📈 Legs Definition in TOM

The legs array defines the individual option positions that make up your strategy structure.
Each leg represents a rule for selecting, sizing, and managing a specific option contract — for example, “sell a 25Δ put 30 days from expiry each month.”

While the Global Settings section controls how the overall strategy behaves,
the Legs Array determines what you actually trade.

🧱 Overview

Every TOM must contain at least one leg:

"legs": [
  {
    "id": "Leg #1",
    "underlying_id": "IND.ES",
    "type": "p",
    "weight_type": "notional",
    "weight": -1,
    "expiry_dte_min": 30,
    "expiry_nearest": "1",
    "strike_type": "delta",
    "strike_value": 0.25,
    "period": "month",
    "period_contract": "All",
    "options_family_id": "IND.ES"
  }
]

Each leg is evaluated and managed independently, unless you choose to synchronize them with
inherit_first_leg_rolls: "yes" in the global settings.

⚙️ Leg Field Reference

🧩 Field Details

id

A descriptive label for the leg.
Used for identification in analytics and reports.
Example: "Short Put", "Long Call Hedge", "Wing #2".

underlying_id

Symbol for the underlying asset whose options are being traded.
This must match a valid instrument code in the platform (e.g., IND.ES, COM.TTF).

options_family_id

Defines which option family to use.
Monthly options usually match the underlying_id.
For weekly options for example it can have a suffix defining another family id, for example: IND.ES_W

type

Specifies whether the leg is a call ("c"), put ("p"), future ("f") or stock ("s")

weight_type

Determines how position size is calculated:

💡 Use "notional" for most systematic tests, since it maintains proportional risk through time.

weight

The leg’s signed size.

• Negative values (-1, -2) → short exposure (sell to open)

• Positive values (1, 2) → long exposure (buy to open)

Example:
"weight": -1 with "weight_type": "notional" means short exposure equal to 1× notional of current equity.

expiry_dte_min

The minimum Days-to-Expiry when selecting contracts.
TOM finds the nearest expiry at or above this threshold.
Example:
If expiry_dte_min = 30 and the available expiries are 15, 45, and 75 days, the engine will consider the 45 and 75-day expiry for the selection.

expiry_nearest

Specifies which qualifying expiry to pick once the DTE filter is applied:

• "1" → the nearest eligible expiry (most common)

• "2" → the second-nearest

• "3" → the third-nearest, etc.

Example:
expiry_dte_min = 30 and expiry_nearest = "2" means the second expiry above 30 DTE.

strike_type and strike_value

These two fields work together to select the option strike.

Example:

"strike_type": "delta",
"strike_value": 0.25

Selects an option closest to 25Δ at entry.

period and period_contract

Define when a new structure is entered if no signals are used.

Example:

"period": "month",
"period_contract": "Jan, Mar, Jun, Sep"

→ Backtest a strategy only on Jan, Mar, Jun, Sep contracts. 

🧠 Multi-Leg Structures

When you include multiple legs in the array, TOM treats them as a linked structure — a combined position made of multiple option legs.

Example: Short Put Spread

"legs": [
  {
    "id": "Short Put",
    "underlying_id": "IND.ES",
    "type": "p",
    "weight_type": "notional",
    "weight": -1,
    "expiry_dte_min": 30,
    "expiry_nearest": "1",
    "strike_type": "delta",
    "strike_value": 0.25,
    "period": "month",
    "period_contract": "All",
    "options_family_id": "IND.ES"
  },
  {
    "id": "Long Put",
    "underlying_id": "IND.ES",
    "type": "p",
    "weight_type": "notional",
    "weight": 1,
    "expiry_dte_min": 30,
    "expiry_nearest": "1",
    "strike_type": "delta",
    "strike_value": 0.05,
    "period": "month",
    "period_contract": "All",
    "options_family_id": "IND.ES"
  }
]

This creates a 25Δ short put hedged with a 5Δ long put, forming a vertical spread.

🔄 Rolls and Synchronisation

Each leg rolls independently when its DTE threshold is reached unless:

• Global inherit_first_leg_rolls = "yes" → all legs roll together based on the first leg’s schedule.

• You explicitly define leg-level DTE roll logic.

This ensures consistency for multi-leg strategies like spreads and condors.