feat: Suppor mid-turn auto compressions so sufficiently large histories don't create deadlocks for max input tokens

assets/config-template.yaml

@@ -186,6 +186,7 @@ save_session: null               # Controls the persistence of the session. If t
 compression_threshold: 4000      # Compress the session when the token count reaches or exceeds this threshold
 compression_keep_last: 0         # Number of most-recent messages to keep visible after compression (0 = compress all messages)
 compression_model: null          # Model to use for session compression (e.g. 'openai:gpt-4o-mini'); defaults to the session's current model
+compression_safety_valve: 0.8    # Force compression when the last real request's prompt size (or the local estimate) reaches this fraction of the model's max_input_tokens, even if compression_threshold would not fire (0 = disabled)
 summarization_prompt: null       # The text prompt used for creating a concise summary of session messages. If null, uses built-in default
 summary_context_prompt: null     # The text prompt used for including the summary of the entire session as context to the model. If null, uses built-in default
 max_tool_result_chars: null      # Cap on tool result characters forwarded to the model per call (null = no cap)

src/client/common.rs

@@ -420,6 +420,15 @@ impl TokenUsage {
             && self.cache_read_input_tokens.is_none()
     }
 
+    /// Prompt-side tokens the API reported for one request: fresh input plus
+    /// cache writes and cache reads. The buckets are disjoint (see
+    /// [`Self::cost_usd`]), so the terms sum without adjustment.
+    pub fn total_prompt_tokens(&self) -> u64 {
+        self.input_tokens.unwrap_or(0)
+            + self.cache_creation_input_tokens.unwrap_or(0)
+            + self.cache_read_input_tokens.unwrap_or(0)
+    }
+
     pub fn accumulate(&mut self, other: &TokenUsage) {
         fn add(acc: &mut Option<u64>, value: Option<u64>) {
             if let Some(value) = value {

src/client/openai.rs

@@ -869,17 +869,22 @@ fn reasoning_thinking_block(item: &Value) -> Option<ThinkingBlock> {
 }
 
 /// OpenAI reports cached input under `input_tokens_details` and has no
-/// cache-creation concept, so that field stays `None`.
+/// cache-creation concept, so that field stays `None`. The reported
+/// `input_tokens` total already includes cached reads, so cached tokens are
+/// subtracted here to keep the buckets disjoint like other providers.
 fn openai_parse_responses_usage(usage: &Value) -> Option<TokenUsage> {
     if !usage.is_object() {
         return None;
     }
 
+    let cached_tokens = usage["input_tokens_details"]["cached_tokens"].as_u64();
     Some(TokenUsage {
-        input_tokens: usage["input_tokens"].as_u64(),
+        input_tokens: usage["input_tokens"]
+            .as_u64()
+            .map(|v| v.saturating_sub(cached_tokens.unwrap_or(0))),
         output_tokens: usage["output_tokens"].as_u64(),
         cache_creation_input_tokens: None,
-        cache_read_input_tokens: usage["input_tokens_details"]["cached_tokens"].as_u64(),
+        cache_read_input_tokens: cached_tokens,
     })
 }
 
@@ -1267,12 +1272,31 @@ mod tests {
         let output = openai_extract_responses(&data).unwrap();
 
         let usage = output.usage.unwrap();
-        assert_eq!(usage.input_tokens, Some(100));
+        assert_eq!(usage.input_tokens, Some(40));
         assert_eq!(usage.output_tokens, Some(20));
         assert_eq!(usage.cache_creation_input_tokens, None);
         assert_eq!(usage.cache_read_input_tokens, Some(60));
     }
 
+    #[test]
+    fn extract_responses_usage_without_cached_details_keeps_input_intact() {
+        let data = json!({
+            "output": [
+                { "type": "message", "content": [{ "type": "output_text", "text": "answer" }] },
+            ],
+            "usage": {
+                "input_tokens": 100,
+                "output_tokens": 20,
+            }
+        });
+
+        let output = openai_extract_responses(&data).unwrap();
+
+        let usage = output.usage.unwrap();
+        assert_eq!(usage.input_tokens, Some(100));
+        assert_eq!(usage.cache_read_input_tokens, None);
+    }
+
     #[test]
     fn extract_responses_without_usage_leaves_it_none() {
         let data = json!({
@@ -1357,7 +1381,7 @@ mod tests {
         assert!(done);
         let (_, _, _, usage) = handler.take();
         let usage = usage.unwrap();
-        assert_eq!(usage.input_tokens, Some(100));
+        assert_eq!(usage.input_tokens, Some(40));
         assert_eq!(usage.output_tokens, Some(20));
         assert_eq!(usage.cache_creation_input_tokens, None);
         assert_eq!(usage.cache_read_input_tokens, Some(60));
@@ -1467,7 +1491,7 @@ mod tests {
         let (text, _, _, usage) = handler.take();
         assert_eq!(text, "partial answer");
         let usage = usage.expect("usage should be recorded for truncated turns");
-        assert_eq!(usage.input_tokens, Some(50));
+        assert_eq!(usage.input_tokens, Some(25));
         assert_eq!(usage.output_tokens, Some(10));
         assert_eq!(usage.cache_read_input_tokens, Some(25));
     }

src/config/app_config.rs

@@ -71,6 +71,7 @@ pub struct AppConfig {
     pub compression_threshold: usize,
     pub compression_keep_last: usize,
     pub compression_model: Option<String>,
+    pub compression_safety_valve: f32,
     pub summarization_prompt: Option<String>,
     pub summary_context_prompt: Option<String>,
     pub max_tool_result_chars: Option<usize>,
@@ -164,6 +165,7 @@ impl Default for AppConfig {
             compression_threshold: 4000,
             compression_keep_last: 0,
             compression_model: None,
+            compression_safety_valve: 0.8,
             summarization_prompt: None,
             summary_context_prompt: None,
             max_tool_result_chars: None,
@@ -270,6 +272,7 @@ impl AppConfig {
             compression_threshold: config.compression_threshold,
             compression_keep_last: config.compression_keep_last,
             compression_model: config.compression_model,
+            compression_safety_valve: config.compression_safety_valve,
             summarization_prompt: config.summarization_prompt,
             summary_context_prompt: config.summary_context_prompt,
             max_tool_result_chars: config.max_tool_result_chars,

src/config/input.rs

@@ -68,6 +68,34 @@ impl Input {
         })
     }
 
+    /// An input carrying only `text`: no session history, no RAG, and no
+    /// function declarations. For auxiliary LLM calls (such as session
+    /// summarization) whose requests must not grow with the session.
+    pub fn detached_text(ctx: &RequestContext, text: &str) -> Self {
+        let mut role = Role::new("", "");
+        role.set_model(ctx.current_model().clone());
+        Self {
+            app_config: Arc::clone(&ctx.app.config),
+            stream_enabled: ctx.app.config.stream,
+            session: None,
+            rag: None,
+            functions: None,
+            text: text.to_string(),
+            raw: (text.to_string(), vec![]),
+            patched_text: None,
+            last_reply: None,
+            continue_output: None,
+            regenerate: false,
+            medias: Default::default(),
+            data_urls: Default::default(),
+            tool_calls: None,
+            role,
+            rag_name: None,
+            with_session: false,
+            with_agent: false,
+        }
+    }
+
     pub async fn from_files(
         ctx: &RequestContext,
         raw_text: &str,
@@ -329,6 +357,14 @@ impl Input {
         }
     }
 
+    /// Re-captures `ctx.session` in place of the snapshot taken at
+    /// construction. Session compression mutates the ctx's session, not the
+    /// snapshot, so an input that outlives a compression must refresh before
+    /// building its next request or it re-sends the pre-compression history.
+    pub fn refresh_session(&mut self, ctx: &RequestContext) {
+        self.session = ctx.session.clone();
+    }
+
     pub fn session_mut<'a>(&self, session: &'a mut Option<Session>) -> Option<&'a mut Session> {
         if self.with_session {
             session.as_mut()
@@ -660,6 +696,60 @@ mod tests {
         assert!(!with_session);
     }
 
+    #[test]
+    fn detached_text_input_carries_no_session_history_or_functions() {
+        let mut ctx = create_test_ctx();
+        ctx.session = Some(Session::default());
+        let earlier = Input::from_str(&ctx, "earlier question", None).unwrap();
+        ctx.session
+            .as_mut()
+            .unwrap()
+            .add_message(&earlier, "earlier answer")
+            .unwrap();
+
+        let detached = Input::detached_text(&ctx, "summarize the above");
+        let messages = detached.build_messages().unwrap();
+        assert_eq!(messages.len(), 1, "must contain only the detached text");
+        assert!(messages[0].role.is_user());
+        assert_eq!(messages[0].content.to_text(), "summarize the above");
+        assert!(detached.declared_function_names().is_empty());
+        assert!(!detached.with_session());
+        assert!(!detached.with_agent());
+    }
+
+    #[test]
+    fn refresh_session_rebuilds_from_compressed_session() {
+        let mut ctx = create_test_ctx();
+        ctx.session = Some(Session::default());
+        for i in 0..4 {
+            let question =
+                Input::from_str(&ctx, &format!("question {i}: {}", "x".repeat(500)), None).unwrap();
+            ctx.session
+                .as_mut()
+                .unwrap()
+                .add_message(&question, &"y".repeat(500))
+                .unwrap();
+        }
+
+        let mut input = Input::from_str(&ctx, "retry me", None).unwrap();
+        let stale = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+
+        ctx.session.as_mut().unwrap().compress("recap".into(), 1);
+
+        assert_eq!(
+            serde_json::to_string(&input.build_messages().unwrap()).unwrap(),
+            stale,
+            "the snapshot captured at construction must be untouched by compression"
+        );
+
+        input.refresh_session(&ctx);
+        let refreshed = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+        assert!(
+            refreshed.len() < stale.len(),
+            "a refreshed input must build a strictly smaller request after compression"
+        );
+    }
+
     #[test]
     fn resolve_paths_detects_last_reply_syntax() {
         let loaders = HashMap::new();

src/config/mod.rs

@@ -286,6 +286,7 @@ pub struct Config {
     pub compression_threshold: usize,
     pub compression_keep_last: usize,
     pub compression_model: Option<String>,
+    pub compression_safety_valve: f32,
     pub summarization_prompt: Option<String>,
     pub summary_context_prompt: Option<String>,
     pub max_tool_result_chars: Option<usize>,
@@ -376,6 +377,7 @@ impl Default for Config {
             compression_threshold: 4000,
             compression_keep_last: 0,
             compression_model: None,
+            compression_safety_valve: 0.8,
             summarization_prompt: None,
             summary_context_prompt: None,
             max_tool_result_chars: None,
@@ -1379,6 +1381,7 @@ clients:
         assert!(cfg.mcp_server_support);
         assert_eq!(cfg.compression_threshold, 4000);
         assert!(cfg.compression_model.is_none());
+        assert_eq!(cfg.compression_safety_valve, 0.8);
         assert_eq!(cfg.rag_top_k, 5);
         assert!(cfg.save_shell_history);
         assert_eq!(cfg.keybindings, "emacs");
@@ -1419,6 +1422,7 @@ clients:
         assert!(cfg.mapping_tools.is_empty());
         assert!(cfg.document_loaders.is_empty());
         assert_eq!(cfg.compression_threshold, 4000);
+        assert_eq!(cfg.compression_safety_valve, 0.8);
         assert!(cfg.theme.is_none());
         assert_eq!(cfg.clients.len(), 1);
     }

src/config/request_context.rs

@@ -16,7 +16,9 @@ use super::{
     list_agents_with_descriptions, memory, paths,
 };
 use super::{MessageContentToolCalls, prompts};
-use crate::client::{Model, ModelType, TokenUsage, list_models};
+use crate::client::{
+    Message, MessageContent, MessageRole, Model, ModelType, TokenUsage, list_models,
+};
 use crate::function::{
     FunctionDeclaration, Functions, ToolCallTracker, ToolResult,
     agents::AGENT_FUNCTION_PREFIX,
@@ -51,7 +53,7 @@ use super::memory::{
 };
 use crate::graph;
 use crate::hooks::{self, HookEvent};
-use anyhow::{Context, Error, Result, bail};
+use anyhow::{Context, Error, Result, anyhow, bail};
 use colored::Colorize;
 use gman::providers::SupportedProvider;
 use hooks::{McpServerHooks, RagSyncHooks};
@@ -68,7 +70,7 @@ use std::io::Write;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::time::Duration;
-use std::{env, fs};
+use std::{env, fs, slice};
 
 pub(crate) fn expand_enabled_mcp_server_ids(
     app: &AppConfig,
@@ -316,6 +318,13 @@ pub struct RequestContext {
     pub last_token_usage: Option<TokenUsage>,
     pub last_cost: Option<f64>,
 
+    /// Prompt-side tokens from the most recent request that reported usage.
+    /// Unlike `last_token_usage`, this is never clobbered by requests that
+    /// report nothing (e.g. a stream rejected before `message_start`), so the
+    /// compression safety valve always sees the last real prompt
+    /// measurement.
+    pub last_prompt_token_usage: Option<usize>,
+
     pub tool_scope: ToolScope,
 
     pub declared_function_names: HashSet<String>,
@@ -385,6 +394,7 @@ impl RequestContext {
             last_message: None,
             last_token_usage: None,
             last_cost: None,
+            last_prompt_token_usage: None,
             tool_scope: ToolScope::default(),
             declared_function_names: Default::default(),
             node_job_scope: None,
@@ -457,6 +467,7 @@ impl RequestContext {
             last_message: None,
             last_token_usage: None,
             last_cost: None,
+            last_prompt_token_usage: None,
             tool_scope: ToolScope {
                 functions,
                 mcp_runtime,
@@ -522,6 +533,7 @@ impl RequestContext {
             last_message: self.last_message.clone(),
             last_token_usage: self.last_token_usage.clone(),
             last_cost: self.last_cost,
+            last_prompt_token_usage: self.last_prompt_token_usage,
             tool_scope: self.tool_scope.clone(),
             declared_function_names: self.declared_function_names.clone(),
             node_job_scope: None,
@@ -577,6 +589,7 @@ impl RequestContext {
             last_message: None,
             last_token_usage: None,
             last_cost: None,
+            last_prompt_token_usage: None,
             tool_scope: ToolScope {
                 functions: Functions::default(),
                 mcp_runtime: McpRuntime::default(),
@@ -2294,6 +2307,14 @@ impl RequestContext {
                     session.accumulate_cost(cost);
                 }
             }
+            // Partial usage from an aborted stream lands here on purpose:
+            // it is a genuine API-reported prompt measurement, not noise.
+            // Only usage-less recordings (failed requests) leave the
+            // previous measurement in place.
+            let prompt_tokens = usage.total_prompt_tokens();
+            if prompt_tokens > 0 {
+                self.last_prompt_token_usage = Some(prompt_tokens as usize);
+            }
         }
         self.last_token_usage = usage;
         self.last_cost = cost;
@@ -5030,7 +5051,7 @@ impl RequestContext {
         } else {
             role_name = self.role.as_ref().map(|v| v.name().to_string());
         }
-        let name = role_name.ok_or_else(|| anyhow::anyhow!("No role"))?;
+        let name = role_name.ok_or_else(|| anyhow!("No role"))?;
         self.upsert_role(app, &name)?;
         self.use_role(app, &name, abort_signal).await
     }
@@ -5243,14 +5264,31 @@ impl RequestContext {
     }
 
     pub async fn compress_session(&mut self) -> Result<()> {
-        match self.session.as_ref() {
+        async fn timed_fetch(input: Input) -> Result<String> {
+            tokio::time::timeout(Duration::from_secs(120), input.fetch_chat_text())
+                .await
+                .map_err(|_| anyhow!("Compression LLM call timed out after 120 s"))?
+        }
+
+        self.compress_session_impl(timed_fetch).await
+    }
+
+    /// The summarization flow with the LLM call injectable, so tests can
+    /// capture and answer each chunk request without a network round trip.
+    async fn compress_session_impl<F, Fut>(&mut self, fetch: F) -> Result<()>
+    where
+        F: Fn(Input) -> Fut,
+        Fut: Future<Output = Result<String>>,
+    {
+        let session_window = match self.session.as_ref() {
             Some(session) => {
                 if !session.has_user_messages() {
                     bail!("No need to compress since there are no messages in the session")
                 }
+                session.model().max_input_tokens()
             }
             None => bail!("No session"),
-        }
+        };
 
         let prompt = self
             .app
@@ -5259,44 +5297,79 @@ impl RequestContext {
             .clone()
             .unwrap_or_else(|| SUMMARIZATION_PROMPT.into());
 
-        async fn timed_fetch(input: &Input) -> Result<String> {
-            tokio::time::timeout(Duration::from_secs(120), input.fetch_chat_text())
-                .await
-                .map_err(|_| anyhow::anyhow!("Compression LLM call timed out after 120 s"))?
-        }
-
-        let mut input = Input::from_str(self, &prompt, None)?;
-        let compression_model_id = self
+        let compression_model = match self
             .compression_model()
-            .filter(|id| *id != self.current_model().id());
-        let summary = match compression_model_id {
+            .filter(|id| *id != self.current_model().id())
+        {
             Some(model_id) => {
                 match Model::retrieve_model(self.app.config.as_ref(), &model_id, ModelType::Chat) {
-                    Ok(model) => {
-                        input.set_role_model(model);
-                        match timed_fetch(&input).await {
-                            Ok(summary) => summary,
-                            Err(err) => {
-                                eprintln!(
-                                    "Warning: compression model '{model_id}' failed: {err}; falling back to the current model '{}' for session compression",
-                                    self.current_model().id()
-                                );
-                                let input = Input::from_str(self, &prompt, None)?;
-                                timed_fetch(&input).await?
-                            }
-                        }
-                    }
+                    Ok(model) => Some(model),
                     Err(err) => {
                         eprintln!(
                             "Warning: compression model '{model_id}' could not be used ({err}); falling back to the current model '{}' for session compression",
                             self.current_model().id()
                         );
-                        timed_fetch(&input).await?
+                        None
                     }
                 }
             }
-            None => timed_fetch(&input).await?,
+            None => None,
         };
+
+        // Cap each chunk well below the summarizer's window so every request
+        // (prior summary + chunk + prompt) fits, even for sessions that have
+        // grown past the window itself.
+        let window = compression_model
+            .as_ref()
+            .and_then(|model| model.max_input_tokens())
+            .or(session_window)
+            .unwrap_or(SUMMARIZATION_WINDOW_FALLBACK_TOKENS);
+        let chunk_budget = (window as f32 * SUMMARIZATION_CHUNK_BUDGET_RATIO) as usize;
+
+        let keep_last = self.compression_keep_last();
+        let chunks: Vec<String> = self
+            .session
+            .as_ref()
+            .map(|session| {
+                // `Session::compress` archives every message when `keep_last`
+                // covers the whole list, so summarize them all in that case.
+                let mut foldable = session.foldable_messages(keep_last);
+                if foldable.is_empty() {
+                    foldable = session.foldable_messages(0);
+                }
+                slice_summarization_chunks(session.model(), foldable, chunk_budget)
+                    .into_iter()
+                    .map(render_summarization_chunk)
+                    .collect()
+            })
+            .unwrap_or_default();
+
+        // Fold forward, oldest chunk first: each call carries the summary of
+        // everything before it, and the summarization prompt instructs the
+        // model to merge that prior summary into the new one.
+        let mut summary = String::new();
+        for chunk in &chunks {
+            let request = compose_summarization_request(&prompt, &summary, chunk);
+            summary = match compression_model.as_ref() {
+                Some(model) => {
+                    let mut input = Input::detached_text(self, &request);
+                    input.set_role_model(model.clone());
+                    match fetch(input).await {
+                        Ok(summary) => summary,
+                        Err(err) => {
+                            eprintln!(
+                                "Warning: compression model '{}' failed: {err}; falling back to the current model '{}' for session compression",
+                                model.id(),
+                                self.current_model().id()
+                            );
+                            fetch(Input::detached_text(self, &request)).await?
+                        }
+                    }
+                }
+                None => fetch(Input::detached_text(self, &request)).await?,
+            };
+        }
+
         let summary_context_prompt = self
             .app
             .config
@@ -5541,6 +5614,73 @@ fn fork_base_name(name: &str) -> &str {
     name
 }
 
+/// Assumed summarizer window when neither the compression model nor the
+/// session model declares `max_input_tokens`.
+const SUMMARIZATION_WINDOW_FALLBACK_TOKENS: usize = 200_000;
+/// Share of the summarizer's window a single history chunk may occupy; the
+/// rest is headroom for the summarization prompt, the folded-forward prior
+/// summary, and the response.
+const SUMMARIZATION_CHUNK_BUDGET_RATIO: f32 = 0.6;
+
+/// Splits `messages` into consecutive runs whose estimated tokens each stay
+/// within `budget`. A message that alone exceeds the budget forms its own
+/// chunk: messages are the smallest unit summarization can fold.
+fn slice_summarization_chunks<'a>(
+    model: &Model,
+    messages: &'a [Message],
+    budget: usize,
+) -> Vec<&'a [Message]> {
+    let mut chunks = vec![];
+    let mut start = 0;
+    let mut tokens = 0;
+    for (i, message) in messages.iter().enumerate() {
+        let message_tokens = model.total_tokens(slice::from_ref(message));
+        if i > start && tokens + message_tokens > budget {
+            chunks.push(&messages[start..i]);
+            start = i;
+            tokens = 0;
+        }
+        tokens += message_tokens;
+    }
+    if start < messages.len() {
+        chunks.push(&messages[start..]);
+    }
+    chunks
+}
+
+/// Renders messages as a role-prefixed transcript for the summarizer. Tool
+/// transcripts are serialized whole rather than dropped: their results often
+/// carry the facts the summary must preserve.
+fn render_summarization_chunk(messages: &[Message]) -> String {
+    messages
+        .iter()
+        .map(|message| {
+            let role = match message.role {
+                MessageRole::System => "SYSTEM",
+                MessageRole::Assistant => "ASSISTANT",
+                MessageRole::User => "USER",
+                MessageRole::Tool => "TOOL",
+            };
+            let content = match &message.content {
+                MessageContent::ToolCalls(tool_calls) => {
+                    serde_json::to_string(tool_calls).unwrap_or_else(|_| tool_calls.text.clone())
+                }
+                content => content.to_text(),
+            };
+            format!("{role}: {content}")
+        })
+        .collect::<Vec<_>>()
+        .join("\n\n")
+}
+
+fn compose_summarization_request(prompt: &str, prior_summary: &str, chunk: &str) -> String {
+    if prior_summary.is_empty() {
+        format!("{chunk}\n\n{prompt}")
+    } else {
+        format!("{SUMMARY_CONTEXT_PROMPT}{prior_summary}\n\n{chunk}\n\n{prompt}")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::super::mcp_factory::McpFactory;
@@ -5755,6 +5895,30 @@ mod tests {
         assert!(!vars.contains_key("total_cost"));
     }
 
+    #[test]
+    fn record_token_usage_keeps_last_prompt_tokens_across_failed_requests() {
+        let mut ctx = create_test_ctx();
+        ctx.record_token_usage(
+            Some(TokenUsage {
+                input_tokens: Some(100),
+                output_tokens: Some(5),
+                cache_creation_input_tokens: Some(40),
+                cache_read_input_tokens: Some(60),
+            }),
+            &priced_model(),
+        );
+
+        assert_eq!(ctx.last_prompt_token_usage, Some(200));
+
+        // A request that fails before the provider reports usage records
+        // `None`: the display field is clobbered by design, but the safety
+        // valve's measurement must survive.
+        ctx.record_token_usage(None, &priced_model());
+
+        assert!(ctx.last_token_usage.is_none());
+        assert_eq!(ctx.last_prompt_token_usage, Some(200));
+    }
+
     #[test]
     fn generate_prompt_context_omits_costs_that_render_as_zero() {
         let mut ctx = create_test_ctx();
@@ -6028,6 +6192,184 @@ mod tests {
         );
     }
 
+    fn windowed_model(max_input_tokens: usize) -> Model {
+        let mut data = ModelData::new("test-window");
+        data.max_input_tokens = Some(max_input_tokens);
+        Model::from_config("openai", &[data]).remove(0)
+    }
+
+    fn text_message(role: MessageRole, text: &str) -> Message {
+        Message::new(role, MessageContent::Text(text.to_string()))
+    }
+
+    #[test]
+    fn summarization_chunks_respect_budget_and_preserve_order() {
+        let model = Model::default();
+        // 1000 ascii chars => ~250 estimated tokens per message.
+        let messages: Vec<Message> = (0..10)
+            .map(|i| {
+                let role = if i % 2 == 0 {
+                    MessageRole::User
+                } else {
+                    MessageRole::Assistant
+                };
+                text_message(role, &format!("msg-{i:02} {}", "x".repeat(993)))
+            })
+            .collect();
+
+        let budget = 600;
+        let chunks = slice_summarization_chunks(&model, &messages, budget);
+
+        assert!(chunks.len() > 1, "a history over budget must be split");
+        for chunk in &chunks {
+            assert!(
+                model.total_tokens(chunk) <= budget,
+                "each chunk must fit the budget"
+            );
+        }
+        let flattened: Vec<&Message> = chunks.iter().flat_map(|chunk| chunk.iter()).collect();
+        assert_eq!(
+            flattened.len(),
+            messages.len(),
+            "chunks must cover every message"
+        );
+        for (original, chunked) in messages.iter().zip(flattened) {
+            assert_eq!(original.content.to_text(), chunked.content.to_text());
+        }
+
+        assert_eq!(
+            slice_summarization_chunks(&model, &messages, 10_000).len(),
+            1,
+            "a history within budget must stay a single chunk"
+        );
+    }
+
+    #[test]
+    fn summarization_chunk_slicing_isolates_oversized_message() {
+        let model = Model::default();
+        let messages = vec![
+            text_message(MessageRole::User, "small question"),
+            text_message(MessageRole::Assistant, &"y".repeat(8000)),
+            text_message(MessageRole::User, "another small question"),
+        ];
+        let chunks = slice_summarization_chunks(&model, &messages, 100);
+        assert_eq!(chunks.len(), 3);
+        assert_eq!(
+            chunks[1].len(),
+            1,
+            "an over-budget message forms its own chunk"
+        );
+    }
+
+    #[test]
+    fn render_summarization_chunk_keeps_tool_transcripts() {
+        let tool_calls = MessageContentToolCalls::new(
+            vec![ToolResult::new(
+                ToolCall::new(
+                    "fs_cat".to_string(),
+                    json!({"path": "notes.txt"}),
+                    Some("call-1".to_string()),
+                ),
+                json!("the file contents"),
+            )],
+            "reading the file".to_string(),
+        );
+        let messages = vec![
+            text_message(MessageRole::User, "read notes.txt"),
+            Message::new(MessageRole::Tool, MessageContent::ToolCalls(tool_calls)),
+        ];
+
+        let rendered = render_summarization_chunk(&messages);
+
+        assert!(rendered.contains("USER: read notes.txt"));
+        assert!(rendered.contains("TOOL: "));
+        assert!(rendered.contains("fs_cat"));
+        assert!(rendered.contains("the file contents"));
+    }
+
+    #[test]
+    fn compress_session_folds_forward_across_chunks() {
+        let mut app = AppState::test_default();
+        app.config = Arc::new(AppConfig {
+            compression_keep_last: 2,
+            ..AppConfig::default()
+        });
+        let mut ctx = RequestContext::new(Arc::new(app), WorkingMode::Cmd);
+        let mut session = Session::default();
+        // A 2_000-token window gives a 1_200-token chunk budget; the session
+        // built below estimates past the window itself.
+        session.set_model(windowed_model(2_000));
+        ctx.session = Some(session);
+        for i in 0..10 {
+            let text = format!("msg-{i:02} {}", "x".repeat(793));
+            let input = Input::from_str(&ctx, &text, Some(Role::new("", ""))).unwrap();
+            ctx.session
+                .as_mut()
+                .unwrap()
+                .add_message(&input, &format!("reply-{i:02}"))
+                .unwrap();
+        }
+        assert!(
+            ctx.session.as_ref().unwrap().tokens() > 2_000,
+            "the session must not fit the summarizer window in one request"
+        );
+
+        let requests = std::cell::RefCell::new(Vec::new());
+        let requests_ref = &requests;
+        tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+            .block_on(ctx.compress_session_impl(move |input: Input| async move {
+                // The real request pipeline, guard_max_input_tokens included.
+                let data = input.prepare_completion_data(input.role().model(), false)?;
+                requests_ref
+                    .borrow_mut()
+                    .push((input.text(), data.messages, data.functions));
+                Ok(format!("folded-summary-{}", requests_ref.borrow().len()))
+            }))
+            .unwrap();
+
+        let requests = requests.into_inner();
+        assert_eq!(requests.len(), 2, "the foldable middle needs two chunks");
+
+        let (first_text, first_messages, first_functions) = &requests[0];
+        assert!(first_text.contains("msg-00"));
+        assert!(!first_text.contains(SUMMARY_CONTEXT_PROMPT));
+        assert_eq!(first_messages.len(), 1, "detached: no session history");
+        assert!(first_messages[0].role.is_user());
+        assert!(
+            first_functions.is_none(),
+            "detached: no function declarations"
+        );
+
+        let (second_text, second_messages, second_functions) = &requests[1];
+        assert!(second_text.contains(SUMMARY_CONTEXT_PROMPT));
+        assert!(second_text.contains("folded-summary-1"));
+        assert!(second_text.contains("msg-05"));
+        assert!(
+            !second_text.contains("msg-00"),
+            "the first chunk reaches the second call only through its summary"
+        );
+        assert!(
+            !second_text.contains("msg-09"),
+            "the keep_last tail is never summarized"
+        );
+        assert_eq!(second_messages.len(), 1, "detached: no session history");
+        assert!(second_functions.is_none());
+
+        let session = ctx.session.as_ref().unwrap();
+        assert_eq!(session.messages().len(), 3, "summary + keep_last tail");
+        assert!(session.messages()[0].role.is_system());
+        assert!(
+            session.messages()[0]
+                .content
+                .to_text()
+                .contains("folded-summary-2"),
+            "the recap must carry the last folded summary"
+        );
+    }
+
     #[test]
     fn should_inject_skill_instructions_requires_function_calling() {
         let app = AppConfig {

src/config/session.rs

@@ -719,17 +719,50 @@ impl Session {
         self.reclaimable_tokens(keep_last) >= (threshold / COMPRESSION_MIN_RECLAIM_DIVISOR).max(1)
     }
 
-    /// Estimated tokens of the messages that compression would actually fold
-    /// away: everything except the leading system message (whose content is
-    /// carried into the summary message) and the `keep_last` tail (kept verbatim).
-    fn reclaimable_tokens(&self, keep_last: usize) -> usize {
+    /// Whether the prompt is close enough to the model's hard input limit
+    /// that compression must run regardless of how `compression_threshold`
+    /// is configured. Sizes the prompt as the larger of the API-reported
+    /// measurement from the last request that reported usage and the local
+    /// estimate, because the estimate can run well below the provider's
+    /// real count.
+    pub fn safety_valve_triggered(
+        &self,
+        valve: f32,
+        keep_last: usize,
+        last_prompt_tokens: Option<usize>,
+    ) -> bool {
+        if self.compressing || !valve.is_finite() || valve <= 0.0 {
+            return false;
+        }
+        let Some(max_input_tokens) = self.model().max_input_tokens() else {
+            return false;
+        };
+        // With nothing to fold away, compressing is a no-op; firing anyway
+        // would spin compress attempts while a single in-flight turn holds
+        // all the tokens.
+        if self.reclaimable_tokens(keep_last) == 0 {
+            return false;
+        }
+        last_prompt_tokens.unwrap_or(0).max(self.tokens())
+            >= (valve * max_input_tokens as f32) as usize
+    }
+
+    /// The messages that compression would actually fold away: everything
+    /// except the leading system message (whose content is carried into the
+    /// summary message) and the `keep_last` tail (kept verbatim).
+    pub fn foldable_messages(&self, keep_last: usize) -> &[Message] {
         let start = usize::from(
             self.messages
                 .first()
                 .is_some_and(|v| v.role == MessageRole::System),
         );
         let end = self.messages.len().saturating_sub(keep_last).max(start);
-        self.model().total_tokens(&self.messages[start..end])
+        &self.messages[start..end]
+    }
+
+    /// Estimated tokens of the messages compression would fold away.
+    fn reclaimable_tokens(&self, keep_last: usize) -> usize {
+        self.model().total_tokens(self.foldable_messages(keep_last))
     }
 
     pub fn compressing(&self) -> bool {
@@ -1109,7 +1142,7 @@ impl AutoName {
 mod tests {
     use super::*;
     use crate::client::{
-        Message, MessageContent, MessageContentToolCalls, MessageRole, Model, TokenUsage,
+        Message, MessageContent, MessageContentToolCalls, MessageRole, Model, ModelData, TokenUsage,
     };
     use crate::config::{AppConfig, AppState, RequestContext, WorkingMode};
     use crate::function::{Functions, ToolCall, ToolResult};
@@ -1578,6 +1611,85 @@ mod tests {
         assert!(session.needs_compression(100, 1));
     }
 
+    /// A session with a system message, a large user message, and a small
+    /// assistant reply, on a model with the given input limit. With
+    /// `keep_last = 1` the user message is reclaimable; with `keep_last = 2`
+    /// nothing is.
+    fn valve_session(max_input_tokens: Option<usize>) -> Session {
+        let mut session = Session::default();
+        let mut data = ModelData::new("valve-model");
+        data.max_input_tokens = max_input_tokens;
+        session.set_model(Model::from_config("provider", &[data]).remove(0));
+        session.messages.push(Message::new(
+            MessageRole::System,
+            MessageContent::Text("x".repeat(400)),
+        ));
+        session.messages.push(Message::new(
+            MessageRole::User,
+            MessageContent::Text("y".repeat(4000)),
+        ));
+        session.messages.push(Message::new(
+            MessageRole::Assistant,
+            MessageContent::Text("ok".to_string()),
+        ));
+        session.update_tokens();
+        session
+    }
+
+    #[test]
+    fn session_safety_valve_fires_from_estimate() {
+        let session = valve_session(Some(1000));
+
+        assert!(session.tokens() >= 800);
+        assert!(session.safety_valve_triggered(0.8, 1, None));
+    }
+
+    #[test]
+    fn session_safety_valve_fires_from_real_usage_over_estimate() {
+        let session = valve_session(Some(1_000_000));
+
+        assert!(!session.safety_valve_triggered(0.8, 1, None));
+        assert!(!session.safety_valve_triggered(0.8, 1, Some(700_000)));
+        assert!(session.safety_valve_triggered(0.8, 1, Some(800_000)));
+    }
+
+    #[test]
+    fn session_safety_valve_disabled_when_zero() {
+        let session = valve_session(Some(1000));
+
+        assert!(!session.safety_valve_triggered(0.0, 1, Some(1_000_000)));
+    }
+
+    #[test]
+    fn session_safety_valve_disabled_when_nan() {
+        let session = valve_session(Some(1000));
+
+        assert!(!session.safety_valve_triggered(f32::NAN, 1, Some(1_000_000)));
+    }
+
+    #[test]
+    fn session_safety_valve_respects_compressing() {
+        let mut session = valve_session(Some(1000));
+        session.set_compressing(true);
+
+        assert!(!session.safety_valve_triggered(0.8, 1, Some(1_000_000)));
+    }
+
+    #[test]
+    fn session_safety_valve_requires_reclaimable_tokens() {
+        let session = valve_session(Some(1000));
+
+        assert!(!session.safety_valve_triggered(0.8, 2, Some(1_000_000)));
+        assert!(session.safety_valve_triggered(0.8, 1, Some(1_000_000)));
+    }
+
+    #[test]
+    fn session_safety_valve_requires_model_max_input_tokens() {
+        let session = valve_session(None);
+
+        assert!(!session.safety_valve_triggered(0.8, 1, Some(1_000_000)));
+    }
+
     #[test]
     fn session_set_compressing_flag() {
         let mut session = Session::default();

src/repl/mod.rs

@@ -1515,6 +1515,8 @@ async fn ask_inner(
 ) -> Result<()> {
     let mut input = input;
     let mut with_embeddings = with_embeddings;
+    // One compress-and-retry per fresh input; tool-loop iterations share it.
+    let mut retried_after_compress = false;
     loop {
         if input.is_empty() {
             return Ok(());
@@ -1530,6 +1532,10 @@ async fn ask_inner(
 
         let app = Arc::clone(&ctx.app.config);
 
+        if maybe_auto_compress(ctx, app.as_ref()).await.succeeded {
+            input.refresh_session(ctx);
+        }
+
         if graph::active_agent_graph_name(ctx).is_some() {
             ctx.before_chat_completion(&input)?;
             let output =
@@ -1560,6 +1566,18 @@ async fn ask_inner(
             match result {
                 Ok(v) => v,
                 Err(err) => {
+                    if compress_for_error_retry(
+                        ctx,
+                        app.as_ref(),
+                        retried_after_compress,
+                        &abort_signal,
+                    )
+                    .await
+                    {
+                        input.refresh_session(ctx);
+                        retried_after_compress = true;
+                        continue;
+                    }
                     ctx.on_chat_completion_error(app.as_ref(), &input);
                     return Err(err);
                 }
@@ -1574,6 +1592,7 @@ async fn ask_inner(
         match check_pending_tasks_guardrail(ctx) {
             GuardrailAction::Inject(prompt) => {
                 input = Input::from_str(ctx, &prompt, None)?;
+                retried_after_compress = false;
                 continue;
             }
             GuardrailAction::ForceTerminate(ids) => {
@@ -1617,6 +1636,7 @@ async fn ask_inner(
                 format!("{prompt}\n\n{todo_state}")
             };
             input = Input::from_str(ctx, &full_prompt, None)?;
+            retried_after_compress = false;
             continue;
         }
 
@@ -1637,63 +1657,45 @@ async fn ask_inner(
             }
         }
 
-        let keep_last = ctx.compression_keep_last();
-        let needs_compression = ctx
-            .session
-            .as_ref()
-            .is_some_and(|s| s.needs_compression(app.compression_threshold, keep_last));
+        let agent_can_continue_after_compress =
+            should_continue(ctx) && !abort_signal.aborted_ctrlc();
 
-        if needs_compression {
-            let agent_can_continue_after_compress =
-                should_continue(ctx) && !abort_signal.aborted_ctrlc();
+        // Auto-continue keys off the compression CONDITION, not the outcome:
+        // a transiently failed compression retries at the next turn end, and
+        // gating on success would stall an unattended agent at the pause
+        // banner with incomplete todos.
+        let compression = maybe_auto_compress(ctx, app.as_ref()).await;
+        if compression.fired && agent_can_continue_after_compress {
+            let full_prompt = {
+                let config = ctx.auto_continue_config();
+                let todo_state = ctx.todo_list.render_for_model();
+                let remaining = ctx.todo_list.incomplete_count();
+                ctx.increment_auto_continue_count();
+                let count = ctx.auto_continue_count;
+                let max = config.max_continues;
 
-            if let Some(session) = ctx.session.as_mut() {
-                session.set_compressing(true);
-            }
+                let prompt = config
+                    .continuation_prompt
+                    .as_deref()
+                    .unwrap_or(DEFAULT_CONTINUATION_PROMPT);
 
-            let color = if app.light_theme() {
-                nu_ansi_term::Color::LightGray
-            } else {
-                nu_ansi_term::Color::DarkGray
-            };
-            eprintln!("\n📢 {}", color.italic().paint("Compressing the session."),);
-
-            auto_compress_session(ctx).await;
-            if let Some(session) = ctx.session.as_mut() {
-                session.set_compressing(false);
-            }
-
-            if agent_can_continue_after_compress {
-                let full_prompt = {
-                    let config = ctx.auto_continue_config();
-                    let todo_state = ctx.todo_list.render_for_model();
-                    let remaining = ctx.todo_list.incomplete_count();
-                    ctx.increment_auto_continue_count();
-                    let count = ctx.auto_continue_count;
-                    let max = config.max_continues;
-
-                    let prompt = config
-                        .continuation_prompt
-                        .as_deref()
-                        .unwrap_or(DEFAULT_CONTINUATION_PROMPT);
-
-                    let color = if app.light_theme() {
-                        nu_ansi_term::Color::LightGray
-                    } else {
-                        nu_ansi_term::Color::DarkGray
-                    };
-                    eprintln!(
-                        "\n📋 {}",
-                        color.italic().paint(format!(
-                            "Auto-continuing after compression ({count}/{max}): {remaining} incomplete todo(s) remain"
-                        ))
-                    );
-
-                    format!("{prompt}\n\n{todo_state}")
+                let color = if app.light_theme() {
+                    nu_ansi_term::Color::LightGray
+                } else {
+                    nu_ansi_term::Color::DarkGray
                 };
-                input = Input::from_str(ctx, &full_prompt, None)?;
-                continue;
-            }
+                eprintln!(
+                    "\n📋 {}",
+                    color.italic().paint(format!(
+                        "Auto-continuing after compression ({count}/{max}): {remaining} incomplete todo(s) remain"
+                    ))
+                );
+
+                format!("{prompt}\n\n{todo_state}")
+            };
+            input = Input::from_str(ctx, &full_prompt, None)?;
+            retried_after_compress = false;
+            continue;
         }
 
         print_turn_divider();
@@ -1709,15 +1711,87 @@ fn print_turn_divider() {
     println!();
 }
 
-async fn auto_compress_session(ctx: &mut RequestContext) {
+struct AutoCompressOutcome {
+    fired: bool,
+    succeeded: bool,
+}
+
+async fn maybe_auto_compress(ctx: &mut RequestContext, app: &AppConfig) -> AutoCompressOutcome {
+    let keep_last = ctx.compression_keep_last();
+    let should_compress = ctx.session.as_ref().is_some_and(|session| {
+        session.needs_compression(app.compression_threshold, keep_last)
+            || session.safety_valve_triggered(
+                app.compression_safety_valve,
+                keep_last,
+                ctx.last_prompt_token_usage,
+            )
+    });
+    if !should_compress {
+        return AutoCompressOutcome {
+            fired: false,
+            succeeded: false,
+        };
+    }
+
+    if let Some(session) = ctx.session.as_mut() {
+        session.set_compressing(true);
+    }
+
+    let color = if app.light_theme() {
+        nu_ansi_term::Color::LightGray
+    } else {
+        nu_ansi_term::Color::DarkGray
+    };
+    eprintln!("\n📢 {}", color.italic().paint("Compressing the session."),);
+
+    let compressed = auto_compress_session(ctx).await;
+    if let Some(session) = ctx.session.as_mut() {
+        session.set_compressing(false);
+    }
+    if compressed {
+        ctx.last_prompt_token_usage = None;
+    }
+    AutoCompressOutcome {
+        fired: true,
+        succeeded: compressed,
+    }
+}
+
+async fn compress_for_error_retry(
+    ctx: &mut RequestContext,
+    app: &AppConfig,
+    already_retried: bool,
+    abort_signal: &AbortSignal,
+) -> bool {
+    if already_retried || abort_signal.aborted() {
+        return false;
+    }
+    let keep_last = ctx.compression_keep_last();
+    let valve_triggered = ctx.session.as_ref().is_some_and(|session| {
+        session.safety_valve_triggered(
+            app.compression_safety_valve,
+            keep_last,
+            ctx.last_prompt_token_usage,
+        )
+    });
+    valve_triggered && maybe_auto_compress(ctx, app).await.succeeded
+}
+
+async fn auto_compress_session(ctx: &mut RequestContext) -> bool {
     match ctx.compress_session().await {
-        Ok(()) => hooks::fire(
-            HookEvent::SessionCompressed,
-            ctx,
-            &hooks::role_extras(ctx),
-            None,
-        ),
-        Err(err) => warn!("Failed to compress the session: {err}"),
+        Ok(()) => {
+            hooks::fire(
+                HookEvent::SessionCompressed,
+                ctx,
+                &hooks::role_extras(ctx),
+                None,
+            );
+            true
+        }
+        Err(err) => {
+            warn!("Failed to compress the session: {err}");
+            false
+        }
     }
 }
 
@@ -2036,10 +2110,12 @@ pub fn split_args_text(line: &str, is_win: bool) -> (Vec<String>, &str) {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::client::{ClientConfig, Model};
+    use crate::client::{ClientConfig, MessageContent, Model, ModelData, ToolCall};
     use crate::config::{AppState, Role, RoleLike, Session, TEMP_ROLE_NAME, WorkingMode};
+    use crate::function::ToolResult;
     use crate::hooks::{HookDef, HooksMap, test_sink};
     use anyhow::anyhow;
+    use serde_json::json;
     use serial_test::serial;
     use std::future::Future;
 
@@ -2357,6 +2433,13 @@ mod tests {
     /// resolvable, and the session already holds a user exchange so
     /// `compress_session` has something to summarize.
     fn compressible_ctx(marker: &str) -> RequestContext {
+        compressible_ctx_with(marker, AppConfig::default())
+    }
+
+    /// `compressible_ctx` with caller-tuned compression settings, so tests
+    /// can drive `needs_compression` and the safety valve; `dry_run` and the
+    /// default client are enforced here to keep compression offline.
+    fn compressible_ctx_with(marker: &str, config: AppConfig) -> RequestContext {
         let mut hooks_map = HooksMap::default();
         hooks_map.insert(
             "session.compressed".to_string(),
@@ -2370,7 +2453,7 @@ mod tests {
             hooks: hooks_map,
             dry_run: true,
             clients: vec![ClientConfig::default()],
-            ..Default::default()
+            ..config
         });
         let mut ctx = RequestContext::new(Arc::new(app), WorkingMode::Repl);
         let mut session = Session::default();
@@ -2382,6 +2465,7 @@ mod tests {
             .unwrap()
             .add_message(&input, "hi")
             .unwrap();
+        ctx.session.as_mut().unwrap().update_tokens();
         ctx
     }
 
@@ -2432,7 +2516,7 @@ mod tests {
         let marker = "auto-compress-ok-f8r";
         run_async(async {
             let mut ctx = compressible_ctx(marker);
-            auto_compress_session(&mut ctx).await;
+            assert!(auto_compress_session(&mut ctx).await);
         });
 
         assert_eq!(
@@ -2494,7 +2578,7 @@ mod tests {
             // An empty session makes compress_session fail; the seam must
             // swallow the error (no panic, no propagation) and fire nothing.
             ctx.session = Some(Session::default());
-            auto_compress_session(&mut ctx).await;
+            assert!(!auto_compress_session(&mut ctx).await);
         });
 
         assert_eq!(
@@ -2504,6 +2588,427 @@ mod tests {
         );
     }
 
+    #[test]
+    #[serial]
+    fn maybe_auto_compress_skips_when_neither_threshold_nor_valve_fires() {
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-skip-q7m";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_keep_last: 1,
+                    ..Default::default()
+                },
+            );
+            let app = Arc::clone(&ctx.app.config);
+
+            assert!(!maybe_auto_compress(&mut ctx, app.as_ref()).await.fired);
+        });
+
+        assert_eq!(session_compressed_count(marker), 0);
+    }
+
+    #[test]
+    #[serial]
+    fn maybe_auto_compress_fires_on_threshold_each_loop_iteration() {
+        // Exercises the shared seam the ask loop calls at the top of every
+        // iteration (tool-loop, guardrail-inject, and auto-continue turns
+        // included) as well as at turn end.
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-threshold-n2x";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_threshold: 1,
+                    compression_keep_last: 1,
+                    ..Default::default()
+                },
+            );
+            let app = Arc::clone(&ctx.app.config);
+
+            assert!(maybe_auto_compress(&mut ctx, app.as_ref()).await.succeeded);
+            assert!(
+                !ctx.is_compressing_session(),
+                "the compressing flag must be released after compression"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn maybe_auto_compress_reports_no_success_when_compression_fails() {
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-fail-g4z";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_threshold: 1,
+                    compression_keep_last: 0,
+                    ..Default::default()
+                },
+            );
+            // A session whose only reclaimable message is non-user: the
+            // threshold fires, but `compress_session` bails on a session
+            // without user messages.
+            {
+                let input = Input::from_str(&ctx, "more context", None).unwrap();
+                let session = ctx.session.as_mut().unwrap();
+                session.add_message(&input, &"y".repeat(400)).unwrap();
+                session.compress("recap".into(), 1);
+            }
+            let app = Arc::clone(&ctx.app.config);
+
+            let outcome = maybe_auto_compress(&mut ctx, app.as_ref()).await;
+            assert!(outcome.fired, "the compression condition must fire");
+            assert!(
+                !outcome.succeeded,
+                "a failed compression must not report success"
+            );
+            assert!(
+                !ctx.is_compressing_session(),
+                "the compressing flag must be released after a failed compression"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 0);
+    }
+
+    #[test]
+    #[serial]
+    fn failed_compression_still_reports_fired_with_kept_tail() {
+        // Pins the turn-end auto-continue gate: with keep_last > 0, a
+        // compression whose condition fired but whose summarization FAILED
+        // must still report `fired`, so an unattended agent auto-continues
+        // and retries the compression at the next turn end instead of
+        // stalling at the pause banner.
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-fired-fail-t8r";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_threshold: 1,
+                    compression_keep_last: 1,
+                    ..Default::default()
+                },
+            );
+            // A session whose foldable middle is a tool transcript and whose
+            // kept tail is an assistant reply: the threshold fires, but
+            // `compress_session` bails on a session without user messages.
+            {
+                let input = Input::from_str(&ctx, "more context", None)
+                    .unwrap()
+                    .merge_tool_results(
+                        "calling a tool to gather context".into(),
+                        vec![ToolResult::new(ToolCall::default(), json!("ok"))],
+                    );
+                let session = ctx.session.as_mut().unwrap();
+                session.add_message(&input, &"y".repeat(400)).unwrap();
+                session.compress("recap".into(), 2);
+            }
+            let app = Arc::clone(&ctx.app.config);
+
+            let outcome = maybe_auto_compress(&mut ctx, app.as_ref()).await;
+            assert!(outcome.fired, "the compression condition must fire");
+            assert!(
+                !outcome.succeeded,
+                "a failed compression must not report success"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 0);
+    }
+
+    #[test]
+    #[serial]
+    fn loop_top_compression_refresh_rebuilds_from_compressed_session() {
+        let _sink = test_sink::install();
+        let marker = "loop-top-refresh-h5w";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_threshold: 1,
+                    compression_keep_last: 1,
+                    ..Default::default()
+                },
+            );
+            let app = Arc::clone(&ctx.app.config);
+            let mut input = Input::from_str(&ctx, "next turn", None).unwrap();
+            let stale = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+
+            assert!(maybe_auto_compress(&mut ctx, app.as_ref()).await.succeeded);
+            input.refresh_session(&ctx);
+
+            let refreshed = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+            assert_ne!(
+                refreshed, stale,
+                "the request built after a loop-top compression must reflect the compressed session"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn maybe_auto_compress_fires_on_safety_valve_despite_huge_threshold() {
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-valve-v5c";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_threshold: 1_000_000,
+                    compression_keep_last: 1,
+                    compression_safety_valve: 0.8,
+                    ..Default::default()
+                },
+            );
+            // A distinct name: `set_model` is a no-op for an unchanged model
+            // id, and the ctx's default session model declares no limit.
+            let mut data = ModelData::new("test-compress-model-limited");
+            data.max_input_tokens = Some(100_000);
+            let model = Model::from_config("openai", &[data]).remove(0);
+            ctx.session.as_mut().unwrap().set_model(model);
+            // The last successful request measured a prompt near the limit
+            // even though the local estimate is tiny.
+            ctx.last_prompt_token_usage = Some(90_000);
+            let app = Arc::clone(&ctx.app.config);
+
+            assert!(maybe_auto_compress(&mut ctx, app.as_ref()).await.succeeded);
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    /// A ctx where only the safety valve can trigger compression: the last
+    /// successful request measured a prompt near the session model's limit
+    /// while the threshold is out of reach.
+    fn valve_tripped_ctx(marker: &str) -> RequestContext {
+        let mut ctx = compressible_ctx_with(
+            marker,
+            AppConfig {
+                compression_threshold: 1_000_000,
+                compression_keep_last: 1,
+                compression_safety_valve: 0.8,
+                ..Default::default()
+            },
+        );
+        let mut data = ModelData::new("test-compress-model-limited");
+        data.max_input_tokens = Some(100_000);
+        let model = Model::from_config("openai", &[data]).remove(0);
+        ctx.session.as_mut().unwrap().set_model(model);
+        ctx.last_prompt_token_usage = Some(90_000);
+        ctx
+    }
+
+    #[test]
+    #[serial]
+    fn successful_compression_clears_stale_prompt_measurement() {
+        let _sink = test_sink::install();
+        let marker = "maybe-compress-stale-p6b";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+
+            assert!(maybe_auto_compress(&mut ctx, app.as_ref()).await.succeeded);
+            assert_eq!(
+                ctx.last_prompt_token_usage, None,
+                "a successful compression must drop the pre-compression prompt measurement"
+            );
+
+            // A subsequent usage-less turn makes tokens reclaimable again;
+            // the valve must not re-fire from the vanished prompt's
+            // measurement.
+            {
+                let input = Input::from_str(&ctx, "next turn", None).unwrap();
+                let session = ctx.session.as_mut().unwrap();
+                session.add_message(&input, "reply").unwrap();
+            }
+            assert!(!maybe_auto_compress(&mut ctx, app.as_ref()).await.fired);
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    /// Counts `on_chat_completion_error` checkpoints in the session; the
+    /// literal must match `INTERRUPTED_RESPONSE_TEXT`, whose presence is
+    /// pinned by the give-up tests below.
+    fn interrupted_checkpoint_count(ctx: &RequestContext) -> usize {
+        ctx.session.as_ref().map_or(0, |session| {
+            session
+                .messages()
+                .iter()
+                .filter(|message| {
+                    matches!(&message.content, MessageContent::Text(text)
+                        if text.ends_with("[Response interrupted due to error]"))
+                })
+                .count()
+        })
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_compresses_once_without_checkpointing() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-valve-k1s";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+            let abort_signal = create_abort_signal();
+
+            assert!(compress_for_error_retry(&mut ctx, app.as_ref(), false, &abort_signal).await);
+            assert!(
+                !ctx.is_compressing_session(),
+                "the compressing flag must be released after compression"
+            );
+            assert_eq!(
+                interrupted_checkpoint_count(&ctx),
+                0,
+                "the retry path must not checkpoint the in-flight turn"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_gives_up_on_second_failure_with_single_checkpoint() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-giveup-p4t";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+            let abort_signal = create_abort_signal();
+            let input = Input::from_str(&ctx, "still too big", None).unwrap();
+
+            assert!(compress_for_error_retry(&mut ctx, app.as_ref(), false, &abort_signal).await);
+            // The retried request failed too: no second compression, and the
+            // give-up path checkpoints the interrupted turn exactly once.
+            assert!(!compress_for_error_retry(&mut ctx, app.as_ref(), true, &abort_signal).await);
+            ctx.on_chat_completion_error(app.as_ref(), &input);
+            assert_eq!(interrupted_checkpoint_count(&ctx), 1);
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_declines_when_valve_is_quiet() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-quiet-m8d";
+        run_async(async {
+            let mut ctx = compressible_ctx_with(
+                marker,
+                AppConfig {
+                    compression_keep_last: 1,
+                    ..Default::default()
+                },
+            );
+            let app = Arc::clone(&ctx.app.config);
+            let abort_signal = create_abort_signal();
+            let input = Input::from_str(&ctx, "transient failure", None).unwrap();
+
+            assert!(!compress_for_error_retry(&mut ctx, app.as_ref(), false, &abort_signal).await);
+            // Valve-false errors keep today's behavior: checkpoint and
+            // surface the error immediately, no compression, no retry.
+            ctx.on_chat_completion_error(app.as_ref(), &input);
+            assert_eq!(interrupted_checkpoint_count(&ctx), 1);
+        });
+
+        assert_eq!(session_compressed_count(marker), 0);
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_flag_blocks_repeat_and_rearms_on_fresh_input() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-flag-c6y";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+            let abort_signal = create_abort_signal();
+
+            // Tool-loop iterations carry the flag forward: a set flag wins
+            // over a still-firing valve.
+            assert!(!compress_for_error_retry(&mut ctx, app.as_ref(), true, &abort_signal).await);
+            assert_eq!(session_compressed_count(marker), 0);
+            // A fresh input rebuild clears the flag and re-arms the retry.
+            assert!(compress_for_error_retry(&mut ctx, app.as_ref(), false, &abort_signal).await);
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_refreshed_input_rebuilds_from_compressed_session() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-refresh-j7q";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+            let abort_signal = create_abort_signal();
+            let mut input = Input::from_str(&ctx, "still too big", None).unwrap();
+            let stale = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+
+            assert!(compress_for_error_retry(&mut ctx, app.as_ref(), false, &abort_signal).await);
+            // The snapshot captured at construction is untouched by the
+            // compression; retrying without a refresh would re-send the
+            // failed prompt byte for byte.
+            assert_eq!(
+                serde_json::to_string(&input.build_messages().unwrap()).unwrap(),
+                stale
+            );
+
+            input.refresh_session(&ctx);
+            let refreshed = serde_json::to_string(&input.build_messages().unwrap()).unwrap();
+            assert_ne!(refreshed, stale);
+            let rebuilt = Input::from_str(&ctx, "still too big", None).unwrap();
+            assert_eq!(
+                refreshed,
+                serde_json::to_string(&rebuilt.build_messages().unwrap()).unwrap(),
+                "a refreshed input must build the same request as one created after compression"
+            );
+        });
+
+        assert_eq!(session_compressed_count(marker), 1);
+    }
+
+    #[test]
+    #[serial]
+    fn error_retry_declines_when_user_aborted() {
+        let _sink = test_sink::install();
+        let marker = "error-retry-abort-b2n";
+        run_async(async {
+            let mut ctx = valve_tripped_ctx(marker);
+            let app = Arc::clone(&ctx.app.config);
+            let input = Input::from_str(&ctx, "killed mid-request", None).unwrap();
+
+            let ctrlc = create_abort_signal();
+            ctrlc.set_ctrlc();
+            assert!(!compress_for_error_retry(&mut ctx, app.as_ref(), false, &ctrlc).await);
+            let ctrld = create_abort_signal();
+            ctrld.set_ctrld();
+            assert!(!compress_for_error_retry(&mut ctx, app.as_ref(), false, &ctrld).await);
+
+            // A user-killed request keeps today's behavior: checkpoint and
+            // surface the error, no compression, no re-send.
+            ctx.on_chat_completion_error(app.as_ref(), &input);
+            assert_eq!(interrupted_checkpoint_count(&ctx), 1);
+        });
+
+        assert_eq!(session_compressed_count(marker), 0);
+    }
+
     #[test]
     fn test_process_command_line() {
         assert_eq!(parse_command(" ."), Some((".", None)));