diff --git a/Cargo.toml b/Cargo.toml
index 8de8027c..80760a4f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,7 +28,7 @@ path = "experiments/test_advanced_math.rs"
 wasm-bindgen = "0.2"
 wasm-bindgen-futures = "0.4"
 js-sys = "0.3"
-web-sys = { version = "0.3", features = ["Window", "Request", "RequestInit", "RequestMode", "Response", "Headers"] }
+web-sys = { version = "0.3", features = ["Window", "WorkerGlobalScope", "Request", "RequestInit", "RequestMode", "Response", "Headers"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 console_error_panic_hook = "0.1"
diff --git a/changelog.d/20260126_140137_fix_worker_fetch.md b/changelog.d/20260126_140137_fix_worker_fetch.md
new file mode 100644
index 00000000..beb7fd59
--- /dev/null
+++ b/changelog.d/20260126_140137_fix_worker_fetch.md
@@ -0,0 +1,11 @@
+---
+bump: patch
+---
+
+### Fixed
+- Currency API fetch now works in Web Worker context (fixes the root cause of issue #18)
+- The `fetch_json` function now uses `js_sys::global()` to detect and use either `Window` or `WorkerGlobalScope` context
+- Previously, `web_sys::window()` returned `None` in Web Workers, causing silent API fetch failures
+
+### Changed
+- Added `WorkerGlobalScope` feature to web-sys dependency in Cargo.toml
diff --git a/docs/case-studies/issue-18/README.md b/docs/case-studies/issue-18/README.md
index ef167977..a4296ecc 100644
--- a/docs/case-studies/issue-18/README.md
+++ b/docs/case-studies/issue-18/README.md
@@ -29,34 +29,37 @@ Users need transparency about currency conversion rates for:
 
 ## Root Cause Analysis
 
-### Primary Issue: Missing Rate Application Pipeline
-
-**Critical Finding**: The web worker successfully fetches exchange rates from the API but **never applies them to the Calculator instance**.
-
-In `web/src/worker.ts`:
-```typescript
-async function fetchExchangeRates() {
-  const responseJson = await fetch_exchange_rates('usd');
-  const response: ExchangeRatesResponse = JSON.parse(responseJson);
-
-  if (response.success) {
-    const rates = JSON.parse(response.rates_json);
-    // PROBLEM: rates are parsed but NEVER applied to calculator!
-    self.postMessage({
-      type: 'ratesLoaded',
-      data: { ... ratesCount: Object.keys(rates).length }
-    });
-  }
+### Primary Issue: Web Worker Context Incompatibility
+
+**Critical Finding**: The currency API fetch code used `web_sys::window()` which returns `None` in a Web Worker context. Web Workers don't have a `window` object - they use `WorkerGlobalScope` instead.
+
+In `src/currency_api.rs` (before fix):
+```rust
+async fn fetch_json(url: &str) -> Result<(String, HashMap<String, f64>), CurrencyApiError> {
+    let window = web_sys::window()
+        .ok_or_else(|| CurrencyApiError::NetworkError("No window object available".to_string()))?;
+    // This always fails in a Web Worker!
+    // ...
 }
 ```
 
-The architecture has all the pieces but they're not connected:
-1. `fetch_exchange_rates()` in WASM returns rates ✅
-2. Worker receives rates and counts them ✅
-3. **Worker never updates Calculator with the rates** ❌
-4. Result: Calculator uses hardcoded fallback rates
+The fix requires checking for both Window and WorkerGlobalScope contexts:
+```rust
+let global = js_sys::global();
+let resp_value = if let Some(window) = global.dyn_ref::<web_sys::Window>() {
+    JsFuture::from(window.fetch_with_request(&request)).await
+} else if let Some(worker) = global.dyn_ref::<web_sys::WorkerGlobalScope>() {
+    JsFuture::from(worker.fetch_with_request(&request)).await
+} else {
+    return Err(CurrencyApiError::NetworkError("Neither Window nor WorkerGlobalScope available".to_string()));
+};
+```
+
+### Secondary Issue: Missing Rate Application Pipeline (Fixed in PR #38)
+
+The web worker originally fetched exchange rates from the API but **never applied them to the Calculator instance**. This was fixed by adding `update_rates_from_api` call in `web/src/worker.ts`.
 
-### Secondary Issue: Hardcoded Fallback Rates
+### Tertiary Issue: Hardcoded Fallback Rates
 
 The calculator uses **hardcoded exchange rates** in `src/types/currency.rs:188`:
 
diff --git a/src/currency_api.rs b/src/currency_api.rs
index fe8ba59c..45c0ddc3 100644
--- a/src/currency_api.rs
+++ b/src/currency_api.rs
@@ -107,10 +107,8 @@ async fn fetch_rates_from_url(
 }
 
 /// Performs the actual fetch and JSON parsing.
+/// Works in both Window and Web Worker contexts.
 async fn fetch_json(url: &str) -> Result<(String, HashMap<String, f64>), CurrencyApiError> {
-    let window = web_sys::window()
-        .ok_or_else(|| CurrencyApiError::NetworkError("No window object available".to_string()))?;
-
     let opts = RequestInit::new();
     opts.set_method("GET");
     opts.set_mode(RequestMode::Cors);
@@ -124,9 +122,21 @@ async fn fetch_json(url: &str) -> Result<(String, HashMap<String, f64>), Currenc
         .set("Accept", "application/json")
         .map_err(|e| CurrencyApiError::NetworkError(format!("Failed to set headers: {:?}", e)))?;
 
-    let resp_value = JsFuture::from(window.fetch_with_request(&request))
-        .await
-        .map_err(|e| CurrencyApiError::NetworkError(format!("Fetch failed: {:?}", e)))?;
+    // Use the global fetch function which works in both Window and Worker contexts.
+    // In a browser window, this is window.fetch; in a worker, this is self.fetch.
+    let global = js_sys::global();
+    let resp_value = if let Some(window) = global.dyn_ref::<web_sys::Window>() {
+        // Running in a Window context
+        JsFuture::from(window.fetch_with_request(&request)).await
+    } else if let Some(worker) = global.dyn_ref::<web_sys::WorkerGlobalScope>() {
+        // Running in a Web Worker context
+        JsFuture::from(worker.fetch_with_request(&request)).await
+    } else {
+        return Err(CurrencyApiError::NetworkError(
+            "Neither Window nor WorkerGlobalScope available".to_string(),
+        ));
+    }
+    .map_err(|e| CurrencyApiError::NetworkError(format!("Fetch failed: {:?}", e)))?;
 
     let resp: Response = resp_value
         .dyn_into()