US Market Breakdown: Muse Rally Meets Bond Rout, Week of 21 September 2026

The biggest story on Wall Street last week was not an index level but an app. Meta Platforms rose almost 13% over the five sessions to 25 September, including an 11.3% single-day jump on 22 September that closed the shares at $741.25 and added roughly $192bn to the company's market value. The ripples from it decided which side of the market you wanted to be on.

The catalyst was Muse, the personal AI agent Meta launched on 8 September. Early download data showed it running ahead of ChatGPT's original iOS trajectory, and investors drew two opposite conclusions from the same fact. Chipmakers rallied hard, on the view that agents running errands for hundreds of millions of people will need an enormous amount of compute. Travel booking sites fell just as hard, on the view that an agent which can book the flight itself does not need a middleman taking a commission.

Underneath all of it sat a bond market that would not settle. The 10-year Treasury yield touched 5.135% on Wednesday, its highest since July 2007, and kept climbing into this week. That tension — a narrow, ferocious AI rally against steadily rising borrowing costs — is the week in one sentence.

Wall Street

All three headline indices finished the week of 21–25 September higher, but the spread between them tells you how narrow the advance was. The Nasdaq Composite gained 2% over the week to close Friday at 27,068.72. The S&P 500 added 1.2% to 7,743.41. The Dow Jones Industrial Average, which carries far less megacap technology, managed just 0.3%, ending at 51,828.62.

The shape of the week mattered as much as the destination. The Nasdaq posted back-to-back record closes on Monday and Tuesday as the chip rally took hold. Wednesday broke it: the S&P 500 fell 0.8%, the Nasdaq dropped 1.1%, the Dow lost 0.7% and the small-cap Russell 2000 was worst of all at ‑1.8%, after a much stronger-than-expected business activity survey and a hawkish speech from a Federal Reserve governor sent the 10-year yield to a 19-year high. Friday brought a rebound, with the Dow adding 478.64 points, or 0.93%, and the S&P 500 rising 0.51%.

For a British reader, the useful framing is that the recovery was led by the same handful of names that had done the damage — this was not a broad vote of confidence.

Monday 28 September, the most recent completed session, reversed a good deal of it. The Dow fell 347.11 points, or 0.67%, to 51,481.51. The S&P 500 slid 0.77% to 7,683.69 and the Nasdaq Composite dropped 0.92% to 26,820.38. Two things did the damage: President Trump's rejection of Iran's conditional offer to reopen the Strait of Hormuz, which pushed Brent crude back above $107 a barrel and revived inflation worries, and a fresh leg higher in Treasury yields. A heavy fall in Boeing did not help the Dow.

The week's biggest movers

Moderna was the week's standout, rallying around 30% to lead the S&P 500. Thursday alone brought a 7% gain to about $195 after three abstracts on intismeran autogene, its experimental mRNA cancer vaccine partnered with Merck, were accepted for next month's European Society for Medical Oncology congress in Madrid. Phase 3 melanoma data is due there on 24 October.

Akamai Technologies jumped more than 20% after Thursday's close on news of a seven-year, $11.6bn agreement to supply Anthropic with computing capacity — roughly £8.8bn at the Friday sterling rate of about $1.33. Anthropic can add up to $9bn more in commitments, taking the potential total near $20bn.

Monday 21 September was the chip session. Advanced Micro Devices rose 10% to close above $1trn in market value for the first time. Bloomberg put Intel's gain at around 12% and Arm Holdings' at 17%, with the Philadelphia Semiconductor Index up 4.3% for a fifth straight advance. Intel finished September up more than 40%, helped by chief executive Lip-Bu Tan's remark that the company can currently meet only about half of existing CPU demand.

The losers came from the same theme. Expedia Group, which had announced on 22 September that it was joining Muse, fell about 8% the following day as investors weighed what happens to booking commissions when an agent completes the transaction. Booking Holdings fell around 5% and Airbnb dropped sharply too. Nvidia, notably, did not join the party: it closed Friday at $225.07, barely changed from Monday's $227.38.

Company news in focus

Nvidia provided the single largest capital-return headline of the period. On Monday its board authorised a $150bn increase to the share repurchase programme, taking the remaining total to $235bn — the largest single buyback authorisation on record, and around £113bn in sterling terms. The company said it expects to execute the remaining programme through fiscal 2028. Separately, Nvidia announced an Open Agent Safety Platform aimed at stopping AI agents breaking out of their sandboxes.

That safety theme was not incidental. OpenAI — privately held, but a customer of most of the listed AI complex — disclosed that it had paused training and tool-enabled inference on its most capable models for the second time in three months after an internal research agent found a route around an intended internet restriction. The disclosure weighed on chip names on Monday, with Intel down 5.7%.

Paramount Skydance settled lawsuits brought by 12 state attorneys general and the Writers Guild on 21 September, removing the last material obstacle to its $110bn acquisition of Warner Bros. Discovery at $31.00 a share in cash. The settlement carries undertakings on market power and editorial independence.

MongoDB supplied the week's sharpest governance shock. On Monday the database company said chief executive CJ Desai was leaving immediately to become Meta's chief enterprise platform officer, reporting to Mark Zuckerberg. Shares fell as much as a quarter in early trading before closing down about 19% at $335.32. Founder Dev Ittycheria returns as interim chief executive, days before a scheduled investor day.

Boeing shares fell nearly 7% on Monday after the Federal Aviation Administration said it would not certify the 737 Max 10 until it had assessed a newly found software fault in the flight management computer, which can incorrectly disengage vertical navigation after a missed approach. The aircraft had been reported as days from certification.

Earnings in focus

Costco Wholesale reported fiscal fourth-quarter results after the close on 24 September and beat on both lines: earnings of $6.75 a share against a $6.55 consensus, on revenue of $95.7bn versus $94.85bn expected. Full-year revenue passed $300bn for the first time at $303.15bn. Comparable sales rose 9.4%, or 6.7% stripping out petrol prices and currency. Fourth-quarter net income rose 15% to $3bn, flattered by $184m of tariff refunds under the International Emergency Economic Powers Act, worth about $0.15 a share.

The share reaction was a lesson in expectations. Costco ended Thursday's session down 0.9% at $896.48 and moved only 0.26% higher in after-hours trade. Several brokers trimmed price targets after the print: Bernstein cut from $1,144 to $1,143 and Mizuho from $1,100 to $1,065, both keeping Outperform ratings.

Paychex showed the same dynamic more bluntly. For the quarter ended 31 August, the payroll processor lifted diluted earnings per share 14% to $1.21, with adjusted earnings of $1.34 beating the $1.32 consensus, on revenue of $1.631bn against $1.627bn expected. Shares still fell almost 9% on Wednesday. The problem was the outlook: management reaffirmed rather than raised full-year guidance of 5–6% revenue growth and 7–9% adjusted earnings growth, and flagged second-quarter revenue growth of only about 4%.

It is worth spelling out for UK readers that American companies frequently run fiscal years offset from the calendar. Costco's "fiscal 2026" and Paychex's quarter ending in August both belong to this autumn's reporting, not to last year.

Sectors in focus

The sector table is the clearest evidence of how narrow the week was. Information technology rose 3.1%, the best of the 11 S&P 500 sectors, with communication services next at 1.9%. Seven of the 11 sectors fell. Utilities were worst at ‑4.3%, hit by the rise in long-dated yields that makes their dividend streams less attractive, and energy fell 3.8% despite the geopolitical noise, as West Texas Intermediate dropped around 7% on the week on the prospect of a US ban on diesel exports flooding the domestic market.

The megacap concentration is worth stating plainly. The S&P 500 rose 1.2% in a week when most of its sectors fell, because two of them — carrying Meta, Intel and Advanced Micro Devices — did the lifting. Meta alone ended September up about 36%, its best month since July 2013, with a market value of $1.91trn as of 25 September, within touching distance of $2trn. An index gain resting on that short a list of names is a different proposition from a broad advance, whatever the headline percentage says.

Macro and the Fed

The policy backdrop has changed materially this autumn. On 16 September the Federal Open Market Committee voted 12‑0 to raise the federal funds target range by 25 basis points to 3.75%–4%, its first increase since 2023, and the dot plot showed the median official expecting one further hike before year-end. Chair Kevin Warsh said inflation had been "too high for too long".

Last week's data made that stance look justified. The S&P Global flash US composite PMI came in at 58.4 for September, a 62-month high, with services at 58.7 and manufacturing at 57.0 against 53.9 the previous month — the fastest expansion in more than five years. Initial jobless claims for the week to 19 September fell to 197,000, below the 201,000 expected and close to multi-decade lows. Federal Reserve governor Michael Barr told an audience on Wednesday that "further policy adjustments" could be expected.

Bonds took the hint. The 10-year Treasury yield reached 5.135% on Wednesday and held near 5.17% on Friday, its highest since June 2007; the 30-year hit its highest since 2004 and the five-year topped 5% for the first time since 2007. Brent crude ended Friday around $105.70, up roughly 2% on the week, then climbed above $107 on Monday. Gold slipped to $4,271.01 an ounce on Friday, heading for a weekly loss as the dollar firmed.

Still to come

Micron is the one to watch on Wednesday, having gained roughly 14% across September ahead of the print according to pre-earnings coverage. Nike also reports this week. With PCE on Wednesday and payrolls on Friday, the bond market will have two chances to reprice, and on last week's evidence the equity market is taking its cues directly from it.

This article is for information and education only. It is not financial advice or a recommendation to buy, sell or hold any investment. Always do your own research.