feat(runtime): add TurnInterruptSignal for graceful turn interruption

Add a shared interrupt flag that the conversation loop polls at safe
points: before each model request, when a stream call fails, and before
each pending tool execution. When the flag is set the turn winds down
instead of failing: tools that have not run yet receive synthesized
error tool_results so every tool_use stays answered and the session
remains valid for the next request.

TurnSummary gains an interrupted field so callers can distinguish a
user-initiated stop from a completed turn, and the session tracer
records a turn_interrupted event (with iteration and phase) instead of
turn_failed / turn_completed.

Groundwork for Esc-to-interrupt (#3196): a follow-up wires this signal
into the streaming API client and the CLI input listener.

rust/crates/runtime/src/conversation.rs

@@ -1,5 +1,9 @@
 use std::collections::BTreeMap;
 use std::fmt::{Display, Formatter};
+use std::sync::{
+    atomic::{AtomicBool, Ordering},
+    Arc,
+};
 
 use serde_json::{Map, Value};
 use telemetry::SessionTracer;
@@ -53,6 +57,41 @@ pub struct PromptCacheEvent {
     pub token_drop: u32,
 }
 
+/// Shared flag used to request graceful interruption of a running turn.
+///
+/// Cloning shares the underlying flag, mirroring
+/// [`HookAbortSignal`](crate::hooks::HookAbortSignal). An input listener
+/// (e.g. Esc or Ctrl+C handling in the CLI) sets the flag while the
+/// conversation loop and the streaming API client poll it at safe points.
+/// When the flag is observed, the turn winds down without treating the
+/// stop as a failure: pending tool calls receive synthesized error
+/// results so the session stays consistent, and [`TurnSummary`] reports
+/// `interrupted: true`.
+#[derive(Debug, Clone, Default)]
+pub struct TurnInterruptSignal {
+    interrupted: Arc<AtomicBool>,
+}
+
+impl TurnInterruptSignal {
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn interrupt(&self) {
+        self.interrupted.store(true, Ordering::SeqCst);
+    }
+
+    #[must_use]
+    pub fn is_interrupted(&self) -> bool {
+        self.interrupted.load(Ordering::SeqCst)
+    }
+
+    pub fn reset(&self) {
+        self.interrupted.store(false, Ordering::SeqCst);
+    }
+}
+
 /// Minimal streaming API contract required by [`ConversationRuntime`].
 pub trait ApiClient {
     fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError>;
@@ -118,6 +157,7 @@ pub struct TurnSummary {
     pub iterations: usize,
     pub usage: TokenUsage,
     pub auto_compaction: Option<AutoCompactionEvent>,
+    pub interrupted: bool,
 }
 
 /// Details about automatic session compaction applied during a turn.
@@ -138,6 +178,7 @@ pub struct ConversationRuntime<C, T> {
     hook_runner: HookRunner,
     auto_compaction_input_tokens_threshold: u32,
     hook_abort_signal: HookAbortSignal,
+    turn_interrupt_signal: TurnInterruptSignal,
     hook_progress_reporter: Option<Box<dyn HookProgressReporter>>,
     session_tracer: Option<SessionTracer>,
 }
@@ -187,6 +228,7 @@ where
             hook_runner: HookRunner::from_feature_config(feature_config),
             auto_compaction_input_tokens_threshold: auto_compaction_threshold_from_env(),
             hook_abort_signal: HookAbortSignal::default(),
+            turn_interrupt_signal: TurnInterruptSignal::default(),
             hook_progress_reporter: None,
             session_tracer: None,
         }
@@ -217,6 +259,15 @@ where
         self
     }
 
+    #[must_use]
+    pub fn with_turn_interrupt_signal(
+        mut self,
+        turn_interrupt_signal: TurnInterruptSignal,
+    ) -> Self {
+        self.turn_interrupt_signal = turn_interrupt_signal;
+        self
+    }
+
     #[must_use]
     pub fn with_hook_progress_reporter(
         mut self,
@@ -350,8 +401,14 @@ where
         let mut prompt_cache_events = Vec::new();
         let mut iterations = 0;
         let mut auto_compaction = None;
+        let mut interrupted = false;
 
         loop {
+            if self.turn_interrupt_signal.is_interrupted() {
+                self.record_turn_interrupted(iterations, "before_request");
+                interrupted = true;
+                break;
+            }
             iterations += 1;
             if iterations > self.max_iterations {
                 let error = RuntimeError::new(
@@ -368,6 +425,14 @@ where
             let events = match self.api_client.stream(request) {
                 Ok(events) => events,
                 Err(error) => {
+                    if self.turn_interrupt_signal.is_interrupted() {
+                        // The client aborted because the user interrupted the
+                        // turn; any partial response is discarded and the stop
+                        // is reported as an interruption rather than a failure.
+                        self.record_turn_interrupted(iterations, "during_request");
+                        interrupted = true;
+                        break;
+                    }
                     self.record_turn_failed(iterations, &error);
                     return Err(error);
                 }
@@ -416,6 +481,25 @@ where
             }
 
             for (tool_use_id, tool_name, input) in pending_tool_uses {
+                if interrupted || self.turn_interrupt_signal.is_interrupted() {
+                    // Every pending tool_use must still receive a tool_result
+                    // so the session stays valid for the next request.
+                    if !interrupted {
+                        self.record_turn_interrupted(iterations, "before_tool");
+                        interrupted = true;
+                    }
+                    let result_message = ConversationMessage::tool_result(
+                        tool_use_id,
+                        tool_name,
+                        "Interrupted by user before this tool could run.",
+                        true,
+                    );
+                    self.session
+                        .push_message(result_message.clone())
+                        .map_err(|error| RuntimeError::new(error.to_string()))?;
+                    tool_results.push(result_message);
+                    continue;
+                }
                 let pre_hook_result = self.run_pre_tool_use_hook(&tool_name, &input);
                 let effective_input = pre_hook_result
                     .updated_input()
@@ -515,6 +599,10 @@ where
                 self.record_tool_finished(iterations, &result_message);
                 tool_results.push(result_message);
             }
+
+            if interrupted {
+                break;
+            }
         }
 
         let summary = TurnSummary {
@@ -524,8 +612,11 @@ where
             iterations,
             usage: self.usage_tracker.cumulative_usage(),
             auto_compaction,
+            interrupted,
         };
-        self.record_turn_completed(&summary);
+        if !interrupted {
+            self.record_turn_completed(&summary);
+        }
 
         Ok(summary)
     }
@@ -689,6 +780,17 @@ where
         session_tracer.record("turn_completed", attributes);
     }
 
+    fn record_turn_interrupted(&self, iteration: usize, phase: &str) {
+        let Some(session_tracer) = &self.session_tracer else {
+            return;
+        };
+
+        let mut attributes = Map::new();
+        attributes.insert("iteration".to_string(), Value::from(iteration as u64));
+        attributes.insert("phase".to_string(), Value::String(phase.to_string()));
+        session_tracer.record("turn_interrupted", attributes);
+    }
+
     fn record_turn_failed(&self, iteration: usize, error: &RuntimeError) {
         let Some(session_tracer) = &self.session_tracer else {
             return;
@@ -850,7 +952,8 @@ mod tests {
     use super::{
         build_assistant_message, parse_auto_compaction_threshold, ApiClient, ApiRequest,
         AssistantEvent, AutoCompactionEvent, ConversationRuntime, PromptCacheEvent, RuntimeError,
-        StaticToolExecutor, ToolExecutor, DEFAULT_AUTO_COMPACTION_INPUT_TOKENS_THRESHOLD,
+        StaticToolExecutor, ToolExecutor, TurnInterruptSignal,
+        DEFAULT_AUTO_COMPACTION_INPUT_TOKENS_THRESHOLD,
     };
     use crate::compact::CompactionConfig;
     use crate::config::{RuntimeFeatureConfig, RuntimeHookConfig};
@@ -1875,4 +1978,165 @@ mod tests {
         // then
         assert_eq!(error.to_string(), "upstream failed");
     }
+
+    #[test]
+    fn interrupt_before_first_request_skips_the_api_call() {
+        struct UnreachableApi;
+
+        impl ApiClient for UnreachableApi {
+            fn stream(
+                &mut self,
+                _request: ApiRequest,
+            ) -> Result<Vec<AssistantEvent>, RuntimeError> {
+                unreachable!("interrupted turn must not reach the API")
+            }
+        }
+
+        // given
+        let interrupt = TurnInterruptSignal::new();
+        interrupt.interrupt();
+        let mut runtime = ConversationRuntime::new(
+            Session::new(),
+            UnreachableApi,
+            StaticToolExecutor::new(),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
+            vec!["system".to_string()],
+        )
+        .with_turn_interrupt_signal(interrupt);
+
+        // when
+        let summary = runtime
+            .run_turn("hello", None)
+            .expect("interruption should not be reported as a failure");
+
+        // then
+        assert!(summary.interrupted);
+        assert_eq!(summary.iterations, 0);
+        assert!(summary.assistant_messages.is_empty());
+        assert!(summary.tool_results.is_empty());
+        assert_eq!(summary.auto_compaction, None);
+        assert_eq!(runtime.session().messages.len(), 1);
+        assert_eq!(runtime.session().messages[0].role, MessageRole::User);
+    }
+
+    #[test]
+    fn interrupt_after_stream_synthesizes_results_for_pending_tools() {
+        struct ToolUseApi {
+            interrupt: TurnInterruptSignal,
+        }
+
+        impl ApiClient for ToolUseApi {
+            fn stream(
+                &mut self,
+                _request: ApiRequest,
+            ) -> Result<Vec<AssistantEvent>, RuntimeError> {
+                // Simulate the user pressing Esc while the response streams in.
+                self.interrupt.interrupt();
+                Ok(vec![
+                    AssistantEvent::TextDelta("Running the tool.".to_string()),
+                    AssistantEvent::ToolUse {
+                        id: "tool-1".to_string(),
+                        name: "add".to_string(),
+                        input: "2,2".to_string(),
+                    },
+                    AssistantEvent::MessageStop,
+                ])
+            }
+        }
+
+        // given
+        let interrupt = TurnInterruptSignal::new();
+        let mut runtime = ConversationRuntime::new(
+            Session::new(),
+            ToolUseApi {
+                interrupt: interrupt.clone(),
+            },
+            StaticToolExecutor::new()
+                .register("add", |_input| panic!("interrupted tool must not run")),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
+            vec!["system".to_string()],
+        )
+        .with_turn_interrupt_signal(interrupt);
+
+        // when
+        let summary = runtime
+            .run_turn("what is 2 + 2?", None)
+            .expect("interruption should not be reported as a failure");
+
+        // then
+        assert!(summary.interrupted);
+        assert_eq!(summary.iterations, 1);
+        assert_eq!(summary.assistant_messages.len(), 1);
+        assert_eq!(summary.tool_results.len(), 1);
+        assert!(matches!(
+            &summary.tool_results[0].blocks[0],
+            ContentBlock::ToolResult {
+                tool_use_id,
+                is_error: true,
+                output,
+                ..
+            } if tool_use_id == "tool-1" && output.contains("Interrupted by user")
+        ));
+        // user text, assistant tool_use, synthesized tool_result
+        assert_eq!(runtime.session().messages.len(), 3);
+        assert!(matches!(
+            runtime.session().messages[2].blocks[0],
+            ContentBlock::ToolResult { is_error: true, .. }
+        ));
+    }
+
+    #[test]
+    fn stream_error_during_interrupt_is_reported_as_interruption() {
+        struct AbortedApi {
+            interrupt: TurnInterruptSignal,
+        }
+
+        impl ApiClient for AbortedApi {
+            fn stream(
+                &mut self,
+                _request: ApiRequest,
+            ) -> Result<Vec<AssistantEvent>, RuntimeError> {
+                // Simulate the streaming client aborting the connection after
+                // observing the interrupt flag mid-stream.
+                self.interrupt.interrupt();
+                Err(RuntimeError::new("request aborted"))
+            }
+        }
+
+        // given
+        let sink = Arc::new(MemoryTelemetrySink::default());
+        let tracer = SessionTracer::new("session-interrupt", sink.clone());
+        let interrupt = TurnInterruptSignal::new();
+        let mut runtime = ConversationRuntime::new(
+            Session::new(),
+            AbortedApi {
+                interrupt: interrupt.clone(),
+            },
+            StaticToolExecutor::new(),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
+            vec!["system".to_string()],
+        )
+        .with_turn_interrupt_signal(interrupt)
+        .with_session_tracer(tracer);
+
+        // when
+        let summary = runtime
+            .run_turn("hello", None)
+            .expect("interrupt-driven aborts should not surface as errors");
+
+        // then
+        assert!(summary.interrupted);
+        assert!(summary.assistant_messages.is_empty());
+        let trace_names = sink
+            .events()
+            .iter()
+            .filter_map(|event| match event {
+                TelemetryEvent::SessionTrace(trace) => Some(trace.name.clone()),
+                _ => None,
+            })
+            .collect::<Vec<_>>();
+        assert!(trace_names.iter().any(|name| name == "turn_interrupted"));
+        assert!(!trace_names.iter().any(|name| name == "turn_failed"));
+        assert!(!trace_names.iter().any(|name| name == "turn_completed"));
+    }
 }

rust/crates/runtime/src/lib.rs

@@ -82,7 +82,7 @@ pub use config_validate::{
 pub use conversation::{
     auto_compaction_threshold_from_env, ApiClient, ApiRequest, AssistantEvent, AutoCompactionEvent,
     ConversationRuntime, PromptCacheEvent, RuntimeError, StaticToolExecutor, ToolError,
-    ToolExecutor, TurnSummary,
+    ToolExecutor, TurnInterruptSignal, TurnSummary,
 };
 pub use file_ops::{
     edit_file, edit_file_in_workspace, glob_search, glob_search_in_workspace, grep_search,