A read-only Postgres role, or Deedglass

If your agent only reads, use a read-only role. You don't need Deedglass. This page is for when that stops being enough.

Last checked 13 October 2026

The short answer

A role with SELECT and nothing else can't change data, whatever the agent sends. That is the boundary.¹
The read-only setting is a default a session can turn off again, so it is a second fence, not the first.²¹
Deedglass is for agents that must write: it decides each write, checks what Postgres committed and keeps a record.

When to choose A read-only role

The agent answers questions from the data and never needs to change it.
You can give it its own role with GRANT SELECT and no other privilege.¹
You want nothing new to install: roles and grants are part of Postgres.

When Deedglass fits

The agent has to write, even a little: ship an order, issue a refund, fix a record.
You want each write decided by a rule first, and checked against what Postgres committed after.
You need a record of what changed that someone else can check offline.

Side by side

Question	A read-only role	Deedglass
Stops all writes	Yes, when the role holds no write privilege¹	Only the writes your rules deny
Asks a person before risky actions	No: a role allows or refuses	Yes: approval in Slack, email or the Console
Checks what Postgres committed	Not needed: nothing is written	Yes: reads Postgres's change log
Signed record of each action, checkable offline	No; Postgres logs what you configure	Yes
Undoes rows	Not needed	Yes, with checks before each undo
Runs as	Part of Postgres	A single binary next to your agent
Price (checked 13 October 2026)	Free	Free self-host · Pro $49 a month

Use them together

Use both. Deedglass's agent connections run under a role you create, so the narrowest grants still apply under its rules. One documented trap is worth knowing: Datadog Security Labs showed that an archived Postgres MCP server's read-only mode ran each query in a read-only transaction that a stacked COMMIT; could end, after which later statements ran with the role's full rights.³ A role without write privileges doesn't have that problem.

What Deedglass doesn't do

It doesn't make a broad role safe. Grants come first; Deedglass decides and checks within them.
It doesn't stop reads. If the agent shouldn't see a table, don't grant SELECT on it.

Sources

1PostgreSQL 18 documentation: Privileges · postgresql.org · checked 13 October 2026
2PostgreSQL 18 documentation: Client connection defaults · postgresql.org · checked 13 October 2026
3Datadog Security Labs: SQL injection in the Postgres MCP server · securitylabs.datadoghq.com · checked 13 October 2026

Last checked 13 October 2026. We re-check these pages every month. If something is out of date, write to hello@deedglass.com.

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