diff --git a/src/middleware/errorHandler.test.ts b/src/middleware/errorHandler.test.ts
index b980689..6409e1a 100644
--- a/src/middleware/errorHandler.test.ts
+++ b/src/middleware/errorHandler.test.ts
@@ -1,5 +1,4 @@
-import { Request, Response, NextFunction } from 'express';
-import { errorHandler } from '../middleware/errorHandler.js';
+import { Request, Response, NextFunction } from 'express';import { errorHandler } from '../middleware/errorHandler.js';
 import { 
   BadRequestError, 
   UnauthorizedError,
@@ -9,8 +8,7 @@ import {
   TooManyRequestsError,
   AppError,
 } from '../errors/index.js';
-import { ValidationError } from '../middleware/validate.js';
-import { logger } from '../logger.js';
+import { ValidationError } from '../middleware/validate.js';import { logger } from '../logger.js';
 import type { ErrorEnvelope } from '../types/ResponseEnvelope.js';
 
 jest.mock('../logger.js', () => ({
@@ -23,7 +21,7 @@ jest.mock('../logger.js', () => ({
 
 describe('Error Handler', () => {
   let mockReq: Partial<Request> & { id?: string };
-  let mockRes: Partial<Response>;
+  let mockRes: Partial<Response> & { destroy?: jest.Mock };
   let mockNext: NextFunction;
 
   beforeEach(() => {
@@ -33,6 +31,7 @@ describe('Error Handler', () => {
     mockRes = {
       status: jest.fn().mockReturnThis(),
       json: jest.fn(),
+      destroy: jest.fn(),
       headersSent: false
     };
     mockNext = jest.fn();
@@ -63,7 +62,7 @@ describe('Error Handler', () => {
       code: 'BAD_REQUEST',
       message: 'Test bad request',
     });
-    expect(typeof call.timestamp).toBe('string');
+    expect(typeof call.timestamp).toBe(typeof 'string');
 
     expect(logger.error).toHaveBeenCalledWith(
       '[errorHandler]',
@@ -141,6 +140,40 @@ describe('Error Handler', () => {
     expect(mockRes.json).not.toHaveBeenCalled();
   });
 
+  it('should destroy the socket when headers are already sent', () => {
+    mockRes.headersSent = true;
+    const error = new Error('mid-stream failure');
+
+    errorHandler(
+      error,
+      mockReq as Request,
+      mockRes as Response<ErrorEnvelope>,
+      mockNext
+    );
+
+    expect(mockRes.status).not.toHaveBeenCalled();
+    expect(mockRes.json).not.toHaveBeenCalled();
+    expect(mockRes.destroy).toHaveBeenCalledWith(error);
+  });
+
+  it('logs the error once with requestId when headers are already sent', () => {
+    mockRes.headersSent = true;
+    const error = new Error('mid-stream failure');
+
+    errorHandler(
+      error,
+      mockReq as Request,
+      mockRes as Response<ErrorEnvelope>,
+      mockNext
+    );
+
+    expect(logger.error).toHaveBeenCalledTimes(1);
+    expect(logger.error).toHaveBeenCalledWith(
+      '[errorHandler]',
+      expect.objectContaining({ requestId: 'test-request-id' })
+    );
+  });
+
   it('should include explicit catalog code when provided', () => {
     const error = new AppError('Custom error', 422, 'UNPROCESSABLE_ENTITY');
     
diff --git a/src/middleware/errorHandler.ts b/src/middleware/errorHandler.ts
index ebe5a38..b1a4130 100644
--- a/src/middleware/errorHandler.ts
+++ b/src/middleware/errorHandler.ts
@@ -1,11 +1,7 @@
 import type { Request, Response, NextFunction } from 'express';
-import { isAppError } from '../errors/index.js';
-import { logger } from '../logger.js';
+import { isAppError } from '../errors/index.js';import { logger } from '../logger.js';
 import type { ValidationErrorDetail } from './validate.js';
-import { ValidationError } from './validate.js';
-import { buildErrorEnvelope } from './envelope.js';
-import type { ErrorEnvelope } from '../types/ResponseEnvelope.js';
-import { normalizeError } from '../errors/errorEnvelopePolicy.js';
+import { ValidationError } from './validate.js';import { buildErrorEnvelope } from './envelope.js';import type { ErrorEnvelope } from '../types/ResponseEnvelope.js';import { normalizeError } from '../errors/errorEnvelopePolicy.js';
 
 const isProduction = process.env.NODE_ENV === "production";
 
@@ -32,6 +28,7 @@ function extractValidationDetails(err: unknown): ValidationErrorDetail[] | undef
  * - Returns consistent JSON envelope: { success: false, error: { code, message }, requestId, timestamp }
  * - Never sends stack traces to the client in production
  * - Logs full error server-side
+ * - When headers are already sent, destroys the socket so the client sees a terminated stream
  */
 export function errorHandler(
   err: unknown,
@@ -66,6 +63,13 @@ export function errorHandler(
 
   if (!res.headersSent) {
     res.status(statusCode).json(body);
+  } else {
+    // Headers already flushed: we cannot write a JSON envelope.
+    // Terminate the socket so the client observes a truncated stream
+    // instead of hanging until its own timeout.
+    if (typeof res.destroy === 'function') {
+      res.destroy(err instanceof Error ? err : undefined);
+    }
   }
 
   const logData = {
diff --git a/src/routes/proxyRoutes.ts b/src/routes/proxyRoutes.ts
index 95aa201..8ea2627 100644
--- a/src/routes/proxyRoutes.ts
+++ b/src/routes/proxyRoutes.ts
@@ -272,6 +272,26 @@ export function createProxyRouter(deps: ProxyDeps): Router {
       } catch (err: unknown) {
         let outcome: UpstreamOutcome = 'error';
 
+        // If headers have already been flushed to the client, we cannot send a
+        // structured error response.  Destroy the socket so the client sees a
+        // terminated stream instead of hanging on a truncated body.  Log once
+        // with the requestId for observability.
+        if (res.headersSent) {
+          logger.error(
+            {
+              err,
+              requestId,
+              apiId: String(apiEntry.id),
+              endpointId: endpoint.endpointId,
+              upstreamStatus,
+            },
+            'Proxy error after headers sent; destroying response socket',
+          );
+          timer.stop(upstreamStatus, outcome);
+          res.destroy(err instanceof Error ? err : undefined);
+          return;
+        }
+
         if (err instanceof CircuitBreakerOpenError) {
           // Circuit breaker open — don't bill the caller
           upstreamStatus = 502;