A Seattle physician pleaded guilty Thursday to killing his partner and their 2½-year-old son in August 2011 at their First Hill penthouse apartment.

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A plea agreement reached Thursday has spared Dr. Louis Chen from the possibility of never getting out of prison for brutally stabbing his partner and slashing their son’s throat in August 2011.

The real battle between the state and defense will be over how much time the now- 43-year-old Chen should serve behind bars during a sentencing hearing that could take four or five days to complete, instead of the usual one to two hours.

Chen’s mental state at the time he killed Eric Cooper, 29, and their 2½-year-old son, Cooper Chen, will be the main focus, with the defense expected to call a number of mental-health experts to testify.

“It’ll be like a little trial,” King County Senior Deputy Prosecutor Don Raz said of Chen’s sentencing hearing, which hasn’t yet been scheduled.

Twenty-five years separates the defense’s sentence recommendation and the state’s, which could mean the difference between Chen gaining his freedom late in life or serving a de facto life sentence.

Chen — who was to stand trial in April — was originally charged with two counts of aggravated first-degree murder. But early on in the case, Prosecutor Dan Satterberg decided not to seek the death penalty, leaving life in prison without the possibility of release as the only possible punishment, had Chen been convicted as charged.

Raz on Thursday told Judge Bill Bowman the state agreed to amend the charges to better reflect Chen’s conduct and because of the risks to the state’s case in having to prove Eric Cooper’s murder was premeditated, and that the two killings were part of a common scheme or plan.

Chen pleaded guilty to premeditated first-degree murder for the death of Cooper Chen and intentional second-degree murder for Eric Cooper’s killing. Each charge also carries a deadly weapon enhancement, adding a total of four years to Chen’s prospective sentence.

He faces a standard-range sentence of 34 to 49 years, with the state planning to recommend he serve a top-end sentence of 49 years, according to Raz. As part of the plea deal, the state agreed to not seek an exceptional sentence above the standard range.

Chen’s defense team, however, will try to convince Bowman that Chen’s significant mental-health issues are mitigating factors that warrant a sentence below the standard range. They will recommend Chen serve 24 years in prison, the lowest possible sentence allowed.

By law, Chen must serve a minimum of 20 years on the first-degree-murder charge, plus another four years for the two deadly weapon enhancements.

“Dr. Chen has accepted responsibility for his conduct,” defense attorney Todd Maybrown said.

In spring 2011, Chen, an endocrinologist, moved to Seattle with his family from North Carolina for a job at Virginia Mason. When he didn’t show up for his first day of work on Aug. 11, 2011, a hospital manager went to Chen’s apartment, court records say.

Chen was naked and covered in dried blood when he answered the door, according to the records. Police found the bodies of Cooper, who had been stabbed more than 100 times, and their son, whose neck had been repeatedly cut, the records say.

Search warrants filed in the case indicate Chen was in the midst of an acrimonious breakup with Cooper.

Inside the apartment, homicide detectives found blood-soaked notes written by Chen, which they say provided evidence of a possible dual motive: that Chen feared Cooper would report his prescription-drug abuse and the abuse of his prescribing powers as a doctor, or use that information against Chen in a future child-custody dispute.

As Chen’s trial date approached, court documents revealed that the defense may have been planning to argue that cough syrup-induced psychosis contributed to the murders.