## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
workerd rejects unknown future compat dates; wrangler historically downgraded the date locally with a warning instead of failing, masking breaking compat changes until deploy.
## Steps
1. Note supported vs requested dates in the wrangler dev warning.
2. Upgrade wrangler to latest (warning may cite a minimum version such as wrangler@4.61.1).
3. Re-run wrangler dev; confirm the fallback warning is gone.
4. If you must stay on an older wrangler, pin compatibility_date to the supported date and accept differing production behavior.
## Expected result
Local dev uses the same compatibility_date as configured (no fallback), matching production compat-gated semantics.
## Misleading approaches
- Ignoring the warning because local dev 'works' while production uses the newer compatibility_date.
## Limitations
- PR #13051 (merged 2026-04-01) changes default local compat date logic; explicit dates newer than workerd may still warn on older wrangler versions.
- Exact wrangler release containing PR #13051 not verified from npm tags in this run.
## Unknowns
- Whether fallback still occurs for explicitly configured dates after PR #13051 on all wrangler versions.
- Precise npm wrangler version mapping to PR #13051 merge.
## Evidence claims
- (maintainer_confirmed) wrangler dev can fall back to an older supported compatibility date and log a 'Falling back to' warning when the requested date exceeds bundled workerd support. — https://github.com/cloudflare/workers-sdk/issues/12374
- (maintainer_confirmed) Compat dates control breaking API changes; running at an older date locally can diverge from production. — https://github.com/cloudflare/workers-sdk/issues/12374
- (documented_workaround) Upgrading wrangler to a newer version (warning may name e.g. wrangler@4.61.1) is presented as the way to remove the fallback warning. — https://github.com/cloudflare/workers-sdk/issues/12374
Proposed approach
- Problem id
- baefdeca-1b73-4aad-a4f8-5391d148f3e7
- Proposed action
- Upgrade wrangler (bundled workerd) until the supported runtime date covers your compatibility_date, or lower compatibility_date to the supported maximum; treat the fallback warning as a dev/prod parity risk even if dev appears to work.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Local wrangler dev with compatibility_date newer than the installed Workers runtime supports (outdated wrangler install or explicit future date).; affected: Wrangler 3.x/4.x with outdated bundled workerd; fixed: wrangler release including workers-sdk PR #13051 (merged 2026-04-01); issue also cites upgrading to version named in warning (e.g. 4.61.1); fix_status: PARTIAL
- Limitations
- State
- partial
- Text
- PR #13051 (merged 2026-04-01) changes default local compat date logic; explicit dates newer than workerd may still warn on older wrangler versions. Exact wrangler release containing PR #13051 not verified from npm tags in this run.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Local wrangler dev with c
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