diff --git a/assistant/src/__tests__/db-proxy-transaction.test.ts b/assistant/src/__tests__/db-proxy-transaction.test.ts
new file mode 100644
index 00000000000..7ced6d2fc9d
--- /dev/null
+++ b/assistant/src/__tests__/db-proxy-transaction.test.ts
@@ -0,0 +1,180 @@
+/**
+ * Tests for the `db_proxy_transaction` IPC handler.
+ *
+ * Verifies all-or-nothing semantics: every step commits together, any
+ * exception or `requireChanges` violation rolls the entire batch back.
+ *
+ * Uses the real DB (via `initializeDb()`); the test preload points
+ * `VELLUM_WORKSPACE_DIR` at a per-file temp dir.
+ */
+
+import { beforeEach, describe, expect, test } from "bun:test";
+
+import { handleDbProxyTransaction } from "../ipc/routes/db-proxy-transaction.js";
+import { getSqlite } from "../memory/db-connection.js";
+import { initializeDb } from "../memory/db-init.js";
+
+initializeDb();
+
+function resetTestTable(): void {
+  const sqlite = getSqlite();
+  sqlite.exec("DROP TABLE IF EXISTS proxy_tx_test");
+  sqlite.exec(
+    "CREATE TABLE proxy_tx_test (id INTEGER PRIMARY KEY, label TEXT NOT NULL, count INTEGER NOT NULL DEFAULT 0)",
+  );
+}
+
+function rowCount(): number {
+  const result = getSqlite()
+    .prepare("SELECT COUNT(*) AS n FROM proxy_tx_test")
+    .get() as { n: number };
+  return result.n;
+}
+
+describe("db_proxy_transaction", () => {
+  beforeEach(() => {
+    resetTestTable();
+  });
+
+  test("commits multiple inserts atomically", () => {
+    const result = handleDbProxyTransaction({
+      steps: [
+        {
+          sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+          bind: [1, "alpha"],
+        },
+        {
+          sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+          bind: [2, "beta"],
+        },
+      ],
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.results).toHaveLength(2);
+      expect(result.results[0].changes).toBe(1);
+      expect(result.results[1].changes).toBe(1);
+    }
+    expect(rowCount()).toBe(2);
+  });
+
+  test("rolls back all writes when a later step throws (SQL constraint)", () => {
+    // Pre-existing row that the transaction will collide with.
+    getSqlite()
+      .prepare("INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)")
+      .run(2, "preexisting");
+
+    expect(() =>
+      handleDbProxyTransaction({
+        steps: [
+          {
+            sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+            bind: [1, "alpha"],
+          },
+          // Primary-key collision triggers a SqliteError mid-transaction.
+          {
+            sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+            bind: [2, "beta"],
+          },
+        ],
+      }),
+    ).toThrow();
+
+    // The first step's insert must NOT have committed.
+    expect(rowCount()).toBe(1);
+    const remaining = getSqlite()
+      .prepare("SELECT label FROM proxy_tx_test WHERE id = ?")
+      .get(2) as { label: string };
+    expect(remaining.label).toBe("preexisting");
+  });
+
+  test("requireChanges aborts the transaction when unmet", () => {
+    // Seed a row to update; condition will not match so changes = 0.
+    getSqlite()
+      .prepare("INSERT INTO proxy_tx_test (id, label, count) VALUES (?, ?, ?)")
+      .run(1, "active", 0);
+
+    const result = handleDbProxyTransaction({
+      steps: [
+        {
+          sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+          bind: [99, "should-rollback"],
+        },
+        {
+          // No row matches label='nonexistent', so changes = 0.
+          sql: "UPDATE proxy_tx_test SET count = count + 1 WHERE label = ?",
+          bind: ["nonexistent"],
+          requireChanges: 1,
+        },
+      ],
+    });
+
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.reason).toBe("require_changes_failed");
+      expect(result.failedStep).toBe(1);
+      expect(result.actualChanges).toBe(0);
+      expect(result.requiredChanges).toBe(1);
+    }
+
+    // The earlier insert must have rolled back.
+    expect(rowCount()).toBe(1);
+    const remaining = getSqlite()
+      .prepare("SELECT label FROM proxy_tx_test WHERE id = ?")
+      .get(1) as { label: string };
+    expect(remaining.label).toBe("active");
+  });
+
+  test("requireChanges allows the transaction to commit when met", () => {
+    getSqlite()
+      .prepare("INSERT INTO proxy_tx_test (id, label, count) VALUES (?, ?, ?)")
+      .run(1, "active", 0);
+
+    const result = handleDbProxyTransaction({
+      steps: [
+        {
+          sql: "INSERT INTO proxy_tx_test (id, label) VALUES (?, ?)",
+          bind: [2, "new"],
+        },
+        {
+          sql: "UPDATE proxy_tx_test SET count = count + 1 WHERE label = ?",
+          bind: ["active"],
+          requireChanges: 1,
+        },
+      ],
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.results[1].changes).toBe(1);
+    }
+    expect(rowCount()).toBe(2);
+    const updated = getSqlite()
+      .prepare("SELECT count FROM proxy_tx_test WHERE id = ?")
+      .get(1) as { count: number };
+    expect(updated.count).toBe(1);
+  });
+
+  test("rejects empty step list", () => {
+    expect(() => handleDbProxyTransaction({ steps: [] })).toThrow(
+      /at least one step/,
+    );
+  });
+
+  test("returns lastInsertRowid for inserts", () => {
+    const result = handleDbProxyTransaction({
+      steps: [
+        {
+          sql: "INSERT INTO proxy_tx_test (label) VALUES (?)",
+          bind: ["solo"],
+        },
+      ],
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.results[0].lastInsertRowid).toBeGreaterThan(0);
+    }
+  });
+});
diff --git a/assistant/src/ipc/assistant-server.ts b/assistant/src/ipc/assistant-server.ts
index b641a337a30..24d4c4b134c 100644
--- a/assistant/src/ipc/assistant-server.ts
+++ b/assistant/src/ipc/assistant-server.ts
@@ -53,6 +53,10 @@ import {
   writeStreamEnd,
 } from "./ipc-framing.js";
 import { type DbProxyParams, handleDbProxy } from "./routes/db-proxy.js";
+import {
+  type DbProxyTransactionParams,
+  handleDbProxyTransaction,
+} from "./routes/db-proxy-transaction.js";
 import { routeDefinitionsToIpcMethods } from "./routes/route-adapter.js";
 import { ensureSocketPathFree } from "./socket-cleanup.js";
 import { resolveIpcSocketPath } from "./socket-path.js";
@@ -167,13 +171,17 @@ export class AssistantIpcServer {
       this.methods.set(route.operationId, route.handler);
     }
 
-    // ⚠️  TEMPORARY — gateway→assistant DB proxy (see ipc/routes/db-proxy.ts).
-    // This is the ONLY route defined directly here; all other routes go in ROUTES.
-    // Remove once contacts/guardian-binding logic is fully migrated to the
-    // gateway's own database.
+    // ⚠️  TEMPORARY — gateway→assistant DB proxies (see ipc/routes/db-proxy.ts
+    // and ipc/routes/db-proxy-transaction.ts). These are the ONLY routes
+    // defined directly here; all other routes go in ROUTES. Remove once
+    // contacts/guardian-binding logic is fully migrated to the gateway's
+    // own database.
     this.methods.set("db_proxy", (params) =>
       handleDbProxy(params as unknown as DbProxyParams),
     );
+    this.methods.set("db_proxy_transaction", (params) =>
+      handleDbProxyTransaction(params as unknown as DbProxyTransactionParams),
+    );
 
     this.watchdog = new SocketWatchdog({
       socketPath: this.socketPath,
diff --git a/assistant/src/ipc/routes/db-proxy-transaction.ts b/assistant/src/ipc/routes/db-proxy-transaction.ts
new file mode 100644
index 00000000000..2b7ace2fd6f
--- /dev/null
+++ b/assistant/src/ipc/routes/db-proxy-transaction.ts
@@ -0,0 +1,139 @@
+/**
+ * ⚠️  TEMPORARY HACK — DO NOT EXTEND ⚠️
+ *
+ * IPC route that lets the gateway execute multiple write statements against
+ * the assistant's SQLite database inside a single atomic transaction.
+ *
+ * Companion to db-proxy.ts. Exists because some gateway-orchestrated writes
+ * (e.g. invite redemption: upsert contact channel + record invite use) must
+ * be all-or-nothing. With the contacts/guardian/invite tables still living
+ * in the assistant DB, the gateway needs a way to commit several writes
+ * atomically there.
+ *
+ * Each step is a write (INSERT/UPDATE/DELETE). All steps run inside a
+ * BEGIN IMMEDIATE transaction. If any step throws — including a step whose
+ * `requireChanges` constraint is unmet — the entire transaction rolls back.
+ *
+ * Read-modify-write across steps is not supported (the IPC is one-shot;
+ * later steps cannot react to earlier step results except via SQL conditions
+ * embedded in the WHERE clause and the optional `requireChanges` guard).
+ *
+ * This route is intentionally NOT in the shared ROUTES array — it is a
+ * private implementation detail between the gateway and assistant IPC
+ * servers and must not be discoverable by clients or the OpenAPI spec.
+ *
+ * Remove once contacts/guardian/invite logic is fully migrated to the
+ * gateway's own database.
+ */
+
+import { getSqlite } from "../../memory/db-connection.js";
+import { getLogger } from "../../util/logger.js";
+
+const log = getLogger("db-proxy-transaction");
+
+/** Column value types that SQLite can return. */
+type SqliteValue = string | number | null | Uint8Array;
+
+export interface DbProxyTransactionStep {
+  /** The SQL write statement to execute. */
+  sql: string;
+  /** Positional bind parameters. */
+  bind?: SqliteValue[];
+  /**
+   * If set, abort the transaction (rollback) when this step's row-change
+   * count is below this threshold. Used for stale-write detection — e.g.
+   * "increment use_count only if status = 'active' AND use_count < max_uses",
+   * with `requireChanges: 1` to abort when no rows match.
+   */
+  requireChanges?: number;
+}
+
+export interface DbProxyTransactionParams {
+  steps: DbProxyTransactionStep[];
+}
+
+export type DbProxyTransactionResult =
+  | {
+      ok: true;
+      results: Array<{ changes: number; lastInsertRowid: number }>;
+    }
+  | {
+      ok: false;
+      reason: "require_changes_failed";
+      failedStep: number;
+      actualChanges: number;
+      requiredChanges: number;
+    };
+
+export function handleDbProxyTransaction(
+  params: DbProxyTransactionParams,
+): DbProxyTransactionResult {
+  const db = getSqlite();
+
+  if (!Array.isArray(params.steps) || params.steps.length === 0) {
+    throw new Error("db_proxy_transaction requires at least one step");
+  }
+
+  // Sentinel used to abort the transaction without leaking through as a generic
+  // SQL error. Better-sqlite3 rolls back when the inner function throws.
+  class RequireChangesFailure extends Error {
+    constructor(
+      public failedStep: number,
+      public actualChanges: number,
+      public requiredChanges: number,
+    ) {
+      super(
+        `Step ${failedStep} affected ${actualChanges} rows, requires ${requiredChanges}`,
+      );
+    }
+  }
+
+  const results: Array<{ changes: number; lastInsertRowid: number }> = [];
+
+  try {
+    db.transaction(() => {
+      for (let i = 0; i < params.steps.length; i++) {
+        const step = params.steps[i];
+        const stmt = db.prepare(step.sql);
+        const result = step.bind ? stmt.run(...step.bind) : stmt.run();
+        const changes = result.changes;
+        results.push({
+          changes,
+          lastInsertRowid: Number(result.lastInsertRowid),
+        });
+
+        if (
+          step.requireChanges !== undefined &&
+          changes < step.requireChanges
+        ) {
+          throw new RequireChangesFailure(i, changes, step.requireChanges);
+        }
+      }
+    }).immediate();
+  } catch (err) {
+    if (err instanceof RequireChangesFailure) {
+      log.debug(
+        {
+          failedStep: err.failedStep,
+          actualChanges: err.actualChanges,
+          requiredChanges: err.requiredChanges,
+        },
+        "db-proxy-transaction aborted by requireChanges guard",
+      );
+      return {
+        ok: false,
+        reason: "require_changes_failed",
+        failedStep: err.failedStep,
+        actualChanges: err.actualChanges,
+        requiredChanges: err.requiredChanges,
+      };
+    }
+    throw err;
+  }
+
+  log.debug(
+    { stepCount: params.steps.length },
+    "db-proxy-transaction committed",
+  );
+  return { ok: true, results };
+}
diff --git a/gateway/src/db/assistant-db-proxy.ts b/gateway/src/db/assistant-db-proxy.ts
index 852d937d96c..8f5aa331214 100644
--- a/gateway/src/db/assistant-db-proxy.ts
+++ b/gateway/src/db/assistant-db-proxy.ts
@@ -66,3 +66,58 @@ export async function assistantDbRun(
 export async function assistantDbExec(sql: string): Promise<void> {
   await dbProxy(sql, "exec");
 }
+
+// ---------------------------------------------------------------------------
+// Transaction helper
+// ---------------------------------------------------------------------------
+
+export interface AssistantDbTransactionStep {
+  /** The SQL write statement to execute. */
+  sql: string;
+  /** Positional bind parameters. */
+  bind?: SqliteValue[];
+  /**
+   * If set, abort the transaction (rollback) when this step's row-change
+   * count is below this threshold. Used for stale-write detection — e.g.
+   * "increment use_count only if status = 'active' AND use_count < max_uses",
+   * with `requireChanges: 1` to abort when no rows match.
+   */
+  requireChanges?: number;
+}
+
+export type AssistantDbTransactionResult =
+  | {
+      ok: true;
+      results: Array<{ changes: number; lastInsertRowid: number }>;
+    }
+  | {
+      ok: false;
+      reason: "require_changes_failed";
+      failedStep: number;
+      actualChanges: number;
+      requiredChanges: number;
+    };
+
+/**
+ * Execute multiple write statements against the assistant's SQLite DB inside
+ * a single atomic transaction (BEGIN IMMEDIATE). All steps commit together;
+ * any throw — including a `requireChanges` constraint failure — rolls back
+ * the entire batch.
+ *
+ * Use this when several writes must succeed or fail as a unit (e.g. invite
+ * redemption: contact-channel upsert + invite-use record).
+ *
+ * Read-modify-write across steps is not supported. Use SQL-level conditions
+ * (WHERE clauses, ON CONFLICT) plus `requireChanges` for stale-write detection.
+ */
+export async function assistantDbTransaction(
+  steps: AssistantDbTransactionStep[],
+): Promise<AssistantDbTransactionResult> {
+  const result = await ipcCallAssistant("db_proxy_transaction", { steps });
+  if (result === undefined) {
+    throw new Error(
+      "db_proxy_transaction IPC call failed — assistant may not be ready",
+    );
+  }
+  return result as AssistantDbTransactionResult;
+}