/manager-portfolio-diff

name: manager-portfolio-diff
description: Diff the two most recent quarterly 13-F filings for an institutional investment manager (Berkshire, Baupost, Renaissance, Pershing Square, Tiger Global, Appaloosa, Scion, etc.) using Massive's pre-parsed 13-F endpoint. Reports initiations, exits, adds (>= 25% share change), trims (<= -25%), and portfolio value change. Answers "what did Buffett/Klarman/Burry do last quarter?" Requires Stocks Basic. Runs on the free tier.

manager-portfolio-diff

You hand over a filer (an alias like "berkshire" or a raw CIK). The skill pulls the two most recent quarterly 13-F filings for that manager, aggregates holdings by CUSIP (handles multi-manager joint filings), and reports the quarter-over-quarter change: initiations, exits, adds, trims, and portfolio value delta.

The Massive endpoint doesn't support an issuer-oriented lookup ("who holds AAPL?"), so this is the correct shape for 13-F work: pick the filer whose decisions you want to see and diff their book.

When to invoke

• "What did Berkshire do this quarter?"
• "Is Baupost still in that name?"
• "Did Pershing Square initiate anything new?"
• Any smart-money-following workflow across a curated cohort of

funds

Not for: "who owns AAPL?" (endpoint doesn't support issuer filter at scale). Not for real-time positioning (13-F is quarterly and lagged ~45 days after quarter-end).

What you need

• Either `--filer` (an alias) OR `--filer-cik` (10-digit CIK)
• `MASSIVE_API_KEY` exported
• Stocks Basic plan minimum

Known aliases:

• `berkshire` / `buffett`
• `baupost` / `klarman`
• `renaissance` / `rentech`
• `bridgewater`
• `third point` / `loeb`
• `pershing` / `pershing square` / `ackman`
• `tiger global` / `coleman`
• `scion` / `burry`
• `appaloosa` / `tepper`

For anyone else, look up their CIK at <https://www.sec.gov/cgi-bin/browse-edgar>.

What you get back

Two output layers from one run.

**Layer 1: canonical JSON** matching [`output-schema.json`](./output-schema.json). Top-level `filer_cik`, `filer_display_name`, `periods`, `summary` (holding counts, portfolio value, activity counts), and `changes` with per-bucket entries (initiation, exit, add, trim, unchanged). Each entry carries `issuer_name`, `cusip`, `prior_shares`, `current_shares`, `prior_market_value`, `current_market_value`, `delta_shares_pct`.

**Layer 2: rendered note**. Header + activity counts, then four buckets (NEW POSITIONS / EXITED / ADDS / TRIMS) capped at 10 entries per bucket, sorted by market value. One-line Take highlighting the biggest new position and biggest exit. See [`references/rendering.md`](./references/rendering.md).

How it works

1. **Pull all 13-F rows** for the filer via `GET /stocks/filings/vX/13-F?filer_cik={CIK}&limit=1000&sort=filing_date.desc`. 2. **Group by `period`** (quarter-end YYYY-MM-DD). Take the two most recent periods as current and prior. 3. **Aggregate by CUSIP** within each period. Sum shares and market value across multiple manager-of-record rows (joint filings produce one row per manager). 4. **Classify each CUSIP:**

• Only in current → initiation
• Only in prior → exit
• In both, share change >= +25% → add
• In both, share change <= -25% → trim
• Otherwise → unchanged

5. **Sort each bucket by market value.** New positions and adds by current value; exits and trims by prior value. Renders top 10 per bucket with overflow count.

Foundations used

• [`massive-api-patterns`](../massive-api-patterns) for REST auth,

retry, pagination.

Output mode: note

Narrative note. A single fund's 13-F is 30-300 rows typically; a grouped-by-bucket note reads better than one wide table.

Endpoints used

• `GET /stocks/filings/vX/13-F?filer_cik={CIK}&limit=1000&sort=filing_date.desc`

All 13-F rows for the filer. Paginated.

Doesn't handle (yet)

• **Fund-of-funds cohort scan.** No "top 20 hedge funds this

quarter." A workflow composite could iterate over aliases.

• **Multi-quarter trajectory.** Only diffs two quarters. A rolling

4-quarter cluster (add-add-add-add vs churn) would be a great extension.

• **Shorts / derivatives.** 13-F is longs + long options + a few

other instruments only. Shorts don't appear.

• **Marked-to-market values.** Uses filer-reported market_value at

the period date. Not marked to today's price.

• **Position-in-portfolio-percent.** Adds are currently ranked on

absolute market value, not share-of-portfolio. Both matter.

These are clean PR extensions. Output schema is forward-compatible.