Pre-buy fit and what-if sandbox before adding a stock or fund (portfolio fit)

[iamsaquib8]

What

Roadmap idea. Before adding a new stock, ETF or mutual fund to a portfolio, investors usually ask: "Will this actually diversify my money, or is it just adding more of what I already own?" A pre-buy sandbox simulates the exact before-and-after impact of a proposed purchase on your portfolio.

Design

• stockwise portfolio fit <SYMBOL|MF:CODE> [AMOUNT] (e.g. portfolio fit PERSISTENT 50000 or portfolio fit MF:120503 100000):
  • Look-Through Overlap Delta: Uses Portfolio X-Ray to show whether the new holding overlaps with existing holdings (especially relevant when buying a new mutual fund or thematic ETF).
  • Sector & Cap Allocation Shifts: Shows before → after percentages (e.g. Technology weight moves from 18.2% → 22.4%; Small-cap weight moves from 12% → 15.5%).
  • Risk & Correlation Impact: Computes the marginal contribution to portfolio risk (MCTR), 1Y correlation against your top 3 holdings, and estimated shift in overall portfolio beta.
  • Downside Drawdown Estimate: Historical max drawdown of your portfolio before vs after the hypothetical addition.
  • Pure simulation sandbox: zero live trades, local calculations only.

Done when

• Simulates before → after shift in sector weights, cap tiers, beta, and top-holding concentration.
• Integrates with xray to catch overlapping underlying stocks in funds/ETFs.
• Conforms to #260 and DISCLAIMER.md: factual risk modeling, never a buy recommendation.
• Supports --json for MCP tool integration.

---

Upvote or comment below with what factors you check before buying a new stock or fund. Ideas with community demand get scheduled on the roadmap.