GitHub Copilot Claude model limits are inaccurate — missing max_prompt_tokens and wrong context values

Problem

The github-copilot provider entries for Claude models have inaccurate limit values. The data currently reports a single context value that doesn't reflect the actual limits exposed by the Copilot API (CAPI).

Source of truth

The authoritative source for Copilot model limits is the CAPI endpoint GET https://api.githubcopilot.com/models. The response includes three distinct token limits per model:

"family": "claude-opus-4.6",
"limits": {
    "max_context_window_tokens": 144000,
    "max_output_tokens": 64000,
    "max_prompt_tokens": 128000,
    "max_non_streaming_output_tokens": 16000
}

Current models.dev values vs CAPI (Claude models)

Every Claude model has discrepancies. The pattern is:

1. context is underreported — models.dev uses 128k, but CAPI reports 144k–216k for max_context_window_tokens
2. output is underreported for sonnet-4.5 and opus-4.5 (16k vs 32k actual)
3. max_prompt_tokens (input limit) is missing entirely — CAPI provides a separate max_prompt_tokens field (128k for all Claude models), which is different from max_context_window_tokens

Impact

This data is consumed by OpenCode (via https://models.dev/api.json) to determine compaction thresholds. The compaction logic in OpenCode uses:

const usable = model.limit.input 
    ? model.limit.input - reserved 
    : context - reserved

Since limit.input is not set (because models.dev doesn't populate it), OpenCode falls back to context - reserved, using the underreported 128k value. This triggers compaction earlier than necessary, wasting ~16k tokens of usable context on every session.

Suggested fix

For github-copilot Claude models, update the limits to match CAPI. The models.dev schema already supports limit.input — it just needs to be populated:

"claude-opus-4.6": {
    "limit": {
        "context": 144000,
        "input": 128000,
        "output": 64000
    }
}

How to verify

Query the CAPI models endpoint with a valid Copilot token:

curl -s "https://api.githubcopilot.com/models" \
  -H "Authorization: Bearer $COPILOT_TOKEN" \
  -H "Copilot-Integration-Id: vscode-chat"

This returns the full model catalog with authoritative limits per model.

Start by locating the github-copilot provider entries and reviewing the existing model limit schema, especially limit.input. Query the CAPI models endpoint with a valid Copilot token and update the listed Claude model records to match its context, prompt, and output limits. Done means all Claude entries reflect the authoritative CAPI values and the resulting model data uses input limits for compaction.