Module Registry Agent Skill

Module Registry Context

When working with any code related to module resolution, compilation, evaluation,

or registration in workerd, you must read the relevant reference documents

before making changes or providing analysis. These documents capture the design

decisions, data flow, thread safety model, and integration points that are not

obvious from reading the source alone.

Reference Documents

Read these documents (using the Read tool) before proceeding with any module

registry work:

• docs/reference/detail/new-module-registry.md — The new jsg::modules::ModuleRegistry

implementation. Covers the two-layer architecture (shared ModuleRegistry +

per-isolate IsolateModuleRegistry), bundle types, resolution priority,

compile cache, evaluation flow, and thread safety model. Read this first

for any work on the new registry.

• docs/reference/detail/legacy-module-registry.md — The legacy

ModuleRegistryImpl<TypeWrapper> implementation. Covers the per-isolate

design, ModuleInfo structure, DynamicImportHandler, and how it interfaces

with V8. Read this when working on code that still uses the legacy path,

or when understanding the dispatch between legacy and new.

• docs/reference/detail/v8-module-internals.md — How V8 handles modules

internally. Covers v8::Module status transitions, InstantiateModule,

Evaluate, SyntheticModule, resolve callbacks, and import.meta. **Read

this** when debugging V8 module state issues, understanding callback

contracts, or working on the boundary between workerd and V8.

When to Read Which

| Task | Documents to read |

| ------------------------------------------------- | ----------------------------------------------- |

| Modifying new registry resolution or caching | new-module-registry, v8 |

| Adding a new built-in module | new-module-registry, legacy-module-registry |

| Debugging module-not-found errors | all three |

| Modifying legacy registry code | legacy-module-registry, v8 |

| Understanding legacy-to-new dispatch | all three |

| Working on import.meta behavior | new-module-registry, v8 |

| Working on CJS/require() interop | new-module-registry, legacy-module-registry |

| Debugging module evaluation failures | new-module-registry, legacy-module-registry, v8 |

| Reviewing a PR that touches module code | all three |

| Working on compile cache or cross-isolate sharing | new-module-registry |

| Working on dynamic import | all three |

| Working on source phase imports (Wasm) | new-module-registry, v8 |

Key Source Files

These are the primary files involved in module handling:

• src/workerd/jsg/modules-new.h / .c++ — New registry implementation
• src/workerd/jsg/modules.h / .c++ — Legacy registry implementation
• src/workerd/jsg/resource.h:1913–1922 — Legacy vs new dispatch at context creation
• src/workerd/jsg/jsg.c++:357–395 — Lock::resolveModule dispatch
• src/workerd/jsg/setup.h:291–297 — isUsingNewModuleRegistry() flag
• src/workerd/io/worker-modules.h — newWorkerModuleRegistry construction
• src/workerd/api/modules.h — Built-in module registration (both legacy and new)
• src/workerd/jsg/observer.h — ResolveObserver metrics interface

Procedure

• Read the relevant reference documents listed above for your task.
• Identify whether the code path uses the new or legacy registry (check

isUsingNewModuleRegistry() usage in the surrounding code).

• Proceed with your work, using the reference documents to inform your

understanding of data flow, thread safety, and integration contracts.

Do not skip reading the reference documents. The module registry has subtle

invariants around thread safety, V8 module status transitions, and evaluation

context (IoContext suppression) that are easy to violate without full context.