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Getting a CP2100 doesn’t mean you’re being audited or penalized. It means the IRS found a mismatch between payee names and TINs on your 1099 filings and is giving you a structured opportunity to fix it. The notice triggers a specific, time-sensitive process, but it’s entirely manageable if you act within the deadlines.
This guide covers what the notice means, what you’re required to do, the first/second B-notice distinction that trips people up, and how filing through IRIS changes the picture.
What is a CP2100 notice?
A CP2100 is the IRS’s way of telling you that one or more payee name and TIN combinations on your filed 1099s don’t match IRS records. The notice includes a list of affected payees, the TINs as you reported them, and instructions on what to do next.
The IRS issues two versions. CP2100 goes to payers who filed 50 or more returns with errors. CP2100A goes to payers with fewer than 50 errors. The content and required response are identical — the only difference is volume. (IRS — Understanding your CP2100 or CP2100A Notice)
When does the IRS send CP2100 notices?
The IRS issues CP2100 and CP2100A notices twice a year — in October and the following April. October notices cover returns processed earlier in the year. April notices catch mismatches from corrected filings submitted after the original deadline.
If you filed 1099s in January or February 2026, expect any mismatches to surface in October 2026 notices. If you filed corrections in March or April, those may appear in the following April’s notices.
What is a B-notice and who sends it?
This is where confusion is common. The IRS does not send B-notices. You do.
A B-notice informs the payee that their name and TIN don’t match IRS records and tells them how to correct it. There are two types, each with different requirements. (IRS Publication 1281 — Backup Withholding for Missing and Incorrect Name/TINs)
First B-Notice
Sent when a payee appears on a CP2100 for the first time within a three-year period. Include a blank Form W-9 and ask the payee to provide their correct name and TIN. Send within 15 business days of the CP2100 notice date or the date you received it, whichever is later. Sample First B-Notice language is provided in Appendix A of IRS Publication 1281. See also: 1099 correction penalties.
Second B-Notice
Sent when the same payee appears on a CP2100 again within three years. Do not include a Form W-9 — the payee must contact the IRS or SSA directly to certify their TIN. For individuals, this means providing an SSA Form 7028 (Social Security Number Verification) or a copy of their Social Security card. For businesses, it means obtaining IRS Letter 147C to validate their EIN. Accepting another W-9 in response to a Second B-Notice does not satisfy the requirement. Sample Second B-Notice language is provided in Appendix B of IRS Publication 1281.
One procedural detail that’s easy to miss: your envelope must be clearly marked “IMPORTANT TAX INFORMATION ENCLOSED” or “IMPORTANT TAX RETURN DOCUMENT ENCLOSED.” This is an IRS requirement, not a suggestion. (IRS Publication 1281)
You’re responsible for tracking first vs. second notice status per payee. The IRS does not tell you which applies, you maintain that record.
The backup withholding clock
Backup withholding is a federal requirement to withhold a percentage of future payments to a payee and remit it directly to the IRS. It exists as an enforcement mechanism: if a payee won’t provide a correct TIN, the IRS ensures tax is collected at source instead.
If a payee doesn’t respond to your B-notice, backup withholding on future payments becomes mandatory.
For a First B-Notice: begin backup withholding no later than 30 business days after the CP2100 notice date or the date you received it, whichever is later, if the payee hasn’t responded. The current backup withholding rate is 24%.
For a Second B-Notice: allow 30 business days for the payee to produce SSA Form 7028 or IRS Letter 147C. Begin backup withholding if neither arrives by that deadline.
Once a payee responds with a valid W-9 (first notice) or official TIN certification (second notice): stop backup withholding no later than 30 calendar days after you receive it.
Backup withholding collected must be reported by filing Form 945, Annual Return of Withheld Federal Income Tax. Deposit amounts under the monthly or semi-weekly schedule per IRS Publication 15.
One important edge case: if a payee doesn’t respond and you make no further payments to them, you are not required to pay backup withholding. But you should document that you sent the B-notice and received no response.
Step-by-step response checklist
As soon as you receive the CP2100:
- Compare each payee on the IRS list against your own records
- If the mismatch is your data entry error — not the payee’s — correct your records and and transmit the correct information to the IRS
- For each remaining payee, determine whether this is their first or second CP2100 appearance within three years
Within 15 business days of the CP2100 notice date or the date you received it, whichever is later:
- Send First B-Notice with a blank Form W-9 (first-time payees)
- Send Second B-Notice without a W-9, directing payee to IRS or SSA (repeat payees within three years)
- Mark all envelopes “IMPORTANT TAX INFORMATION ENCLOSED”
- Retain copies of all notices sent
By 30 business days after the CP2100 notice date or receipt date, whichever is later, if no response:
- Begin 24% backup withholding on future reportable payments
Once the payee responds:
- Verify the corrected TIN actually matches IRS records before closing out the notice — a new W-9 with an unverified TIN doesn’t resolve the problem
- Stop backup withholding within 30 calendar days of receiving valid certification
Ongoing:
- File Form 945 to report backup withholding collected
- Retain all records for at least four years
Common mistakes
Sending a Second B-Notice with a W-9
Not permitted. The payee must certify their TIN through SSA or IRS — self-certification on a W-9 is not accepted for a second notice.
Missing the 15-business-day window
The clock starts from the CP2100 date, not the date you opened it. Check your mail regularly — the IRS mails these to your address of record.
Not marking the envelope correctly
A B-notice sent in an unmarked envelope doesn’t satisfy the IRS requirement, even if the contents are correct.
Accepting a new W-9 without verifying the TIN
If a payee provides updated information but the name/TIN combination still doesn’t match IRS records, you haven’t resolved the issue. Another CP2100 notice is coming. Always run TIN matching after receiving a corrected W-9.
Sending two B-notices when you receive two CP2100 notices for the same vendor in the same calendar year
If the same payee appears on both the October and April notices within a 12-month period, you only send one B-notice — the second CP2100 appearance counts as the second in the three-year cycle for tracking purposes, but does not require a second mailing. Sending a duplicate B-notice is unnecessary and can confuse the payee.
Using the wrong backup withholding rate
The current rate is 24%. The old rate was 28%. Using the old rate is still a common error in organizations that haven’t updated their procedures.
What changes if you filed through IRIS
With FIRE, TIN mismatches were invisible until the CP2100 arrived months after filing. IRIS validates TINs at submission — so many of the errors that would have generated an October CP2100 now surface immediately as “Accepted with Errors” when you file in January or February. For a full breakdown of how IRIS differs from FIRE, see our FIRE to IRIS transition guide.
The practical implication: organizations that filed 2025 returns through IRIS and corrected “Accepted with Errors” records at submission may receive fewer CP2100 notices this October. The mismatches were caught earlier in the process.
But IRIS doesn’t eliminate CP2100 notices. The IRS matching program runs its own validation after processing, and can flag discrepancies that IRIS’s submission-time check missed. CP2100 remains the IRS’s downstream catch; it covers mismatches across all filing systems, including records IRIS accepted without flagging. If you’re on IRIS, think of CP2100 as a secondary layer, not a system you’ve bypassed.
How to prevent CP2100 notices before next filing season
The best time to address TIN accuracy is before you file, not after a CP2100 arrives.
Collect Form W-9 before issuing any payment. Chasing a W-9 after the fact is harder and creates gaps in your records. Make it a condition of onboarding.
Use the IRS TIN Matching program. Free for most 1099 filers, it lets you verify name/TIN combinations against IRS records before filing. Bulk TIN matching in the fall — before filing season — catches errors while there’s still time to solicit corrections without penalty pressure.
Re-verify recurring vendors annually. Business names change, EINs get reassigned, individuals marry and change names. A vendor who matched correctly last year may not match this year.
Audit sole proprietors specifically. The most common source of TIN mismatches is a sole proprietor whose DBA name appears on Line 1 of the W-9 instead of their legal personal name. The IRS matches against the individual’s legal name registered with the SSA (not the business name). This is the same issue that generates IRIS Error 015 at filing. Getting the name format right on the W-9 prevents both.
How Tab Service handles B-notices on your behalf
Tab Service manages B-notice processing as part of the TAB1099 annual compliance service. When a CP2100 arrives, we identify affected payees, generate compliant First and Second B-Notice letters, handle the mailing, and monitor responses. Clients receive a clean audit trail of all notices sent, responses received, and backup withholding status. All of this documentation matters if the IRS ever issues a 972CG penalty notice.
For organizations that want to prevent CP2100 notices from arriving in the first place, our TIN verification service runs bulk matching against IRS records before filing season — catching mismatches before they become notices.
Sources: IRS — Understanding your CP2100 or CP2100A Notice; IRS Publication 1281 — Backup Withholding for Missing and Incorrect Name/TINs; IRS Publication 15 — Employer’s Tax Guide; IRS Form 945.
