version: "1.0.1" name: fhir-api description: Expert guidance for implementing FHIR RESTful API servers and clients following the HL7 FHIR specification. Use this skill when implementing a FHIR server with REST endpoints, building a FHIR client, designing FHIR API routes and handlers, implementing FHIR operations (read, create, update, delete, search, history), working with FHIR bundles, batch requests, or transactions, handling FHIR content negotiation, headers, and versioning, or implementing conditional operations. Trigger keywords include "FHIR REST", "FHIR API", "FHIR server", "FHIR client", "FHIR endpoint", "FHIR operations", "RESTful FHIR", "implement FHIR".

FHIR REST API implementation

This skill provides guidance for implementing FHIR RESTful APIs according to the HL7 FHIR specification (R4/R5).

URL structure

All FHIR REST URLs follow the pattern:

[base]/[type]/[id]

• [base]: Service base URL (e.g., https://fhir.example.org/r4)
• [type]: Resource type (e.g., Patient, Observation)
• [id]: Logical resource ID

URLs are case-sensitive and use UTF-8 encoding.

Core operations

For detailed specifications of each operation, see references/operations.md.

Content negotiation

MIME types

Use the Accept header for response format and Content-Type for request body format.

The _format query parameter overrides Accept when clients cannot set headers.

FHIR version

Specify version via MIME type parameter:

Accept: application/fhir+json; fhirVersion=4.0

Version mappings: 1.0 (R2), 3.0 (R3), 4.0 (R4), 4.3 (R4B), 5.0 (R5).

Required headers

Request headers

Response headers

Versioning and optimistic locking

FHIR uses weak ETags for version tracking:

1. Server returns ETag: W/"[versionId]" with responses
2. Client sends If-Match: W/"[versionId]" with updates
3. Server returns 412 Precondition Failed if version mismatch

Implement version-aware updates when CapabilityStatement.rest.resource.versioning is versioned-update.

Error handling

Return OperationOutcome resources for all errors:

{
    "resourceType": "OperationOutcome",
    "issue": [
        {
            "severity": "error",
            "code": "invalid",
            "diagnostics": "Patient.birthDate: Invalid date format"
        }
    ]
}

Status codes

Prefer header

Control response content with Prefer:

For async operations, use Prefer: respond-async to get 202 Accepted with status polling URL.

Implementation checklist

Server implementations should:

1. Implement CapabilityStatement at /metadata
2. Support content negotiation (JSON at minimum)
3. Return proper ETags for versioned resources
4. Include Location header on create/update
5. Return OperationOutcome for all errors
6. Support _format parameter fallback
7. Honour Prefer header for response content

References

• operations.md: Detailed operation specifications
• search.md: Search parameter and result handling
• batch-transaction.md: Batch and transaction processing