WASHINGTON — THE Obama administration has often stated its commitment to open government. So why is it keeping such tight wraps on the contents of the Trans-Pacific Partnership, the most significant international commercial agreement since the creation of the World Trade Organization in 1995?

The agreement, under negotiation since 2008, would set new rules for everything from food safety and financial markets to medicine prices and Internet freedom. It would include at least 12 of the countries bordering the Pacific and be open for more to join. President Obama has said he wants to sign it by October.

Although Congress has exclusive constitutional authority to set the terms of trade, so far the executive branch has managed to resist repeated requests by members of Congress to see the text of the draft agreement and has denied requests from members to attend negotiations as observers — reversing past practice.

While the agreement could rewrite broad sections of nontrade policies affecting Americans’ daily lives, the administration also has rejected demands by outside groups that the nearly complete text be publicly released. Even the George W. Bush administration, hardly a paragon of transparency, published online the draft text of the last similarly sweeping agreement, called the Free Trade Area of the Americas, in 2001.