Swansea striker Oli McBurnie, Hibernian midfielder Dylan McGeouch and Wolves full-back Barry Douglas are among six new faces in the Scotland squad.
New manager Alex McLeish has also called up Hearts keeper Jon McLaughlin and Aberdeen defender Scott McKenna.
McLeish had already confirmed Manchester United midfielder Scott McTominay would be involved.
Costa Rica visit Hampden on 23 March, with another friendly in Hungary four days later.
McLeish, beginning his second spell as national manager, has to select a new captain after Celtic’s Scott Brown retired from international football.
Former skipper Darren Fletcher has been left out of the squad.
Brown’s Celtic team-mates Leigh Griffiths and Craig Gordon are currently injured, but Stuart Armstrong is included in the 27-man pool having only just returned to fitness.
Fulham midfielder Kevin McDonald, Jamie Murphy, on loan at Rangers, and goalkeeper Jordan Archer are uncapped players who have been in previous squads.
In the absence of Gordon, Hull City keeper Allan McGregor, 36, may play for the first time since winning his 36th cap against Canada a year ago.
McLeish gave McGregor his first international start during his 10-match reign in 2007.
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Experienced trio James Morrison, Barry Bannan and Steven Fletcher are injured but Ikechi Anya, James McArthur, Robert Snodgrass and Chris Martin, all regulars under previous manager Gordon Strachan, are not selected.
McBurnie, 21, joined Barnsley on loan in January and has scored five goals in seven appearances.
Former Dundee United defender Douglas, 28, has been impressive since moving to Wolves last summer following spells in Poland and Turkey. He has scored four goals and provided 11 assists for the Championship leaders.
McKenna, 21, was on loan with Ayr United last season but is now an established starter at Pittodrie, while McLaughlin, 30, has turned in a string of good displays for Hearts this season.
English-born McTominay, 21, whose father is Scottish, pledged his international future to Scotland following talks with McLeish.
The imposing central midfielder has played 17 times for Manchester United this season, starting the last five games.
Scotland will play two more friendlies at the end of the season, travelling to play Peru and Mexico.
Goalkeepers: Jordan Archer (Millwall), Allan McGregor (Hull City) Jon McLaughlin (Hearts)
Defenders: Barry Douglas (Wolves), Grant Hanley (Norwich), Russell Martin (Norwich, on loan at Rangers), Scott McKenna (Aberdeen), Charlie Mulgrew (Blackburn), Callum Paterson (Cardiff), Andrew Robertson (Liverpool), Kieran Tierney (Celtic).
Midfielders: Stuart Armstrong (Celtic), James Forrest (Celtic), Tom Cairney (Fulham), Ryan Fraser (Bournemouth), Kevin McDonald (Fulham), Kenny McLean (Aberdeen), Dylan McGeouch (Hibernian). John McGinn (Hibernian), Callum McGregor (Celtic), Scott McTominay (Manchester United), Jamie Murphy (Brighton, on loan at Rangers), Matt Ritchie (Newcastle), Ryan Christie (Celtic, on loan at Aberdeen)
Forwards: Oli McBurnie (Swansea, on loan at Barnsley), Jason Cummings (Nottingham Forest, on loan at Rangers), Matt Phillips (West Brom)