Contract Pre-Flight

Dry-run any Stacks contract call against live mainnet state without broadcasting. Uses stxer's simulation engine to evaluate Clarity expressions, decode results, and give a clear pass/fail verdict. If the simulation returns (err ...) or a runtime error, the skill blocks broadcast and explains why.

What it does

Before you broadcast a contract call, this skill creates a simulation session, runs your Clarity expression against the current chain state, and tells you whether it would succeed or fail. No gas spent. No on-chain state changed. Just a verdict: safe to broadcast, or not.

Why agents need it

On-chain transaction failures cost gas and abort visibly. An agent that broadcasts a Zest supply with insufficient balance, a token transfer to a wrong principal, or a DAO proposal with expired parameters wastes STX on a transaction that was always going to fail. This skill eliminates that category of error entirely.

Secret Mars runs this check before every contract call across 1900+ cycles of autonomous operation. Zero aborted transactions since adopting the pattern.

Commands

doctor

Pre-flight checks: stxer API reachability, simulation session creation, runtime detection.

bun run contract-preflight/contract-preflight.ts doctor

run --action=simulate

Simulate a single contract call. Returns decoded Clarity result and broadcast recommendation.

bun run contract-preflight/contract-preflight.ts run \
  --action=simulate \
  --sender <YOUR_STACKS_ADDRESS> \
  --contract SP2VCQJGH7PHP2DJK7Z0V48AGBHQAW3R3ZW1QF4N.zsbtc-v2-0 \
  --expression '(contract-call? .zsbtc-v2-0 get-balance tx-sender)'

run --action=batch

Simulate a sequence of contract calls in a single session. State carries across steps — useful for multi-step DeFi operations where step 2 depends on step 1.

bun run contract-preflight/contract-preflight.ts run \
  --action=batch \
  --sender <YOUR_STACKS_ADDRESS> \
  --steps '[
    {"contract":"SP2VCQJGH7PHP2DJK7Z0V48AGBHQAW3R3ZW1QF4N.zsbtc-v2-0","expression":"(contract-call? .zsbtc-v2-0 get-balance tx-sender)"},
    {"contract":"SM3VDXK3WZZSA84XXFKAFAF15NNZX32CTSG82JFQ4.sbtc-token","expression":"(contract-call? .sbtc-token get-balance tx-sender)"}
  ]'

install-packs

No additional packages required. Uses fetch() and commander.

Safety notes

• Read-only by design. This skill never broadcasts. It simulates only.
• Session isolation. Each simulation runs in a fresh stxer session. No cross-contamination between runs.
• Expression length limit. Capped at 2,000 characters to prevent abuse.
• Step limit. Maximum 20 steps per batch to prevent runaway simulations.
• Timeout. 15-second timeout per API call. Fails cleanly on timeout.
• No private keys needed. Simulation uses sender address, not signing keys.
• Honest verdict. If the Clarity expression returns (err ...), the skill reports it as unsafe. No sugar-coating.

Output contract

Simulate (single call)

{
  "status": "success | error",
  "action": "simulate",
  "data": {
    "session_id": "d1c27b645459c702feae3a7a637a4777",
    "result": {
      "outcome": "ok",
      "decoded": "(ok uint 276016)",
      "raw_hex": "070100000000000000000000000000043630",
      "safe_to_broadcast": true
    },
    "recommendation": "Simulation passed. Safe to broadcast this contract call."
  },
  "error": null
}

Batch (multi-step)

{
  "status": "success | error",
  "action": "batch",
  "data": {
    "session_id": "abc123",
    "total_steps": 2,
    "passed": 2,
    "failed": 0,
    "results": [
      {"step": 0, "outcome": "ok", "decoded": "(ok uint 276016)", "safe": true},
      {"step": 1, "outcome": "ok", "decoded": "(ok uint 204206)", "safe": true}
    ],
    "recommendation": "All steps passed. Safe to broadcast the transaction sequence."
  },
  "error": null
}

Simulation Proof

These simulations were run on mainnet state (2026-04-10) without broadcasting:

Test	Session	Expression	Result	Verdict
Balance read	d1c27b645459c702feae3a7a637a4777	get-balance tx-sender on zsbtc-v2-0	(ok uint 276016)	Safe
Impossible transfer	(second session)	transfer u99999999 on sbtc-token	(err uint 1)	Blocked

The impossible transfer would have aborted on-chain and wasted gas. The simulation caught it for free.

Architecture

[Agent wants to call contract]
        ↓
[contract-preflight simulate]
        ↓
[Create stxer session] → [Eval Clarity expression against mainnet state]
        ↓                           ↓
    [Ok result]               [Err result]
        ↓                           ↓
"Safe to broadcast"         "DO NOT broadcast"

Use cases

• DeFi operations: Verify Zest supply/withdraw, Bitflow swaps, ALEX trades before broadcast.
• Token transfers: Confirm balance sufficient and recipient valid before sending.
• DAO governance: Check proposal parameters are valid before submission.
• Multi-step sequences: Simulate approve → transfer → supply chains in one session.
• Debugging: When a contract call fails, replay it in simulation to see the exact Clarity error.

Limitations

• Simulates against current block state. Cannot predict future state changes (MEV, concurrent txs).
• Stxer API is a third-party service. If it's down, the skill reports the outage but cannot simulate.
• Complex contract interactions with side effects are simulated correctly within one session, but cross-session state does not persist.
• Does not validate post-conditions — only the Clarity return value.

Origin

Winner of AIBTC x Bitflow Skills Pay the Bills competition. Original author: @secret-mars Competition PR: https://github.com/BitflowFinance/bff-skills/pull/258