Event Gaps: GPT-5.6 Sol's picks for 2026-09-08
GPT-5.6 Sol picked BE, HUBS, P, ILMN, BLDR for a 1-day horizon. Intraday alpha vs SPY: -0.77%. Picks with positive alpha: 40%.
| Experiment | Event Gaps |
|---|---|
| Model | GPT-5.6 Sol |
| Prompt version | event-gap-v1 |
| Asked | |
| Horizon | 1-day |
| Measured from | 2026-09-04 |
| Measured to | 2026-09-08 |
| Benchmark | SPY |
| Status | Settled |
| Intraday alpha vs SPY | -0.77% |
| Picks with positive alpha | 40% |
| Benchmark return | -0.40% |
The picks
| Rank | Symbol | Gap direction | Price change | Return in the called direction | Confidence | vs benchmark | Called right |
|---|---|---|---|---|---|---|---|
| 1 | BE | up | +3.53% | +3.53% | 82% | +3.94% | Yes |
| 2 | HUBS | up | -1.97% | -1.97% | 76% | -1.57% | No |
| 3 | P | up | +0.64% | +0.64% | 73% | +1.04% | Yes |
| 4 | ILMN | up | -3.47% | -3.47% | 72% | -3.07% | No |
| 5 | BLDR | down | -4.60% | -4.60% | 62% | -4.20% | No |
The model's market view
The strongest verified pre-target-session gap catalysts available before the cutoff are S&P Dow Jones Indices' September 4 quarterly-rebalance announcements. The announcement was posted at 5:15 PM Eastern, after the September 4 regular close, and September 7 is the Labor Day market holiday, so September 8 is the first regular session in which these changes can be fully incorporated into opening prices. Index additions should attract positioning ahead of the September 21 effective date, while deletions can face anticipated passive outflows. For the experiment's equal-weight LONG open-to-close score, the best setups are additions whose gaps appear large enough to confirm demand but not so extreme that immediate profit-taking becomes dominant. A gap-down name is included only where an opening overshoot could create a long-side intraday reversal. The main portfolio risk is that index-related moves are substantially pre-priced in extended trading and fade after Tuesday's opening auction.
Why each pick
BE
Reported fact: S&P Dow Jones Indices announced at 5:15 PM Eastern on September 4 that Bloom Energy will join the S&P 500 effective before the September 21 open. Independent post-announcement reporting showed BE rising roughly 7% to 7.5% in extended trading after already gaining strongly during Friday's regular session. Expectation: the September 8 regular open should retain a large positive gap because Monday is a market holiday and Tuesday is the first opening auction after the announcement. For the open-to-close LONG score, continuation is plausible because S&P 500 inclusion creates anticipated index-fund demand and BE already has strong AI-data-center power momentum. Emotional continuation risk is very high, however: the combination of a large pre-existing rally and a 7%+ after-hours jump can produce an opening climax followed by profit-taking, so the expected intraday continuation is less certain than the gap itself.
HUBS
Reported fact: the same September 4 S&P rebalance announcement promoted HubSpot to the S&P MidCap 400, and contemporaneous extended-hours reporting showed HUBS gaining about 4%. Expectation: I expect a sizable positive September 8 opening gap as investors price expected benchmark-related demand following the three-day weekend. For the open-to-close LONG score, HUBS may offer a better continuation profile than the most extreme S&P 500 addition because its approximately 4% initial reaction is meaningful without being as stretched. Emotional continuation risk is moderate: index-promotion enthusiasm can attract momentum buying, but the change is mechanical and does not itself improve HubSpot's operating fundamentals, making a post-open fade possible if buyers exhaust early.
P
Reported fact: S&P Dow Jones Indices announced Everpure's promotion from the S&P MidCap 400 to the S&P 500, effective before the September 21 open. Contemporaneous reporting showed P gaining approximately 2.2% in extended trading following the announcement. Expectation: Tuesday's regular opening gap should be positive and could exceed the initial extended-hours reaction as more institutional investors participate after the holiday. For the open-to-close LONG score, the moderate initial gap leaves room for incremental index-related positioning rather than presenting an obviously exhausted move. Emotional continuation risk is moderate: S&P 500 inclusion is a powerful sentiment catalyst, but if the opening auction immediately incorporates most anticipated passive demand, arbitrage and profit-taking could cause the shares to drift lower afterward.
ILMN
Reported fact: S&P Dow Jones Indices announced Illumina's move from the S&P MidCap 400 into the S&P 500. Post-announcement reports showed ILMN gaining about 2% after hours. Expectation: I expect a positive September 8 gap because the stock has not yet had a regular session to absorb the index promotion. The long-side open-to-close thesis is that a relatively restrained initial reaction can attract additional institutional and momentum demand during Tuesday's session. Emotional continuation risk is lower than for BE but still material: the catalyst is mechanical rather than an earnings upgrade, and traders may sell into strength once the opening inclusion premium is established.
BLDR
Reported fact: S&P Dow Jones Indices announced that Builders FirstSource will be removed from the S&P 500 and moved to the S&P SmallCap 600 effective before the September 21 open. Contemporaneous extended-hours reporting showed BLDR falling approximately 1.8% after the announcement. Expectation: the September 8 regular open should gap down as investors price expected index-related selling. This is nevertheless a LONG selection for the experiment: the open-to-close thesis is reversal, not short continuation. Because the catalyst is a mechanical index deletion rather than newly reported deterioration in BLDR's underlying business, a sufficiently negative opening auction could overshoot the fundamental information and permit a rebound through the session. Emotional continuation risk is high: anticipated passive selling is real and may keep pressure on the shares, so if the opening decline is modest rather than capitulatory, further downside after the open is a significant risk.
Other models on the same run
| Model | Picks | Intraday alpha vs SPY | Picks with positive alpha |
|---|---|---|---|
| Muse Spark 1.1 | LULU, AVGO, PATH, GWRE, ZS | -2.12% | 40% |
| Grok 4.5 | UNFI, ABM, CAN, DLNG, CASY | +1.70% | 40% |
| Gemini 2.5 Pro | BE, TTD, BLDR, TAP, ILMN | -1.62% | 20% |
| DeepSeek R1 | UNFI, CASY, GME, ABM, BRZE | +0.53% | 20% |
Written up on ForecastNest Insights: Event Gaps: GPT-5.6 Sol's picks for 2026-09-08.
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