Instance reads (GET /{type}/{id}) ignore Accept: application/fhir+xml and return JSON; search honors it

[jmandel]

Summary

A FHIR instance read (GET /{type}/{id}) ignores Accept: application/fhir+xml and returns JSON, even though the server does serve XML — a search (?url=…) for the same resource correctly returns XML. So content negotiation is honored on search but silently dropped on instance reads.

Repro (live)

# instance read — asks for XML, gets JSON
curl -s -o /dev/null -w 'instance  -> %{http_code} %{content_type}\n' \
  -H 'Accept: application/fhir+xml' 'https://tx.fhir.org/r4/CodeSystem/v3-ActCode'
# => 200 application/fhir+json   (BUG)

# search — asks for XML, gets XML
curl -s -o /dev/null -w 'search    -> %{http_code} %{content_type}\n' \
  -H 'Accept: application/fhir+xml' 'https://tx.fhir.org/r4/CodeSystem?url=http://terminology.hl7.org/CodeSystem/v3-ActCode'
# => 200 application/fhir+xml    (correct)

(Same on ConceptMap/{id}. ValueSet/$expand and the search paths honor XML; instance reads do not.)

Expected

Per http.html content negotiation, the server SHALL return the requested representation (or 406 Not Acceptable if it cannot) — not silently switch to JSON. Since search already returns XML, instance reads should too.

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Confirmed live against https://tx.fhir.org on 2026-06-06 (software FHIRsmith 0.9.6). Source line refs are from the published main branch.

[grahamegrieve]

fixed