How Module Duplication Breaks SolidJS Reactivity

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If you encounter mysterious reactivity problems in SolidJS or similar frameworks, remember to check if you’re working with properly deduplicated dependencies. It might save you hours of debugging!

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When working with modern JavaScript frameworks, reactivity is the magic that makes our UIs respond seamlessly to data changes. But sometimes that magic breaks in mysterious ways, and the culprit is hiding in plain sight.

The Mystery

Recently, while integrating the Vercel AI SDK into our Chat Widget, we encountered a baffling issue. We had two applications: one where reactivity worked perfectly, and another where it appeared to be broken—despite using identical code patterns.

In the failing application, our chat interface wasn’t updating with new messages, even though we could see the data was being received correctly in the console logs:

// We could see this log from our useChat hook
onUpdate message: {"id":"ClKqs9YaSdlOe3lN", "role":"assistant", "content":"Hello! How can I assist you today?", ...}

// But our component's effect wasn't triggering
createEffect(() => {
  const msgs = messages();
  console.log("Effect triggered with messages:", msgs); // This never ran after updates
});

The most perplexing part? Both applications were using the same locally-built version of the AI SDK package with identical implementations. Yet one worked, and one didn’t.

The Debugging Journey

We spent half a day meticulously debugging, exploring various possibilities:

1. We checked for issues with component structure and prop passing
2. We analyzed the reactivity chain to find where it might be breaking
3. We tried alternative approaches to consuming the messages signal
4. We compared implementations between the working and non-working applications
5. We added manual signals and effects to try to force reactivity

Nothing worked. With each failed attempt, the mystery deepened.

The Breakthrough

The next morning, with fresh eyes, we looked at something we hadn’t considered important before: the exact structure of our dependencies.

Running npm ls on both projects revealed a crucial difference:

Working project:

@agent-embed/js@0.0.21
├─┬ @ai-sdk/solid@1.1.23
│ └── @ai-sdk/ui-utils@1.1.19 deduped
└── @ai-sdk/ui-utils@1.1.19

Non-working project:

@agent-embed/js@0.0.21
├─┬ @ai-sdk/solid@1.1.23 -> ./../../ai/packages/solid
│ └── @ai-sdk/ui-utils@1.1.19 -> ./../../ai/packages/ui-utils
└── @ai-sdk/ui-utils@1.1.19 -> ./../../ai/packages/ui-utils

That single word made all the difference: deduped.

Why This Matters for SolidJS

SolidJS, like many modern reactive frameworks, relies on a single instance of its reactivity system. When you have multiple copies of SolidJS or related packages in your dependency tree, you effectively create isolated islands of reactivity that can’t communicate with each other.

In our case, the local filesystem links were causing Node.js to load separate instances of the @ai-sdk/ui-utils package—one for our application and another for the @ai-sdk/solid package. Since the reactivity system exists within these packages, signals created in one instance couldn’t trigger effects registered in the other.

This explains why we could see data being updated internally in the SDK (logging was working), but our component never received notification of these changes.

The Solution: Yalc to the Rescue

After identifying the problem, we turned to Yalc, a tool designed for local package development that handles dependency deduplication better than plain filesystem links.

We followed these steps:

# Install Yalc globally
npm install -g yalc

# Publish our local packages to the Yalc store
cd ../../ai/packages/solid
yalc publish
cd ../../ai/packages/ui-utils
yalc publish

# Add these packages to our project
cd /Users/jai/work/agent-embed/js
yalc add @ai-sdk/solid @ai-sdk/ui-utils

After reinstalling dependencies and rebuilding, we confirmed that deduplication was happening correctly:

@agent-embed/js@0.0.21
├─┬ @ai-sdk/solid@1.1.23 -> ./.yalc/@ai-sdk/solid
│ └── @ai-sdk/ui-utils@1.1.19 deduped -> ./.yalc/@ai-sdk/ui-utils
└── @ai-sdk/ui-utils@1.1.19 -> ./.yalc/@ai-sdk/ui-utils

And sure enough, our reactivity was working again! Messages were flowing through the system correctly, and our UI was updating as expected.

Lessons Learned

This experience highlights several important lessons for working with modern JavaScript frameworks:

1. Dependency deduplication matters - Especially for frameworks that rely on shared internal state or singleton patterns.

2. Local development can introduce subtle issues - The convenience of direct filesystem links comes with hidden costs.

3. Tools like Yalc exist for a reason - They solve real problems that arise in local package development.

4. When debugging reactivity, check your dependencies - Sometimes the issue isn’t in your code but in how your code is being loaded.

The next time you encounter mysterious reactivity problems in SolidJS or similar frameworks, remember to check if you’re working with properly deduplicated dependencies. It might save you hours of debugging!