How can I boost the field length norm in elasticsearch function score?

It looks like you could achieve that using a field of type token_count together with a field_value_factor function score.

So, something like this in the field mapping:

"name": {   "type": "string",  "fields": {    "length": {       "type":     "token_count",      "analyzer": "standard"    }  }}

This will use the number of tokens in the field. If you want to use the number of characters, you can change the analyzer from standard to a custom one that tokenizes each character.

Then in the query:

"function_score": {  ...,  "field_value_factor": {    "field": "name.length",    "modifier": "reciprocal"  }}

I have something that kind of works. With the following, I deduct the length of a field of my interest from the score.

{ "query": {   "function_score": {     "boost_mode": "replace",     "query": {...},     "script_score": {         "script": "_score  - doc['<field_name>'].value.length()"     }   } }}

Hovever, I cannot control the relative weight of this number I am subtracting, compared to the old score. That's why I am not accepting my answer: I'll wait for better ones for a while. Ideally, I'd love to have a way to access the field length norm function within the script_score, or to get an equivalent result.