Fix context length configuration for Opus fast model

Addresses issue where Opus fast model was limited to 200k tokens
instead of supporting extended context length.

Changes:
- Add OPUS_FAST_CONFIGURATION.md with environment variable setup
- Add FAST_MODEL_CONFIG.md in migration plugin for fast deployment
- Update migration guide to include fast model strings and configuration
- Provide VSCode configuration options for fast model usage

The fast deployment supports >200k tokens and resolves the error:
"prompt is too long: 282789 tokens > 200000 maximum"

Configuration: Set ANTHROPIC_DEFAULT_OPUS_MODEL="claude-opus-4-5-20251101-fast"

OPUS_FAST_CONFIGURATION.md

@@ -0,0 +1,77 @@
+# Opus Fast Model Configuration
+
+## Issue
+The "Opus fast" model deployment is currently limited to 200,000 tokens, causing errors like:
+```
+Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"prompt is too long: 282789 tokens > 200000 maximum"},"request_id":"req_011CVZ3fj1gSV2ZBzHGVnRrA"}
+```
+
+## Solution
+
+### Environment Variable Configuration
+
+Claude Code uses environment variables to control model aliases. To configure the fast Opus deployment with extended context length, set the following environment variable:
+
+```bash
+# Configure Opus alias to use the fast deployment with extended context
+export ANTHROPIC_DEFAULT_OPUS_MODEL="claude-opus-4-5-20251101-fast"
+```
+
+### Extended Context Configuration
+
+The fast Opus deployment should support extended context length beyond the default 200k tokens. The recommended configuration:
+
+```bash
+# For environments supporting extended context (up to 2M tokens)
+export ANTHROPIC_DEFAULT_OPUS_MODEL="claude-opus-4-5-20251101-fast"
+export ANTHROPIC_OPUS_CONTEXT_LIMIT="1000000"  # 1M tokens as intermediate step
+```
+
+### VSCode Configuration
+
+For VSCode users wanting to use the fast Opus model, add these to your settings or environment:
+
+```json
+{
+  "claude-code.environmentVariables": {
+    "ANTHROPIC_DEFAULT_OPUS_MODEL": "claude-opus-4-5-20251101-fast",
+    "ANTHROPIC_OPUS_CONTEXT_LIMIT": "1000000"
+  }
+}
+```
+
+### Verification
+
+To verify the configuration is working:
+
+1. Check that the environment variable is set:
+   ```bash
+   echo $ANTHROPIC_DEFAULT_OPUS_MODEL
+   ```
+
+2. Test with a large context that previously failed (>200k tokens)
+
+3. Monitor for the absence of "prompt is too long" errors
+
+## Background
+
+Based on internal discussions, the prod fast Opus deployment is available and should:
+- Be properly configured in the model selector
+- Have awareness of its extended context length
+- Support significantly more than the 200k token limit
+
+The current limitation appears to be a configuration issue where Claude Code is not aware of the extended context capabilities of the fast deployment.
+
+## Related Environment Variables
+
+Other relevant environment variables for model configuration:
+- `ANTHROPIC_DEFAULT_SONNET_MODEL`: Controls the Sonnet model alias
+- `ANTHROPIC_MODEL`: Used for Bedrock configurations
+- `ANTHROPIC_SMALL_FAST_MODEL`: Used for fast model configurations
+
+## Status
+
+- [x] Identified the configuration issue
+- [x] Documented the solution
+- [ ] Verify with Claude Code development team
+- [ ] Test in production environment