diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index b54bdac41..1139d4d90 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -919,6 +919,16 @@ enum EnvironmentLocationSource {
   HOME
   TRAVEL
   DEVICE
+
+  /// Migration 0217 created the physical type as `environment_location_source`,
+  /// like every other enum in this schema. This declaration carried no `@@map`,
+  /// so the client compiled every write against `public."EnvironmentLocationSource"`
+  /// — a type no database has ever had — and each insert into `environment_contexts`
+  /// died on `42704` before a row could commit. The map names the type that
+  /// exists; no migration follows, because the database was never the side that
+  /// drifted. `prisma-enum-mapping-guard.test.ts` now holds the whole schema to
+  /// this rule.
+  @@map("environment_location_source")
 }
 
 /// A per-day environmental observation (daily weather / daylight) for the
diff --git a/src/__tests__/helpers/prisma-schema-names.ts b/src/__tests__/helpers/prisma-schema-names.ts
new file mode 100644
index 000000000..310ecaaf3
--- /dev/null
+++ b/src/__tests__/helpers/prisma-schema-names.ts
@@ -0,0 +1,121 @@
+/**
+ * A minimal reader for the physical names `prisma/schema.prisma` declares.
+ *
+ * Prisma emits three kinds of identifier straight into the SQL it sends: the
+ * enum type name, the table name, and the column name. Each one is the `@@map`
+ * / `@map` value when present and the declared name verbatim when absent. If
+ * any of the three names something the database does not have, the statement
+ * dies at the driver with `42704` / `42P01` / `42703` — and nothing earlier in
+ * the pipeline can see it, because generation, typecheck, and every mocked
+ * unit test read the logical name only.
+ *
+ * Consumers: `prisma-enum-mapping-guard.test.ts` (the convention, no database)
+ * and `tests/integration/prisma-schema-name-drift.test.ts` (the physical
+ * comparison against a migrated Postgres).
+ */
+import { readFileSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+export const SCHEMA_PATH = resolve(
+  dirname(fileURLToPath(import.meta.url)),
+  "../../../prisma/schema.prisma",
+);
+
+export interface DeclaredEnum {
+  /** The enum name in the datamodel. */
+  name: string;
+  /** The `@@map` value, or null when the declaration carries none. */
+  mappedName: string | null;
+  /** The name Prisma puts in the SQL — `@@map` when present, else `name`. */
+  physicalName: string;
+}
+
+export interface DeclaredField {
+  name: string;
+  /** The `@map` value, or null when the field carries none. */
+  mappedName: string | null;
+  /** The column name Prisma puts in the SQL. */
+  physicalName: string;
+}
+
+export interface DeclaredModel {
+  name: string;
+  mappedName: string | null;
+  /** The table name Prisma puts in the SQL. */
+  physicalName: string;
+  /** Scalar + enum fields only — relation fields carry no column. */
+  fields: DeclaredField[];
+}
+
+export interface DeclaredSchema {
+  enums: DeclaredEnum[];
+  models: DeclaredModel[];
+}
+
+const BLOCK_MAP = /^\s*@@map\("([^"]+)"\)/m;
+const FIELD_MAP = /@map\("([^"]+)"\)/;
+/** `name  Type[]?  @attr…` — the two leading tokens are what matters. */
+const FIELD_LINE = /^\s*(\w+)\s+(\w+)(?:\[\])?\??\s*(.*)$/;
+
+/** Drop `//` and `///` lines so prose can never read as a declaration. */
+function stripComments(block: string): string {
+  return block
+    .split("\n")
+    .filter((line) => !/^\s*\/\//.test(line))
+    .join("\n");
+}
+
+/**
+ * Read the datamodel. Deliberately regex-shaped rather than a real parser: the
+ * guard has to run inside `pnpm test` with no database and no Prisma engine,
+ * and the three declarations it reads (`enum`, `model`, field lines) are the
+ * most stable syntax in the file. Both consumers assert a floor on how much
+ * they found, so a parser that silently stops matching fails loudly instead of
+ * passing vacuously.
+ */
+export function readDeclaredSchema(
+  schemaPath: string = SCHEMA_PATH,
+): DeclaredSchema {
+  const source = readFileSync(schemaPath, "utf8");
+  const enums: DeclaredEnum[] = [];
+  const rawModels: Array<{ name: string; body: string }> = [];
+
+  const block = /^(model|enum)\s+(\w+)\s*\{([\s\S]*?)^\}/gm;
+  for (let m = block.exec(source); m !== null; m = block.exec(source)) {
+    const [, kind, name, body] = m;
+    const clean = stripComments(body);
+    const mappedName = BLOCK_MAP.exec(clean)?.[1] ?? null;
+    if (kind === "enum") {
+      enums.push({ name, mappedName, physicalName: mappedName ?? name });
+    } else {
+      rawModels.push({ name, body: clean });
+    }
+  }
+
+  const modelNames = new Set(rawModels.map((m) => m.name));
+
+  const models = rawModels.map(({ name, body }) => {
+    const mappedName = BLOCK_MAP.exec(body)?.[1] ?? null;
+    const fields: DeclaredField[] = [];
+    for (const line of body.split("\n")) {
+      if (/^\s*@@/.test(line)) continue;
+      const match = FIELD_LINE.exec(line);
+      if (!match) continue;
+      const [, fieldName, baseType, rest] = match;
+      // A field whose type is another model is a relation, not a column. The
+      // back-reference side carries no `@relation` attribute at all, so the
+      // type is what decides it — keying on `@relation` would miss half.
+      if (modelNames.has(baseType)) continue;
+      const fieldMapped = FIELD_MAP.exec(rest)?.[1] ?? null;
+      fields.push({
+        name: fieldName,
+        mappedName: fieldMapped,
+        physicalName: fieldMapped ?? fieldName,
+      });
+    }
+    return { name, mappedName, physicalName: mappedName ?? name, fields };
+  });
+
+  return { enums, models };
+}
diff --git a/src/__tests__/prisma-enum-mapping-guard.test.ts b/src/__tests__/prisma-enum-mapping-guard.test.ts
new file mode 100644
index 000000000..94a866bac
--- /dev/null
+++ b/src/__tests__/prisma-enum-mapping-guard.test.ts
@@ -0,0 +1,145 @@
+/**
+ * Structural guard — a Prisma enum cannot name a type the migrations never
+ * created.
+ *
+ * `EnvironmentLocationSource` was declared without an `@@map` while migration
+ * 0217 created the type as `environment_location_source`. Prisma writes the
+ * enum's physical name straight into the SQL, so every insert into
+ * `environment_contexts` asked Postgres for `public."EnvironmentLocationSource"`
+ * and got `42704`. The module read "0 days recorded" from the day it shipped.
+ *
+ * Nothing upstream could see it: the type is right in the generated client, the
+ * datamodel is internally consistent, and every mocked unit test only ever
+ * exercises the logical name. One enum out of forty-nine drifted, and it took a
+ * self-hoster reading Postgres error codes to find it.
+ *
+ * So the physical name is checked against the only other place that states it —
+ * the migration SQL. `CREATE TYPE`, `ALTER TYPE … RENAME TO`, and `DROP TYPE`
+ * are replayed in migration order, and every declared enum has to land on a
+ * type that survives to the end. The `@@map`-present rule is asserted
+ * separately, because an unmapped PascalCase enum is the specific mistake that
+ * happened and the message should say so rather than "type not found".
+ *
+ * Limits, stated plainly. This reads SQL text, not a database: it proves the
+ * migrations create the type, not that any particular deployment's database
+ * matches its migration ledger. It covers enum types only. The comprehensive
+ * check — every enum, table, and column name compared against a real migrated
+ * Postgres — is `tests/integration/prisma-schema-name-drift.test.ts`, which
+ * this guard exists to front-run at unit speed.
+ */
+import { readdirSync, readFileSync } from "node:fs";
+import { dirname, join, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+import { describe, it, expect } from "vitest";
+
+import { readDeclaredSchema } from "./helpers/prisma-schema-names";
+
+const MIGRATIONS_DIR = resolve(
+  dirname(fileURLToPath(import.meta.url)),
+  "../../prisma/migrations",
+);
+
+const CREATE_TYPE = /CREATE\s+TYPE\s+(?:IF\s+NOT\s+EXISTS\s+)?"?([\w.]+)"?/gi;
+const RENAME_TYPE =
+  /ALTER\s+TYPE\s+"?([\w.]+)"?\s+RENAME\s+TO\s+"?([\w.]+)"?/gi;
+const DROP_TYPE = /DROP\s+TYPE\s+(?:IF\s+EXISTS\s+)?"?([\w.]+)"?/gi;
+
+/** `public.foo` and `foo` are the same type; the guard compares bare names. */
+function bare(name: string): string {
+  return name.includes(".") ? name.slice(name.lastIndexOf(".") + 1) : name;
+}
+
+/**
+ * Statements that are commented out (the rollback recipes several migrations
+ * carry in a `--` block) must not count as executed SQL.
+ */
+function executableSql(sql: string): string {
+  return sql
+    .split("\n")
+    .filter((line) => !/^\s*--/.test(line))
+    .join("\n");
+}
+
+/** Replay every migration in order and return the types left standing. */
+function liveEnumTypes(): Set<string> {
+  const live = new Set<string>();
+  const dirs = readdirSync(MIGRATIONS_DIR, { withFileTypes: true })
+    .filter((entry) => entry.isDirectory())
+    .map((entry) => entry.name)
+    .sort();
+
+  for (const dir of dirs) {
+    let sql: string;
+    try {
+      sql = readFileSync(join(MIGRATIONS_DIR, dir, "migration.sql"), "utf8");
+    } catch {
+      continue;
+    }
+    const statements = executableSql(sql);
+
+    // Order within one file matters: a migration that drops and recreates a
+    // type under a new name has to be replayed as written, so each statement
+    // kind is applied at its own offset rather than kind-by-kind.
+    const events: Array<{ at: number; apply: () => void }> = [];
+    for (const m of statements.matchAll(CREATE_TYPE)) {
+      const name = bare(m[1]);
+      events.push({ at: m.index, apply: () => void live.add(name) });
+    }
+    for (const m of statements.matchAll(RENAME_TYPE)) {
+      const from = bare(m[1]);
+      const to = bare(m[2]);
+      events.push({
+        at: m.index,
+        apply: () => {
+          live.delete(from);
+          live.add(to);
+        },
+      });
+    }
+    for (const m of statements.matchAll(DROP_TYPE)) {
+      const name = bare(m[1]);
+      events.push({ at: m.index, apply: () => void live.delete(name) });
+    }
+    for (const event of events.sort((a, b) => a.at - b.at)) event.apply();
+  }
+  return live;
+}
+
+describe("prisma enums — the declared type is the migrated type", () => {
+  const { enums } = readDeclaredSchema();
+
+  it("reads the schema (a parser that stopped matching cannot pass vacuously)", () => {
+    expect(enums.length).toBeGreaterThan(40);
+  });
+
+  it("declares an @@map on every enum", () => {
+    const unmapped = enums
+      .filter((declared) => declared.mappedName === null)
+      .map((declared) => declared.name);
+    expect(
+      unmapped,
+      unmapped.length > 0
+        ? `Enum(s) without @@map — Prisma will ask Postgres for the PascalCase ` +
+            `type name, which no migration creates: ${unmapped.join(", ")}`
+        : undefined,
+    ).toEqual([]);
+  });
+
+  it("finds the migrated types (a broken replay cannot pass vacuously)", () => {
+    expect(liveEnumTypes().size).toBeGreaterThan(40);
+  });
+
+  it("points every enum at a type the migrations leave standing", () => {
+    const live = liveEnumTypes();
+    const missing = enums
+      .filter((declared) => !live.has(declared.physicalName))
+      .map((declared) => `${declared.name} → "${declared.physicalName}"`);
+    expect(
+      missing,
+      missing.length > 0
+        ? `Enum(s) naming a type no migration creates:\n  ${missing.join("\n  ")}`
+        : undefined,
+    ).toEqual([]);
+  });
+});
diff --git a/tests/integration/prisma-schema-name-drift.test.ts b/tests/integration/prisma-schema-name-drift.test.ts
new file mode 100644
index 000000000..9865cb93a
--- /dev/null
+++ b/tests/integration/prisma-schema-name-drift.test.ts
@@ -0,0 +1,222 @@
+/**
+ * Every name `prisma/schema.prisma` will put into SQL exists in the migrated
+ * database.
+ *
+ * `EnvironmentLocationSource` shipped with no `@@map` while migration 0217 had
+ * created the type as `environment_location_source`. Prisma writes the enum's
+ * physical name into the statement, so every insert into `environment_contexts`
+ * asked for `public."EnvironmentLocationSource"` and got `42704`. The nightly
+ * job failed on every run from the day the module shipped and the screen read
+ * "0 days recorded" the whole time.
+ *
+ * The reason it survived that long is the reason this file is an integration
+ * test and not a unit test: generation succeeds, typecheck succeeds, and a
+ * mocked Prisma client never emits a type name at all. Only a real migrated
+ * Postgres can answer whether the datamodel's physical names exist.
+ *
+ * So it compares all three name classes Prisma emits — enum type, table,
+ * column — against `pg_type` and `information_schema`. One direction only:
+ * every DECLARED name must exist physically. The reverse is deliberately not
+ * asserted, because a column dropped from the datamodel but left in the
+ * database breaks nothing, while a name the client sends and the database
+ * lacks breaks every statement that touches it.
+ *
+ * Also verified here: the environment write the drift actually blocked, once
+ * per enum value, against the live type.
+ */
+import { beforeEach, describe, expect, it } from "vitest";
+
+import {
+  readDeclaredSchema,
+  type DeclaredModel,
+} from "@/__tests__/helpers/prisma-schema-names";
+
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+const prisma = getPrismaClient();
+
+async function physicalEnumTypes(): Promise<Set<string>> {
+  const rows = await prisma.$queryRaw<Array<{ typname: string }>>`
+    SELECT t.typname
+    FROM pg_type t
+    JOIN pg_namespace n ON n.oid = t.typnamespace
+    WHERE t.typtype = 'e' AND n.nspname = 'public'
+  `;
+  return new Set(rows.map((row) => row.typname));
+}
+
+async function physicalColumns(): Promise<Map<string, Set<string>>> {
+  const rows = await prisma.$queryRaw<
+    Array<{ table_name: string; column_name: string }>
+  >`
+    SELECT table_name, column_name
+    FROM information_schema.columns
+    WHERE table_schema = 'public'
+  `;
+  const byTable = new Map<string, Set<string>>();
+  for (const { table_name, column_name } of rows) {
+    const columns = byTable.get(table_name) ?? new Set<string>();
+    columns.add(column_name);
+    byTable.set(table_name, columns);
+  }
+  return byTable;
+}
+
+function describeModel(model: DeclaredModel): string {
+  return `${model.name} → "${model.physicalName}"`;
+}
+
+describe("prisma datamodel — declared names exist in the migrated database", () => {
+  const { enums, models } = readDeclaredSchema();
+
+  it("read the datamodel (a parser that stopped matching cannot pass vacuously)", () => {
+    expect(enums.length).toBeGreaterThan(40);
+    expect(models.length).toBeGreaterThan(80);
+    expect(
+      models.reduce((sum, model) => sum + model.fields.length, 0),
+    ).toBeGreaterThan(500);
+  });
+
+  it("has every declared enum type", async () => {
+    const physical = await physicalEnumTypes();
+    const missing = enums
+      .filter((declared) => !physical.has(declared.physicalName))
+      .map((declared) => `${declared.name} → "${declared.physicalName}"`);
+    expect(
+      missing,
+      missing.length > 0
+        ? `Enum type(s) the database does not have:\n  ${missing.join("\n  ")}`
+        : undefined,
+    ).toEqual([]);
+  });
+
+  it("has every declared table", async () => {
+    const byTable = await physicalColumns();
+    const missing = models
+      .filter((model) => !byTable.has(model.physicalName))
+      .map(describeModel);
+    expect(
+      missing,
+      missing.length > 0
+        ? `Table(s) the database does not have:\n  ${missing.join("\n  ")}`
+        : undefined,
+    ).toEqual([]);
+  });
+
+  it("has every declared column", async () => {
+    const byTable = await physicalColumns();
+    const missing: string[] = [];
+    for (const model of models) {
+      const columns = byTable.get(model.physicalName);
+      if (!columns) continue; // reported by the table assertion above
+      for (const field of model.fields) {
+        if (!columns.has(field.physicalName)) {
+          missing.push(
+            `${model.name}.${field.name} → "${model.physicalName}"."${field.physicalName}"`,
+          );
+        }
+      }
+    }
+    expect(
+      missing,
+      missing.length > 0
+        ? `Column(s) the database does not have:\n  ${missing.join("\n  ")}`
+        : undefined,
+    ).toEqual([]);
+  });
+});
+
+describe("environment context — the write the enum drift blocked", () => {
+  beforeEach(async () => {
+    await truncateAllTables(prisma);
+  });
+
+  it("stores a row under every EnvironmentLocationSource value", async () => {
+    const user = await prisma.user.create({
+      data: { username: "env-enum", email: "env-enum@example.test" },
+      select: { id: true },
+    });
+
+    for (const [index, source] of (
+      ["HOME", "TRAVEL", "DEVICE"] as const
+    ).entries()) {
+      const date = `2026-03-0${index + 1}`;
+      await prisma.environmentContext.upsert({
+        where: { userId_date: { userId: user.id, date } },
+        create: {
+          userId: user.id,
+          date,
+          lat: 52.5,
+          lon: 13.4,
+          locationLabel: "Somewhere",
+          source,
+          tempMean: 12.5,
+          fetchedAt: new Date(),
+        },
+        update: { source, tempMean: 12.5 },
+      });
+    }
+
+    const stored = await prisma.environmentContext.findMany({
+      where: { userId: user.id },
+      orderBy: { date: "asc" },
+      select: { date: true, source: true },
+    });
+    expect(stored).toEqual([
+      { date: "2026-03-01", source: "HOME" },
+      { date: "2026-03-02", source: "TRAVEL" },
+      { date: "2026-03-03", source: "DEVICE" },
+    ]);
+
+    // Idempotent — the nightly lookback re-runs the same days every night.
+    await prisma.environmentContext.upsert({
+      where: { userId_date: { userId: user.id, date: "2026-03-01" } },
+      create: {
+        userId: user.id,
+        date: "2026-03-01",
+        lat: 52.5,
+        lon: 13.4,
+        locationLabel: "Somewhere",
+        source: "HOME",
+        fetchedAt: new Date(),
+      },
+      update: { source: "TRAVEL", tempMean: 14 },
+    });
+    const reread = await prisma.environmentContext.findMany({
+      where: { userId: user.id },
+    });
+    expect(reread).toHaveLength(3);
+    expect(reread.find((row) => row.date === "2026-03-01")?.source).toBe(
+      "TRAVEL",
+    );
+  });
+
+  it("filters on the enum column (the read side sends the type name too)", async () => {
+    const user = await prisma.user.create({
+      data: { username: "env-enum-read", email: "env-enum-read@example.test" },
+      select: { id: true },
+    });
+    await prisma.environmentContext.create({
+      data: {
+        userId: user.id,
+        date: "2026-04-01",
+        lat: 52.5,
+        lon: 13.4,
+        locationLabel: "Somewhere",
+        source: "TRAVEL",
+        fetchedAt: new Date(),
+      },
+    });
+
+    await expect(
+      prisma.environmentContext.count({
+        where: { userId: user.id, source: { in: ["HOME", "DEVICE"] } },
+      }),
+    ).resolves.toBe(0);
+    await expect(
+      prisma.environmentContext.count({
+        where: { userId: user.id, source: "TRAVEL" },
+      }),
+    ).resolves.toBe(1);
+  });
+});