cloud: fix Freestyle VM fork via snapshot clone by lawrencecchen · Pull Request #12320 · manaflow-ai/cmux

lawrencecchen · 2026-09-11T11:26:33Z

Problem

cmux vm fork selected the gateway's always-present fork() method on Freestyle, then returned 501 vm_operation_unsupported. Freestyle has no native fork API.

Fix

Expose driver capabilities from the provider gateway and select the native path only when the driver declares fork: true. Freestyle now uses the existing documented snapshot-then-create clone path.

The PR keeps the regression test in its own first commit, followed by the fix.

Validation

bun test --isolate tests/vm-fork-capability.test.ts passes.
ESLint passes for the changed files.
Full bun run typecheck reaches two existing unrelated test errors in tests/docs-search-cache.test.ts and tests/vercel-ignore-build.test.ts.
Live Freestyle validation was completed on the prior PR's vmfx1 stack: fork created a new VM from a fresh snapshot, exec worked, and delete worked.

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Note

Medium Risk
Changes fork routing in the VM workflow for Freestyle and any provider without native fork; wrong capability wiring could still pick the wrong clone path or skip billing/create semantics on the native branch.

Overview
Fixes cmux vm fork on Freestyle, which was incorrectly treated as a native clone because the provider gateway always exposes a fork method that fails with vm_operation_unsupported when the driver has no real fork API.

The gateway now exposes optional capabilities(provider) (backed by vmCapabilitiesFor), and forkVm picks the native path only when the driver reports fork: true via providerForksNatively. Freestyle (fork: false) goes through the existing snapshot → create clone flow instead of calling provider fork. A regression test asserts Freestyle never hits the fork call and reaches snapshot first.

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Summary by cubic

Fixes cmux vm fork on Freestyle, which previously always selected the gateway's fork() method and failed with vm_operation_unsupported. The gateway now exposes driver capabilities, and the native fork path is used only when the driver declares fork: true. Freestyle uses the existing snapshot-then-create clone path instead, with usage events recording which fork path was taken. A regression test locks in the capability decision.

^{Written for commit af7025c. Summary will update on new commits.}

Summary by CodeRabbit

Bug Fixes
- Improved VM forking behavior by respecting provider-declared capabilities.
- Freestyle providers now correctly use the snapshot-based fallback when native forking is unsupported.
- Prevented unsupported native fork attempts from causing VM operations to fail unnecessarily.
- Improved reliability when providers report that native VM cloning is unavailable.
New Features
- Added capability detection for VM providers to support more accurate operation handling.

…ive fork The live provider gateway always defines fork() and answers unsupported from inside it when the driver has no native fork. forkVm read the presence of that function as native support, so on Freestyle every fork failed with vm_operation_unsupported and the snapshot-based path was unreachable.

vercel · 2026-09-11T11:26:35Z

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Project	Deployment	Actions	Updated
cmux166	Ready	Preview	Sep 12, 2026 2:57am UTC
cmux41	Ready	Preview	Sep 12, 2026 2:57am UTC

github-actions · 2026-09-11T11:26:53Z

All contributors have signed the CLA ✍️ ✅
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coderabbitai · 2026-09-11T11:27:08Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: ec0ea2c8-aa7e-4910-9976-de786ed0fad8

📥 Commits

Reviewing files that changed from the base of the PR and between 18ca953 and af7025c.

📒 Files selected for processing (1)

web/services/vms/workflows.ts

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📝 Walkthrough

Walkthrough

The VM provider gateway now reports provider capabilities. forkVm checks the fork capability before selecting native cloning and uses the snapshot path when native fork is unsupported. Tests cover the capability value and routing behavior.

Changes

VM fork capability handling

Layer / File(s)	Summary
Provider capability reporting `web/services/vms/providerGateway.ts`	The gateway shape and live implementation now expose provider capabilities through `vmCapabilitiesFor`.
Capability-aware fork selection `web/services/vms/workflows.ts`, `web/tests/vm-fork-capability.test.ts`	`forkVm` checks the provider fork capability before using native fork. Tests verify that Freestyle uses the snapshot path when native fork is unsupported.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: austinywang

Merge Risk: ⚪ Minimal · up to af702

The capability-aware fork routing is mergeable with no identified current-head risk.

🚥 Pre-merge checks | ✅ 24 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (24 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: fixing Freestyle VM forks by using snapshot cloning.
Description check	✅ Passed	The description clearly explains the problem, fix, regression test, lint validation, typecheck limitation, and live validation. It does not include the template's Demo Video, Review Trigger, or Checkl…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Cmux Swift Actor Isolation	✅ Passed	The pull request changes only TypeScript files: `web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and a TypeScript test. The authoritative diff contains no Swift paths or Swift a…
Cmux Swift Blocking Runtime	✅ Passed	PASS. The authoritative pull-request diff changes only three TypeScript files. It adds no production Swift code and introduces no Swift blocking or timing synchronization. The `Effect.sync` calls are …
Cmux Browser Automation Off-Main	✅ Passed	PASS. The pull request changes only `web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and `web/tests/vm-fork-capability.test.ts`. It does not change browser socket automation, `S…
Cmux Expensive Synchronous Load	✅ Passed	PASS. The authoritative pull-request diff changes only three TypeScript files: `web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and `web/tests/vm-fork-capability.test.ts`. It ad…
Cmux Cache Substitution Correctness	✅ Passed	PASS: The PR does not replace an authoritative read with a cached or opportunistic value. `capabilities` delegates to `vmCapabilitiesFor`, which derives flags from the registered driver and declaratio…
Cmux No Hacky Sleeps	✅ Passed	PASS: The pull request adds capability lookup and changes fork routing, but it adds no sleep, timer, delayed dispatch, polling loop, or wall-clock wait. The only `Effect.sleep` and polling logic found…
Cmux Algorithmic Complexity	✅ Passed	PASS: The production diff adds only constant-time capability resolution and a single capability branch. `providerForksNatively` performs fixed scalar checks at `web/services/vms/workflows.ts:1511-1513…
Cmux Swift Concurrency	✅ Passed	The authoritative pull-request diff changes only TypeScript files under `web/services/vms` and one TypeScript test. It introduces no Swift code or Swift concurrency patterns, so this check is not appl…
Cmux Swift `@Concurrent`	✅ Passed	PASS: The authoritative PR diff changes only TypeScript files (`providerGateway.ts`, `workflows.ts`, and `vm-fork-capability.test.ts`). It introduces no Swift files or Swift concurrency annotations, s…
Cmux Swift Package Boundaries	✅ Passed	The authoritative pull-request diff changes only two TypeScript production files and one TypeScript test file. It contains no Swift changes, so the Swift package boundary rule is not applicable.
Cmux Swiftpm Lockfiles	✅ Passed	PASS. The review-scoped diff changes only three TypeScript files: `web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and `web/tests/vm-fork-capability.test.ts`. It changes no `Pac…
Cmux Swift Logging	✅ Passed	PASS: The PR changes only three TypeScript files (`providerGateway.ts`, `workflows.ts`, and a test file); it adds no Swift file or Swift code. The added lines contain no `print`, `debugPrint`, `dump`,…
Cmux User-Facing Error Privacy	✅ Passed	PASS. The production diff adds an internal gateway capability method, changes fork routing, and records `native`/`snapshotId` in usage-event metadata. It does not add or modify user-facing error text,…
Cmux Full Internationalization	✅ Passed	PASS. The diff changes only VM service logic and adds a regression test. It adds no Swift text, web UI copy, API response text, rendered markdown, locale files, or message keys. The new `capabilities`…
Cmux Swiftui State Layout	✅ Passed	PASS: The reviewed diff changes only three TypeScript files under web/services/vms and web/tests. It introduces no SwiftUI code, ObservableObject/@published state, GeometryReader layout measurement, l…
Cmux Architecture Rethink	✅ Passed	PASS. The authoritative pull-request diff changes only two TypeScript service files and one TypeScript test file. It contains no Swift changes, so the Swift architectural-rethink failure conditions do…
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	PASS. The reviewed diff changes only three TypeScript files: web/services/vms/providerGateway.ts, web/services/vms/workflows.ts, and web/tests/vm-fork-capability.test.ts. The patch contains no Swift f…
Cmux Source Artifacts	✅ Passed	The diff changes only two hand-written TypeScript source files and one hand-written regression test: `web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and `web/tests/vm-fork-capa…
Cmux No Test Or Debug Seam In Production Source	✅ Passed	The authoritative pull-request diff changes only two TypeScript production files and one TypeScript test file. It contains no Swift files under a production `Sources/` path, so the specified Swift tes…
Cmux No Ambient Global State	✅ Passed	PASS: The reviewed range changes only three TypeScript files (`web/services/vms/providerGateway.ts`, `web/services/vms/workflows.ts`, and `web/tests/vm-fork-capability.test.ts`). It contains no Swift …

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cursor · 2026-09-12T02:48:17Z

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f23e719 fix: add nightly floor for beta builds (manaflow-ai#12389) b317b35 cloud: fix Freestyle VM fork via snapshot clone (manaflow-ai#12320) a7bd0c3 Fix Nightly compatibility warnings (manaflow-ai#12385)

lawrencecchen added 2 commits September 11, 2026 04:21

cloud: select Freestyle fork path from driver capabilities

18ca953

vercel Bot deployed to Preview – cmux41 September 11, 2026 11:27 View deployment

vercel Bot deployed to Preview – cmux166 September 11, 2026 11:27 View deployment

cloud: record snapshot fork path in usage events

af7025c

vercel Bot deployed to Preview – cmux41 September 12, 2026 02:56 View deployment

vercel Bot deployed to Preview – cmux166 September 12, 2026 02:57 View deployment

lawrencecchen merged commit b317b35 into main Sep 12, 2026
23 of 25 checks passed

lawrencecchen deleted the fix-cloud-vm-fork-capability-main branch September 12, 2026 02:59

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cloud: fix Freestyle VM fork via snapshot clone - #12320

cloud: fix Freestyle VM fork via snapshot clone#12320
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