The HSR Trap: How Antitrust Review and Premerger Filing Requirements Can Delay or Kill Your Startup Acquisition

Your startup is being acquired. The LOI is signed, diligence is proceeding, and the closing timeline looks tight but manageable. Then your M&A counsel asks a question that stops the deal momentum cold: "Does this transaction trigger an HSR filing?" You have never heard of HSR. You assumed antitrust review was something that happened to Fortune 500 mergers, not startup acquisitions. But the Hart-Scott-Rodino Antitrust Improvements Act of 1976 applies to transactions of any size — including startup acquisitions — that meet its reporting thresholds. And the consequences of getting it wrong are severe: civil penalties of up to $51,744 per day for failing to file, a mandatory 30-day waiting period that can stretch into months if the government issues a "second request," and gun-jumping rules that prohibit pre-closing integration activities that most founders assume are standard diligence preparation.

We have written about the CFIUS foreign investment review process — the national-security counterpart to antitrust review. We have also covered the buy-side due diligence checklist and deal protection provisions that founders negotiate in the LOI. This guide fills the domestic regulatory gap: how HSR premerger notification requirements, the 2023 Merger Guidelines, gun-jumping prohibitions, and second-request risk can delay, block, or impose conditions on your startup acquisition — and how to allocate that risk before signing.

When HSR Filing Is Required: The 2025 Thresholds

The HSR Act requires parties to certain mergers and acquisitions to file a notification form with the Federal Trade Commission and the Department of Justice and observe a waiting period before closing. The filing obligation is triggered by two tests: the size of transaction and the size of person. The thresholds are adjusted annually based on changes in gross national product.

For 2025, the FTC announced revised thresholds effective in mid-to-late February 2025. According to Davis Polk's analysis of the FTC announcement, the key thresholds are:

• Size of transaction: A transaction is reportable if, after closing, the acquiring person will hold voting securities, assets, or non-corporate interests valued above $126.4 million — provided the size-of-person test is also satisfied.
• Size of person: One party must have at least $25.3 million in total assets or annual sales, and the other party must have at least $252.9 million.
• Threshold above which size-of-person test does not apply: Transactions valued above $505.8 million are reportable regardless of the parties' size.

Filing fees range from $30,000 for transactions under $179.4 million to $2.39 million for deals valued at $5.555 billion or more. The critical point for founders: any acquisition with a deal value above $126.4 million potentially triggers an HSR filing obligation. Many startup acquisitions — particularly in AI, fintech, and health tech — now exceed this threshold. If your deal crosses it and you close without filing, both the acquiring and acquired parties face civil penalties of up to $51,744 per day of noncompliance, adjusted annually for inflation.

The 30-Day Waiting Period and What Happens During It

Once both parties file their HSR notification forms, a statutory waiting period begins. For most transactions, the initial waiting period is 30 calendar days (15 days for cash tender offers and bankruptcy transactions). During this period, the FTC and DOJ review the filing to determine whether the transaction may substantially lessen competition in any relevant market. The agencies divide review responsibility based on industry expertise — the FTC typically reviews tech, pharmaceuticals, and consumer products, while the DOJ reviews telecommunications, financial services, and agriculture.

At the end of the 30-day waiting period, one of three things happens:

1. The waiting period expires without action. The parties are free to close. This is the most common outcome — the vast majority of HSR filings clear without any agency action.
2. The parties "pull and refile." Before the waiting period expires, the parties voluntarily withdraw their filing and refile it, restarting the 30-day clock. This is a common tactic when the reviewing agency has concerns but the parties want to give the agency more time without triggering a formal second request. Pulling and refiling adds 30 days to the timeline but avoids the substantial cost and burden of a second request.
3. The agency issues a second request. If the reviewing agency determines it needs more information to assess competitive effects, it issues a "Request for Additional Information and Documentary Materials" — commonly called a "second request." This extends the waiting period and triggers a burdensome document-production process.

For founders, the practical implication is straightforward: if your deal requires an HSR filing, build at least 30 days into your closing timeline for the initial waiting period — and substantially more if the agency is likely to scrutinize the transaction.

Second Requests: What They Are and What They Cost

A second request is the antitrust agencies' most powerful pre-merger investigative tool. When issued, it stops the HSR clock and requires both parties to produce extensive documentation — internal emails, strategic plans, competitive analyses, customer data, pricing information, and executive testimony. The production typically covers millions of documents and can take three to six months to complete, even with dedicated legal teams and e-discovery vendors.

After the parties substantially comply with the second request, the agency has an additional 30-day period to review the production and decide whether to clear the transaction, negotiate a consent agreement requiring divestitures or behavioral remedies, or sue to block the deal. The total timeline from second request to final agency action frequently exceeds six months.

Second requests are more likely when the transaction involves:

• Horizontal overlap — the parties compete in the same product market, particularly if their combined market share exceeds 30%
• Vertical integration — the acquirer controls a supply chain input or distribution channel that the target relies on
• Nascent competitor acquisition — the target is a small but disruptive entrant in a market dominated by the acquirer
• Tech platform consolidation — acquisitions involving AI, data platforms, or digital infrastructure that raise concerns about data aggregation or foreclosure

The 2023 Merger Guidelines — released jointly by the DOJ and FTC on December 18, 2023 — significantly expanded the analytical frameworks the agencies use to evaluate these factors. As the DOJ's official announcement explains, the guidelines describe "factors and frameworks the Agencies often utilize when reviewing mergers and acquisitions." The guidelines introduced greater emphasis on nascent competition, potential competition theories, and vertical foreclosure — frameworks that make it easier for the agencies to challenge tech and platform acquisitions even when the target has minimal current revenue.

The 2023 Merger Guidelines: Why They Matter for Startup Acquisitions

The 2023 Merger Guidelines replaced the 2010 Horizontal Merger Guidelines and signaled a fundamentally more aggressive enforcement posture. While the guidelines are technically non-binding — they are a statement of agency practice, not law — they shape how agency staff evaluate transactions and decide whether to investigate, negotiate remedies, or litigate. For startup acquisitions, three aspects of the 2023 guidelines are particularly consequential.

First, the guidelines expand the focus on nascent competition. The agencies now more aggressively scrutinize acquisitions of startups that are not yet direct competitors but are on a trajectory to enter the acquirer's market. This is precisely the theory the DOJ used to block Visa's attempted acquisition of Plaid. As the DOJ's own account of the case explains, Visa announced its $5.3 billion acquisition of Plaid in January 2020. The DOJ sued to block the deal in November 2020, alleging that Plaid was developing a product that would compete with Visa's online debit network. Internal Visa communications described the acquisition as an "insurance policy" to neutralize a threat to Visa's debit business. Visa and Plaid abandoned the merger on January 12, 2021.

Second, the guidelines lower the implicit threshold for agency concern. The 2023 guidelines state that the agencies may challenge mergers when the evidence shows the transaction threatens competition — a broader standard than the 2010 guidelines' emphasis on concrete, quantifiable market share data. This means acquisitions of early-stage startups with limited revenue but significant technology, data, or user bases are more likely to draw scrutiny.

Third, the guidelines emphasize multi-platform and data-driven markets. The agencies are increasingly concerned about acquisitions that combine data sets or platform capabilities in ways that could foreclose competitors. For AI startups whose value lies in training data, model capabilities, or user access, this scrutiny is particularly relevant — and it intersects with the AI model licensing traps we have covered elsewhere.

Real-World Impact: Deals That Died Under Antitrust Scrutiny

The Visa-Plaid case is not an outlier. The Adobe-Figma acquisition provides an even more direct lesson for startup founders. In September 2022, Adobe announced its intent to acquire Figma for approximately $20 billion. The deal drew intense regulatory scrutiny from both U.S. and U.K. authorities. On December 18, 2023 — the same day the DOJ and FTC released the 2023 Merger Guidelines — Adobe and Figma announced they were abandoning the transaction. As DOJ Antitrust Division Assistant Attorney General Jonathan Kanter stated, "The Antitrust Division remains committed to protecting competition in technology markets." Adobe paid Figma a $1 billion reverse termination fee — the cost of walking away from a deal that regulators would not allow to proceed.

For founders, these cases illustrate a critical reality: even when the buyer is willing and the price is agreed, antitrust review can kill the deal. The question is not whether the agencies will review your transaction — if it crosses the HSR threshold, they will. The question is whether the transaction presents competitive concerns that trigger a second request, a consent negotiation, or a litigation threat. And if the deal fails on antitrust grounds, who bears the cost.

Gun-Jumping: The Rules Most Founders Violate Unknowingly

Even if you file correctly and observe the waiting period, the HSR Act imposes a separate set of restrictions that most founders have never heard of: gun-jumping prohibitions. The HSR rules, codified at 16 CFR Part 803, prohibit the parties from taking control of each other's operations or transferring beneficial ownership before the waiting period expires. The purpose is to preserve the competitive status quo while the agencies review the transaction.

In practice, gun-jumping violations fall into two categories:

1. Substantive gun-jumping — exercising operational control of the target before closing. This includes halting production, coordinating pricing decisions, reassigning employees to the acquirer's reporting structure, or redirecting customer relationships. The FTC's January 2025 settlement with XCL Resources, Verdun Oil, and EP Energy illustrates the consequences. According to Akerman's analysis of the case, the companies paid a record $5.6 million civil penalty — the largest ever for gun-jumping — after the purchase agreement allowed the acquirer to assume operational control of the target during the HSR waiting period. The violations included halting production on certain wells, requiring target employees to report directly to acquirer counterparts, coordinating on pricing and customer contracts, and exchanging competitively sensitive information without adequate safeguards. The parties were in violation for 94 days between signing and the amendment that returned operational control to the target.

2. Procedural gun-jumping — failing to file an HSR notification or closing before the waiting period expires. The penalty for procedural gun-jumping is the same: up to $51,744 per day of noncompliance. A deal that closes 30 days early because the parties did not realize an HSR filing was required can generate over $1.5 million in civil penalties.

The gun-jumping rules create a tension that founders must navigate carefully. Buyers want to begin integration planning immediately after signing — mapping systems, planning employee transitions, identifying cost savings. But the HSR Act prohibits the parties from implementing any operational changes before the waiting period expires. The line between permissible planning and impermissible gun-jumping is fact-specific and requires antitrust counsel to navigate. Permissible activities typically include: developing integration plans (without implementing them), conducting due diligence, and obtaining regulatory approvals. Impermissible activities include: combining sales forces, coordinating pricing, transferring employees, sharing competitively sensitive customer or pricing data without clean-room protocols, and making operational decisions for the target.

Allocating Antitrust Risk in the LOI

Antitrust risk does not affect both parties equally. If the deal is blocked on antitrust grounds, the seller loses the acquisition — and potentially months of deal momentum, employee focus, and market opportunity. The buyer may lose a strategic objective, but it retains its capital and can pursue other targets. This asymmetry means that antitrust risk allocation is primarily a seller-protection issue, and it should be negotiated at the LOI stage — not discovered during diligence.

Three LOI provisions are essential for allocating antitrust risk:

Reverse Termination Fees (Antitrust Break Fee)

A reverse termination fee (RTF) requires the buyer to pay the seller a defined amount if the deal fails to close due to antitrust blocking. The Adobe-Figma case illustrates the concept: Adobe paid Figma $1 billion when the deal was abandoned due to regulatory resistance. For startup acquisitions, RTFs for antitrust failure typically range from 3% to 8% of deal value, depending on the assessed antitrust risk. If your startup operates in a market where the acquirer has significant market share, or if the acquirer has a history of blocked acquisitions, the RTF should be at the higher end. Negotiate the RTF as a specific, fixed dollar amount — not a formula — and ensure it is payable promptly upon termination, not subject to escrow or conditions.

Hell-or-Highwater Clauses

A hell-or-highwater clause requires the buyer to take any and all actions necessary to obtain antitrust clearance — including divesting assets, accepting behavioral remedies, and litigating against the agencies if needed. The clause eliminates the buyer's ability to walk away from the deal by claiming that the required remedies are too burdensome. For founders, a hell-or-highwater provision is the strongest possible protection against antitrust failure: it commits the buyer to closing regardless of the cost of regulatory clearance.

Buyers resist hell-or-highwater clauses, particularly when the required divestitures could gut the strategic rationale for the acquisition. A compromise position is a "reasonable efforts" clause requiring the buyer to use commercially reasonable efforts to obtain clearance, paired with a reverse termination fee if those efforts fail. The trade-off: a reasonable efforts clause gives the buyer an escape hatch, but the RTF compensates the seller for the failed deal. Founders should push for the strongest efforts standard the buyer will accept, and size the RTF to reflect the residual risk.

Closing Conditions and Outside Date

The LOI should specify that closing is conditioned on HSR clearance (either expiration of the waiting period or affirmative agency approval) and set an outside date — a "drop-dead date" after which either party can terminate. If antitrust review extends beyond the outside date, the deal terminates and the RTF (if negotiated) becomes payable. Founders should negotiate an outside date long enough to accommodate the full HSR timeline — at least 90 days for routine filings and 12+ months for transactions with significant antitrust risk. An outside date that is too short gives the buyer an easy exit; one that is too long leaves the seller in indefinite limbo.

Actionable Next Steps

1. Determine whether your deal triggers HSR before signing the LOI. If the transaction value exceeds $126.4 million (2025 threshold), an HSR filing may be required. Assess whether any exemptions apply — such as the intrapersonal exemption for certain entity reorganizations — and confirm the filing obligation with antitrust counsel. Do not assume that "startup deals" are below the threshold; many Series B and later acquisitions now exceed it.
2. Assess antitrust risk early. Evaluate whether the transaction presents horizontal overlap, vertical foreclosure concerns, or nascent competitor theories. If the acquirer is a market leader in a space where your startup is building a disruptive product, the risk of a second request or agency challenge is materially higher. Factor this risk into your deal timeline and LOI negotiations.
3. Negotiate antitrust risk allocation in the LOI. Include a reverse termination fee sized to compensate for deal failure (3-8% of deal value depending on antitrust risk), an efforts obligation on the buyer (push for hell-or-highwater; accept reasonable efforts only with a substantial RTF), and an outside date that accommodates the full HSR timeline. Do not leave antitrust risk unallocated.
4. Implement gun-jumping compliance protocols immediately after signing. Before the HSR waiting period expires, establish clear rules for what the parties can and cannot do. Use clean-room protocols for any competitively sensitive information sharing. Require antitrust counsel review of all pre-closing integration activities. The $5.6 million XCL/Verdun penalty demonstrates that gun-jumping enforcement is real and escalating.
5. Build the HSR timeline into your deal schedule. Assume 30 days for the initial waiting period. If the reviewing agency is likely to have concerns, build in time for a pull-and-refile (an additional 30 days). If a second request is possible, assume 6+ months from filing to clearance. Communicate this timeline to your board, investors, and employees.
6. Engage antitrust counsel before the LOI is signed. HSR analysis requires specialized expertise in antitrust law, market definition, and agency interaction. The cost of a pre-LOI antitrust assessment is trivial compared to the cost of discovering an HSR obligation after closing — or discovering that your deal is unchallengeable on antitrust grounds only after the buyer walks away without paying a termination fee you never negotiated.

HSR premerger notification is not a formality. It is a regulatory regime that can delay your closing by months, block your transaction entirely, and impose six-figure penalties for noncompliance. The founders who navigate antitrust review successfully are the ones who assessed their HSR obligations before signing, allocated antitrust risk in the LOI, and implemented gun-jumping compliance protocols from day one — not the ones who discovered the HSR Act during the third week of diligence, when the 30-day clock had already started running against a deadline nobody planned for.