generate_image

Instructions

Implementation Reference

• src/nano_banana_mcp/server.py:24-24 (registration)

  The @mcp.tool() decorator registers 'generate_image' as an MCP tool on the FastMCP server instance.

  @mcp.tool()

• src/nano_banana_mcp/server.py:25-68 (handler)

  The main handler function that takes a prompt, output_path, and optional model, generates an image using Google's Gemini API (genai), extracts the image bytes from the response, and writes them to disk. Handles file collision by appending a numeric suffix and returns the final path.

  def generate_image(prompt: str, output_path: str, model: str | None = None) -> str:
      """Generate an image with Gemini and save it to output_path.
  
      Known model values (override via `model`):
        - gemini-2.5-flash-image           (default — Nano Banana, fastest/cheapest)
        - gemini-3.1-flash-image-preview   (Nano Banana 2, higher quality)
        - gemini-3-pro-image-preview       (Nano Banana Pro, top quality)
  
      If output_path already exists, a numeric suffix is appended
      (foo.png -> foo-1.png) and the final path is returned.
      """
      api_key = os.environ.get("GEMINI_API_KEY")
      if not api_key:
          raise RuntimeError("GEMINI_API_KEY is not set in the MCP server environment.")
  
      target = Path(output_path).expanduser().resolve()
      target.parent.mkdir(parents=True, exist_ok=True)
      final = _resolve_collision(target)
  
      client = genai.Client(api_key=api_key)
      response = client.models.generate_content(
          model=model or DEFAULT_MODEL,
          contents=prompt,
      )
  
      image_bytes = None
      for part in response.candidates[0].content.parts:
          inline = getattr(part, "inline_data", None)
          if inline and inline.data:
              image_bytes = inline.data
              break
  
      if image_bytes is None:
          text_parts = [
              p.text for p in response.candidates[0].content.parts
              if getattr(p, "text", None)
          ]
          raise RuntimeError(
              f"No image in response. Model said: {' '.join(text_parts) or '(empty)'}"
          )
  
      final.write_bytes(image_bytes)
      note = "" if final == target else f" (renamed from {target.name} to avoid overwrite)"
      return f"wrote: {final}{note}"

• src/nano_banana_mcp/server.py:7-7 (helper)

  Default model constant used when no model override is provided.

  DEFAULT_MODEL = "gemini-2.5-flash-image"

• src/nano_banana_mcp/server.py:12-21 (helper)

  Helper function _resolve_collision that appends a numeric suffix (e.g., -1, -2) to avoid overwriting existing files.

  def _resolve_collision(path: Path) -> Path:
      if not path.exists():
          return path
      stem, suffix, parent = path.stem, path.suffix, path.parent
      i = 1
      while True:
          candidate = parent / f"{stem}-{i}{suffix}"
          if not candidate.exists():
              return candidate
          i += 1