06 update types

Module 06: Update Types

Comprehensive coverage of all SessionUpdate types in ACP.

What You’ll Learn

• All SessionUpdate types and when they appear
• Dispatching on update types with instanceof
• Building rich UIs that show agent activity

The Code

The client registers a sessionUpdateConsumer and uses instanceof to handle each type:

AcpSyncClient client = AcpClient.sync(transport)
    .sessionUpdateConsumer(notification -> {
        SessionUpdate update = notification.update();
        if (update instanceof AgentMessageChunk msg) {
            System.out.print(((TextContent) msg.content()).text());
        } else if (update instanceof AgentThoughtChunk thought) {
            System.out.println("[Thought] " +
                ((TextContent) thought.content()).text());
        } else if (update instanceof ToolCall tc) {
            System.out.println("[Tool] " + tc.title() +
                " | " + tc.kind() + " | " + tc.status());
        } else if (update instanceof ToolCallUpdateNotification tcUpdate) {
            System.out.println("[ToolUpdate] " +
                tcUpdate.toolCallId() + " -> " + tcUpdate.status());
        } else if (update instanceof Plan plan) {
            System.out.println("[Plan] " + plan.entries().size() + " entries:");
            plan.entries().forEach(entry ->
                System.out.println("  - " + entry.content() +
                    " [" + entry.status() + "]"));
        } else if (update instanceof AvailableCommandsUpdate commands) {
            System.out.println("[Commands] " +
                commands.availableCommands().size() + " available");
        } else if (update instanceof CurrentModeUpdate mode) {
            System.out.println("[Mode] " + mode.currentModeId());
        } else if (update instanceof UsageUpdate usage) {
            System.out.printf("[Usage] %d/%d tokens%n", usage.used(), usage.size());
            if (usage.cost() != null) {
                System.out.printf("  Cost: %.4f %s%n",
                    usage.cost().amount(), usage.cost().currency());
            }
        }
    })
    .build();

Update Types Reference

Type	Content	Typical Use
AgentMessageChunk	Incremental response text	Main output, streamed word by word
AgentThoughtChunk	Agent’s thinking process	Show reasoning in a collapsible panel
ToolCall	Tool execution start	Show tool name, kind, and status
ToolCallUpdateNotification	Tool progress	Update status of in-progress tool
Plan	Agent’s planned steps	Show step list with priorities and completion
AvailableCommandsUpdate	Slash commands	Populate command palette
CurrentModeUpdate	Mode change	Update UI mode indicator
UsageUpdate	Token usage and cost	Show context window usage bar (unstable)

This module extends Module 05 by handling every update type rather than just messages.

Source Code

View on GitHub

Running the Example

export GEMINI_API_KEY=your-key-here
./mvnw exec:java -pl module-06-update-types

Next Module

Module 07: Agent Requests — handle file read/write requests from agents.