On Thursday, Japan's auction of 30-year government bonds will serve as a critical test of investor demand resilience, with global bond selloffs having already pushed long-term yields to levels not seen in nearly two decades. A weak result could send ripples through international financial markets and further elevate borrowing costs worldwide. Domestically, concerns over Prime Minister Takaichi Sanae's expansionary fiscal policies are amplifying these pressures.

Heading into Thursday's auction, the yield on Japan's 30-year government bond fell 7 basis points to 4.095%, pulling back from its highest level since the instrument's introduction in 1999. This decline followed strength in U.S. Treasuries overnight, driven by falling oil prices. While Tuesday's 10-year bond auction progressed smoothly, the 30-year tenor faces a more formidable challenge—global long-term yields remain elevated, and ultra-long bonds are particularly sensitive to fiscal conditions and shifts in supply-demand dynamics.

Barclays strategists, including Ayao Ehara, noted in a report: "We expect the overall result to be weak-to-soft. Given the recent rise in yields, current rate levels are already high, and from a long-term driver perspective, they are near fair value, but fiscal concerns continue to weigh."

Meanwhile, Japanese ministries have submitted record budget requests for the next fiscal year, intensifying market scrutiny over how new spending will be financed and how much additional debt the government might issue. This concern coincides with Japan's benchmark 10-year government bond yield reaching 3% for the first time in three decades. Additionally, Bank of Japan Governor Kazuo Ueda has hinted at a potential rate hike at this month's policy meeting, saying decisions will be made with upside price risks in mind. This follows repeated comments from U.S. Treasury Secretary Scott Bessent emphasizing the need for corresponding action.

Strategist Mark Cranfield observed: "Thursday's 30-year JGB auction could become another sore point for the U.S. Treasury market, with the risk of the Japan-U.S. long-term yield spread falling below 100 basis points."

"Weak auction conditions are gradually taking shape: USD/JPY has risen above the 160 level, G-10 bond markets are under broad pressure, and Japan's 30-year yield is only a few basis points away from its May peak. Once it breaks into a higher trading range, the Japan-U.S. yield spread will narrow to below 1%, making Japanese bonds more attractive on a relative value basis and potentially intensifying selling pressure on U.S. Treasuries," the strategist added.

Market watchers point out that a weak auction result could have repercussions beyond Japan, amplifying the global selloff and potentially complicating U.S. authorities' efforts to curb long-term Treasury yield increases. Prashant Newnaha, senior rates strategist for Asia-Pacific at TD Securities, remarked: "JGBs have long been the anchor of global fixed-income markets, but that role has now reversed. Continued JGB selling could trigger a repricing of global fixed-income assets."

Newnaha also noted that rising 30-year yields could "shift market focus back to fiscal policy rather than just monetary policy," emphasizing that Japan's current debt-to-GDP ratio is significantly higher than the last time 10-year JGB yields were around 3%.

However, there are reasons to anticipate buying interest. According to Den Miki, senior rates strategist at Sumitomo Mitsui Nikko Securities, the previous two 30-year bond auctions—both conducted when yields were near 4%—recorded relatively high bid-to-cover ratios, and life and non-life insurers have recently accelerated purchases of ultra-long bonds. Yet she noted that investors remain reluctant to significantly extend duration while the endpoint of the current yield rise remains unclear. Den expects the auction result to range between average and slightly weak.