Gemini fails with Custom Prompt action

[tonivanhala]

The default Custom Prompt action (invoked from Action Palette) fails with gemini-2.5-flash-preview-05-20 and gemini-2.5-pro models. The reported error is No placement returned. Inline Assistant works fine out-of-the-box.

Gemini returns a response which does not adhere to the required response format. The root cause appears to be multiple system prompts in the request, which causes Gemini to ignore some of them. In this case, the Custom Prompt in the prompt library defines a system prompt that is sent along with the general inline system prompt, which specifies the required response format. Gemini seems to ignore the first system prompt and return raw response text, e.g., just the code to be added.

Inline Assistant does not include the second system prompt in the request, so it doesn't suffer from this issue.

Overriding the Custom Prompt in my plugin configuration fixes the issue.

      prompt_library = {
        ["Custom Prompt"] = {
          strategy = "inline",
          description = "Prompt the LLM from Neovim",
          opts = {
            is_default = true,
            is_slash_cmd = false,
            user_prompt = true,
          },
          prompts = {
            {
              role = "user", -- CHANGED FROM "system" TO "user"
              content = function(context)
                return string.format(
                  [[I want you to act as a senior %s developer. I will ask you specific questions and I want you to return raw code only (no codeblocks and no explanations). If you can't respond with code, respond with nothing]],
                  context.filetype
                )
              end,
            },
          },
        },
      },

I tried to look into making Gemini models to work out-of-the-box with the Custom Prompt action, but was unsure what would be the cleanest way to do this.