Visual Test Plan: Agent Panel Image Rendering

🎯 The Goal

We want a visual regression test that catches bugs in how read_file displays images.

If someone changes the code in ReadFileTool or the UI rendering in thread_view.rs, this test should fail and show us visually what changed.

⚠️ Current Problem: The Test is Useless

The current test in crates/zed/src/visual_test_runner.rs does NOT test the real code!

Here's what it does now (WRONG):

1. Creates a StubAgentConnection
2. Hard-codes a fake tool call response with pre-baked image data
3. Injects that directly into AcpThread
4. Takes a screenshot

Why this is useless: If you change how ReadFileTool produces its output (in crates/agent/src/tools/read_file_tool.rs), the test will still pass because it never runs that code! The test bypasses the entire tool execution pipeline.

✅ What We Actually Need

The test should:

1. Create a real project with a real image file
2. Actually run the real ReadFileTool::run() method
3. Let the tool produce its real output via event_stream.update_fields()
4. Have that real output flow through to AcpThread and render in the UI
5. Take a screenshot of the real rendered result

This way, if someone changes ReadFileTool or the UI rendering, the test will catch it.

📚 Architecture Background (For Newcomers)

Here's how the agent system works:

The Two "Thread" Types

• Thread (in crates/agent/src/thread.rs) - Runs tools, talks to LLMs, produces events
• AcpThread (in crates/acp_thread/src/acp_thread.rs) - Receives events and stores data for UI rendering

How Tools Work

1. Thread has registered tools (like ReadFileTool)
2. When a tool runs, it gets a ToolCallEventStream
3. The tool calls event_stream.update_fields(...) to send updates
4. Those updates become ThreadEvent::ToolCallUpdate events
5. Events flow to AcpThread via handle_thread_events() in NativeAgentConnection
6. AcpThread stores the data and the UI renders it

The Key File Locations

• Tool implementation: crates/agent/src/tools/read_file_tool.rs
  • Lines 163-188: Image file handling (calls event_stream.update_fields())
• Event stream: crates/agent/src/thread.rs
  • ToolCallEventStream::update_fields() - sends updates
  • ToolCallEventStream::test() - creates a test event stream
• UI rendering: crates/agent_ui/src/acp/thread_view.rs
  • render_image_output() - renders images in tool call output
• Current (broken) test: crates/zed/src/visual_test_runner.rs
  • run_agent_thread_view_test() - the function that needs fixing

🔧 Implementation Plan

Option A: Direct Tool Invocation (Recommended)

Run the real tool and capture its output:

// 1. Create a project with a real image file
let fs = FakeFs::new(cx.executor());
fs.insert_file("/project/test-image.png", EMBEDDED_TEST_IMAGE.to_vec()).await;
let project = Project::test(fs.clone(), ["/project"], cx).await;

// 2. Create the ReadFileTool (needs Thread, ActionLog)
let action_log = cx.new(|_| ActionLog::new(project.clone()));
// ... create Thread with project ...
let tool = Arc::new(ReadFileTool::new(thread.downgrade(), project.clone(), action_log));

// 3. Run the tool and capture events
let (event_stream, mut event_receiver) = ToolCallEventStream::test();
let input = ReadFileToolInput {
    path: "project/test-image.png".to_string(),
    start_line: None,
    end_line: None,
};
tool.run(input, event_stream, cx).await?;

// 4. Collect the ToolCallUpdateFields that the tool produced
let updates = event_receiver.collect_updates();

// 5. Create an AcpThread and inject the real tool output
// ... create AcpThread ...
acp_thread.update(cx, |thread, cx| {
    // First create the tool call entry
    thread.upsert_tool_call(initial_tool_call, cx)?;
    // Then update it with the real output from the tool
    for update in updates {
        thread.update_tool_call(update, cx)?;
    }
})?;

// 6. Render and screenshot

Required Exports

The agent crate needs to export these for the visual test:

• ReadFileTool and ReadFileToolInput
• ToolCallEventStream::test() (already has #[cfg(feature = "test-support")])
• Thread (to create the tool)

Check crates/agent/src/lib.rs and add exports if needed.

Required Dependencies in crates/zed/Cargo.toml

The visual-tests feature needs:

"agent/test-support"  # For ToolCallEventStream::test() and tool exports

Option B: Use NativeAgentConnection with Fake Model

Alternatively, use the full agent flow with a fake LLM:

1. Create NativeAgentServer with a FakeLanguageModel
2. Program the fake model to return a tool call for read_file
3. Let the real agent flow execute the tool
4. The tool runs, produces output, flows through to UI

This is more complex but tests more of the real code path.

📋 Step-by-Step Implementation Checklist

Phase 1: Enable Tool Access

• Add agent/test-support to visual-tests feature in crates/zed/Cargo.toml
• Verify ReadFileTool, ReadFileToolInput, ToolCallEventStream::test() are exported
• Added additional required features: language_model/test-support, fs/test-support, action_log

Phase 2: Rewrite the Test

• In run_agent_thread_view_test(), remove the fake stub response
• Create a real temp directory with a real image file (FakeFs doesn't work in visual test runner)
• Create the real ReadFileTool with Thread, ActionLog, etc.
• Run the tool with ToolCallEventStream::test()
• Capture the ToolCallUpdateFields it produces
• Use the real tool output to populate the stub connection's response

Phase 3: Verify It Works

• Run UPDATE_BASELINE=1 cargo run -p zed --bin visual_test_runner --features visual-tests
• Check the screenshot shows the real tool output
• Intentionally break read_file_tool.rs (comment out event_stream.update_fields)
• Verified the test fails with: "ReadFileTool did not produce any content - the tool is broken!"
• Restored the code and verified test passes again

🧪 How to Verify the Test is Actually Testing Real Code

After implementing, do this sanity check:

1. In crates/agent/src/tools/read_file_tool.rs, comment out lines 181-185:

   // event_stream.update_fields(ToolCallUpdateFields::new().content(vec![
   //     acp::ToolCallContent::Content(acp::Content::new(acp::ContentBlock::Image(
   //         acp::ImageContent::new(language_model_image.source.clone(), "image/png"),
   //     ))),
   // ]));
   

2. Run the visual test - it should FAIL or produce a visibly different screenshot

3. Restore the code - test should pass again

If commenting out the real tool code doesn't affect the test, the test is still broken!

📁 Files Modified

Note: No changes needed to crates/agent/src/lib.rs - all necessary exports were already public.

✅ Already Completed (Don't Redo These)

These changes have already been made and are working:

1. read_file tool sends image content - crates/agent/src/tools/read_file_tool.rs now calls event_stream.update_fields() with image content blocks (lines 181-185)

2. UI renders images - crates/agent_ui/src/acp/thread_view.rs has render_image_output() that shows dimensions ("512×512 PNG") and a "Go to File" button

3. Image tool calls auto-expand - The UI automatically expands tool calls that return images

4. Visual test infrastructure exists - The test runner, baseline comparison, etc. all work

The only thing broken is that the test doesn't actually run the real tool code!

🔗 Related Code References

• Tool implementation: read_file_tool.rs
• Event stream: thread.rs lines 2501-2596
• UI rendering: thread_view.rs render_image_output
• Current test: visual_test_runner.rs run_agent_thread_view_test