The CP2100 notice is the IRS letter no founder wants to open. It arrives nine months after the 1099 you filed, lists every name and TIN that did not match IRS records, and starts the penalty clock for every one of them. The fix is to never file the bad TIN in the first place, which means catching the W-9 error at intake.
This post catalogs the seven specific W-9 mistakes responsible for most CP2100 notices. Each one is preventable. Most can be caught in under a minute if you know what to look for. The ones that cannot be caught visually are exactly the ones a TIN verification tool catches before the form ever enters your records.
Mistake 1: Personal Name on Line 1, EIN in Part I (or Vice Versa)
The most common W-9 error of all. The contractor writes their personal name on Line 1 ("Maria Santos") and puts their LLC's EIN ("XX-XXXXXXX") in the TIN section. Or the inverse: LLC name on Line 1 ("Santos Consulting LLC"), personal SSN in Part I.
The IRS treats the name and the TIN as a paired identity. A personal name belongs with an SSN. An entity name belongs with an EIN. Mix them and the IRS responds with "no match" because the name and the TIN belong to different taxpayers in their records.
The fix at intake: read Line 1 and Part I together. If Line 1 looks personal and Part I is an EIN, send it back. If Line 1 looks entity-shaped and Part I is an SSN, send it back. A 30-second visual check resolves this one without any tool.
Why this catches you anyway: contractors who own single-member LLCs are routinely confused about whose name goes where. The IRS rule is that single-member LLCs are disregarded entities for tax purposes, so the owner's personal name and SSN are required even though the LLC has its own EIN. The contractor will swear they did it right.
Mistake 2: Maiden Name or Married Name Mismatch
The contractor recently got married or divorced. Their Social Security card still shows their birth name but they wrote their married name on the W-9. Or vice versa.
The Social Security Administration tracks legal name changes separately from the IRS. If the contractor has not yet updated their SSA record (or has but the IRS database has not caught up), the name on their W-9 will not match what the IRS has on file.
The fix at intake: there is no visual signal for this mistake. The W-9 looks correct. The contractor believes they wrote their legal name. The only way to catch this before filing is real-time TIN verification, which returns "match on TIN, no match on name" and identifies the issue. Without verification, the mismatch surfaces in the CP2100 notice nine months later.
The conversation with the contractor is: "The IRS database shows your name differently from what is on the W-9. Can you confirm what is on your most recent Social Security card?" This usually surfaces the legal-name-change history in under a minute.
Mistake 3: Nickname or Preferred Name Instead of Legal Name
Robert puts "Bob" on the W-9. Catherine writes "Cathy". Jonathan signs as "Jon". The Social Security records all use the full legal first name. The IRS database matches what Social Security has.
This one accounts for a surprising share of name-mismatch errors. It is not malicious or careless on the contractor's side, since most people genuinely use their preferred name in professional contexts. But the W-9 requires the legal name, not the preferred one.
The fix at intake: any time the Line 1 first name reads like a common shortening (Bob, Jim, Mike, Liz, Jen, Sam, Dan, Tim, Tom, Ron, Bill, Ted, Sue), ask the contractor to confirm whether the full legal first name is something different. About one in three flags a real shortening. The clarification is friendly, since the contractor is not at fault for using their normal name.
Real-time verification catches this exact pattern when it returns "no match on name". TinCheck will not return the legal name (the IRS does not expose that), but the mismatch signal alone is enough to know to ask.
Mistake 4: ITIN Submitted as SSN, or SSN Submitted as ITIN
The TIN field accepts both SSNs (XXX-XX-XXXX format) and ITINs (9XX-XX-XXXX format, where the first digit is always 9). They look similar, and a contractor with both an SSN and an ITIN might submit the wrong one.
ITINs are issued to non-resident individuals who are not eligible for an SSN, or to certain dependents and spouses on tax filings. If a contractor has both numbers (some do, particularly green card holders who got an ITIN before their SSN), the W-9 should carry the SSN. Their work authorization changed; the active tax identifier did too.
The fix at intake: look at the first digit of the nine-digit TIN. If it is 9, the contractor submitted an ITIN. Confirm with them whether they meant to. For US-resident contractors with work authorization, the answer is almost always "I should have used my SSN."
This is the kind of error that compounds: the contractor uses the same wrong number across many W-9s over years, so every payor reporting on them generates a CP2100 against the same identity mismatch.
Mistake 5: Old or Inactive EIN
The contractor's LLC dissolved, then they restarted under a new entity. They wrote the OLD EIN on the W-9 because that is the one they remember.
Dissolved-entity EINs are not reused, but they are also not always flagged immediately in the IRS system. The 1099 you file with that EIN will get rejected at filing time or trigger a CP2100 with a "TIN not valid" code.
The fix at intake: this is invisible visually. The EIN format looks correct. Only real-time verification surfaces it, since TinCheck and the IRS TIN Matching service check against the live active-entity database. A check that returns "no match" on an EIN that the contractor swears is correct often points to an inactive entity in the IRS records.
Mistake 6: Box on Line 3 Does Not Match the TIN Type
Line 3 of the W-9 asks the contractor to check a federal tax classification: Individual / sole proprietor, C Corporation, S Corporation, Partnership, Trust/estate, LLC (with sub-classification), or Other.
The mismatch error: contractor checks "S Corporation" on Line 3 but provides an SSN in Part I. S Corporations file under EINs, not SSNs. The classification and the TIN type do not match.
This one is dangerous because Line 3 also drives the contractor's exemption status for backup withholding. A mis-checked Line 3 paired with a mismatched TIN type creates a paperwork problem that propagates through your 1099 filing and your backup withholding obligations at the same time.
The fix at intake: cross-check Line 3 against Part I. Individual or sole-prop on Line 3 means SSN expected. Any entity classification (C-Corp, S-Corp, Partnership, Trust) means EIN expected. LLC checkbox requires the sub-classification (single-member, partnership, or corporation) which then dictates the TIN type.
Mistake 7: Address on Line 5/6 Is Undeliverable
The contractor submits an old address, a typo'd street name, or an address that USPS cannot deliver to (vacant lot, demolished building, an apartment without the unit number).
This one does not trigger CP2100 directly. CP2100 is about TIN matching. But undeliverable addresses cause a downstream problem: the 1099-NEC you mail to the contractor in January gets returned, the contractor never receives their copy, and if they were planning to use it to file their own taxes they are now coming to you in April asking for a reissue. Multiply across 20 contractors and you spend February doing nothing but tracking down good addresses.
The fix at intake: USPS Address Validation. TinCheck includes this as a free byproduct of every TIN check. The dashboard returns "deliverable" or "non-deliverable" alongside the TIN result. The IRS portal does not do this.
For founders running the free IRS portal alone, the workaround is to manually verify addresses through the USPS address-lookup tool. It works but it adds a step per contractor.
What No Visual Check Will Catch
Five of the seven mistakes above can be caught visually if you read the W-9 carefully. Two cannot: the legal-name mismatch (mistakes 2 and 3) and the inactive-EIN problem (mistake 5). For those, real-time TIN verification is the only intake-time signal.
That is the operational case for paying for a verification tool when you could have used the free IRS portal. The free portal does the verification but with a 5-to-14-day enrollment lead time and no real-time API. For a contractor who needs to be paid this week, the verification has to happen now or it does not happen at all.
The full comparison of TinCheck against the free IRS option is in the TinCheck vs IRS post. The 5-minute workflow for running a verification is in the how-to post. The penalty math is in the IRS penalty guide.
Building the Intake Checklist
The cleanest implementation of the seven-mistake check at intake is a one-page checklist your accounts payable lead runs before approving any new contractor. The checklist:
- 1Line 1 name and Part I TIN type match (personal name + SSN, or entity name + EIN)
- 2First name on Line 1 is the full legal name, not a nickname
- 3Line 3 federal tax classification matches the TIN type
- 4TIN first digit confirms the type (9 indicates ITIN, anything else SSN; entity gets EIN)
- 5Address on Lines 5/6 is current and deliverable
- 6Real-time TIN verification returns "match" on the name and TIN pair
- 7The verification result is attached to the contractor record
Steps 1 to 5 are visual checks taking about a minute. Step 6 is the TinCheck API call, taking about five seconds. Step 7 is the file-attachment step, taking about thirty seconds. Total time: under three minutes per contractor at intake. Total saving: one CP2100 notice nine months later, which can cost between $60 and $660 per form depending on when corrected.
The economics of this checklist favor running it on every contractor. The marginal cost of a check is well under a dollar. The marginal cost of a missed mistake is a four-figure penalty plus the operational drag of resolving a CP2100 case.
FAQ
Next Step
Pick the W-9 from your most recently onboarded contractor. Run it against the seven-mistake checklist. If you have not done a TIN verification on them yet, run that as your seventh check.
Claim the free first TinCheck verification through SaaSOffers.
Software engineer and product builder with 13+ years of experience across software engineering, product development, and startup operations. Built SaaSOffers to make every startup deal discoverable and verified for founders worldwide.
Ready to unlock these deals?
Free account. No credit card required.