diff --git a/crates/relayburn-sdk/Cargo.toml b/crates/relayburn-sdk/Cargo.toml
index 4e8e305a..d5e28dd3 100644
--- a/crates/relayburn-sdk/Cargo.toml
+++ b/crates/relayburn-sdk/Cargo.toml
@@ -7,6 +7,14 @@ license.workspace = true
 repository.workspace = true
 description = "Embedding API for relayburn — published to crates.io as the supported Rust surface."
 
+[features]
+# Opt-in feature for downstream integration tests that need the
+# test-only API (`set_ingest_gap_writer` / `restore_ingest_gap_writer`).
+# In-crate unit tests reach the same items via `cfg(test)`; external
+# crates wanting the same hooks enable this feature explicitly so the
+# items are NOT part of the SDK's default public surface.
+test-utils = []
+
 [dependencies]
 # Deps absorbed from the four former lower crates
 # (`relayburn-{reader,ledger,analyze,ingest}`) when the monolith
diff --git a/crates/relayburn-sdk/src/ingest.rs b/crates/relayburn-sdk/src/ingest.rs
index 00264dcb..5ae13ada 100644
--- a/crates/relayburn-sdk/src/ingest.rs
+++ b/crates/relayburn-sdk/src/ingest.rs
@@ -66,9 +66,14 @@ pub use cursors::{
 };
 pub use gap::{
     count_new_tool_calls, count_new_tool_results, count_tool_call_gaps, emit_gap_warning,
-    record_session_gap, reset_ingest_gap_warnings, restore_ingest_gap_writer,
-    set_ingest_gap_writer, AdapterName, ToolCallGapCounts,
+    record_session_gap, reset_ingest_gap_warnings, AdapterName, ToolCallGapCounts,
 };
+// Test-only writer-override hooks. Gated to `cfg(test)` for in-crate
+// tests and to the `test-utils` feature for downstream integration
+// tests; deliberately NOT part of the default SDK surface so embedders
+// can't hijack the global gap-warning writer for the whole process.
+#[cfg(any(test, feature = "test-utils"))]
+pub use gap::{restore_ingest_gap_writer, set_ingest_gap_writer};
 pub use ingest::{
     ingest_all, ingest_claude_projects, ingest_claude_session, ingest_codex_sessions,
     ingest_opencode_sessions, IngestOptions, IngestReport, IngestRoots,
diff --git a/crates/relayburn-sdk/src/ingest/gap.rs b/crates/relayburn-sdk/src/ingest/gap.rs
index b3224002..7d50dcc5 100644
--- a/crates/relayburn-sdk/src/ingest/gap.rs
+++ b/crates/relayburn-sdk/src/ingest/gap.rs
@@ -135,6 +135,12 @@ pub fn reset_ingest_gap_warnings() {
 /// back to `set_ingest_gap_writer` in a `defer`-equivalent so a panicking
 /// test leaves the global state restored on the next test that pulls the
 /// guard.
+///
+/// Gated to `cfg(test)` for in-crate tests and to the `test-utils`
+/// feature for downstream integration tests so the hook is NOT part of
+/// the SDK's default public surface — otherwise an embedder could
+/// hijack stderr writes for the whole process.
+#[cfg(any(test, feature = "test-utils"))]
 pub fn set_ingest_gap_writer<F>(write: F) -> WriterFn
 where
     F: Fn(&str) + Send + Sync + 'static,
@@ -146,6 +152,9 @@ where
 /// Restore a previously captured sink. Convenience for the
 /// `let prev = set_ingest_gap_writer(...); ...; restore_ingest_gap_writer(prev);`
 /// idiom — equivalent to `setIngestGapWriter(prev)` in TS.
+///
+/// Gated alongside `set_ingest_gap_writer` — see that function.
+#[cfg(any(test, feature = "test-utils"))]
 pub fn restore_ingest_gap_writer(write: WriterFn) {
     state_lock().write = write;
 }