core/tools: show images instead of describing them

New `view_image` reads a local picture (JPEG, PNG, GIF, WebP, sniffed from its
bytes) and hands it to the model as an image part, with an optional `detail`
level — `low` measured 3.4x cheaper on a 1200x900 picture. `read_file` now
points at it instead of returning decode noise.

`fetch_local` returns an image response as the picture itself, and MCP image
content blocks become image parts instead of `[image: ...]` placeholders, so a
screenshot server finally reaches the model.

tests/test_tools/test_catalog.py

@@ -59,6 +59,7 @@ def test_read_only_classification() -> None:
     read_only = {t.name for t in default_tool_definitions(surface="local") if t.tags & ToolTag.READ_ONLY}
     assert read_only == {
         "read_file",
+        "view_image",
         "get_truncated",
         "glob_paths",
         "regex_files",

tests/test_tools/test_fetch_local.py

@@ -2,12 +2,14 @@
 
 from __future__ import annotations
 
+import base64
 import threading
 from http.server import BaseHTTPRequestHandler, HTTPServer
 from typing import TYPE_CHECKING, override
 
 import pytest
 
+from plyngent.agent import ToolOutput
 from plyngent.tools.context import read_only_context
 from plyngent.tools.danger import classify_danger
 from plyngent.tools.net import fetch_local, grant_private_host
@@ -29,6 +31,12 @@ if TYPE_CHECKING:
     from pathlib import Path
 
 
+# A real 1x1 PNG: the body must be sniffed by its bytes, not by the header.
+_PNG_BYTES = base64.b64decode(
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg=="
+)
+
+
 class _Handler(BaseHTTPRequestHandler):
     """Minimal echo server for fetch_local tests."""
 
@@ -66,6 +74,9 @@ class _Handler(BaseHTTPRequestHandler):
         if self.path.startswith("/bin"):
             self._send(200, b"\x00\x01\x02\xffbinary", content_type="application/octet-stream")
             return
+        if self.path.startswith("/png"):
+            self._send(200, _PNG_BYTES, content_type="image/png")
+            return
         if self.path.startswith("/ua"):
             ua = (self.headers.get("User-Agent") or "").encode()
             self._send(200, ua)
@@ -200,6 +211,20 @@ async def test_fetch_get_post_put_delete(workspace: Path, http_server: str) -> N
     assert "status=204" in out_del
 
 
+async def test_an_image_response_comes_back_as_the_picture(workspace: Path, http_server: str) -> None:
+    """A PNG body is handed to the model as an image part, not a hex preview."""
+    del workspace
+    grant_private_host("127.0.0.1", int(http_server.rsplit(":", 1)[1]))
+
+    output: object = await fetch_local.handler(f"{http_server}/png")
+
+    assert isinstance(output, ToolOutput)
+    assert "body_kind=image" in output.text
+    assert "status=200" in output.text
+    part = output.images[0]
+    assert part.image_url.url.startswith("data:image/png;base64,")
+
+
 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

@@ -0,0 +1,68 @@
+"""``view_image``: a local picture reaches the model as a picture."""
+
+from __future__ import annotations
+
+import base64
+from typing import TYPE_CHECKING
+
+from plyngent.agent import ToolOutput
+from plyngent.tools.file.read import read_file
+from plyngent.tools.file.view_image import view_image
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+# A real 1x1 PNG: the format is sniffed from these bytes.
+PNG = base64.b64decode(
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg=="
+)
+
+
+async def test_view_image_attaches_the_picture(workspace: Path) -> None:
+    (workspace / "shot.png").write_bytes(PNG)
+
+    output = await view_image.handler("shot.png")
+
+    assert isinstance(output, ToolOutput)
+    assert output.text == "image: shot.png (attached below)"
+    url = output.images[0].image_url.url
+    assert url.startswith("data:image/png;base64,")
+    assert base64.b64decode(url.partition(",")[2]) == PNG
+
+
+async def test_the_detail_level_reaches_the_part(workspace: Path) -> None:
+    (workspace / "shot.png").write_bytes(PNG)
+
+    output = await view_image.handler("shot.png", detail="low")
+
+    assert isinstance(output, ToolOutput)
+    assert output.images[0].image_url.detail == "low"
+
+
+async def test_an_unknown_detail_level_is_an_error(workspace: Path) -> None:
+    (workspace / "shot.png").write_bytes(PNG)
+
+    assert "detail must be one of" in await view_image.handler("shot.png", detail="huge")
+
+
+async def test_a_non_image_file_is_an_error(workspace: Path) -> None:
+    (workspace / "notes.txt").write_text("hello")
+
+    result = await view_image.handler("notes.txt")
+
+    assert isinstance(result, str)
+    assert "not a JPEG, PNG, GIF, or WebP image" in result
+
+
+async def test_a_missing_file_is_an_error(workspace: Path) -> None:
+    del workspace
+
+    assert await view_image.handler("nope.png") == "error: file not found: nope.png"
+
+
+async def test_read_file_points_at_view_image_for_a_picture(workspace: Path) -> None:
+    (workspace / "shot.png").write_bytes(PNG)
+
+    result = await read_file.handler("shot.png")
+
+    assert result == "error: shot.png is an image (image/png); use view_image to look at it"