[travsr-mcp] resolve the embed hook backend per repo, not machine-wide

crates/travsr-mcp/src/lib.rs

@@ -132,7 +132,7 @@ fn inject_embed_hook(store: &mut SqliteStore, db_path: &Path) {
 
     use travsr_error::StoreError;
     use travsr_plugin_host::{
-        active_backend_id, embed_backends, lookup_embed_backend, EmbedQueryHook, EmbedSupervisor,
+        embed_backends, lookup_embed_backend, EmbedQueryHook, EmbedSupervisor,
     };
     use travsr_store::{EmbedKnnHook, EmbedReadiness, EmbedScoreHook};
 
@@ -143,7 +143,13 @@ fn inject_embed_hook(store: &mut SqliteStore, db_path: &Path) {
     }
 
     let Some(home) = dirs::home_dir() else { return };
-    let backend = active_backend_id()
+    // Prefer the repo's own `.travsr/embed.toml` override, then the user's
+    // machine-wide active backend from ~/.travsr/embed.toml, then the catalog
+    // default so a fresh install without `travsr embed switch` still works.
+    // Mirrors `resolve_backend`'s resolution order (travsr-plugin-host) — this
+    // is a per-repo embedding decision, not an install/list/hint path, so it
+    // must not resolve on `active_backend_id()` alone (#547).
+    let backend = hook_backend_id(db_path)
         .as_deref()
         .and_then(lookup_embed_backend)
         .or_else(|| embed_backends().first())
@@ -286,3 +292,117 @@ fn inject_embed_hook(store: &mut SqliteStore, db_path: &Path) {
     store.set_embed_score_hook(meta_score);
     tracing::info!("embed plugin hook installed (lazy, sidecar starting in background)");
 }
+
+/// Resolve the embed backend id to use for hook injection at `db_path`,
+/// preferring the repo's own `.travsr/embed.toml` override over the
+/// machine-wide `~/.travsr/embed.toml` default. Mirrors `resolve_backend`'s
+/// resolution order in travsr-plugin-host — hook injection is a per-repo
+/// embedding decision, so it must not resolve on `active_backend_id()` alone
+/// (#547: a repo-level `travsr embed switch` was silently ignored, causing
+/// `travsr mcp --stdio` to spawn the wrong sidecar model and disable Step 4).
+///
+/// Same helper as `travsr-daemon`'s `hook_backend_id` (#526), which fixed the
+/// identical resolution order in the daemon's own hook injection.
+///
+/// `parent().parent()` assumes `db_path` is `<repo>/.travsr/graph.db`, which
+/// holds for the default path. An explicit `travsr mcp --stdio --db <path>`
+/// outside a repo yields no `embed.toml` there and falls through to the
+/// machine default, which is the pre-existing behaviour and not a regression.
+/// `resolve_backend_paths` on the spawn side makes the same assumption from
+/// the same input, so hook and spawn still agree in that case, which is the
+/// invariant that matters (#770 review).
+fn hook_backend_id(db_path: &Path) -> Option<String> {
+    use travsr_plugin_host::{active_backend_id, repo_backend_id};
+
+    let repo_root = db_path.parent().and_then(|p| p.parent());
+    repo_root
+        .and_then(repo_backend_id)
+        .or_else(active_backend_id)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // HOME is process-global and Rust tests run in parallel, so every test
+    // that mutates it must serialize on ONE lock. Reuses the existing
+    // `seed::DOCS_ENV_LOCK` rather than declaring a second: these tests and
+    // the docs-lane tests share the `travsr_mcp` lib test binary, so two
+    // disjoint mutexes would serialize each group against itself while still
+    // racing the other. That is not hypothetical here; DOCS_ENV_LOCK's own doc
+    // records it happening once already, and #770's review demonstrated this
+    // pair interleaving (#770 review).
+    use crate::seed::DOCS_ENV_LOCK as ENV_LOCK;
+
+    /// #547: hook injection must prefer a repo's own `.travsr/embed.toml`
+    /// override over the machine-wide `~/.travsr/embed.toml` default, the
+    /// same resolution order `resolve_backend` (travsr-plugin-host) uses.
+    #[test]
+    fn hook_backend_id_prefers_repo_config_over_global() {
+        let _guard = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        let home = tempfile::tempdir().unwrap();
+        let repo = tempfile::tempdir().unwrap();
+        let old_home = std::env::var_os("HOME");
+        std::env::set_var("HOME", home.path());
+
+        std::fs::create_dir_all(home.path().join(".travsr")).unwrap();
+        std::fs::write(
+            home.path().join(".travsr").join("embed.toml"),
+            "active = \"global-backend\"\n",
+        )
+        .unwrap();
+
+        let repo_travsr = repo.path().join(".travsr");
+        std::fs::create_dir_all(&repo_travsr).unwrap();
+        std::fs::write(
+            repo_travsr.join("embed.toml"),
+            "active = \"repo-backend\"\n",
+        )
+        .unwrap();
+
+        let resolved = hook_backend_id(&repo_travsr.join("graph.db"));
+
+        match old_home {
+            Some(h) => std::env::set_var("HOME", h),
+            None => std::env::remove_var("HOME"),
+        }
+
+        assert_eq!(
+            resolved.as_deref(),
+            Some("repo-backend"),
+            "repo's .travsr/embed.toml must win over the machine-wide default"
+        );
+    }
+
+    /// #547: with no repo-level override the machine-wide default still applies,
+    /// so the fix narrows nothing that used to work.
+    #[test]
+    #[cfg_attr(
+        windows,
+        ignore = "dirs::home_dir() on Windows ignores HOME/USERPROFILE entirely (SHGetKnownFolderPath) - this test's isolation cannot work there, see crates/travsr-cli/tests/embed_switch.rs's module doc comment"
+    )]
+    fn hook_backend_id_falls_back_to_global_when_repo_unconfigured() {
+        let _guard = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        let home = tempfile::tempdir().unwrap();
+        let repo = tempfile::tempdir().unwrap();
+        let old_home = std::env::var_os("HOME");
+        std::env::set_var("HOME", home.path());
+
+        std::fs::create_dir_all(home.path().join(".travsr")).unwrap();
+        std::fs::write(
+            home.path().join(".travsr").join("embed.toml"),
+            "active = \"global-backend\"\n",
+        )
+        .unwrap();
+
+        // No repo .travsr/embed.toml written at all.
+        let resolved = hook_backend_id(&repo.path().join(".travsr").join("graph.db"));
+
+        match old_home {
+            Some(h) => std::env::set_var("HOME", h),
+            None => std::env::remove_var("HOME"),
+        }
+
+        assert_eq!(resolved.as_deref(), Some("global-backend"));
+    }
+}