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The policy engine lets you attach business descriptions to tables and columns via a YAML file. These descriptions are merged into describe_table and discover responses, giving the AI model context about what your data means — not just what types it has.

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Enabling the policy engine

Set the POLICY_FILE environment variable or --policy-file CLI flag to the path of your YAML file: 
POLICY_FILE=./policy.yaml isthmus
Or in your MCP client config: 
{
  "mcpServers": {
    "isthmus": {
      "command": "isthmus",
      "env": {
        "DATABASE_URL": "postgres://user:pass@localhost:5432/mydb",
        "POLICY_FILE": "/path/to/policy.yaml"
      }
    }
  }
}

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YAML format

context:
  tables:
    schema.table_name:
      description: "Business description of the table"
      columns:
        column_name: "Business description of the column"
Keys are fully qualified: schema.table_name. Descriptions only fill empty comments — Postgres COMMENT ON values always take precedence.

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Full example

context:
  tables:
    public.customers:
      description: "Registered platform customers. One row per customer account."
      columns:
        id: "Unique customer identifier (UUID)"
        email: "Primary email address, used for login and notifications"
        name: "Full display name"
        created_at: "Account creation timestamp (UTC)"

    public.orders:
      description: "Purchase orders. Lifecycle: draft -> pending -> paid -> shipped -> delivered."
      columns:
        id: "Unique order identifier (UUID)"
        customer_id: "FK to customers.id — the buyer"
        status: "Order status: draft, pending, paid, shipped, delivered, cancelled"
        amount_cents: "Order total in cents (USD), before tax"
        created_at: "Order creation timestamp (UTC)"

    public.products:
      description: "Product catalog. Each row is a distinct SKU."
      columns:
        id: "Unique product identifier (UUID)"
        name: "Product display name"
        price_cents: "Unit price in cents (USD)"
        category: "Product category for filtering and reporting"
        stock: "Current inventory count"

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How enrichment works

Without a policy file, describe_table returns raw schema information: 
{
  "name": "amount_cents",
  "data_type": "integer",
  "is_nullable": false,
  "is_primary_key": false
}
With the policy file above, the same column includes a comment: 
{
  "name": "amount_cents",
  "data_type": "integer",
  "is_nullable": false,
  "is_primary_key": false,
  "comment": "Order total in cents (USD), before tax"
}
This tells the AI model that amount_cents is in cents and in USD — so it knows to divide by 100 when displaying amounts, and not to confuse it with other currencies.

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Tips for writing descriptions

  • Be specific about units — “Price in cents (USD)” is better than “Price”
  • Document enum values — “Status: draft, pending, paid, shipped, delivered, cancelled”
  • Explain relationships — “FK to customers.id — the buyer”
  • Describe lifecycles — “Lifecycle: draft -> pending -> paid -> shipped -> delivered”
  • Clarify ambiguous names — “created_at: Account creation timestamp (UTC)” vs. just “timestamp”
  • Think like an AI — what would you need to know to write correct SQL against this table?

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Column masking

The policy file also supports per-column masking to protect PII and sensitive data. Add a mask directive to any column: 
context:
  tables:
    public.customers:
      columns:
        email:
          description: "Primary email address"
          mask: "redact"
Four mask types are available: redact, hash, partial, and null. Masking is applied to both query results and describe_table sample rows. See Column Masking for the full reference — mask types, examples, conflict detection, and best practices.

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Validation

The policy file is validated at startup. Isthmus will reject files with:
  • Empty table keys
  • Empty column keys within a table
  • Invalid mask values (allowed: redact, hash, partial, null)
  • Conflicting masks for the same column name across different tables
If the policy file is invalid, Isthmus exits with an error before starting the MCP server.