Through trial and error, I was able to determine that Salesforce's limit on the URI length is 16,088 characters.
Beyond that, Salesforce begins returning
System.HttpResponse[Status=Request Header Fields Too Large, StatusCode=431]
At 16,410 characters long, Salesforce returns
System.HttpResponse[Status=URI Too Long, StatusCode=414]
And finally, once the URI hits 32,435 characters, Salesforce croaks and we get the following exception
System.CalloutException: Unexpected end of file from server
This was determined with the following anonymous apex
// Setting up things for the JWT OAuth flow
// My connected app is set for admins selecting who is pre-authorized
String iss = '<client key of connected app>';
String aud = 'https://test.salesforce.com';
String sub = '<username for sandbox>';
Auth.JWT jwt = new Auth.JWT();
// 'Sandbox_Data_Populator' is the unique name of a self-signed certificate stored
// in Salesforce.
// This same cert is used as the digital signature for my connected app
Auth.JWS jws = new Auth.JWS(jwt, 'Sandbox_Data_Populator');
String tokenEndpoint = 'https://cs52.salesforce.com/services/oauth2/token';
Auth.JWTBearerTokenExchange bearer = new Auth.JWTBearerTokenExchange(tokenEndpoint, jws);
String accessToken = bearer.getAccessToken();
// Time to build the REST request OF DOOM!
// I have an Apex webservice whose URI is /services/apexrest/recordType
String endpt = '/recordType?s=';
// Start making the value to place in the query string
String qs = 'repeat';
// make it nice and long
endpt += qs.repeat(2725);
// and add some fine grain control
endpt += 'a'.repeat(1);
// Make the callout
// In this case, I'm calling out to the same org that I'm running this anonymous apex in
req.setEndpoint('https://cs52.salesforce.com/services/apexrest' + endpt);
req.setHeader('Authorization', 'Bearer ' + accessToken);
res = httpObj.send(req);
The precise values at which Salesforce returns the different HTTP codes and exception were determined by tuning
qs.repeat(2725); using a binary-ish search approach.
I started at
qs.repeat(5000);, went down to
qs.repeat(2500);, up to
qs.repeat(2750);, etc... until I found the coarse limit, then I adjusted
'a'.repeat(1); (only need to go from 0 to 5) to find the exact limit.
Keep in mind that this limit appears to be for the entire URI. If you're using a longer URI, like say "https://my.custom.domain.salesforce.com", that'll eat up a few more characters than my "https://cs52.salesforce.com" does.
We can have up to 800 custom fields (or even more if you have managed packages installed) at a maximum of 60 characters each (40 for field label +
__c + 15 for namespace +
__), so it is possible to hit this limit if you attempt to query all fields on an object through the REST API...but that's not something most people will need to worry about.