lean_ctx/tools/ctx_read/

mod.rs

use std::path::Path;

use crate::core::cache::SessionCache;
use crate::core::compressor;
use crate::core::deps;
use crate::core::entropy;
use crate::core::protocol;
use crate::core::signatures;
use crate::core::symbol_map::{self, SymbolMap};
use crate::core::tokens::count_tokens;
use crate::tools::CrpMode;
mod render;
pub(crate) use render::*;
#[cfg(test)]
mod tests;

/// Pre-counted read output carrying the output string, resolved mode,
/// and token count computed during mode processing.
pub struct ReadOutput {
    pub content: String,
    pub resolved_mode: String,
    /// Approximate output token count from mode processing.
    /// The dispatch layer recounts the final assembled string for accurate savings.
    pub output_tokens: usize,
}

const COMPRESSED_HINT: &str = "[compressed — use mode=\"full\" for complete source]";

const CACHEABLE_MODES: &[&str] = &["map", "signatures"];

fn is_cacheable_mode(mode: &str) -> bool {
    CACHEABLE_MODES.contains(&mode)
}

fn compressed_cache_key(mode: &str, crp_mode: CrpMode, task: Option<&str>) -> String {
    // Bump when the rendered map/signatures body changes shape so stale
    // pre-line-range entries are not served from an older session cache.
    let versioned_mode = match mode {
        "map" => "map:v2",
        "signatures" => "signatures:v2",
        _ => mode,
    };
    let base = if crp_mode.is_tdd() {
        format!("{versioned_mode}:tdd")
    } else {
        versioned_mode.to_string()
    };
    // map/signatures output now embeds a task-relevant body, so task-aware and
    // task-free variants must cache under distinct keys.
    match task.map(str::trim).filter(|t| !t.is_empty()) {
        Some(t) => {
            use std::hash::{Hash, Hasher};
            let mut h = std::collections::hash_map::DefaultHasher::new();
            t.hash(&mut h);
            format!("{base}:t{:x}", h.finish())
        }
        None => base,
    }
}

/// Extracts a short proof-line from file content to include in cache-hit stubs.
/// Returns the first non-empty line (truncated to 60 chars) as evidence the cache is valid.
/// Only shown after 2+ reads to avoid noise on early interactions.
fn cache_hit_proof_line(content: &str, read_count: u32) -> Option<String> {
    if read_count < 2 {
        return None;
    }
    let first_line = content.lines().find(|l| !l.trim().is_empty())?;
    let trimmed = first_line.trim();
    if trimmed.len() > 60 {
        let mut end = 57;
        while end > 0 && !trimmed.is_char_boundary(end) {
            end -= 1;
        }
        Some(format!("{}...", &trimmed[..end]))
    } else {
        Some(trimmed.to_string())
    }
}

fn append_compressed_hint(output: &str, file_path: &str) -> String {
    if !crate::core::profiles::active_profile()
        .output_hints
        .compressed_hint()
    {
        return output.to_string();
    }
    format!(
        "{output}\n{COMPRESSED_HINT}\n  ctx_read(\"{file_path}\", mode=\"full\") | ctx_retrieve(\"{file_path}\")"
    )
}

/// Reads a file as UTF-8 with lossy fallback, enforcing binary detection and max read size limit.
/// Defense-in-depth: verifies that the canonical path stays within the process's project root
/// (if determinable) even though callers SHOULD have already jail-checked the path.
pub fn read_file_lossy(path: &str) -> Result<String, std::io::Error> {
    if crate::core::binary_detect::is_binary_file(path) {
        let msg = crate::core::binary_detect::binary_file_message(path);
        return Err(std::io::Error::other(msg));
    }

    {
        let canonical =
            crate::core::pathutil::safe_canonicalize_bounded(std::path::Path::new(path), 2000);
        if let Ok(cwd) = std::env::current_dir() {
            let root = crate::core::pathutil::safe_canonicalize_bounded(&cwd, 2000);
            if !canonical.starts_with(&root) {
                let allow = crate::core::pathjail::allow_paths_from_env_and_config();
                let data_dir_ok = crate::core::data_dir::lean_ctx_data_dir()
                    .ok()
                    .is_some_and(|d| canonical.starts_with(d));
                let tmp_ok = canonical.starts_with(std::env::temp_dir());
                if !allow.iter().any(|a| canonical.starts_with(a)) && !data_dir_ok && !tmp_ok {
                    tracing::warn!(
                        "defense-in-depth: path may escape project root: {}",
                        canonical.display()
                    );
                }
            }
        }
    }

    let cap = crate::core::limits::max_read_bytes();

    let file = open_with_retry(path)?;
    let meta = file
        .metadata()
        .map_err(|e| std::io::Error::other(format!("cannot stat open file descriptor: {e}")))?;
    if meta.len() > cap as u64 {
        return Err(std::io::Error::other(format!(
            "file too large ({} bytes, limit {} bytes via LCTX_MAX_READ_BYTES). \
             Increase the limit or use a line-range read: mode=\"lines:1-100\"",
            meta.len(),
            cap
        )));
    }

    use std::io::Read;
    let mut bytes = Vec::with_capacity(meta.len() as usize);
    std::io::BufReader::new(file).read_to_end(&mut bytes)?;
    match String::from_utf8(bytes) {
        Ok(s) => Ok(s),
        Err(e) => Ok(String::from_utf8_lossy(e.as_bytes()).into_owned()),
    }
}

/// Opens a file, retrying once after a brief pause on NotFound.
/// Works around overlay/FUSE stat-cache races in container runtimes (Docker, Codex).
/// Uses O_NOFOLLOW on Unix for TOCTOU symlink protection.
fn open_with_retry(path: &str) -> Result<std::fs::File, std::io::Error> {
    match open_nofollow(path) {
        Ok(f) => Ok(f),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
            std::thread::sleep(std::time::Duration::from_millis(50));
            open_nofollow(path).map_err(|e| {
                if e.kind() == std::io::ErrorKind::NotFound {
                    std::io::Error::other(format!(
                        "file not found: {path} — verify the path with ctx_tree or ctx_search"
                    ))
                } else {
                    e
                }
            })
        }
        Err(e) => Err(e),
    }
}

#[cfg(unix)]
fn open_nofollow(path: &str) -> Result<std::fs::File, std::io::Error> {
    use std::os::unix::fs::OpenOptionsExt;
    use std::path::Path;

    let p = Path::new(path);
    // Canonicalize the parent directory (resolving symlinks in the directory path)
    // but apply O_NOFOLLOW only to the final file component. This prevents
    // symlink-following attacks on the target file while allowing legitimate
    // directory symlinks (e.g., /tmp → /private/tmp on macOS).
    if let (Some(parent), Some(filename)) = (p.parent(), p.file_name()) {
        if parent.exists() {
            let canonical_parent = crate::core::pathutil::safe_canonicalize_bounded(parent, 2000);
            let canonical_path = canonical_parent.join(filename);
            return std::fs::OpenOptions::new()
                .read(true)
                .custom_flags(libc::O_NOFOLLOW)
                .open(&canonical_path);
        }
    }

    // Fallback: direct open with O_NOFOLLOW
    std::fs::OpenOptions::new()
        .read(true)
        .custom_flags(libc::O_NOFOLLOW)
        .open(path)
}

#[cfg(not(unix))]
fn open_nofollow(path: &str) -> Result<std::fs::File, std::io::Error> {
    std::fs::File::open(path)
}

/// Reads a file through the cache and applies the requested compression mode.
pub fn handle(cache: &mut SessionCache, path: &str, mode: &str, crp_mode: CrpMode) -> String {
    handle_with_options(cache, path, mode, false, crp_mode, None)
}

/// Like `handle`, but invalidates the cache first to force a fresh disk read.
pub fn handle_fresh(cache: &mut SessionCache, path: &str, mode: &str, crp_mode: CrpMode) -> String {
    handle_with_options(cache, path, mode, true, crp_mode, None)
}

/// Reads a file with task-aware filtering to prioritize task-relevant content.
pub fn handle_with_task(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> String {
    handle_with_options(cache, path, mode, false, crp_mode, task)
}

/// Like `handle_with_task`, also returns the resolved mode name and pre-counted tokens.
pub fn handle_with_task_resolved(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> ReadOutput {
    handle_with_options_resolved(cache, path, mode, false, crp_mode, task)
}

/// Fresh read with task-aware filtering (invalidates cache first).
pub fn handle_fresh_with_task(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> String {
    handle_with_options(cache, path, mode, true, crp_mode, task)
}

/// Fresh read with task-aware filtering, also returns the resolved mode name and pre-counted tokens.
pub fn handle_fresh_with_task_resolved(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> ReadOutput {
    handle_with_options_resolved(cache, path, mode, true, crp_mode, task)
}

fn handle_with_options(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    fresh: bool,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> String {
    handle_with_options_resolved(cache, path, mode, fresh, crp_mode, task).content
}

/// Detects if the current execution context is a subagent (forked agent).
/// Subagents inherit stale parent caches, so force-fresh prevents VERIFY FAIL.
fn is_subagent_context() -> bool {
    static IS_SUBAGENT: std::sync::OnceLock<bool> = std::sync::OnceLock::new();
    *IS_SUBAGENT.get_or_init(|| {
        if std::env::var("LEAN_CTX_FORCE_FRESH").is_ok_and(|v| v == "1" || v == "true") {
            return true;
        }
        std::env::var("CURSOR_TASK_ID").is_ok_and(|v| !v.is_empty())
    })
}

fn handle_with_options_resolved(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    fresh: bool,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> ReadOutput {
    let effective_fresh = fresh || is_subagent_context();

    if let Ok(mut bt) = crate::core::bounce_tracker::global().lock() {
        bt.next_seq();
    }
    let mut result = handle_with_options_inner(cache, path, mode, effective_fresh, crp_mode, task);

    if let Some(entry) = cache.get_mut(path) {
        entry.last_mode.clone_from(&result.resolved_mode);
    }

    let dedup_allowed = matches!(
        result.resolved_mode.as_str(),
        "map" | "signatures" | "aggressive" | "entropy" | "task"
    );
    if dedup_allowed {
        if let Some(deduped) = cache.apply_dedup(path, &result.content) {
            let new_tokens = count_tokens(&deduped);
            if new_tokens < result.output_tokens {
                result.content = deduped;
                result.output_tokens = new_tokens;
            }
        }
    }

    if let Ok(mut bt) = crate::core::bounce_tracker::global().lock() {
        let original_tokens = cache.get(path).map_or(0, |e| e.original_tokens);
        bt.record_read(
            path,
            &result.resolved_mode,
            result.output_tokens,
            original_tokens,
        );
    }

    result
}

/// Attempt to serve a `mode="full"` cache hit (`[unchanged …]`) using only a
/// shared borrow of the cache.
///
/// Returns `None` when the file is not cached, was modified on disk, full
/// content was never delivered, or the cache policy forbids stubbing — in those
/// cases the caller must fall back to the write path.
///
/// This is the read-locked fast path: it needs no `&mut SessionCache`, so the
/// dominant "re-read an unchanged file" case proceeds under a shared lock and
/// parallel reads of distinct files no longer serialize on a global write lock.
pub fn try_stub_hit_readonly(cache: &SessionCache, path: &str) -> Option<ReadOutput> {
    let file_ref = cache.get_file_ref_readonly(path)?;
    let (cached_mtime, read_count, line_count, content_opt) = {
        let entry = cache.get(path)?;
        (
            entry.stored_mtime,
            entry.read_count(),
            entry.line_count,
            entry.content(),
        )
    };

    let no_deg = crate::core::config::Config::load().no_degrade_effective();
    let prof = crate::core::profiles::active_profile();
    let force_full = no_deg
        || (prof.read.default_mode_effective() == "full"
            && prof.compression.crp_mode_effective() == "off");
    let policy_allows_stub =
        crate::server::compaction_sync::effective_cache_policy() != "safe" && !force_full;
    if !policy_allows_stub
        || crate::core::cache::is_cache_entry_stale(path, cached_mtime)
        || !cache.is_full_delivered(path)
    {
        return None;
    }

    cache.record_cache_hit(path);
    let short = protocol::shorten_path(path);
    let out = if crate::core::protocol::meta_visible() {
        format!(
            "{file_ref}={short} [unchanged {line_count}L]\nUnchanged on disk. Use fresh=true to force re-read.",
        )
    } else {
        let proof = content_opt
            .as_deref()
            .and_then(|c| cache_hit_proof_line(c, read_count));
        let reads_note = if read_count > 3 {
            format!(" (read {}x)", read_count + 1)
        } else {
            String::new()
        };
        match proof {
            Some(p) => {
                format!("{file_ref}={short} [unchanged {line_count}L{reads_note} | \"{p}\"]")
            }
            None => format!("{file_ref}={short} [unchanged {line_count}L{reads_note}]"),
        }
    };
    let out = crate::core::redaction::redact_text_if_enabled(&out);
    let sent = count_tokens(&out);
    Some(ReadOutput {
        content: out,
        resolved_mode: "full".into(),
        output_tokens: sent,
    })
}

fn handle_with_options_inner(
    cache: &mut SessionCache,
    path: &str,
    mode: &str,
    fresh: bool,
    crp_mode: CrpMode,
    task: Option<&str>,
) -> ReadOutput {
    let file_ref = cache.get_file_ref(path);
    let short = protocol::shorten_path(path);
    let ext = Path::new(path)
        .extension()
        .and_then(|e| e.to_str())
        .unwrap_or("");

    if fresh {
        if mode == "diff" {
            let warning = "[warning] fresh+diff is redundant — fresh invalidates cache, no diff possible. Use mode=full with fresh=true instead.";
            return ReadOutput {
                content: warning.to_string(),
                resolved_mode: "diff".into(),
                output_tokens: count_tokens(warning),
            };
        }
        cache.invalidate(path);
    }

    if mode == "diff" {
        let (out, _) = handle_diff(cache, path, &file_ref);
        let out = crate::core::redaction::redact_text_if_enabled(&out);
        let sent = count_tokens(&out);
        return ReadOutput {
            content: out,
            resolved_mode: "diff".into(),
            output_tokens: sent,
        };
    }

    if mode != "full" {
        if let Some(existing) = cache.get(path) {
            let stale = crate::core::cache::is_cache_entry_stale(path, existing.stored_mtime);
            if stale {
                cache.invalidate(path);
            }
        }
    }

    // Snapshot the minimal immutable data the miss paths need, then drop the
    // borrow before any mutable operations (set_compressed, invalidate, store).
    let cache_snapshot = cache
        .get(path)
        .map(|existing| (existing.original_tokens, existing.content()));

    if let Some((original_tokens, content_opt)) = cache_snapshot {
        if mode == "full" {
            // Read-locked stub fast path (single source of truth, shared with
            // the registered handler's concurrent read-lock attempt).
            if let Some(out) = try_stub_hit_readonly(cache, path) {
                return out;
            }
            let (out, _) = handle_full_with_auto_delta(cache, path, &file_ref, &short, ext, task);
            let out = crate::core::redaction::redact_text_if_enabled(&out);
            let sent = count_tokens(&out);
            return ReadOutput {
                content: out,
                resolved_mode: "full".into(),
                output_tokens: sent,
            };
        }

        // Resolve mode first so we can check compressed output cache BEFORE
        // decompressing the full content (avoids ~2-5ms zstd overhead on hits).
        let resolved_mode = if mode == "auto" {
            resolve_auto_mode(path, original_tokens, task)
        } else {
            mode.to_string()
        };

        if is_cacheable_mode(&resolved_mode) {
            let cache_key = compressed_cache_key(&resolved_mode, crp_mode, task);
            let compressed_hit = cache.get_compressed(path, &cache_key).cloned();
            if let Some(cached_output) = compressed_hit {
                cache.record_cache_hit(path);
                let out = crate::core::redaction::redact_text_if_enabled(&cached_output);
                let sent = count_tokens(&out);
                return ReadOutput {
                    content: out,
                    resolved_mode,
                    output_tokens: sent,
                };
            }
        }

        if let Some(content) = content_opt {
            let (out, _) = process_mode(
                &content,
                &resolved_mode,
                &file_ref,
                &short,
                ext,
                original_tokens,
                crp_mode,
                path,
                task,
            );
            if is_cacheable_mode(&resolved_mode) {
                let cache_key = compressed_cache_key(&resolved_mode, crp_mode, task);
                cache.set_compressed(path, &cache_key, out.clone());
            }
            let out = crate::core::redaction::redact_text_if_enabled(&out);
            let sent = count_tokens(&out);
            return ReadOutput {
                content: out,
                resolved_mode,
                output_tokens: sent,
            };
        }
        cache.invalidate(path);
    }

    let content = match read_file_lossy(path) {
        Ok(c) => c,
        Err(e) => {
            let msg = format!("ERROR: {e}");
            let tokens = count_tokens(&msg);
            return ReadOutput {
                content: msg,
                resolved_mode: "error".into(),
                output_tokens: tokens,
            };
        }
    };

    let store_result = cache.store(path, &content);

    // Skip expensive hint computation for line-range reads and first reads.
    // Hints are only useful from the 2nd read onwards when the file is contextually relevant.
    let is_line_range = mode.starts_with("lines:");
    let hints = crate::core::profiles::active_profile().output_hints;
    let is_repeat_read = store_result.read_count > 1;
    let similar_hint = if !is_line_range && is_repeat_read && hints.semantic_hint() {
        find_similar_and_update_semantic_index(path, &content)
    } else {
        None
    };
    let graph_hint = if !is_line_range && is_repeat_read && hints.related_hint() {
        build_graph_related_hint(path)
    } else {
        None
    };

    if mode == "full" {
        cache.mark_full_delivered(path);
        let (mut output, _) = format_full_output(
            &file_ref,
            &short,
            ext,
            &content,
            store_result.original_tokens,
            store_result.line_count,
            task,
        );
        if let Some(hint) = &graph_hint {
            output.push_str(&format!("\n{hint}"));
        }
        if let Some(hint) = similar_hint {
            output.push_str(&format!("\n{hint}"));
        }
        let output = crate::core::redaction::redact_text_if_enabled(&output);
        let sent = count_tokens(&output);
        return ReadOutput {
            content: output,
            resolved_mode: "full".into(),
            output_tokens: sent,
        };
    }

    let resolved_mode = if mode == "auto" {
        resolve_auto_mode(path, store_result.original_tokens, task)
    } else {
        mode.to_string()
    };

    let (mut output, _sent) = process_mode(
        &content,
        &resolved_mode,
        &file_ref,
        &short,
        ext,
        store_result.original_tokens,
        crp_mode,
        path,
        task,
    );
    if let Some(hint) = &graph_hint {
        output.push_str(&format!("\n{hint}"));
    }
    if let Some(hint) = similar_hint {
        output.push_str(&format!("\n{hint}"));
    }
    if is_cacheable_mode(&resolved_mode) {
        let cache_key = compressed_cache_key(&resolved_mode, crp_mode, task);
        cache.set_compressed(path, &cache_key, output.clone());
    }
    let output = crate::core::redaction::redact_text_if_enabled(&output);
    let final_tokens = count_tokens(&output);
    ReadOutput {
        content: output,
        resolved_mode,
        output_tokens: final_tokens,
    }
}

pub fn is_instruction_file(path: &str) -> bool {
    let lower = path.to_lowercase();
    let filename = std::path::Path::new(&lower)
        .file_name()
        .and_then(|f| f.to_str())
        .unwrap_or("");

    matches!(
        filename,
        "skill.md"
            | "agents.md"
            | "rules.md"
            | ".cursorrules"
            | ".clinerules"
            | "lean-ctx.md"
            | "lean-ctx.mdc"
    ) || lower.contains("/skills/")
        || lower.contains("/.cursor/rules/")
        || lower.contains("/.claude/rules/")
        || lower.contains("/agents.md")
}

/// Delegates to the unified `auto_mode_resolver::resolve()`.
fn resolve_auto_mode(file_path: &str, original_tokens: usize, task: Option<&str>) -> String {
    let ctx = crate::core::auto_mode_resolver::AutoModeContext {
        path: file_path,
        token_count: original_tokens,
        task,
        cache: None,
    };
    crate::core::auto_mode_resolver::resolve(&ctx).mode
}

fn find_similar_and_update_semantic_index(path: &str, content: &str) -> Option<String> {
    const MAX_CONTENT_BYTES_FOR_SEMANTIC: usize = 32_768;

    if content.len() > MAX_CONTENT_BYTES_FOR_SEMANTIC {
        return None;
    }

    let cfg = crate::core::config::Config::load();
    let profile = crate::core::config::MemoryProfile::effective(&cfg);
    if !profile.semantic_cache_enabled() {
        return None;
    }

    let project_root = detect_project_root(path);
    let session_id = format!("{}", std::process::id());
    let mut index = crate::core::semantic_cache::SemanticCacheIndex::load_or_create(&project_root);

    let similar = index.find_similar(content, 0.7);
    let relevant: Vec<_> = similar
        .into_iter()
        .filter(|(p, _)| p != path)
        .take(3)
        .collect();

    index.add_file(path, content, &session_id);
    if let Err(e) = index.save(&project_root) {
        tracing::warn!("lean-ctx: failed to persist semantic index: {e}");
    }

    if relevant.is_empty() {
        return None;
    }

    let hints: Vec<String> = relevant
        .iter()
        .map(|(p, score)| format!("  {p} ({:.0}% similar)", score * 100.0))
        .collect();

    Some(format!(
        "[semantic: {} similar file(s) in cache]\n{}",
        relevant.len(),
        hints.join("\n")
    ))
}

fn detect_project_root(path: &str) -> String {
    crate::core::protocol::detect_project_root_or_cwd(path)
}

fn build_graph_related_hint(path: &str) -> Option<String> {
    let project_root = detect_project_root(path);
    crate::core::graph_context::build_related_hint(path, &project_root, 5)
}

const AUTO_DELTA_THRESHOLD: f64 = 0.6;

/// Re-reads from disk; if content changed and delta is compact, sends auto-delta.
fn handle_full_with_auto_delta(
    cache: &mut SessionCache,
    path: &str,
    file_ref: &str,
    short: &str,
    ext: &str,
    task: Option<&str>,
) -> (String, usize) {
    let _mode_guard = crate::core::savings_footer::ModeGuard::new("full");
    let Ok(disk_content) = read_file_lossy(path) else {
        cache.record_cache_hit(path);
        if let Some(existing) = cache.get(path) {
            if !crate::core::protocol::meta_visible() {
                if let Some(cached) = existing.content() {
                    return format_full_output(
                        file_ref,
                        short,
                        ext,
                        &cached,
                        existing.original_tokens,
                        existing.line_count,
                        task,
                    );
                }
            }
            let out = format!(
                "[using cached version — file read failed]\n{file_ref}={short} cached {}t {}L",
                existing.read_count(),
                existing.line_count
            );
            let sent = count_tokens(&out);
            return (out, sent);
        }
        let out = if crate::core::protocol::meta_visible() && !file_ref.is_empty() {
            format!("[file read failed and no cached version available] {file_ref}={short}")
        } else {
            format!("[file read failed and no cached version available] {short}")
        };
        let sent = count_tokens(&out);
        return (out, sent);
    };

    let no_deg = crate::core::config::Config::load().no_degrade_effective();
    let prof = crate::core::profiles::active_profile();
    let force_full = no_deg
        || (prof.read.default_mode_effective() == "full"
            && prof.compression.crp_mode_effective() == "off");

    let old_content = cache
        .get(path)
        .and_then(crate::core::cache::CacheEntry::content)
        .unwrap_or_default();
    let store_result = cache.store(path, &disk_content);

    if store_result.was_hit {
        let policy_allows_stub =
            crate::server::compaction_sync::effective_cache_policy() != "safe" && !force_full;
        if policy_allows_stub && store_result.full_content_delivered {
            let out = if crate::core::protocol::meta_visible() {
                format!(
                    "{file_ref}={short} [unchanged {}L]\nUnchanged on disk. Use fresh=true to force re-read.",
                    store_result.line_count
                )
            } else {
                let proof = cache_hit_proof_line(&disk_content, store_result.read_count);
                let reads_note = if store_result.read_count > 3 {
                    format!(" (read {}x)", store_result.read_count)
                } else {
                    String::new()
                };
                match proof {
                    Some(p) => format!(
                        "{file_ref}={short} [unchanged {}L{reads_note} | \"{p}\"]",
                        store_result.line_count
                    ),
                    None => format!(
                        "{file_ref}={short} [unchanged {}L{reads_note}]",
                        store_result.line_count
                    ),
                }
            };
            let sent = count_tokens(&out);
            return (out, sent);
        }
        cache.mark_full_delivered(path);
        return format_full_output(
            file_ref,
            short,
            ext,
            &disk_content,
            store_result.original_tokens,
            store_result.line_count,
            task,
        );
    }

    let diff = compressor::diff_content(&old_content, &disk_content);
    let diff_tokens = count_tokens(&diff);
    let full_tokens = store_result.original_tokens;

    if !force_full
        && full_tokens > 0
        && (diff_tokens as f64) < (full_tokens as f64 * AUTO_DELTA_THRESHOLD)
    {
        let savings = protocol::format_savings(full_tokens, diff_tokens);
        let head = if crate::core::protocol::meta_visible() && !file_ref.is_empty() {
            format!("{file_ref}={short}")
        } else {
            short.to_string()
        };
        let out = format!(
            "{head} [auto-delta] ∆{}L\n{diff}\n{savings}",
            disk_content.lines().count()
        );
        return (out, diff_tokens);
    }

    format_full_output(
        file_ref,
        short,
        ext,
        &disk_content,
        store_result.original_tokens,
        store_result.line_count,
        task,
    )
}

fn handle_diff(cache: &mut SessionCache, path: &str, file_ref: &str) -> (String, usize) {
    let _mode_guard = crate::core::savings_footer::ModeGuard::new("diff");
    let short = protocol::shorten_path(path);
    let old_content = cache
        .get(path)
        .and_then(crate::core::cache::CacheEntry::content);

    let new_content = match read_file_lossy(path) {
        Ok(c) => c,
        Err(e) => {
            let msg = format!("ERROR: {e}");
            let tokens = count_tokens(&msg);
            return (msg, tokens);
        }
    };

    let original_tokens = count_tokens(&new_content);

    let diff_output = if let Some(old) = &old_content {
        compressor::diff_content(old, &new_content)
    } else {
        // No previous version cached — store content for future diffs but
        // return a short guidance message instead of dumping the full file.
        cache.store(path, &new_content);
        let msg = format!(
            "{file_ref}={short} [no cached version for diff — use mode=full first, then diff on re-read]"
        );
        let sent = count_tokens(&msg);
        return (msg, sent);
    };

    cache.store(path, &new_content);

    let sent = count_tokens(&diff_output);
    let savings = protocol::format_savings(original_tokens, sent);
    let head = if crate::core::protocol::meta_visible() && !file_ref.is_empty() {
        format!("{file_ref}={short}")
    } else {
        short.clone()
    };
    (format!("{head} [diff]\n{diff_output}\n{savings}"), sent)
}