Under the plea agreement approved by U.S. District Judge Todd Campbell, Tammy Silas of Ardmore, Ala., faces 20 years in prison.
Yahir Anthony Carrillo was kidnapped on Sept. 29, 2009. The woman posing as the agent beat and stabbed his mother, police said, but Silas was never charged in the attack.
Yahir was found unharmed three days later at Silas' home in Ardmore, 80 miles south of Nashville. Silas, who was 39 at the time of the attack, was to have gone on trial Tuesday, but now won't, and her sentencing date was not listed in court papers.
The boy's mother, Maria Gurrola, was hospitalized for one day for her injuries, which included a collapsed lung.
Silas targeted Gurrola and followed her for a day before the kidnapping, authorities say.
Prosecutors have never said why Silas was not accused in the attack. Without the plea deal, she faced a maximum sentence of life in prison.
According to court documents, besides being beaten, Gurrola was attacked with beer bottles and a knife. Her 3-year-old daughter was not harmed.
Authorities said they traced the baby to Silas based on records for a vehicle she rented the same day as the abduction.
"The motivation for the crime was the defendant's desire to have a child," court documents said.
Silas had told reporters she purchased the baby, but authorities soon figured out that wasn't true.
Gurrola told investigators she had never seen the woman who stabbed her. According to the arrest warrant, Gurrola was targeted while she and a cousin, identified only as "JS," were running errands and visiting a state food assistance office.
The baby and his siblings were in foster care for a few days before being returned to their mother when the baby-selling allegations turned out not to be true.