diff --git a/speedwagon/src/store/description.rs b/speedwagon/src/store/description.rs
new file mode 100644
index 0000000..5b5de60
--- /dev/null
+++ b/speedwagon/src/store/description.rs
@@ -0,0 +1,316 @@
+//! KB-level description for a routing agent. Built from each doc's `purpose`
+//! (titles are kept for the fallback). Output stays self-contained — no
+//! mention of peer KBs — so a renamed or removed neighbor cannot stale it.
+//! Korean output lands ~1/3 the chars of English at the same budget;
+//! per-language budgets are deferred until a near-domain Korean KB shows up.
+
+use ailoy::{
+    agent::{Agent, AgentProvider, AgentSpec},
+    message::{Message, Part, Role},
+};
+use anyhow::{Context as _, Result};
+use futures::StreamExt as _;
+
+const MODEL: &str = "openai/gpt-5.4-mini";
+
+const DESCRIPTION_INSTRUCTION: &str = concat!(
+    "You write a self-contained description of a knowledge base. ",
+    "This description will be read by a routing agent that picks the right ",
+    "knowledge base for a user's question. ",
+    "Inputs: KB name, optional instruction, and a list of one-line document purposes. ",
+    "Describe what is INSIDE this knowledge base — its document types, ",
+    "the entities and time periods covered, and the topics it can answer. ",
+    "Lead with the collective identity of the documents. ",
+    "Describe this KB on its own terms — do not compare it to other KBs, ",
+    "list what it excludes, or mention neighboring KB names. ",
+    "Output must NOT mention dataset names, QA pairs, paper IDs, contract IDs, ",
+    "or any metadata about how this knowledge base was assembled. ",
+    "Describe ONLY what documents are inside, as if a curator wrote it. ",
+    "Write the description in English regardless of the document language. ",
+    "Length: ~200 characters. Output a JSON object: {\"description\": \"<text>\"}."
+);
+
+pub struct DescriptionAgent {
+    spec: AgentSpec,
+    provider: Option<AgentProvider>,
+}
+
+impl DescriptionAgent {
+    pub fn new(provider: Option<AgentProvider>) -> Self {
+        Self {
+            spec: AgentSpec::new(MODEL).instruction(DESCRIPTION_INSTRUCTION),
+            provider,
+        }
+    }
+
+    /// Only `purpose` is sent to the LLM; `title` is kept in the signature
+    /// so the caller can feed the same slice to `fallback_description`.
+    pub async fn generate(
+        &self,
+        kb_name: &str,
+        instruction: Option<&str>,
+        docs: &[(&str, &str)],
+    ) -> Result<String> {
+        let user = build_user_message(kb_name, instruction, docs);
+        let query = Message::new(Role::User).with_contents([Part::text(user)]);
+
+        let mut agent = match &self.provider {
+            Some(provider) => Agent::try_with_provider(self.spec.clone(), provider).await?,
+            None => Agent::try_new(self.spec.clone()).await?,
+        };
+
+        let mut text_parts: Vec<String> = Vec::new();
+        {
+            let mut stream = agent.run(query);
+            while let Some(result) = stream.next().await {
+                let output = result?;
+                for part in &output.message.contents {
+                    if let Some(text) = part.as_text() {
+                        text_parts.push(text.to_string());
+                    }
+                }
+            }
+        }
+        let raw = text_parts.join("");
+        Ok(parse_description_response(&raw))
+    }
+}
+
+fn build_user_message(
+    kb_name: &str,
+    instruction: Option<&str>,
+    docs: &[(&str, &str)],
+) -> String {
+    let mut s = String::new();
+    s.push_str(&format!("KB name: {kb_name}\n"));
+    if let Some(instr) = instruction {
+        if !instr.trim().is_empty() {
+            s.push_str(&format!("KB instruction: {instr}\n"));
+        }
+    }
+    s.push_str(&format!("\nDocuments ({}):\n", docs.len()));
+    for (_title, purpose) in docs {
+        let p = if purpose.is_empty() { "(no purpose)" } else { *purpose };
+        s.push_str(&format!("- {p}\n"));
+    }
+    s
+}
+
+/// Empty return signals the caller to use `fallback_description`.
+fn parse_description_response(raw: &str) -> String {
+    let trimmed = raw.trim();
+    if trimmed.is_empty() {
+        return String::new();
+    }
+
+    if let Ok(value) = serde_json::from_str::<serde_json::Value>(trimmed) {
+        if let Some(d) = value.get("description").and_then(|v| v.as_str()) {
+            return d.trim().to_string();
+        }
+    }
+
+    if let (Some(start), Some(end)) = (trimmed.find('{'), trimmed.rfind('}')) {
+        if start < end {
+            if let Ok(value) = serde_json::from_str::<serde_json::Value>(&trimmed[start..=end]) {
+                if let Some(d) = value.get("description").and_then(|v| v.as_str()) {
+                    return d.trim().to_string();
+                }
+            }
+        }
+    }
+
+    String::new()
+}
+
+/// Reads `OPENAI_API_KEY` from the environment, runs `DescriptionAgent`, and
+/// substitutes `fallback_description` if the LLM body is empty. Transport
+/// errors (missing key, network) propagate.
+pub async fn get_description(
+    kb_name: &str,
+    instruction: Option<&str>,
+    docs: &[(&str, &str)],
+) -> Result<String> {
+    dotenvy::dotenv().ok();
+
+    let mut provider = AgentProvider::new();
+    provider.model_openai(
+        std::env::var("OPENAI_API_KEY").context("OPENAI_API_KEY not set in environment")?,
+    );
+
+    let agent = DescriptionAgent::new(Some(provider));
+    let result = agent.generate(kb_name, instruction, docs).await?;
+    if result.is_empty() {
+        log::warn!("description generation returned empty string; using fallback");
+        let titles: Vec<&str> = docs.iter().map(|(t, _)| *t).collect();
+        Ok(fallback_description(docs.len(), &titles))
+    } else {
+        Ok(result)
+    }
+}
+
+/// Deterministic fallback when the LLM call fails or returns empty.
+pub fn fallback_description(doc_count: usize, top_titles: &[&str]) -> String {
+    if doc_count == 0 {
+        return String::new();
+    }
+    let titles: Vec<&str> = top_titles
+        .iter()
+        .filter(|t| !t.is_empty())
+        .take(5)
+        .copied()
+        .collect();
+    if titles.is_empty() {
+        format!("{doc_count} documents")
+    } else {
+        format!("{doc_count} documents including: {}", titles.join("; "))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_direct_json() {
+        let raw = r#"{"description": "hello"}"#;
+        assert_eq!(parse_description_response(raw), "hello");
+    }
+
+    #[test]
+    fn parse_json_with_surrounding_text() {
+        let raw = r#"Here you go: {"description": "hello"} done."#;
+        assert_eq!(parse_description_response(raw), "hello");
+    }
+
+    #[test]
+    fn parse_trims_inner_whitespace() {
+        let raw = r#"{"description": "   hello   "}"#;
+        assert_eq!(parse_description_response(raw), "hello");
+    }
+
+    #[test]
+    fn parse_empty_input() {
+        assert_eq!(parse_description_response(""), "");
+        assert_eq!(parse_description_response("   "), "");
+    }
+
+    #[test]
+    fn parse_missing_field() {
+        let raw = r#"{"other": "value"}"#;
+        assert_eq!(parse_description_response(raw), "");
+    }
+
+    #[test]
+    fn parse_malformed_json() {
+        assert_eq!(parse_description_response("{not json"), "");
+    }
+
+    #[test]
+    fn fallback_zero_docs() {
+        assert_eq!(fallback_description(0, &[]), "");
+    }
+
+    #[test]
+    fn fallback_no_titles_falls_back_to_count_only() {
+        assert_eq!(fallback_description(7, &[]), "7 documents");
+        assert_eq!(fallback_description(7, &["", ""]), "7 documents");
+    }
+
+    #[test]
+    fn fallback_with_titles_takes_top_five() {
+        let titles = ["A", "B", "C", "D", "E", "F", "G"];
+        assert_eq!(
+            fallback_description(7, &titles),
+            "7 documents including: A; B; C; D; E"
+        );
+    }
+
+    #[test]
+    fn user_message_skips_blank_instruction() {
+        let docs = [("T1", "P1"), ("T2", "")];
+        let msg = build_user_message("kb1", Some("   "), &docs);
+        assert!(msg.contains("KB name: kb1"));
+        assert!(!msg.contains("KB instruction"));
+        assert!(msg.contains("- P1"));
+        assert!(msg.contains("- (no purpose)"));
+    }
+
+    #[test]
+    fn user_message_renders_purpose_only_not_title() {
+        let docs = [("Apple 10-K", "Apple FY2021 annual report")];
+        let msg = build_user_message("finance", None, &docs);
+        // purpose is in, title is not
+        assert!(msg.contains("Apple FY2021 annual report"));
+        assert!(!msg.contains("Apple 10-K"));
+    }
+
+    // ---- Store integration ----
+
+    #[tokio::test]
+    async fn describe_returns_empty_for_empty_store() {
+        // Empty store → describe short-circuits before any LLM call,
+        // so this runs without OPENAI_API_KEY.
+        let tmp = tempfile::tempdir().expect("tempdir");
+        let store = crate::store::Store::new(tmp.path()).expect("open store");
+        let result = store
+            .describe("kb-empty", None)
+            .await
+            .expect("describe should not fail on empty store");
+        assert_eq!(result, "");
+    }
+
+    #[tokio::test]
+    #[ignore = "requires OPENAI_API_KEY"]
+    async fn describe_round_trips_against_pre_populated_index() {
+        // Pre-populate the index with deterministic (title, purpose) pairs
+        // so this test only exercises the description path, not PurposeAgent
+        // / TitleAgent. Mirrors the indexer round-trip tests in spirit.
+        let tmp = tempfile::tempdir().expect("tempdir");
+        let root = tmp.path();
+        let index_dir = root.join("index");
+        let index = crate::store::indexer::open_or_create(&index_dir).expect("open index");
+        crate::store::indexer::add_document(
+            &index,
+            "doc1",
+            "Apple FY2021 10-K",
+            "Apple Inc. 2021 Form 10-K annual report — revenue, iPhone, services, SEC filing",
+            "body 1",
+        )
+        .expect("add doc1");
+        crate::store::indexer::add_document(
+            &index,
+            "doc2",
+            "Walmart FY2023 10-K",
+            "Walmart Inc. FY2023 Form 10-K annual report — omnichannel, retail, SEC filing",
+            "body 2",
+        )
+        .expect("add doc2");
+        // drop the writer-backed Index so Store::new can reopen the same dir.
+        drop(index);
+
+        // We need origin/ and corpus/ to exist for Store::new.
+        std::fs::create_dir_all(root.join("origin")).unwrap();
+        std::fs::create_dir_all(root.join("corpus")).unwrap();
+
+        let store = crate::store::Store::new(root).expect("open store");
+        let description = store
+            .describe("finance", Some("public-company financial filings"))
+            .await
+            .expect("describe");
+
+        assert!(!description.is_empty(), "description should not be empty");
+        assert!(
+            description.chars().count() < 600,
+            "description should stay short: {description:?}"
+        );
+        // The model should pick up at least one of the entity tokens we fed in.
+        let lowered = description.to_lowercase();
+        let mentions_finance_token = ["apple", "walmart", "10-k", "filing", "financial"]
+            .iter()
+            .any(|t| lowered.contains(t));
+        assert!(
+            mentions_finance_token,
+            "description did not mention any expected token: {description:?}"
+        );
+    }
+}
diff --git a/speedwagon/src/store/mod.rs b/speedwagon/src/store/mod.rs
index 4e8f024..b1e37db 100644
--- a/speedwagon/src/store/mod.rs
+++ b/speedwagon/src/store/mod.rs
@@ -1,3 +1,4 @@
+mod description;
 mod document;
 mod indexer;
 mod parser;
@@ -15,6 +16,7 @@ use anyhow::{Context as _, Result};
 use tantivy::Index;
 use uuid::Uuid;
 
+pub use description::{DescriptionAgent, fallback_description};
 pub use document::{Document, FindResult};
 pub use searcher::{SearchPage, SearchResult};
 
@@ -254,6 +256,26 @@ impl Store {
             context_bytes,
         ))
     }
+
+    /// One LLM call over every doc's `(title, purpose)` in the index. Input
+    /// is proportional to doc count (~24K chars at N=200), so don't run this
+    /// synchronously on indexing hot paths — use a finalize hook or
+    /// background job. Empty LLM body falls back to a deterministic string.
+    pub async fn describe(
+        &self,
+        kb_name: &str,
+        instruction: Option<&str>,
+    ) -> Result<String> {
+        let docs = indexer::list_documents(&self.index, false)?;
+        if docs.is_empty() {
+            return Ok(String::new());
+        }
+        let pairs: Vec<(&str, &str)> = docs
+            .iter()
+            .map(|d| (d.title.as_str(), d.purpose.as_str()))
+            .collect();
+        description::get_description(kb_name, instruction, &pairs).await
+    }
 }
 
 #[cfg(test)]