a3s-code-core 8.3.0

A3S Code Core - Embeddable AI agent library with tool execution
Documentation
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//! zvec-rust FTS/BM25 workspace search.

mod ranking;
#[cfg(test)]
mod tests;

use self::ranking::{build_lexical_index, query_terms, tokenize, LexicalIndex};
use crate::text::truncate_utf8;
use crate::tools::types::{Tool, ToolContext, ToolOutput};
use crate::workspace::{
    escape_control_chars_for_display, LexicalSearchRequest, LexicalSearchResult,
    WorkspaceGlobRequest, WorkspaceGrepRequest, WorkspacePath,
};
use anyhow::Result;
use async_trait::async_trait;
use futures::stream::{self, StreamExt};
use std::collections::{HashMap, HashSet};

const DEFAULT_LIMIT: usize = 10;
const MAX_LIMIT: usize = 25;
const DEFAULT_CONTEXT_LINES: usize = 2;
const MAX_CONTEXT_LINES: usize = 8;
const MAX_QUERY_BYTES: usize = 2_048;
const MAX_QUERY_TERMS: usize = 32;
const MAX_CANDIDATE_FILES: usize = 256;
const MAX_FILE_BYTES: usize = 512 * 1_024;
const MAX_TOTAL_BYTES: usize = 16 * 1_024 * 1_024;
const CHUNK_LINES: usize = 80;
const MAX_CHUNKS_PER_FILE: usize = 128;
const MAX_RESULTS_PER_FILE: usize = 2;
const READ_CONCURRENCY: usize = 8;
const MAX_RENDERED_LINE_BYTES: usize = 500;

pub struct Bm25Tool;

#[derive(Debug)]
struct CandidateSelection {
    paths: Vec<WorkspacePath>,
    matching_files: usize,
    backend_truncated: bool,
    candidate_truncated: bool,
    used_glob_fallback: bool,
}

#[derive(Debug)]
struct ChunkSource {
    path: WorkspacePath,
    start_line: usize,
    lines: Vec<String>,
}

#[derive(Debug, Default)]
struct ScanOutcome {
    sources: Vec<ChunkSource>,
    read_files: usize,
    failed_reads: usize,
    oversized_files: usize,
    scanned_bytes: usize,
    truncated: bool,
}

#[derive(Debug)]
struct RenderedResult {
    content: String,
    metadata: serde_json::Value,
    source_anchor: String,
}

#[async_trait]
impl Tool for Bm25Tool {
    fn name(&self) -> &str {
        "bm25"
    }

    fn description(&self) -> &str {
        "Rank workspace text chunks with zvec-rust FTS/BM25 lexical relevance. Use for multi-term or natural-language repository searches; use grep for exact strings and regular expressions."
    }

    fn parameters(&self) -> serde_json::Value {
        serde_json::json!({
            "type": "object",
            "additionalProperties": false,
            "properties": {
                "query": {
                    "type": "string",
                    "minLength": 1,
                    "maxLength": MAX_QUERY_BYTES,
                    "description": "Required. Plain-text relevance query. Code identifiers are expanded into camelCase and snake_case subterms."
                },
                "path": {
                    "type": "string",
                    "description": "Optional. Directory or file to search. Default: workspace root."
                },
                "glob": {
                    "type": "string",
                    "description": "Optional. File glob filter, for example '*.rs' or '*.{ts,tsx}'."
                },
                "limit": {
                    "type": "integer",
                    "minimum": 1,
                    "maximum": MAX_LIMIT,
                    "description": "Optional. Maximum ranked chunks to return. Default: 10; maximum: 25."
                },
                "context": {
                    "type": "integer",
                    "minimum": 0,
                    "maximum": MAX_CONTEXT_LINES,
                    "description": "Optional. Context lines before and after the strongest matching line in each chunk. Default: 2; maximum: 8."
                }
            },
            "required": ["query"],
            "examples": [
                {
                    "query": "session permission policy"
                },
                {
                    "query": "workspace path validation",
                    "path": "core/src",
                    "glob": "*.rs",
                    "limit": 8,
                    "context": 3
                }
            ]
        })
    }

    fn capabilities(&self, _args: &serde_json::Value) -> crate::tools::ToolCapabilities {
        crate::tools::ToolCapabilities::parallel_safe_read(2)
    }

    async fn execute(&self, args: &serde_json::Value, ctx: &ToolContext) -> Result<ToolOutput> {
        let query = match args.get("query").and_then(serde_json::Value::as_str) {
            Some(query) if !query.trim().is_empty() => query.trim(),
            _ => return Ok(ToolOutput::error("query parameter is required")),
        };
        if query.len() > MAX_QUERY_BYTES {
            return Ok(ToolOutput::error(format!(
                "query exceeds the {MAX_QUERY_BYTES}-byte limit"
            )));
        }
        let terms = query_terms(query, MAX_QUERY_TERMS);
        if terms.is_empty() {
            return Ok(ToolOutput::error(
                "query must contain at least one letter, number, underscore, or CJK character; use grep for punctuation-only searches",
            ));
        }

        let limit = match bounded_usize(args, "limit", DEFAULT_LIMIT, 1, MAX_LIMIT) {
            Ok(value) => value,
            Err(error) => return Ok(ToolOutput::error(error)),
        };
        let context_lines =
            match bounded_usize(args, "context", DEFAULT_CONTEXT_LINES, 0, MAX_CONTEXT_LINES) {
                Ok(value) => value,
                Err(error) => return Ok(ToolOutput::error(error)),
            };
        let path = match ctx.resolve_workspace_path(
            args.get("path")
                .and_then(serde_json::Value::as_str)
                .unwrap_or("."),
        ) {
            Ok(path) => path,
            Err(error) => {
                return Ok(ToolOutput::error(format!(
                    "Failed to resolve path: {error}"
                )))
            }
        };
        let glob = args
            .get("glob")
            .and_then(serde_json::Value::as_str)
            .map(str::trim)
            .filter(|glob| !glob.is_empty());

        let persistent_requested = args
            .get("_persistent_index")
            .and_then(serde_json::Value::as_bool)
            .unwrap_or(false);
        if persistent_requested {
            if let Some(output) =
                search_persistent_catalog(query, path.clone(), glob, limit, context_lines, ctx)
                    .await
            {
                return Ok(output);
            }
            // Transparent `bm25` calls continue through the catalog scorer
            // while the durable generation is absent or building.
        }

        if let Some(output) = search_incremental_catalog(
            query,
            path.clone(),
            glob,
            limit,
            context_lines,
            persistent_requested,
            ctx,
        )
        .await
        {
            return Ok(output);
        }

        let Some(search) = ctx.workspace_services.search() else {
            return Ok(ToolOutput::error(
                "bm25 is not available: this workspace backend did not provide search",
            ));
        };
        let candidates = match select_candidates(&terms, &path, glob, search, ctx).await {
            Ok(candidates) => candidates,
            Err(error) => {
                return Ok(ToolOutput::error(format!(
                    "BM25 candidate search failed: {error}"
                )))
            }
        };
        if candidates.paths.is_empty() {
            return Ok(no_matches_output(
                query,
                &terms,
                &candidates,
                &ScanOutcome::default(),
            ));
        }

        let scan = scan_candidates(candidates.paths.clone(), ctx).await;
        if scan.sources.is_empty() {
            if scan.read_files == 0 && scan.failed_reads > 0 {
                return Ok(ToolOutput::error(format!(
                    "BM25 could not read any of the {} candidate file(s)",
                    candidates.paths.len()
                ))
                .with_metadata(search_metadata(&terms, &candidates, &scan)));
            }
            return Ok(no_matches_output(query, &terms, &candidates, &scan));
        }

        let lexical = match build_scan_index(&scan) {
            Ok(index) => index,
            Err(error) => {
                return Ok(
                    ToolOutput::error(format!("BM25 lexical index failed: {error}"))
                        .with_metadata(search_metadata(&terms, &candidates, &scan)),
                )
            }
        };
        let mut ranked = lexical
            .search(&terms, scan.sources.len())
            .map_err(|error| anyhow::anyhow!("lexical FTS query failed: {error}"))?
            .into_iter()
            .filter(|(_, score)| score.is_finite() && *score > 0.0)
            .collect::<Vec<_>>();
        ranked.sort_by(|(left_index, left_score), (right_index, right_score)| {
            right_score
                .total_cmp(left_score)
                .then_with(|| {
                    scan.sources[*left_index]
                        .path
                        .as_str()
                        .cmp(scan.sources[*right_index].path.as_str())
                })
                .then_with(|| {
                    scan.sources[*left_index]
                        .start_line
                        .cmp(&scan.sources[*right_index].start_line)
                })
        });

        let mut per_file = HashMap::new();
        let mut rendered = Vec::new();
        for (index, score) in ranked {
            let source = &scan.sources[index];
            let count = per_file.entry(source.path.as_str()).or_insert(0usize);
            if *count >= MAX_RESULTS_PER_FILE {
                continue;
            }
            *count += 1;
            rendered.push(render_result(
                rendered.len() + 1,
                source,
                score,
                &terms,
                context_lines,
            ));
            if rendered.len() >= limit {
                break;
            }
        }
        if rendered.is_empty() {
            return Ok(no_matches_output(query, &terms, &candidates, &scan));
        }

        let safe_query = escape_control_chars_for_display(truncate_utf8(query, 256));
        let mut content = format!("BM25 results for: {safe_query}\n\n");
        content.push_str(
            &rendered
                .iter()
                .map(|result| result.content.as_str())
                .collect::<Vec<_>>()
                .join("\n\n"),
        );
        content.push_str(&format!(
            "\n\n{} result(s); {} file(s) read; {} chunk(s) scored",
            rendered.len(),
            scan.read_files,
            scan.sources.len()
        ));
        if candidates.backend_truncated || candidates.candidate_truncated || scan.truncated {
            content.push_str("\nWarning: search limits truncated the candidate corpus.");
        }

        let mut seen_anchors = HashSet::new();
        let source_anchors = rendered
            .iter()
            .filter(|result| seen_anchors.insert(result.source_anchor.clone()))
            .map(|result| result.source_anchor.clone())
            .collect::<Vec<_>>();
        let results = rendered
            .into_iter()
            .map(|result| result.metadata)
            .collect::<Vec<_>>();
        let mut metadata = search_metadata(&terms, &candidates, &scan);
        metadata["source_anchors"] = serde_json::json!(source_anchors);
        metadata["results"] = serde_json::json!(results);
        metadata["returned_results"] = serde_json::json!(results.len());

        Ok(ToolOutput::success(content).with_metadata(metadata))
    }
}

async fn search_persistent_catalog(
    query: &str,
    path: WorkspacePath,
    glob: Option<&str>,
    limit: usize,
    context_lines: usize,
    ctx: &ToolContext,
) -> Option<ToolOutput> {
    let index = ctx.workspace_services.persistent_index()?;
    if !index.is_ready() {
        return None;
    }
    let mut request = LexicalSearchRequest::new(query);
    request.path = path.clone();
    request.glob = glob.map(str::to_owned);
    // Overfetch so stale top-ranked chunks can be removed without making a
    // fresh lower-ranked result disappear from the requested page.
    request.limit = limit.saturating_mul(4).min(MAX_LIMIT);
    request.max_candidate_files = MAX_CANDIDATE_FILES;
    request.max_results_per_file = MAX_RESULTS_PER_FILE;
    let query_task = tokio::task::spawn_blocking(move || index.search(&request));
    let mut result = match query_task.await {
        Ok(Ok(result)) => result,
        Ok(Err(error)) => {
            tracing::debug!(%error, "workspace persistent bm25 query failed; falling back");
            return None;
        }
        Err(error) => {
            tracing::debug!(%error, "workspace persistent bm25 query task failed; falling back");
            return None;
        }
    };
    let cancellation = ctx.cancellation_token();
    let (verified, filtered_stale, verification_truncated) =
        match crate::workspace::retrieval::retain_verified(
            result.hits,
            limit,
            |hit| hit.chunk.as_ref(),
            ctx.workspace_services.fs().as_ref(),
            ctx.workspace_services.operation_timeout(),
            &cancellation,
            &cancellation,
        )
        .await
        {
            Ok(result) => result,
            Err(error) => {
                tracing::debug!(%error, "workspace persistent bm25 verification failed; falling back");
                return None;
            }
        };
    result.hits = verified;
    // A generation can remain readable while the manifest has already
    // published a newer source revision. Returning that generation would make
    // edits appear to disappear until the native rebuild completes. Fall back
    // to the catalog in this window; it is the live admission authority and
    // keeps the model-visible `bm25` call available during native churn.
    let catalog_source_revision = ctx
        .workspace_services
        .chunk_catalog()
        .and_then(|catalog| catalog.snapshot().ok())
        .map(|snapshot| snapshot.source_revision())
        .unwrap_or(result.source_revision);
    let catalog_is_newer = catalog_source_revision > result.source_revision;
    // If verification observed a stale native hit, prefer a catalog fallback
    // only after verifying its candidates against the live filesystem too. If
    // that fallback is not ready, the filtered native response remains safe.
    if catalog_is_newer && (result.hits.is_empty() || filtered_stale) {
        if let Some(output) =
            search_incremental_catalog(query, path, glob, limit, context_lines, true, ctx).await
        {
            return Some(output);
        }
    }
    let mut output = render_incremental_result(query, result, context_lines);
    if let Some(serde_json::Value::Object(metadata)) = output.metadata.as_mut() {
        metadata.insert(
            "index_kind".to_owned(),
            serde_json::json!("persistent_zvec_fts"),
        );
        metadata.insert(
            "execution_mode".to_owned(),
            serde_json::json!("persistent_zvec_fts"),
        );
        metadata.insert("source_verified".to_owned(), serde_json::json!(true));
        metadata.insert(
            "freshness".to_owned(),
            serde_json::json!(if catalog_is_newer {
                "rebuilding"
            } else if filtered_stale {
                "possibly_stale"
            } else {
                "ready"
            }),
        );
        metadata.insert(
            "catalog_source_revision".to_owned(),
            serde_json::json!(catalog_source_revision),
        );
        metadata.insert(
            "filtered_stale".to_owned(),
            serde_json::json!(filtered_stale),
        );
        metadata.insert(
            "verification_truncated".to_owned(),
            serde_json::json!(verification_truncated),
        );
    }
    Some(output)
}

async fn search_incremental_catalog(
    query: &str,
    path: WorkspacePath,
    glob: Option<&str>,
    limit: usize,
    context_lines: usize,
    verify_sources: bool,
    ctx: &ToolContext,
) -> Option<ToolOutput> {
    let catalog = ctx.workspace_services.chunk_catalog()?;
    let snapshot = match catalog.snapshot() {
        Ok(snapshot) if snapshot.source_revision() > 0 => snapshot,
        Ok(_) => return None,
        Err(error) => {
            tracing::warn!(%error, "BM25 catalog snapshot failed; using query-time fallback");
            return None;
        }
    };
    let mut request = LexicalSearchRequest::new(query);
    request.path = path;
    request.glob = glob.map(str::to_owned);
    request.limit = limit;
    request.max_candidate_files = MAX_CANDIDATE_FILES;
    request.max_results_per_file = MAX_RESULTS_PER_FILE;
    let mut result = match snapshot.lexical_search(&request) {
        Ok(result) => result,
        Err(error) => {
            return Some(ToolOutput::error(format!(
                "BM25 catalog search failed: {error}"
            )))
        }
    };
    if verify_sources {
        let cancellation = ctx.cancellation_token();
        let (verified, filtered_stale, verification_truncated) =
            match crate::workspace::retrieval::retain_verified(
                result.hits,
                limit,
                |hit| hit.chunk.as_ref(),
                ctx.workspace_services.fs().as_ref(),
                ctx.workspace_services.operation_timeout(),
                &cancellation,
                &cancellation,
            )
            .await
            {
                Ok(result) => result,
                Err(error) => {
                    tracing::debug!(%error, "BM25 catalog verification failed during native fallback");
                    return None;
                }
            };
        result.hits = verified;
        let mut output = render_incremental_result(query, result, context_lines);
        if let Some(serde_json::Value::Object(metadata)) = output.metadata.as_mut() {
            metadata.insert("source_verified".to_owned(), serde_json::json!(true));
            metadata.insert(
                "filtered_stale".to_owned(),
                serde_json::json!(filtered_stale),
            );
            metadata.insert(
                "verification_truncated".to_owned(),
                serde_json::json!(verification_truncated),
            );
            metadata.insert(
                "freshness".to_owned(),
                serde_json::json!(if filtered_stale {
                    "possibly_stale"
                } else {
                    "ready"
                }),
            );
        }
        return Some(output);
    }

    Some(render_incremental_result(query, result, context_lines))
}

fn render_incremental_result(
    query: &str,
    result: LexicalSearchResult,
    context_lines: usize,
) -> ToolOutput {
    let candidate_truncated = result.candidate_truncated;
    let terms = result.query_terms.clone();
    let mut rendered = Vec::with_capacity(result.hits.len());
    for (rank, hit) in result.hits.iter().enumerate() {
        let source = ChunkSource {
            path: WorkspacePath::from_normalized(hit.chunk.path.as_ref()),
            start_line: hit.chunk.start_line.saturating_sub(1),
            lines: hit.chunk.text.lines().map(str::to_owned).collect(),
        };
        rendered.push(render_result(
            rank + 1,
            &source,
            hit.score,
            &terms,
            context_lines,
        ));
    }
    let metadata = incremental_search_metadata(&result, &rendered);
    if rendered.is_empty() {
        let safe_query = escape_control_chars_for_display(truncate_utf8(query, 256));
        return ToolOutput::success(format!("No BM25 matches found for query: {safe_query}"))
            .with_metadata(metadata);
    }

    let safe_query = escape_control_chars_for_display(truncate_utf8(query, 256));
    let mut content = format!("BM25 results for: {safe_query}\n\n");
    content.push_str(
        &rendered
            .iter()
            .map(|result| result.content.as_str())
            .collect::<Vec<_>>()
            .join("\n\n"),
    );
    content.push_str(&format!(
        "\n\n{} result(s); 0 file(s) read; {} indexed chunk(s) considered",
        rendered.len(),
        result.scored_chunks
    ));
    if candidate_truncated {
        content.push_str("\nWarning: search limits truncated the candidate corpus.");
    }
    ToolOutput::success(content).with_metadata(metadata)
}

fn incremental_search_metadata(
    result: &LexicalSearchResult,
    rendered: &[RenderedResult],
) -> serde_json::Value {
    let mut seen_anchors = HashSet::new();
    let source_anchors = rendered
        .iter()
        .filter(|result| seen_anchors.insert(result.source_anchor.clone()))
        .map(|result| result.source_anchor.clone())
        .collect::<Vec<_>>();
    let results = rendered
        .iter()
        .map(|result| result.metadata.clone())
        .collect::<Vec<_>>();
    serde_json::json!({
        "algorithm": "bm25",
        "mode": "incremental_catalog",
        "parameters": {
            "engine": result.lexical_engine.stable_id(),
            "tokenizer": "whitespace",
            "chunk_lines": CHUNK_LINES,
        },
        "query_terms": result.query_terms,
        "catalog": {
            "revision": result.catalog_revision,
            "source_revision": result.source_revision,
        },
        "candidate_search": {
            "matching_files": result.matching_files,
            "selected_files": result.selected_files,
            "backend_truncated": false,
            "candidate_truncated": result.candidate_truncated,
            "used_glob_fallback": false,
        },
        "scan": {
            "read_files": 0,
            "failed_reads": 0,
            "oversized_files": 0,
            "scanned_bytes": 0,
            "scored_chunks": result.scored_chunks,
            "truncated": result.candidate_truncated,
        },
        "source_anchors": source_anchors,
        "results": results,
        "returned_results": results.len(),
    })
}

fn bounded_usize(
    args: &serde_json::Value,
    name: &str,
    default: usize,
    minimum: usize,
    maximum: usize,
) -> std::result::Result<usize, String> {
    let Some(value) = args.get(name) else {
        return Ok(default);
    };
    let Some(value) = value.as_u64().and_then(|value| usize::try_from(value).ok()) else {
        return Err(format!(
            "{name} must be an integer from {minimum} to {maximum}"
        ));
    };
    if !(minimum..=maximum).contains(&value) {
        return Err(format!("{name} must be from {minimum} to {maximum}"));
    }
    Ok(value)
}

async fn select_candidates(
    terms: &[String],
    path: &WorkspacePath,
    glob: Option<&str>,
    search: std::sync::Arc<dyn crate::workspace::WorkspaceSearch>,
    ctx: &ToolContext,
) -> Result<CandidateSelection> {
    let mut pattern_terms = terms.to_vec();
    pattern_terms.sort_by_key(|term| std::cmp::Reverse(term.len()));
    let pattern = pattern_terms
        .iter()
        .map(|term| regex::escape(term))
        .collect::<Vec<_>>()
        .join("|");
    let request = WorkspaceGrepRequest {
        base: path.clone(),
        pattern,
        glob: glob.map(str::to_string),
        context_lines: 0,
        case_insensitive: true,
        max_output_size: 0,
    };
    let grep_search = std::sync::Arc::clone(&search);
    let outcome = ctx
        .workspace_services
        .run_with_timeout("bm25 candidate search", async move {
            grep_search.grep_with_sources(request).await
        })
        .await?;
    let matching_files = outcome.result.file_count;
    let backend_truncated = outcome.result.truncated;

    let (raw_paths, used_glob_fallback) = match outcome.matched_paths {
        Some(paths) => (paths, false),
        None => {
            let request = WorkspaceGlobRequest {
                base: path.clone(),
                pattern: glob.unwrap_or("**/*").to_string(),
            };
            let paths = ctx
                .workspace_services
                .run_with_timeout(
                    "bm25 candidate glob",
                    async move { search.glob(request).await },
                )
                .await?
                .matches;
            (paths, true)
        }
    };
    let raw_path_count = raw_paths.len();
    let mut seen = HashSet::new();
    let paths = raw_paths
        .into_iter()
        .filter_map(|path| ctx.resolve_workspace_path(path.as_str()).ok())
        .filter(|path| !path.is_root() && seen.insert(path.as_str().to_string()))
        .take(MAX_CANDIDATE_FILES)
        .collect::<Vec<_>>();

    Ok(CandidateSelection {
        paths,
        matching_files,
        backend_truncated,
        candidate_truncated: raw_path_count > MAX_CANDIDATE_FILES,
        used_glob_fallback,
    })
}

async fn scan_candidates(paths: Vec<WorkspacePath>, ctx: &ToolContext) -> ScanOutcome {
    let calls = paths.into_iter().map(|path| {
        let services = ctx.workspace_services.clone();
        async move {
            let fs = services.fs();
            let read_path = path.clone();
            let result = services
                .run_with_timeout("bm25 read", async move { fs.read_text(&read_path).await })
                .await;
            (path, result)
        }
    });
    let mut reads = stream::iter(calls).buffered(READ_CONCURRENCY);
    let mut outcome = ScanOutcome::default();

    while let Some((path, result)) = reads.next().await {
        let content = match result {
            Ok(content) => content,
            Err(_) => {
                outcome.failed_reads += 1;
                continue;
            }
        };
        if content.len() > MAX_FILE_BYTES {
            outcome.oversized_files += 1;
            continue;
        }
        if outcome.scanned_bytes.saturating_add(content.len()) > MAX_TOTAL_BYTES {
            outcome.truncated = true;
            break;
        }
        outcome.scanned_bytes += content.len();
        outcome.read_files += 1;

        let lines = content.lines().map(str::to_string).collect::<Vec<_>>();
        let chunk_count = lines.len().div_ceil(CHUNK_LINES);
        if chunk_count > MAX_CHUNKS_PER_FILE {
            outcome.truncated = true;
        }
        for (chunk_index, chunk_lines) in lines
            .chunks(CHUNK_LINES)
            .take(MAX_CHUNKS_PER_FILE)
            .enumerate()
        {
            let text = chunk_lines.join("\n");
            if tokenize(&text).is_empty() {
                continue;
            }
            outcome.sources.push(ChunkSource {
                path: path.clone(),
                start_line: chunk_index * CHUNK_LINES,
                lines: chunk_lines.to_vec(),
            });
        }
    }
    outcome
}

fn build_scan_index(scan: &ScanOutcome) -> Result<LexicalIndex> {
    build_lexical_index(
        scan.sources
            .iter()
            .enumerate()
            .map(|(index, source)| (format!("scan-{index}"), source.lines.join("\n"))),
        crate::workspace::WorkspaceLexicalEngine::default(),
    )
    .map_err(|error| anyhow::anyhow!(error.to_string()))
}

fn render_result(
    rank: usize,
    source: &ChunkSource,
    score: f64,
    terms: &[String],
    context_lines: usize,
) -> RenderedResult {
    let query_terms = terms.iter().map(String::as_str).collect::<HashSet<_>>();
    let best_line = source
        .lines
        .iter()
        .enumerate()
        .max_by(|(left_index, left), (right_index, right)| {
            line_match_count(left, &query_terms)
                .cmp(&line_match_count(right, &query_terms))
                .then_with(|| right_index.cmp(left_index))
        })
        .map(|(index, _)| index)
        .unwrap_or_default();
    let local_start = best_line.saturating_sub(context_lines);
    let local_end = (best_line + context_lines + 1).min(source.lines.len());
    let start_line = source.start_line + local_start + 1;
    let end_line = source.start_line + local_end;
    let safe_path = escape_control_chars_for_display(source.path.as_str());
    let mut content = format!("{rank}. {safe_path}:{start_line}-{end_line} (score {score:.4})");
    for (offset, line) in source.lines[local_start..local_end].iter().enumerate() {
        let safe_line = escape_control_chars_for_display(line);
        let rendered = truncate_utf8(&safe_line, MAX_RENDERED_LINE_BYTES);
        content.push_str(&format!("\n{:>6} | {rendered}", start_line + offset));
        if rendered.len() < safe_line.len() {
            content.push('');
        }
    }

    RenderedResult {
        content,
        metadata: serde_json::json!({
            "path": source.path.as_str(),
            "start_line": start_line,
            "end_line": end_line,
            "score": score,
        }),
        source_anchor: source.path.as_str().to_string(),
    }
}

fn line_match_count(line: &str, query_terms: &HashSet<&str>) -> usize {
    tokenize(line)
        .iter()
        .filter(|term| query_terms.contains(term.as_str()))
        .count()
}

fn no_matches_output(
    query: &str,
    terms: &[String],
    candidates: &CandidateSelection,
    scan: &ScanOutcome,
) -> ToolOutput {
    let safe_query = escape_control_chars_for_display(truncate_utf8(query, 256));
    ToolOutput::success(format!("No BM25 matches found for query: {safe_query}"))
        .with_metadata(search_metadata(terms, candidates, scan))
}

fn search_metadata(
    terms: &[String],
    candidates: &CandidateSelection,
    scan: &ScanOutcome,
) -> serde_json::Value {
    serde_json::json!({
        "algorithm": "bm25",
        "parameters": {
            "engine": crate::workspace::WorkspaceLexicalEngine::default().stable_id(),
            "tokenizer": "whitespace",
            "chunk_lines": CHUNK_LINES,
        },
        "query_terms": terms,
        "candidate_search": {
            "matching_files": candidates.matching_files,
            "selected_files": candidates.paths.len(),
            "backend_truncated": candidates.backend_truncated,
            "candidate_truncated": candidates.candidate_truncated,
            "used_glob_fallback": candidates.used_glob_fallback,
        },
        "scan": {
            "read_files": scan.read_files,
            "failed_reads": scan.failed_reads,
            "oversized_files": scan.oversized_files,
            "scanned_bytes": scan.scanned_bytes,
            "scored_chunks": scan.sources.len(),
            "truncated": scan.truncated,
        },
    })
}