diff --git a/tests/unit/test_eva_prompter_host_only.py b/tests/unit/test_eva_prompter_host_only.py
new file mode 100644
index 000000000..9bb3b3309
--- /dev/null
+++ b/tests/unit/test_eva_prompter_host_only.py
@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import os
+import sys
+import types
+
+import pytest
+
+# Add the project root to the Python path (matches existing unit tests style).
+project_root = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..", ".."))
+sys.path.insert(0, project_root)
+
+# ufo.utils conditionally imports pywinauto on Windows. This unit test doesn't
+# need the real dependency, so we stub the minimal symbol to keep the test
+# environment lightweight.
+if "pywinauto" not in sys.modules:
+    pywinauto_mod = types.ModuleType("pywinauto")
+    win32structures_mod = types.ModuleType("pywinauto.win32structures")
+
+    class RECT:  # pragma: no cover
+        pass
+
+    win32structures_mod.RECT = RECT
+    sys.modules["pywinauto"] = pywinauto_mod
+    sys.modules["pywinauto.win32structures"] = win32structures_mod
+
+if "fastmcp" not in sys.modules:
+    fastmcp_mod = types.ModuleType("fastmcp")
+    fastmcp_client_mod = types.ModuleType("fastmcp.client")
+    fastmcp_client_transports_mod = types.ModuleType("fastmcp.client.transports")
+
+    class FastMCP:  # pragma: no cover
+        pass
+
+    class StdioTransport:  # pragma: no cover
+        pass
+
+    fastmcp_mod.FastMCP = FastMCP
+    fastmcp_client_transports_mod.StdioTransport = StdioTransport
+
+    sys.modules["fastmcp"] = fastmcp_mod
+    sys.modules["fastmcp.client"] = fastmcp_client_mod
+    sys.modules["fastmcp.client.transports"] = fastmcp_client_transports_mod
+
+import ufo.utils
+from ufo.prompter.eva_prompter import EvaluationAgentPrompter
+
+
+class _FakeTrajectory:
+    def __init__(self, app_agent_log, final_screenshot_image):
+        self.app_agent_log = app_agent_log
+        self.final_screenshot_image = final_screenshot_image
+
+
+def _mk_prompter(monkeypatch, trajectory: _FakeTrajectory):
+    p = EvaluationAgentPrompter(is_visual=False, prompt_template="", example_prompt_template="")
+
+    # Avoid needing prompt templates; we only care about screenshot handling.
+    monkeypatch.setattr(p, "user_prompt_construction", lambda request, log_eva: "ok", raising=True)
+    monkeypatch.setattr(p, "load_logs", lambda log_path: trajectory, raising=True)
+
+    calls = []
+
+    def _encode_image(path):
+        calls.append(path)
+        return f"encoded:{path}"
+
+    monkeypatch.setattr(ufo.utils, "encode_image", _encode_image, raising=True)
+    return p, calls
+
+
+def test_head_tail_does_not_crash_when_app_agent_log_empty(monkeypatch):
+    traj = _FakeTrajectory(app_agent_log=[], final_screenshot_image="final.png")
+    p, calls = _mk_prompter(monkeypatch, traj)
+
+    content = p.user_content_construction_head_tail(log_path="unused", request="req")
+
+    assert calls == ["final.png"]
+    assert any(item.get("type") == "text" and item.get("text") == "ok" for item in content)
+
+
+def test_head_tail_encodes_initial_when_app_agent_log_present(monkeypatch):
+    traj = _FakeTrajectory(
+        app_agent_log=[{"ScreenshotImages": {"clean_screenshot_path": "init.png"}}],
+        final_screenshot_image="final.png",
+    )
+    p, calls = _mk_prompter(monkeypatch, traj)
+
+    content = p.user_content_construction_head_tail(log_path="unused", request="req")
+
+    assert calls == ["init.png", "final.png"]
+    assert any(item.get("type") == "text" and item.get("text") == "ok" for item in content)
diff --git a/ufo/prompter/eva_prompter.py b/ufo/prompter/eva_prompter.py
index 4d0c2fc17..10fca479a 100644
--- a/ufo/prompter/eva_prompter.py
+++ b/ufo/prompter/eva_prompter.py
@@ -103,14 +103,14 @@ def user_content_construction_head_tail(
 
         trajectory = self.load_logs(log_path)
 
-        if len(trajectory.app_agent_log) >= 0:
-            first_screenshot_str = ufo.utils.encode_image(
-                trajectory.app_agent_log[0]
-                .get("ScreenshotImages")
-                .get("clean_screenshot_path")
-            )
-        else:
-            first_screenshot_str = ""
+        first_screenshot_str = ""
+        if trajectory.app_agent_log:
+            # Host-only trajectories can have no AppAgent steps. Guard against
+            # empty app_agent_log so evaluation doesn't crash with IndexError.
+            screenshot_images = trajectory.app_agent_log[0].get("ScreenshotImages") or {}
+            first_path = screenshot_images.get("clean_screenshot_path")
+            if first_path:
+                first_screenshot_str = ufo.utils.encode_image(first_path)
 
         last_screenshot_str = ufo.utils.encode_image(trajectory.final_screenshot_image)