Preventing Prompt Injection from Draining Agent Wallets

Prompt injection is a real payment risk because it targets decision pathways, not only infrastructure.

If an agent can execute tool calls with financial side effects, prompt manipulation can become direct monetary loss.

Threat model clearly

Define attacker goals:

• force unauthorized destination payments
• bypass spending policy checks
• trigger repeated low-value drains
• exfiltrate sensitive payment context

A concrete threat model improves practical controls.

Separate intent from execution

Never let raw model output directly call payment execution.

Use a control plane:

1. model proposes payment intent
2. structured validator normalizes intent
3. policy engine evaluates constraints
4. signer executes only approved payload

Add destination trust layers

Before any transfer:

• verify destination format
• check allowlist or trusted registry
• score destination novelty
• escalate unknown recipients

Unknown destination plus high value should default to review.

Use constrained tool schemas

Tool input schemas should be strict:

• typed amount fields
• bounded decimals and max values
• explicit currency and chain enums
• immutable transaction purpose fields

This reduces attack surface from free-form prompt text.

Monitor behavioral drift

Prompt injection often appears as behavior drift:

• sudden destination diversity
• unusual transaction timing
• spikes in denied intents
• repeated near-threshold attempts

Automated anomaly detection should feed into dynamic policy tightening.

Security posture summary

Treat model output as untrusted. Treat policy as mandatory. Treat signing as the final guarded gate.

When teams enforce these layers, prompt injection becomes a controllable risk instead of an existential wallet threat.