# FHIR Facade Implementation — Plan Mode

You are a senior healthcare integration engineer. Your task is to **analyze provided artifacts and produce a detailed implementation plan** for a FHIR R4 facade layer before writing any code.

## Inputs provided
- One or more **HAR files** (browser network captures) showing real HTTP traffic from the underlying proprietary API
- An **OpenAPI specification** describing the underlying API's endpoints, schemas, and authentication

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## Phase 1 — Analysis (complete before any planning)

### HAR file analysis
For each HAR file:
1. Identify all unique API endpoints called (method + path)
2. Extract request headers and authentication patterns (tokens, cookies, API keys)
   - **Note:** If the HAR contains login/auth flows (e.g. `/login`, `/oauth/token`, `/auth`), you may **skip detailed analysis of those flows** unless they reveal something structurally important about how session tokens are used downstream. Just note that auth exists and what credential type is passed on subsequent requests.
3. Document request/response payload shapes with representative field names and value types
4. Note pagination patterns, error response formats, and any behaviors not reflected in the OpenAPI spec
5. Flag discrepancies between actual HAR traffic and the OpenAPI spec

### OpenAPI spec analysis
1. List all available endpoints and operations
2. Identify data models and their fields
3. Note authentication/authorization schemes (summary only — implementation detail deferred)
4. Flag endpoints that appear unused or absent in the HAR captures

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## Phase 2 — FHIR Resource Identification & Mapping

**Infer the appropriate FHIR R4 resources from the HAR and OpenAPI data.** Do not assume a fixed resource set — let the underlying data model drive what FHIR resources make sense to expose.

For each identified mapping:

| Underlying resource (HAR/OpenAPI) | FHIR R4 Resource | Confidence | Mapping notes / gaps |
|-----------------------------------|------------------|------------|----------------------|
| ...                               | ...              | High/Med/Low | ...                |

For each FHIR resource:
- Which underlying API calls are needed to populate it?
- What field-level transformations are required (renaming, type coercion, unit conversion, code system mapping)?
- Which mandatory FHIR fields cannot be populated from available data? (mark for `extension`, stub value, or omission with justification)
- Which FHIR search parameters (`_id`, `subject`, `date`, etc.) can be realistically supported?

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## Phase 3 — Tech Stack Recommendation

Based on the complexity, payload shapes, and patterns observed in the HAR/OpenAPI data, recommend the most appropriate tech stack for this facade. Consider:

- **Ecosystem maturity** for FHIR (existing libraries, validators, terminology servers)
- **Transformation complexity** — if mappings are heavy, prefer a language with strong data manipulation ergonomics
- **Operational simplicity** — prefer something deployable as a single service without heavy infrastructure
- **Team familiarity signals** — if the OpenAPI spec or HAR reveals tech clues (e.g. JSON conventions, versioning style), factor that in

Provide a brief justification for your recommendation and name any key libraries (e.g. `fhir.js`, `fhirpy`, `HAPI FHIR`, `medplum`).

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## Phase 4 — Architecture Plan

Outline the facade's internal structure:

1. **Layer breakdown**
   - FHIR REST layer — handles incoming FHIR requests, routing, and basic validation
   - Mapping/translation layer — bidirectional transforms between FHIR and proprietary models
   - Upstream client layer — calls the underlying API; handles auth token injection, retries, and error normalization
2. **Authentication strategy**
   - How the facade authenticates *to* the upstream API (based on HAR evidence — token type, header name, refresh pattern)
   - How the facade authenticates *its own callers* (recommend a sensible default; note this can be swapped out)
   - Flag whether credential handling can be simplified or stubbed for an initial implementation
3. **Caching strategy** — where caching adds value given observed traffic patterns
4. **Error handling** — how upstream errors map to FHIR `OperationOutcome` responses
5. **Capability statement** — what the `/metadata` endpoint will advertise

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## Phase 5 — Implementation Roadmap

Break work into ordered milestones. For each, list the modules/files to be created:

- **Milestone 1:** Project scaffold + upstream API client (raw calls working, auth token injected)
- **Milestone 2:** First FHIR resource end-to-end — pick the simplest/most complete mapping from Phase 2
- **Milestone 3:** Remaining FHIR resources in priority order
- **Milestone 4:** Search parameter support
- **Milestone 5:** Validation, error normalization, `/metadata` conformance statement
- **Milestone 6:** Tests — unit for mappers, integration against a FHIR validator, contract tests

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## Flags & Decisions Required

Before finishing the plan, surface:
- **Ambiguities** in the HAR/OpenAPI that need a human decision before implementation can proceed
- **Mandatory FHIR fields** that are unavailable from the source system and require a policy decision
- **Security observations** from the HAR (tokens in query strings, missing headers, overly broad scopes)
- **Scope recommendations** — if the HAR reveals the system is large, suggest a phased scope rather than boiling the ocean

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**Do not write implementation code yet.**
Deliver the full analysis and plan, then pause and wait for review and approval before proceeding.