heartbit_core/agent/

context.rs

//! Context management strategy for long-running agent sessions.

use crate::llm::types::{
    CompletionRequest, ContentBlock, Message, ReasoningEffort, Role, ToolDefinition, ToolResult,
};

use super::token_estimator::{estimate_message_tokens, estimate_tokens};

/// Strategy for managing the context window.
#[derive(Debug, Clone, PartialEq)]
pub enum ContextStrategy {
    /// No trimming — all messages are sent (current default behavior).
    Unlimited,
    /// Keep first message + as many recent messages as fit in `max_tokens`.
    SlidingWindow {
        /// Maximum token budget for the sliding window.
        max_tokens: u32,
    },
}

/// Conversation context for an agent run.
pub(crate) struct AgentContext {
    system: String,
    messages: Vec<Message>,
    tools: Vec<ToolDefinition>,
    max_turns: usize,
    max_tokens: u32,
    current_turn: usize,
    context_strategy: ContextStrategy,
    reasoning_effort: Option<ReasoningEffort>,
}

impl AgentContext {
    pub(crate) fn new(
        system: impl Into<String>,
        task: impl Into<String>,
        tools: Vec<ToolDefinition>,
    ) -> Self {
        Self {
            system: system.into(),
            messages: vec![Message::user(task)],
            tools,
            max_turns: 10,
            max_tokens: 4096,
            current_turn: 0,
            context_strategy: ContextStrategy::Unlimited,
            reasoning_effort: None,
        }
    }

    /// Create a context from pre-built content blocks (for multimodal messages).
    pub(crate) fn from_content(
        system: impl Into<String>,
        content: Vec<ContentBlock>,
        tools: Vec<ToolDefinition>,
    ) -> Self {
        Self {
            system: system.into(),
            messages: vec![Message {
                role: Role::User,
                content,
            }],
            tools,
            max_turns: 10,
            max_tokens: 4096,
            current_turn: 0,
            context_strategy: ContextStrategy::Unlimited,
            reasoning_effort: None,
        }
    }

    /// Replace `ContentBlock::Image` and `ContentBlock::Audio` blocks in all
    /// messages except the last user message with text placeholders. Prevents
    /// large base64 payloads from accumulating in the conversation history.
    pub(crate) fn evict_media(&mut self) {
        // Find the index of the last user message
        let last_user_idx = self.messages.iter().rposition(|m| m.role == Role::User);

        for (i, msg) in self.messages.iter_mut().enumerate() {
            if Some(i) == last_user_idx {
                continue;
            }
            for block in &mut msg.content {
                match block {
                    ContentBlock::Image { .. } => {
                        *block = ContentBlock::Text {
                            text: "[image previously sent]".into(),
                        };
                    }
                    ContentBlock::Audio { .. } => {
                        *block = ContentBlock::Text {
                            text: "[audio previously sent]".into(),
                        };
                    }
                    _ => {}
                }
            }
        }
    }

    pub(crate) fn with_max_turns(mut self, max_turns: usize) -> Self {
        self.max_turns = max_turns;
        self
    }

    pub(crate) fn with_max_tokens(mut self, max_tokens: u32) -> Self {
        self.max_tokens = max_tokens;
        self
    }

    pub(crate) fn with_context_strategy(mut self, strategy: ContextStrategy) -> Self {
        self.context_strategy = strategy;
        self
    }

    pub(crate) fn with_reasoning_effort(mut self, effort: Option<ReasoningEffort>) -> Self {
        self.reasoning_effort = effort;
        self
    }

    pub(crate) fn message_count(&self) -> usize {
        self.messages.len()
    }

    pub(crate) fn current_turn(&self) -> usize {
        self.current_turn
    }

    pub(crate) fn max_turns(&self) -> usize {
        self.max_turns
    }

    pub(crate) fn increment_turn(&mut self) {
        self.current_turn += 1;
    }

    pub(crate) fn add_assistant_message(&mut self, message: Message) {
        self.messages.push(message);
    }

    pub(crate) fn add_user_message(&mut self, text: impl Into<String>) {
        self.messages.push(Message::user(text));
    }

    pub(crate) fn add_tool_results(&mut self, results: Vec<ToolResult>) {
        self.messages.push(Message::tool_results(results));
    }

    /// Get the text from the last assistant message (avoids re-cloning the response).
    pub(crate) fn last_assistant_text(&self) -> Option<String> {
        self.messages.iter().rev().find_map(|m| {
            if m.role == Role::Assistant {
                let text: String = m
                    .content
                    .iter()
                    .filter_map(|b| match b {
                        ContentBlock::Text { text } => Some(text.as_str()),
                        _ => None,
                    })
                    .collect();
                Some(text)
            } else {
                None
            }
        })
    }

    /// Estimate total tokens across all messages.
    pub(crate) fn total_tokens(&self) -> u32 {
        self.messages
            .iter()
            .map(estimate_message_tokens)
            .sum::<u32>()
            + estimate_tokens(&self.system)
    }

    /// Check whether the context exceeds a token threshold and needs compaction.
    pub(crate) fn needs_compaction(&self, max_tokens: u32) -> bool {
        self.total_tokens() > max_tokens
    }

    /// Replace old messages with a summary, keeping the initial task context
    /// and the last `keep_last_n` messages.
    ///
    /// The summary is merged into the first user message to maintain the required
    /// alternating user/assistant role sequence (Anthropic API constraint).
    ///
    /// If there aren't enough messages to compact (first + keep_last_n >= total),
    /// this is a no-op.
    pub(crate) fn inject_summary(&mut self, summary: String, keep_last_n: usize) {
        // Extract the original task text from the first message
        let Some(first) = self.messages.first() else {
            return;
        };
        let original_task: String = first
            .content
            .iter()
            .filter_map(|b| match b {
                ContentBlock::Text { text } => Some(text.as_str()),
                _ => None,
            })
            .collect();

        inject_summary_into_messages(&mut self.messages, &original_task, &summary, keep_last_n);
    }

    /// Render all messages as a plain text transcript for summarization.
    pub(crate) fn conversation_text(&self) -> String {
        messages_to_text(&self.messages)
    }

    /// Return the messages that would be discarded by `inject_summary(keep_last_n)`.
    ///
    /// This is the "middle" of the message list (excluding first and last N messages).
    pub(crate) fn messages_to_be_compacted(&self, keep_last_n: usize) -> &[Message] {
        if self.messages.len() <= 1 + keep_last_n {
            return &[];
        }
        let tail_start = self.messages.len().saturating_sub(keep_last_n);
        // Middle = messages[1..tail_start]
        if tail_start <= 1 {
            return &[];
        }
        &self.messages[1..tail_start]
    }

    pub(crate) fn to_request(&self) -> CompletionRequest {
        let messages = match &self.context_strategy {
            ContextStrategy::Unlimited => self.messages.clone(),
            ContextStrategy::SlidingWindow { max_tokens } => {
                apply_sliding_window(&self.messages, *max_tokens)
            }
        };

        CompletionRequest {
            system: self.system.clone(),
            messages,
            tools: self.tools.clone(),
            max_tokens: self.max_tokens,
            tool_choice: None,
            reasoning_effort: self.reasoning_effort,
        }
    }
}

/// Inject a summary into a message list, replacing middle messages.
///
/// Keeps the original task from `messages[0]` and merges it with the summary
/// into a single User message. Then appends the last `keep_last_n` messages.
/// Adjusts the tail start to ensure User/Assistant alternation is preserved.
///
/// Shared between standalone (`AgentContext`) and durable (`AgentWorkflow`) paths.
pub fn inject_summary_into_messages(
    messages: &mut Vec<Message>,
    original_task: &str,
    summary: &str,
    keep_last_n: usize,
) {
    if messages.is_empty() {
        return;
    }
    let total = messages.len();
    // Need at least: first(1) + something_to_summarize(1) + keep_last_n
    if total <= 1 + keep_last_n {
        return;
    }

    let combined = Message::user(format!(
        "{original_task}\n\n[Previous conversation summary]\n{summary}"
    ));

    // Determine tail start, then adjust to maintain alternating User/Assistant roles.
    // After the combined User message, the tail must start with an Assistant message.
    let mut tail_start = total.saturating_sub(keep_last_n);
    // Guard: `tail_start > 1` because index 0 is the original first message being
    // replaced — including it in the tail would duplicate content. In valid conversations,
    // messages[1] is always Assistant (first LLM response after the user task), so
    // `tail_start == 1 && User` cannot occur with well-formed input.
    while tail_start < total && messages[tail_start].role == Role::User && tail_start > 1 {
        tail_start -= 1;
    }
    let last_messages: Vec<Message> = messages[tail_start..].to_vec();

    messages.clear();
    messages.push(combined);
    messages.extend(last_messages);
}

/// Render a message list as a plain text transcript for summarization.
///
/// Shared between standalone (`AgentContext`) and durable (`AgentWorkflow`) paths.
pub fn messages_to_text(messages: &[Message]) -> String {
    let mut parts = Vec::with_capacity(messages.len());
    for msg in messages {
        let role = match msg.role {
            Role::User => "User",
            Role::Assistant => "Assistant",
        };
        let text: String = msg
            .content
            .iter()
            .map(|b| match b {
                ContentBlock::Text { text } => text.as_str().into(),
                ContentBlock::ToolUse { name, input, .. } => {
                    format!("[Tool call: {name}({input})]")
                }
                ContentBlock::ToolResult { content, .. } => {
                    format!("[Tool result: {content}]")
                }
                ContentBlock::Image { media_type, .. } => {
                    format!("[Image: {media_type}]")
                }
                ContentBlock::Audio { format, .. } => {
                    format!("[Audio: {format}]")
                }
            })
            .collect::<Vec<String>>()
            .join(" ");
        parts.push(format!("{role}: {text}"));
    }
    parts.join("\n")
}

/// Apply sliding window to a message list: always keep the first message (initial task),
/// then include as many recent messages as fit within `max_tokens`.
///
/// Tool use/result pairs are kept together to avoid orphaned tool references.
///
/// Shared between standalone (`AgentContext`) and durable (`AgentWorkflow`) paths.
pub fn apply_sliding_window(messages: &[Message], max_tokens: u32) -> Vec<Message> {
    if messages.len() <= 1 {
        return messages.to_vec();
    }

    let first = &messages[0];
    let first_tokens = estimate_message_tokens(first);
    if first_tokens >= max_tokens {
        return vec![first.clone()];
    }

    let mut budget = max_tokens - first_tokens;
    let tail = &messages[1..];

    // Walk backward, accumulating messages. Keep tool_use/tool_result pairs together.
    let mut included_from = tail.len();
    let mut i = tail.len();
    while i > 0 {
        i -= 1;
        let msg = &tail[i];
        let msg_tokens = estimate_message_tokens(msg);

        // Check if this message is a tool_result (User with ToolResult blocks)
        // and the previous message is the corresponding tool_use (Assistant with ToolUse).
        // If so, they must be included together.
        let is_tool_result = msg.role == Role::User
            && msg
                .content
                .iter()
                .any(|b| matches!(b, ContentBlock::ToolResult { .. }));

        if is_tool_result && i > 0 {
            let prev = &tail[i - 1];
            let prev_tokens = estimate_message_tokens(prev);
            let pair_tokens = msg_tokens + prev_tokens;

            if pair_tokens <= budget {
                budget -= pair_tokens;
                i -= 1;
                included_from = i;
            } else {
                break;
            }
        } else if msg_tokens <= budget {
            budget -= msg_tokens;
            included_from = i;
        } else {
            break;
        }
    }

    let mut result = vec![first.clone()];
    result.extend_from_slice(&tail[included_from..]);
    result
}

#[cfg(test)]
mod tests {
    use super::*;
    use serde_json::json;

    #[test]
    fn new_context_has_user_message() {
        let ctx = AgentContext::new("system", "do something", vec![]);
        let req = ctx.to_request();

        assert_eq!(req.system, "system");
        assert_eq!(req.messages.len(), 1);
        assert_eq!(req.messages[0].role, Role::User);
    }

    #[test]
    fn with_max_turns_overrides_default() {
        let ctx = AgentContext::new("sys", "task", vec![]).with_max_turns(5);
        assert_eq!(ctx.max_turns(), 5);
    }

    #[test]
    fn with_max_tokens_overrides_default() {
        let ctx = AgentContext::new("sys", "task", vec![]).with_max_tokens(8192);
        let req = ctx.to_request();
        assert_eq!(req.max_tokens, 8192);
    }

    #[test]
    fn default_max_tokens_is_4096() {
        let ctx = AgentContext::new("sys", "task", vec![]);
        let req = ctx.to_request();
        assert_eq!(req.max_tokens, 4096);
    }

    #[test]
    fn turn_tracking() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        assert_eq!(ctx.current_turn(), 0);
        ctx.increment_turn();
        assert_eq!(ctx.current_turn(), 1);
    }

    #[test]
    fn add_user_message_creates_user_message() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_user_message("follow up question");

        let req = ctx.to_request();
        assert_eq!(req.messages.len(), 2); // initial + added
        assert_eq!(req.messages[1].role, Role::User);
    }

    #[test]
    fn add_tool_results_creates_user_message() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_tool_results(vec![ToolResult::success("call-1", "result")]);

        let req = ctx.to_request();
        assert_eq!(req.messages.len(), 2);
        assert_eq!(req.messages[1].role, Role::User);
    }

    #[test]
    fn request_includes_tools() {
        let tools = vec![ToolDefinition {
            name: "search".into(),
            description: "Search".into(),
            input_schema: json!({"type": "object"}),
        }];
        let ctx = AgentContext::new("sys", "task", tools);
        let req = ctx.to_request();
        assert_eq!(req.tools.len(), 1);
        assert_eq!(req.tools[0].name, "search");
    }

    #[test]
    fn default_is_unlimited() {
        let ctx = AgentContext::new("sys", "task", vec![]);
        assert!(matches!(ctx.context_strategy, ContextStrategy::Unlimited));
    }

    #[test]
    fn unlimited_passes_all() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("response 1"));
        ctx.add_assistant_message(Message::assistant("response 2"));
        ctx.add_assistant_message(Message::assistant("response 3"));

        let req = ctx.to_request();
        assert_eq!(req.messages.len(), 4); // 1 user + 3 assistant
    }

    #[test]
    fn sliding_window_preserves_first() {
        let mut ctx = AgentContext::new("sys", "initial task", vec![])
            .with_context_strategy(ContextStrategy::SlidingWindow { max_tokens: 20 });

        ctx.add_assistant_message(Message::assistant("a".repeat(100)));
        ctx.add_assistant_message(Message::assistant("recent"));

        let req = ctx.to_request();
        // First message must always be preserved
        assert_eq!(req.messages[0].role, Role::User);
        assert!(
            req.messages[0]
                .content
                .iter()
                .any(|b| matches!(b, ContentBlock::Text { text } if text == "initial task"))
        );
    }

    #[test]
    fn sliding_window_trims_old() {
        let mut ctx = AgentContext::new("sys", "task", vec![])
            .with_context_strategy(ContextStrategy::SlidingWindow { max_tokens: 50 });

        // Add many messages to exceed the window
        for i in 0..10 {
            ctx.add_assistant_message(Message::assistant(format!("response {i} with some text")));
        }

        let req = ctx.to_request();
        // Should have fewer messages than the full 11
        assert!(req.messages.len() < 11);
        // First message always preserved
        assert_eq!(req.messages[0].role, Role::User);
    }

    #[test]
    fn sliding_window_keeps_tool_pairs() {
        let mut ctx = AgentContext::new("sys", "task", vec![])
            .with_context_strategy(ContextStrategy::SlidingWindow { max_tokens: 200 });

        // Add a tool use + result pair
        ctx.add_assistant_message(Message {
            role: Role::Assistant,
            content: vec![ContentBlock::ToolUse {
                id: "c1".into(),
                name: "search".into(),
                input: json!({"q": "test"}),
            }],
        });
        ctx.add_tool_results(vec![ToolResult::success("c1", "found it")]);
        ctx.add_assistant_message(Message::assistant("Based on the search results..."));

        let req = ctx.to_request();
        // Check that tool_use and tool_result are both present or both absent
        let has_tool_use = req.messages.iter().any(|m| {
            m.content
                .iter()
                .any(|b| matches!(b, ContentBlock::ToolUse { .. }))
        });
        let has_tool_result = req.messages.iter().any(|m| {
            m.content
                .iter()
                .any(|b| matches!(b, ContentBlock::ToolResult { .. }))
        });
        assert_eq!(
            has_tool_use, has_tool_result,
            "tool_use and tool_result must be kept together"
        );
    }

    #[test]
    fn sliding_window_single_message() {
        let ctx = AgentContext::new("sys", "task", vec![])
            .with_context_strategy(ContextStrategy::SlidingWindow { max_tokens: 10 });

        let req = ctx.to_request();
        assert_eq!(req.messages.len(), 1);
    }

    #[test]
    fn needs_compaction_below_threshold() {
        let ctx = AgentContext::new("sys", "task", vec![]);
        assert!(!ctx.needs_compaction(10000));
    }

    #[test]
    fn needs_compaction_above_threshold() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        for _ in 0..50 {
            ctx.add_assistant_message(Message::assistant("a".repeat(200)));
        }
        assert!(ctx.needs_compaction(100));
    }

    #[test]
    fn inject_summary_replaces_middle() {
        let mut ctx = AgentContext::new("sys", "initial task", vec![]);
        ctx.add_assistant_message(Message::assistant("msg 1"));
        ctx.add_assistant_message(Message::assistant("msg 2"));
        ctx.add_assistant_message(Message::assistant("msg 3"));
        ctx.add_assistant_message(Message::assistant("msg 4"));
        ctx.add_assistant_message(Message::assistant("msg 5"));

        ctx.inject_summary("summary of earlier conversation".into(), 2);

        // Should have: combined_first(1) + last 2 = 3 messages
        assert_eq!(ctx.messages.len(), 3);
        // First message contains both original task and summary
        let first_text: String = ctx.messages[0]
            .content
            .iter()
            .filter_map(|b| match b {
                ContentBlock::Text { text } => Some(text.as_str()),
                _ => None,
            })
            .collect::<Vec<_>>()
            .join("");
        assert!(first_text.contains("initial task"));
        assert!(first_text.contains("summary of earlier"));
    }

    #[test]
    fn inject_summary_preserves_first_and_last() {
        let mut ctx = AgentContext::new("sys", "first task", vec![]);
        ctx.add_assistant_message(Message::assistant("old 1"));
        ctx.add_assistant_message(Message::assistant("old 2"));
        ctx.add_assistant_message(Message::assistant("recent 1"));
        ctx.add_assistant_message(Message::assistant("recent 2"));
        ctx.add_assistant_message(Message::assistant("recent 3"));

        ctx.inject_summary("compressed".into(), 3);

        // combined_first(1) + last 3 = 4
        assert_eq!(ctx.messages.len(), 4);
        // Last message should be "recent 3"
        assert!(
            ctx.messages[3]
                .content
                .iter()
                .any(|b| matches!(b, ContentBlock::Text { text } if text == "recent 3"))
        );
    }

    #[test]
    fn inject_summary_noop_few_messages() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("only one"));

        ctx.inject_summary("summary".into(), 4);

        // Not enough messages to compact (total=2, need > 1 + 4)
        assert_eq!(ctx.messages.len(), 2);
    }

    #[test]
    fn inject_summary_maintains_alternating_roles() {
        // After summarization, message roles must alternate (user/assistant)
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("a1"));
        ctx.add_assistant_message(Message::assistant("a2"));
        ctx.add_assistant_message(Message::assistant("a3"));
        ctx.add_assistant_message(Message::assistant("a4"));

        ctx.inject_summary("summary".into(), 2);

        // First message is User (combined task+summary)
        assert_eq!(ctx.messages[0].role, Role::User);
        // The remaining messages should start with assistant
        assert_eq!(ctx.messages[1].role, Role::Assistant);
    }

    #[test]
    fn inject_summary_adjusts_tail_when_starting_with_user() {
        // Regression: if keep_last_n tail starts with a User message (tool_result),
        // the combined User + User sequence violates the alternating-role invariant.
        // inject_summary must include the preceding Assistant to maintain alternation.
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("a1"));
        ctx.add_tool_results(vec![ToolResult::success("c1", "result1")]);
        ctx.add_assistant_message(Message::assistant("a2"));
        ctx.add_tool_results(vec![ToolResult::success("c2", "result2")]);
        ctx.add_assistant_message(Message::assistant("a3"));
        // Messages: User, A, U(tool), A, U(tool), A
        // Total = 6, keep_last_n = 2 → tail_start = 4 → messages[4] = U(tool)
        // Without fix: combined(U) + U(tool) + A → role violation
        // With fix: combined(U) + A + U(tool) + A → correct alternation

        ctx.inject_summary("summary".into(), 2);

        // First must be User, second must be Assistant
        assert_eq!(ctx.messages[0].role, Role::User);
        assert_eq!(ctx.messages[1].role, Role::Assistant);
        // Verify alternation throughout
        for w in ctx.messages.windows(2) {
            assert_ne!(w[0].role, w[1].role, "adjacent messages have same role");
        }
    }

    #[test]
    fn total_tokens_grows_with_messages() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        let initial = ctx.total_tokens();

        ctx.add_assistant_message(Message::assistant("a".repeat(100)));
        assert!(ctx.total_tokens() > initial);
    }

    #[test]
    fn shared_inject_summary_preserves_alternation() {
        // Test the shared function directly (used by both standalone and Restate paths)
        let mut messages = vec![
            Message::user("original task"),
            Message::assistant("a1"),
            Message::tool_results(vec![ToolResult::success("c1", "result1")]),
            Message::assistant("a2"),
            Message::tool_results(vec![ToolResult::success("c2", "result2")]),
            Message::assistant("a3"),
        ];

        inject_summary_into_messages(&mut messages, "original task", "summary of conversation", 2);

        // First must be User (combined), then alternating
        assert_eq!(messages[0].role, Role::User);
        assert_eq!(messages[1].role, Role::Assistant);
        for w in messages.windows(2) {
            assert_ne!(w[0].role, w[1].role, "adjacent messages have same role");
        }
        // Combined message contains both task and summary
        let first_text: String = messages[0]
            .content
            .iter()
            .filter_map(|b| match b {
                ContentBlock::Text { text } => Some(text.as_str()),
                _ => None,
            })
            .collect::<Vec<_>>()
            .join("");
        assert!(first_text.contains("original task"));
        assert!(first_text.contains("summary of conversation"));
    }

    #[test]
    fn inject_summary_tail_start_near_beginning() {
        // With total = 4 and keep_last_n = 2, tail_start = 2.
        // messages[2] is Assistant (valid), so no role adjustment needed.
        // This tests the boundary near the start of the conversation.
        let mut messages = vec![
            Message::user("original task"),
            Message::assistant("first response"),
            Message::assistant("second response"),
            Message::assistant("third response"),
        ];

        inject_summary_into_messages(&mut messages, "original task", "summary", 2);

        // combined(User) + last 2 = 3 messages
        assert_eq!(messages.len(), 3);
        assert_eq!(messages[0].role, Role::User);
        assert_eq!(messages[1].role, Role::Assistant);
        // Combined message has task + summary
        let first_text: String = messages[0]
            .content
            .iter()
            .filter_map(|b| match b {
                ContentBlock::Text { text } => Some(text.as_str()),
                _ => None,
            })
            .collect::<Vec<_>>()
            .join("");
        assert!(first_text.contains("original task"));
        assert!(first_text.contains("summary"));
    }

    #[test]
    fn from_content_creates_multimodal_message() {
        let content = vec![
            ContentBlock::Text {
                text: "describe this".into(),
            },
            ContentBlock::Image {
                media_type: "image/jpeg".into(),
                data: "base64data".into(),
            },
        ];
        let ctx = AgentContext::from_content("system", content, vec![]);
        let req = ctx.to_request();
        assert_eq!(req.messages.len(), 1);
        assert_eq!(req.messages[0].role, Role::User);
        assert_eq!(req.messages[0].content.len(), 2);
        assert!(matches!(
            &req.messages[0].content[1],
            ContentBlock::Image { .. }
        ));
    }

    #[test]
    fn evict_media_replaces_old_images_with_placeholder() {
        let mut ctx = AgentContext::from_content(
            "sys",
            vec![
                ContentBlock::Text {
                    text: "describe this".into(),
                },
                ContentBlock::Image {
                    media_type: "image/jpeg".into(),
                    data: "data1".into(),
                },
            ],
            vec![],
        );
        ctx.add_assistant_message(Message::assistant("It shows a cat."));
        // Add a second user message with another image (this is now the "last" user)
        ctx.messages.push(Message {
            role: Role::User,
            content: vec![ContentBlock::Image {
                media_type: "image/png".into(),
                data: "data2".into(),
            }],
        });

        ctx.evict_media();

        // First user message's image should be replaced
        assert_eq!(
            ctx.messages[0].content[1],
            ContentBlock::Text {
                text: "[image previously sent]".into()
            }
        );
        // Last user message's image should be preserved
        assert!(matches!(
            &ctx.messages[2].content[0],
            ContentBlock::Image { media_type, .. } if media_type == "image/png"
        ));
    }

    #[test]
    fn evict_media_replaces_old_audio_with_placeholder() {
        let mut ctx = AgentContext::from_content(
            "sys",
            vec![
                ContentBlock::Text {
                    text: "listen to this".into(),
                },
                ContentBlock::Audio {
                    format: "ogg".into(),
                    data: "audiodata1".into(),
                },
            ],
            vec![],
        );
        ctx.add_assistant_message(Message::assistant("I heard it."));
        ctx.messages.push(Message {
            role: Role::User,
            content: vec![ContentBlock::Audio {
                format: "mp3".into(),
                data: "audiodata2".into(),
            }],
        });

        ctx.evict_media();

        // First user message's audio should be replaced
        assert_eq!(
            ctx.messages[0].content[1],
            ContentBlock::Text {
                text: "[audio previously sent]".into()
            }
        );
        // Last user message's audio should be preserved
        assert!(matches!(
            &ctx.messages[2].content[0],
            ContentBlock::Audio { format, .. } if format == "mp3"
        ));
    }

    #[test]
    fn evict_media_noop_when_no_media() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("reply"));
        let msg_count = ctx.message_count();
        ctx.evict_media();
        assert_eq!(ctx.message_count(), msg_count);
    }

    #[test]
    fn inject_summary_empty_messages_is_noop() {
        let mut messages = vec![];
        inject_summary_into_messages(&mut messages, "task", "summary", 2);
        assert!(messages.is_empty());
    }

    #[test]
    fn inject_summary_while_loop_steps_back_to_assistant() {
        // The `while` loop (vs single `if`) ensures the tail always starts with
        // an Assistant message even when keep_last_n produces a tail starting with User.
        let mut messages = vec![
            Message::user("original task"),
            Message::assistant("a1"),
            Message::tool_results(vec![ToolResult::success("c1", "r1")]),
            Message::assistant("a2"),
            Message::tool_results(vec![ToolResult::success("c2", "r2")]),
            Message::assistant("a3"),
            Message::tool_results(vec![ToolResult::success("c3", "r3")]),
            Message::assistant("a4"),
        ];
        // total=8, keep_last_n=2 → tail_start=6 → messages[6]=U(tool) → step back
        // messages[5]=A → stop. Correct.
        inject_summary_into_messages(&mut messages, "original task", "summary", 2);

        assert_eq!(messages[0].role, Role::User);
        assert_eq!(messages[1].role, Role::Assistant);
        for w in messages.windows(2) {
            assert_ne!(w[0].role, w[1].role, "adjacent messages have same role");
        }
    }

    #[test]
    fn messages_to_be_compacted_returns_middle() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("a1"));
        ctx.add_assistant_message(Message::assistant("a2"));
        ctx.add_assistant_message(Message::assistant("a3"));
        ctx.add_assistant_message(Message::assistant("a4"));

        // total=5, keep_last_n=2 → tail_start=3 → middle=[1..3]
        let compacted = ctx.messages_to_be_compacted(2);
        assert_eq!(compacted.len(), 2);
    }

    #[test]
    fn messages_to_be_compacted_empty_when_few_messages() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("a1"));

        // total=2, 1 + keep_last_n=2 = 3 > 2 → empty
        let compacted = ctx.messages_to_be_compacted(2);
        assert!(compacted.is_empty());
    }

    #[test]
    fn messages_to_be_compacted_excludes_first_and_last() {
        let mut ctx = AgentContext::new("sys", "task", vec![]);
        ctx.add_assistant_message(Message::assistant("old1"));
        ctx.add_assistant_message(Message::assistant("old2"));
        ctx.add_assistant_message(Message::assistant("recent1"));
        ctx.add_assistant_message(Message::assistant("recent2"));

        let compacted = ctx.messages_to_be_compacted(2);
        // Should contain old1 and old2 (indices 1,2), not task (0) or recent (3,4)
        for msg in compacted {
            let text: String = msg
                .content
                .iter()
                .filter_map(|b| match b {
                    ContentBlock::Text { text } => Some(text.as_str()),
                    _ => None,
                })
                .collect();
            assert!(
                text.starts_with("old"),
                "compacted messages should be old ones, got: {text}"
            );
        }
    }
}