experimental_
Secret for HMAC-signing tool-approval requests (upstream v6.0.202 experimental_toolApprovalSecret). When set, every approval request the loop issues carries a signature over (approvalId, toolCallId, toolName, input), and a replayed approval is re-validated FAIL-CLOSED before the tool executes: missing/invalid signature throws AgentError.InvalidToolApprovalSignature, the input is re-validated against the tool's schema, and a tool that no longer requires approval is denied rather than run — so a client cannot forge, re-target, or input-swap an approval. Experimental upstream (can break in patches).