diff --git a/.changeset/fix-sandbox-deferred-promise-pump.md b/.changeset/fix-sandbox-deferred-promise-pump.md
new file mode 100644
index 0000000000..aaff021baf
--- /dev/null
+++ b/.changeset/fix-sandbox-deferred-promise-pump.md
@@ -0,0 +1,9 @@
+---
+"@objectstack/runtime": patch
+---
+
+fix(runtime): drive sandbox host calls with deferred promises and a deadline-bounded pump
+
+The QuickJS sandbox exposed `ctx.api.object(x).find/update/...` via `newAsyncifiedFunction`, which unwinds the WASM stack per host call and forbids a second call while the first is unwound. A script awaiting two host calls in sequence (e.g. an action doing `findOne()` then `update()`) drove the second call from a continuation resumed inside `executePendingJobs`, corrupting the wasm heap (`memory access out of bounds` / `p->ref_count == 0`) or exhausting the fixed 1000-iteration pump budget — surfacing as `action '…' did not resolve after 1000 pump iterations`.
+
+Host API methods are now exposed as deferred QuickJS promises (`vm.newPromise()`), so sequential `await`s compose with no stack unwinding, and the pump loop is bounded by the configured `timeoutMs` instead of a fixed iteration cap. Host **method** calls now require `await` (the `.object(name)` proxy getter stays synchronous); a stuck/never-settling host call is cut off with a clear timeout error.
diff --git a/packages/runtime/src/sandbox/body-runner.test.ts b/packages/runtime/src/sandbox/body-runner.test.ts
index ee00ff9915..38b8b812f0 100644
--- a/packages/runtime/src/sandbox/body-runner.test.ts
+++ b/packages/runtime/src/sandbox/body-runner.test.ts
@@ -81,7 +81,7 @@ describe('hookBodyRunnerFactory', () => {
       events: ['afterInsert'],
       body: {
         language: 'js',
-        source: 'return { opportunity_count: ctx.api.object("opportunity").count() };',
+        source: 'return { opportunity_count: await ctx.api.object("opportunity").count() };',
         capabilities: ['api.read'],
       },
     } as any);
diff --git a/packages/runtime/src/sandbox/quickjs-runner.test.ts b/packages/runtime/src/sandbox/quickjs-runner.test.ts
index ce1b1d4c28..71897c4156 100644
--- a/packages/runtime/src/sandbox/quickjs-runner.test.ts
+++ b/packages/runtime/src/sandbox/quickjs-runner.test.ts
@@ -212,3 +212,103 @@ describe('QuickJSScriptRunner — async host APIs', () => {
     expect(r.mutatedInput).toMatchObject({ raw: 'abc-9', normalized: 'ABC-9' });
   });
 });
+
+describe('QuickJSScriptRunner — long-running async host work (pump budget)', () => {
+  // Regression: an action's single `ctx.api.update(...)` can synchronously drive
+  // a large amount of awaited host work — e.g. a record-change flow that the
+  // engine runs inline inside the afterUpdate hook chain (see tianshun-mtc
+  // `lead_apply_convert` → `lead_convert_approval`). From the sandbox's view
+  // that is ONE asyncified host call that takes many event-loop turns to settle.
+  //
+  // The pump loop must bound that wait by the configured `timeoutMs`, NOT by a
+  // fixed iteration count: a legitimately-progressing call that needs >1000
+  // event-loop turns but finishes well within the timeout must still resolve.
+  // The old fixed `pumps < 1000` cap fired in ~tens of ms and surfaced as
+  // "did not resolve after 1000 pump iterations" — the exact production error.
+
+  it('resolves an action whose host call settles after >1000 event-loop turns', async () => {
+    const TURNS = 1500; // comfortably exceeds the old 1000-pump cap
+    let observed = 0;
+    const api = {
+      object: () => ({
+        // One asyncified host call that internally needs many macrotask turns
+        // before its promise settles — mirrors a CRUD write that synchronously
+        // runs a long downstream automation.
+        update: async () => {
+          for (let i = 0; i < TURNS; i++) {
+            await new Promise<void>((resolve) => setImmediate(resolve));
+          }
+          observed = TURNS;
+          return { ok: true };
+        },
+      }),
+    };
+
+    const r = await runner.runScript(
+      {
+        language: 'js',
+        source: "return await ctx.api.object('wid').update({ id: 'x', status: 'pending' });",
+        capabilities: ['api.write'],
+        timeoutMs: 30000,
+      },
+      ctx({ api }),
+      actionOpts,
+    );
+
+    expect(r.value).toEqual({ ok: true });
+    expect(observed).toBe(TURNS);
+  }, 40000);
+
+  it('resolves an action that makes >1000 sequential host calls', async () => {
+    const N = 1500;
+    let calls = 0;
+    const api = {
+      object: () => ({
+        update: async () => {
+          calls++;
+          return { i: calls };
+        },
+      }),
+    };
+
+    const r = await runner.runScript(
+      {
+        language: 'js',
+        source: `
+          const o = ctx.api.object('wid');
+          for (let i = 0; i < ${N}; i++) { await o.update({ id: 'x', i }); }
+          return { calls: ${N} };
+        `,
+        capabilities: ['api.write'],
+        timeoutMs: 30000,
+      },
+      ctx({ api }),
+      actionOpts,
+    );
+
+    expect(r.value).toEqual({ calls: N });
+    expect(calls).toBe(N);
+  }, 40000);
+
+  it('still enforces the timeout on a host call that never settles', async () => {
+    const api = {
+      object: () => ({
+        // Never resolves — must be killed by the deadline, not hang forever.
+        update: () => new Promise<never>(() => {}),
+      }),
+    };
+
+    await expect(
+      runner.runScript(
+        {
+          language: 'js',
+          source: "return await ctx.api.object('wid').update({ id: 'x' });",
+          capabilities: ['api.write'],
+          timeoutMs: 300,
+        },
+        ctx({ api }),
+        actionOpts,
+      ),
+    ).rejects.toThrow(/timeout/i);
+  }, 10000);
+});
diff --git a/packages/runtime/src/sandbox/quickjs-runner.ts b/packages/runtime/src/sandbox/quickjs-runner.ts
index 6e3e6d1d93..a5d64a9ee4 100644
--- a/packages/runtime/src/sandbox/quickjs-runner.ts
+++ b/packages/runtime/src/sandbox/quickjs-runner.ts
@@ -156,7 +156,7 @@ export class QuickJSScriptRunner implements ScriptRunner {
 
       // L2 scripts: wrap as async IIFE and use side-channel + asyncified pump.
       // Each pump iteration:
-      //   1. yield to the host event loop (lets asyncified host calls resolve)
+      //   1. yield to the host event loop (lets host promises settle)
       //   2. drain QuickJS pending jobs (advances the .then chain)
       //   3. read __result/__error from the VM
       const wrapped = args.origin.kind === 'hook'
@@ -179,9 +179,20 @@ export class QuickJSScriptRunner implements ScriptRunner {
       }
       evalRes.value.dispose();
 
+      // Drive the script's async continuations to completion. Each iteration
+      // yields to the host event loop (so in-flight host promises settle and
+      // resolve their VM-side deferred handles) and then drains the QuickJS job
+      // queue. The ONLY bound on how long we wait is the deadline: a slow but
+      // progressing script — many sequential host writes, or one write that
+      // synchronously drives a downstream record-change automation — must be
+      // allowed to finish within its timeout, and a stuck / never-settling host
+      // call is cut off here (the QuickJS interrupt handler can't fire while we
+      // are parked on a host promise, so this deadline check is the backstop).
+      // The previous fixed `pumps < 1000` cap fired in ~tens of ms on legitimate
+      // work and surfaced as "did not resolve after 1000 pump iterations".
       let pumps = 0;
-      while (pumps < 1000) {
-        // Yield to host event loop so any in-flight asyncified host promises resolve.
+      for (;;) {
+        // Yield to host event loop so any in-flight host promises resolve.
         await new Promise<void>((resolve) => setImmediate(resolve));
 
         const pending = runtime.executePendingJobs();
@@ -210,14 +221,11 @@ export class QuickJSScriptRunner implements ScriptRunner {
 
         if (Date.now() > deadline) {
           throw new SandboxError(
-            `${args.origin.kind} '${args.origin.name}' exceeded timeout of ${args.timeoutMs}ms`,
+            `${args.origin.kind} '${args.origin.name}' exceeded timeout of ${args.timeoutMs}ms (after ${pumps} pump iterations)`,
           );
         }
         pumps++;
       }
-      throw new SandboxError(
-        `${args.origin.kind} '${args.origin.name}' did not resolve after ${pumps} pump iterations`,
-      );
     } finally {
       // newAsyncContext() owns its WASM module; disposing the context disposes
       // the runtime + module together.
@@ -230,8 +238,9 @@ export class QuickJSScriptRunner implements ScriptRunner {
    * the body declared it; missing methods throw at call-time inside the VM
    * with a clear diagnostic.
    *
-   * Host API methods are installed via {@link QuickJSAsyncContext.newAsyncifiedFunction}
-   * so they may return Promises (real ObjectQL `find/count/insert/...` are async).
+   * Host API methods are installed as deferred-promise functions (see
+   * {@link installApiMethod}) so they may return Promises (real ObjectQL
+   * `find/count/insert/...` are async) without asyncify's single-unwind limit.
    */
   private installCtx(
     vm: QuickJSAsyncContext,
@@ -320,11 +329,25 @@ export class QuickJSScriptRunner implements ScriptRunner {
 }
 
 /**
- * Asyncified host-bound API method.
+ * Host-bound API method, exposed to the VM as an async function.
+ *
+ * IMPORTANT: this deliberately does NOT use `newAsyncifiedFunction`. Asyncify
+ * unwinds the WASM stack while a host call is in flight, and the engine forbids
+ * one asyncified call from running while another is unwound ("the stack cannot
+ * be unwound twice"). A script that awaits two host calls in sequence — e.g. the
+ * real `lead_apply_convert` action doing `findOne()` then `update()` — trips
+ * exactly that: the second call is driven from a resumed continuation inside
+ * `executePendingJobs` (a non-async frame), which corrupted the wasm heap
+ * (`memory access out of bounds` / `p->ref_count == 0`) and, when it limped
+ * along, blew the pump budget ("did not resolve after 1000 pump iterations").
  *
- * Awaits Promise return values from the host implementation and marshals the
- * resolved value back into the VM. Capability check happens at call time and
- * surfaces inside the VM as a thrown error with a clear diagnostic.
+ * Instead we hand the VM a real QuickJS promise (a deferred) and settle it from
+ * the host event loop. Sequential `await`s are then ordinary promises with no
+ * stack unwinding, so any number of host calls compose safely; the pump loop in
+ * {@link QuickJSScriptRunner.execute} drains the resulting jobs.
+ *
+ * The capability check runs synchronously at call time and surfaces inside the
+ * VM as a thrown error with a clear diagnostic.
  */
 function installApiMethod(
   vm: QuickJSAsyncContext,
@@ -336,7 +359,9 @@ function installApiMethod(
   required: HookBodyCapability,
   origin: ScriptOrigin,
 ): void {
-  const fn = vm.newAsyncifiedFunction(method, async (...argHandles) => {
+  const fn = vm.newFunction(method, (...argHandles) => {
+    // Capability gate — throw synchronously so the VM sees a normal exception at
+    // the call site (mirrors ctx.log / ctx.crypto gating).
     if (!caps.has(required)) {
       throw new SandboxError(
         `capability '${required}' not granted to ${origin.kind} '${origin.name}' (called ctx.api.object('${objectName}').${method})`,
@@ -346,14 +371,37 @@ function installApiMethod(
     if (!apiAny || typeof apiAny.object !== 'function') {
       throw new SandboxError(`ctx.api unavailable in ${origin.kind} '${origin.name}'`);
     }
+    // Dump args now, while the handles are alive — they are freed when this
+    // function returns, long before the async work below runs.
     const args = argHandles.map((h) => vm.dump(h));
-    const proxy = (apiAny.object as (n: string) => Record<string, unknown>)(objectName);
-    const m = proxy[method] as ((...a: unknown[]) => unknown) | undefined;
-    if (typeof m !== 'function') {
-      throw new SandboxError(`ctx.api.object('${objectName}').${method} not implemented`);
-    }
-    const ret = await Promise.resolve(m.apply(proxy, args));
-    return jsonToHandle(vm, ret);
+
+    const deferred = vm.newPromise();
+    void (async () => {
+      try {
+        const proxy = (apiAny.object as (n: string) => Record<string, unknown>)(objectName);
+        const m = proxy[method] as ((...a: unknown[]) => unknown) | undefined;
+        if (typeof m !== 'function') {
+          throw new SandboxError(`ctx.api.object('${objectName}').${method} not implemented`);
+        }
+        const ret = await Promise.resolve(m.apply(proxy, args));
+        if (!vm.alive) return; // VM disposed (e.g. timed out) before we settled.
+        const h = jsonToHandle(vm, ret);
+        deferred.resolve(h);
+        h.dispose();
+      } catch (err) {
+        if (!vm.alive) return;
+        const errH =
+          err instanceof Error
+            ? vm.newError({ name: err.name || 'Error', message: err.message })
+            : vm.newError({ name: 'Error', message: String(err) });
+        deferred.reject(errH);
+        errH.dispose();
+      }
+    })();
+    // The pump loop is the sole driver of executePendingJobs, so the resolution
+    // propagates into the VM on a subsequent pump iteration — no nudge here, to
+    // avoid any re-entrant executePendingJobs.
+    return deferred.handle;
   });
   vm.setProp(parent, method, fn);
   fn.dispose();