respect context length for anthropic

use the context length as per the config, rather than just hard-coded values.

test_anthropic_fix.md

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+# Anthropic max_tokens Error Fix - Test Plan
+
+## Changes Made
+
+### 1. Fixed Context Window Size Detection
+- **Problem**: Code used hardcoded 200k limit for Anthropic instead of configured max_tokens
+- **Fix**: Modified `determine_context_length()` to check configured max_tokens first before falling back to defaults
+- **Files**: `crates/g3-core/src/lib.rs` lines 923-945, 967-985
+
+### 2. Added Thinning Before Summarization
+- **Problem**: Code attempted summarization even when context window was nearly full
+- **Fix**: Added logic to try thinning first when context usage is between 80-90%
+- **Files**: `crates/g3-core/src/lib.rs` lines 2415-2439
+
+### 3. Added Capacity Checks Before Summarization
+- **Problem**: No validation that sufficient tokens remained for summarization
+- **Fix**: Added capacity checks for all provider types with helpful error messages
+- **Files**: `crates/g3-core/src/lib.rs` lines 2480-2520
+
+### 4. Improved Error Messages
+- **Problem**: Generic errors when summarization failed
+- **Fix**: Specific error messages suggesting `/thinnify` and `/compact` commands
+- **Files**: Multiple locations in summarization logic
+
+### 5. Dynamic Buffer Calculation
+- **Problem**: Fixed 5k buffer regardless of model size
+- **Fix**: Proportional buffer (2.5% of model limit, min 1k, max 10k)
+- **Files**: `crates/g3-core/src/lib.rs` line 2487
+
+## Test Cases
+
+### Test 1: Configured max_tokens Respected
+```toml
+# In g3.toml
+[providers.anthropic]
+api_key = "your-key"
+model = "claude-3-5-sonnet-20241022"
+max_tokens = 50000  # Should use this instead of 200k default
+```
+
+### Test 2: Thinning Before Summarization
+- Fill context to 85% capacity
+- Verify thinning is attempted before summarization
+- Check that summarization is skipped if thinning resolves the issue
+
+### Test 3: Capacity Error Handling
+- Fill context to 98% capacity
+- Verify helpful error message is shown instead of API error
+- Check that `/thinnify` and `/compact` commands are suggested
+
+### Test 4: Provider-Specific Handling
+- Test with different providers (anthropic, databricks, embedded)
+- Verify each uses appropriate capacity checks and buffers
+
+## Expected Behavior
+
+1. **No more max_tokens API errors** from Anthropic when context window is full
+2. **Automatic thinning** when approaching capacity (80-90%)
+3. **Clear error messages** with actionable suggestions when at capacity
+4. **Respect configured limits** instead of hardcoded defaults
+5. **Graceful degradation** with helpful user guidance
+
+## Manual Testing Commands
+
+```bash
+# Test with small max_tokens to trigger the issue quickly
+g3 --chat
+# Then paste large amounts of text to fill context window
+# Verify thinning and error handling work correctly
+```