Return HTML charts as EmbeddedResource instead of server-side files

HTML charts were being written to the server filesystem and returning
a path the client couldn't access. Now returns the HTML content
inline as an MCP EmbeddedResource with text/html MIME type, so the
client receives the full interactive chart over the wire.

Dockerfile.mcp

@@ -15,7 +15,6 @@ WORKDIR /app
 ENV UV_COMPILE_BYTECODE=1
 ENV PATH="/app/.venv/bin:$PATH"
 ENV MPLCONFIGDIR=/tmp/matplotlib
-ENV MCNOAA_CHARTS_DIR=/tmp/charts
 
 COPY --from=deps /app/.venv /app/.venv
 COPY src/ src/

src/mcnoaa_tides/tools/charts.py

@@ -2,12 +2,11 @@
 
 import asyncio
 from datetime import datetime, timezone
-from pathlib import Path
 from typing import Literal
 
 from fastmcp import Context, FastMCP
 from fastmcp.utilities.types import Image
-from mcp.types import ToolAnnotations
+from mcp.types import EmbeddedResource, TextResourceContents, ToolAnnotations
 
 from mcnoaa_tides.charts import check_deps
 from mcnoaa_tides.client import NOAAClient
@@ -23,15 +22,16 @@ def register(mcp: FastMCP) -> None:
         hours: int = 48,
         include_observed: bool = True,
         format: Literal["png", "html"] = "png",
-    ) -> Image | str:
+    ) -> Image | EmbeddedResource:
         """Generate a tide prediction chart with high/low markers.
 
         Creates a visual chart of tide predictions showing the water level curve
         with high (H) and low (L) tide markers. Optionally overlays observed
         water levels as a dashed line for comparison.
 
-        PNG format returns an inline image. HTML format saves an interactive
-        chart to artifacts/charts/ and returns the file path.
+        PNG format returns an inline image. HTML format returns an interactive
+        Plotly chart as an embedded resource (text/html) that the client can
+        save or render directly.
 
         Requires mcnoaa-tides[viz] to be installed.
         """
@@ -97,8 +97,7 @@ def register(mcp: FastMCP) -> None:
             from mcnoaa_tides.charts.tides import render_tide_chart_html
 
             html = render_tide_chart_html(predictions, observed, station_name)
-            path = _save_html(html, station_id, "tides")
-            return f"Interactive tide chart saved to: {path}"
+            return _html_resource(html, station_id, "tides")
 
     @mcp.tool(tags={"visualization"}, annotations=_ANNOTATIONS)
     async def visualize_conditions(
@@ -106,7 +105,7 @@ def register(mcp: FastMCP) -> None:
         station_id: str,
         hours: int = 24,
         format: Literal["png", "html"] = "png",
-    ) -> Image | str:
+    ) -> Image | EmbeddedResource:
         """Generate a multi-panel marine conditions dashboard.
 
         Creates a dashboard with up to 4 panels:
@@ -117,8 +116,9 @@ def register(mcp: FastMCP) -> None:
 
         Products unavailable at a station are simply omitted from the dashboard.
 
-        PNG format returns an inline image. HTML format saves an interactive
-        chart to artifacts/charts/ and returns the file path.
+        PNG format returns an inline image. HTML format returns an interactive
+        Plotly dashboard as an embedded resource (text/html) that the client
+        can save or render directly.
 
         Requires mcnoaa-tides[viz] to be installed.
         """
@@ -184,23 +184,16 @@ def register(mcp: FastMCP) -> None:
             from mcnoaa_tides.charts.conditions import render_conditions_html
 
             html = render_conditions_html(snapshot, station_name)
-            path = _save_html(html, station_id, "conditions")
-            return f"Interactive conditions dashboard saved to: {path}"
+            return _html_resource(html, station_id, "conditions")
 
 
-def _save_html(html: str, station_id: str, chart_type: str) -> Path:
-    """Save HTML chart and return the path.
-
-    Uses $MCNOAA_CHARTS_DIR if set, otherwise falls back to
-    artifacts/charts/ (relative to cwd).  The container sets
-    MCNOAA_CHARTS_DIR=/tmp/charts so the nobody user can write.
-    """
-    import os
-
-    timestamp = datetime.now(timezone.utc).strftime("%Y%m%d_%H%M%S")
-    base = os.environ.get("MCNOAA_CHARTS_DIR", "artifacts/charts")
-    out_dir = Path(base)
-    out_dir.mkdir(parents=True, exist_ok=True)
-    path = out_dir / f"{station_id}_{chart_type}_{timestamp}.html"
-    path.write_text(html, encoding="utf-8")
-    return path
+def _html_resource(html: str, station_id: str, chart_type: str) -> EmbeddedResource:
+    """Wrap an HTML chart as an MCP EmbeddedResource for inline delivery."""
+    return EmbeddedResource(
+        type="resource",
+        resource=TextResourceContents(
+            uri=f"chart://{chart_type}/{station_id}",
+            mimeType="text/html",
+            text=html,
+        ),
+    )