The stock market's toughest test of the autumn arrives after the close Wednesday: Facebook parent Meta Platforms' third-quarter earnings report could define sentiment toward tech for months to come.

A wave of defensive trading over the past month -- largely linked to worries over the pace of AI spending and the fate of its investment boom -- has weighed on the market's most important sector. Quickening inflationary pressures and the surge in bond market volatility haven't helped, either.

The Nasdaq Composite has fallen 2.5% since mid-September, while the broader S&P 500 has slipped just 0.7%. Healthcare, energy, and consumer staples stocks, meanwhile, have all recorded solid gains, suggesting investors are seeing more than a little fatigue in the AI trade heading into the final months of the year.

That backdrop makes Meta's results feel like a bit of a make-or-break moment for the stock market, particularly as the word "bubble" has started to appear in discussions about AI spending. The company is the first of the so-called Magnificent Seven tech firms to report third-quarter earnings this season, making its outlook for AI investments (and how it plans to finance them) a critical bellwether for the sector.

"There are no signs that demand for AI is slowing," said Angelo Kourkafas, senior global strategist at Edward Jones to Bloomberg Surveillance TV. "What has changed is that financing costs are becoming higher, and that is putting pressure on some of the more marginal players."

"We're seeing healthy skepticism and a maturation of the theme, moving away from an environment where everything AI-related goes up toward a greater focus on quality and the companies with the deepest pockets to finance the investment needed to meet demand," he added.

Meta is very likely to either support or challenge that thesis in a market that continues to digest Anthropic's pending initial public offering, an impressive profit report from Elon Musk's SpaceX, and the ongoing slump in shares of neocloud leader CoreWeave.

And it has been here before.

Last autumn, Meta's third-quarter updates sparked an 11% slide in the stock as its growth in capital spending outweighed gains in overall revenue. It also boosted capital expenditure plans and extended its all-in bet on AI advancements. The S&P 500's tech sector fell 10.5% over the following three weeks, while an index of the Magnificent Seven giants slumped 8.7%.

The year before that, another capex increase and plans for a "significant" acceleration in spending, triggered a similar, albeit shorter-lived, market reaction.

This year's update, however, feels edgier. Investors are balking at the practically unlimited spending plans that have defined the core of the AI trade, and are increasingly demanding more clarity on the link between expansion and profits.

And the "bubble" word is starting to be used more freely in describing the fate of the AI investment boom, now entering its fifth year, than even before. Bank of America's latest Fund Managers' Survey, in fact, showed that 45% of those polled expect an AI bubble event next year. They see it as a greater risk to the market than issues tied to soaring government bond yields.

That puts Meta's capital spending plans, which are expected to reach at least $140 billion this year and rise past $202 billion in 2027, in sharp focus.

The company's recent success with the launch of AI agent Muse -- which triggered a 20% pop in the stock over two weeks in September -- might give CEO Mark Zuckerberg the confidence to boost those figures even further. After all, for top players like Meta, Microsoft, Alphabet, and Amazon.com, the race for AI dominance in the real economy shows no signs of abating.

SpaceX, in fact, said Tuesday that its current quarter spending could reach $40 billion, roughly double what it shelled out over the previous three months. Investors didn't seem troubled, of course, and the group's impressive earnings beat pushed the stock nearly 10% higher in early Wednesday trading.

"SpaceX's blowout report showed that AI agents, and the compute behind them, are already making real money -- and that reframes the capex debate entirely," said Luke Longo, technology analyst and publisher of the Innovation Investor. "When compute is this scarce and this valuable, a bigger spending plan isn't recklessness, it's a land grab."

But are markets primed to accept that kind of bullishness outside of Musk-controlled companies? AI doesn't exist in a vacuum: Investors also have to consider the broader decline in chips stocks (the principal beneficiaries of AI spending) and the mounting concerns in bond and commodity markets tied to the U.S. war in Iran.

This time last year, when Meta unveiled its amped-up spending plans, 10-year Treasury yields were trading firmly south of 4%, and investment-grade borrowing costs were hovering at the lowest levels in decades.

Today, of course, 10-year Treasury yields are well north of 5%, the Federal Reserve is likely to lift its benchmark borrowing rate past 4% before the end of the year, and corporate borrowing costs have surged as a result of record AI borrowing.

That's a massive change in the investment landscape, which is also compromised by big questions over the debt financing of AI companies like CoreWeave and Oracle. There are also worries about the impact of new listings from Anthropic and OpenAI, which will both test investor appetite.

Such anxiety could become the norm, however. Siebert Financial's Chief Investment Officer Mark Malek says AI spending is a given. The question, rather, is how long will it take to see the returns on those investments.

"The question for AI spenders is no longer whether they'll spend," he says. "It's whether the revenue shows up on the same schedule as the bills. Get used to that common theme. It isn't going anywhere."

Meta could offer the latest example of that later today. We'll see how the market feels about it.