get_amzn_stock_info

A plain (non-temporal) permit showing that an MCP-manifest input field validates at context.input.stock. The tool’s stock argument comes from the MCP manifest’s inputSchema, so in the generated Drupe schema it lands at context.input.stock — meaning an ordinary Cedar when clause type-checks against it. The rule allows GetStockInfo only when the requested stock is AMZN; everything else is denied.

The trace shows both cases:

• @0 — alice requests GetStockInfo for AMZN → allow.
• @100 — bob requests GetStockInfo for MSFT → deny.

Referenced by guide/11-mcp-schema-generation.md.

Policy

// Permit reading stock info only for AMZN. The tool's `stock` argument comes
// from the MCP manifest's inputSchema, so in the generated Drupe schema it
// lands at `context.input.stock` — meaning a plain Cedar `when` clause validates
// against it. This is the policy from guide chapter 11 ("What you get"),
// promoted to a complete rule with an @id.
@id("get_amzn_stock_info")
permit (
    principal,
    action == Drupe::Action::"GetStockInfo",
    resource
)
when { context.input.stock == "AMZN" };

Schema

namespace Drupe {
  type ApproveSaleInput = {
    shares: Long,
    stock: String
  };

  type ApproveSaleOutput = {
    approved: Bool
  };

  type ContentFilterFinding = {
    severityScore: decimal
  };

  type GetStockInfoInput = {
    stock: String
  };

  type GetStockInfoOutput = {
    info: String
  };

  type PromptAttackFinding = {
    severityScore: decimal
  };

  type SellSharesInput = {
    shares: Long,
    stock: String
  };

  type SellSharesOutput = {
    proceeds: decimal
  };

  type SensitiveInfoFinding = {
    confidenceScore: decimal
  };

  type SystemContext = {
    now: datetime
  };

  entity Gateway;

  entity IamEntity = {
    id: String
  };

  entity OAuthUser = {
    id: String
  } tags String;

  entity UnauthenticatedUser;

  action "ApproveSale" in [Action::"CallTool"] appliesTo {
    principal: [IamEntity, OAuthUser, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      input: ApproveSaleInput,
      output?: ApproveSaleOutput,
      system: SystemContext
    }
  };

  action "CallTool" in [Action::"Mcp"] appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      system: SystemContext
    }
  };

  action "GetStockInfo" in [Action::"CallTool"] appliesTo {
    principal: [IamEntity, OAuthUser, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      input: GetStockInfoInput,
      output?: GetStockInfoOutput,
      system: SystemContext
    }
  };

  action "Http" appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      system: SystemContext
    }
  };

  action "InvokeAgent" in [Action::"Http"] appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      input?: {      },
      system: SystemContext
    }
  };

  action "InvokeLLM" in [Action::"Http"] appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      input?: {      },
      system: SystemContext
    }
  };

  action "Mcp" appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      system: SystemContext
    }
  };

  action "SellShares" in [Action::"CallTool"] appliesTo {
    principal: [IamEntity, OAuthUser, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      input: SellSharesInput,
      output?: SellSharesOutput,
      system: SystemContext
    }
  };

  action "UnknownTool" in [Action::"CallTool"] appliesTo {
    principal: [OAuthUser, IamEntity, UnauthenticatedUser],
    resource: [Gateway],
    context: {
      system: SystemContext
    }
  };
}

Trace

@0 scope(principal: Drupe::OAuthUser::"alice", resource: Drupe::Gateway::"gw1") request_context(input: { stock: "AMZN" }) Drupe::Action::"GetStockInfo"::request(input: { stock: "AMZN" }, callerPrincipal: Drupe::OAuthUser::"alice", callerResource: Drupe::Gateway::"gw1", requestId: "u1")
@100 scope(principal: Drupe::OAuthUser::"bob", resource: Drupe::Gateway::"gw1") request_context(input: { stock: "MSFT" }) Drupe::Action::"GetStockInfo"::request(input: { stock: "MSFT" }, callerPrincipal: Drupe::OAuthUser::"bob", callerResource: Drupe::Gateway::"gw1", requestId: "u2")

Expected Output

@0 (time point 0): ALLOW  [rules: 0]
@100 (time point 1): DENY