Oracle Database / Connections & Oracle Net
ORA-12520: check the requested server type and available handlers
A descriptor requesting a server type that the listener cannot provide can fail even when other applications connect. Read the complete connect descriptor.
Environment and inputs: Oracle Database 19c baseline · SQL*Plus-compatible SQL unless RMAN, DGMGRL, ADRCI or a host shell is specified. Check release, patch level, edition and container first.
Start with this diagnostic
Use the stated SQL, RMAN, DGMGRL or host shell and replace sample names with approved targets. DBA and V$ views need authorized catalog access in the correct container. Do not grant broad privileges for an example.
lsnrctl services LISTENERRead the result
The listener shows handler types and availability for registered services. Compare the application's DEDICATED or SHARED requirement with actual handlers.
Resolve and verify
Confirm whether the application requires shared or dedicated server mode, then align the supported descriptor and instance configuration through the DBA. Retest with the original driver.
Operational precautions
Do not switch server mode globally to repair one client. Shared-server behavior and limits differ from dedicated-server behavior.