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Reg SHO Rule 201 (SSR) Slippage Playbook

2026-03-12 · finance

Reg SHO Rule 201 (SSR) Slippage Playbook

Date: 2026-03-12
Category: research
Scope: US NMS short execution when the Rule 201 short-sale price test is active

1) Why this matters

Most slippage models assume both sides of the touch are symmetrically executable.

Under Rule 201 SSR (alternative uptick), that assumption breaks for short orders: execution/display at or below current NBB is restricted (unless an exception applies). The result is a structural execution wedge:

If your scheduler ignores SSR state, it systematically underestimates short-side tail slippage on stress days.

2) Constraint-aware cost objective

Let parent short order have remaining size (R_t), horizon (H), and SSR state (z_t \in {0,1}).

Define action set (a_t) (rest, improve, cross, pause, reroute) and expected cost:

[ J(a_t\mid s_t,z_t)=\mathbb{E}[C_{spread}+C_{impact}+C_{delay}+C_{opportunity}+C_{constraint}\mid s_t,z_t,a_t] ]

Under SSR, add explicit constraint term:

[ C_{constraint}=C_{blocked}+C_{queue_drift}+C_{catchup_convexity} ]

where:

3) Effective executable depth under SSR

Define baseline executable short depth at time (t): (D^{base}_t).

Under SSR, only price levels strictly above NBB are generally eligible for non-exempt short execution. Let eligibility filter (\phi(p_t,\text{NBB}_t)\in{0,1}).

[ D^{ssr}t = \sum{\ell \in LOB_t} q_{t,\ell},\phi(p_{t,\ell},\text{NBB}_t) ]

with depth shrink ratio:

[ \text{DSR}_t = 1 - \frac{D^{ssr}_t}{D^{base}_t} ]

High (\text{DSR}) means displayed liquidity is mostly unusable for shorts under SSR.

4) Fill-toxicity tradeoff changes sign

Without SSR, waiting passively can improve spread capture.

With SSR active, waiting often has a different profile:

Use competing hazards:

and optimize for q95/CVaR cost, not only mean IS.

5) SSR-specific metric stack

Healthy trend on SSR days: BCR↓, QDR↓, SDR↓, SCC↓ while completion reliability stays within guardrails.

6) State controller for live execution

NORMAL

No SSR trigger. Standard short policy.

SSR_WATCH

Price approaches trigger zone; pre-arm SSR-safe tactics (smaller clips, fallback templates).

SSR_ACTIVE

SSR triggered; enforce constraint-aware routing:

SSR_RECOVERY

SSR lifted; unwind defensive throttles gradually (hysteresis) to avoid overreaction.

SAFE

Activated on repeated rejects, stale quote risk, or budget-burn breach; preserve completion integrity over micro-alpha.

7) Practical policy updates

  1. Constraint-first child generation
    Validate SSR compliance before routing, not after reject.

  2. SSR-aware aggressiveness ladder
    Increase urgency only when SDR and remaining horizon justify it.

  3. Deficit smoothing
    Repay schedule deficits continuously; avoid end-window panic cross bursts.

  4. Venue- and order-type exception handling
    Keep explicit handling for permissible exemptions (where policy/legal framework allows) with audit logs.

  5. TCA split by SSR state
    Report IS separately for non-SSR vs SSR-active intervals.

8) Calibration workflow

Offline

  1. Label SSR active intervals from market-center trigger data.
  2. Reconstruct child-intent lifecycle (intent -> accepted/rejected -> fill -> markout).
  3. Estimate BCR/UER/QDR/SDR/SCC by symbol regime (ADV, volatility, spread class).
  4. Fit quantile models for shortfall with SSR interaction terms.
  5. Stress-test with synthetic high-DSR and high-SDR scenarios.

Online

9) Guardrails

10) Implementation checklist

11) References

One-line takeaway

On SSR days, short execution is a constrained-control problem: model the constraint cost explicitly or you will underprice tail slippage and overpay in deadline catch-up.