All outputs are real tool calls to the Equibles MCP server from 21 October 2026, trimmed.
"What did the big funds buy?" is one of the first questions people ask an AI agent with market data. The answer comes from 13F filings: every quarter, managers with over $100M in US equities report their long positions. The data is public and exact. Reading it is where agents, and people, go wrong.
Here are the three mistakes I see most, with real outputs, and the prompt I use to avoid them.
Connect it first
In Claude, open Customize → Connectors → Discover, search for Equibles, click Connect to Claude and Authorize, then switch it on in a chat from + → Connectors. In ChatGPT: Plugins, search Equibles, Install plugin. In Claude Code, claude mcp add --transport http equibles https://mcp.equibles.com/mcp and then /mcp to sign in. The free plan covers 100 requests a day; other clients are in the setup docs.
Mistake 1: reading the newest quarter before it's filed
Funds have 45 days after the quarter ends to file. For the quarter that ended on 30 September, that's 14 November. Ask today "Which institutions bought the most Natera (NTRA) last quarter?" and GetTopInstitutionalBuyersSellers returns the 30 September board, with a warning on top:
Top buyers and sellers of Natera, Inc. (NTRA) as of 2026-09-30
Note: the 2026-09-30 filing window is still open.
Excluded 752 holders with no observed 2026-09-30 13F
(138,371,102 shares as of 2026-06-30); largest: JPMORGAN CHASE & CO,
BlackRock, Inc., PRICE T ROWE ASSOCIATES INC /MD/.
| # | Top buyer so far | Δ Shares | Prior → New |
|---|---|---|---|
| 1 | HWG Holdings LP | +23,772 | 0 → 23,772 |
| 2 | Riverbend Wealth Management, LLC | +13,000 | 0 → 13,000 |
| 3 | Triasima Portfolio Management inc. | +10,991 | 0 → 10,991 |
That's a list of the early filers, not of the biggest buyers. The quarter before, with every filing in, looks completely different (reportDate=2026-06-30):
| # | Top buyer, Q2 2026 | Δ Shares | Prior → New |
|---|---|---|---|
| 1 | FMR LLC | +2,897,142 | 2,554,254 → 5,451,396 |
| 2 | GOLDMAN SACHS GROUP INC | +1,307,121 | 822,831 → 2,129,952 |
| 3 | JANE STREET GROUP, LLC | +943,609 | 499,977 → 1,443,586 |
| 4 | D. E. Shaw & Co., Inc. | +787,919 | 491,336 → 1,279,255 |
| 5 | VIKING GLOBAL INVESTORS LP | +390,694 | 0 → 390,694 |
Until mid-November, "last quarter" should mean the June quarter. I tell the agent to check the filing-window note and fall back a quarter when it's open.
Mistake 2: treating value change as buying
The same Q2 board, sellers side:
| Top seller, Q2 2026 | Δ Shares | Δ Position Value |
|---|---|---|
| PRICE T ROWE ASSOCIATES INC /MD/ | -3,162,981 | +$12.1M |
| WELLINGTON MANAGEMENT GROUP LLP | -727,855 | +$143.2M |
| JPMORGAN CHASE & CO | -521,321 | +$815.1M |
JPMorgan cut its Natera stake by over half a million shares, and the position's value still rose by $815M, because the stock went up during the quarter. Value change mixes trading with price movement. Only the share change says what the fund did, so I ask agents to sort and describe by Δ Shares and treat Δ Value as context.
Mistake 3: ignoring the Type column
One manager's quarter, from GetInstitutionQuarterlyActivity for Duquesne Family Office (Stanley Druckenmiller), as of 30 June 2026:
- 48 positions initiated, 16 increased, 11 reduced, 23 exited.
- The largest new position by value is Insmed (INSM), +$143.9M. Its Type is Call: 1,350,000 shares' worth of call options, not stock.
- Natera was increased by 122,700 shares, about 4%, and shows +$252.2M in value. Most of that is the price.
- Sandisk was reduced by 3,255 shares and still shows +$55.1M.
"Druckenmiller's biggest new buy was Insmed" is true only if you add "through call options". 13F filings list put and call positions by the shares underlying them, and an agent summarizing the table will flatten that unless you ask it not to.
One more check before comparing managers: the superinvestor list marks stale filers. Today Bill Ackman's latest report is 31 March 2026 and Michael Burry's is 30 September 2025, while most others are at 30 June 2026. Putting those side by side mixes different quarters.
The prompt I reuse
Using Equibles, for [TICKER]:
1. Show the top 13F buyers and sellers for the latest quarter. If the filing
window is still open, say how many holders are missing and repeat for the
previous quarter.
2. Rank by change in shares, not change in value, and show both.
3. Flag option positions (Type Call or Put) and any filer whose latest report
is older than the quarter you're describing.
What 13F can't tell you
- Shorts. 13F covers long positions only.
- Today. It's a quarter-end snapshot filed up to 45 days later; the fund may have traded since.
- Everything. Only US-listed securities on the SEC's 13F list, and only managers above the $100M threshold.
- Why. A position change has no rationale attached.
Each stock's holder history is also on its page, for example Natera's holdings.
Nothing here is investment advice.
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