Any one of these ends the purchase
The COA guide told you what a good document contains. This is the shorter, more useful list: what makes you close the tab.
1. No COA available at all
Walk away. There is no version of this where the absence of a certificate is fine. A vendor who will not publish testing has either not tested, or has tested and does not like the result.
A peptide without a certificate of analysis is a gamble with no way to verify purity, identity, or the absence of contaminants.
2. In-house testing only
If the laboratory that ran the test is owned by the company selling the product, you have a conflict of interest, not an audit.
Third-party means an independent laboratory with its own name, address and reputation on the document. This is not pedantry. The entire value of a COA comes from the tester having nothing to gain from the result.
3. Purity below 95%
Unacceptable for research peptides. The standard is ≥98%, and 95% is already the floor rather than a target.
Note that some vendors will publish a 92% or 93% result and present it as transparency. Transparency about a substandard product is still a substandard product.
4. Missing identity confirmation
No mass spectrometry data means you do not know what is in the vial. You may know it is 98% pure. You do not know 98% pure what.
This is the most commonly missing test on otherwise professional-looking COAs, and it is the one that would catch the worst failure mode: receiving an entirely different compound.
5. A generic PDF not matched to a batch number
A COA describes one specific manufacturing lot. If the document has no batch number, or the batch number does not match the one printed on your vial, it is a certificate about somebody else's product.
Batch-specific testing is the difference between "this company has produced a clean batch at some point" and "the vial in your hand was tested".
The pattern underneath all five
Each red flag is a way of appearing to provide verification without actually providing it. No COA is honest failure. The other four are the appearance of rigour without the substance, which is more dangerous, because it disarms exactly the scrutiny a certificate is meant to invite.
When you find one of these, the correct response is not to negotiate or ask for clarification. It is to find another source. There are enough vendors meeting the full standard that compromising on it is a choice rather than a necessity.