render_chart

Instructions

Implementation Reference

• hermesforge_mcp/server.py:117-184 (handler)

  Main handler function for the render_chart tool. Accepts a Chart.js JSON config, width, height, and format, then posts to the Hermesforge chart rendering API and returns a base64-encoded image.

  @mcp.tool()
  def render_chart(
      chart_config: str,
      width: int = 800,
      height: int = 600,
      format: str = "png",
  ) -> str:
      """
      Render a Chart.js configuration as a chart image.
  
      Use this when you need to:
      - Visualize data as bar, line, pie, scatter, or other Chart.js chart types
      - Generate charts programmatically from data
      - Create charts for reports or documentation
  
      Args:
          chart_config: A JSON string containing a valid Chart.js configuration object.
              Example: '{"type":"bar","data":{"labels":["A","B","C"],"datasets":[{"label":"Values","data":[1,2,3]}]}}'
          width: Chart width in pixels (default: 800)
          height: Chart height in pixels (default: 600)
          format: Image format - 'png' or 'jpeg' (default: 'png')
  
      Returns:
          Base64-encoded chart image with data URI prefix.
  
      Rate limits: Shared with screenshot API. Get a free API key at https://hermesforge.dev/api/keys
      """
      import json
  
      # Validate JSON
      try:
          config = json.loads(chart_config)
      except json.JSONDecodeError as e:
          return f"Error: chart_config is not valid JSON: {e}"
  
      payload = {
          "config": config,
          "width": width,
          "height": height,
          "format": format,
      }
  
      try:
          resp = requests.post(
              f"{API_BASE}/api/charts/render",
              json=payload,
              headers={**_auth_headers(), "Content-Type": "application/json"},
              timeout=30,
          )
      except requests.RequestException as e:
          return f"Error: Could not reach Hermesforge API: {e}"
  
      if resp.status_code == 200:
          img_bytes = resp.content
          b64 = base64.b64encode(img_bytes).decode()
          mime = "image/jpeg" if format == "jpeg" else "image/png"
          return f"data:{mime};base64,{b64}"
      elif resp.status_code == 429:
          try:
              msg = resp.json().get("message", "")
          except Exception:
              msg = ""
          return (
              f"Rate limit reached. {msg} "
              f"Get a free API key at https://hermesforge.dev/api/keys"
          )
      else:
          return f"Error: API returned {resp.status_code}: {resp.text[:200]}"

• hermesforge_mcp/server.py:118-123 (schema)

  Type signature/input schema for render_chart. chart_config (str, required), width (int, default 800), height (int, default 600), format (str, default 'png'). Returns str.

  def render_chart(
      chart_config: str,
      width: int = 800,
      height: int = 600,
      format: str = "png",
  ) -> str:

• hermesforge_mcp/server.py:117-117 (registration)

  Registration decorator registering render_chart as an MCP tool via the FastMCP instance.

  @mcp.tool()