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Spec references: Validation | OperationOutcome | $validate
Validation answers a deceptively simple question:
“Is this resource acceptable according to FHIR (and the profiles it claims to follow)?”
In practice, validation is layered.

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What gets validated

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1) Structural correctness

Basic “is this shaped like a FHIR resource?” checks:
  • resourceType is present and known
  • element names and datatypes match the definition
  • required elements and cardinalities are satisfied

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2) Profile constraints

If a resource is expected to conform to a profile (or declares one via meta.profile[]), validators can enforce:
  • additional required elements
  • fixed values and patterns
  • slicing rules
  • invariants (FHIRPath constraints)
See Profiles and FHIRPath.

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3) Terminology rules

Coded fields (Coding, CodeableConcept, etc.) can be bound to ValueSets. Validation may check the binding strength (required, extensible, …) and whether the codes are permitted. See Terminology.

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How validation errors are reported

FHIR uses OperationOutcome. Each problem is an issue with:
  • severity: fatal, error, warning, information
  • code: a standardized category (e.g. structure, value, invariant)
  • diagnostics: human-readable details (often server/tool specific)
  • expression[]: FHIRPath expressions pointing to the element(s) involved (when available)
Tiny example: 
{
  "resourceType": "OperationOutcome",
  "issue": [
    {
      "severity": "error",
      "code": "required",
      "diagnostics": "Patient.name is required",
      "expression": ["Patient.name"]
    }
  ]
}

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When validation happens

Common places you’ll see validation:
  • Client-side (form/SDK validation before sending)
  • Server-side on create, update, patch, transaction
  • Explicit validation via the $validate operation (if the server supports it)
If you get unexpected validation failures, confirm you’re using the right FHIR version (R4 vs R5) and the right IG package versions.

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Discovering server support

Validation capabilities vary by server. The fastest way to check is the CapabilityStatement (GET /fhir/metadata), which can list supported profiles, terminology capabilities, and operations such as $validate.

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