A demurrer asks one question: taking the complaint's own allegations as true, does it state a claim. Whether outside evidence might undercut those allegations is a question for summary judgment, later in the case.
The rule
Blank v. Kirwan, 39 Cal.3d 311 (1985) is the controlling authority: a demurrer can't reach outside the pleading to resolve a factual dispute. Two real California rulings show how this plays out when defense counsel tries anyway.
In Tsirtsis v. Nordean Law, APC, Case No. 25-01478811 (Orange County Superior Court, Dept. W15), the defense asked the court to use a prior summary judgment ruling from the underlying case (something outside the malpractice complaint itself) to argue the plaintiff couldn't show damages or causation. The court's response: "The record before the Court at this stage in the proceedings is insufficient to make such a finding... the demurrer... is OVERRULED."
In Gordy v. Shernoff, Bidart, Echeverria, LLP, Case No. 26STCV00683 (Los Angeles County Superior Court, Dept. 516), the defense argued that the attorneys had withdrawn from the plaintiff's prior case, relying on documents outside the complaint to make the point. The court, citing Blank v. Kirwan by name: "the Court cannot rely on the items for Requests for Judicial Notice... arguments relating to events that took place outside the four corners of the FAC are inapplicable."
Why this keeps happening
The outside evidence is often persuasive: a prior ruling, a documented withdrawal, a fact that would matter a great deal at trial. That's exactly what makes it tempting to raise on demurrer. The test is whether it's in the complaint. If it isn't, a demurrer is the wrong vehicle to raise it, no matter how strong it looks.
The bottom line
If you're defending a malpractice claim on demurrer, keep your argument inside the four corners of the complaint, and save the outside facts for summary judgment, where they belong. If you're the plaintiff and a demurrer against you leans on anything not alleged in your own complaint, that's a real, citable basis to push back, regardless of how compelling that outside fact might otherwise be.
Methodology note: illustrated by two real California trial-court rulings (Case No. 25-01478811, Orange County Superior Court; Case No. 26STCV00683, Los Angeles County Superior Court), both non-precedential tentatives you can look up on their county dockets, cited here for the binding authority the courts themselves invoked (Blank v. Kirwan), which is the published, citable law. This is a statement of established doctrine, not a corpus-frequency claim. Nothing here is legal advice.