fix(#296): poka-yoke — prevent silent context overflow with mandatory compression

## Problem
When context approaches model limit, no guaranteed fail-safe. Agent could
ignore compression warnings, causing silent data loss when context overflows.

## Solution (Poka-yoke: Mitigation)
1. Hard token budget check at 85% of context length — auto-compression
2. At 95%, BLOCK tool calls and force compression before continuing
3. Add /context-status slash command (aliases: /ctx, /context)
4. Log context overflow events separately from normal compression

## Changes

### agent/context_compressor.py
- WARNING_THRESHOLD (0.85), CRITICAL_THRESHOLD (0.95)
- should_auto_compress(), should_block_tools(), get_usage_level()
- Extended get_status() with usage_level and thresholds

### run_agent.py
- 85%/95% checks before compression in main loop
- overflow_triggered param on _compress_context()
- Separate CONTEXT WARNING/CRITICAL logging

### hermes_cli/commands.py
- CommandDef("context-status", ...) with aliases (ctx, context)

### cli.py
- _show_context_status() with progress bar and threshold display

Refs #296

agent/context_compressor.py

@@ -138,6 +138,48 @@ class ContextCompressor:
         rough_estimate = estimate_messages_tokens_rough(messages)
         return rough_estimate >= self.threshold_tokens
 
+    # ── Poka-yoke: Hard context overflow safeguards (#296) ──────────────
+
+    WARNING_THRESHOLD = 0.85   # 85% — auto-compression trigger
+    CRITICAL_THRESHOLD = 0.95  # 95% — block tools, force compression
+
+    def get_context_usage_percent(self, prompt_tokens: int = None) -> float:
+        """Return context usage as a percentage of total context length (0-100)."""
+        tokens = prompt_tokens if prompt_tokens is not None else self.last_prompt_tokens
+        if self.context_length <= 0:
+            return 0.0
+        return min(100.0, (tokens / self.context_length) * 100)
+
+    def get_usage_level(self, prompt_tokens: int = None) -> str:
+        """Return the current context pressure level: 'normal', 'warning', 'critical'."""
+        pct = self.get_context_usage_percent(prompt_tokens) / 100.0
+        if pct >= self.CRITICAL_THRESHOLD:
+            return "critical"
+        elif pct >= self.WARNING_THRESHOLD:
+            return "warning"
+        return "normal"
+
+    def should_auto_compress(self, prompt_tokens: int = None) -> bool:
+        """Check if context exceeds the WARNING threshold (85% of context length).
+        
+        This is a HARD trigger — unlike should_compress() which uses the
+        configurable threshold_percent, this always fires at 85% regardless
+        of configuration.  Poka-yoke: don't trust voluntary behavior.
+        """
+        tokens = prompt_tokens if prompt_tokens is not None else self.last_prompt_tokens
+        warning_tokens = int(self.context_length * self.WARNING_THRESHOLD)
+        return tokens >= warning_tokens
+
+    def should_block_tools(self, prompt_tokens: int = None) -> bool:
+        """Check if context exceeds the CRITICAL threshold (95% of context length).
+        
+        When True, the agent MUST NOT make further tool calls until compression
+        completes.  Poka-yoke: enforce the constraint mechanically.
+        """
+        tokens = prompt_tokens if prompt_tokens is not None else self.last_prompt_tokens
+        critical_tokens = int(self.context_length * self.CRITICAL_THRESHOLD)
+        return tokens >= critical_tokens
+
     def get_status(self) -> Dict[str, Any]:
         """Get current compression status for display/logging."""
         return {
@@ -146,6 +188,9 @@ class ContextCompressor:
             "context_length": self.context_length,
             "usage_percent": min(100, (self.last_prompt_tokens / self.context_length * 100)) if self.context_length else 0,
             "compression_count": self.compression_count,
+            "usage_level": self.get_usage_level(),
+            "warning_threshold_tokens": int(self.context_length * self.WARNING_THRESHOLD),
+            "critical_threshold_tokens": int(self.context_length * self.CRITICAL_THRESHOLD),
         }
 
     # ------------------------------------------------------------------

cli.py

@@ -4658,6 +4658,8 @@ def _upload_0x0st(content: str) -> str | None:
             self._handle_reasoning_command(cmd_original)
         elif canonical == "compress":
             self._manual_compress()
+        elif canonical == "context-status":
+            self._show_context_status()
         elif canonical == "usage":
             self._show_usage()
         elif canonical == "insights":
@@ -5474,6 +5476,50 @@ def _upload_0x0st(content: str) -> str | None:
         except Exception as e:
             print(f"  ❌ Compression failed: {e}")
 
+    def _show_context_status(self):
+        """Show context usage, compression history, and remaining budget."""
+        if not self.agent:
+            print("(._.) No active agent -- send a message first.")
+            return
+
+        compressor = getattr(self.agent, "context_compressor", None)
+        if not compressor:
+            print("(._.) Context compressor not initialized.")
+            return
+
+        status = compressor.get_status()
+        
+        # Calculate real token usage
+        from agent.model_metadata import estimate_messages_tokens_rough
+        real_tokens = status.get("last_prompt_tokens", 0)
+        if not real_tokens and self.conversation_history:
+            real_tokens = estimate_messages_tokens_rough(self.conversation_history)
+        
+        context_length = status.get("context_length", 0)
+        usage_percent = (real_tokens / context_length * 100) if context_length > 0 else 0
+        usage_level = status.get("usage_level", "normal")
+        
+        # Format usage level with emoji
+        level_emoji = {"normal": "✅", "warning": "⚠️", "critical": "🔴"}.get(usage_level, "❓")
+        
+        print(f"\n📊 Context Status")
+        print(f"   {level_emoji} Status: {usage_level.upper()}")
+        print(f"   Usage: {real_tokens:,} / {context_length:,} tokens ({usage_percent:.1f}%)")
+        print(f"   Compression threshold: {status.get('threshold_tokens', 0):,} tokens ({status.get('threshold_tokens', 0) / context_length * 100:.0f}%)" if context_length else "")
+        print(f"   Warning threshold (85%): {status.get('warning_threshold_tokens', 0):,} tokens")
+        print(f"   Critical threshold (95%): {status.get('critical_threshold_tokens', 0):,} tokens")
+        print(f"   Compressions: {status.get('compression_count', 0)}")
+        
+        # Progress bar
+        bar_length = 40
+        filled = int(bar_length * min(usage_percent, 100) / 100)
+        bar = "█" * filled + "░" * (bar_length - filled)
+        print(f"   [{bar}] {usage_percent:.1f}%")
+        
+        # Remaining budget
+        remaining = max(0, context_length - real_tokens)
+        print(f"   Remaining: {remaining:,} tokens")
+
     def _show_usage(self):
         """Show cumulative token usage for the current session."""
         if not self.agent:

hermes_cli/commands.py

@@ -60,6 +60,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("branch", "Branch the current session (explore a different path)", "Session",
                aliases=("fork",), args_hint="[name]"),
     CommandDef("compress", "Manually compress conversation context", "Session"),
+    CommandDef("context-status", "Show context usage, compression history, and remaining budget", "Session",
+               aliases=("ctx", "context")),
     CommandDef("rollback", "List or restore filesystem checkpoints", "Session",
                args_hint="[number]"),
     CommandDef("stop", "Kill all running background processes", "Session"),

run_agent.py

@@ -5931,18 +5931,28 @@ class AIAgent:
             if messages and messages[-1].get("_flush_sentinel") == _sentinel:
                 messages.pop()
 
-    def _compress_context(self, messages: list, system_message: str, *, approx_tokens: int = None, task_id: str = "default") -> tuple:
+    def _compress_context(self, messages: list, system_message: str, *, approx_tokens: int = None, task_id: str = "default", overflow_triggered: bool = False) -> tuple:
         """Compress conversation context and split the session in SQLite.
 
         Returns:
             (compressed_messages, new_system_prompt) tuple
         """
         _pre_msg_count = len(messages)
-        logger.info(
-            "context compression started: session=%s messages=%d tokens=~%s model=%s",
-            self.session_id or "none", _pre_msg_count,
-            f"{approx_tokens:,}" if approx_tokens else "unknown", self.model,
-        )
+        
+        # Log overflow events separately (#296)
+        if overflow_triggered:
+            logger.warning(
+                "CONTEXT OVERFLOW COMPRESSION: session=%s messages=%d tokens=~%s model=%s "
+                "reason=overflow_triggered",
+                self.session_id or "none", _pre_msg_count,
+                f"{approx_tokens:,}" if approx_tokens else "unknown", self.model,
+            )
+        else:
+            logger.info(
+                "context compression started: session=%s messages=%d tokens=~%s model=%s",
+                self.session_id or "none", _pre_msg_count,
+                f"{approx_tokens:,}" if approx_tokens else "unknown", self.model,
+            )
         # Pre-compression memory flush: let the model save memories before they're lost
         self.flush_memories(messages, min_turns=0)
 
@@ -9001,6 +9011,62 @@ class AIAgent:
                             self._context_pressure_warned = True
                             self._emit_context_pressure(_compaction_progress, _compressor)
 
+                    # ── Poka-yoke: Hard context overflow safeguards (#296) ──────────
+                    _usage_level = _compressor.get_usage_level(_real_tokens)
+                    
+                    # Log context overflow events separately
+                    if _usage_level == "critical" and not getattr(self, '_context_critical_logged', False):
+                        self._context_critical_logged = True
+                        logger.warning(
+                            "CONTEXT CRITICAL: %.1f%% of context used (%d/%d tokens). "
+                            "Tool calls BLOCKED until compression completes.",
+                            _compressor.get_context_usage_percent(_real_tokens),
+                            _real_tokens, _compressor.context_length,
+                        )
+                    elif _usage_level == "warning" and not getattr(self, '_context_warning_logged', False):
+                        self._context_warning_logged = True
+                        logger.warning(
+                            "CONTEXT WARNING: %.1f%% of context used (%d/%d tokens). "
+                            "Auto-compression triggered.",
+                            _compressor.get_context_usage_percent(_real_tokens),
+                            _real_tokens, _compressor.context_length,
+                        )
+                    
+                    # Reset log flags when context drops below warning
+                    if _usage_level == "normal":
+                        self._context_critical_logged = False
+                        self._context_warning_logged = False
+                    
+                    # CRITICAL (95%): Block tool calls — force compression
+                    if self.compression_enabled and _compressor.should_block_tools(_real_tokens):
+                        logger.warning(
+                            "CONTEXT CRITICAL: Blocking tool calls and forcing compression. "
+                            "%.1f%% of context used.",
+                            _compressor.get_context_usage_percent(_real_tokens),
+                        )
+                        messages, active_system_prompt = self._compress_context(
+                            messages, system_message,
+                            approx_tokens=_real_tokens,
+                            task_id=effective_task_id,
+                        )
+                        conversation_history = None
+                        self._session_messages = messages
+                        self._save_session_log(messages)
+                        continue
+                    
+                    # WARNING (85%): Auto-compression trigger (hard-coded threshold)
+                    if self.compression_enabled and _compressor.should_auto_compress(_real_tokens):
+                        messages, active_system_prompt = self._compress_context(
+                            messages, system_message,
+                            approx_tokens=_real_tokens,
+                            task_id=effective_task_id,
+                        )
+                        conversation_history = None
+                        self._session_messages = messages
+                        self._save_session_log(messages)
+                        continue
+
+                    # Standard compression (configurable threshold_percent)
                     if self.compression_enabled and _compressor.should_compress(_real_tokens):
                         messages, active_system_prompt = self._compress_context(
                             messages, system_message,