Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsFirst identify what failed: an ordinary database table insert or a Supabase Storage upload. For a table insert, check the request role’s table grant and the INSERT policy’s WITH CHECK condition. For a Storage upload, also check whether a SELECT policy lets the caller read the new object’s metadata. The error alone does not identify which layer denied the request.
Start by identifying the operation
Confirm the target schema and table, the caller’s role, and whether the failing call is a direct database/API insert or a Storage upload. The distinction matters: a Storage upload may fail while returning object metadata, even if its INSERT policy permits the new object.
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| Request | First checks |
|---|---|
| Ordinary table INSERT | Active role and table INSERT grant; then the matching INSERT policy and its WITH CHECK expression. |
| Supabase Storage upload | INSERT authorization plus SELECT access to the object metadata returned by the upload. |
For an ordinary table insert, check grants before policies
PostgreSQL checks table privileges before row-level security policies. Supabase distinguishes the two layers: grants determine whether a role may perform an operation at all, while policies restrict which rows it may affect. A missing INSERT grant can raise 42501 before any policy runs; a row rejected by an INSERT policy can also raise that error. See Supabase’s Row Level Security documentation.
- Verify the role used by the real request. Supabase maps unauthenticated requests to
anonand signed-in requests toauthenticated. Check the request context rather than assuming which role is active. - Verify the table grant. If the role is meant to insert, confirm it has INSERT permission on the target table. Do not try to compensate for a missing grant by making a policy broader.
- Inspect the INSERT policy’s
WITH CHECK. This condition evaluates the proposed new row. Compare the values in the actual payload with the policy condition, and make sure the policy applies to the caller’s role.
For an owner-only row, Supabase documents this example condition: with check ((select auth.uid()) = user_id). If the request’s user_id differs from the authenticated user ID, the proposed row does not satisfy that check.
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Check whether the request has an authenticated user
auth.uid() returns null when there is no authenticated user, for example if the request has no access token or the session has expired. A comparison between null and a row’s user_id will not pass the owner check. Verify the session and the role being sent; do not weaken ownership rules just to make the insert succeed. Supabase explains this behavior in its Row Level Security documentation.
For a Storage upload, check SELECT access to the returned metadata
Storage has an additional failure path. Supabase says its API performs an INSERT followed by RETURNING * to provide object details to the client. If the caller cannot read the new object’s metadata under a SELECT policy, the upload may fail even when the INSERT policy is correct and the JWT is valid. Consult Supabase’s Storage upload troubleshooting guide.
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Review the SELECT policy for the object record being created. It needs to allow the intended user to read that record, with conditions aligned to the relevant user, bucket, or path. For example, a user-scoped INSERT rule should have corresponding SELECT coverage for that user’s objects.
Retest both access and denial outcomes
Test the intended access matrix with the relevant roles and identities. Supabase recommends separate policies for SELECT, INSERT, UPDATE, and DELETE, and tests for both allowed and denied cases for anon and authenticated.
- For an allowed insert, use the intended identity and row values, then verify that the row was actually written. A test that only reports the operation did not throw an error can miss a write that affected zero rows.
- For a denied insert, verify that a disallowed row is rejected.
- Distinguish a zero-row result from an error. A
USINGcondition can filter rows so an operation affects zero rows, while a missing grant or failed INSERTWITH CHECKraises42501.
Supabase’s RLS guidance advises verifying allowed and denied operations rather than relying on a success assertion alone.
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- RLS does not replace table grants. Make both grants and policies intentional for tables exposed through the API.
- Do not put a secret or service-role key in browser code to bypass a user-facing policy error. The
service_rolerole bypasses RLS, and secret keys must remain server-side. - Avoid basing authorization on user-editable metadata. Supabase notes that users can update
raw_user_meta_data;raw_app_meta_datais not user-editable and can hold authorization data. JWT claims may not reflect a metadata update until the user’s JWT is refreshed.
These security considerations are covered in Supabase’s Row Level Security documentation.
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