How to meet it
OWASP AISVS v1.01
Model Context Protocol (MCP) Security · Schema, Message, and Input Validation
Sign MCP tool responses
Verify that MCP servers sign tool responses with a unique nonce and timestamp so MCP clients can detect replay attempts.
OWASP AISVS v1.01, requirement 10.4.6
In plain English
Sign tool responses with a unique nonce and timestamp, so clients can spot replayed responses.
What good looks like
- Responses carry a signature, nonce and timestamp.
- Clients verify them and reject replays.
- Clock tolerance is small.
How teams usually do it
- Server-side signing with a published key, and a nonce cache in the client.
Evidence an auditor would accept
- Response format and replay tests.
Common mistakes
- Signing responses that clients never check.
Not sure which requirements apply to your system? Take the assessment.
Requirement text from the OWASP AI Security Verification Standard, licensed under CC BY-SA 4.0. This guidance is shared under the same licence.