Short answer. Yes. The law itself tells an agent not to carry out the agency if doing so would manifestly result in loss or damage to the principal, so a clearly harmful instruction is not one your agent is obligated to blindly execute, even where you gave broad authority.
What the law says
An agent shall not carry out an agency if its execution would manifestly result in loss or damage to the principal.
Civil Code, Article 1888 — No Execution That Damages the Principal. Read the full provision →
The rule runs against blind obedience
Ordinarily an agent's job is to follow the principal's instructions, and an agent who deviates without cause takes on real risk. This article carves out the opposite situation: an agent shall not carry out an agency if its execution would manifestly result in loss or damage to the principal. Rather than making obedience the highest value, the law puts the principal's own interest ahead of mechanical compliance, on the theory that an agency exists to serve the principal, not to be executed for its own sake.
"Manifestly" is doing real work in this sentence
The article does not free an agent to second-guess every instruction they personally dislike or think is unwise. The word manifestly sets a high bar — the resulting loss or damage has to be obvious, not a matter of the agent's private judgment call about what would work out better. An agent who has doubts about whether an instruction is a good idea is in a very different position from one facing an instruction whose harm to the principal is plain on its face.
This is about the execution, not a general veto power
Read carefully, the article is narrow: it addresses what the agent should not carry out, not what the agent may unilaterally decide to do instead. It stops the agent from following through on a manifestly damaging instruction; it does not, by its own text, hand the agent open-ended authority to substitute their own plan for yours. Where the line falls between refusing to execute and taking some other action on your behalf will depend heavily on the specific instruction and what the rest of your agency agreement says.
Why this matters if you feel your agent overstepped, or underdelivered
This article cuts both ways. If your agent went ahead with something manifestly damaging to you, its text is squarely relevant to whether they should be answerable for that. If instead your agent refused an instruction you think was reasonable, the same article is your agent's likely justification — and whether that justification holds up turns on whether the harm really was manifest, not just arguable. Either way, this is the specific rule you are working from, not a general standard of reasonableness you can invoke on its own.