Fix type safety for header handling in server routing

src/routing/server.ts

@@ -3,7 +3,7 @@ import { DatapromptStore } from '../core/dataprompt.js';
 import { events } from '../core/events.js';
 import { getLogManager } from '../utils/logging.js';
 import { createPromptFlow, FlowDefinition } from './flow-builder.js';
-import { createRequestContext } from '../utils/helpers/request.js';
+import { createRequestContext, convertHeaders } from '../utils/helpers/request.js';
 
 export interface DatapromptRoute {
   flowDef: FlowDefinition;
@@ -24,8 +24,7 @@ function createRouteHandler({
     const url = `${req.protocol}://${req.get('host')}${req.originalUrl}`;
     const standardRequest = new Request(url, {
       method: req.method,
-      // TODO: Improve type safety for headers. req.headers can contain string[] which might not be compatible with Record<string, string>.
-      headers: req.headers as Record<string, string>,
+      headers: convertHeaders(req.headers),
       body: req.method !== 'GET' ? JSON.stringify(req.body) : undefined,
     });
 

src/utils/helpers/request.ts

@@ -1,5 +1,27 @@
 import { RequestContext, RequestContextSchema } from "../../core/interfaces.js";
 import { MatchResult } from "../../routing/route-matcher.js";
+import { IncomingHttpHeaders } from 'http';
+
+/**
+ * Converts Node.js/Express headers to Fetch API Headers.
+ * Handles strings, string arrays, and undefined values safely.
+ * @param headers Node.js IncomingHttpHeaders
+ * @returns Standard Fetch API Headers object
+ */
+export function convertHeaders(headers: IncomingHttpHeaders): Headers {
+  const fetchHeaders = new Headers();
+  for (const [key, value] of Object.entries(headers)) {
+    if (value === undefined) {
+      continue;
+    }
+    if (Array.isArray(value)) {
+      value.forEach((v) => fetchHeaders.append(key, v));
+    } else {
+      fetchHeaders.append(key, value);
+    }
+  }
+  return fetchHeaders;
+}
 
 /**
  * A universal helper to create a valid RequestContext from various inputs.

tests/unit/request.helper.test.ts

@@ -1,7 +1,49 @@
 import { describe, it, expect } from 'vitest';
-import { createRequestContext } from '../../src/utils/helpers/request.js';
+import { createRequestContext, convertHeaders } from '../../src/utils/helpers/request.js';
 import { RequestContext } from '../../src/core/interfaces';
 import { MatchResult } from '../../src/routing/route-matcher.js';
+import { IncomingHttpHeaders } from 'http';
+
+describe('convertHeaders', () => {
+    it('should correctly handle string headers', () => {
+        const headers: IncomingHttpHeaders = {
+            'content-type': 'application/json',
+            'x-api-key': '12345'
+        };
+        const fetchHeaders = convertHeaders(headers);
+        expect(fetchHeaders.get('content-type')).toBe('application/json');
+        expect(fetchHeaders.get('x-api-key')).toBe('12345');
+    });
+
+    it('should correctly handle array headers', () => {
+        const headers: IncomingHttpHeaders = {
+            'x-custom-list': ['item1', 'item2']
+        };
+        const fetchHeaders = convertHeaders(headers);
+        // Headers.get returns values comma-separated
+        expect(fetchHeaders.get('x-custom-list')).toBe('item1, item2');
+    });
+
+    it('should ignore undefined headers', () => {
+        const headers: IncomingHttpHeaders = {
+            'x-valid': 'true',
+            'x-undefined': undefined
+        };
+        const fetchHeaders = convertHeaders(headers);
+        expect(fetchHeaders.has('x-valid')).toBe(true);
+        expect(fetchHeaders.has('x-undefined')).toBe(false);
+    });
+
+    it('should handle mixed string and array headers', () => {
+        const headers: IncomingHttpHeaders = {
+            'x-single': 'value',
+            'x-multiple': ['v1', 'v2']
+        };
+        const fetchHeaders = convertHeaders(headers);
+        expect(fetchHeaders.get('x-single')).toBe('value');
+        expect(fetchHeaders.get('x-multiple')).toBe('v1, v2');
+    });
+});
 
 describe('createRequestContext', () => {
   it('should handle a simple URL string', async () => {