diff --git a/tests/tools/test_computer_use_capture_routing.py b/tests/tools/test_computer_use_capture_routing.py
new file mode 100644
index 00000000000..44084fabbea
--- /dev/null
+++ b/tests/tools/test_computer_use_capture_routing.py
@@ -0,0 +1,431 @@
+"""End-to-end regression for #24015 — capture routing via auxiliary.vision.
+
+When ``computer_use(action='capture', mode='som'|'vision')`` returns a
+screenshot, ``_capture_response`` previously always returned a
+``_multimodal`` envelope. For non-vision main models, or when the user
+explicitly configured ``auxiliary.vision`` in ``config.yaml``, that
+envelope tripped HTTP 404 / 400 at the provider boundary even though a
+perfectly good vision backend was sitting in config waiting to be used.
+
+This file exercises the integrated ``_capture_response`` flow with
+deterministic stubs for:
+
+* ``should_route_capture_to_aux_vision`` (the policy decision)
+* ``_run_async`` (sync->async bridge)
+* ``vision_analyze_tool`` (the aux LLM call)
+* ``hermes_constants.get_hermes_dir`` (cache path)
+
+…so the full code path is covered without a live cua-driver, a real
+auxiliary client, or network access.
+"""
+
+from __future__ import annotations
+
+import base64
+import json
+import os
+from pathlib import Path
+from typing import Any
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# Fixtures / helpers
+# ---------------------------------------------------------------------------
+
+# 1×1 PNG (transparent) — minimal bytes that decode cleanly.
+_PNG_B64 = (
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42m"
+    "NkYAAAAAYAAjCB0C8AAAAASUVORK5CYII="
+)
+
+# 1×1 JPEG — used to verify mime detection works for either stream type.
+_JPEG_B64 = (
+    "/9j/4AAQSkZJRgABAQEAYABgAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQEBAQEB"
+    "AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQH/"
+)
+
+
+@pytest.fixture
+def tmp_cache_dir(tmp_path):
+    """Override get_hermes_dir so cache writes land under tmp_path."""
+    cache_dir = tmp_path / "cache_vision"
+    cache_dir.mkdir()
+
+    def _fake_get(*_args, **_kw):
+        return cache_dir
+
+    with patch("hermes_constants.get_hermes_dir", _fake_get):
+        yield cache_dir
+
+
+def _make_capture(
+    *,
+    png_b64: str = _PNG_B64,
+    mode: str = "som",
+    elements=None,
+    app: str = "Safari",
+    window_title: str = "GitHub – Issue #24015",
+    width: int = 1280,
+    height: int = 800,
+):
+    from tools.computer_use.backend import CaptureResult, UIElement
+
+    elements = list(elements or [
+        UIElement(index=0, role="AXButton", label="Sign in",
+                  bounds=(10, 20, 80, 30)),
+        UIElement(index=1, role="AXTextField", label="username",
+                  bounds=(10, 60, 200, 24)),
+    ])
+    raw = base64.b64decode(png_b64, validate=False)
+    return CaptureResult(
+        mode=mode,
+        width=width,
+        height=height,
+        png_b64=png_b64,
+        elements=elements,
+        app=app,
+        window_title=window_title,
+        png_bytes_len=len(raw),
+    )
+
+
+def _stub_aux_analysis(text: str):
+    """Return a fake vision_analyze_tool coroutine result (JSON envelope)."""
+    return json.dumps({"success": True, "analysis": text})
+
+
+# ---------------------------------------------------------------------------
+# _capture_response: routing OFF (current/native behaviour)
+# ---------------------------------------------------------------------------
+
+class TestCaptureResponseDefaultPath:
+    """When routing helper says 'native', the existing multimodal envelope wins."""
+
+    def test_som_capture_returns_multimodal_envelope_when_native(self):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(png_b64=_PNG_B64, mode="som")
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=False):
+            resp = cu_tool._capture_response(cap)
+
+        assert isinstance(resp, dict)
+        assert resp.get("_multimodal") is True
+        # Image part must use image/png MIME for a PNG payload.
+        image_part = next(
+            p for p in resp["content"] if p.get("type") == "image_url"
+        )
+        url = image_part["image_url"]["url"]
+        assert url.startswith("data:image/png;base64,")
+        assert "vision_analysis" not in resp
+
+    def test_jpeg_capture_returns_image_jpeg_mime_when_native(self):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(png_b64=_JPEG_B64, mode="som")
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=False):
+            resp = cu_tool._capture_response(cap)
+
+        url = next(p for p in resp["content"] if p.get("type") == "image_url")
+        assert url["image_url"]["url"].startswith("data:image/jpeg;base64,")
+
+    def test_ax_only_capture_returns_text_regardless_of_routing(self):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="ax", png_b64="")
+        # ax mode never has a PNG so neither path matters; assert pure text.
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True) as routing:
+            resp = cu_tool._capture_response(cap)
+
+        # ax never even consults the routing helper — short-circuited above
+        # the image branch.
+        routing.assert_not_called()
+        assert isinstance(resp, str)
+        body = json.loads(resp)
+        assert body["mode"] == "ax"
+
+
+# ---------------------------------------------------------------------------
+# _capture_response: routing ON (the #24015 fix)
+# ---------------------------------------------------------------------------
+
+class TestCaptureResponseRoutedToAuxVision:
+    """When routing helper says 'aux', the PNG is pre-analysed and a text
+    response is returned with no image_url parts at all."""
+
+    def test_som_capture_returns_text_with_vision_analysis(
+        self, tmp_cache_dir,
+    ):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+
+        captured_calls = {}
+
+        def _fake_run_async(coro):
+            captured_calls["called"] = True
+            return _stub_aux_analysis(
+                "A Safari window showing a GitHub issue page with a 'Sign "
+                "in' button and a 'username' text field."
+            )
+
+        # vision_analyze_tool is async; force a sync MagicMock so we can
+        # assert positional args without dealing with awaitables.
+        fake_vat = MagicMock(return_value="<coro>")
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            resp = cu_tool._capture_response(cap)
+
+        # Must be a JSON string, NOT a multimodal envelope. This is exactly
+        # the contract that prevents #24015's HTTP 404 from firing on the
+        # next agent turn.
+        assert isinstance(resp, str)
+        body = json.loads(resp)
+        assert body["mode"] == "som"
+        assert body["app"] == "Safari"
+        assert "Sign in" in body["vision_analysis"]
+        assert body["vision_analysis_routed_via"] == "auxiliary.vision"
+        # The original AX-only metadata (window title, element index, app)
+        # is preserved alongside the new vision analysis so the agent loses
+        # no context vs the multimodal path.
+        assert body["window_title"] == "GitHub – Issue #24015"
+        assert len(body["elements"]) == 2
+
+        assert captured_calls.get("called") is True
+        # vision_analyze_tool was invoked with a path under the patched cache
+        # and a non-empty prompt.
+        args, _kwargs = fake_vat.call_args
+        path_arg, prompt_arg = args[0], args[1]
+        assert str(tmp_cache_dir) in path_arg
+        assert "macOS application screenshot" in prompt_arg
+        # AX summary is included so the aux model can ground its description
+        # against the same set-of-mark index the agent will see.
+        assert "Sign in" in prompt_arg
+
+    def test_temp_screenshot_file_is_cleaned_up_after_routing(
+        self, tmp_cache_dir,
+    ):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+        # We capture the path the aux call sees so we can assert it's gone
+        # after _capture_response returns.
+        observed_path = {}
+
+        def _fake_run_async(_coro):
+            return _stub_aux_analysis("description goes here")
+
+        def _fake_vat(image_path, _prompt):
+            observed_path["path"] = image_path
+            # File must exist while aux is being arranged.
+            assert os.path.exists(image_path)
+            return "<coro>"
+
+        fake_vat = MagicMock(side_effect=_fake_vat)
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            cu_tool._capture_response(cap)
+
+        # File must be unlinked after _capture_response returns.
+        assert observed_path["path"]
+        assert not os.path.exists(observed_path["path"])
+
+    def test_temp_file_cleaned_up_even_when_aux_call_raises(
+        self, tmp_cache_dir,
+    ):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+        observed_path = {}
+
+        def _fake_vat(image_path, _prompt):
+            observed_path["path"] = image_path
+            return "<coro>"
+
+        def _fake_run_async(_coro):
+            raise RuntimeError("aux LLM down")
+
+        fake_vat = MagicMock(side_effect=_fake_vat)
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            resp = cu_tool._capture_response(cap)
+
+        # Aux failure → fall back to multimodal envelope (so the user still
+        # gets *something* useful even if vision is broken).
+        assert isinstance(resp, dict)
+        assert resp.get("_multimodal") is True
+        # Temp file must still be cleaned up.
+        assert observed_path["path"]
+        assert not os.path.exists(observed_path["path"])
+
+    def test_empty_aux_analysis_falls_back_to_multimodal(self, tmp_cache_dir):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+
+        def _fake_run_async(_coro):
+            return _stub_aux_analysis("")
+
+        fake_vat = MagicMock(return_value="<coro>")
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            resp = cu_tool._capture_response(cap)
+
+        # Empty analysis is treated as failure — we'd rather show pixels
+        # than embed an empty 'vision_analysis' string into the result.
+        assert isinstance(resp, dict)
+        assert resp.get("_multimodal") is True
+
+    def test_invalid_aux_response_falls_back_to_multimodal(self, tmp_cache_dir):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+
+        def _fake_run_async(_coro):
+            return 1234  # not a string at all
+
+        fake_vat = MagicMock(return_value="<coro>")
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            resp = cu_tool._capture_response(cap)
+
+        assert isinstance(resp, dict)
+        assert resp.get("_multimodal") is True
+
+
+# ---------------------------------------------------------------------------
+# _should_route_through_aux_vision: end-to-end with real config plumbing
+# ---------------------------------------------------------------------------
+
+class TestRoutingDecisionWiring:
+    """Verify _should_route_through_aux_vision wires the right config + helper."""
+
+    def test_explicit_aux_vision_in_config_routes_to_aux(self):
+        from tools.computer_use import tool as cu_tool
+
+        cfg = {
+            "model": {"default": "tencent/hy3-preview", "provider": "openrouter"},
+            "auxiliary": {
+                "vision": {
+                    "provider": "openrouter",
+                    "model": "google/gemini-2.5-flash",
+                }
+            },
+        }
+        with patch("agent.auxiliary_client._read_main_provider",
+                   return_value="openrouter"), \
+             patch("agent.auxiliary_client._read_main_model",
+                   return_value="tencent/hy3-preview"), \
+             patch("hermes_cli.config.load_config", return_value=cfg):
+            assert cu_tool._should_route_through_aux_vision() is True
+
+    def test_no_explicit_aux_and_vision_capable_main_keeps_multimodal(self):
+        from tools.computer_use import tool as cu_tool
+
+        cfg = {
+            "model": {"default": "claude-opus-4-5", "provider": "anthropic"},
+        }
+        with patch("agent.auxiliary_client._read_main_provider",
+                   return_value="anthropic"), \
+             patch("agent.auxiliary_client._read_main_model",
+                   return_value="claude-opus-4-5"), \
+             patch("hermes_cli.config.load_config", return_value=cfg), \
+             patch("tools.computer_use.vision_routing._lookup_supports_vision",
+                   return_value=True), \
+             patch("tools.computer_use.vision_routing."
+                   "_provider_accepts_multimodal_tool_result",
+                   return_value=True):
+            assert cu_tool._should_route_through_aux_vision() is False
+
+    def test_config_load_failure_disables_routing_safely(self):
+        from tools.computer_use import tool as cu_tool
+
+        with patch("hermes_cli.config.load_config",
+                   side_effect=RuntimeError("config.yaml unreadable")):
+            # No exception should bubble up — fail open by returning False
+            # so the legacy multimodal envelope continues to work.
+            assert cu_tool._should_route_through_aux_vision() is False
+
+    def test_helper_decision_exception_is_swallowed(self):
+        from tools.computer_use import tool as cu_tool
+        from tools.computer_use import vision_routing as vr_mod
+
+        with patch("agent.auxiliary_client._read_main_provider",
+                   return_value="openrouter"), \
+             patch("agent.auxiliary_client._read_main_model",
+                   return_value="x"), \
+             patch("hermes_cli.config.load_config", return_value={}), \
+             patch.object(vr_mod, "should_route_capture_to_aux_vision",
+                          side_effect=ValueError("policy bug")):
+            assert cu_tool._should_route_through_aux_vision() is False
+
+
+# ---------------------------------------------------------------------------
+# Bug reproduction marker — proves the fix is needed.
+# ---------------------------------------------------------------------------
+
+class TestBugReproductionAnchor:
+    """Without the fix, this test would assert the wrong thing.
+
+    On upstream/main HEAD prior to this branch, _capture_response returns a
+    multimodal envelope unconditionally — so when a non-vision main model
+    is configured, the captured PNG is delivered to the main provider as
+    image_url content and the request is rejected with HTTP 404. We don't
+    have a live provider here, but we can pin the contract: with routing
+    enabled the response MUST be a JSON string with no image_url parts.
+    """
+
+    def test_non_vision_main_model_never_returns_image_url_when_routed(
+        self, tmp_cache_dir,
+    ):
+        from tools.computer_use import tool as cu_tool
+
+        cap = _make_capture(mode="som")
+
+        def _fake_run_async(_coro):
+            return _stub_aux_analysis(
+                "Screenshot showing a GitHub.com window with a sign-in "
+                "form."
+            )
+
+        fake_vat = MagicMock(return_value="<coro>")
+
+        with patch.object(cu_tool, "_should_route_through_aux_vision",
+                          return_value=True), \
+             patch("model_tools._run_async", side_effect=_fake_run_async), \
+             patch("tools.vision_tools.vision_analyze_tool",
+                   new_callable=lambda: fake_vat):
+            resp = cu_tool._capture_response(cap)
+
+        # Must be a string (text-only result).
+        assert isinstance(resp, str)
+        # Must NOT contain a base64 image URL anywhere — that's what tripped
+        # 'No endpoints found that support image input' on the reporter's
+        # main provider in #24015.
+        assert "data:image" not in resp
+        assert "image_url" not in resp