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San Francisco Supervisor John Avalos is expected to introduce on Tuesday a scaled-back version of a mandatory local hiring bill after reaching a tentative agreement with labor unions, Avalos and a union official said Monday.

In its first year, the legislation would require construction firms contracted by San Francisco to hire at least 25 percent local residents for city jobs. The minimum would be raised by 5 percentage points every year thereafter until it is capped at 50 percent in 2016.

In addition, the city, contractors and unions would have an opportunity to review the program and make changes after three years — effectively an “abort” button if the plan does not go smoothly.

The mandated hiring levels in the amended version have been substantially trimmed since Avalos last month introduced the original bill, which would have initially required contractors to hire 30 percent local workers, ramping up to 50 percent within three years.

A union official who reviewed the amendments said the changes made the legislation acceptable — if only barely.

“There are some relief valves to prevent this legislation from doing actual harm,” said Michael Theriault, the secretary-treasurer of the San Francisco Building & Construction Trades Council, a group of 28 construction unions affiliated with the AFL-CIO. “It’s gone from being a potentially fatal illness to a low-grade fever.”

Avalos sounded cautious on Monday.

“I’m just hoping that we’ll come to some consensus,” he said. “The ones who are opposed to this will just say no. Hopefully they won’t take other action.”

The debate over the ordinance reflects a growing economic disconnect between those at the center of San Francisco's thriving, information-based economy and the other skilled workers who are left out of it.

For years, city rules have required building firms to show only that they’ve made “good-faith efforts” to hire at least half local workers. But surveys conducted by the city and by independent groups found that residents occupied just a quarter of city-funded building jobs.

So when Avalos initially introduced his bill in October after working on it for a year, it was lauded by community groups and supervisors representing some of the most economically troubled neighborhoods of San Francisco — such as Sophie Maxwell of District 10, which includes the Bayview — as a measure to spur job growth.

But union officials said that the local hiring plan was unrealistic simply because there were not enough qualified residents to meet the hiring mandate. Theriault said that union locals drew their members from all over the Bay Area, and only a small fraction lived in San Francisco.

Thierault told The Bay Citizen last month that San Francisco "would have to pull more than 1,000 journey-level workers out of thin air" to comply with the law.

Although he supported Avalos’ effort to bolster local employment, Thierault said Monday that the bill still failed to address what he viewed as an underlying problem: There are not enough schools with shop classes, and not enough apprenticeship programs, he said.

“I have a basic conceptual difficulty with any local hiring bill that deals with journey-level hires as opposed to advancing apprenticeship,” he said. “We should strengthen the pipeline.”