Replace ContextWindowWarning with generic Warning event

Introduce a reusable Warning { kind, message, details } variant in
AgentEvent so non-fatal warnings (context window usage, deprecation,
etc.) share a single event shape. The context_window warning preserves
all original fields inside the JSON details object.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs-internal/events-strategy.md

@@ -253,7 +253,7 @@ All agent events include `node_id` and `node_label`.
 | `Agent.ToolCallOutputDelta` | `delta` |
 | `Agent.ToolCallCompleted` | `tool_name`, `tool_call_id`, `output`, `is_error` |
 | `Agent.Error` | `error` |
-| `Agent.ContextWindowWarning` | `estimated_tokens`, `context_window_size`, `usage_percent` |
+| `Agent.Warning` | `kind`, `message`, `details` |
 | `Agent.LoopDetected` | _(none)_ |
 | `Agent.TurnLimitReached` | `max_turns` |
 | `Agent.SkillExpanded` | `skill_name` |

docs/execution/context.mdx

@@ -240,6 +240,6 @@ Compaction emits three events to the [event stream](/execution/observability#eve
 
 | Event | When | Key fields |
 |---|---|---|
-| `Agent.ContextWindowWarning` | Token estimate exceeds 80% of context window | `estimated_tokens`, `context_window_size`, `usage_percent` |
+| `Agent.Warning` (kind: `context_window`) | Token estimate exceeds 80% of context window | `kind`, `message`, `details` (contains `estimated_tokens`, `context_window_size`, `usage_percent`) |
 | `Agent.CompactionStarted` | Compaction begins | `estimated_tokens`, `context_window_size` |
 | `Agent.CompactionCompleted` | Summary generated and history replaced | `original_turn_count`, `preserved_turn_count`, `summary_token_estimate`, `tracked_file_count` |

docs/execution/observability.mdx

@@ -45,7 +45,7 @@ Events fall into several categories:
 | `Agent.Error` | `stage`, `error` | Agent-level error |
 | `Agent.LoopDetected` | `stage` | Repeated tool call pattern detected |
 | `Agent.SteeringInjected` | `stage`, `text` | Human steering message injected |
-| `Agent.ContextWindowWarning` | `stage`, `estimated_tokens`, `context_window_size`, `usage_percent` | Token usage exceeds context window threshold |
+| `Agent.Warning` | `stage`, `kind`, `message`, `details` | Non-fatal warning (e.g. context window usage) |
 | `Agent.CompactionStarted` | `stage`, `estimated_tokens`, `context_window_size` | Context compaction triggered |
 | `Agent.CompactionCompleted` | `stage`, `original_turn_count`, `preserved_turn_count`, `summary_token_estimate`, `tracked_file_count` | Context compaction finished |
 | `Agent.LlmRetry` | `stage`, `provider`, `model`, `attempt`, `delay_secs` | LLM API call retried |

lib/crates/fabro-agent/README.md

@@ -227,7 +227,7 @@ profile.register_subagent_tools(manager, factory, 0);
 ## Safety Features
 
 - **Loop detection** -- Detects repeating tool call patterns (period 1, 2, or 3) and injects a steering warning
-- **Context window monitoring** -- Emits `ContextWindowWarning` events when estimated usage exceeds 80%
+- **Context window monitoring** -- Emits `Warning` events (kind `"context_window"`) when estimated usage exceeds 80%
 - **Tool argument validation** -- Validates arguments against JSON Schema before execution
 - **Tool output truncation** -- Per-tool character and line limits with head/tail or tail-only truncation modes
 - **Environment variable filtering** -- `LocalSandbox` strips secrets (`*_API_KEY`, `*_SECRET`, `*_TOKEN`, `*_PASSWORD`, `*_CREDENTIAL`) from subprocess environments

lib/crates/fabro-agent/src/compaction.rs

@@ -9,7 +9,7 @@ use fabro_llm::types::{Message, Request};
 use tracing::debug;
 
 /// Check whether the context window usage exceeds the configured threshold.
-/// Emits a `ContextWindowWarning` event when over the threshold.
+/// Emits a `Warning` event with kind `"context_window"` when over the threshold.
 /// Returns `true` if the threshold is exceeded.
 pub fn check_context_usage(
     system_prompt: &str,
@@ -26,10 +26,17 @@ pub fn check_context_usage(
     if estimated_tokens > threshold {
         emitter.emit(
             session_id.to_owned(),
-            AgentEvent::ContextWindowWarning {
-                estimated_tokens,
-                context_window_size: context_window,
-                usage_percent: estimated_tokens * 100 / context_window,
+            AgentEvent::Warning {
+                kind: "context_window".into(),
+                message: format!(
+                    "Context window usage: {}%",
+                    estimated_tokens * 100 / context_window
+                ),
+                details: serde_json::json!({
+                    "estimated_tokens": estimated_tokens,
+                    "context_window_size": context_window,
+                    "usage_percent": estimated_tokens * 100 / context_window,
+                }),
             },
         );
         true
@@ -345,11 +352,8 @@ mod tests {
         let over = check_context_usage("prompt", &history, &profile, 80, &emitter, "sess");
         assert!(over);
 
-        // Should have emitted a ContextWindowWarning
+        // Should have emitted a Warning
         let event = rx.try_recv().unwrap();
-        assert!(matches!(
-            event.event,
-            AgentEvent::ContextWindowWarning { .. }
-        ));
+        assert!(matches!(event.event, AgentEvent::Warning { .. }));
     }
 }

lib/crates/fabro-agent/src/session.rs

@@ -1608,13 +1608,9 @@ mod tests {
 
         let mut found_warning = false;
         while let Ok(event) = rx.try_recv() {
-            if let AgentEvent::ContextWindowWarning {
-                context_window_size,
-                ..
-            } = &event.event
-            {
+            if let AgentEvent::Warning { details, .. } = &event.event {
                 found_warning = true;
-                assert_eq!(*context_window_size, 100);
+                assert_eq!(details["context_window_size"], 100);
             }
         }
         assert!(found_warning);
@@ -1660,7 +1656,7 @@ mod tests {
 
         let mut found_warning = false;
         while let Ok(event) = rx.try_recv() {
-            if matches!(event.event, AgentEvent::ContextWindowWarning { .. }) {
+            if matches!(event.event, AgentEvent::Warning { .. }) {
                 found_warning = true;
             }
         }

lib/crates/fabro-agent/src/types.rs

@@ -130,10 +130,10 @@ pub enum AgentEvent {
     Error {
         error: crate::error::AgentError,
     },
-    ContextWindowWarning {
-        estimated_tokens: usize,
-        context_window_size: usize,
-        usage_percent: usize,
+    Warning {
+        kind: String,
+        message: String,
+        details: serde_json::Value,
     },
     LoopDetected,
     TurnLimitReached {
@@ -264,17 +264,12 @@ impl AgentEvent {
             Self::Error { error } => {
                 error!(session_id, error = %error, "Agent error");
             }
-            Self::ContextWindowWarning {
-                estimated_tokens,
-                context_window_size,
-                usage_percent,
-            } => {
+            Self::Warning { kind, message, .. } => {
                 warn!(
                     session_id,
-                    estimated_tokens,
-                    context_window_size,
-                    usage_percent,
-                    "Context window usage high"
+                    kind = kind.as_str(),
+                    message = message.as_str(),
+                    "Warning"
                 );
             }
             Self::LoopDetected => {

lib/crates/fabro-cli/src/commands/run_progress.rs

@@ -1380,7 +1380,13 @@ impl ProgressUI {
                 }
                 self.on_tool_call_completed(stage_node_id, tool_call_id, *is_error);
             }
-            AgentEvent::ContextWindowWarning { usage_percent, .. } if self.verbose => {
+            AgentEvent::Warning { kind, details, .. }
+                if kind == "context_window" && self.verbose =>
+            {
+                let usage_percent = details
+                    .get("usage_percent")
+                    .and_then(|v| v.as_u64())
+                    .unwrap_or(0);
                 let yellow = Style::new().yellow();
                 self.insert_info_line_for_stage(
                     stage_node_id,