Fix legend images: rewrite GIBS SVG URLs to PNG for API compatibility

GIBS capabilities list .svg legend URLs, but the Anthropic API only
accepts raster image types. GIBS hosts PNG versions at the same path,
so we rewrite .svg → .png and validate content-type on both legend
fetch paths (direct URL and WMS GetLegendGraphic fallback).

src/mcgibs/client.py

@@ -38,6 +38,9 @@ _INIT_RETRY_DELAYS = [2.0, 4.0, 8.0]
 # Maximum image dimensions to prevent OOM from LLM requests
 MAX_IMAGE_DIMENSION = 4096
 
+# Content types accepted for legend images (SVG is not API-compatible)
+_RASTER_IMAGE_TYPES = {"image/png", "image/jpeg", "image/gif", "image/webp"}
+
 
 class _LRUCache(OrderedDict):
     """Simple LRU cache backed by OrderedDict."""
@@ -421,12 +424,22 @@ class GIBSClient:
         layer_id: str,
         orientation: str = "horizontal",
     ) -> bytes | None:
-        """Fetch the pre-rendered legend image for a layer."""
+        """Fetch the pre-rendered legend image for a layer.
+
+        GIBS capabilities list SVG legend URLs, but PNG versions exist
+        at the same path. We prefer PNG for broad client compatibility
+        (the Anthropic API and most image viewers don't render SVG).
+        """
         layer = self.layer_index.get(layer_id)
         if layer and layer.legend_url:
+            url = layer.legend_url
+            if url.endswith(".svg"):
+                url = url[:-4] + ".png"
             try:
-                resp = await self.http.get(layer.legend_url)
+                resp = await self.http.get(url)
                 resp.raise_for_status()
+                content_type = resp.headers.get("content-type", "").split(";")[0].strip()
+                if content_type in _RASTER_IMAGE_TYPES:
                     return resp.content
             except httpx.HTTPError:
                 pass
@@ -446,8 +459,8 @@ class GIBSClient:
         try:
             resp = await self.http.get(url, params=params)
             resp.raise_for_status()
-            content_type = resp.headers.get("content-type", "")
+            content_type = resp.headers.get("content-type", "").split(";")[0].strip()
-            if content_type.startswith("image/"):
+            if content_type in _RASTER_IMAGE_TYPES:
                 return resp.content
         except httpx.HTTPError as exc:
             log.debug("Legend not available for %s: %s", layer_id, exc)

tests/test_client.py

@@ -407,6 +407,74 @@ async def test_client_caches_successful_geocode(capabilities_xml):
     await client.close()
 
 
+# ---------------------------------------------------------------------------
+# Legend SVG → PNG rewriting (live API compatibility fix)
+# ---------------------------------------------------------------------------
+
+
+@respx.mock
+async def test_legend_rewrites_svg_to_png(capabilities_xml):
+    """Legend URLs ending in .svg should be rewritten to .png."""
+    # Modify the capabilities XML so AMSR2's legend URL ends in .svg
+    patched_xml = capabilities_xml.replace(
+        "AMSR2_Sea_Ice_Concentration_12km.png",
+        "AMSR2_Sea_Ice_Concentration_12km.svg",
+    )
+    respx.get(url__regex=r".*WMTSCapabilities\.xml").mock(
+        return_value=httpx.Response(200, text=patched_xml)
+    )
+
+    buf = BytesIO()
+    Image.new("RGB", (200, 20), "white").save(buf, format="PNG")
+    png_route = respx.get(url__regex=r".*legends/.*\.png").mock(
+        return_value=httpx.Response(
+            200, content=buf.getvalue(), headers={"content-type": "image/png"}
+        )
+    )
+
+    client = GIBSClient()
+    await client.initialize()
+
+    result = await client.get_legend_image("AMSR2_Sea_Ice_Concentration_12km_Monthly")
+    assert result is not None
+    # Verify the .png URL was requested, not .svg
+    assert png_route.call_count == 1
+    assert ".png" in str(png_route.calls.last.request.url)
+
+    await client.close()
+
+
+@respx.mock
+async def test_legend_rejects_svg_content(capabilities_xml):
+    """If the legend URL returns SVG content, it should be rejected."""
+    respx.get(url__regex=r".*WMTSCapabilities\.xml").mock(
+        return_value=httpx.Response(200, text=capabilities_xml)
+    )
+    respx.get(url__regex=r".*legends/.*\.png").mock(
+        return_value=httpx.Response(
+            200,
+            content=b'<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"...',
+            headers={"content-type": "image/svg+xml"},
+        )
+    )
+    # WMS fallback also returns non-image
+    respx.get(url__regex=r".*wms\.cgi.*").mock(
+        return_value=httpx.Response(
+            200,
+            text="<ServiceException>not supported</ServiceException>",
+            headers={"content-type": "application/xml"},
+        )
+    )
+
+    client = GIBSClient()
+    await client.initialize()
+
+    result = await client.get_legend_image("AMSR2_Sea_Ice_Concentration_12km_Monthly")
+    assert result is None
+
+    await client.close()
+
+
 # ---------------------------------------------------------------------------
 # Client.http property guard
 # ---------------------------------------------------------------------------