Japan's Wednesday auction of 20-year government bonds faces the toughest test in years as yields on that tenor surged to their highest since 1996. The sale comes amid a global bond rout, with super-long Japanese yields climbing to multi-year peaks. A similar auction a year ago drew the weakest demand in over a decade, and traders are bracing for thin participation.

Rising energy prices from the Middle East conflict have stoked inflation fears, pushing super-long yields higher. Prime Minister Sanae Takaichi called for a supplementary budget, and Reuters reported the government will likely issue fresh debt to cover new spending. Overseas investors are demanding higher yields, according to Yusuke Ikawa at BNP Paribas Securities Japan.

Traders now price a 76% chance of a Bank of Japan rate hike at its June meeting. Strategists at Barclays warned even elevated yields may not shore up demand. The bid-to-cover ratio will serve as the key gauge of whether the market can absorb new supply at these levels.