Error: Failed to decode current backend config

TL;DR: The .terragrunt-cache holds a backend record written by an older version that the current binary can't decode. The error suggests -reconfigure, but the real fix is deleting the stale .terragrunt-cache so init starts clean.

Error: Failed to decode current backend config

The backend configuration created by the most recent run of "terraform init"
could not be decoded: unsupported attribute "lock_table". The configuration
may have been initialized by an earlier version that used an incompatible
configuration structure. Run "terraform init -reconfigure" to force
re-initialization of the backend.

Steps

1. Delete the unit's cache: rm -rf .terragrunt-cache (from the unit directory).

Expected: the stale backend record is gone.

1. Re-run terragrunt init.

Expected: Initializing the backend... completes without the decode error.

1. If MANY units are affected (e.g. after a version bump across the repo): find . -type d -name ".terragrunt-cache" -prune -exec rm -rf {} +.

Expected: all units re-init clean.

When this applies

• Failed to decode current backend config / unsupported attribute right after upgrading the tofu/terraform version or the backend schema.
• The config itself didn't change; only the tool version did.

When it doesn't apply

• You actually CHANGED the backend config (bucket, key, region): then -reconfigure vs -migrate-state is the real decision, not cache clearing.
• Backend initialization required: that's a missing init, not a stale one.

Tool versions

All Terragrunt versions (cache layout unchanged across 1.x).

Why it happens

Terragrunt copies the module into .terragrunt-cache and init records the backend config there. After a version upgrade, the cached record uses a schema the new binary can't decode, and init fails before it ever re-reads your current config.

Edge cases

• There is no terragrunt clear-cache command (removed in the 1.0 redesign); deleting the directory yourself IS the supported operation.
• Set TG_DOWNLOAD_DIR to move the cache outside the working tree if stale caches keep biting in CI.