I am trying to set the default header parameters for org source blocks in `plantuml-mode`. 

My goal is to generate the result file in the org attachment directory with a generic name by default, and display it as an inline image in the result drawer.

This is my first attempt:

```lisp
(setq org-babel-default-header-args:plantuml
         `((:results . "file link") (:output-dir . ,(org-attach-dir-get-create)) (:file-ext . "png") (:file . "diagram.png") (:exports . "results")))
```

This works fine the first time, but then, the `org-attach-dir` string is prepended to the `file` path string at every code block execution.

So for example, if `org-attach-dir-get-create` returns "mydir/", the first time I execute the block, the `:file` value is `mydir/diagram.png` as intended. The second time I execute the block, I get `mydir/mydir/diagram.png`, the third time, `mydir/mydir/mydir/diagram.png`, and so on.

Please note that I am keeping `file-ext`, `file`, and `output-dir` as separated parameters in the hope of being able to override them individually if needed in specific cases.

I understood from that failure that the header-defaults are set at each block execution, so I tried to pass them all as a function:

```lisp
(defun my/org-babel-plantuml-header-args ()
  "Set dynamic header arguments for PlantUML source blocks."
  (let ((output-dir (org-attach-dir-get-create)))
    `((:results . "file link")
      (:output-dir . ,output-dir)
      (:file-ext . "png")
      (:file . "diagram.png")
      (:exports . "results"))))

  (setq org-babel-default-header-args:plantuml
         (my/org-babel-plantuml-header-args))
```

This has exactly the same outcome. It does not make sense to me, but my tests are very stubborn at proving me wrong.

I would appreciate some help in gettng it to work, because I used some of my most advanced  Elisp tricks for that one :) 

=== EDIT ===

After further testing, I jumped to the wrong conclusion when blaming this buggy behavior on the dynamic setting of the output directory.

I can reproduce it every time I set both `output-dir` and `file` within the `org-babel-default-header-args` variable even with a static string.

Regarding my initial goal, which was to have the diagram files created in the attachment directory, and displayed in the buffer, it is not possible to achieve using the header default arguments.

As @NickD commented, any function used there to set an option is only called once upon setting the `org-babel-default-header-args:plantuml` variable. This approach even had other adverse effects as I am using an org config file.

I managed to achieve the same results by using a custom function in a Yasnippet snippet to build the `output-dir` string. It works fine, just has a bit of a cluttered header.

Sorry if this thread is a bit all over the place. If future readers are looking to achieve the same kind of thing, I would gladly share the snippet code, and the custom hook function to set the attachment tag to the appropriate org node. I am not posting it because it uses Org-roam which is a niche use case I think, and I think it is off-topic considering how the thread started.