diff --git a/mcp-server/src/__tests__/db-client.test.ts b/mcp-server/src/__tests__/db-client.test.ts
new file mode 100644
index 0000000..4c82993
--- /dev/null
+++ b/mcp-server/src/__tests__/db-client.test.ts
@@ -0,0 +1,128 @@
+// Boot-factory tests for services/db-client.ts. This is the foundation the
+// upcoming service refactor (each `(supabaseUrl, supabaseKey)` constructor
+// migrating to a shared `DbClient` instance) will lean on, so the env-flag
+// behaviour is pinned here before any service starts depending on it.
+//
+// Adapter contract is tested separately (pglite-adapter.test.ts walks every
+// query verb shape against a real PGlite instance); these tests only assert
+// the boot path returns the right backend kind and a usable handle.
+
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import os from "node:os";
+import path from "node:path";
+import fs from "node:fs/promises";
+
+import { PostgrestClient } from "@supabase/postgrest-js";
+
+import { createDbClient } from "../services/db-client.js";
+import { PGliteAdapter } from "../native/pglite.js";
+
+async function withEnv<T>(env: Record<string, string | undefined>, run: () => Promise<T>): Promise<T> {
+  const prev: Record<string, string | undefined> = {};
+  for (const k of Object.keys(env)) {
+    prev[k] = process.env[k];
+    if (env[k] === undefined) delete process.env[k];
+    else process.env[k] = env[k];
+  }
+  try {
+    return await run();
+  } finally {
+    for (const k of Object.keys(prev)) {
+      if (prev[k] === undefined) delete process.env[k];
+      else process.env[k] = prev[k];
+    }
+  }
+}
+
+test("createDbClient: PostgREST mode (default — env unset)", async () => {
+  await withEnv({ MYCELIUM_USE_PGLITE: undefined }, async () => {
+    const boot = await createDbClient({
+      supabaseUrl: "http://localhost:54321",
+      supabaseKey: "test-key",
+    });
+    try {
+      assert.equal(boot.mode, "postgrest");
+      assert.ok(boot.client instanceof PostgrestClient);
+    } finally {
+      await boot.close();
+    }
+  });
+});
+
+test("createDbClient: PostgREST mode is selected for any non-1 value", async () => {
+  for (const value of ["0", "false", "true", "yes", ""]) {
+    await withEnv({ MYCELIUM_USE_PGLITE: value }, async () => {
+      const boot = await createDbClient({
+        supabaseUrl: "http://localhost:54321",
+        supabaseKey: "",
+      });
+      try {
+        assert.equal(boot.mode, "postgrest", `value=${JSON.stringify(value)} should not select pglite`);
+      } finally {
+        await boot.close();
+      }
+    });
+  }
+});
+
+test("createDbClient: PGlite mode boots an adapter with usable .from()", async () => {
+  const tmp = await fs.mkdtemp(path.join(os.tmpdir(), "mycelium-db-client-test-"));
+  await withEnv(
+    {
+      MYCELIUM_USE_PGLITE: "1",
+      MYCELIUM_DATA_DIR: tmp,
+    },
+    async () => {
+      const boot = await createDbClient({
+        supabaseUrl: "ignored-when-pglite",
+        supabaseKey: "ignored-when-pglite",
+        pglite: { skipMigrations: true },
+      });
+      try {
+        assert.equal(boot.mode, "pglite");
+        assert.ok(boot.client instanceof PGliteAdapter);
+
+        // Smoke: adapter responds to a trivial query through the chain.
+        // No migrations were applied, so we touch a temporary table to
+        // prove .from() / .insert() / .select() round-trip.
+        const adapter = boot.client as PGliteAdapter;
+        await adapter.raw().exec(`
+          CREATE TABLE db_client_smoke (id INT PRIMARY KEY, label TEXT)
+        `);
+        const ins = await adapter
+          .from("db_client_smoke")
+          .insert({ id: 1, label: "ok" })
+          .select()
+          .single();
+        assert.equal(ins.error, null);
+        assert.equal((ins.data as { label: string }).label, "ok");
+      } finally {
+        await boot.close();
+      }
+    },
+  );
+  await fs.rm(tmp, { recursive: true, force: true });
+});
+
+test("createDbClient: close() in PGlite mode releases the underlying handle", async () => {
+  const tmp = await fs.mkdtemp(path.join(os.tmpdir(), "mycelium-db-client-test-"));
+  await withEnv(
+    {
+      MYCELIUM_USE_PGLITE: "1",
+      MYCELIUM_DATA_DIR: tmp,
+    },
+    async () => {
+      const boot = await createDbClient({
+        supabaseUrl: "x",
+        supabaseKey: "y",
+        pglite: { skipMigrations: true },
+      });
+      const adapter = boot.client as PGliteAdapter;
+      await boot.close();
+      // After close, the underlying PGlite handle reports closed.
+      assert.equal(adapter.raw().closed, true);
+    },
+  );
+  await fs.rm(tmp, { recursive: true, force: true });
+});
diff --git a/mcp-server/src/services/db-client.ts b/mcp-server/src/services/db-client.ts
new file mode 100644
index 0000000..86fd1c7
--- /dev/null
+++ b/mcp-server/src/services/db-client.ts
@@ -0,0 +1,82 @@
+// One-stop boot for the database client services in src/services/ depend on.
+//
+// Two backends share the same PostgREST surface (`.from(table).select|insert|
+// update|delete().eq|in|or|order|limit().single|maybeSingle()` plus
+// `.rpc(name, args)` returning `{ data, error }`):
+//
+//   - `PostgrestClient` from @supabase/postgrest-js — Docker / hosted mode,
+//     hits the PostgREST endpoint at SUPABASE_URL.
+//   - `PGliteAdapter`  from src/native/pglite.ts        — native mode,
+//     speaks the same chain shape directly to an in-process PGlite WASM
+//     Postgres. The adapter contract tests (pglite-adapter.test.ts) gate
+//     the surface so it stays drop-in compatible.
+//
+// Why this lives separate from src/index.ts:
+//   The next native-app PR migrates ~30 service constructors away from
+//   `(supabaseUrl, supabaseKey)` and onto a shared `DbClient` instance.
+//   Keeping the factory here means each migrated service imports a single
+//   stable type and the env-flag plumbing is exercised by a small unit
+//   test, not the MCP server boot smoke test.
+//
+// Selection: MYCELIUM_USE_PGLITE=1 picks the native adapter; anything else
+// (unset, "0", "false", any other value) keeps PostgREST.
+
+import { PostgrestClient } from "@supabase/postgrest-js";
+
+import type { PGliteAdapter } from "../native/pglite.js";
+
+export type DbClient = PostgrestClient | PGliteAdapter;
+export type DbClientMode = "pglite" | "postgrest";
+
+export interface DbBootOptions {
+  supabaseUrl: string;
+  supabaseKey: string;
+  /**
+   * Pass-through to createNativeDb() when MYCELIUM_USE_PGLITE=1. Tests use
+   * `skipMigrations: true` to keep the adapter wakeup under a second; the
+   * 79-migration walk is exercised separately in pglite-migration-walk.test.ts.
+   */
+  pglite?: {
+    dataDir?: string;
+    migrationsDir?: string;
+    skipMigrations?: boolean;
+  };
+}
+
+export interface DbBootResult {
+  client: DbClient;
+  mode: DbClientMode;
+  /** Tear down any owned resources (PGlite handle, etc). No-op in PostgREST mode. */
+  close(): Promise<void>;
+}
+
+/**
+ * MYCELIUM_USE_PGLITE=1 routes through the in-process PGlite adapter; any
+ * other value keeps the PostgREST client pointed at SUPABASE_URL. Returns a
+ * `close()` so callers can tear the native handle down on shutdown.
+ */
+export async function createDbClient(opts: DbBootOptions): Promise<DbBootResult> {
+  if (process.env.MYCELIUM_USE_PGLITE === "1") {
+    const { createNativeDb } = await import("../native/factory.js");
+    const db = await createNativeDb(opts.pglite ?? {});
+    return {
+      client: db.adapter,
+      mode: "pglite",
+      close: () => db.close(),
+    };
+  }
+
+  const client = new PostgrestClient(opts.supabaseUrl, {
+    headers: opts.supabaseKey
+      ? { Authorization: `Bearer ${opts.supabaseKey}`, apikey: opts.supabaseKey }
+      : {},
+  });
+  return {
+    client,
+    mode: "postgrest",
+    close: async () => {
+      // PostgrestClient holds no sockets of its own — fetch handles those
+      // per-request — so there is nothing to release.
+    },
+  };
+}