:    ant.MessageRoleUser,
+			Content: append(canceledToolUseContents, ant.StringContent(cancelToolUseMessage)),
 		}
 		// Set up expected behaviors
 		waitForSendMessage := make(chan any)