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Fix prompt caching round-trip on cache miss#3666

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jlowin merged 1 commit intomainfrom
fix/caching-prompt-roundtrip
Mar 28, 2026
Merged

Fix prompt caching round-trip on cache miss#3666
jlowin merged 1 commit intomainfrom
fix/caching-prompt-roundtrip

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ResponseCachingMiddleware.on_get_prompt has an inconsistency with the other cache-miss paths in on_call_tool and on_read_resource. Those methods wrap the result into a cacheable form, store it, and then unwrap() before returning — ensuring the caller always gets a value that has been through the same serialization round-trip regardless of whether it came from cache or from upstream. on_get_prompt skips the unwrap() on the initial miss path, returning the raw PromptResult directly. This means the first call returns a structurally different object than subsequent cached calls, which can surface as subtle type or equality mismatches downstream.

The fix aligns on_get_prompt with the existing pattern:

# Before — raw value returned on miss, unwrapped value on hit
value: PromptResult = await call_next(context=context)
await self._get_prompt_cache.put(key=cache_key, value=CachablePromptResult.wrap(value), ...)
return value

# After — unwrapped value returned in both paths
value: PromptResult = await call_next(context=context)
cached_value = CachablePromptResult.wrap(value)
await self._get_prompt_cache.put(key=cache_key, value=cached_value, ...)
return cached_value.unwrap()

Closes #3664

🤖 Generated with Claude Code

@marvin-context-protocol marvin-context-protocol Bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. server Related to FastMCP server implementation or server-side functionality. labels Mar 27, 2026
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Clean fix, aligns on_get_prompt with the established wrap/store/unwrap pattern.

@jlowin jlowin merged commit 2491993 into main Mar 28, 2026
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@jlowin jlowin deleted the fix/caching-prompt-roundtrip branch March 28, 2026 01:42
@strawgate strawgate changed the title Fix prompt caching round-trip on cache miss [DO NOT MERGE] Fix prompt caching round-trip on cache miss Mar 28, 2026
@strawgate strawgate changed the title [DO NOT MERGE] Fix prompt caching round-trip on cache miss Fix prompt caching round-trip on cache miss Mar 28, 2026
jlowin added a commit that referenced this pull request Mar 30, 2026
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Jeremiah Lowin <[email protected]>
Co-authored-by: Marvin Context Protocol <41898282+Marvin Context [email protected]>
Co-authored-by: voidborne-d <[email protected]>
Co-authored-by: marvin-context-protocol[bot] <225465937+marvin-context-protocol[bot]@users.noreply.github.com>
Co-authored-by: Claude <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: d 🔹 <[email protected]>
Co-authored-by: Jeremiah Lowin <[email protected]>
Co-authored-by: nightcityblade <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
Co-authored-by: Claude Sonnet 4.6 <[email protected]>
Co-authored-by: Bill Easton <[email protected]>
Co-authored-by: Sumanshu Nankana <[email protected]>
Co-authored-by: Eric Robinson <[email protected]>
Co-authored-by: Martim Santos <[email protected]>
Co-authored-by: d 🔹 <[email protected]>
Co-authored-by: Matthieu B <[email protected]>
Co-authored-by: Sascha Buehrle <[email protected]>
Co-authored-by: Hakancan <[email protected]>
Co-authored-by: nightcityblade <[email protected]>
Co-authored-by: Matt Hallowell <[email protected]>
Co-authored-by: nate nowack <[email protected]>
Co-authored-by: Bill Easton <[email protected]>
Co-authored-by: Marcus Shu <[email protected]>
Co-authored-by: Rushabh Doshi <[email protected]>
Co-authored-by: AIKAWA Shigechika <[email protected]>
Co-authored-by: Jeremy Simon <[email protected]>
Co-authored-by: Miguel Miranda Dias <[email protected]>
Co-authored-by: Anthony James Padavano <[email protected]>
Co-authored-by: Mostafa Kamal <[email protected]>
Fix auto-close MRE script posting comment without closing (#3386)
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fix: increase sleep duration in proxy cache tests (#3567)
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ResponseCachingMiddleware.on_get_prompt doesn't round-trip through wrap/unwrap on cache miss

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