FHIR Developer Skill

Quick Reference

HTTP Status Codes

Code	When to Use
`200 OK`	Successful read, update, or search
`201 Created`	Successful create (include `Location` header)
`204 No Content`	Successful delete
`400 Bad Request`	Malformed JSON, wrong resourceType
`401 Unauthorized`	Missing, expired, revoked, or malformed token (RFC 6750)
`403 Forbidden`	Valid token but insufficient scopes
`404 Not Found`	Resource doesn't exist
`412 Precondition Failed`	If-Match ETag mismatch (NOT 400!)
`422 Unprocessable Entity`	Missing required fields, invalid enum values, business rule violations

Required Fields by Resource (FHIR R4)

Resource	Required Fields	Everything Else
Patient	(none)	All optional
Observation	`status` , `code`	Optional
Encounter	`status` , `class`	Optional (including `subject` , `period` )
Condition	`subject`	Optional (including `code` , `clinicalStatus` )
MedicationRequest	`status` , `intent` , `medication[x]` , `subject`	Optional
Medication	(none)	All optional
Bundle	`type`	Optional

Required vs Optional Fields (CRITICAL)

Only validate fields with cardinality starting with "1" as required.

Cardinality	Required?
`0..1` , `0..*`	NO
`1..1` , `1..*`	YES

Common mistake: Making

subject

period

required on Encounter. They are 0..1 (optional).

Value Sets (Enum Values)

Invalid enum values must return

422 Unprocessable Entity

Patient.gender

male | female | other | unknown

Observation.status

registered | preliminary | final | amended | corrected | cancelled | entered-in-error | unknown

Encounter.status

planned | arrived | triaged | in-progress | onleave | finished | cancelled | entered-in-error | unknown

Encounter.class (Common Codes)

Code	Display	Use
`AMB`	ambulatory	Outpatient visits
`IMP`	inpatient encounter	Hospital admissions
`EMER`	emergency	Emergency department
`VR`	virtual	Telehealth

Condition.clinicalStatus

active | recurrence | relapse | inactive | remission | resolved

Condition.verificationStatus

unconfirmed | provisional | differential | confirmed | refuted | entered-in-error

MedicationRequest.status

active | on-hold | cancelled | completed | entered-in-error | stopped | draft | unknown

MedicationRequest.intent

proposal | plan | order | original-order | reflex-order | filler-order | instance-order | option

Bundle.type

document | message | transaction | transaction-response | batch | batch-response | history | searchset | collection

Validation Pattern

Python/FastAPI:

python

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

def operation_outcome(severity: str, code: str, diagnostics: str):
    return {
        "resourceType": "OperationOutcome",
        "issue": [{"severity": severity, "code": code, "diagnostics": diagnostics}]
    }

VALID_OBS_STATUS = {"registered", "preliminary", "final", "amended",
                    "corrected", "cancelled", "entered-in-error", "unknown"}

@app.post("/Observation", status_code=201)
async def create_observation(data: dict):
    if not data.get("status"):
        return JSONResponse(status_code=422, content=operation_outcome(
            "error", "required", "Observation.status is required"
        ), media_type="application/fhir+json")

    if data["status"] not in VALID_OBS_STATUS:
        return JSONResponse(status_code=422, content=operation_outcome(
            "error", "value", f"Invalid status '{data['status']}'"
        ), media_type="application/fhir+json")
    # ... create resource

TypeScript/Express:

typescript

const VALID_OBS_STATUS = new Set(['registered', 'preliminary', 'final', 'amended',
  'corrected', 'cancelled', 'entered-in-error', 'unknown']);

app.post('/Observation', (req, res) => {
  if (!req.body.status) {
    return res.status(422).contentType('application/fhir+json')
      .json(operationOutcome('error', 'required', 'Observation.status is required'));
  }
  if (!VALID_OBS_STATUS.has(req.body.status)) {
    return res.status(422).contentType('application/fhir+json')
      .json(operationOutcome('error', 'value', `Invalid status '${req.body.status}'`));
  }
  // ... create resource
});

Pydantic v2 Models (use

Literal

, not

const=True

python

from typing import Literal
from pydantic import BaseModel

class Patient(BaseModel):
    resourceType: Literal["Patient"] = "Patient"
    id: str | None = None
    gender: Literal["male", "female", "other", "unknown"] | None = None

Coding Systems (URLs)

System	URL
LOINC	`http://loinc.org`
SNOMED CT	`http://snomed.info/sct`
RxNorm	`http://www.nlm.nih.gov/research/umls/rxnorm`
ICD-10	`http://hl7.org/fhir/sid/icd-10`
v3-ActCode	`http://terminology.hl7.org/CodeSystem/v3-ActCode`
Observation Category	`http://terminology.hl7.org/CodeSystem/observation-category`
Condition Clinical	`http://terminology.hl7.org/CodeSystem/condition-clinical`
Condition Ver Status	`http://terminology.hl7.org/CodeSystem/condition-ver-status`

Common LOINC Codes (Vital Signs)

Code	Description
`8867-4`	Heart rate
`8480-6`	Systolic blood pressure
`8462-4`	Diastolic blood pressure
`8310-5`	Body temperature
`2708-6`	Oxygen saturation (SpO2)

Data Type Patterns

Coding (direct) vs CodeableConcept (wrapped)

Coding - Used by

Encounter.class

json

{"system": "http://terminology.hl7.org/CodeSystem/v3-ActCode", "code": "AMB"}

CodeableConcept - Used by

Observation.code

Condition.code

json

{"coding": [{"system": "http://loinc.org", "code": "8480-6"}], "text": "Systolic BP"}

Reference

json

{"reference": "Patient/123", "display": "John Smith"}

Identifier

json

{"system": "http://hospital.example.org/mrn", "value": "12345"}

Common Mistakes

Mistake	Correct Approach
Making `subject` or `period` required on Encounter	Both are 0..1 (optional). Only `status` and `class` are required
Using CodeableConcept for `Encounter.class`	`class` uses Coding directly: `{"system": "...", "code": "AMB"}`
Returning 400 for ETag mismatch	Use `412 Precondition Failed` for If-Match failures
Returning 400 for invalid enum values	Use `422 Unprocessable Entity` for validation errors
Forgetting Content-Type header	Always set `Content-Type: application/fhir+json`
Missing Location header on create	Return `Location: /Patient/{id}` with 201 Created

Resource Structures

For complete JSON examples of all resources, see references/resource-examples.md.

Quick reference for error responses:

json

{
  "resourceType": "OperationOutcome",
  "issue": [{"severity": "error", "code": "not-found", "diagnostics": "Patient/123 not found"}]
}

RESTful Endpoints

POST   /[ResourceType]              # Create (returns 201 + Location header)
GET    /[ResourceType]/[id]         # Read
PUT    /[ResourceType]/[id]         # Update
DELETE /[ResourceType]/[id]         # Delete (returns 204)
GET    /[ResourceType]?param=value  # Search (returns Bundle)
GET    /metadata                    # CapabilityStatement
POST   /                            # Bundle transaction/batch

Conditional Operations

If-Match (optimistic locking):

Client sends:
```
If-Match: W/"1"
```
Mismatch returns
```
412 Precondition Failed
```

If-None-Exist (conditional create):

Client sends:

If-None-Exist: identifier=http://mrn|12345

Match exists: return existing (200)
No match: create new (201)

Reference Files

For detailed guidance, see:

Resource Examples: Complete JSON structures for Patient, Observation, Encounter, Condition, MedicationRequest, OperationOutcome, CapabilityStatement
SMART on FHIR Authorization: OAuth flows, scope syntax (v1/v2), backend services, scope enforcement
Pagination: Search result pagination,
```
_count
```
/
```
_offset
```
parameters, link relations
Bundle Operations: Transaction vs batch semantics, atomicity, processing order

Implementation Checklist

Set
```
Content-Type: application/fhir+json
```
on all responses
Return
```
meta.versionId
```
and
```
meta.lastUpdated
```
on resources
Return
```
Location
```
header on create:
```
/Patient/{id}
```
Return
```
ETag
```
header:
```
W/"{versionId}"
```
Use OperationOutcome for all error responses
Validate required fields → 422 for missing
Validate enum values → 422 for invalid
Search returns Bundle with
```
type: "searchset"
```

Quick Start Script

To scaffold a new FHIR API project with correct Pydantic v2 patterns:

bash

python scripts/setup_fhir_project.py my_fhir_api

Creates a FastAPI project with correct models, OperationOutcome helpers, and Patient CRUD endpoints.

fhir-developer-skill

NPX Install

Tags

SKILL.md Content

FHIR Developer Skill

Quick Reference

HTTP Status Codes

Required Fields by Resource (FHIR R4)

Required vs Optional Fields (CRITICAL)

Value Sets (Enum Values)

Patient.gender

Observation.status

Encounter.status

Encounter.class (Common Codes)

Condition.clinicalStatus

Condition.verificationStatus

MedicationRequest.status

MedicationRequest.intent

Bundle.type

Validation Pattern

Coding Systems (URLs)

Common LOINC Codes (Vital Signs)

Data Type Patterns

Coding (direct) vs CodeableConcept (wrapped)

Reference

Identifier

Common Mistakes

Resource Structures

RESTful Endpoints

Conditional Operations

Reference Files

Implementation Checklist

Quick Start Script