Short answer. Yes, but not for acts beyond authority. Under Article 1895, where solidarity has been agreed upon, each agent is responsible for the non-fulfilment of the agency and for the fault or negligence of his fellow agents — except, in the latter case, when the fellow agents acted beyond the scope of their authority.
What the law says
each of the agents is responsible for the non-fulfillment of agency, and for the fault or negligence
Civil Code, Article 1895 — Effect of Agreed Solidarity. Read the full provision →
Solidarity makes each answer for the others
When agents have agreed to be solidarily liable, the protective default that each answers only for himself is replaced by a much heavier regime. Article 1895 spells it out: if solidarity has been agreed upon, each of the agents is responsible for the non-fulfillment of agency, and for the fault or negligence of his fellow agents. So under an agreed solidarity, one agent can be held for the whole — not only for his own failure to perform, but for a co-agent's fault or negligence in the task.
The exception: acts beyond authority
There is one important carve-out, and it is precise. The responsibility for a fellow agent's fault or negligence does not extend to acts the fellow agent did beyond the scope of their authority. So if a co-agent went outside the mandate — did something he was never authorised to do — the other agents are not solidarily liable for that, even under an agreed solidarity. The solidarity binds them to answer for one another's faults committed within the agency; it does not make each a guarantor of a co-agent's frolic outside it.
Non-fulfilment versus a fellow's fault
It is worth separating the two limbs of the responsibility, because the exception attaches to only one. The first limb — responsibility for the non-fulfilment of the agency — makes each solidary agent answerable if the agency is simply not carried out; that is the core of what solidarity means and it has no authority-based exception. The second limb — responsibility for a fellow agent's fault or negligence — is the one qualified by the beyond-authority carve-out. So each agent guarantees that the agency gets performed, full stop; but he guarantees a co-agent's careless or faulty conduct only where that conduct was within authority. Keeping the two apart matters when a loss is traced to a specific act: was it a failure to perform, or a fellow's fault, and was that fault within his authority?
If your agents are solidarily bound
If you stipulated that your agents would be solidarily liable, you gained a powerful position: you may hold any one of them for the whole loss, whether it came from the agency not being performed or from a co-agent's fault within the task. You need not apportion blame before recovering. But note the limit — an act a co-agent did outside his authority is not something the others answer for, even here. For an agent who has agreed to solidarity, the message is that you are now exposed to your co-agents' mistakes within the agency, so choose your co-agents carefully and keep them within their authority.