Truncate long chat sessions

[OaenHed]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Visual Studio

Body

After any meaningfully long session with a chat host, there will always come a time when an exception is generated due to too many tokens, forcing users to start a new session ... and lose the built up context. This behaviour makes little sense, since chat hosts don't (can't?) actively look at the beginning of the session anyways. This isn't a guess; I've tested this multiple times. Anything written at the start of a long-ish session cannot be directly retrieved even if explicit instructions are given that this is going to happen.

Would it be possible to simply truncate the top x% of the tokens when the limit is reached, to allow the session to continue?

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[asaddevx]

Hi @OaenHed, this is a known limitation with LLM-based chat systems, and I understand how frustrating it is to lose context mid-session.

Why This Happens

Copilot Chat (and similar AI assistants) use a context window — a limited number of tokens (roughly 8k-128k depending on the model) that the AI can "see" at once. Once you exceed this limit, the system has to drop older messages to make room for new ones.

Currently, when the limit is reached, the session throws an error rather than automatically truncating. You're correct that chat hosts don't actively use the earliest messages after a certain point — but the system still keeps them in the context buffer until it overflows.

Current Workarounds

While the product doesn't yet support automatic truncation, here are some ways to mitigate this:

1. Manual Truncation

• Periodically start a new session and summarize the key context from your previous session
• Example prompt: "Here's what we've established so far: [summary]. Let's continue from here."

2. Use @workspace References

Instead of letting context build up naturally, explicitly reference relevant files using @workspace or file paths. This keeps the context focused on what matters.

3. Break Complex Tasks into Steps

• Complete one logical task per session
• Document decisions as you go (in a markdown file or comments)
• Reference that documentation in new sessions

How to Get This Feature Added

Since this is a product feature request, the best way to get it implemented is to:

1. File an issue in the vscode-copilot-release repository
2. Tag it as a feature request and explain the user impact
3. Reference this discussion to show others are experiencing the same pain point

Product teams prioritize features based on user demand, so the more visibility this gets, the more likely it is to be addressed.

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Hope this helps explain the situation and gives you some useful workarounds in the meantime. Let me know if you'd like help crafting a feature request issue!