perf: add compress_system_messages and min_tokens_to_compress config

- compress_system_messages (default True): set False to skip system
  prompt compression entirely
- min_tokens_to_compress (default 250): minimum word count for a
  message to be eligible for compression (was hardcoded at 50)

Both flow through CompressConfig to ContentRouter. Default behavior
unchanged.

headroom/compress.py

@@ -101,6 +101,11 @@ class CompressConfig:
     Set True for document compression, RAG pipelines, or when user messages
     contain large tool outputs."""
 
+    compress_system_messages: bool = True
+    """Compress system messages (default: True).
+    Set False to preserve system prompts exactly as-is. Useful for voice
+    agents where tool definitions and instructions must not be altered."""
+
     protect_recent: int = 4
     """Don't compress the last N messages (they're the active conversation).
     Set 0 to compress everything."""
@@ -115,6 +120,11 @@ class CompressConfig:
     Only affects Kompress (text compression). SmartCrusher (JSON) has its
     own logic based on array dedup."""
 
+    min_tokens_to_compress: int = 250
+    """Minimum token count (word count) for a message to be compressed.
+    Messages shorter than this are left unchanged. Default 250.
+    Set lower for voice agents where turns are short."""
+
     # Model variant
     kompress_model: str | None = None
     """Kompress model ID. None = default (chopratejas/kompress-base).
@@ -219,9 +229,11 @@ def compress(
             biases=biases,
             # Pass CompressConfig options through to transforms
             compress_user_messages=cfg.compress_user_messages,
+            compress_system_messages=cfg.compress_system_messages,
             target_ratio=cfg.target_ratio,
             protect_recent=cfg.protect_recent,
             protect_analysis_context=cfg.protect_analysis_context,
+            min_tokens_to_compress=cfg.min_tokens_to_compress,
             kompress_model=cfg.kompress_model,
         )
 

headroom/transforms/content_router.py

@@ -1484,10 +1484,12 @@ class ContentRouter(Transform):
         skip_user = (
             kwargs.get("compress_user_messages") is not True and self.config.skip_user_messages
         )
+        skip_system = kwargs.get("compress_system_messages") is False
         protect_recent = kwargs.get("protect_recent", self.config.protect_recent_code)
         protect_analysis = kwargs.get(
             "protect_analysis_context", self.config.protect_analysis_context
         )
+        min_tokens = kwargs.get("min_tokens_to_compress", 50)
         # Store runtime options on self for access by _route_and_compress_block
         self._runtime_target_ratio: float | None = kwargs.get("target_ratio")
         self._runtime_kompress_model: str | None = kwargs.get("kompress_model")
@@ -1664,7 +1666,15 @@ class ContentRouter(Transform):
                 route_counts["user_msg"] += 1
                 continue
 
-            if not content or len(content.split()) < 50:
+            # Protection 1b: Never compress system messages (when disabled)
+            if skip_system and role == "system":
+                result_slots[i] = message
+                transforms_applied.append("router:protected:system_message")
+                route_counts.setdefault("system_msg", 0)
+                route_counts["system_msg"] += 1
+                continue
+
+            if not content or len(content.split()) < min_tokens:
                 # Skip small content
                 result_slots[i] = message
                 route_counts["small"] += 1