Check the tail (next sequence number) of a stream.
AI agents call check_tail to retrieve information from S2 StreamStore MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
The tool performs a read-only operation to query the next sequence number of a stream, which is metadata retrieval with no capability to modify, delete, or execute arbitrary operations. This is consistent with Read category tools that retrieve or query data with no side effects.
From the tool's definition Tool name 'check_tail' and description 'Check the tail (next sequence number) of a stream' indicate a query operation that retrieves metadata about stream state without modification or side effects.
Attacks that exploit this kind of access
Check the tail (next sequence number) of a stream. It is categorised as a Read tool in the S2 StreamStore MCP Server MCP Server, which means it retrieves data without modifying state.
Register the S2 StreamStore MCP Server MCP server in PolicyLayer and add a rule for check_tail: 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 S2 StreamStore MCP Server. Nothing to install.
check_tail is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the check_tail 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 check_tail. 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.
check_tail is provided by the S2 StreamStore MCP Server MCP server (s2-streamstore/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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