diff --git a/packages/backend/src/connectors/engines/rest.engine.spec.ts b/packages/backend/src/connectors/engines/rest.engine.spec.ts
index a02dbcd2..5a72f609 100644
--- a/packages/backend/src/connectors/engines/rest.engine.spec.ts
+++ b/packages/backend/src/connectors/engines/rest.engine.spec.ts
@@ -190,6 +190,22 @@ describe('RestEngine', () => {
     expect(sent.params).not.toHaveProperty('__rawquery');
   });
 
+  it('drops __rawquery keys that would reach the prototype chain', async () => {
+    // The fragment comes from a tool argument, so from the model.
+    mockedAxios.mockResolvedValue({ data: {} });
+
+    await engine.execute(
+      { baseUrl: 'https://api.example.com', authType: 'NONE' },
+      { method: 'GET', path: '/article', queryParams: { __rawquery: '$filter' } },
+      { filter: '__proto__=x&constructor=y&prototype=z&name-eq=ok' },
+    );
+
+    const sent = mockedAxios.mock.calls[0][0] as unknown as { params: Record<string, unknown> };
+    expect(Object.keys(sent.params)).toEqual(['name-eq']);
+    expect(Object.getPrototypeOf(sent.params)).toBe(Object.prototype);
+    expect(({} as Record<string, unknown>).x).toBeUndefined();
+  });
+
   it('omits __rawquery entirely when the source param is absent', async () => {
     mockedAxios.mockResolvedValue({ data: {} });
 
diff --git a/packages/backend/src/connectors/engines/rest.engine.ts b/packages/backend/src/connectors/engines/rest.engine.ts
index 0e6bc3c0..195d464c 100644
--- a/packages/backend/src/connectors/engines/rest.engine.ts
+++ b/packages/backend/src/connectors/engines/rest.engine.ts
@@ -182,6 +182,7 @@ export class RestEngine {
         const raw = String(mappedQuery['__rawquery']);
         delete mappedQuery['__rawquery'];
         for (const [k, v] of new URLSearchParams(raw)) {
+          if (isUnsafeKey(k)) continue;
           mappedQuery[k] = v;
         }
       }
@@ -658,6 +659,7 @@ export class RestEngine {
     ) {
       bodyParams = {};
       for (const [k, v] of new URLSearchParams(axiosConfig.data)) {
+        if (isUnsafeKey(k)) continue;
         bodyParams[k] = v;
       }
     }
@@ -759,6 +761,15 @@ export class RestEngine {
   }
 }
 
+/**
+ * Keys that would reach an object's prototype chain instead of naming a
+ * parameter. The query and form strings parsed here come from tool arguments,
+ * so from the model; no API names a parameter like this.
+ */
+function isUnsafeKey(key: string): boolean {
+  return key === '__proto__' || key === 'constructor' || key === 'prototype';
+}
+
 /**
  * axios's own percent-encoding for a query component, reproduced so that
  * swapping in a custom serializer changes ONLY the array shape and nothing
diff --git a/packages/backend/src/connectors/graphql-builtins.spec.ts b/packages/backend/src/connectors/graphql-builtins.spec.ts
index bdfa748a..844003a2 100644
--- a/packages/backend/src/connectors/graphql-builtins.spec.ts
+++ b/packages/backend/src/connectors/graphql-builtins.spec.ts
@@ -98,3 +98,36 @@ describe('buildGraphqlBuiltinTools', () => {
     },
   );
 });
+
+describe('slugifyForPrefix without a backtracking regex', () => {
+  // The previous implementation, kept here as the reference the new one must
+  // match. Its trim regex was polynomial on long runs of underscores.
+  const reference = (name: string) =>
+    name.toLowerCase().replace(/[^a-z0-9]+/g, '_').replace(/^_+|_+$/g, '') || 'graphql';
+
+  it('matches the previous output on awkward and random inputs', () => {
+    const fixed = ['', '___', '--a--', 'A__B', ' x ', 'Ärger Öl', 'İstanbul API', '9lives', '..', 'a/b\\c'];
+    const alphabet = 'aZ9_-. /\\Äéİߣ\t\n@';
+    const random = Array.from({ length: 2000 }, (_, i) => {
+      let str = '';
+      let seed = i * 2654435761;
+      const len = (i % 17) + 1;
+      for (let j = 0; j < len; j++) {
+        seed = (seed * 1103515245 + 12345) >>> 0;
+        str += alphabet[seed % alphabet.length];
+      }
+      return str;
+    });
+    for (const input of [...fixed, ...random]) {
+      expect(slugifyForPrefix(input)).toBe(reference(input));
+    }
+  });
+
+  it('stays linear on the input that made the old regex crawl', () => {
+    const hostile = '_'.repeat(50_000) + '!';
+    const started = Date.now();
+    expect(slugifyForPrefix(hostile)).toBe('graphql');
+    expect(Date.now() - started).toBeLessThan(200);
+  });
+});
+
diff --git a/packages/backend/src/connectors/graphql-builtins.ts b/packages/backend/src/connectors/graphql-builtins.ts
index 17f869be..73a80ad2 100644
--- a/packages/backend/src/connectors/graphql-builtins.ts
+++ b/packages/backend/src/connectors/graphql-builtins.ts
@@ -49,18 +49,34 @@ export interface GraphqlBuiltinOptions {
  *   slugifyForPrefix("")                    → "graphql" (safe fallback)
  */
 export function slugifyForPrefix(name: string): string {
-  const slug = name
-    .toLowerCase()
-    .replace(/[^a-z0-9]+/g, '_')
-    .replace(/^_+|_+$/g, '');
+  // One pass instead of `.replace(/^_+|_+$/g, '')`, which backtracks
+  // polynomially on a long run of underscores (CodeQL js/polynomial-redos).
+  let slug = '';
+  let pendingUnderscore = false;
+  for (const ch of name.toLowerCase()) {
+    if ((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')) {
+      if (pendingUnderscore && slug) slug += '_';
+      slug += ch;
+      pendingUnderscore = false;
+    } else {
+      pendingUnderscore = true;
+    }
+  }
   return slug || 'graphql';
 }
 
+/** `url` without trailing slashes, without the backtracking `/\/+$/`. */
+function trimTrailingSlashes(url: string): string {
+  let end = url.length;
+  while (end > 0 && url[end - 1] === '/') end--;
+  return url.slice(0, end);
+}
+
 export function buildGraphqlBuiltinTools(
   opts: GraphqlBuiltinOptions,
 ): GraphqlBuiltinTool[] {
   const { prefix, displayName, baseUrl } = opts;
-  const schemaUrl = opts.schemaUrl || `${baseUrl.replace(/\/+$/, '')}/schema`;
+  const schemaUrl = opts.schemaUrl || `${trimTrailingSlashes(baseUrl)}/schema`;
 
   const variablesSchema = {
     type: 'object',
diff --git a/packages/backend/src/mcp-server/mcp-endpoint.controller.ts b/packages/backend/src/mcp-server/mcp-endpoint.controller.ts
index 2d93423c..725ace4f 100644
--- a/packages/backend/src/mcp-server/mcp-endpoint.controller.ts
+++ b/packages/backend/src/mcp-server/mcp-endpoint.controller.ts
@@ -432,6 +432,9 @@ export class McpEndpointController {
       res.status(200);
       res.setHeader('Content-Type', 'text/event-stream');
       res.setHeader('Cache-Control', 'no-cache');
+      // The body echoes the request id; nosniff keeps any browser from
+      // reading this stream as anything but the event stream it is.
+      res.setHeader('X-Content-Type-Options', 'nosniff');
       res.end(`event: message\ndata: ${JSON.stringify(payload)}\n\n`);
     }
     return true;