Search for posts within a specific newsletter
AI agents call search_newsletter to retrieve information from Substack Reader MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
Searching for posts is a retrieval operation that queries data without side effects. It does not create, modify, delete, or execute any actions. The low severity reflects the minimal blast radius—the worst outcome is retrieving unintended newsletter content the user has access to, which is a read-only concern.
From the tool's definition Tool name and description indicate search functionality: 'Search for posts within a specific newsletter'. The server description states it 'enables users to manage and read Substack content' and supports 'searching within newsletters'.
Attacks that exploit this kind of access
Search for posts within a specific newsletter. It is categorised as a Read tool in the Substack Reader MCP Server MCP Server, which means it retrieves data without modifying state.
Register the Substack Reader MCP Server MCP server in PolicyLayer and add a rule for search_newsletter: 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 Substack Reader MCP Server. Nothing to install.
search_newsletter 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 search_newsletter 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 search_newsletter. 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.
search_newsletter is provided by the Substack Reader MCP Server MCP server (kealu-inc/substack-reader-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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