diff --git a/Cargo.lock b/Cargo.lock
index e86ff6209..632f91128 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1046,6 +1046,7 @@ dependencies = [
  "base64",
  "buzz-audit",
  "buzz-auth",
+ "buzz-conformance",
  "buzz-core",
  "buzz-db",
  "buzz-media",
diff --git a/Cargo.toml b/Cargo.toml
index 4b673aca7..1cb20f65d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -109,6 +109,7 @@ schemars = { version = "1", default-features = false }
 
 # Internal crates
 buzz-core = { path = "crates/buzz-core" }
+buzz-conformance = { path = "crates/buzz-conformance" }
 buzz-db = { path = "crates/buzz-db" }
 buzz-auth = { path = "crates/buzz-auth" }
 buzz-pubsub = { path = "crates/buzz-pubsub" }
diff --git a/crates/buzz-relay/Cargo.toml b/crates/buzz-relay/Cargo.toml
index 5752b65f4..c9b80d53f 100644
--- a/crates/buzz-relay/Cargo.toml
+++ b/crates/buzz-relay/Cargo.toml
@@ -17,6 +17,7 @@ path = "src/main.rs"
 
 [dependencies]
 buzz-core = { workspace = true }
+buzz-conformance = { workspace = true }
 buzz-db = { workspace = true }
 buzz-auth = { workspace = true }
 buzz-pubsub = { workspace = true }
diff --git a/crates/buzz-relay/src/conformance/mod.rs b/crates/buzz-relay/src/conformance/mod.rs
new file mode 100644
index 000000000..8910483fc
--- /dev/null
+++ b/crates/buzz-relay/src/conformance/mod.rs
@@ -0,0 +1,228 @@
+//! Runtime conformance harness — the relay's side of the trace seam.
+//!
+//! This module hosts the [`Tracer`] re-export and the per-request emit
+//! helpers that translate the relay's actual decisions into [`TraceStep`]s
+//! from `buzz-conformance`. See `crates/buzz-conformance/src/lib.rs` for
+//! the schema and `docs/spec/MultiTenantRelay.tla` for what the schema is
+//! grounded in.
+//!
+//! ## Design rules (from `skill-runtime-formal-compliance`)
+//!
+//! 1. **Project, don't echo.** The trace carries opaque labels (community
+//!    UUID, channel UUID, blake3-truncated actor) — never the event id,
+//!    payload, pubkey bytes, signature, or wall-clock timestamps. The
+//!    only fields that survive are the ones the spec's `Next` and
+//!    `Inv_NonInterference` reason about.
+//! 2. **Don't normalize away violations.** The emitter records
+//!    `claimed_community` (from the event's `h` tag) SEPARATELY from
+//!    `resolved_community` (from `TenantContext::community()`). The
+//!    checker's M2 bite depends on seeing both.
+//! 3. **Drop guard is load-bearing.** Every entry to a critical seam must
+//!    construct an [`EmitGuard`]; if the seam exits without an emit, the
+//!    guard's `Drop` records [`TraceAction::ImplBug`] which the checker
+//!    treats as a coverage breach.
+//!
+//! ## Wire points
+//!
+//! - **ingest.rs:** AuthCheck at `check_channel_membership` call site;
+//!   WriteInsert / WriteInsertGlobal / WriteDuplicate at the two
+//!   `dispatch_persistent_event` sites; SanitizedError at the outer
+//!   wrapper based on the IngestError variant.
+//! - **req.rs / event.rs:** (held back as additive patch for Eva to apply
+//!   onto Max's req.rs writes — see thread `c882c9b1…`).
+
+use std::sync::Arc;
+
+use buzz_core::tenant::TenantContext;
+use nostr::PublicKey;
+use uuid::Uuid;
+
+pub use buzz_conformance::{
+    AbstractState, ActorLabel, ChannelLabel, CommunityLabel, HostLabel, OpaqueId, SanitizedReason,
+    TraceAction, TraceStep, Tracer, Verdict,
+};
+
+mod tracers;
+pub use tracers::{JsonlTracer, NoopTracer};
+
+/// Build the [`AbstractState`] for a request from its resolved tenant
+/// context and authenticated public key.
+///
+/// `community` and `host` come straight from `TenantContext` — server-
+/// resolved, never client input. `actor` is the lower 16 bytes of
+/// `blake3(pubkey_bytes)` as a hex string, opaque and stable across the
+/// run.
+pub fn state_for_request(tenant: &TenantContext, actor: &PublicKey) -> AbstractState {
+    AbstractState {
+        resolved_community: CommunityLabel::from_uuid(*tenant.community().as_uuid()),
+        bound_host: HostLabel(tenant.host().to_string()),
+        actor: actor_label(actor),
+    }
+}
+
+/// Opaque actor label: first 16 hex chars of the pubkey. The pubkey is
+/// already a hash from the client's POV (Schnorr X-only) — equality of
+/// the prefix is equivalent to equality of the pubkey for tracing
+/// purposes, and the relay already prints full pubkey hexes elsewhere,
+/// so the prefix discloses nothing the rest of the log doesn't already.
+/// Using the pubkey directly also avoids dragging in a hash dep for what
+/// is observability code.
+fn actor_label(actor: &PublicKey) -> ActorLabel {
+    let hex = actor.to_hex();
+    let n = hex.len().min(16);
+    ActorLabel(hex[..n].to_string())
+}
+
+/// Opaque message id label: first 16 hex chars of the event id. Same
+/// rationale as actor labels — the id is already a sha256 hash.
+pub fn msg_id_label(event_id: &[u8]) -> OpaqueId {
+    let mut out = String::with_capacity(16);
+    for b in event_id.iter().take(8) {
+        use std::fmt::Write;
+        let _ = write!(&mut out, "{b:02x}");
+    }
+    OpaqueId(out)
+}
+
+/// Map a UUID channel id into a [`ChannelLabel`]. Channels are not secret
+/// — they appear in event `h` tags — so this is a direct wrap.
+pub fn channel_label(ch: Uuid) -> ChannelLabel {
+    ChannelLabel(ch)
+}
+
+/// Extract the *client-claimed* community from an event's `h` tag. Used
+/// to populate [`TraceAction`]'s `claimed_community` field. The relay
+/// does NOT trust this value for resolution — the resolver uses the
+/// server-owned channel→community map. Recording it separately is what
+/// makes the M2 (claim≠resolved) bite visible to the checker.
+///
+/// Returns `None` if there is no `h` tag, or the `h` tag does not parse
+/// as a UUID.
+pub fn claimed_community_from_event(event: &nostr::Event) -> Option<CommunityLabel> {
+    for tag in event.tags.iter() {
+        // The relay's existing convention: `h` tag carries the community
+        // uuid (or channel uuid, ambiguous — but on the WRITE path the h
+        // tag's documented use is the community claim).
+        let raw = tag.as_slice();
+        if raw.first().map(|s| s.as_str()) == Some("h") {
+            if let Some(val) = raw.get(1) {
+                if let Ok(parsed) = Uuid::parse_str(val) {
+                    return Some(CommunityLabel::from_uuid(parsed));
+                }
+            }
+            return None;
+        }
+    }
+    None
+}
+
+/// Build a [`TraceStep::new`] with a freshly-computed [`AbstractState`].
+/// Convenience wrapper to keep the call sites in ingest.rs short.
+pub fn step(action: TraceAction, state: AbstractState) -> TraceStep {
+    TraceStep::new(action, state)
+}
+
+/// Record one step on the tracer. Equivalent to `tracer.record(step(...))`.
+/// Kept inline so the call sites stay tight and self-documenting.
+pub fn emit(tracer: &Arc<dyn Tracer>, action: TraceAction, state: AbstractState) {
+    tracer.record(TraceStep::new(action, state));
+}
+
+/// RAII coverage-breach guard. Constructed at the top of any critical
+/// seam (currently: `ingest_event`); the guard observes a [`Tracer`]
+/// wrapper that counts emits. If the seam exits without any emit
+/// reaching the underlying tracer, `Drop` records a synthetic
+/// [`TraceAction::ImplBug`] step — the checker treats that as a
+/// coverage breach.
+///
+/// The guard wraps the original tracer so production code paths never
+/// need to "disarm" or pass anything around — they just call
+/// `tracer.record(...)` as before, and the wrapper bumps a counter. If
+/// at drop time the counter is zero, the guard emits ImplBug onto the
+/// underlying tracer.
+pub struct EmitGuard {
+    /// The inner tracer, used both for the production emits during the
+    /// request AND for the synthetic ImplBug on Drop if the request
+    /// emitted nothing.
+    inner: Arc<dyn Tracer>,
+    state: AbstractState,
+    counter: Arc<std::sync::atomic::AtomicUsize>,
+    kind: &'static str,
+}
+
+/// Wrapper tracer that bumps a counter on every record. Returned by
+/// [`EmitGuard::counting_tracer`].
+struct CountingTracer {
+    inner: Arc<dyn Tracer>,
+    counter: Arc<std::sync::atomic::AtomicUsize>,
+}
+
+impl std::fmt::Debug for CountingTracer {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("CountingTracer").finish_non_exhaustive()
+    }
+}
+
+impl Tracer for CountingTracer {
+    fn record(&self, step: TraceStep) {
+        self.counter
+            .fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+        self.inner.record(step);
+    }
+}
+
+impl EmitGuard {
+    /// Arm a new guard for the given seam name (e.g.
+    /// `"ingest_exited_without_trace"`). Returns the guard along with a
+    /// counting wrapper around `tracer` that callers should pass into
+    /// the request path instead of the original `tracer`. Every emit
+    /// against the wrapper bumps the guard's counter; if the count is
+    /// still zero at Drop, the guard records an `ImplBug` on the
+    /// original tracer.
+    pub fn arm(
+        tracer: Arc<dyn Tracer>,
+        state: AbstractState,
+        kind: &'static str,
+    ) -> (Self, Arc<dyn Tracer>) {
+        let counter = Arc::new(std::sync::atomic::AtomicUsize::new(0));
+        let counting: Arc<dyn Tracer> = Arc::new(CountingTracer {
+            inner: tracer.clone(),
+            counter: counter.clone(),
+        });
+        let guard = Self {
+            inner: tracer,
+            state,
+            counter,
+            kind,
+        };
+        (guard, counting)
+    }
+}
+
+impl Drop for EmitGuard {
+    fn drop(&mut self) {
+        if self.counter.load(std::sync::atomic::Ordering::Relaxed) == 0 {
+            let step = TraceStep::new(
+                TraceAction::ImplBug {
+                    kind: self.kind.to_string(),
+                },
+                self.state.clone(),
+            );
+            self.inner.record(step);
+        }
+    }
+}
+
+/// Map an `IngestError` variant onto the closed `SanitizedReason`
+/// alphabet (spec line 778, `Inv_SanitizedErrors`). The alphabet is
+/// asserted 1:1 with the relay's error variants — if a fourth variant
+/// is ever added to `IngestError` this match goes non-exhaustive and
+/// CI catches it.
+pub fn sanitized_reason_for(err: &crate::handlers::ingest::IngestError) -> SanitizedReason {
+    use crate::handlers::ingest::IngestError as E;
+    match err {
+        E::Rejected(_) => SanitizedReason::Invalid,
+        E::AuthFailed(_) => SanitizedReason::Restricted,
+        E::Internal(_) => SanitizedReason::ServerError,
+    }
+}
diff --git a/crates/buzz-relay/src/conformance/tracers.rs b/crates/buzz-relay/src/conformance/tracers.rs
new file mode 100644
index 000000000..682c1714e
--- /dev/null
+++ b/crates/buzz-relay/src/conformance/tracers.rs
@@ -0,0 +1,73 @@
+//! Concrete [`Tracer`] implementations. Production uses [`NoopTracer`];
+//! conformance tests + the CI replay job use [`JsonlTracer`].
+
+use std::fs::{File, OpenOptions};
+use std::io::{BufWriter, Write};
+use std::path::Path;
+use std::sync::Mutex;
+
+use buzz_conformance::{TraceStep, Tracer};
+
+/// Zero-cost tracer used in production builds. Records nothing — the
+/// emitter call still constructs the action arguments, but the build can
+/// have the compiler eliminate them entirely behind a feature flag if
+/// the cost ever shows up in benches.
+#[derive(Debug, Default, Clone, Copy)]
+pub struct NoopTracer;
+
+impl Tracer for NoopTracer {
+    fn record(&self, _step: TraceStep) {}
+}
+
+/// JSONL-to-file tracer for tests + the CI replay job. Each `record` call
+/// serializes the step as one line of JSON and appends it. The file is
+/// opened in append mode so multiple test runs accumulate; consumers are
+/// expected to truncate between runs.
+///
+/// The internal `Mutex<BufWriter<File>>` serializes writes — concurrent
+/// requests producing interleaved JSONL is fine on the read side because
+/// the spec doesn't model emission order, only set membership.
+pub struct JsonlTracer {
+    out: Mutex<BufWriter<File>>,
+}
+
+impl JsonlTracer {
+    /// Open a new JSONL tracer writing to `path`. Truncates any existing
+    /// file at that path so a fresh test run starts clean.
+    pub fn create<P: AsRef<Path>>(path: P) -> std::io::Result<Self> {
+        let file = OpenOptions::new()
+            .create(true)
+            .write(true)
+            .truncate(true)
+            .open(path)?;
+        Ok(Self {
+            out: Mutex::new(BufWriter::new(file)),
+        })
+    }
+}
+
+impl std::fmt::Debug for JsonlTracer {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("JsonlTracer").finish_non_exhaustive()
+    }
+}
+
+impl Tracer for JsonlTracer {
+    fn record(&self, step: TraceStep) {
+        // Acquire-and-write. If the lock is poisoned we accept the panic
+        // — this is observability code and a poisoned lock means a worse
+        // bug landed elsewhere.
+        let mut guard = match self.out.lock() {
+            Ok(g) => g,
+            Err(e) => e.into_inner(),
+        };
+        // Best-effort: a write failure here loses one trace step but
+        // must NOT take down the request path. The Drop guard's
+        // coverage-breach action is the safety net for systemic loss.
+        if let Ok(line) = serde_json::to_string(&step) {
+            let _ = guard.write_all(line.as_bytes());
+            let _ = guard.write_all(b"\n");
+            let _ = guard.flush();
+        }
+    }
+}
diff --git a/crates/buzz-relay/src/handlers/event.rs b/crates/buzz-relay/src/handlers/event.rs
index 3633acd50..aa5b3a914 100644
--- a/crates/buzz-relay/src/handlers/event.rs
+++ b/crates/buzz-relay/src/handlers/event.rs
@@ -245,6 +245,12 @@ pub(crate) async fn dispatch_persistent_event(
     kind_u32: u32,
     actor_pubkey_hex: &str,
 ) -> usize {
+    // No `crate::conformance` emit here — the spec doesn't have a
+    // separate fan-out action. Acceptance was already recorded at the
+    // ingest seam (`crates/buzz-relay/src/handlers/ingest.rs`'s
+    // WriteInsert/WriteInsertGlobal/WriteDuplicate emit). The fan-out
+    // surfaces as `ReadMessageRows` observations on the subscriber side
+    // (read seam in req.rs, emitted by the held-back read-seam diff).
     let event_id_hex = stored_event.event.id.to_hex();
 
     let topic = match stored_event.channel_id {
diff --git a/crates/buzz-relay/src/handlers/ingest.rs b/crates/buzz-relay/src/handlers/ingest.rs
index 7e9b95026..2e400cf32 100644
--- a/crates/buzz-relay/src/handlers/ingest.rs
+++ b/crates/buzz-relay/src/handlers/ingest.rs
@@ -43,6 +43,11 @@ use crate::state::AppState;
 
 use super::event::dispatch_persistent_event;
 
+use crate::conformance::{
+    self as conf, channel_label, claimed_community_from_event, emit, msg_id_label,
+    state_for_request, EmitGuard, TraceAction, Verdict,
+};
+
 /// How the HTTP caller authenticated (for [`IngestAuth::Http`]).
 #[derive(Debug, Clone)]
 pub enum HttpAuthMethod {
@@ -1138,11 +1143,57 @@ fn validate_event_reminder(event: &Event) -> Result<(), &'static str> {
 /// Shared by WebSocket and HTTP transports. The caller constructs [`IngestAuth`]
 /// from their transport-specific auth mechanism and maps the result to their
 /// transport-specific response format.
+///
+/// Builds a [`crate::conformance::EmitGuard`] around the actual ingest
+/// logic so the trace seam has fail-closed coverage: any exit path that
+/// doesn't emit a Write*/SanitizedError action will be caught by the
+/// guard's Drop → `ImplBug` → CoverageBreach. The wrapper also maps
+/// `IngestError` → SanitizedError in one place, sparing every individual
+/// `return Err(...)` from having to emit explicitly. See
+/// `crates/buzz-relay/src/conformance/mod.rs` and
+/// `docs/spec/MultiTenantRelay.tla`.
 pub async fn ingest_event(
     state: &Arc<AppState>,
     tenant: &TenantContext,
     event: Event,
     auth: IngestAuth,
+) -> Result<IngestResult, IngestError> {
+    let abstract_state = state_for_request(tenant, auth.pubkey());
+    let (_guard, tracer) = EmitGuard::arm(
+        state.tracer.clone(),
+        abstract_state.clone(),
+        "ingest_event_exited_without_trace",
+    );
+
+    let result = ingest_event_inner(state, &tracer, tenant, event, auth).await;
+
+    // Map terminal error variants onto the closed SanitizedReason
+    // alphabet (spec line 778). The inner fn's success path emits
+    // WriteInsert/WriteInsertGlobal/WriteDuplicate explicitly at its
+    // dispatch points — so on Ok we don't emit here.
+    if let Err(err) = &result {
+        let reason = conf::sanitized_reason_for(err);
+        emit(
+            &tracer,
+            TraceAction::SanitizedError { reason },
+            abstract_state.clone(),
+        );
+    }
+
+    // _guard drops here. If `tracer` received no records during the
+    // request (a panic before the first emit, or a future new exit
+    // path that forgets to emit), Drop records an ImplBug step on
+    // the underlying tracer — the checker treats that as
+    // CoverageBreach.
+    result
+}
+
+async fn ingest_event_inner(
+    state: &Arc<AppState>,
+    tracer: &Arc<dyn buzz_conformance::Tracer>,
+    tenant: &TenantContext,
+    event: Event,
+    auth: IngestAuth,
 ) -> Result<IngestResult, IngestError> {
     let event_id_hex = event.id.to_hex();
     let kind_u32 = event_kind_u32(&event);
@@ -1348,9 +1399,31 @@ pub async fn ingest_event(
             || kind_u32 == KIND_NIP29_CREATE_GROUP
             || auth.has_proxy_scope();
         if !skip_membership {
-            check_channel_membership(tenant, state, ch_id, &pubkey_bytes)
-                .await
-                .map_err(IngestError::Rejected)?;
+            // Spec AuthCheck (line 794): emit the verdict at the actual
+            // call site. claimed_community comes from the event's h tag
+            // (recorded separately to bite M2 / M8 — claim or A-host
+            // driving a B-channel verdict — at the checker). The verdict
+            // basis is `tenant.community()` server-resolved, confirmed
+            // at `check_channel_membership`'s `is_member_cached(tenant
+            // .community(), …)` call (see crates/buzz-relay/src/handlers
+            // /ingest.rs:424).
+            let auth_result = check_channel_membership(tenant, state, ch_id, &pubkey_bytes).await;
+            let claimed = claimed_community_from_event(&event);
+            let verdict = if auth_result.is_ok() {
+                Verdict::Allow
+            } else {
+                Verdict::Deny
+            };
+            emit(
+                tracer,
+                TraceAction::AuthCheck {
+                    channel: channel_label(ch_id),
+                    claimed_community: claimed,
+                    verdict,
+                },
+                state_for_request(tenant, auth.pubkey()),
+            );
+            auth_result.map_err(IngestError::Rejected)?;
         }
     }
 
@@ -1815,6 +1888,26 @@ pub async fn ingest_event(
         }
 
         let pubkey_hex = auth.pubkey().to_hex();
+        // Spec WriteInsert (line 514) / WriteDuplicate (line 606): emit
+        // the abstract write action. The persist API returns
+        // `was_inserted` (true → Insert, false → Duplicate). This branch
+        // is the reaction path; channel_id is always Some here, so
+        // WriteInsertGlobal does not apply.
+        let claimed = claimed_community_from_event(&event);
+        let action = if was_inserted {
+            TraceAction::WriteInsert {
+                msg_id: msg_id_label(event.id.as_bytes()),
+                channel: channel_label(channel_id.expect("reaction path has channel")),
+                claimed_community: claimed,
+            }
+        } else {
+            TraceAction::WriteDuplicate {
+                msg_id: msg_id_label(event.id.as_bytes()),
+                channel: channel_label(channel_id.expect("reaction path has channel")),
+                claimed_community: claimed,
+            }
+        };
+        emit(tracer, action, state_for_request(tenant, auth.pubkey()));
         dispatch_persistent_event(tenant, state, &stored_event, kind_u32, &pubkey_hex).await;
 
         info!(event_id = %event_id_hex, kind = kind_u32, "Event ingested via pipeline");
@@ -1912,6 +2005,35 @@ pub async fn ingest_event(
     }
 
     let pubkey_hex = auth.pubkey().to_hex();
+    // Spec WriteInsert (line 514) / WriteInsertGlobal (line 559) /
+    // WriteDuplicate (line 606): emit the abstract write at the trailing
+    // dispatch site. `channel_id.is_some()` distinguishes channel-bearing
+    // (Insert/Duplicate) from channel-less (InsertGlobal); `was_inserted`
+    // distinguishes accepted-new (Insert/Global) from no-op-on-conflict
+    // (Duplicate). The WriteInsertGlobal duplicate case is not modeled
+    // separately in the spec (channel-less duplicates collapse to the
+    // same observation shape as channel-less inserts at this seam);
+    // see docs/spec/MultiTenantRelay.tla lines 559-595.
+    {
+        let claimed = claimed_community_from_event(&event);
+        let action = match (channel_id, was_inserted) {
+            (Some(ch), true) => TraceAction::WriteInsert {
+                msg_id: msg_id_label(event.id.as_bytes()),
+                channel: channel_label(ch),
+                claimed_community: claimed,
+            },
+            (Some(ch), false) => TraceAction::WriteDuplicate {
+                msg_id: msg_id_label(event.id.as_bytes()),
+                channel: channel_label(ch),
+                claimed_community: claimed,
+            },
+            (None, _) => TraceAction::WriteInsertGlobal {
+                msg_id: msg_id_label(event.id.as_bytes()),
+                claimed_community: claimed,
+            },
+        };
+        emit(tracer, action, state_for_request(tenant, auth.pubkey()));
+    }
     dispatch_persistent_event(tenant, state, &stored_event, kind_u32, &pubkey_hex).await;
 
     info!(event_id = %event_id_hex, kind = kind_u32, "Event ingested via pipeline");
diff --git a/crates/buzz-relay/src/lib.rs b/crates/buzz-relay/src/lib.rs
index 9a7aa50f8..17d56dc2f 100644
--- a/crates/buzz-relay/src/lib.rs
+++ b/crates/buzz-relay/src/lib.rs
@@ -8,6 +8,11 @@ pub mod api;
 pub mod audio;
 /// Relay configuration from environment variables.
 pub mod config;
+/// Runtime conformance harness — abstract trace emission at the
+/// ingest/read accept-reject boundary, replayed against
+/// `docs/spec/MultiTenantRelay.tla` by the independent `buzz-conformance`
+/// checker.
+pub mod conformance;
 /// WebSocket connection lifecycle and state.
 pub mod connection;
 /// Relay error types.
diff --git a/crates/buzz-relay/src/state.rs b/crates/buzz-relay/src/state.rs
index 08c13e843..c9a4559bd 100644
--- a/crates/buzz-relay/src/state.rs
+++ b/crates/buzz-relay/src/state.rs
@@ -278,6 +278,13 @@ pub struct AppState {
     /// Prevents repeated DB lookups from bursty observer traffic.
     #[allow(clippy::type_complexity)]
     pub observer_owner_cache: Arc<moka::sync::Cache<(Vec<u8>, Vec<u8>), bool>>,
+
+    /// Runtime conformance tracer. Production binds [`crate::conformance::NoopTracer`]
+    /// (zero cost). Conformance tests bind [`crate::conformance::JsonlTracer`] to
+    /// record traces for replay against `docs/spec/MultiTenantRelay.tla`.
+    /// See `crates/buzz-conformance/` and `crate::conformance` for the
+    /// schema, emitter helpers, and the independent checker.
+    pub tracer: Arc<dyn buzz_conformance::Tracer>,
 }
 
 impl AppState {
@@ -406,6 +413,11 @@ impl AppState {
                     .time_to_live(std::time::Duration::from_secs(300))
                     .build(),
             ),
+            // Default to NoopTracer: production builds pay zero cost.
+            // Conformance tests overwrite this with a JsonlTracer after
+            // construction (see test helpers in
+            // `crates/buzz-test-client` once those land).
+            tracer: Arc::new(crate::conformance::NoopTracer),
         };
         (
             state,