test: cover the image gate

The gate has four entry points and each now has a test: `view_image` and
`fetch_local` refuse with the message naming the config key, the registry drops
a plugin tool's picture and says why, and the workspace helpers keep `None` (no
host verdict) permissive. On the CLI side the verdict is asserted to follow the
selected model, and to reach the tools once an instance is bound.

tests/test_tools/test_fetch_local.py

@@ -10,7 +10,7 @@ from typing import TYPE_CHECKING, override
 import pytest
 
 from plyngent.agent import ToolOutput
-from plyngent.tools.context import read_only_context
+from plyngent.tools.context import get_instance, read_only_context
 from plyngent.tools.danger import classify_danger
 from plyngent.tools.net import fetch_local, grant_private_host
 from plyngent.tools.net.policy import (
@@ -225,6 +225,24 @@ async def test_an_image_response_comes_back_as_the_picture(workspace: Path, http
     assert part.image_url.url.startswith("data:image/png;base64,")
 
 
+async def test_a_text_only_model_keeps_the_metadata_and_the_reason(
+    workspace: Path,
+    http_server: str,
+) -> None:
+    """The picture stays behind, and the model is told why (``image_in``)."""
+    grant_private_host("127.0.0.1", int(http_server.rsplit(":", 1)[1]))
+    instance = get_instance()
+    assert instance is not None
+    instance.workspace.image_in = False
+    instance.workspace.image_model = ("ds", "text-only")
+
+    output = await fetch_local.handler(f"{http_server}/png")
+
+    assert isinstance(output, str)
+    assert "body_kind=image" in output
+    assert "error: model 'text-only' does not accept image input" in output
+
+
 async def test_fetch_read_only_context_blocks_non_get(workspace: Path, http_server: str) -> None:
     """In a read-only context fetch_local only allows GET; outside it POST works."""
     del workspace

tests/test_tools/test_view_image.py

@@ -6,6 +6,7 @@ import base64
 from typing import TYPE_CHECKING
 
 from plyngent.agent import ToolOutput
+from plyngent.tools.context import get_instance
 from plyngent.tools.file.read import read_file
 from plyngent.tools.file.view_image import view_image
 
@@ -60,6 +61,21 @@ async def test_a_missing_file_is_an_error(workspace: Path) -> None:
     assert await view_image.handler("nope.png") == "error: file not found: nope.png"
 
 
+async def test_a_text_only_model_gets_the_reason_instead_of_the_picture(workspace: Path) -> None:
+    """``image_in`` gates the tool exactly like an ``@path`` mention."""
+    (workspace / "shot.png").write_bytes(PNG)
+    instance = get_instance()
+    assert instance is not None
+    instance.workspace.image_in = False
+    instance.workspace.image_model = ("deepseek", "deepseek-v4-pro")
+
+    result = await view_image.handler("shot.png")
+
+    assert isinstance(result, str)
+    assert result.startswith("error: model 'deepseek-v4-pro' does not accept image input")
+    assert 'set image_in = true under [providers.deepseek.models."deepseek-v4-pro"]' in result
+
+
 async def test_read_file_points_at_view_image_for_a_picture(workspace: Path) -> None:
     (workspace / "shot.png").write_bytes(PNG)
 

tests/test_tools/test_workspace.py

@@ -9,10 +9,12 @@ from plyngent.tools import (
     check_command_allowed,
     get_trash_enabled,
     get_workspace_root,
+    image_input_refusal,
     mode_covers,
     parse_access_mode,
     resolve_path,
     set_command_denylist,
+    set_image_in,
     set_path_denylist,
     set_trash_enabled,
 )
@@ -67,6 +69,34 @@ def test_trash_switch(workspace: object) -> None:
     assert get_trash_enabled() is True
 
 
+def test_image_switch(workspace: object) -> None:
+    """A picture is refused once the host says the model cannot take one."""
+    del workspace
+    assert image_input_refusal() is None
+
+    set_image_in(image_in=False)
+    unnamed = image_input_refusal()
+    assert unnamed is not None
+    assert "does not accept image input" in unnamed
+
+    set_image_in(image_in=False, provider="ds", model="text-only")
+    named = image_input_refusal()
+    assert named is not None
+    assert "model 'text-only' does not accept image input" in named
+    assert '[providers.ds.models."text-only"]' in named
+
+    set_image_in(image_in=True, provider="ds", model="vision")
+    assert image_input_refusal() is None
+
+
+def test_the_image_switch_needs_a_declaration(workspace: object) -> None:
+    """No host verdict (``None``) sends the picture: only ``False`` refuses."""
+    del workspace
+    policy = active_workspace_policy()
+    assert policy.image_in is None
+    assert image_input_refusal() is None
+
+
 def test_config_grant_modes(workspace: object, tmp_path_factory: pytest.TempPathFactory) -> None:
     del workspace
     outside = tmp_path_factory.mktemp("granted")