fix: large sessions summarize/truncate (#2846)

crates/goose/src/agents/context.rs

@@ -27,11 +27,20 @@ impl Agent {
             &OldestFirstTruncation,
         )?;
 
-        // Add an assistant message to the truncated messages
-        // to ensure the assistant's response is included in the context.
+        // Only add an assistant message if we have room for it and it won't cause another overflow
         let assistant_message = Message::assistant().with_text("I had run into a context length exceeded error so I truncated some of the oldest messages in our conversation.");
-        new_messages.push(assistant_message.clone());
-        new_token_counts.push(token_counter.count_chat_tokens("", &[assistant_message], &[]));
+        let assistant_tokens =
+            token_counter.count_chat_tokens("", &[assistant_message.clone()], &[]);
+
+        let current_total: usize = new_token_counts.iter().sum();
+        if current_total + assistant_tokens <= target_context_limit {
+            new_messages.push(assistant_message);
+            new_token_counts.push(assistant_tokens);
+        } else {
+            // If we can't fit the assistant message, at least log what happened
+            tracing::warn!("Cannot add truncation notice message due to context limits. Current: {}, Assistant: {}, Limit: {}", 
+                          current_total, assistant_tokens, target_context_limit);
+        }
 
         Ok((new_messages, new_token_counts))
     }
@@ -54,8 +63,18 @@ impl Agent {
             let assistant_message = Message::assistant().with_text(
                 "I had run into a context length exceeded error so I summarized our conversation.",
             );
-            new_messages.push(assistant_message.clone());
-            new_token_counts.push(token_counter.count_chat_tokens("", &[assistant_message], &[]));
+            let assistant_tokens =
+                token_counter.count_chat_tokens("", &[assistant_message.clone()], &[]);
+
+            let current_total: usize = new_token_counts.iter().sum();
+            if current_total + assistant_tokens <= target_context_limit {
+                new_messages.push(assistant_message);
+                new_token_counts.push(assistant_tokens);
+            } else {
+                // If we can't fit the assistant message, at least log what happened
+                tracing::warn!("Cannot add summarization notice message due to context limits. Current: {}, Assistant: {}, Limit: {}", 
+                              current_total, assistant_tokens, target_context_limit);
+            }
         }
 
         Ok((new_messages, new_token_counts))

crates/goose/src/context_mgmt/truncate.rs

@@ -1,8 +1,166 @@
-use crate::message::Message;
+use crate::message::{Message, MessageContent};
 use anyhow::{anyhow, Result};
-use mcp_core::Role;
+use mcp_core::{Content, ResourceContents, Role};
 use std::collections::HashSet;
-use tracing::debug;
+use tracing::{debug, warn};
+
+/// Maximum size for truncated content in characters
+const MAX_TRUNCATED_CONTENT_SIZE: usize = 5000;
+
+/// Handles messages that are individually larger than the context limit
+/// by truncating their content rather than removing them entirely
+fn handle_oversized_messages(
+    messages: &[Message],
+    token_counts: &[usize],
+    context_limit: usize,
+    strategy: &dyn TruncationStrategy,
+) -> Result<(Vec<Message>, Vec<usize>), anyhow::Error> {
+    let mut truncated_messages = Vec::new();
+    let mut truncated_token_counts = Vec::new();
+    let mut any_truncated = false;
+
+    // Create a basic token counter for re-estimating truncated content
+    // Note: This is a rough approximation since we don't have access to the actual tokenizer here
+    let estimate_tokens = |text: &str| -> usize {
+        // Rough approximation: 1 token per 4 characters for English text
+        (text.len() / 4).max(1)
+    };
+
+    for (i, (message, &original_tokens)) in messages.iter().zip(token_counts.iter()).enumerate() {
+        if original_tokens > context_limit {
+            warn!(
+                "Message {} has {} tokens, exceeding context limit of {}",
+                i, original_tokens, context_limit
+            );
+
+            // Try to truncate the message content
+            let truncated_message = truncate_message_content(message, MAX_TRUNCATED_CONTENT_SIZE)?;
+            let estimated_new_tokens =
+                estimate_message_tokens(&truncated_message, &estimate_tokens);
+
+            if estimated_new_tokens > context_limit {
+                // Even truncated message is too large, skip it entirely
+                warn!("Skipping message {} as even truncated version ({} tokens) exceeds context limit", i, estimated_new_tokens);
+                any_truncated = true;
+                continue;
+            }
+
+            truncated_messages.push(truncated_message);
+            truncated_token_counts.push(estimated_new_tokens);
+            any_truncated = true;
+        } else {
+            truncated_messages.push(message.clone());
+            truncated_token_counts.push(original_tokens);
+        }
+    }
+
+    if any_truncated {
+        debug!("Truncated large message content, now attempting normal truncation");
+        // After content truncation, try normal truncation if still needed
+        return truncate_messages(
+            &truncated_messages,
+            &truncated_token_counts,
+            context_limit,
+            strategy,
+        );
+    }
+
+    Ok((truncated_messages, truncated_token_counts))
+}
+
+/// Truncates the content within a message while preserving its structure
+fn truncate_message_content(message: &Message, max_content_size: usize) -> Result<Message> {
+    let mut new_message = message.clone();
+
+    for content in &mut new_message.content {
+        match content {
+            MessageContent::Text(text_content) => {
+                if text_content.text.len() > max_content_size {
+                    let truncated = format!(
+                        "{}\n\n[... content truncated from {} to {} characters ...]",
+                        &text_content.text[..max_content_size.min(text_content.text.len())],
+                        text_content.text.len(),
+                        max_content_size
+                    );
+                    text_content.text = truncated;
+                }
+            }
+            MessageContent::ToolResponse(tool_response) => {
+                if let Ok(ref mut result) = tool_response.tool_result {
+                    for content_item in result {
+                        if let Content::Text(ref mut text_content) = content_item {
+                            if text_content.text.len() > max_content_size {
+                                let truncated = format!(
+                                    "{}\n\n[... tool response truncated from {} to {} characters ...]",
+                                    &text_content.text[..max_content_size.min(text_content.text.len())],
+                                    text_content.text.len(),
+                                    max_content_size
+                                );
+                                text_content.text = truncated;
+                            }
+                        }
+                        // Handle Resource content which might contain large text
+                        else if let Content::Resource(ref mut resource_content) = content_item {
+                            if let ResourceContents::TextResourceContents { text, .. } =
+                                &mut resource_content.resource
+                            {
+                                if text.len() > max_content_size {
+                                    let truncated = format!(
+                                        "{}\n\n[... resource content truncated from {} to {} characters ...]",
+                                        &text[..max_content_size.min(text.len())],
+                                        text.len(),
+                                        max_content_size
+                                    );
+                                    *text = truncated;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            // Other content types are typically smaller, but we could extend this if needed
+            _ => {}
+        }
+    }
+
+    Ok(new_message)
+}
+
+/// Estimates token count for a message using a simple heuristic
+fn estimate_message_tokens(message: &Message, estimate_fn: &dyn Fn(&str) -> usize) -> usize {
+    let mut total_tokens = 10; // Base overhead for message structure
+
+    for content in &message.content {
+        match content {
+            MessageContent::Text(text_content) => {
+                total_tokens += estimate_fn(&text_content.text);
+            }
+            MessageContent::ToolResponse(tool_response) => {
+                if let Ok(ref result) = tool_response.tool_result {
+                    for content_item in result {
+                        match content_item {
+                            Content::Text(text_content) => {
+                                total_tokens += estimate_fn(&text_content.text);
+                            }
+                            Content::Resource(resource_content) => {
+                                match &resource_content.resource {
+                                    ResourceContents::TextResourceContents { text, .. } => {
+                                        total_tokens += estimate_fn(text);
+                                    }
+                                    _ => total_tokens += 5, // Small overhead for other resource types
+                                }
+                            }
+                            _ => total_tokens += 5, // Small overhead for other content types
+                        }
+                    }
+                }
+            }
+            _ => total_tokens += 5, // Small overhead for other content types
+        }
+    }
+
+    total_tokens
+}
 
 /// Truncates the messages to fit within the model's context window.
 /// Mutates the input messages and token counts in place.
@@ -31,6 +189,17 @@ pub fn truncate_messages(
     debug!("Total tokens before truncation: {}", total_tokens);
 
     // Check if any individual message is larger than the context limit
+    // First, check for any message that's too large
+    let max_message_tokens = token_counts.iter().max().copied().unwrap_or(0);
+    if max_message_tokens > context_limit {
+        // Try to handle large messages by truncating their content
+        debug!(
+            "Found oversized message with {} tokens, attempting content truncation",
+            max_message_tokens
+        );
+        return handle_oversized_messages(&messages, &token_counts, context_limit, strategy);
+    }
+
     let min_user_msg_tokens = messages
         .iter()
         .zip(token_counts.iter())
@@ -41,7 +210,7 @@ pub fn truncate_messages(
     // If there are no valid user messages, or the smallest one is too big for the context
     if min_user_msg_tokens.is_none() || min_user_msg_tokens.unwrap() > context_limit {
         return Err(anyhow!(
-            "Not possible to truncate messages within context limit"
+            "Not possible to truncate messages within context limit: no suitable user messages found"
         ));
     }
 
@@ -53,6 +222,28 @@ pub fn truncate_messages(
     let indices_to_remove =
         strategy.determine_indices_to_remove(&messages, &token_counts, context_limit)?;
 
+    // Circuit breaker: if we can't remove enough messages, fail gracefully
+    let tokens_to_remove: usize = indices_to_remove
+        .iter()
+        .map(|&i| token_counts.get(i).copied().unwrap_or(0))
+        .sum();
+
+    if total_tokens - tokens_to_remove > context_limit && !indices_to_remove.is_empty() {
+        debug!(
+            "Standard truncation insufficient: {} tokens remain after removing {} tokens",
+            total_tokens - tokens_to_remove,
+            tokens_to_remove
+        );
+        // Try more aggressive truncation or content truncation
+        return handle_oversized_messages(&messages, &token_counts, context_limit, strategy);
+    }
+
+    if indices_to_remove.is_empty() && total_tokens > context_limit {
+        return Err(anyhow!(
+            "Cannot truncate any messages: all messages may be essential or too large individually"
+        ));
+    }
+
     // Step 3: Remove the marked messages
     // Vectorize the set and sort in reverse order to avoid shifting indices when removing
     let mut indices_to_remove = indices_to_remove.iter().cloned().collect::<Vec<usize>>();
@@ -212,6 +403,14 @@ mod tests {
         (Message::user().with_tool_response(id, Ok(result)), tokens)
     }
 
+    // Helper function to create a large tool response with massive content
+    fn large_tool_response(id: &str, large_text: String, tokens: usize) -> (Message, usize) {
+        (
+            Message::user().with_tool_response(id, Ok(vec![Content::text(large_text)])),
+            tokens,
+        )
+    }
+
     // Helper function to create messages with alternating user and assistant
     // text messages of a fixed token count
     fn create_messages_with_counts(
@@ -237,22 +436,70 @@ mod tests {
         (messages, token_counts)
     }
 
+    #[test]
+    fn test_handle_oversized_single_message() -> Result<()> {
+        // Create a scenario similar to the real issue: one very large tool response
+        let large_content = "A".repeat(50000); // Very large content
+        let messages = vec![
+            user_text(1, 10).0,
+            assistant_tool_request(
+                "tool1",
+                ToolCall::new("read_file", json!({"path": "large_file.txt"})),
+                20,
+            )
+            .0,
+            large_tool_response("tool1", large_content, 100000).0, // Massive tool response
+            user_text(2, 10).0,
+        ];
+        let token_counts = vec![10, 20, 100000, 10]; // One message is huge
+        let context_limit = 5000; // Much smaller than the large message
+
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
+            context_limit,
+            &OldestFirstTruncation,
+        );
+
+        // Should succeed by truncating the large content
+        assert!(
+            result.is_ok(),
+            "Should handle oversized message by content truncation"
+        );
+        let (truncated_messages, truncated_counts) = result.unwrap();
+
+        // Should have some messages remaining
+        assert!(
+            !truncated_messages.is_empty(),
+            "Should have some messages left"
+        );
+
+        // Total should be within limit
+        let total_tokens: usize = truncated_counts.iter().sum();
+        assert!(
+            total_tokens <= context_limit,
+            "Total tokens {} should be <= context limit {}",
+            total_tokens,
+            context_limit
+        );
+
+        Ok(())
+    }
+
     #[test]
     fn test_oldest_first_no_truncation() -> Result<()> {
         let (messages, token_counts) = create_messages_with_counts(1, 10, false);
         let context_limit = 25;
 
-        let mut messages_clone = messages.clone();
-        let mut token_counts_clone = token_counts.clone();
-        truncate_messages(
-            &mut messages_clone,
-            &mut token_counts_clone,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
 
-        assert_eq!(messages_clone, messages);
-        assert_eq!(token_counts_clone, token_counts);
+        assert_eq!(result.0, messages);
+        assert_eq!(result.1, token_counts);
         Ok(())
     }
 
@@ -287,28 +534,27 @@ mod tests {
         let token_counts = vec![15, 20, 10, 25, 10, 30, 20, 35, 5];
         let context_limit = 100; // Force truncation while preserving some tool interactions
 
-        let mut messages_clone = messages.clone();
-        let mut token_counts_clone = token_counts.clone();
-        truncate_messages(
-            &mut messages_clone,
-            &mut token_counts_clone,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
+        let (truncated_messages, truncated_counts) = result;
 
         // Verify that tool pairs are kept together and the conversation remains coherent
-        assert!(messages_clone.len() >= 3); // At least one complete interaction should remain
-        assert!(messages_clone.last().unwrap().role == Role::User); // Last message should be from user
+        assert!(truncated_messages.len() >= 3); // At least one complete interaction should remain
+        assert!(truncated_messages.last().unwrap().role == Role::User); // Last message should be from user
 
         // Verify tool pairs are either both present or both removed
-        let tool_ids: HashSet<_> = messages_clone
+        let tool_ids: HashSet<_> = truncated_messages
             .iter()
             .flat_map(|m| m.get_tool_ids())
             .collect();
 
         // Each tool ID should appear 0 or 2 times (request + response)
         for id in tool_ids {
-            let count = messages_clone
+            let count = truncated_messages
                 .iter()
                 .flat_map(|m| m.get_tool_ids().into_iter())
                 .filter(|&tool_id| tool_id == id)
@@ -316,21 +562,26 @@ mod tests {
             assert!(count == 0 || count == 2, "Tool pair was split: {}", id);
         }
 
+        // Total should be within limit
+        let total_tokens: usize = truncated_counts.iter().sum();
+        assert!(total_tokens <= context_limit);
+
         Ok(())
     }
 
     #[test]
     fn test_edge_case_context_window() -> Result<()> {
         // Test case where we're exactly at the context limit
-        let (mut messages, mut token_counts) = create_messages_with_counts(2, 25, false);
+        let (messages, token_counts) = create_messages_with_counts(2, 25, false);
         let context_limit = 100; // Exactly matches total tokens
 
-        (messages, token_counts) = truncate_messages(
-            &mut messages,
-            &mut token_counts,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
+        let (mut messages, mut token_counts) = result;
 
         assert_eq!(messages.len(), 4); // No truncation needed
         assert_eq!(token_counts.iter().sum::<usize>(), 100);
@@ -339,12 +590,13 @@ mod tests {
         messages.push(user_text(5, 1).0);
         token_counts.push(1);
 
-        (messages, token_counts) = truncate_messages(
-            &mut messages,
-            &mut token_counts,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
+        let (messages, token_counts) = result;
 
         assert!(token_counts.iter().sum::<usize>() <= context_limit);
         assert!(messages.last().unwrap().role == Role::User);
@@ -376,30 +628,29 @@ mod tests {
         }
 
         let context_limit = 50; // Force partial truncation
-        let mut messages_clone = messages.clone();
-        let mut token_counts_clone = token_counts.clone();
 
-        (messages_clone, _) = truncate_messages(
-            &mut messages_clone,
-            &mut token_counts_clone,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
+        let (truncated_messages, _) = result;
 
         // Verify that remaining tool chains are complete
-        let remaining_tool_ids: HashSet<_> = messages_clone
+        let remaining_tool_ids: HashSet<_> = truncated_messages
             .iter()
             .flat_map(|m| m.get_tool_ids())
             .collect();
 
         for _id in remaining_tool_ids {
             // Count request/response pairs
-            let requests = messages_clone
+            let requests = truncated_messages
                 .iter()
                 .flat_map(|m| m.get_tool_request_ids().into_iter())
                 .count();
 
-            let responses = messages_clone
+            let responses = truncated_messages
                 .iter()
                 .flat_map(|m| m.get_tool_response_ids().into_iter())
                 .count();
@@ -414,22 +665,23 @@ mod tests {
     #[test]
     fn test_truncation_with_image_content() -> Result<()> {
         // Create a conversation with image content mixed in
-        let mut messages = vec![
+        let messages = vec![
             Message::user().with_image("base64_data", "image/png"), // 50 tokens
             Message::assistant().with_text("I see the image"),      // 10 tokens
             Message::user().with_text("Can you describe it?"),      // 10 tokens
             Message::assistant().with_text("It shows..."),          // 20 tokens
             Message::user().with_text("Thanks!"),                   // 5 tokens
         ];
-        let mut token_counts = vec![50, 10, 10, 20, 5];
+        let token_counts = vec![50, 10, 10, 20, 5];
         let context_limit = 45; // Force truncation
 
-        (messages, token_counts) = truncate_messages(
-            &mut messages,
-            &mut token_counts,
+        let result = truncate_messages(
+            &messages,
+            &token_counts,
             context_limit,
             &OldestFirstTruncation,
         )?;
+        let (messages, token_counts) = result;
 
         // Verify the conversation still makes sense
         assert!(messages.len() >= 1);
@@ -442,24 +694,19 @@ mod tests {
     #[test]
     fn test_error_cases() -> Result<()> {
         // Test impossibly small context window
-        let (mut messages, mut token_counts) = create_messages_with_counts(1, 10, false);
+        let (messages, token_counts) = create_messages_with_counts(1, 10, false);
         let result = truncate_messages(
-            &mut messages,
-            &mut token_counts,
+            &messages,
+            &token_counts,
             5, // Impossibly small context
             &OldestFirstTruncation,
         );
         assert!(result.is_err());
 
         // Test unmatched token counts
-        let mut messages = vec![user_text(1, 10).0];
-        let mut token_counts = vec![10, 10]; // Mismatched length
-        let result = truncate_messages(
-            &mut messages,
-            &mut token_counts,
-            100,
-            &OldestFirstTruncation,
-        );
+        let messages = vec![user_text(1, 10).0];
+        let token_counts = vec![10, 10]; // Mismatched length
+        let result = truncate_messages(&messages, &token_counts, 100, &OldestFirstTruncation);
         assert!(result.is_err());
 
         Ok(())

crates/goose/src/session/storage.rs

@@ -211,7 +211,20 @@ pub fn generate_session_id() -> String {
 ///
 /// Creates the file if it doesn't exist, reads and deserializes all messages if it does.
 /// The first line of the file is expected to be metadata, and the rest are messages.
+/// Large messages are automatically truncated to prevent memory issues.
 pub fn read_messages(session_file: &Path) -> Result<Vec<Message>> {
+    read_messages_with_truncation(session_file, Some(50000)) // 50KB limit per message content
+}
+
+/// Read messages from a session file with optional content truncation
+///
+/// Creates the file if it doesn't exist, reads and deserializes all messages if it does.
+/// The first line of the file is expected to be metadata, and the rest are messages.
+/// If max_content_size is Some, large message content will be truncated during loading.
+pub fn read_messages_with_truncation(
+    session_file: &Path,
+    max_content_size: Option<usize>,
+) -> Result<Vec<Message>> {
     let file = fs::OpenOptions::new()
         .read(true)
         .write(true)
@@ -231,18 +244,163 @@ pub fn read_messages(session_file: &Path) -> Result<Vec<Message>> {
             // Metadata successfully parsed, continue with the rest of the lines as messages
         } else {
             // This is not metadata, it's a message
-            messages.push(serde_json::from_str::<Message>(&line)?);
+            let message = parse_message_with_truncation(&line, max_content_size)?;
+            messages.push(message);
         }
     }
 
     // Read the rest of the lines as messages
     for line in lines {
-        messages.push(serde_json::from_str::<Message>(&line?)?);
+        let line = line?;
+        let message = parse_message_with_truncation(&line, max_content_size)?;
+        messages.push(message);
     }
 
     Ok(messages)
 }
 
+/// Parse a message from JSON string with optional content truncation
+fn parse_message_with_truncation(
+    json_str: &str,
+    max_content_size: Option<usize>,
+) -> Result<Message> {
+    // First try to parse normally
+    match serde_json::from_str::<Message>(json_str) {
+        Ok(mut message) => {
+            // If we have a size limit, check and truncate if needed
+            if let Some(max_size) = max_content_size {
+                truncate_message_content_in_place(&mut message, max_size);
+            }
+            Ok(message)
+        }
+        Err(e) => {
+            // If parsing fails and the string is very long, it might be due to size
+            if json_str.len() > 100000 {
+                tracing::warn!(
+                    "Failed to parse very large message ({}KB), attempting truncation",
+                    json_str.len() / 1024
+                );
+
+                // Try to truncate the JSON string itself before parsing
+                let truncated_json = if let Some(max_size) = max_content_size {
+                    truncate_json_string(json_str, max_size)
+                } else {
+                    json_str.to_string()
+                };
+
+                match serde_json::from_str::<Message>(&truncated_json) {
+                    Ok(message) => {
+                        tracing::info!("Successfully parsed message after JSON truncation");
+                        Ok(message)
+                    }
+                    Err(_) => {
+                        tracing::error!("Failed to parse message even after truncation, skipping");
+                        // Return a placeholder message indicating the issue
+                        Ok(Message::user()
+                            .with_text("[Message too large to load - content truncated]"))
+                    }
+                }
+            } else {
+                Err(e.into())
+            }
+        }
+    }
+}
+
+/// Truncate content within a message in place
+fn truncate_message_content_in_place(message: &mut Message, max_content_size: usize) {
+    use crate::message::MessageContent;
+    use mcp_core::{Content, ResourceContents};
+
+    for content in &mut message.content {
+        match content {
+            MessageContent::Text(text_content) => {
+                if text_content.text.len() > max_content_size {
+                    let truncated = format!(
+                        "{}\n\n[... content truncated during session loading from {} to {} characters ...]",
+                        &text_content.text[..max_content_size.min(text_content.text.len())],
+                        text_content.text.len(),
+                        max_content_size
+                    );
+                    text_content.text = truncated;
+                }
+            }
+            MessageContent::ToolResponse(tool_response) => {
+                if let Ok(ref mut result) = tool_response.tool_result {
+                    for content_item in result {
+                        match content_item {
+                            Content::Text(ref mut text_content) => {
+                                if text_content.text.len() > max_content_size {
+                                    let truncated = format!(
+                                        "{}\n\n[... tool response truncated during session loading from {} to {} characters ...]",
+                                        &text_content.text[..max_content_size.min(text_content.text.len())],
+                                        text_content.text.len(),
+                                        max_content_size
+                                    );
+                                    text_content.text = truncated;
+                                }
+                            }
+                            Content::Resource(ref mut resource_content) => {
+                                if let ResourceContents::TextResourceContents { text, .. } =
+                                    &mut resource_content.resource
+                                {
+                                    if text.len() > max_content_size {
+                                        let truncated = format!(
+                                            "{}\n\n[... resource content truncated during session loading from {} to {} characters ...]",
+                                            &text[..max_content_size.min(text.len())],
+                                            text.len(),
+                                            max_content_size
+                                        );
+                                        *text = truncated;
+                                    }
+                                }
+                            }
+                            _ => {} // Other content types are typically smaller
+                        }
+                    }
+                }
+            }
+            _ => {} // Other content types are typically smaller
+        }
+    }
+}
+
+/// Attempt to truncate a JSON string by finding and truncating large text values
+fn truncate_json_string(json_str: &str, max_content_size: usize) -> String {
+    // This is a heuristic approach - look for large text values in the JSON
+    // and truncate them. This is not perfect but should handle the common case
+    // of large tool responses.
+
+    if json_str.len() <= max_content_size * 2 {
+        return json_str.to_string();
+    }
+
+    // Try to find patterns that look like large text content
+    // Look for "text":"..." patterns and truncate the content
+    let mut result = json_str.to_string();
+
+    // Simple regex-like approach to find and truncate large text values
+    if let Some(start) = result.find("\"text\":\"") {
+        let text_start = start + 8; // Length of "text":"
+        if let Some(end) = result[text_start..].find("\",") {
+            let text_end = text_start + end;
+            let text_content = &result[text_start..text_end];
+
+            if text_content.len() > max_content_size {
+                let truncated_text = format!(
+                    "{}\n\n[... content truncated during JSON parsing from {} to {} characters ...]",
+                    &text_content[..max_content_size.min(text_content.len())],
+                    text_content.len(),
+                    max_content_size
+                );
+                result.replace_range(text_start..text_end, &truncated_text);
+            }
+        }
+    }
+
+    result
+}
+
 /// Read session metadata from a session file
 ///
 /// Returns default empty metadata if the file doesn't exist or has no metadata.
@@ -462,12 +620,13 @@ mod tests {
         let dir = tempdir()?;
         let file_path = dir.path().join("special.jsonl");
 
-        // Insert some problematic JSON-like content between long text
+        // Insert some problematic JSON-like content between moderately long text
+        // (keeping under truncation limit to test serialization/deserialization)
         let long_text = format!(
             "Start_of_message\n{}{}SOME_MIDDLE_TEXT{}End_of_message",
-            "A".repeat(100_000),
+            "A".repeat(10_000), // Reduced from 100_000 to stay under 50KB limit
             "\"}]\n",
-            "A".repeat(100_000)
+            "A".repeat(10_000) // Reduced from 100_000 to stay under 50KB limit
         );
 
         let special_chars = vec![
@@ -562,6 +721,56 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn test_large_content_truncation() -> Result<()> {
+        let dir = tempdir()?;
+        let file_path = dir.path().join("large_content.jsonl");
+
+        // Create a message with content larger than the 50KB truncation limit
+        let very_large_text = "A".repeat(100_000); // 100KB of text
+        let messages = vec![
+            Message::user().with_text(&very_large_text),
+            Message::assistant().with_text("Small response"),
+        ];
+
+        // Write messages
+        persist_messages(&file_path, &messages, None).await?;
+
+        // Read them back - should be truncated
+        let read_messages = read_messages(&file_path)?;
+
+        assert_eq!(messages.len(), read_messages.len());
+
+        // First message should be truncated
+        if let Some(MessageContent::Text(read_text)) = read_messages[0].content.first() {
+            assert!(
+                read_text.text.len() < very_large_text.len(),
+                "Content should be truncated"
+            );
+            assert!(
+                read_text
+                    .text
+                    .contains("content truncated during session loading"),
+                "Should contain truncation notice"
+            );
+            assert!(
+                read_text.text.starts_with("AAAA"),
+                "Should start with original content"
+            );
+        } else {
+            panic!("Expected text content in first message");
+        }
+
+        // Second message should be unchanged
+        if let Some(MessageContent::Text(read_text)) = read_messages[1].content.first() {
+            assert_eq!(read_text.text, "Small response");
+        } else {
+            panic!("Expected text content in second message");
+        }
+
+        Ok(())
+    }
+
     #[tokio::test]
     async fn test_metadata_special_chars() -> Result<()> {
         let dir = tempdir()?;