Degrade transient optional list failures in Modern enumeration - #6348
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lorenzozanee wants to merge 1 commit into
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Degrade transient optional list failures in Modern enumeration#6348lorenzozanee wants to merge 1 commit into
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A rate-limited Modern backend that answers an optional capability enumeration (resources/list, resources/templates/list or prompts/list) with a transient HTTP 429 was misclassified as ErrBackendUnavailable, flipping a reachable backend to unhealthy/degraded and aggregate Ready=false even though initialize and tools/list succeeded. Degrade a transient failure on an optional list to an empty result instead of failing the whole enumeration. tools/list stays fatal; the -32601 not-implemented degradation is unchanged. Fixes stacklok#6347 Signed-off-by: lorenzozanee <wyz0707@proton.me>
lorenzozanee
requested review from
ChrisJBurns,
JAORMX,
amirejaz,
jerm-dro,
jhrozek and
tgrunnagle
as code owners
August 16, 2026 05:19
reyortiz3
approved these changes
Aug 17, 2026
Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #6348 +/- ##
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+ Coverage 72.96% 73.01% +0.05%
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Files 742 742
Lines 78236 78250 +14
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+ Hits 57085 57138 +53
+ Misses 17172 17117 -55
- Partials 3979 3995 +16 ☔ View full report in Codecov by Harness. 🚀 New features to boost your workflow:
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Summary
A reachable Modern backend can be marked unavailable when an optional capability enumeration (resources/templates/list, resources/list or prompts/list) returns a transient HTTP 429. The 429 is classified as transient in the Modern layer, but the enumeration treated every error other than -32601 as fatal, so the transient failure surfaced as ErrBackendUnavailable and flipped the backend to unhealthy/degraded even though initialize and tools/list had succeeded. With several VirtualMCPServers probing the same rate-limited backend, aggregate Ready flips to False (BackendsDegraded).
A transient failure on an optional list now degrades to an empty result (with a WARN) instead of failing the whole enumeration. tools/list stays fatal: a backend that cannot list its tools is genuinely unavailable. The existing -32601 not-implemented degradation is unchanged.
Fixes #6347
Type of change
Test plan
task test)task test-e2e)task lint-fix)The regression tests fail on the base commit with the exact error from the report ("backend unavailable: failed to list resource templates ... modern backend returned a transient error: HTTP 429") and pass with the fix; the affected vmcp packages are green.
Special notes for reviewers
This fix targets the Modern path reported in #6347. The Legacy path still treats a 429 on optional enumeration as fatal; detecting the transient class there would require reaching into go-sdk internals and is left for a follow-up. This is orthogonal to #6246 (partialFailureMode wiring), which does not touch error classification.