diff --git a/crates/application/src/conversation/mod.rs b/crates/application/src/conversation/mod.rs
new file mode 100644
index 0000000..c2e1ee7
--- /dev/null
+++ b/crates/application/src/conversation/mod.rs
@@ -0,0 +1,18 @@
+//! Conversation persistence use cases (cadrage « persistance conversationnelle »,
+//! ARCHITECTURE §19, lot P6).
+//!
+//! Where [`crate::agent::lifecycle`] owns the *runtime* of an agent (spawn, PTY,
+//! sessions), this module owns the **durable conversational memory** of a
+//! conversation (paire) : the canonical append-only log and its incremental
+//! handoff. The single use case here, [`RecordTurn`], materialises the
+//! **end-of-turn checkpoint** (D19-5) by talking **only** to the three driven/driving
+//! ports of §19 ([`domain::ConversationLog`], [`domain::HandoffStore`],
+//! [`domain::HandoffSummarizer`]) — never to a concrete adapter.
+//!
+//! This is the **isolated logic** of P6 (lot P6a) : no wiring to the orchestrator,
+//! the PTY or `app-tauri` (that is P6b). It is fully testable with in-memory fakes
+//! of the three ports.
+
+mod record;
+
+pub use record::RecordTurn;
diff --git a/crates/application/src/conversation/record.rs b/crates/application/src/conversation/record.rs
new file mode 100644
index 0000000..b6d5109
--- /dev/null
+++ b/crates/application/src/conversation/record.rs
@@ -0,0 +1,100 @@
+//! [`RecordTurn`] — the end-of-turn checkpoint use case (ARCHITECTURE §19, lot P6a).
+
+use std::sync::Arc;
+
+use domain::{ConversationId, ConversationLog, ConversationTurn, HandoffStore, HandoffSummarizer};
+
+use crate::error::AppError;
+
+/// Records one conversation turn at a checkpoint: appends it to the canonical
+/// log, then advances the incremental handoff (ARCHITECTURE §19, décision D19-5).
+///
+/// **Single Responsibility**: materialise *one* end-of-turn checkpoint. It is the
+/// only place that couples the append-only log (source of truth, [`ConversationLog`])
+/// with the cumulative resume summary ([`HandoffStore`]) via the incremental
+/// [`HandoffSummarizer`]. It consumes **exactly** those three ports (Interface
+/// Segregation), injected as `Arc<dyn …>` at the composition root.
+///
+/// ## Incremental by contract (no full re-read)
+///
+/// Each call folds **only the new turn** into the previous handoff
+/// (`fold(prev, &[turn])`), never re-reading the whole thread — exactly the seam
+/// [`HandoffSummarizer`] was designed for (§19, lot P4). The resulting handoff's
+/// [`Handoff::up_to`](domain::Handoff::up_to) is the recorded turn's id (guaranteed
+/// by the summarizer, P4).
+///
+/// ## Best-effort fold, honest `Result`
+///
+/// The fold itself cannot fail (it returns no `Result`, P4 — a future LLM
+/// summarizer falls back rather than erroring), so the only failures are the
+/// store operations (`append` / `load` / `save`), which propagate as [`AppError`].
+/// This use case stays **honest** and surfaces them; the live wiring (P6b) decides
+/// whether to swallow a handoff failure rather than block the turn.
+///
+/// ## No debounce here (deferred)
+///
+/// One `append` + one `save` per recorded turn. The debounce optimisation (coalesce
+/// several rapid turns into a single handoff recompute) is intentionally **deferred**
+/// — it belongs to the live wiring (P6b), not to this isolated logic.
+pub struct RecordTurn {
+    log: Arc<dyn ConversationLog>,
+    handoffs: Arc<dyn HandoffStore>,
+    summarizer: Arc<dyn HandoffSummarizer>,
+}
+
+impl RecordTurn {
+    /// Builds the use case from its three injected ports.
+    #[must_use]
+    pub fn new(
+        log: Arc<dyn ConversationLog>,
+        handoffs: Arc<dyn HandoffStore>,
+        summarizer: Arc<dyn HandoffSummarizer>,
+    ) -> Self {
+        Self {
+            log,
+            handoffs,
+            summarizer,
+        }
+    }
+
+    /// Records `turn` in `conversation` at an end-of-turn checkpoint.
+    ///
+    /// Steps (ordering is contractual):
+    /// 1. **append** the turn to the canonical log (source of truth) ;
+    /// 2. **load** the previous handoff (`None` on a first checkpoint) ;
+    /// 3. **fold** `prev` with the single new turn (incremental, best-effort) ;
+    /// 4. **save** the advanced handoff.
+    ///
+    /// The append happens **first** so the durable source of truth is never behind
+    /// the derived handoff. The turn is cloned for the append because the fold (step
+    /// 3) borrows it as the incremental slice.
+    ///
+    /// # Errors
+    /// [`AppError::Store`] (or [`AppError::NotFound`] for a missing store item) when
+    /// the canonical log append, the handoff load, or the handoff save fails — every
+    /// [`domain::ports::StoreError`] is mapped through the existing `From` impl. The
+    /// fold never fails (P4).
+    pub async fn record(
+        &self,
+        conversation: ConversationId,
+        turn: ConversationTurn,
+    ) -> Result<(), AppError> {
+        // 1. Append to the canonical append-only log (source of truth, D19-1a).
+        self.log.append(conversation, turn.clone()).await?;
+
+        // 2. Load the previous resume point (absence is never an error, P3).
+        let prev = self.handoffs.load(conversation).await?;
+
+        // 3. Fold the previous handoff with *only* the new turn (incremental, P4):
+        //    no full re-read of the log; best-effort, so no `Result` to propagate.
+        let handoff = self
+            .summarizer
+            .fold(prev, std::slice::from_ref(&turn))
+            .await;
+
+        // 4. Persist the advanced handoff (overwrites the previous resume point).
+        self.handoffs.save(conversation, handoff).await?;
+
+        Ok(())
+    }
+}
diff --git a/crates/application/src/lib.rs b/crates/application/src/lib.rs
index f6cd7bf..042ce85 100644
--- a/crates/application/src/lib.rs
+++ b/crates/application/src/lib.rs
@@ -12,6 +12,7 @@
 #![warn(missing_docs)]
 
 pub mod agent;
+pub mod conversation;
 pub mod embedder;
 pub mod error;
 pub mod git;
@@ -40,6 +41,7 @@ pub use agent::{
     SaveProfileInput, SaveProfileOutput, StructuredSessionDescriptor, UpdateAgentContext,
     UpdateAgentContextInput, send_blocking, AGENT_MEMORY_RECALL_BUDGET,
 };
+pub use conversation::RecordTurn;
 pub use embedder::{
     CheckEmbedderSuggestion, CheckEmbedderSuggestionInput, CheckEmbedderSuggestionOutput,
     DeleteEmbedderProfile, DeleteEmbedderProfileInput, DescribeEmbedderEngines, DismissChoice,
diff --git a/crates/application/tests/conversation_record.rs b/crates/application/tests/conversation_record.rs
new file mode 100644
index 0000000..c0642e8
--- /dev/null
+++ b/crates/application/tests/conversation_record.rs
@@ -0,0 +1,360 @@
+//! P6a tests for the [`RecordTurn`] use case (ARCHITECTURE §19) with in-memory
+//! port fakes (no real store/FS).
+//!
+//! Coverage:
+//! - single `record`: log contains the turn; handoff `up_to == turn.id`; summary
+//!   contains the turn text;
+//! - incremental over two `record` calls: log order [t1, t2]; `up_to == t2.id`;
+//!   summary contains both texts; **proof of incrementality** — the summarizer
+//!   saw `prev = None` then `prev = Some(_)`, and `new_turns.len() == 1` on *both*
+//!   calls (never 2 → the log is never fully re-read);
+//! - error propagation: a store whose `append` / `save` / `load` returns a
+//!   non-`NotFound` `StoreError` makes `record` return `AppError::Store(_)`;
+//!   a failing `append` performs **no** `save` (no side effect after failure);
+//! - isolation by `ConversationId`.
+
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+
+use async_trait::async_trait;
+use domain::ports::StoreError;
+use domain::{
+    ConversationId, ConversationLog, ConversationTurn, Handoff, HandoffStore, HandoffSummarizer,
+    InputSource, TurnId, TurnRole,
+};
+
+use application::{AppError, RecordTurn};
+
+// ---------------------------------------------------------------------------
+// Deterministic constructors
+// ---------------------------------------------------------------------------
+
+fn conv_id(n: u128) -> ConversationId {
+    ConversationId::from_uuid(uuid::Uuid::from_u128(n))
+}
+
+fn turn_id(n: u128) -> TurnId {
+    TurnId::from_uuid(uuid::Uuid::from_u128(n))
+}
+
+fn turn(conv: ConversationId, id: TurnId, text: &str) -> ConversationTurn {
+    ConversationTurn::new(id, conv, 1_000, InputSource::Human, TurnRole::Prompt, text)
+}
+
+// ---------------------------------------------------------------------------
+// Fakes — Vec/Map backed, Mutex never held across an `.await`.
+// ---------------------------------------------------------------------------
+
+#[derive(Default)]
+struct InMemoryConversationLog {
+    threads: Mutex<HashMap<ConversationId, Vec<ConversationTurn>>>,
+    /// When set, `append` returns this error instead of appending.
+    fail_append: Mutex<Option<StoreError>>,
+}
+
+impl InMemoryConversationLog {
+    fn failing_append(err: StoreError) -> Self {
+        Self {
+            threads: Mutex::new(HashMap::new()),
+            fail_append: Mutex::new(Some(err)),
+        }
+    }
+
+    fn thread(&self, conv: ConversationId) -> Vec<ConversationTurn> {
+        self.threads.lock().unwrap().get(&conv).cloned().unwrap_or_default()
+    }
+}
+
+#[async_trait]
+impl ConversationLog for InMemoryConversationLog {
+    async fn append(
+        &self,
+        conversation: ConversationId,
+        turn: ConversationTurn,
+    ) -> Result<(), StoreError> {
+        if let Some(err) = self.fail_append.lock().unwrap().clone() {
+            return Err(err);
+        }
+        self.threads
+            .lock()
+            .unwrap()
+            .entry(conversation)
+            .or_default()
+            .push(turn);
+        Ok(())
+    }
+
+    async fn read(
+        &self,
+        conversation: ConversationId,
+        since: Option<TurnId>,
+    ) -> Result<Vec<ConversationTurn>, StoreError> {
+        let all = self.thread(conversation);
+        let out = match since {
+            None => all,
+            Some(cursor) => {
+                let pos = all.iter().position(|t| t.id == cursor);
+                match pos {
+                    Some(i) => all[i + 1..].to_vec(),
+                    None => all,
+                }
+            }
+        };
+        Ok(out)
+    }
+
+    async fn last(
+        &self,
+        conversation: ConversationId,
+        n: usize,
+    ) -> Result<Vec<ConversationTurn>, StoreError> {
+        let all = self.thread(conversation);
+        let start = all.len().saturating_sub(n);
+        Ok(all[start..].to_vec())
+    }
+}
+
+#[derive(Default)]
+struct InMemoryHandoffStore {
+    store: Mutex<HashMap<ConversationId, Handoff>>,
+    fail_load: Mutex<Option<StoreError>>,
+    fail_save: Mutex<Option<StoreError>>,
+    /// Number of completed `save` calls (used to prove no save-after-append-fail).
+    saves: Mutex<usize>,
+}
+
+impl InMemoryHandoffStore {
+    fn failing_load(err: StoreError) -> Self {
+        Self {
+            fail_load: Mutex::new(Some(err)),
+            ..Self::default()
+        }
+    }
+
+    fn failing_save(err: StoreError) -> Self {
+        Self {
+            fail_save: Mutex::new(Some(err)),
+            ..Self::default()
+        }
+    }
+
+    fn loaded(&self, conv: ConversationId) -> Option<Handoff> {
+        self.store.lock().unwrap().get(&conv).cloned()
+    }
+
+    fn save_count(&self) -> usize {
+        *self.saves.lock().unwrap()
+    }
+}
+
+#[async_trait]
+impl HandoffStore for InMemoryHandoffStore {
+    async fn load(&self, conversation: ConversationId) -> Result<Option<Handoff>, StoreError> {
+        if let Some(err) = self.fail_load.lock().unwrap().clone() {
+            return Err(err);
+        }
+        Ok(self.store.lock().unwrap().get(&conversation).cloned())
+    }
+
+    async fn save(
+        &self,
+        conversation: ConversationId,
+        handoff: Handoff,
+    ) -> Result<(), StoreError> {
+        if let Some(err) = self.fail_save.lock().unwrap().clone() {
+            return Err(err);
+        }
+        self.store.lock().unwrap().insert(conversation, handoff);
+        *self.saves.lock().unwrap() += 1;
+        Ok(())
+    }
+}
+
+/// Records, on every `fold` call, the arguments it received so the test can prove
+/// incrementality. Deterministic: `up_to = last new turn id`, and
+/// `summary_md = prev.summary_md + each new turn's text`.
+#[derive(Default)]
+struct RecordingSummarizer {
+    /// One entry per `fold` call: `(prev_was_some, new_turns_len)`.
+    calls: Mutex<Vec<(bool, usize)>>,
+}
+
+impl RecordingSummarizer {
+    fn calls(&self) -> Vec<(bool, usize)> {
+        self.calls.lock().unwrap().clone()
+    }
+}
+
+#[async_trait]
+impl HandoffSummarizer for RecordingSummarizer {
+    async fn fold(&self, prev: Option<Handoff>, new_turns: &[ConversationTurn]) -> Handoff {
+        self.calls
+            .lock()
+            .unwrap()
+            .push((prev.is_some(), new_turns.len()));
+
+        let mut summary = prev.map(|h| h.summary_md).unwrap_or_default();
+        for t in new_turns {
+            summary.push_str(&t.text);
+        }
+        let up_to = new_turns
+            .last()
+            .map(|t| t.id)
+            .expect("fold must receive at least one new turn in these tests");
+        Handoff::new(summary, up_to, None)
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[tokio::test]
+async fn single_record_appends_turn_and_advances_handoff() {
+    let log = Arc::new(InMemoryConversationLog::default());
+    let handoffs = Arc::new(InMemoryHandoffStore::default());
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let conv = conv_id(1);
+    let t1 = turn(conv, turn_id(10), "hello world");
+
+    uc.record(conv, t1.clone()).await.expect("record should succeed");
+
+    // Log contains the turn.
+    assert_eq!(log.thread(conv), vec![t1.clone()]);
+
+    // Handoff advanced: up_to == turn id, summary contains the turn text.
+    let h = handoffs.loaded(conv).expect("handoff should be saved");
+    assert_eq!(h.up_to, t1.id);
+    assert!(h.summary_md.contains("hello world"), "summary={:?}", h.summary_md);
+
+    // First fold: prev = None, exactly one new turn.
+    assert_eq!(summarizer.calls(), vec![(false, 1)]);
+    assert_eq!(handoffs.save_count(), 1);
+}
+
+#[tokio::test]
+async fn two_records_fold_incrementally_without_full_reread() {
+    let log = Arc::new(InMemoryConversationLog::default());
+    let handoffs = Arc::new(InMemoryHandoffStore::default());
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let conv = conv_id(1);
+    let t1 = turn(conv, turn_id(10), "first-text");
+    let t2 = turn(conv, turn_id(20), "second-text");
+
+    uc.record(conv, t1.clone()).await.expect("record t1");
+    uc.record(conv, t2.clone()).await.expect("record t2");
+
+    // Log: [t1, t2] in order.
+    assert_eq!(log.thread(conv), vec![t1.clone(), t2.clone()]);
+
+    // Handoff up_to == t2.id; summary contains both texts.
+    let h = handoffs.loaded(conv).expect("handoff saved");
+    assert_eq!(h.up_to, t2.id);
+    assert!(h.summary_md.contains("first-text"), "summary={:?}", h.summary_md);
+    assert!(h.summary_md.contains("second-text"), "summary={:?}", h.summary_md);
+
+    // Proof of incrementality: prev None then Some, and len == 1 on BOTH calls
+    // (never 2 → the whole log is never re-read into the fold).
+    assert_eq!(summarizer.calls(), vec![(false, 1), (true, 1)]);
+    assert_eq!(handoffs.save_count(), 2);
+}
+
+#[tokio::test]
+async fn append_error_propagates_as_store_and_does_not_save() {
+    let log = Arc::new(InMemoryConversationLog::failing_append(StoreError::Io(
+        "disk full".to_owned(),
+    )));
+    let handoffs = Arc::new(InMemoryHandoffStore::default());
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let conv = conv_id(1);
+    let err = uc
+        .record(conv, turn(conv, turn_id(10), "x"))
+        .await
+        .expect_err("append failure must surface");
+
+    assert!(matches!(err, AppError::Store(_)), "got {err:?}");
+
+    // No side effect after a failed append: handoff never saved, fold never called.
+    assert_eq!(handoffs.save_count(), 0);
+    assert!(handoffs.loaded(conv).is_none());
+    assert!(summarizer.calls().is_empty());
+}
+
+#[tokio::test]
+async fn load_error_propagates_as_store() {
+    let log = Arc::new(InMemoryConversationLog::default());
+    let handoffs = Arc::new(InMemoryHandoffStore::failing_load(StoreError::Io(
+        "load boom".to_owned(),
+    )));
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let conv = conv_id(1);
+    let err = uc
+        .record(conv, turn(conv, turn_id(10), "x"))
+        .await
+        .expect_err("load failure must surface");
+
+    assert!(matches!(err, AppError::Store(_)), "got {err:?}");
+    // Append happened (source of truth first), but no save and no fold.
+    assert_eq!(log.thread(conv).len(), 1);
+    assert_eq!(handoffs.save_count(), 0);
+    assert!(summarizer.calls().is_empty());
+}
+
+#[tokio::test]
+async fn save_error_propagates_as_store() {
+    let log = Arc::new(InMemoryConversationLog::default());
+    let handoffs = Arc::new(InMemoryHandoffStore::failing_save(StoreError::Serialization(
+        "save boom".to_owned(),
+    )));
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let conv = conv_id(1);
+    let err = uc
+        .record(conv, turn(conv, turn_id(10), "x"))
+        .await
+        .expect_err("save failure must surface");
+
+    assert!(matches!(err, AppError::Store(_)), "got {err:?}");
+    // Append + fold happened, but the save itself failed → nothing stored.
+    assert_eq!(log.thread(conv).len(), 1);
+    assert_eq!(handoffs.save_count(), 0);
+    assert!(handoffs.loaded(conv).is_none());
+    assert_eq!(summarizer.calls(), vec![(false, 1)]);
+}
+
+#[tokio::test]
+async fn records_are_isolated_per_conversation() {
+    let log = Arc::new(InMemoryConversationLog::default());
+    let handoffs = Arc::new(InMemoryHandoffStore::default());
+    let summarizer = Arc::new(RecordingSummarizer::default());
+    let uc = RecordTurn::new(log.clone(), handoffs.clone(), summarizer.clone());
+
+    let a = conv_id(1);
+    let b = conv_id(2);
+    let ta = turn(a, turn_id(10), "alpha");
+    let tb = turn(b, turn_id(20), "beta");
+
+    uc.record(a, ta.clone()).await.expect("record a");
+    uc.record(b, tb.clone()).await.expect("record b");
+
+    // Logs are kept apart.
+    assert_eq!(log.thread(a), vec![ta.clone()]);
+    assert_eq!(log.thread(b), vec![tb.clone()]);
+
+    // Handoffs are kept apart: each covers only its own conversation's turn.
+    let ha = handoffs.loaded(a).expect("handoff a");
+    let hb = handoffs.loaded(b).expect("handoff b");
+    assert_eq!(ha.up_to, ta.id);
+    assert!(ha.summary_md.contains("alpha") && !ha.summary_md.contains("beta"));
+    assert_eq!(hb.up_to, tb.id);
+    assert!(hb.summary_md.contains("beta") && !hb.summary_md.contains("alpha"));
+}