Skip to content

Load the code-execution stack on demand in the MCP session DO #1699

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
RhysSullivan merged 1 commit into from
Aug 19, 2026
Merged

Load the code-execution stack on demand in the MCP session DO #1699

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
28 changes: 25 additions & 3 deletions apps/cloud/src/mcp/session-durable-object.ts
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -66,9 +66,6 @@ import {
type DrizzleDb,
type DbServiceShape,
} from "../db/db";
import { makeExecutionStack } from "../engine/execution-stack";
import { preloadQuickJs } from "../quickjs";
import { CloudMeteredExecutionStackLayer } from "../engine/execution-stack-metered";
import { AutumnService } from "../extensions/billing/service";
import { DoTelemetryLive, flushTracerProvider } from "../observability/telemetry";
import {
Expand Down Expand Up @@ -242,6 +239,31 @@ export class McpSessionDOSqlite extends McpAgentSessionDOBase<Env, CloudSessionD
): Effect.Effect<BuiltMcpServer> {
const self = this;
return Effect.gen(function* () {
// Imported here rather than at module scope. Cloudflare requires a
// Durable Object class to be exported from the Worker entry, so every
// static import this module makes is evaluated by *every* cold isolate —
// including the ones that only render a page or forward a passthrough
// proxy and never open an MCP session. These three roots pull the whole
// code-execution stack (sucrase, ajv, QuickJS-WASM): measured at 1.9 MB
// of the Worker's startup closure, for code only a real session runs.
// `apps/cloud/scripts/start-closure.mjs` reports that number and will
// show it moving back if these become static again.
const [{ preloadQuickJs }, { makeExecutionStack }, { CloudMeteredExecutionStackLayer }] =
yield* Effect.promise(
() =>
Promise.all([
import("../quickjs"),
import("../engine/execution-stack"),
import("../engine/execution-stack-metered"),
]) as Promise<
[
typeof import("../quickjs"),
typeof import("../engine/execution-stack"),
typeof import("../engine/execution-stack-metered"),
]
>,
);

// QuickJS-WASM must be loaded before anything asks for a sandbox: the
// default variant cannot fetch its own `.wasm` on Workers. Cloud runs
// user `execute` code on the dynamic-worker runtime, but the artifact
Expand Down
Loading