diff --git a/src/main.rs b/src/main.rs
index 63b3199..0aab491 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,6 +7,7 @@ mod filter_parser;
 mod index_key;
 mod ingest;
 mod metric_primitive;
+mod profile;
 mod settings;
 mod timestamp;
 
@@ -17,6 +18,7 @@ use clap::Parser;
 use tokio::net::TcpListener;
 use tokio::sync::mpsc;
 
+use profile::ProfileStore;
 use settings::Settings;
 
 /// Observability dashboard from terminal output.
@@ -48,6 +50,38 @@ async fn main() {
 
     let (engine_tx, engine_rx) = mpsc::channel(1024);
     tokio::spawn(engine::run(engine_rx));
+
+    // Ticket 009: reload the active profile's param/metric declarations into the
+    // engine BEFORE starting ingest below. Ordering matters: declarations must be
+    // in place before any line is ingested so that backfill/live-derivation
+    // semantics (ticket 003) apply uniformly to every line, including ones that
+    // arrive immediately once ingest starts (there's no "declare after the fact
+    // and go back to reprocess early lines" step, so doing it after ingest starts
+    // could silently miss lines that raced ahead of the declarations).
+    {
+        let (active_profile_name, profiles_dir) = {
+            let settings_guard = settings.lock().unwrap();
+            (
+                settings_guard.profiles.active_profile.clone(),
+                profile::resolve_profiles_dir(&settings_guard, &settings_path),
+            )
+        };
+        if let Some(profile_name) = active_profile_name {
+            let store = ProfileStore::load(&profile::profiles_store_path(&profiles_dir));
+            match store.get(&profile_name) {
+                Some(active_profile) => {
+                    profile::load_into_engine(&engine_tx, active_profile).await;
+                }
+                None => {
+                    eprintln!(
+                        "warning: active profile '{profile_name}' not found under {}; skipping",
+                        profiles_dir.display()
+                    );
+                }
+            }
+        }
+    }
+
     tokio::spawn(ingest::run(engine_tx.clone()));
 
     let app = api::router(engine_tx, &args.static_dir, settings, settings_path);
diff --git a/src/profile.rs b/src/profile.rs
new file mode 100644
index 0000000..4ed28a5
--- /dev/null
+++ b/src/profile.rs
@@ -0,0 +1,761 @@
+//! Named, persistable bundles of parsing/indexing rules ("profiles" / "presets"),
+//! ticket 009.
+//!
+//! A `Profile` bundles the param declarations (ticket 001) and metric
+//! declarations (ticket 002) needed to derive a given CLI app's structure from
+//! its raw log output, plus any saved dashboards for that app. Profiles are
+//! collected in a `ProfileStore`, persisted as a single `profiles.json` file
+//! under `Settings.profiles.profiles_dir` (resolved relative to the settings
+//! file's directory — the same convention ticket 010 uses for CSS storage). A
+//! single file (rather than one file per profile) is simplest and avoids
+//! filename-sanitization concerns for profile names.
+//!
+//! `Settings.profiles` (see `settings.rs`) only tracks lightweight pointers —
+//! known profile names and which one is active — while this module owns the
+//! actual declaration content. That mirrors how `Settings.css_entries` holds
+//! lightweight `CssEntry` records while the CSS content itself lives in
+//! separate files (ticket 010).
+//!
+//! On startup, `main.rs` loads the active profile's declarations into a fresh
+//! engine by replaying `EngineMessage::DeclareParam`/`DeclareMetric` for each
+//! stored declaration (see `load_into_engine` below) — this reproduces the
+//! same parsing/indexing rules the profile had when last saved, without
+//! requiring the user to re-declare anything by hand after a restart.
+//!
+//! Several public methods here (e.g. `ProfileStore::delete`,
+//! `record_param_declaration`) have no caller yet within this worktree — CRUD
+//! over profiles isn't exposed over HTTP until ticket 004 lands the API layer
+//! and calls into this module. `#![allow(dead_code)]` mirrors how
+//! `engine.rs`/`data_value.rs` handle the same not-yet-wired situation for
+//! tickets 001/002.
+#![allow(dead_code)]
+
+use std::fmt;
+use std::fs;
+use std::io;
+use std::path::{Path, PathBuf};
+
+use serde::{Deserialize, Serialize};
+
+use crate::data_value::{MetricPrimitiveKind, RegexToMetricPrimitiveMatcher};
+use crate::engine::{DeclareMetricRequest, DeclareParamRequest, EngineHandle, EngineMessage};
+use crate::settings::Settings;
+use tokio::sync::oneshot;
+
+/// Mirrors `data_value::MetricPrimitiveKind` with `Serialize`/`Deserialize` so
+/// profiles can round-trip through JSON. `MetricPrimitiveKind` itself only
+/// derives `Serialize` (it's an engine-internal type owned by tickets
+/// 001/002/003, out of scope for this ticket to modify), so this module keeps
+/// its own mirror and converts at the boundary.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum ProfileMetricPrimitiveKind {
+    String,
+    Number,
+    Timestamp,
+}
+
+impl From<MetricPrimitiveKind> for ProfileMetricPrimitiveKind {
+    fn from(kind: MetricPrimitiveKind) -> Self {
+        match kind {
+            MetricPrimitiveKind::String => ProfileMetricPrimitiveKind::String,
+            MetricPrimitiveKind::Number => ProfileMetricPrimitiveKind::Number,
+            MetricPrimitiveKind::Timestamp => ProfileMetricPrimitiveKind::Timestamp,
+        }
+    }
+}
+
+impl From<ProfileMetricPrimitiveKind> for MetricPrimitiveKind {
+    fn from(kind: ProfileMetricPrimitiveKind) -> Self {
+        match kind {
+            ProfileMetricPrimitiveKind::String => MetricPrimitiveKind::String,
+            ProfileMetricPrimitiveKind::Number => MetricPrimitiveKind::Number,
+            ProfileMetricPrimitiveKind::Timestamp => MetricPrimitiveKind::Timestamp,
+        }
+    }
+}
+
+/// A stored param declaration, matching `DeclareParamRequest`'s shape (ticket 001).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct ParamDeclarationRecord {
+    pub name: String,
+    pub message_match_regex: String,
+    pub capture_group: String,
+    pub kind: ProfileMetricPrimitiveKind,
+}
+
+impl From<&DeclareParamRequest> for ParamDeclarationRecord {
+    fn from(request: &DeclareParamRequest) -> Self {
+        ParamDeclarationRecord {
+            name: request.name.clone(),
+            message_match_regex: request.message_match_regex.clone(),
+            capture_group: request.capture_group.clone(),
+            kind: request.kind.into(),
+        }
+    }
+}
+
+impl From<&ParamDeclarationRecord> for DeclareParamRequest {
+    fn from(record: &ParamDeclarationRecord) -> Self {
+        DeclareParamRequest {
+            name: record.name.clone(),
+            message_match_regex: record.message_match_regex.clone(),
+            capture_group: record.capture_group.clone(),
+            kind: record.kind.into(),
+        }
+    }
+}
+
+/// A stored metric param matcher, matching `RegexToMetricPrimitiveMatcher`'s shape.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct MetricParamRecord {
+    pub param_name: String,
+    pub capture_group: String,
+    pub kind: ProfileMetricPrimitiveKind,
+}
+
+/// A stored metric declaration, matching `DeclareMetricRequest`'s shape (ticket 002).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct MetricDeclarationRecord {
+    pub name: String,
+    pub message_match_regex: String,
+    pub params: Vec<MetricParamRecord>,
+}
+
+impl From<&DeclareMetricRequest> for MetricDeclarationRecord {
+    fn from(request: &DeclareMetricRequest) -> Self {
+        MetricDeclarationRecord {
+            name: request.name.clone(),
+            message_match_regex: request.message_match_regex.clone(),
+            params: request
+                .params
+                .iter()
+                .map(|matcher| MetricParamRecord {
+                    param_name: matcher.param_name.clone(),
+                    capture_group: matcher.capture_group.clone(),
+                    kind: matcher.kind.into(),
+                })
+                .collect(),
+        }
+    }
+}
+
+impl From<&MetricDeclarationRecord> for DeclareMetricRequest {
+    fn from(record: &MetricDeclarationRecord) -> Self {
+        DeclareMetricRequest {
+            name: record.name.clone(),
+            message_match_regex: record.message_match_regex.clone(),
+            params: record
+                .params
+                .iter()
+                .map(|param| RegexToMetricPrimitiveMatcher {
+                    param_name: param.param_name.clone(),
+                    capture_group: param.capture_group.clone(),
+                    kind: param.kind.into(),
+                })
+                .collect(),
+        }
+    }
+}
+
+/// A named bundle of parsing/indexing rules for a particular CLI app: which
+/// params and metrics to derive from its log output, plus any saved
+/// dashboards.
+///
+/// `dashboards` is stubbed as opaque JSON: dashboards (ticket 014) don't exist
+/// yet, so there's no concrete schema to store. Ticket 014 should replace this
+/// with whatever concrete `Dashboard` type it introduces (or keep it loosely
+/// typed, if that turns out to be the right call for that ticket).
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Profile {
+    pub name: String,
+    #[serde(default)]
+    pub params: Vec<ParamDeclarationRecord>,
+    #[serde(default)]
+    pub metrics: Vec<MetricDeclarationRecord>,
+    /// Placeholder for ticket 014's dashboard storage. See the struct doc above.
+    #[serde(default)]
+    pub dashboards: Vec<serde_json::Value>,
+}
+
+impl Profile {
+    /// A new, empty profile with the given name.
+    pub fn new(name: String) -> Profile {
+        Profile {
+            name,
+            params: Vec::new(),
+            metrics: Vec::new(),
+            dashboards: Vec::new(),
+        }
+    }
+}
+
+/// Sends every param/metric declaration stored in `profile` to the engine at
+/// `engine`, as `EngineMessage::DeclareParam`/`DeclareMetric`. Used on startup
+/// to reload the active profile's rules into a fresh engine (see `main.rs`),
+/// and in tests to simulate that reload.
+///
+/// A stored declaration should generally already be valid — it round-tripped
+/// through the engine's own validation when originally declared (see
+/// `record_param_declaration`/`record_metric_declaration` below) — but this
+/// doesn't assume that: any declaration the engine rejects (or any send/recv
+/// failure, e.g. the engine channel closing) is logged to stderr and skipped
+/// rather than panicking, so one bad stored declaration can't take the whole
+/// process down.
+pub async fn load_into_engine(engine: &EngineHandle, profile: &Profile) {
+    for record in &profile.params {
+        let (respond_to, response) = oneshot::channel();
+        let request: DeclareParamRequest = record.into();
+        let name = request.name.clone();
+        if engine
+            .send(EngineMessage::DeclareParam { request, respond_to })
+            .await
+            .is_err()
+        {
+            eprintln!("warning: engine unavailable while loading profile '{}'", profile.name);
+            return;
+        }
+        match response.await {
+            Ok(Ok(())) => {}
+            Ok(Err(err)) => eprintln!(
+                "warning: profile '{}' param declaration '{name}' rejected by engine: {err}",
+                profile.name
+            ),
+            Err(_) => eprintln!(
+                "warning: engine dropped response while declaring param '{name}' from profile '{}'",
+                profile.name
+            ),
+        }
+    }
+
+    for record in &profile.metrics {
+        let (respond_to, response) = oneshot::channel();
+        let request: DeclareMetricRequest = record.into();
+        let name = request.name.clone();
+        if engine
+            .send(EngineMessage::DeclareMetric { request, respond_to })
+            .await
+            .is_err()
+        {
+            eprintln!("warning: engine unavailable while loading profile '{}'", profile.name);
+            return;
+        }
+        match response.await {
+            Ok(Ok(())) => {}
+            Ok(Err(err)) => eprintln!(
+                "warning: profile '{}' metric declaration '{name}' rejected by engine: {err}",
+                profile.name
+            ),
+            Err(_) => eprintln!(
+                "warning: engine dropped response while declaring metric '{name}' from profile '{}'",
+                profile.name
+            ),
+        }
+    }
+}
+
+/// Error returned by `ProfileStore` mutations and lookups.
+#[derive(Debug)]
+pub enum ProfileError {
+    /// No profile with the given name exists.
+    NotFound,
+    /// A profile with the given name already exists.
+    DuplicateName,
+    /// Underlying I/O failure reading/writing the profile store or settings file.
+    Io(io::Error),
+}
+
+impl fmt::Display for ProfileError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ProfileError::NotFound => write!(f, "no profile with that name"),
+            ProfileError::DuplicateName => write!(f, "a profile with that name already exists"),
+            ProfileError::Io(err) => write!(f, "I/O error: {err}"),
+        }
+    }
+}
+
+impl std::error::Error for ProfileError {}
+
+impl From<io::Error> for ProfileError {
+    fn from(err: io::Error) -> Self {
+        ProfileError::Io(err)
+    }
+}
+
+/// Resolves `settings.profiles.profiles_dir` relative to `settings_path`'s
+/// parent directory (or treats it as absolute if it already is one) — the
+/// same directory convention ticket 010 uses for CSS storage.
+pub fn resolve_profiles_dir(settings: &Settings, settings_path: &Path) -> PathBuf {
+    let profiles_dir = Path::new(&settings.profiles.profiles_dir);
+    if profiles_dir.is_absolute() {
+        profiles_dir.to_path_buf()
+    } else {
+        settings_path
+            .parent()
+            .map(|parent| parent.join(profiles_dir))
+            .unwrap_or_else(|| profiles_dir.to_path_buf())
+    }
+}
+
+/// Path to the single file holding all profiles, within `profiles_dir` (see
+/// `resolve_profiles_dir`).
+pub fn profiles_store_path(profiles_dir: &Path) -> PathBuf {
+    profiles_dir.join("profiles.json")
+}
+
+/// On-disk collection of all known profiles, persisted as a single JSON file
+/// (see module docs for why one file rather than one-per-profile).
+#[derive(Debug, Clone, Default)]
+pub struct ProfileStore {
+    profiles: Vec<Profile>,
+}
+
+impl ProfileStore {
+    /// Loads the store from `store_path`. A missing file is treated as an
+    /// empty store (not an error) — there's no reason to fail startup just
+    /// because no profiles have been created yet. Any I/O or parse error
+    /// while reading an existing file is logged to stderr and treated as an
+    /// empty store, mirroring `Settings::load`'s non-fatal-error handling.
+    pub fn load(store_path: &Path) -> ProfileStore {
+        match fs::read_to_string(store_path) {
+            Ok(contents) => match serde_json::from_str::<Vec<Profile>>(&contents) {
+                Ok(profiles) => ProfileStore { profiles },
+                Err(err) => {
+                    eprintln!(
+                        "warning: failed to parse profile store at {}: {err}; using empty store",
+                        store_path.display()
+                    );
+                    ProfileStore::default()
+                }
+            },
+            Err(err) if err.kind() == io::ErrorKind::NotFound => ProfileStore::default(),
+            Err(err) => {
+                eprintln!(
+                    "warning: failed to read profile store at {}: {err}; using empty store",
+                    store_path.display()
+                );
+                ProfileStore::default()
+            }
+        }
+    }
+
+    /// Persists the store to `store_path` as pretty-printed JSON, creating
+    /// any missing parent directories first.
+    pub fn save(&self, store_path: &Path) -> io::Result<()> {
+        if let Some(parent) = store_path.parent() {
+            fs::create_dir_all(parent)?;
+        }
+        let contents = serde_json::to_string_pretty(&self.profiles)
+            .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))?;
+        fs::write(store_path, contents)
+    }
+
+    /// All known profiles.
+    pub fn list(&self) -> &[Profile] {
+        &self.profiles
+    }
+
+    /// Looks up a profile by name.
+    pub fn get(&self, name: &str) -> Option<&Profile> {
+        self.profiles.iter().find(|p| p.name == name)
+    }
+
+    /// Creates a new, empty profile named `name`, persists the store to
+    /// `store_path`, and records the name in `settings.profiles.names`
+    /// (persisting `settings` to `settings_path` too, keeping the two files in
+    /// sync). Fails with `ProfileError::DuplicateName` if a profile with that
+    /// name already exists.
+    pub fn create(
+        &mut self,
+        store_path: &Path,
+        settings: &mut Settings,
+        settings_path: &Path,
+        name: String,
+    ) -> Result<(), ProfileError> {
+        if self.get(&name).is_some() {
+            return Err(ProfileError::DuplicateName);
+        }
+        self.profiles.push(Profile::new(name.clone()));
+        self.save(store_path)?;
+
+        if !settings.profiles.names.contains(&name) {
+            settings.profiles.names.push(name);
+        }
+        settings.save(settings_path)?;
+        Ok(())
+    }
+
+    /// Deletes the profile named `name`, persists the store to `store_path`,
+    /// and drops it from `settings.profiles.names` (persisting `settings` to
+    /// `settings_path` too). If the deleted profile was active, clears
+    /// `settings.profiles.active_profile`. Fails with `ProfileError::NotFound`
+    /// if no such profile exists.
+    pub fn delete(
+        &mut self,
+        store_path: &Path,
+        settings: &mut Settings,
+        settings_path: &Path,
+        name: &str,
+    ) -> Result<(), ProfileError> {
+        let index = self.profiles.iter().position(|p| p.name == name).ok_or(ProfileError::NotFound)?;
+        self.profiles.remove(index);
+        self.save(store_path)?;
+
+        settings.profiles.names.retain(|n| n != name);
+        if settings.profiles.active_profile.as_deref() == Some(name) {
+            settings.profiles.active_profile = None;
+        }
+        settings.save(settings_path)?;
+        Ok(())
+    }
+
+    /// Sets (or clears, with `name: None`) the active profile: updates
+    /// `settings.profiles.active_profile` and persists `settings` to
+    /// `settings_path`. Fails with `ProfileError::NotFound` if `name` is
+    /// `Some` and no such profile exists in this store.
+    ///
+    /// This only updates `Settings`; it does not itself reload the newly
+    /// active profile's declarations into a running engine (that happens on
+    /// the next startup — see `load_into_engine` and `main.rs`).
+    pub fn set_active(
+        &self,
+        settings: &mut Settings,
+        settings_path: &Path,
+        name: Option<String>,
+    ) -> Result<(), ProfileError> {
+        if let Some(name) = &name {
+            if self.get(name).is_none() {
+                return Err(ProfileError::NotFound);
+            }
+        }
+        settings.profiles.active_profile = name;
+        settings.save(settings_path)?;
+        Ok(())
+    }
+
+    /// Records a newly-declared param into `profile_name`'s stored
+    /// declarations (replacing any existing record with the same param name,
+    /// so re-declaring updates it in place), persisting the store to
+    /// `store_path`.
+    ///
+    /// **Integration hook for ticket 004 (HTTP API)**: when the HTTP API
+    /// declares a param against the engine (sending
+    /// `EngineMessage::DeclareParam`), it should also call this — with the
+    /// same `DeclareParamRequest` — against the currently-active profile, so
+    /// the declaration survives a restart (see `load_into_engine`). This
+    /// ticket (009) cannot wire that call itself: there's no declare-param
+    /// HTTP route yet for it to hook into. Call shape:
+    ///
+    /// ```ignore
+    /// profile_store.record_param_declaration(&store_path, &active_profile_name, &request)?;
+    /// ```
+    pub fn record_param_declaration(
+        &mut self,
+        store_path: &Path,
+        profile_name: &str,
+        request: &DeclareParamRequest,
+    ) -> Result<(), ProfileError> {
+        let profile = self
+            .profiles
+            .iter_mut()
+            .find(|p| p.name == profile_name)
+            .ok_or(ProfileError::NotFound)?;
+        let record: ParamDeclarationRecord = request.into();
+        match profile.params.iter_mut().find(|r| r.name == record.name) {
+            Some(existing) => *existing = record,
+            None => profile.params.push(record),
+        }
+        self.save(store_path)?;
+        Ok(())
+    }
+
+    /// Records a newly-declared metric into `profile_name`'s stored
+    /// declarations (replacing any existing record with the same metric name),
+    /// persisting the store to `store_path`.
+    ///
+    /// **Integration hook for ticket 004 (HTTP API)**: same as
+    /// `record_param_declaration` above, but for
+    /// `EngineMessage::DeclareMetric`/`DeclareMetricRequest`.
+    pub fn record_metric_declaration(
+        &mut self,
+        store_path: &Path,
+        profile_name: &str,
+        request: &DeclareMetricRequest,
+    ) -> Result<(), ProfileError> {
+        let profile = self
+            .profiles
+            .iter_mut()
+            .find(|p| p.name == profile_name)
+            .ok_or(ProfileError::NotFound)?;
+        let record: MetricDeclarationRecord = request.into();
+        match profile.metrics.iter_mut().find(|r| r.name == record.name) {
+            Some(existing) => *existing = record,
+            None => profile.metrics.push(record),
+        }
+        self.save(store_path)?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::data_value::MetricPrimitiveKind;
+    use crate::dto::LogsParams;
+    use crate::engine::{EngineMessage, EngineQuery, EngineResponse, IngestedLine};
+    use crate::timestamp::Timestamp;
+    use tokio::sync::mpsc;
+
+    fn param_request(name: &str, pattern: &str, capture_group: &str) -> DeclareParamRequest {
+        DeclareParamRequest {
+            name: name.to_string(),
+            message_match_regex: pattern.to_string(),
+            capture_group: capture_group.to_string(),
+            kind: MetricPrimitiveKind::String,
+        }
+    }
+
+    fn metric_request(name: &str, pattern: &str, param_name: &str, capture_group: &str) -> DeclareMetricRequest {
+        DeclareMetricRequest {
+            name: name.to_string(),
+            message_match_regex: pattern.to_string(),
+            params: vec![RegexToMetricPrimitiveMatcher {
+                param_name: param_name.to_string(),
+                capture_group: capture_group.to_string(),
+                kind: MetricPrimitiveKind::Number,
+            }],
+        }
+    }
+
+    async fn ask(engine: &EngineHandle, query: EngineQuery) -> EngineResponse {
+        let (respond_to, response) = oneshot::channel();
+        engine.send(EngineMessage::Query { query, respond_to }).await.unwrap();
+        response.await.unwrap()
+    }
+
+    #[test]
+    fn resolve_profiles_dir_is_relative_to_settings_parent() {
+        let settings_path = Path::new("/config/ottyl/settings.json");
+        let mut settings = Settings::default();
+        settings.profiles.profiles_dir = "profiles".to_string();
+
+        let dir = resolve_profiles_dir(&settings, settings_path);
+        assert_eq!(dir, Path::new("/config/ottyl/profiles"));
+    }
+
+    #[test]
+    fn resolve_profiles_dir_respects_absolute_path() {
+        let settings_path = Path::new("/config/ottyl/settings.json");
+        let mut settings = Settings::default();
+        settings.profiles.profiles_dir = "/elsewhere/profiles".to_string();
+
+        let dir = resolve_profiles_dir(&settings, settings_path);
+        assert_eq!(dir, Path::new("/elsewhere/profiles"));
+    }
+
+    #[test]
+    fn create_list_get_delete_round_trip() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        let settings_path = dir.path().join("settings.json");
+        let mut settings = Settings::default();
+
+        let mut store = ProfileStore::load(&store_path);
+        store.create(&store_path, &mut settings, &settings_path, "web".to_string()).unwrap();
+        store.create(&store_path, &mut settings, &settings_path, "worker".to_string()).unwrap();
+
+        assert_eq!(store.list().len(), 2);
+        assert!(store.get("web").is_some());
+        assert_eq!(settings.profiles.names, vec!["web".to_string(), "worker".to_string()]);
+
+        // Duplicate creation is rejected.
+        let dup = store.create(&store_path, &mut settings, &settings_path, "web".to_string());
+        assert!(matches!(dup, Err(ProfileError::DuplicateName)));
+
+        // Reload from disk: profiles and settings both persisted.
+        let reloaded = ProfileStore::load(&store_path);
+        assert_eq!(reloaded.list().len(), 2);
+        let reloaded_settings = Settings::load(&settings_path);
+        assert_eq!(reloaded_settings.profiles.names, settings.profiles.names);
+
+        store.delete(&store_path, &mut settings, &settings_path, "worker").unwrap();
+        assert!(store.get("worker").is_none());
+        assert_eq!(settings.profiles.names, vec!["web".to_string()]);
+
+        let missing = store.delete(&store_path, &mut settings, &settings_path, "worker");
+        assert!(matches!(missing, Err(ProfileError::NotFound)));
+    }
+
+    #[test]
+    fn set_active_updates_and_persists_settings() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        let settings_path = dir.path().join("settings.json");
+        let mut settings = Settings::default();
+
+        let mut store = ProfileStore::load(&store_path);
+        store.create(&store_path, &mut settings, &settings_path, "web".to_string()).unwrap();
+        store.create(&store_path, &mut settings, &settings_path, "worker".to_string()).unwrap();
+
+        store.set_active(&mut settings, &settings_path, Some("web".to_string())).unwrap();
+        assert_eq!(settings.profiles.active_profile, Some("web".to_string()));
+
+        // Switching to another known profile works and persists.
+        store.set_active(&mut settings, &settings_path, Some("worker".to_string())).unwrap();
+        let reloaded = Settings::load(&settings_path);
+        assert_eq!(reloaded.profiles.active_profile, Some("worker".to_string()));
+
+        // Setting an unknown profile active is rejected, leaving the previous value.
+        let err = store.set_active(&mut settings, &settings_path, Some("ghost".to_string()));
+        assert!(matches!(err, Err(ProfileError::NotFound)));
+        assert_eq!(settings.profiles.active_profile, Some("worker".to_string()));
+
+        // Clearing the active profile works.
+        store.set_active(&mut settings, &settings_path, None).unwrap();
+        assert_eq!(settings.profiles.active_profile, None);
+    }
+
+    #[test]
+    fn record_param_and_metric_declarations_persist_into_profile() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        let settings_path = dir.path().join("settings.json");
+        let mut settings = Settings::default();
+
+        let mut store = ProfileStore::load(&store_path);
+        store.create(&store_path, &mut settings, &settings_path, "web".to_string()).unwrap();
+
+        let param_req = param_request("level", r"level=(?P<level>\w+)", "level");
+        store.record_param_declaration(&store_path, "web", &param_req).unwrap();
+
+        let metric_req = metric_request(
+            "requests",
+            r"duration=(?P<duration>\d+)",
+            "duration",
+            "duration",
+        );
+        store.record_metric_declaration(&store_path, "web", &metric_req).unwrap();
+
+        let profile = store.get("web").unwrap();
+        assert_eq!(profile.params.len(), 1);
+        assert_eq!(profile.params[0].name, "level");
+        assert_eq!(profile.metrics.len(), 1);
+        assert_eq!(profile.metrics[0].name, "requests");
+
+        // Re-declaring the same name updates in place rather than duplicating.
+        let param_req_v2 = param_request("level", r"lvl=(?P<level>\w+)", "level");
+        store.record_param_declaration(&store_path, "web", &param_req_v2).unwrap();
+        let profile = store.get("web").unwrap();
+        assert_eq!(profile.params.len(), 1);
+        assert_eq!(profile.params[0].message_match_regex, r"lvl=(?P<level>\w+)");
+
+        // Reload from disk: recorded declarations survive a restart.
+        let reloaded = ProfileStore::load(&store_path);
+        let profile = reloaded.get("web").unwrap();
+        assert_eq!(profile.params.len(), 1);
+        assert_eq!(profile.metrics.len(), 1);
+    }
+
+    /// Acceptance criterion: declaring params/metrics into a profile, "restarting"
+    /// (dropping and reloading the store from disk, rather than a literal process
+    /// restart), and replaying the saved profile against a fresh engine reproduces
+    /// the same declarations and still ingests lines correctly — without
+    /// re-declaring anything by hand.
+    ///
+    /// Note: ticket 003 (metric bucket derivation) isn't implemented yet in this
+    /// worktree, so there's no bucket index to compare directly. What this test
+    /// verifies instead is the part ticket 009 owns: that a profile's declarations
+    /// round-trip through disk byte-for-byte and, once replayed via
+    /// `load_into_engine`, produce identical engine-visible declarations (which is
+    /// exactly the state ticket 003's derivation logic will read from) — and that
+    /// the engine keeps ingesting lines normally afterwards.
+    #[tokio::test]
+    async fn profile_round_trip_reproduces_declarations_in_fresh_engine() {
+        let dir = tempfile::tempdir().unwrap();
+        let store_path = dir.path().join("profiles.json");
+        let settings_path = dir.path().join("settings.json");
+        let mut settings = Settings::default();
+
+        // Declare directly into a profile (as ticket 004's HTTP API will do via the
+        // `record_*_declaration` hooks once it exists).
+        let mut store = ProfileStore::load(&store_path);
+        store.create(&store_path, &mut settings, &settings_path, "web".to_string()).unwrap();
+        store
+            .record_param_declaration(
+                &store_path,
+                "web",
+                &param_request("level", r"level=(?P<level>\w+)", "level"),
+            )
+            .unwrap();
+        store
+            .record_metric_declaration(
+                &store_path,
+                "web",
+                &metric_request(
+                    "requests",
+                    r"request duration=(?P<duration>\d+)",
+                    "duration",
+                    "duration",
+                ),
+            )
+            .unwrap();
+        store.set_active(&mut settings, &settings_path, Some("web".to_string())).unwrap();
+
+        // Simulate a restart: drop everything in memory, reload from disk.
+        drop(store);
+        drop(settings);
+        let settings = Settings::load(&settings_path);
+        let active_name = settings.profiles.active_profile.clone().unwrap();
+        let reloaded_store = ProfileStore::load(&store_path);
+        let profile = reloaded_store.get(&active_name).unwrap().clone();
+
+        // Spawn a fresh engine and replay the profile's declarations into it.
+        let (engine_tx, engine_rx) = mpsc::channel(16);
+        tokio::spawn(crate::engine::run(engine_rx));
+        load_into_engine(&engine_tx, &profile).await;
+
+        // Ingest a line and confirm the engine is functioning with the replayed
+        // declarations in place (no re-declaration needed).
+        engine_tx
+            .send(EngineMessage::Ingest(IngestedLine {
+                sequence: 0,
+                timestamp: Timestamp::from_millis(0),
+                text: "level=info request duration=42".to_string(),
+            }))
+            .await
+            .unwrap();
+
+        let params = match ask(&engine_tx, EngineQuery::ParamDeclarations).await {
+            EngineResponse::ParamDeclarations(params) => params,
+            _ => panic!("unexpected response"),
+        };
+        assert_eq!(params.len(), 1);
+        assert_eq!(params[0].name, "level");
+        assert_eq!(params[0].message_match_regex, r"level=(?P<level>\w+)");
+        assert_eq!(params[0].capture_group, "level");
+        assert!(matches!(params[0].kind, MetricPrimitiveKind::String));
+
+        let metrics = match ask(&engine_tx, EngineQuery::MetricDeclarations).await {
+            EngineResponse::MetricDeclarations(metrics) => metrics,
+            _ => panic!("unexpected response"),
+        };
+        assert_eq!(metrics.len(), 1);
+        assert_eq!(metrics[0].name, "requests");
+        assert_eq!(metrics[0].message_match_regex, r"request duration=(?P<duration>\d+)");
+        assert_eq!(metrics[0].params.len(), 1);
+        assert_eq!(metrics[0].params[0].param_name, "duration");
+
+        let logs = match ask(
+            &engine_tx,
+            EngineQuery::Logs(LogsParams { since_sequence: None, start_ms: None, end_ms: None, limit: None }),
+        )
+        .await
+        {
+            EngineResponse::Logs(logs) => logs,
+            _ => panic!("unexpected response"),
+        };
+        assert_eq!(logs.len(), 1);
+        assert_eq!(logs[0].text, "level=info request duration=42");
+    }
+}