diff --git a/crates/fbuild-cli/src/daemon_client.rs b/crates/fbuild-cli/src/daemon_client.rs
index 22d0467d..50b89dfc 100644
--- a/crates/fbuild-cli/src/daemon_client.rs
+++ b/crates/fbuild-cli/src/daemon_client.rs
@@ -1032,6 +1032,11 @@ async fn ensure_direct_daemon_running() -> fbuild_core::Result<()> {
         DaemonProbe::Restarted => true,
         DaemonProbe::NotRunning => false,
     };
+    if !restarted {
+        if let Some(error) = foreign_root_owner_error() {
+            return Err(fbuild_core::FbuildError::DaemonError(error));
+        }
+    }
     let spawned_pid = start_daemon(&client).await?;
     if restarted {
         // FastLED/fbuild#1476: whichever branch `start_daemon` took (spawned,
@@ -1042,6 +1047,40 @@ async fn ensure_direct_daemon_running() -> fbuild_core::Result<()> {
     Ok(())
 }
 
+/// A live daemon of another version or identity owns this cache root on a
+/// port this CLI does not use. A daemon spawned now would only exit on
+/// `root-owner.lock busy`, and three such spawns used to end in a misleading
+/// "daemon did not become healthy" error while the owner was merely busy.
+fn foreign_root_owner_error() -> Option<String> {
+    let claim = fbuild_paths::daemon_ownership::read_owner_claim()?;
+    let own_port = fbuild_paths::get_daemon_port();
+    let live = fbuild_core::platform::process::pid_is_alive(claim.pid)
+        && fbuild_core::platform::process::pid_exe_stem_matches(
+            claim.pid,
+            fbuild_paths::daemon_ownership::DAEMON_EXE_STEM,
+        );
+    (live && claim.port != own_port).then(|| foreign_root_owner_message(&claim, own_port))
+}
+
+fn foreign_root_owner_message(
+    claim: &fbuild_paths::daemon_ownership::OwnerClaim,
+    own_port: u16,
+) -> String {
+    format!(
+        "the fbuild cache root is owned by another fbuild-daemon (pid {}, version {}, mode {}, \
+         port {}, exe {}), which this fbuild {} cannot share; it would use port {}. \
+         No compilation was attempted. Wait for that daemon's work to finish and stop it \
+         (`fbuild daemon stop`), or run the same fbuild build as the client that started it.",
+        claim.pid,
+        claim.version,
+        claim.mode,
+        claim.port,
+        claim.exe.display(),
+        env!("CARGO_PKG_VERSION"),
+        own_port
+    )
+}
+
 enum DaemonProbe {
     /// A daemon is running and should be kept.
     Keep,
diff --git a/crates/fbuild-cli/src/daemon_client/tests.rs b/crates/fbuild-cli/src/daemon_client/tests.rs
index 2430bc59..1f39fcb7 100644
--- a/crates/fbuild-cli/src/daemon_client/tests.rs
+++ b/crates/fbuild-cli/src/daemon_client/tests.rs
@@ -3,8 +3,9 @@
 
 use super::{
     DaemonAcquisition, DaemonClient, DaemonInfoResponse, HealthResponseFull,
-    broker_refusal_is_fatal, daemon_cache_identity_error, launcher_path,
-    restart_diag::SiblingDaemon, same_running_image, should_restart_daemon, wedged_daemon_note,
+    broker_refusal_is_fatal, daemon_cache_identity_error, foreign_root_owner_message,
+    launcher_path, restart_diag::SiblingDaemon, same_running_image, should_restart_daemon,
+    wedged_daemon_note,
 };
 use running_process::broker::client::RefusalKind::{VersionBlocked, VersionUnsupported};
 
@@ -222,3 +223,28 @@ fn a_live_unresponsive_daemon_is_named_along_with_its_recovery() {
 fn no_live_daemon_produces_no_hint() {
     assert_eq!(wedged_daemon_note(None), "");
 }
+
+/// A busy daemon of another build that owns the cache root must be named,
+/// not reported as an unhealthy daemon after three doomed spawns.
+#[test]
+fn foreign_root_owner_message_names_the_owner() {
+    let claim = fbuild_paths::daemon_ownership::OwnerClaim {
+        pid: 98258,
+        exe: fbuild_core::path::NormalizedPath::new("/venv/bin/fbuild-daemon"),
+        version: "2.5.44".to_string(),
+        mode: "prod".to_string(),
+        cache_root_key: "/c".to_string(),
+        port: 60903,
+    };
+    let message = foreign_root_owner_message(&claim, 19142);
+    for needle in [
+        "98258",
+        "2.5.44",
+        "60903",
+        "19142",
+        "/venv/bin/fbuild-daemon",
+        "No compilation",
+    ] {
+        assert!(message.contains(needle), "{needle} missing from: {message}");
+    }
+}
diff --git a/crates/fbuild-paths/src/lib.rs b/crates/fbuild-paths/src/lib.rs
index 4992ae19..4be3aaf2 100644
--- a/crates/fbuild-paths/src/lib.rs
+++ b/crates/fbuild-paths/src/lib.rs
@@ -13,6 +13,9 @@ pub mod executable_hash;
 pub mod long_paths;
 pub mod running_process;
 
+#[cfg(test)]
+mod owner_port_tests;
+
 /// The project-local and home-local fbuild directory segment: `.fbuild`.
 ///
 /// This is the canonical spelling. Nothing outside `fbuild-paths` /
@@ -386,7 +389,10 @@ fn read_port_from_file(path: &Path) -> Option<u16> {
 /// Priority:
 /// 1. `FBUILD_DAEMON_PORT` environment variable (if set and valid 1–65535)
 /// 2. Port file for this endpoint (if it exists and is valid)
-/// 3. [`default_daemon_port`] — a deterministic per-(version, cache-identity)
+/// 3. The port of the live daemon that owns this cache root (its
+///    `root-owner.json` claim), when it is the same version, mode and cache
+///    root
+/// 4. [`default_daemon_port`] — a deterministic per-(version, cache-identity)
 ///    port
 ///
 /// FastLED/fbuild#1009: the endpoint is keyed by version+identity (via
@@ -417,10 +423,57 @@ pub fn get_daemon_port() -> u16 {
         // `recorded_daemon_owner_is_dead`.
     }
 
-    // Priority 3: deterministic per-endpoint default.
+    // Priority 3: the live daemon that owns this cache root, when it is the
+    // same version and identity. FastLED/fbuild#1009 keys the endpoint on
+    // version+identity, but two builds that both report the same version can
+    // still derive different ports (e.g. a 2.5.44 wheel built before the
+    // #1700 port-window change and a 2.5.44 dev build after it). The
+    // version-blind root-ownership lock lets only one of them run, so a
+    // client that ignored the owner would spawn daemons that exit on
+    // `root-owner.lock busy` and then report the busy owner as unhealthy.
+    if let Some(port) = daemon_ownership::read_owner_claim().and_then(|claim| {
+        let identity = crate::running_process::DaemonCacheIdentity::discover();
+        adoptable_owner_port(
+            &claim,
+            env!("CARGO_PKG_VERSION"),
+            identity.mode,
+            &identity.cache_root_key,
+            owner_claim_is_live,
+        )
+    }) {
+        return port;
+    }
+
+    // Priority 4: deterministic per-endpoint default.
     default_daemon_port()
 }
 
+/// The owner claim's port, if the claim describes a live daemon this client
+/// may share: same version, mode and cache root. Pure apart from `is_live`.
+fn adoptable_owner_port(
+    claim: &daemon_ownership::OwnerClaim,
+    version: &str,
+    mode: &str,
+    cache_root_key: &str,
+    is_live: impl FnOnce(&daemon_ownership::OwnerClaim) -> bool,
+) -> Option<u16> {
+    (claim.port > 0
+        && claim.version == version
+        && claim.mode == mode
+        && claim.cache_root_key == cache_root_key
+        && is_live(claim))
+    .then_some(claim.port)
+}
+
+/// Positive evidence that the claim's pid is a running `fbuild-daemon`.
+fn owner_claim_is_live(claim: &daemon_ownership::OwnerClaim) -> bool {
+    fbuild_core::platform::process::pid_is_alive(claim.pid)
+        && fbuild_core::platform::process::pid_exe_stem_matches(
+            claim.pid,
+            daemon_ownership::DAEMON_EXE_STEM,
+        )
+}
+
 /// Can we *prove* the daemon that wrote this endpoint's records is gone?
 ///
 /// Fails safe: anything short of positive evidence of death returns `false`,
diff --git a/crates/fbuild-paths/src/owner_port_tests.rs b/crates/fbuild-paths/src/owner_port_tests.rs
new file mode 100644
index 00000000..3f0bb97e
--- /dev/null
+++ b/crates/fbuild-paths/src/owner_port_tests.rs
@@ -0,0 +1,51 @@
+//! Tests for `adoptable_owner_port` (split from lib.rs for the 1000-LOC gate).
+
+use super::{adoptable_owner_port, daemon_ownership};
+
+fn claim(version: &str, mode: &str, cache: &str, port: u16) -> daemon_ownership::OwnerClaim {
+    daemon_ownership::OwnerClaim {
+        pid: std::process::id(),
+        exe: fbuild_core::path::NormalizedPath::new("/x/fbuild-daemon"),
+        version: version.to_string(),
+        mode: mode.to_string(),
+        cache_root_key: cache.to_string(),
+        port,
+    }
+}
+
+/// Two same-version builds that derive different default ports must
+/// still find the live daemon that owns their shared cache root.
+#[test]
+fn owner_claim_port_is_adopted_for_same_version_identity() {
+    let c = claim("2.5.44", "prod", "/c", 60903);
+    assert_eq!(
+        adoptable_owner_port(&c, "2.5.44", "prod", "/c", |_| true),
+        Some(60903)
+    );
+}
+
+#[test]
+fn owner_claim_port_is_not_adopted_across_identity_or_when_dead() {
+    let c = claim("2.5.44", "prod", "/c", 60903);
+    assert_eq!(
+        adoptable_owner_port(&c, "2.5.44", "prod", "/c", |_| false),
+        None
+    );
+    assert_eq!(
+        adoptable_owner_port(&c, "2.5.45", "prod", "/c", |_| true),
+        None
+    );
+    assert_eq!(
+        adoptable_owner_port(&c, "2.5.44", "dev", "/c", |_| true),
+        None
+    );
+    assert_eq!(
+        adoptable_owner_port(&c, "2.5.44", "prod", "/d", |_| true),
+        None
+    );
+    let zero = claim("2.5.44", "prod", "/c", 0);
+    assert_eq!(
+        adoptable_owner_port(&zero, "2.5.44", "prod", "/c", |_| true),
+        None
+    );
+}