The 30-year Japanese government bond yield jumped 30 basis points last week, rising above 3.8 percent. The 40-year JGB yield climbed above 4 percent, marking record levels for both maturities.

Prime Minister Sanae Takaichi announced snap elections for Japan's lower house next month. Her platform includes tax cuts and higher defense spending, with a pledge to break free of excessive fiscal austerity.

The fiscal expansion outlook fueled concerns among investors about Japan's public finances. Some press reports drew comparisons to the United Kingdom's 2022 fiscal crisis, though Takaichi denied the parallel applies to Japan.

Japan's gross government debt has exceeded 200 percent of GDP for the past 15 years. Net government debt has remained well above 100 percent of GDP during that period.

Real-time analysis suggests Japan's fiscal position may be less severe than headline figures imply. The International Monetary Fund estimated Japan's government ran a deficit of about 2.5 percent of GDP last year, though other estimates place it closer to 0.5 percent.

Capital Economics projects that net government debt could fall to around 100 percent of GDP by the end of next year, a forecast that incorporates Takaichi's proposed tax cuts.

A competing explanation for rising JGB yields centers on a reassessment of Japan's economic growth, inflation and monetary policy outlook—one that accounts for a larger share of recent yield movements than fiscal fears alone.

Bond yields typically decompose into inflation compensation and real yields. For the 10-year JGB, most of the increase in nominal yields over recent months reflects higher inflation compensation. Since 2020, inflation compensation accounts for nearly all of the rise in the 10-year JGB yield, which currently stands around 2.82 percent, with roughly 2.0 percentage points of a prior 2.3 percent yield attributed to inflation compensation.

Japan's bond market has seen increased activity since the Bank of Japan abandoned yield curve control in March. For years, the central bank's policy kept rates suppressed, punishing investors who bet on rising yields in what was known as the widow-maker trade.

Yields have eased slightly since their peak, partly on speculation about a joint U.S.-Japan intervention to support the yen. Charles Gave said last week that Japanese government bond yields were trading above levels justified by fundamentals.