## Fix incorrect context size limits for GitHub Copilot Chat models
Fixes#44909
### Problem
The agent panel was displaying incorrect token limits for GitHub Copilot
models. Users reported that:
- The agent panel always showed a **128K token limit** for all GitHub
Copilot models, regardless of their actual context window size
- Claude models (e.g., Claude 3.7 Sonnet, Claude Opus 4.5) were showing
~90K instead of their actual 200K context window
- GPT-4o was showing 110K instead of its actual 128K context window
- Users could continue using models beyond the displayed limit, which
worked but was confusing
### Root Cause
The `max_token_count()` method in `copilot_chat.rs` was returning
`max_prompt_tokens` instead of `max_context_window_tokens`:
```rust
// Before (incorrect)
pub fn max_token_count(&self) -> u64 {
self.capabilities.limits.max_prompt_tokens
}
```
GitHub's API returns three different token-related fields:
- `max_context_window_tokens`: The **full context window size** (e.g.,
200K for Claude 3.7)
- `max_prompt_tokens`: GitHub's limit for prompt input (e.g., 90K for
Claude 3.7)
- `max_output_tokens`: Maximum output tokens (e.g., 16K)
The `max_token_count()` method in the `LanguageModel` trait is expected
to return the **full context window size** — this is consistent with all
other providers (Anthropic returns 200K for Claude, OpenAI returns 128K
for GPT-4o, etc.).
### Solution
<img width="583" height="132" alt="Screenshot 2026-01-25 at 1 07 53 AM"
src="https://github.com/user-attachments/assets/847e2fdb-635d-44bc-a630-2d4867ba8c32"
/>
Changed `max_token_count()` to return `max_context_window_tokens`:
```rust
// After (correct)
pub fn max_token_count(&self) -> u64 {
self.capabilities.limits.max_context_window_tokens as u64
}
```
### Impact
| Model | Before | After |
|-------|--------|-------|
| Claude 3.7 Sonnet | 90,000 | **200,000** |
| Claude Opus 4.5 | 90,000 | **200,000** |
| GPT-4o | 110,000 | **128,000** |
### Testing
Added a new test
`test_max_token_count_returns_context_window_not_prompt_tokens` that:
1. Deserializes model JSON with distinct `max_context_window_tokens` and
`max_prompt_tokens` values
2. Verifies Claude 3.7 Sonnet returns 200,000 (context window), not
90,000 (prompt tokens)
3. Verifies GPT-4o returns 128,000 (context window), not 110,000 (prompt
tokens)
All existing tests continue to pass:
```
running 4 tests
test tests::test_unknown_vendor_resilience ... ok
test tests::test_max_token_count_returns_context_window_not_prompt_tokens ... ok
test tests::test_resilient_model_schema_deserialize ... ok
test result: ok. 4 passed; 0 failed
```
Release Notes:
- copilot: Fixed incorrect context window size displayed for GitHub
Copilot Chat models in the agent panel.
Anil Pai
13ad175c8d