diff --git a/package-lock.json b/package-lock.json
index 882ea853..ca9bd433 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -24593,7 +24593,7 @@
         "helmet": "^8.3.0",
         "https-proxy-agent": "^7.0.6",
         "ioredis": "^5.11.1",
-        "js-yaml": "^4.3.2",
+        "js-yaml": "^5.4.1",
         "mongodb": "^7.6.0",
         "mssql": "^12.7.2",
         "mysql2": "^3.24.4",
@@ -24814,6 +24814,28 @@
         "url": "https://github.com/sponsors/epoberezkin"
       }
     },
+    "packages/backend/node_modules/js-yaml": {
+      "version": "5.4.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-5.4.1.tgz",
+      "integrity": "sha512-28R/k+NAjeuf7+CKlTxWZVExJGwVVLwY06DgEnOMz2gEpfNkDcD7QvyiVPT0xy0XXhU8vHsd4Ot42OOPdJG7dQ==",
+      "funding": [
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/puzrin"
+        },
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/nodeca"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "argparse": "^2.0.1"
+      },
+      "bin": {
+        "js-yaml": "bin/js-yaml.mjs"
+      }
+    },
     "packages/backend/node_modules/json-schema-traverse": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/json-schema-traverse/-/json-schema-traverse-1.0.0.tgz",
diff --git a/packages/backend/package.json b/packages/backend/package.json
index acc2893c..83cfabd8 100644
--- a/packages/backend/package.json
+++ b/packages/backend/package.json
@@ -62,7 +62,7 @@
     "helmet": "^8.3.0",
     "https-proxy-agent": "^7.0.6",
     "ioredis": "^5.11.1",
-    "js-yaml": "^4.3.2",
+    "js-yaml": "^5.4.1",
     "mongodb": "^7.6.0",
     "mssql": "^12.7.2",
     "mysql2": "^3.24.4",
diff --git a/packages/backend/src/connectors/parsers/openapi.parser.spec.ts b/packages/backend/src/connectors/parsers/openapi.parser.spec.ts
index 94aebe30..06e41ce3 100644
--- a/packages/backend/src/connectors/parsers/openapi.parser.spec.ts
+++ b/packages/backend/src/connectors/parsers/openapi.parser.spec.ts
@@ -25,6 +25,33 @@ describe('OpenApiParser', () => {
     expect(tools).toHaveLength(0);
   });
 
+  it('should parse a YAML spec with anchors and merge keys', async () => {
+    const yamlSpec = [
+      'openapi: 3.0.0',
+      'info: { title: Test API, version: 1.0.0 }',
+      'x-common: &paging',
+      '  in: query',
+      '  schema: { type: integer }',
+      'paths:',
+      '  /points:',
+      '    get:',
+      '      operationId: listPoints',
+      '      parameters:',
+      '        - <<: *paging',
+      '          name: y',
+      '          description: Vertical offset',
+      '      responses:',
+      "        '200': { description: OK }",
+    ].join('\n');
+    const tools = await parser.parse(yamlSpec);
+    expect(tools).toHaveLength(1);
+    const params = tools[0].parameters as any;
+    // The merged keys land on the parameter, and `y` stays a name, not `true`.
+    expect(params.properties.y).toBeDefined();
+    expect(params.properties.y.type).toBe('integer');
+    expect(tools[0].endpointMapping.queryParams).toEqual(expect.objectContaining({ y: '$y' }));
+  });
+
   // ── GET endpoints ──────────────────────────────────────────────────────
 
   it('should parse a simple GET endpoint', async () => {
diff --git a/packages/backend/src/connectors/parsers/openapi.parser.ts b/packages/backend/src/connectors/parsers/openapi.parser.ts
index 9b52c144..2608e801 100644
--- a/packages/backend/src/connectors/parsers/openapi.parser.ts
+++ b/packages/backend/src/connectors/parsers/openapi.parser.ts
@@ -4,10 +4,16 @@ import { capToolName } from './tool-name.util';
 const SwaggerParser = require('swagger-parser');
 import axios from 'axios';
  
-const yaml = require('js-yaml') as { load: (s: string) => unknown };
+import { CORE_SCHEMA, load as loadYaml, mergeTag, timestampTag } from 'js-yaml';
 import { assertSafeOutboundUrl } from '../../common/ssrf.util';
 import { normalizeOpenApi31 } from './openapi-3.1-normalizer';
 
+// js-yaml 5 loads with the bare YAML 1.2 core schema. Real-world specs use
+// `<<: *anchor` merge keys, which it would keep as a literal "<<" property,
+// so add merge (and timestamps) back: the same result js-yaml 4 gave. Not
+// YAML11_SCHEMA: its 1.1 booleans turn a property named `y` or `on` into `true`.
+const SPEC_YAML_SCHEMA = CORE_SCHEMA.withTags(mergeTag, timestampTag);
+
 export interface ParsedTool {
   name: string;
   /**
@@ -169,7 +175,7 @@ export class OpenApiParser {
       return JSON.parse(input);
     }
     try {
-      return yaml.load(input);
+      return loadYaml(input, { schema: SPEC_YAML_SCHEMA });
     } catch (yamlErr) {
       // Last-resort: try JSON anyway so the original error surfaces if the
       // input is genuinely malformed.