Sample
Negotiate response content types with res.format, automatic Vary and Content-Type header injection, wildcard fallback, and 406 Not Acceptable error delegation
sha256:2752bf7c02fd809432943b420ccc39a51bafb6c2fadccf23663bfaac4281c1d9
PUBLISHED
L3_CONTRACT_PASS
MIT-0
Execution evidence
Declared environment and signed verification runs are separated so you can see exactly what this sample proves.
Evidence basisSigned contract pass
Verification receipts1
Verification levelL3_CONTRACT_PASS
Declared environment
- Execution context
- node
- Operating system
- linux
- Architecture
- x64
- Runtime
- node
- Language
- node
- Package manager
- npm
Verification-run environments
- Execution context
- node 22
- Operating system
- linux alpine · musl
- Architecture
- x64
- Runtime
- node 22
- Language
- javascript
- Package manager
- npm
- Execution
- container · docker
CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · node-typescript@1 · 2026-08-17
Case
- Goal
- Negotiate response content types with res.format, automatic Vary and Content-Type header injection, wildcard fallback, and 406 Not Acceptable error delegation HOW
- Packages
-
express 4.21.0
- Environment
- node
- Created
- 2026-08-17T06:38:47Z
Commonly assumed
res.format requires handlers to set their own Content-Type header and falls back to a 200 response or uncaught exception when the Accept header is unmatched
The sample's author recorded this as what a developer or model would expect here. The contract below is what actually ran.
Contract
- res.format automatically sets the Vary: Accept header and pre-populates Content-Type matching the negotiated format before executing the handler
- res.format with wildcard Accept */* selects the first property key enumerated in the format object
- res.format negotiates formats by request Accept q-factor weights regardless of object key definition order
- res.format without a matching format or default property forwards an HTTP 406 Not Acceptable Error with err.types to next()
- res.format translates extension shortcuts like text and json into canonical MIME types in err.types
- res.format executes the default callback property when no format matches the Accept header without triggering a 406 error
Files
- NOTES.md
- csx.json
- package-lock.json
- package.json
- test/contract.mjs
Download the source artifact (tar.gz)
Origin Seeder
csx-seed
Verification receipts
- node 22 · linux alpine/x64 · docker · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · node-typescript@1 · 2026-08-17 · ed25519:d91480838ac982c9