diff --git a/README.md b/README.md
index 3f763dd..fe412de 100644
--- a/README.md
+++ b/README.md
@@ -41,11 +41,17 @@ Detalle del despliegue y la secuencia de cutover: [`docs/rollout.md`](docs/rollo
 
 ```bash
 npm ci
-npm test            # 47 tests (vitest + mongodb-memory-server, sin infra externa)
+npm test            # 47 tests (vitest + fake-repo en memoria, sin infra externa)
 npm run typecheck
 npm run build       # -> dist/ (+ plantillas)
 ```
 
+> **Nota MongoDB**: producción corre **MongoDB 3.2** (EOL), así que el servicio
+> usa el driver `mongodb` **v3.7** (el moderno no conecta a 3.2) y no hay change
+> streams (fase 1b usará polling). Los tests van contra un fake en memoria
+> porque no hay mongod tan antiguo ejecutable en hosts modernos. Detalle en
+> [`docs/legacy-behavior.md`](docs/legacy-behavior.md) §0.
+
 ## Ejecución
 
 Config por env (ver [`.env.example`](.env.example)) o `config.json` (ver
diff --git a/docs/legacy-behavior.md b/docs/legacy-behavior.md
index dedd7e9..508d0a0 100644
--- a/docs/legacy-behavior.md
+++ b/docs/legacy-behavior.md
@@ -13,6 +13,24 @@
 > **campo a campo**. Los tests de caracterización (`test/legacy-behavior.test.ts`)
 > lo verifican.
 
+## 0. Entorno de datos (crítico)
+
+- **Producción corre MongoDB 3.2.11** (verificado en shiva vía `buildInfo`,
+  jul-2026). Es EOL desde 2018. Implica:
+  - El servicio usa el **driver `mongodb` v3.7** (el v6 exige servidor ≥ 4.2 y
+    **no conecta** a 3.2). No subir el driver sin subir antes el servidor.
+  - **No hay change streams** (requieren 3.6+) → la fase 1b debe usar el
+    **polling** de respaldo, no change streams.
+  - Los tests de integración corren contra un **fake en memoria**
+    (`test/fake-repo.ts`), porque en hosts modernos no se puede
+    descargar/ejecutar un mongod tan antiguo (glibc/libssl).
+- Cifras reales del import (jul-2026): ~413 docs `notifications` pendientes de
+  FCM, 0 de email, **8176 users con `fireBaseToken`** (muchos llevarán ~2 años
+  muertos → alta tasa de token inválido en el primer envío real).
+- Recomendación de fondo (fuera de fase 1a): planificar un **upgrade de MongoDB**
+  como fase propia; 3.2 bloquea drivers modernos, change streams y es un riesgo
+  de seguridad. Afecta a Meteor 1.6 y node-red, que dependen de él.
+
 ## 1. Esquema del doc `notifications`
 
 Colección `notifications` (db `fuegos`, `idGeneration: 'MONGO'` → `_id` es
diff --git a/package-lock.json b/package-lock.json
index 5d3b7b9..269c96c 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -14,16 +14,16 @@
         "ioredis": "^5.4.2",
         "juice": "^11.0.0",
         "luxon": "^3.5.0",
-        "mongodb": "^6.12.0",
+        "mongodb": "^3.7.4",
         "nodemailer": "^6.9.16",
         "pino": "^9.5.0",
         "pino-pretty": "^13.0.0"
       },
       "devDependencies": {
         "@types/luxon": "^3.4.2",
+        "@types/mongodb": "^3.6.20",
         "@types/node": "^22.10.2",
         "@types/nodemailer": "^6.4.17",
-        "mongodb-memory-server": "^11.2.0",
         "typescript": "^5.7.2",
         "vitest": "^2.1.8"
       },
@@ -728,15 +728,6 @@
         "url": "https://opencollective.com/js-sdsl"
       }
     },
-    "node_modules/@mongodb-js/saslprep": {
-      "version": "1.4.12",
-      "resolved": "https://registry.npmjs.org/@mongodb-js/saslprep/-/saslprep-1.4.12.tgz",
-      "integrity": "sha512-QAfAMwNgnYxZ2C6D1HgeP7Gc4i/uvJRim415PCIL9ptRxWMNbWeLBYb2/9R4pGKny/s1FVu2JA2cxCUBUOggrA==",
-      "license": "MIT",
-      "dependencies": {
-        "sparse-bitfield": "^3.0.3"
-      }
-    },
     "node_modules/@msgpackr-extract/msgpackr-extract-darwin-arm64": {
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/@msgpackr-extract/msgpackr-extract-darwin-arm64/-/msgpackr-extract-darwin-arm64-3.0.4.tgz",
@@ -1270,6 +1261,16 @@
         "node": ">= 10"
       }
     },
+    "node_modules/@types/bson": {
+      "version": "4.0.5",
+      "resolved": "https://registry.npmjs.org/@types/bson/-/bson-4.0.5.tgz",
+      "integrity": "sha512-vVLwMUqhYJSQ/WKcE60eFqcyuWse5fGH+NMAXHuKrUAPoryq3ATxk5o4bgYNtg5aOM4APVg7Hnb3ASqUYG0PKg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/caseless": {
       "version": "0.12.5",
       "resolved": "https://registry.npmjs.org/@types/caseless/-/caseless-0.12.5.tgz",
@@ -1308,6 +1309,17 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/mongodb": {
+      "version": "3.6.20",
+      "resolved": "https://registry.npmjs.org/@types/mongodb/-/mongodb-3.6.20.tgz",
+      "integrity": "sha512-WcdpPJCakFzcWWD9juKoZbRtQxKIMYF/JIAM4JrNHrMcnJL6/a2NWjXxW7fo9hxboxxkg+icff8d7+WIEvKgYQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/bson": "*",
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/ms": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/@types/ms/-/ms-2.1.0.tgz",
@@ -1353,21 +1365,6 @@
       "license": "MIT",
       "optional": true
     },
-    "node_modules/@types/webidl-conversions": {
-      "version": "7.0.3",
-      "resolved": "https://registry.npmjs.org/@types/webidl-conversions/-/webidl-conversions-7.0.3.tgz",
-      "integrity": "sha512-CiJJvcRtIgzadHCYXw7dqEnMNRjhGZlYK05Mj9OyktqV8uVT8fD2BFOB7S1uwBE3Kj2Z+4UyPmFw/Ixgw/LAlA==",
-      "license": "MIT"
-    },
-    "node_modules/@types/whatwg-url": {
-      "version": "11.0.5",
-      "resolved": "https://registry.npmjs.org/@types/whatwg-url/-/whatwg-url-11.0.5.tgz",
-      "integrity": "sha512-coYR071JRaHa+xoEvvYqvnIHaVqaYrLPbsufM9BF63HkwI5Lgmy2QR8Q5K/lYDYo5AK82wOvSOS0UsLTpTG7uQ==",
-      "license": "MIT",
-      "dependencies": {
-        "@types/webidl-conversions": "*"
-      }
-    },
     "node_modules/@vitest/expect": {
       "version": "2.1.9",
       "resolved": "https://registry.npmjs.org/@vitest/expect/-/expect-2.1.9.tgz",
@@ -1571,16 +1568,6 @@
         "node": ">=12"
       }
     },
-    "node_modules/async-mutex": {
-      "version": "0.5.0",
-      "resolved": "https://registry.npmjs.org/async-mutex/-/async-mutex-0.5.0.tgz",
-      "integrity": "sha512-1A94B18jkJ3DYq284ohPxoXbfTA5HsQ7/Mf4DEhcyLx3Bz27Rh59iScbB6EPiP+B+joue6YCxcMXSbFC1tZKwA==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
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     "node_modules/async-retry": {
       "version": "1.3.3",
       "resolved": "https://registry.npmjs.org/async-retry/-/async-retry-1.3.3.tgz",
@@ -1607,106 +1594,6 @@
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       }
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-      "dev": true,
-      "license": "Apache-2.0",
-      "peerDependencies": {
-        "react-native-b4a": "*"
-      },
-      "peerDependenciesMeta": {
-        "react-native-b4a": {
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-    "node_modules/bare-events": {
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-      "resolved": "https://registry.npmjs.org/bare-events/-/bare-events-2.9.1.tgz",
-      "integrity": "sha512-Z0oHEHAFDZkffN8Qc39zNZjQlMDkPJRyyyZieU1VH7u8c5S+qHZ2S8ixdKIAxEjfHO7FJxXmJWgteOghVanIsg==",
-      "dev": true,
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-      "dev": true,
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-      "dependencies": {
-        "bare-events": "^2.5.4",
-        "bare-path": "^3.0.0",
-        "bare-stream": "^2.6.4",
-        "bare-url": "^2.2.2",
-        "fast-fifo": "^1.3.2"
-      },
-      "engines": {
-        "bare": ">=1.16.0"
-      },
-      "peerDependencies": {
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-      },
-      "peerDependenciesMeta": {
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-      "resolved": "https://registry.npmjs.org/bare-path/-/bare-path-3.1.1.tgz",
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-      "dev": true,
-      "license": "Apache-2.0",
-      "dependencies": {
-        "b4a": "^1.8.1",
-        "streamx": "^2.25.0",
-        "teex": "^1.0.1"
-      },
-      "peerDependencies": {
-        "bare-abort-controller": "*",
-        "bare-buffer": "*",
-        "bare-events": "*"
-      },
-      "peerDependenciesMeta": {
-        "bare-abort-controller": {
-          "optional": true
-        },
-        "bare-buffer": {
-          "optional": true
-        },
-        "bare-events": {
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-        }
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-    "node_modules/bare-url": {
-      "version": "2.4.5",
-      "resolved": "https://registry.npmjs.org/bare-url/-/bare-url-2.4.5.tgz",
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-      "dev": true,
-      "license": "Apache-2.0",
-      "dependencies": {
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     "node_modules/base64-js": {
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       "resolved": "https://registry.npmjs.org/base64-js/-/base64-js-1.5.1.tgz",
@@ -1736,6 +1623,46 @@
         "node": "*"
       }
     },
+    "node_modules/bl": {
+      "version": "2.2.1",
+      "resolved": "https://registry.npmjs.org/bl/-/bl-2.2.1.tgz",
+      "integrity": "sha512-6Pesp1w0DEX1N550i/uGV/TqucVL4AM/pgThFSN/Qq9si1/DF9aIHs1BxD8V/QU0HoeHO6cQRTAuYnLPKq1e4g==",
+      "license": "MIT",
+      "dependencies": {
+        "readable-stream": "^2.3.5",
+        "safe-buffer": "^5.1.1"
+      }
+    },
+    "node_modules/bl/node_modules/readable-stream": {
+      "version": "2.3.8",
+      "resolved": "https://registry.npmjs.org/readable-stream/-/readable-stream-2.3.8.tgz",
+      "integrity": "sha512-8p0AUk4XODgIewSi0l8Epjs+EVnWiK7NoDIEGU0HhE7+ZyY8D1IMY7odu5lRrFXGg71L15KG8QrPmum45RTtdA==",
+      "license": "MIT",
+      "dependencies": {
+        "core-util-is": "~1.0.0",
+        "inherits": "~2.0.3",
+        "isarray": "~1.0.0",
+        "process-nextick-args": "~2.0.0",
+        "safe-buffer": "~5.1.1",
+        "string_decoder": "~1.1.1",
+        "util-deprecate": "~1.0.1"
+      }
+    },
+    "node_modules/bl/node_modules/safe-buffer": {
+      "version": "5.1.2",
+      "resolved": "https://registry.npmjs.org/safe-buffer/-/safe-buffer-5.1.2.tgz",
+      "integrity": "sha512-Gd2UZBJDkXlY7GbJxfsE8/nvKkUEU1G38c1siN6QP6a9PT9MmHB8GnpscSmMJSoF8LOIrt8ud/wPtojys4G6+g==",
+      "license": "MIT"
+    },
+    "node_modules/bl/node_modules/string_decoder": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.1.1.tgz",
+      "integrity": "sha512-n/ShnvDi6FHbbVfviro+WojiFzv+s8MPMHBczVePfUpDJLwoLT0ht1l4YwBCbi8pJAveEEdnkHyPyTP/mzRfwg==",
+      "license": "MIT",
+      "dependencies": {
+        "safe-buffer": "~5.1.0"
+      }
+    },
     "node_modules/boolbase": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/boolbase/-/boolbase-1.0.0.tgz",
@@ -1743,12 +1670,12 @@
       "license": "ISC"
     },
     "node_modules/bson": {
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-      "resolved": "https://registry.npmjs.org/bson/-/bson-6.10.4.tgz",
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+      "version": "1.1.6",
+      "resolved": "https://registry.npmjs.org/bson/-/bson-1.1.6.tgz",
+      "integrity": "sha512-EvVNVeGo4tHxwi8L6bPj3y3itEvStdwvvlojVxxbyYfoaxJ6keLgrTuKdyfEAszFK+H3olzBuafE0yoh0D1gdg==",
       "license": "Apache-2.0",
       "engines": {
-        "node": ">=16.20.1"
+        "node": ">=0.6.19"
       }
     },
     "node_modules/buffer-equal-constant-time": {
@@ -1836,19 +1763,6 @@
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@@ -1990,11 +1904,10 @@
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-      "dev": true,
+    "node_modules/core-util-is": {
+      "version": "1.0.3",
+      "resolved": "https://registry.npmjs.org/core-util-is/-/core-util-is-1.0.3.tgz",
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     },
     "node_modules/cron-parser": {
@@ -2393,16 +2306,6 @@
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@@ -2584,27 +2448,6 @@
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-      "resolved": "https://registry.npmjs.org/tr46/-/tr46-0.0.3.tgz",
-      "integrity": "sha512-N3WMsuqV66lT30CrXNbEjx4GEwlow3v6rr4mCcv6prnfwhS01rkgyFdjPNBYd9br7LpXV1+Emh01fHnq2Gdgrw==",
-      "license": "MIT",
-      "optional": true
-    },
-    "node_modules/node-fetch/node_modules/webidl-conversions": {
-      "version": "3.0.1",
-      "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-3.0.1.tgz",
-      "integrity": "sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==",
-      "license": "BSD-2-Clause",
-      "optional": true
-    },
-    "node_modules/node-fetch/node_modules/whatwg-url": {
-      "version": "5.0.0",
-      "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-5.0.0.tgz",
-      "integrity": "sha512-saE57nupxk6v3HY35+jzBwYa0rKSy0XR8JSxZPwgLr7ys0IBzhGviA1/TUGJLmSVqs8pb9AnvICXEuOHLprYTw==",
-      "license": "MIT",
-      "optional": true,
-      "dependencies": {
-        "tr46": "~0.0.3",
-        "webidl-conversions": "^3.0.0"
-      }
-    },
     "node_modules/node-gyp-build-optional-packages": {
       "version": "5.2.2",
       "resolved": "https://registry.npmjs.org/node-gyp-build-optional-packages/-/node-gyp-build-optional-packages-5.2.2.tgz",
@@ -3982,6 +3542,18 @@
         "wrappy": "1"
       }
     },
+    "node_modules/optional-require": {
+      "version": "1.1.10",
+      "resolved": "https://registry.npmjs.org/optional-require/-/optional-require-1.1.10.tgz",
+      "integrity": "sha512-0r3OB9EIQsP+a5HVATHq2ExIy2q/Vaffoo4IAikW1spCYswhLxqWQS0i3GwS3AdY/OIP4SWZHLGz8CMU558PGw==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "require-at": "^1.0.6"
+      },
+      "engines": {
+        "node": ">=4"
+      }
+    },
     "node_modules/p-limit": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/p-limit/-/p-limit-3.1.0.tgz",
@@ -3998,45 +3570,6 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
-    "node_modules/p-locate": {
-      "version": "4.1.0",
-      "resolved": "https://registry.npmjs.org/p-locate/-/p-locate-4.1.0.tgz",
-      "integrity": "sha512-R79ZZ/0wAxKGu3oYMlz8jy/kbhsNrS7SKZ7PxEHBgJ5+F2mtFW2fK2cOtBh1cHYkQsbzFV7I+EoRKe6Yt0oK7A==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "p-limit": "^2.2.0"
-      },
-      "engines": {
-        "node": ">=8"
-      }
-    },
-    "node_modules/p-locate/node_modules/p-limit": {
-      "version": "2.3.0",
-      "resolved": "https://registry.npmjs.org/p-limit/-/p-limit-2.3.0.tgz",
-      "integrity": "sha512-//88mFWSJx8lxCzwdAABTJL2MyWB12+eIY7MDL2SqLmAkeKU9qxRvWuSyTjm3FUmpBEMuFfckAIqEaVGUDxb6w==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "p-try": "^2.0.0"
-      },
-      "engines": {
-        "node": ">=6"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/sindresorhus"
-      }
-    },
-    "node_modules/p-try": {
-      "version": "2.2.0",
-      "resolved": "https://registry.npmjs.org/p-try/-/p-try-2.2.0.tgz",
-      "integrity": "sha512-R4nPAVTAU0B9D35/Gk3uJf/7XYbQcyohSKdvAxIRSNghFl4e71hVoGnBNQz9cWaXxO2I10KTC+3jMdvvoKw6dQ==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=6"
-      }
-    },
     "node_modules/parse5": {
       "version": "7.3.0",
       "resolved": "https://registry.npmjs.org/parse5/-/parse5-7.3.0.tgz",
@@ -4086,16 +3619,6 @@
         "url": "https://github.com/fb55/entities?sponsor=1"
       }
     },
-    "node_modules/path-exists": {
-      "version": "4.0.0",
-      "resolved": "https://registry.npmjs.org/path-exists/-/path-exists-4.0.0.tgz",
-      "integrity": "sha512-ak9Qy5Q7jYb2Wwcey5Fpvg2KoAc/ZIhLSLOSBmRmygPsGwkVVt0fZa0qrtMz+m6tJTAHfZQ8FnmB4MG4LWy7/w==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "node_modules/path-expression-matcher": {
       "version": "1.6.2",
       "resolved": "https://registry.npmjs.org/path-expression-matcher/-/path-expression-matcher-1.6.2.tgz",
@@ -4129,13 +3652,6 @@
         "node": ">= 14.16"
       }
     },
-    "node_modules/pend": {
-      "version": "1.2.0",
-      "resolved": "https://registry.npmjs.org/pend/-/pend-1.2.0.tgz",
-      "integrity": "sha512-F3asv42UuXchdzt+xXqfW1OGlVBe+mxa2mqI0pg5yAHZPvFmY3Y6drSf/GQ1A86WgWEN9Kzh/WrgKa6iGcHXLg==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -4213,19 +3729,6 @@
       "integrity": "sha512-BndPH67/JxGExRgiX1dX0w1FvZck5Wa4aal9198SrRhZjH3GxKQUKIBnYJTdj2HDN3UQAS06HlfcSbQj2OHmaw==",
       "license": "MIT"
     },
-    "node_modules/pkg-dir": {
-      "version": "4.2.0",
-      "resolved": "https://registry.npmjs.org/pkg-dir/-/pkg-dir-4.2.0.tgz",
-      "integrity": "sha512-HRDzbaKjC+AOWVXxAU/x54COGeIv9eb+6CkDSQoNTt4XyWoIJvuPsXizxu/Fr23EiekbtZwmh1IcIG/l/a10GQ==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "find-up": "^4.0.0"
-      },
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "node_modules/postcss": {
       "version": "8.5.18",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.5.18.tgz",
@@ -4255,6 +3758,12 @@
         "node": "^10 || ^12 || >=14"
       }
     },
+    "node_modules/process-nextick-args": {
+      "version": "2.0.1",
+      "resolved": "https://registry.npmjs.org/process-nextick-args/-/process-nextick-args-2.0.1.tgz",
+      "integrity": "sha512-3ouUOpQhtgrbOa17J7+uxOTpITYWaGP7/AhoR3+A+/1e9skrzelGi/dXzEYyvbxubEF6Wn2ypscTKiKJFFn1ag==",
+      "license": "MIT"
+    },
     "node_modules/process-warning": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/process-warning/-/process-warning-5.0.0.tgz",
@@ -4318,15 +3827,6 @@
         "once": "^1.3.1"
       }
     },
-    "node_modules/punycode": {
-      "version": "2.3.1",
-      "resolved": "https://registry.npmjs.org/punycode/-/punycode-2.3.1.tgz",
-      "integrity": "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg==",
-      "license": "MIT",
-      "engines": {
-        "node": ">=6"
-      }
-    },
     "node_modules/quick-format-unescaped": {
       "version": "4.0.4",
       "resolved": "https://registry.npmjs.org/quick-format-unescaped/-/quick-format-unescaped-4.0.4.tgz",
@@ -4378,6 +3878,15 @@
         "node": ">=4"
       }
     },
+    "node_modules/require-at": {
+      "version": "1.0.6",
+      "resolved": "https://registry.npmjs.org/require-at/-/require-at-1.0.6.tgz",
+      "integrity": "sha512-7i1auJbMUrXEAZCOQ0VNJgmcT2VOKPRl2YGJwgpHpC9CE91Mv4/4UYIUm4chGJaI381ZDq1JUicFii64Hapd8g==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=4"
+      }
+    },
     "node_modules/require-directory": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/require-directory/-/require-directory-2.1.1.tgz",
@@ -4493,6 +4002,19 @@
       "integrity": "sha512-YZo3K82SD7Riyi0E1EQPojLz7kpepnSQI9IyPbHHg1XXXevb5dJI7tpyN2ADxGcQbHG7vcyRHk0cbwqcQriUtg==",
       "license": "MIT"
     },
+    "node_modules/saslprep": {
+      "version": "1.0.3",
+      "resolved": "https://registry.npmjs.org/saslprep/-/saslprep-1.0.3.tgz",
+      "integrity": "sha512-/MY/PEMbk2SuY5sScONwhUDsV2p77Znkb/q3nSVstq/yQzYJOH/Azh29p9oJLsl3LnQwSvZDKagDGBsBwSooag==",
+      "license": "MIT",
+      "optional": true,
+      "dependencies": {
+        "sparse-bitfield": "^3.0.3"
+      },
+      "engines": {
+        "node": ">=6"
+      }
+    },
     "node_modules/secure-json-parse": {
       "version": "4.1.0",
       "resolved": "https://registry.npmjs.org/secure-json-parse/-/secure-json-parse-4.1.0.tgz",
@@ -4570,6 +4092,7 @@
       "resolved": "https://registry.npmjs.org/sparse-bitfield/-/sparse-bitfield-3.0.3.tgz",
       "integrity": "sha512-kvzhi7vqKTfkh0PZU+2D2PIllw2ymqJKujUcyPMd9Y75Nv4nPbGJZXNhxsgdQab2BmlDct1YnfQCguEvHr7VsQ==",
       "license": "MIT",
+      "optional": true,
       "dependencies": {
         "memory-pager": "^1.0.2"
       }
@@ -4620,18 +4143,6 @@
       "license": "MIT",
       "optional": true
     },
-    "node_modules/streamx": {
-      "version": "2.28.0",
-      "resolved": "https://registry.npmjs.org/streamx/-/streamx-2.28.0.tgz",
-      "integrity": "sha512-1Yowhzjf0ivGMrTIkY9hav5TxobO9qIVqUE41fiCGMGgc3CLlf4MY+9AHmZqBWgDTue0fY9zWjYFVyf6Diuobw==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "events-universal": "^1.0.0",
-        "fast-fifo": "^1.3.2",
-        "text-decoder": "^1.1.0"
-      }
-    },
     "node_modules/string_decoder": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.3.0.tgz",
@@ -4705,19 +4216,6 @@
       "license": "MIT",
       "optional": true
     },
-    "node_modules/tar-stream": {
-      "version": "3.2.0",
-      "resolved": "https://registry.npmjs.org/tar-stream/-/tar-stream-3.2.0.tgz",
-      "integrity": "sha512-ojzvCvVaNp6aOTFmG7jaRD0meowIAuPc3cMMhSgKiVWws1GyHbGd/xvnyuRKcKlMpt3qvxx6r0hreCNITP9hIg==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "b4a": "^1.6.4",
-        "bare-fs": "^4.5.5",
-        "fast-fifo": "^1.2.0",
-        "streamx": "^2.15.0"
-      }
-    },
     "node_modules/teeny-request": {
       "version": "9.0.0",
       "resolved": "https://registry.npmjs.org/teeny-request/-/teeny-request-9.0.0.tgz",
@@ -4762,26 +4260,6 @@
         "node": ">= 6"
       }
     },
-    "node_modules/teex": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/teex/-/teex-1.0.1.tgz",
-      "integrity": "sha512-eYE6iEI62Ni1H8oIa7KlDU6uQBtqr4Eajni3wX7rpfXD8ysFx8z0+dri+KWEPWpBsxXfxu58x/0jvTVT1ekOSg==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "streamx": "^2.12.5"
-      }
-    },
-    "node_modules/text-decoder": {
-      "version": "1.2.7",
-      "resolved": "https://registry.npmjs.org/text-decoder/-/text-decoder-1.2.7.tgz",
-      "integrity": "sha512-vlLytXkeP4xvEq2otHeJfSQIRyWxo/oZGEbXrtEEF9Hnmrdly59sUbzZ/QgyWuLYHctCHxFF4tRQZNQ9k60ExQ==",
-      "dev": true,
-      "license": "Apache-2.0",
-      "dependencies": {
-        "b4a": "^1.6.4"
-      }
-    },
     "node_modules/thread-stream": {
       "version": "3.2.0",
       "resolved": "https://registry.npmjs.org/thread-stream/-/thread-stream-3.2.0.tgz",
@@ -4836,16 +4314,11 @@
       }
     },
     "node_modules/tr46": {
-      "version": "5.1.1",
-      "resolved": "https://registry.npmjs.org/tr46/-/tr46-5.1.1.tgz",
-      "integrity": "sha512-hdF5ZgjTqgAntKkklYw0R03MG2x/bSzTtkxmIRw/sTNV8YXsCJ1tfLAX23lhxhHJlEf3CRCOCGGWw3vI3GaSPw==",
+      "version": "0.0.3",
+      "resolved": "https://registry.npmjs.org/tr46/-/tr46-0.0.3.tgz",
+      "integrity": "sha512-N3WMsuqV66lT30CrXNbEjx4GEwlow3v6rr4mCcv6prnfwhS01rkgyFdjPNBYd9br7LpXV1+Emh01fHnq2Gdgrw==",
       "license": "MIT",
-      "dependencies": {
-        "punycode": "^2.3.1"
-      },
-      "engines": {
-        "node": ">=18"
-      }
+      "optional": true
     },
     "node_modules/tslib": {
       "version": "2.8.1",
@@ -4899,8 +4372,7 @@
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/util-deprecate/-/util-deprecate-1.0.2.tgz",
       "integrity": "sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw==",
-      "license": "MIT",
-      "optional": true
+      "license": "MIT"
     },
     "node_modules/uuid": {
       "version": "9.0.1",
@@ -5113,13 +4585,11 @@
       }
     },
     "node_modules/webidl-conversions": {
-      "version": "7.0.0",
-      "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-7.0.0.tgz",
-      "integrity": "sha512-VwddBukDzu71offAQR975unBIGqfKZpM+8ZX6ySk8nYhVoo5CYaZyzt3YBvYtRtO+aoGlqxPg/B87NGVZ/fu6g==",
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-3.0.1.tgz",
+      "integrity": "sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==",
       "license": "BSD-2-Clause",
-      "engines": {
-        "node": ">=12"
-      }
+      "optional": true
     },
     "node_modules/websocket-driver": {
       "version": "0.7.5",
@@ -5167,16 +4637,14 @@
       }
     },
     "node_modules/whatwg-url": {
-      "version": "14.2.0",
-      "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-14.2.0.tgz",
-      "integrity": "sha512-De72GdQZzNTUBBChsXueQUnPKDkg/5A5zp7pFDuQAj5UFoENpiACU0wlCvzpAGnTkj++ihpKwKyYewn/XNUbKw==",
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-5.0.0.tgz",
+      "integrity": "sha512-saE57nupxk6v3HY35+jzBwYa0rKSy0XR8JSxZPwgLr7ys0IBzhGviA1/TUGJLmSVqs8pb9AnvICXEuOHLprYTw==",
       "license": "MIT",
+      "optional": true,
       "dependencies": {
-        "tr46": "^5.1.0",
-        "webidl-conversions": "^7.0.0"
-      },
-      "engines": {
-        "node": ">=18"
+        "tr46": "~0.0.3",
+        "webidl-conversions": "^3.0.0"
       }
     },
     "node_modules/why-is-node-running": {
@@ -5287,19 +4755,6 @@
         "node": ">=12"
       }
     },
-    "node_modules/yauzl": {
-      "version": "3.4.0",
-      "resolved": "https://registry.npmjs.org/yauzl/-/yauzl-3.4.0.tgz",
-      "integrity": "sha512-jIH9yLR9wqr0wOS0TpBvo/g/2UgZH5qePVbjgRliiF0BYvOZyaBknKsF+x9Iht0O6sqgnB93rCICdOZFecJuDw==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "pend": "~1.2.0"
-      },
-      "engines": {
-        "node": ">=12"
-      }
-    },
     "node_modules/yocto-queue": {
       "version": "0.1.0",
       "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",
diff --git a/package.json b/package.json
index ba106fd..8dfdf21 100644
--- a/package.json
+++ b/package.json
@@ -22,16 +22,16 @@
     "ioredis": "^5.4.2",
     "juice": "^11.0.0",
     "luxon": "^3.5.0",
-    "mongodb": "^6.12.0",
+    "mongodb": "^3.7.4",
     "nodemailer": "^6.9.16",
     "pino": "^9.5.0",
     "pino-pretty": "^13.0.0"
   },
   "devDependencies": {
     "@types/luxon": "^3.4.2",
+    "@types/mongodb": "^3.6.20",
     "@types/node": "^22.10.2",
     "@types/nodemailer": "^6.4.17",
-    "mongodb-memory-server": "^11.2.0",
     "typescript": "^5.7.2",
     "vitest": "^2.1.8"
   }
diff --git a/src/db.ts b/src/db.ts
index 4d50fd2..c2aa0c8 100644
--- a/src/db.ts
+++ b/src/db.ts
@@ -12,9 +12,12 @@ export interface Repos {
 }
 
 export async function connect(cfg: Config): Promise<Repos> {
+  // Production MongoDB is 3.2 (see docs/legacy-behavior.md). We pin the v3.x
+  // driver; useUnifiedTopology is required, and retryWrites (3.6+) is omitted.
   const client = new MongoClient(cfg.mongoUrl, {
-    // The service only writes provenance/idempotency; prefer safety.
-    retryWrites: true,
+    useUnifiedTopology: true,
+    useNewUrlParser: true,
+    serverSelectionTimeoutMS: 10_000,
   });
   await client.connect();
   const db = client.db(cfg.dbName);
diff --git a/src/poller.ts b/src/poller.ts
index 1fe15eb..9bf339f 100644
--- a/src/poller.ts
+++ b/src/poller.ts
@@ -1,5 +1,5 @@
 import type { Queue } from 'bullmq';
-import type { Filter } from 'mongodb';
+import type { FilterQuery } from 'mongodb';
 import type { Config } from './config.js';
 import type { Repos } from './db.js';
 import type { Logger } from './logger.js';
@@ -8,7 +8,7 @@ import { jobId } from './queue.js';
 import type { Channel, NotificationDoc, SendJob } from './types.js';
 
 /** Pending-doc filter for a channel — the CORRECT fields (old cron had a typo). */
-export function pendingFilter(channel: Channel): Filter<NotificationDoc> {
+export function pendingFilter(channel: Channel): FilterQuery<NotificationDoc> {
   return channel === 'fcm'
     ? { type: 'mobile', notified: { $ne: true } }
     : { type: 'web', emailNotified: { $ne: true } };
diff --git a/src/sends.ts b/src/sends.ts
index 0babd5c..285f322 100644
--- a/src/sends.ts
+++ b/src/sends.ts
@@ -1,7 +1,12 @@
-import { MongoServerError, type ObjectId } from 'mongodb';
+import type { ObjectId } from 'mongodb';
 import type { Repos } from './db.js';
 import type { Channel, OperatingMode, SendRecord } from './types.js';
 
+/** Duplicate-key error (unique index violated). Works across driver versions. */
+function isDuplicateKey(err: unknown): boolean {
+  return typeof err === 'object' && err !== null && (err as { code?: number }).code === 11000;
+}
+
 /**
  * Persistent idempotency. `reserve()` inserts a placeholder into
  * `notification_sends`; the unique index (notificationId, channel) makes a
@@ -28,7 +33,7 @@ export async function reserve(
     await repos.sends.insertOne(rec);
     return true;
   } catch (err) {
-    if (err instanceof MongoServerError && err.code === 11000) {
+    if (isDuplicateKey(err)) {
       return false; // already reserved/sent — idempotent skip
     }
     throw err;
diff --git a/test/fake-repo.ts b/test/fake-repo.ts
new file mode 100644
index 0000000..5d77ac1
--- /dev/null
+++ b/test/fake-repo.ts
@@ -0,0 +1,165 @@
+/**
+ * In-memory fake of the Mongo collections used by the service. Production runs
+ * MongoDB 3.2, for which no mongod binary is downloadable/runnable on modern
+ * dev/CI hosts (glibc/libssl). So the DB integration tests run against this
+ * fake, which faithfully emulates the subset of behaviour the code relies on:
+ * the query operators used ($ne/$in/$gte/$exists/equality), $set/$unset updates,
+ * projection/limit, and — crucially — unique-index duplicate-key errors (11000)
+ * that back the idempotency guarantee.
+ */
+import type { Repos } from '../src/db.js';
+
+type Doc = Record<string, any>;
+
+function isObjectId(v: any): boolean {
+  return v != null && typeof v === 'object' && typeof v.equals === 'function' && v._bsontype === 'ObjectID';
+}
+
+function valEq(a: any, b: any): boolean {
+  if (a === b) return true;
+  if (a == null || b == null) return a === b;
+  if (isObjectId(a)) {
+    try {
+      return a.equals(b);
+    } catch {
+      return false;
+    }
+  }
+  if (isObjectId(b)) return b.equals(a);
+  if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
+  return false;
+}
+
+function isOperatorObject(cond: any): boolean {
+  if (cond == null || typeof cond !== 'object') return false;
+  if (cond instanceof Date || isObjectId(cond)) return false;
+  const keys = Object.keys(cond);
+  return keys.length > 0 && keys.every((k) => k.startsWith('$'));
+}
+
+function matchField(docVal: any, cond: any): boolean {
+  if (isOperatorObject(cond)) {
+    return Object.entries(cond).every(([op, v]) => {
+      switch (op) {
+        case '$ne':
+          return !valEq(docVal, v);
+        case '$in':
+          return Array.isArray(v) && v.some((x) => valEq(docVal, x));
+        case '$gte':
+          return docVal != null && docVal >= (v as any);
+        case '$lte':
+          return docVal != null && docVal <= (v as any);
+        case '$exists':
+          return (docVal !== undefined) === v;
+        default:
+          throw new Error(`fake-repo: unsupported operator ${op}`);
+      }
+    });
+  }
+  return valEq(docVal, cond);
+}
+
+function matches(doc: Doc, filter: Doc): boolean {
+  return Object.entries(filter).every(([k, cond]) => matchField(doc[k], cond));
+}
+
+function applyUpdate(doc: Doc, update: Doc): void {
+  if (update.$set) Object.assign(doc, update.$set);
+  if (update.$unset) for (const k of Object.keys(update.$unset)) delete doc[k];
+}
+
+class FakeCollection {
+  docs: Doc[] = [];
+  private uniqueFields: string[] | null = null;
+
+  async createIndex(spec: Doc, opts?: { unique?: boolean }): Promise<string> {
+    if (opts?.unique) this.uniqueFields = Object.keys(spec);
+    return 'idx';
+  }
+
+  private violatesUnique(doc: Doc): boolean {
+    if (!this.uniqueFields) return false;
+    return this.docs.some((d) => this.uniqueFields!.every((f) => valEq(d[f], doc[f])));
+  }
+
+  async insertOne(doc: Doc): Promise<{ insertedId: any }> {
+    if (this.violatesUnique(doc)) {
+      const err: any = new Error('E11000 duplicate key error');
+      err.code = 11000;
+      throw err;
+    }
+    const copy = { ...doc };
+    this.docs.push(copy);
+    return { insertedId: copy._id };
+  }
+
+  async insertMany(docs: Doc[]): Promise<{ insertedCount: number }> {
+    for (const d of docs) await this.insertOne(d);
+    return { insertedCount: docs.length };
+  }
+
+  async findOne(filter: Doc = {}): Promise<Doc | null> {
+    return this.docs.find((d) => matches(d, filter)) ?? null;
+  }
+
+  async updateOne(filter: Doc, update: Doc): Promise<{ modifiedCount: number }> {
+    const d = this.docs.find((x) => matches(x, filter));
+    if (!d) return { modifiedCount: 0 };
+    applyUpdate(d, update);
+    return { modifiedCount: 1 };
+  }
+
+  async updateMany(filter: Doc, update: Doc): Promise<{ modifiedCount: number }> {
+    const ds = this.docs.filter((x) => matches(x, filter));
+    for (const d of ds) applyUpdate(d, update);
+    return { modifiedCount: ds.length };
+  }
+
+  async deleteOne(filter: Doc): Promise<{ deletedCount: number }> {
+    const i = this.docs.findIndex((d) => matches(d, filter));
+    if (i === -1) return { deletedCount: 0 };
+    this.docs.splice(i, 1);
+    return { deletedCount: 1 };
+  }
+
+  async deleteMany(filter: Doc = {}): Promise<{ deletedCount: number }> {
+    const before = this.docs.length;
+    this.docs = this.docs.filter((d) => !matches(d, filter));
+    return { deletedCount: before - this.docs.length };
+  }
+
+  async countDocuments(filter: Doc = {}): Promise<number> {
+    return this.docs.filter((d) => matches(d, filter)).length;
+  }
+
+  find(filter: Doc = {}, _opts?: Doc) {
+    let result = this.docs.filter((d) => matches(d, filter));
+    const cursor = {
+      project() {
+        return cursor;
+      },
+      limit(n: number) {
+        result = result.slice(0, n);
+        return cursor;
+      },
+      async toArray() {
+        return result.map((d) => ({ ...d }));
+      },
+    };
+    return cursor;
+  }
+}
+
+export function makeRepos(): Repos {
+  const notifications = new FakeCollection();
+  const users = new FakeCollection();
+  const sends = new FakeCollection();
+  return {
+    client: {} as any,
+    db: {} as any,
+    notifications: notifications as any,
+    users: users as any,
+    sends: sends as any,
+    close: async () => {},
+  };
+}
diff --git a/test/handlers.test.ts b/test/handlers.test.ts
index dca5263..5220415 100644
--- a/test/handlers.test.ts
+++ b/test/handlers.test.ts
@@ -1,30 +1,14 @@
-import { afterAll, beforeAll, beforeEach, describe, expect, it } from 'vitest';
-import type { MongoMemoryServer } from 'mongodb-memory-server';
+import { beforeEach, describe, expect, it } from 'vitest';
 import type { Repos } from '../src/db.js';
 import type { Config } from '../src/config.js';
 import { processEmailJob, processFcmJob, type Deps } from '../src/handlers.js';
-import { aNotif, aUser, fakeFcm, fakeMailer, silentLog, startMongo } from './helpers.js';
+import { aNotif, aUser, fakeFcm, fakeMailer, freshRepos, silentLog, testCfg } from './helpers.js';
 
-let server: MongoMemoryServer;
 let repos: Repos;
-let baseCfg: Config;
-
-beforeAll(async () => {
-  const s = await startMongo();
-  server = s.server;
-  repos = s.repos;
-  baseCfg = s.cfg;
-});
-
-afterAll(async () => {
-  await repos.close();
-  await server.stop();
-});
+const baseCfg: Config = testCfg();
 
 beforeEach(async () => {
-  await repos.notifications.deleteMany({});
-  await repos.users.deleteMany({});
-  await repos.sends.deleteMany({});
+  repos = await freshRepos();
 });
 
 function deps(over: Partial<Deps> = {}): Deps {
diff --git a/test/helpers.ts b/test/helpers.ts
index 411cc44..ed870d5 100644
--- a/test/helpers.ts
+++ b/test/helpers.ts
@@ -1,21 +1,20 @@
-import { MongoMemoryServer } from 'mongodb-memory-server';
 import { pino } from 'pino';
 import { ObjectId } from 'mongodb';
-import { connect, ensureIndexes, type Repos } from '../src/db.js';
+import type { Repos } from '../src/db.js';
 import type { Config } from '../src/config.js';
 import type { FcmClient, FcmSendResult } from '../src/fcm.js';
 import type { Mailer } from '../src/mailer.js';
 import type { EmailContent, PushPayload } from '../src/messages.js';
 import type { NotificationDoc, UserDoc } from '../src/types.js';
+import { makeRepos } from './fake-repo.js';
 
 export const silentLog = pino({ level: 'silent' });
 
-export async function startMongo(): Promise<{ server: MongoMemoryServer; repos: Repos; cfg: Config }> {
-  const server = await MongoMemoryServer.create();
-  const cfg = testCfg({ mongoUrl: server.getUri(), dbName: 'fuegos' });
-  const repos = await connect(cfg);
-  await ensureIndexes(repos);
-  return { server, repos, cfg };
+/** Fresh in-memory repos with the idempotency unique index applied. */
+export async function freshRepos(): Promise<Repos> {
+  const repos = makeRepos();
+  await repos.sends.createIndex({ notificationId: 1, channel: 1 }, { unique: true, name: 'uniq' });
+  return repos;
 }
 
 export function testCfg(over: Partial<Config> = {}): Config {
diff --git a/test/poller.test.ts b/test/poller.test.ts
index b5229bd..ae41c0f 100644
--- a/test/poller.test.ts
+++ b/test/poller.test.ts
@@ -1,29 +1,17 @@
-import { afterAll, beforeAll, beforeEach, describe, expect, it } from 'vitest';
+import { beforeEach, describe, expect, it } from 'vitest';
 import type { Queue } from 'bullmq';
-import type { MongoMemoryServer } from 'mongodb-memory-server';
 import type { Repos } from '../src/db.js';
 import type { Config } from '../src/config.js';
 import { createPoller } from '../src/poller.js';
 import { pendingFilter } from '../src/poller.js';
 import type { Channel, SendJob } from '../src/types.js';
-import { aNotif, silentLog, startMongo } from './helpers.js';
+import { aNotif, freshRepos, silentLog, testCfg } from './helpers.js';
 
-let server: MongoMemoryServer;
 let repos: Repos;
-let baseCfg: Config;
+const baseCfg: Config = testCfg();
 
-beforeAll(async () => {
-  const s = await startMongo();
-  server = s.server;
-  repos = s.repos;
-  baseCfg = s.cfg;
-});
-afterAll(async () => {
-  await repos.close();
-  await server.stop();
-});
 beforeEach(async () => {
-  await repos.notifications.deleteMany({});
+  repos = await freshRepos();
 });
 
 /** Fake queue recording add() calls. */