feat: add STRANDS_ANTHROPIC_BETA environment variable (#34)

* feat: add STRANDS_ANTHROPIC_BETA environment variable

* docs(readme): document environment variables

Author: awsarron

README.md

@@ -205,6 +205,24 @@ export STRANDS_SYSTEM_PROMPT="You are a Python expert."
 echo "You are a security expert." > .prompt
 ```
 
+## 🌍 Environment Variables Configuration
+
+Strands Agent Builder also provides customization through environment variables:
+
+| Environment Variable | Description | Default | 
+|----------------------|-------------|---------|
+| STRANDS_MODEL_ID | Claude model ID to use for inference | us.anthropic.claude-sonnet-4-20250514-v1:0 |
+| STRANDS_MAX_TOKENS | Maximum tokens for agent responses | 32768 |
+| STRANDS_BUDGET_TOKENS | Token budget for agent thinking/reasoning | 2048 |
+| STRANDS_THINKING_TYPE | Type of thinking capability | enabled |
+| STRANDS_ANTHROPIC_BETA | Anthropic beta features (comma-separated) | interleaved-thinking-2025-05-14 |
+| STRANDS_CACHE_TOOLS | Tool caching strategy | default |
+| STRANDS_CACHE_PROMPT | Prompt caching strategy | default |
+| STRANDS_SYSTEM_PROMPT | Custom system prompt (overrides .prompt file) | None |
+| STRANDS_KNOWLEDGE_BASE_ID | Default Knowledge Base ID | None |
+| STRANDS_TOOL_CONSOLE_MODE | Enable rich console UI | enabled |
+| BYPASS_TOOL_CONSENT | Skip tool confirmation prompts | false |
+
 ## Exit
 
 Type `exit`, `quit`, or press `Ctrl+C`/`Ctrl+D`

src/strands_agents_builder/utils/model_utils.py

@@ -19,7 +19,6 @@
         retries=dict(max_attempts=3, mode="adaptive"),
     ),
     "additional_request_fields": {
-        "anthropic_beta": ["interleaved-thinking-2025-05-14"],
         "thinking": {
             "type": os.getenv("STRANDS_THINKING_TYPE", "enabled"),
             "budget_tokens": int(os.getenv("STRANDS_BUDGET_TOKENS", "2048")),
@@ -28,6 +27,9 @@
     "cache_tools": os.getenv("STRANDS_CACHE_TOOLS", "default"),
     "cache_prompt": os.getenv("STRANDS_CACHE_PROMPT", "default"),
 }
+ANTHROPIC_BETA_FEATURES = os.getenv("STRANDS_ANTHROPIC_BETA", "interleaved-thinking-2025-05-14")
+if len(ANTHROPIC_BETA_FEATURES) > 0:
+    DEFAULT_MODEL_CONFIG["additional_request_fields"]["anthropic_beta"] = ANTHROPIC_BETA_FEATURES.split(",")
 
 
 def load_path(name: str) -> pathlib.Path: