A Case Note: What Japan’s Pension Simulator Shows About Sharing Government Code

May 25, 2026

In my previous blog post, I wrote about 18F’s “Eligibility APIs” initiative in the United States. The idea was to make social benefit eligibility logic reusable by state governments through APIs, but it ran into the division of responsibilities between the federal and state governments.

The initiative eventually shifted toward prescreener tools, whose code was made open source and reused outside government. If the 18F project is new to you, my previous post provides the background.

Japan has a different, but related, case of government code being made reusable outside government.

The Ministry of Health, Labour and Welfare (MHLW) shares the source code of its Public Pension Simulator with private companies, allowing them to modify and integrate it into their own applications.

However, this does not fit neatly into the familiar models of full open source or public API access. It is a third option: MHLW shares the source code only after each company has accepted the terms of use, held a prior discussion with the ministry, and submitted a formal application.

A Pension Estimation Tool Used More Than 11 Million Times
Since its launch in 2022, the Public Pension Simulator has been used more than 11 million times by the general public.

The simulator is not a system for determining legal pension entitlements. It is a low-friction, public-facing estimation tool designed to help users form a rough picture of their future pension.

Japan already has a separate service for more detailed pension estimates, where users are asked to enter detailed personal records such as work histories and contribution data. The Public Pension Simulator, by contrast, does not require personal information, allowing users to get a rough estimate more quickly and easily.

To make that possible, MHLW implemented a simplified estimation logic based on the pension system: it used approximations, excluded certain exceptions, and applied standardized figures.

MHLW provides “Important Notes for Users” for users to explain those assumptions and limitations behind the calculation. But written explanation is not the same as executable logic. To truly understand how those choices actually work in the calculation, you would need to be able to test inputs against outputs.

Why Reuse Became Conditional
Between 2022 and 2023, MHLW conducted a trial to explore whether the simulator’s code could eventually be released as open source and incorporated into private-sector services, such as financial apps and household budgeting tools.

According to publicly available materials from MHLW, several private operators were able to analyze and implement the code, and for a moment it looked as though a public-private ecosystem around pension estimation might emerge.

Yet MHLW did not proceed with an unconditional open-source release. The final arrangement was the conditional source-code provision described above.

A cautious reading is that the ministry could not fully distance itself from how code based on a public pension system would be used in private services.

Even with disclaimers and terms of use, a pension estimate shown in a private app could still be understood by users as connected to a government tool.

MHLW also had reason to worry that multiple pension estimates circulating through private-sector tools could confuse users about which results they should trust.

“Reusable Code” is Not “Free Code”
This case shows a practical truth: reusable code is not free code.

For private companies, receiving government code may look like an advantage. In practice, however, adapting it for their own products is only the first step.

They must also take on the burden of keeping the logic updated as pension amounts and rules change frequently, and continuing to verify the accuracy of the estimates.

They would also need to explain to users what kinds of assumptions and simplifications the estimate is based on. All of that would come at their own risk and expense.

The Key Takeaway
Policy is increasingly implemented through public-facing software. In some cases, governments may want that software to be reused outside government — not only through APIs, but by allowing private services to incorporate the code itself.

The Public Pension Simulator case shows what happens when government code is reused inside private services. The code in question is not an official computational model of the statutes, regulations, notices, and operational rules that make up Japan’s public pension system. It is a simplified estimation tool, based on that system and designed for public use.

But once private-sector reuse became plausible, the issue was no longer just whether companies could understand or integrate the code. It was what would happen after integration: how the logic would be updated, explained, and verified outside the original government service.

Japan’s Public Pension Simulator offers a concrete case study of that shift: from whether government code can be reused to what must be put in place once it is.

References
English titles of Japanese sources are informal translations by the author.

A Case Note: What Japan’s Pension Simulator Shows About Sharing Government Code

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