"I work through an agency."

Do not accept "it's the other one's problem"

That is the standard opening move, and it is frequently wrong in both directions.

Labor Code § 2810.3 makes a client employer share civil legal responsibility for payment of wages and for failure to secure valid workers' compensation coverage. A waiver of § 2810.3 is void as against public policy, and retaliation for reporting a violation is prohibited.

Exemptions are narrow: businesses with fewer than 25 total workers, or using five or fewer workers from labor contractors.

And you may have two employers

Separately from § 2810.3, the company you were sent to may be your employer too under California's special-employment analysis — based on who directed your tasks, supervised you, set your hours, provided equipment and training, and could remove you from the assignment.

That cuts both ways, and honest advice has to say so.

It can make the client company liable for benefits. It can also make the client company immune from a civil suit, because workers' compensation is exclusive against an employer.

Sometimes the better answer is "the client is not a special employer, and it is a third-party defendant." The control facts decide it. The full analysis.

Gather this immediately

Both company names, exactly. Assignment paperwork, timecards, badge, pay stubs — and note which entity issued the check.

Who actually supervised you, by name. Who trained you. Who provided the equipment.

Photographs of the site, including signage or vehicles showing the client company's name.

If neither carried coverage

The Uninsured Employers Benefits Trust Fund pays benefits — but it is procedurally unforgiving, and the employer must be personally served and properly joined.

And § 3706 permits a civil action where § 3708 presumes negligence. The full route.

§ 3357 presumes you are an employee, and immigration status does not affect eligibility.