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<!-- markdownlint-disable MD024 -->
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# Capability Spec Delta: currency-provider-fixer (currency-provider-fixer)
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**Package**: `@suaveplan/currency-provider-fixer`
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**Status**: PROPOSED
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> Every requirement below carries a `**Verified by:**` line at authoring time.
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> No exceptions.
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## ADDED Requirements
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### Requirement: Provider identity flags
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The package SHALL export a `FixerProvider` class that implements the
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`CurrencyProvider` contract from `@suaveplan/currency` and exposes the stable
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identity flags `name = "fixer"`, `supportsFiat = true`, and
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`supportsCrypto = false`.
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#### Scenario: identity flags are stable
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- **WHEN** a caller constructs `new FixerProvider({ apiKey: "x" })`
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- **THEN** `provider.name === "fixer"`, `provider.supportsFiat === true`,
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and `provider.supportsCrypto === false`
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**Verified by:** `src/fixer/fixer.test.ts::"exposes the stable identity flags name/supportsFiat/supportsCrypto"`
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---
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### Requirement: API key required at construction
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The constructor SHALL raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.MISSING_CONFIG` when `config.apiKey` is the empty string,
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before any network request is issued.
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#### Scenario: empty api key is rejected at construction
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- **WHEN** a caller invokes `new FixerProvider({ apiKey: "" })`
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- **THEN** a `CurrencyProviderError` is thrown synchronously with
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`code === CURRENCY_PROVIDER.MISSING_CONFIG` and `providerName === "fixer"`
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**Verified by:** `src/fixer/fixer.test.ts::"raises MISSING_CONFIG when apiKey is empty at construction"`
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---
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### Requirement: Default base URL
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The provider SHALL default the request base URL to
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`https://data.fixer.io/api` and SHALL allow the caller to override it via
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`config.baseUrl`.
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#### Scenario: default base URL used when none provided
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- **WHEN** `new FixerProvider({ apiKey: "k", fetch }).fetchRates()` is called
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- **THEN** the recorded `fetch` URL starts with `https://data.fixer.io/api/latest?`
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**Verified by:** `src/fixer/fixer.test.ts::"defaults the request base URL to data.fixer.io when no override is provided"`
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#### Scenario: custom base URL is honoured
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- **WHEN** `new FixerProvider({ apiKey: "k", baseUrl: "https://proxy/api", fetch }).fetchRates()` is called
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- **THEN** the recorded `fetch` URL starts with `https://proxy/api/latest?`
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**Verified by:** `src/fixer/fixer.test.ts::"honours a custom baseUrl override"`
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---
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### Requirement: Query construction
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`fetchRates(options?)` SHALL build the request query with `access_key` set
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to the configured api key, `base` defaulting to `EUR` when
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`options.base` is unset, and `symbols` set to a comma-joined list when
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`options.targets` is a non-empty array. When `options.targets` is omitted or
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empty, no `symbols` parameter SHALL be added.
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#### Scenario: default base EUR and access_key are forwarded
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- **WHEN** `fetchRates()` is called without options
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- **THEN** the URL query contains `access_key=k` and `base=EUR` and does
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not contain `symbols`
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**Verified by:** `src/fixer/fixer.test.ts::"defaults base to EUR and forwards access_key, omits symbols when targets is unset"`
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#### Scenario: targets list is comma-joined into symbols
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- **WHEN** `fetchRates({ base: "USD", targets: ["EUR","GBP"] })` is called
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- **THEN** the URL query contains `base=USD` and `symbols=EUR%2CGBP`
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**Verified by:** `src/fixer/fixer.test.ts::"forwards a custom base and comma-joins targets into the symbols param"`
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---
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### Requirement: Injectable FetchLike
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The provider SHALL honour the optional `config.fetch` (`FetchLike` from
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`@suaveplan/types`) and use it for every outbound request. When omitted, the
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provider SHALL fall back to `globalThis.fetch`.
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#### Scenario: custom fetch is invoked
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- **WHEN** a recording `FetchLike` is passed via `config.fetch` and
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`fetchRates()` is called
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- **THEN** the recorded fetch closure receives exactly one call for that
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invocation
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**Verified by:** `src/fixer/fixer.test.ts::"invokes the injected FetchLike instead of globalThis.fetch"`
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---
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### Requirement: Successful envelope projection
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The provider SHALL resolve `fetchRates` with `{ base, rates, timestamp:
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data.timestamp * 1000 }` when the upstream responds with
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`{ success: true, base, rates, timestamp }`. The upstream `timestamp` is
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Unix seconds; the returned `timestamp` SHALL be milliseconds since epoch.
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#### Scenario: successful envelope is returned with millisecond timestamp
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- **WHEN** the upstream returns `{ success: true, base: "EUR", rates: { GBP: 0.85 }, timestamp: 1_700_000_000 }` with HTTP 200
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- **THEN** `fetchRates` resolves to `{ base: "EUR", rates: { GBP: 0.85 }, timestamp: 1_700_000_000_000 }`
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**Verified by:** `src/fixer/fixer.test.ts::"returns the canonical RateFetchResult with the timestamp converted from seconds to milliseconds"`
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---
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### Requirement: UNSUPPORTED_BASE mapping
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The provider SHALL raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.UNSUPPORTED_BASE`, `providerName === "fixer"`, and the
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upstream `error.info` carried on `upstreamMessage` whenever the upstream
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responds with `{ success: false, error: { code: 105 } }`
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(`base_currency_access_restricted` — free tier rejecting a non-EUR base).
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#### Scenario: free-tier base restriction maps to UNSUPPORTED_BASE
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- **WHEN** the upstream returns `{ success: false, error: { code: 105, type: "base_currency_access_restricted", info: "Access Restricted" } }`
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- **THEN** `fetchRates({ base: "USD" })` rejects with
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`code === CURRENCY_PROVIDER.UNSUPPORTED_BASE` and
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`upstreamMessage === "Access Restricted"`
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**Verified by:** `src/fixer/fixer.test.ts::"maps upstream error.code 105 to UNSUPPORTED_BASE"`
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---
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### Requirement: UNKNOWN_CURRENCY mapping
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The provider SHALL raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.UNKNOWN_CURRENCY` and propagate the upstream
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`error.info` on `upstreamMessage` whenever the upstream responds with
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`{ success: false, error: { code: 201 } }` (`invalid_base_currency`) or
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`{ ..., error: { code: 202 } }` (`invalid_currency_codes`).
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#### Scenario: invalid base currency maps to UNKNOWN_CURRENCY
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- **WHEN** the upstream returns `{ success: false, error: { code: 201, info: "invalid base" } }`
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- **THEN** `fetchRates({ base: "ZZZ" })` rejects with
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`code === CURRENCY_PROVIDER.UNKNOWN_CURRENCY` and
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`upstreamMessage === "invalid base"`
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**Verified by:** `src/fixer/fixer.test.ts::"maps upstream error.code 201 to UNKNOWN_CURRENCY"`
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#### Scenario: invalid symbol codes maps to UNKNOWN_CURRENCY
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- **WHEN** the upstream returns `{ success: false, error: { code: 202, info: "invalid symbols" } }`
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- **THEN** `fetchRates({ targets: ["ZZZ"] })` rejects with
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`code === CURRENCY_PROVIDER.UNKNOWN_CURRENCY` and
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`upstreamMessage === "invalid symbols"`
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**Verified by:** `src/fixer/fixer.test.ts::"maps upstream error.code 202 to UNKNOWN_CURRENCY"`
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---
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### Requirement: UPSTREAM_ERROR fallback
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The provider SHALL raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.UPSTREAM_ERROR`, `providerName === "fixer"`, and
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`upstreamMessage` populated with the upstream `error.info` (or the string
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`"<unknown>"` when absent) whenever the upstream responds with
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`{ success: false }` and any code that is not one of the discriminated
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`105` / `201` / `202` codes — including when the `error` field is missing
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entirely.
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#### Scenario: unknown upstream error code maps to UPSTREAM_ERROR
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- **WHEN** the upstream returns `{ success: false, error: { code: 999, info: "rate limit exceeded" } }`
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- **THEN** `fetchRates()` rejects with
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`code === CURRENCY_PROVIDER.UPSTREAM_ERROR` and
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`upstreamMessage === "rate limit exceeded"`
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**Verified by:** `src/fixer/fixer.test.ts::"maps any other success:false code to UPSTREAM_ERROR with upstreamMessage"`
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#### Scenario: missing error field still maps to UPSTREAM_ERROR
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- **WHEN** the upstream returns `{ success: false }` with no `error` field
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- **THEN** `fetchRates()` rejects with
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`code === CURRENCY_PROVIDER.UPSTREAM_ERROR` and
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`upstreamMessage === "<unknown>"`
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**Verified by:** `src/fixer/fixer.test.ts::"maps success:false with no error field to UPSTREAM_ERROR with <unknown>"`
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---
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### Requirement: PARSE_ERROR on malformed JSON
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The provider SHALL raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.PARSE_ERROR` and `providerName === "fixer"` whenever
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the upstream body cannot be parsed as JSON, when the parsed value is not
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a non-null object (for example `null`, a string, a number, or an array),
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or when a `success: true` envelope is missing any required field.
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#### Scenario: invalid JSON body raises PARSE_ERROR
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- **WHEN** the upstream returns the literal body `"<html>"` with HTTP 200
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- **THEN** `fetchRates()` rejects with `code === CURRENCY_PROVIDER.PARSE_ERROR`
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**Verified by:** `src/fixer/fixer.test.ts::"raises PARSE_ERROR when the response body is not valid JSON"`
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#### Scenario: non-object JSON raises PARSE_ERROR
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- **WHEN** the upstream returns the literal body `"null"` with HTTP 200
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- **THEN** `fetchRates()` rejects with `code === CURRENCY_PROVIDER.PARSE_ERROR`
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**Verified by:** `src/fixer/fixer.test.ts::"raises PARSE_ERROR when the parsed JSON is null or a non-object"`
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#### Scenario: success:true envelope with missing required fields raises PARSE_ERROR
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- **WHEN** the upstream returns `{ success: true }` with `base` / `rates` /
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`timestamp` absent
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- **THEN** `fetchRates()` rejects with `code === CURRENCY_PROVIDER.PARSE_ERROR`
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**Verified by:** `src/fixer/fixer.test.ts::"raises PARSE_ERROR when a success envelope is missing required fields"`
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---
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### Requirement: HTTP_ERROR via slim-core httpRequest
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The provider SHALL delegate HTTP transport to the slim core's `httpRequest`
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helper so that non-2xx responses raise `CurrencyProviderError` with code
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`CURRENCY_PROVIDER.HTTP_ERROR`, `providerName === "fixer"`, and `status`
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populated to the upstream HTTP status code.
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#### Scenario: 5xx upstream raises HTTP_ERROR
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- **WHEN** the upstream returns HTTP 503 with body `"Service Unavailable"`
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- **THEN** `fetchRates()` rejects with
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`code === CURRENCY_PROVIDER.HTTP_ERROR`, `status === 503`, and
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`providerName === "fixer"`
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**Verified by:** `src/fixer/fixer.test.ts::"raises HTTP_ERROR with status and providerName on a non-2xx response"`
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---
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### Requirement: Caller AbortSignal propagation
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When `options.signal` is supplied to `fetchRates`, the provider SHALL
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forward it through `httpRequest` so the underlying fetch is cancelled when
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the caller aborts.
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#### Scenario: aborting the caller signal rejects the fetchRates promise
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- **WHEN** `fetchRates({ signal })` is called with a signal whose controller
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is aborted before the upstream resolves
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- **THEN** the returned promise rejects with the caller's abort reason
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**Verified by:** `src/fixer/fixer.test.ts::"forwards the caller AbortSignal to the underlying fetch"`
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---
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### Requirement: No auto-register top-level side effect
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The package SHALL NOT register an instance of `FixerProvider` with any
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global registry at import time. Importing the package SHALL be free of
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side effects and SHALL leave `"sideEffects": false` honoured for
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tree-shaking.
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#### Scenario: import has no observable side effect
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- **WHEN** the package is imported via `await import("@suaveplan/currency-provider-fixer")`
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- **THEN** no `CurrencyProviderRegistry` instance is constructed or
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mutated by the module, and the only public surface is the `FixerProvider`
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class
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**Verified by:** `src/fixer/fixer.test.ts::"has no side effect at import time and exposes only FixerProvider"`
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---
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### Requirement: Universal-tier purity
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The package SHALL be universal-tier — it SHALL NOT import any
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`node:` / `bun:` / DOM-only module, and SHALL run unchanged under Node 18+,
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Bun, modern browsers, and Web Workers.
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#### Scenario: no banned imports in source
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- **WHEN** the source tree under `src/` is scanned
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- **THEN** zero matches are found for `from "node:`, `from "bun:`,
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`from "fs"`, `from "path"`, or any DOM-only global
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**Verified by:** `src/fixer/fixer.test.ts::"imports only universal-tier modules — no node:, bun:, fs, or DOM-only globals"`
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