startWebAuthnLoginWithId
Start a WebAuthn authentication ceremony by generating a new challenge for the user
This record as markdown: /tools/io-fusionauth-mcp-api/startwebauthnloginwithid.md
What startWebAuthnLoginWithId does on Fusionauth
AI agents invoke startWebAuthnLoginWithId to trigger actions in Fusionauth. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call: builds kicked off, notifications sent, workflows started.
Why startWebAuthnLoginWithId is rated High
This tool executes an authentication ceremony startup operation rather than passively reading data. While it does not directly modify user data or have destructive effects, it triggers a state-changing operation (challenge generation) on the server that affects subsequent authentication flows.
From the tool's definition Generates a new challenge for the user in a WebAuthn authentication ceremony; this is an active operation that initiates a cryptographic process with side effects (challenge creation/state change on the server).
Attacks that exploit this kind of access
The rule that runs startWebAuthnLoginWithId safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Fusionauth, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For startWebAuthnLoginWithId, this is the rule to start with:
startWebAuthnLoginWithId stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Fusionauth, apply this rule, and every startWebAuthnLoginWithId call is checked against it from then on.
Questions about startWebAuthnLoginWithId
Start a WebAuthn authentication ceremony by generating a new challenge for the user. It is categorised as a Execute tool in the Fusionauth MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Fusionauth MCP server in PolicyLayer and add a rule for startWebAuthnLoginWithId: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Fusionauth. Nothing to install.
startWebAuthnLoginWithId is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the startWebAuthnLoginWithId rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for startWebAuthnLoginWithId. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
startWebAuthnLoginWithId is provided by the Fusionauth MCP server (@fusionauth/mcp-api). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
More on Fusionauth, and thousands of servers like it.
Across the catalogue