hive-c0re: fail on an unparseable resource-limits or topology file, write both atomically

resource-limits.json and topology.json were read with parse errors
folded into an empty map, and written in place with std::fs::write. One
truncated resource-limits.json followed by a single set_limits call
rewrote the file with only that agent's entry, erasing every other
agent's CPU and memory overrides without a log line. topology.json had
the same shape: reconcile rebuilt it from the live set, losing pending
(provisioned, never spawned) names.

- agent_config::read_map / write_map are generic over the stored type.
  tool-groups and capabilities behave as before.
- resource_limits::read / effective return an error for an existing but
  unreadable file; a missing file is still the empty map. set_limits
  fails without writing on such a file, and writes atomically.
- topology: reconcile fails without writing on an unreadable file and
  writes atomically. all_agents logs the error and returns no agents,
  so a ManageRootAgent holder starts without cross-agent mounts.

Read-path behaviour on an unreadable resource-limits.json, per caller:
- write_dropins (every spawn / swap / WriteDropin): logs the error and
  keeps the limits drop-in already under /run; the agent still starts.
  With no drop-in yet (first start since boot) it writes the hive
  defaults, because no drop-in means an uncapped container.
- render_flake: propagates, so sync_agents (and spawn/rebuild/destroy
  jobs) fail. An empty map would give tighter-capped agents the hive
  memoryMaxBytes.
- container_view::build_all: logs the error each scan and renders the
  rows at the hive defaults (no ContainerView wire change).
- set_resource_limits reply: propagates.

Closes #4731

hive-c0re/src/agent_config/mod.rs

@@ -14,18 +14,20 @@ pub mod resource_limits;
 pub mod tool_groups;
 pub mod topology;
 
-use std::collections::BTreeMap;
 use std::io::{self, Write as _};
 use std::path::Path;
 
-/// Read a per-agent `name → [string]` JSON map. A missing file is the
-/// empty map; any other read failure, or content that doesn't parse, is
-/// an error, so a read-modify-write can't write back a map that has lost
-/// every other agent's entry.
-fn read_map(path: &Path) -> io::Result<BTreeMap<String, Vec<String>>> {
+use serde::Serialize;
+use serde::de::DeserializeOwned;
+
+/// Read one of these JSON registries. A missing file is `T::default()`
+/// (the empty map or set); any other read failure, or content that
+/// doesn't parse, is an error, so a read-modify-write can't write back a
+/// registry that has lost every other agent's entry.
+fn read_map<T: DeserializeOwned + Default>(path: &Path) -> io::Result<T> {
     let raw = match std::fs::read_to_string(path) {
         Ok(raw) => raw,
-        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(BTreeMap::new()),
+        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(T::default()),
         Err(e) => {
             return Err(io::Error::new(
                 e.kind(),
@@ -41,13 +43,13 @@ fn read_map(path: &Path) -> io::Result<BTreeMap<String, Vec<String>>> {
     })
 }
 
-/// Serialise `map` as pretty JSON and replace `path` with it atomically:
+/// Serialise `value` as pretty JSON and replace `path` with it atomically:
 /// a temp file in the same directory (`rename(2)` is only atomic within a
 /// filesystem), fsynced, renamed over `path`, then the directory fsynced
 /// so the rename itself survives a crash. A reader sees the old file or
 /// the new one, never a truncated one.
-fn write_map(path: &Path, map: &BTreeMap<String, Vec<String>>) -> io::Result<()> {
-    let text = serde_json::to_string_pretty(map)
+fn write_map<T: Serialize + ?Sized>(path: &Path, value: &T) -> io::Result<()> {
+    let text = serde_json::to_string_pretty(value)
         .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
     let (Some(dir), Some(name)) = (path.parent(), path.file_name()) else {
         return Err(io::Error::new(
@@ -67,8 +69,12 @@ fn write_map(path: &Path, map: &BTreeMap<String, Vec<String>>) -> io::Result<()>
 
 #[cfg(test)]
 mod tests {
+    use std::collections::BTreeMap;
+
     use super::*;
 
+    type Map = BTreeMap<String, Vec<String>>;
+
     #[test]
     fn write_map_round_trips_and_leaves_no_temp_file() {
         let dir = tempfile::tempdir().expect("tempdir");
@@ -77,7 +83,7 @@ mod tests {
         write_map(&path, &map).expect("first write");
         let replaced = BTreeMap::from([("bob".to_owned(), vec!["inbox".to_owned()])]);
         write_map(&path, &replaced).expect("replacing write");
-        assert_eq!(read_map(&path).expect("read"), replaced);
+        assert_eq!(read_map::<Map>(&path).expect("read"), replaced);
         let entries: Vec<_> = std::fs::read_dir(dir.path())
             .expect("read_dir")
             .map(|e| e.expect("entry").file_name())
@@ -88,7 +94,8 @@ mod tests {
     #[test]
     fn read_map_of_a_missing_file_is_empty() {
         let dir = tempfile::tempdir().expect("tempdir");
-        let map = read_map(&dir.path().join("absent.json")).expect("missing file is not an error");
+        let map =
+            read_map::<Map>(&dir.path().join("absent.json")).expect("missing file is not an error");
         assert!(map.is_empty());
     }
 
@@ -97,7 +104,7 @@ mod tests {
         let dir = tempfile::tempdir().expect("tempdir");
         let path = dir.path().join("map.json");
         std::fs::write(&path, "{\n  \"alice\": [\"messag").expect("seed");
-        let err = read_map(&path).expect_err("truncated JSON must not read as a map");
+        let err = read_map::<Map>(&path).expect_err("truncated JSON must not read as a map");
         assert_eq!(err.kind(), io::ErrorKind::InvalidData);
     }
 }

hive-c0re/src/agent_config/resource_limits.rs

@@ -28,7 +28,7 @@
 //! a resource *cap* should not be sourced from the capped party.
 
 use std::collections::BTreeMap;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 const RESOURCE_LIMITS_FILE: &str = "resource-limits.json";
 
@@ -58,28 +58,35 @@ pub fn resource_limits_path() -> PathBuf {
     crate::paths::meta_root().join(RESOURCE_LIMITS_FILE)
 }
 
-/// Read the per-agent limit map. Returns an empty map when the file is
-/// absent or unparsable — callers treat a missing entry as "hive-wide
-/// defaults", which is also the safe failure mode for a malformed file.
-#[must_use]
-pub fn read() -> BTreeMap<String, AgentLimits> {
-    let path = resource_limits_path();
-    let Ok(raw) = std::fs::read_to_string(&path) else {
-        return BTreeMap::new();
-    };
-    serde_json::from_str(&raw).unwrap_or_default()
+/// Read the per-agent limit map. An absent file is the empty map —
+/// callers treat a missing entry as "hive-wide defaults". A file that
+/// exists but can't be read or parsed is an error: an override can be
+/// tighter than the hive default, so reading it as empty is not a safe
+/// fallback.
+pub fn read() -> std::io::Result<BTreeMap<String, AgentLimits>> {
+    super::read_map(&resource_limits_path())
 }
 
-/// Resolve the effective values for an agent, filling each unset field
+/// Resolve the effective values for an agent from the override file at
+/// `path` (normally [`resource_limits_path`]), filling each unset field
 /// from the hive-wide default. This is the single place the fallback
 /// rule lives; `write_dropins` calls it and passes the result straight
-/// to systemd.
+/// to systemd. Errors as [`read`] does.
 ///
-/// Reads the override file. Use [`effective_from`] when resolving more
-/// than one agent in a row.
-#[must_use]
-pub fn effective(name: &str, hive_cpu_quota: &str, hive_memory_max: &str) -> (String, String) {
-    effective_from(&read(), name, hive_cpu_quota, hive_memory_max)
+/// Use [`effective_from`] when resolving more than one agent in a row.
+pub fn effective(
+    path: &Path,
+    name: &str,
+    hive_cpu_quota: &str,
+    hive_memory_max: &str,
+) -> std::io::Result<(String, String)> {
+    let limits = super::read_map(path)?;
+    Ok(effective_from(
+        &limits,
+        name,
+        hive_cpu_quota,
+        hive_memory_max,
+    ))
 }
 
 /// [`effective`] against an already-loaded map — the multi-agent form.
@@ -200,42 +207,27 @@ pub fn parse_bytes(value: &str) -> Option<u64> {
     u64::try_from(bytes).ok()
 }
 
-/// Persist the full map. Sorted JSON output keeps meta-repo diffs
-/// minimal.
+/// Set one agent's overrides and persist the map atomically. An entry
+/// with both fields unset is removed rather than stored, so "reset to
+/// hive defaults" and "never configured" are the same state on disk.
 ///
 /// # Errors
 ///
-/// Returns an `io::Error` when the meta dir can't be created or the
-/// file can't be written (permissions, disk full). Serialization
-/// failure is surfaced as `InvalidData`, though it can't happen for
-/// this type — it's a plain map of strings.
-pub fn write(map: &BTreeMap<String, AgentLimits>) -> std::io::Result<()> {
-    let path = resource_limits_path();
-    if let Some(parent) = path.parent() {
-        std::fs::create_dir_all(parent)?;
-    }
-    let text = serde_json::to_string_pretty(map)
-        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
-    std::fs::write(&path, format!("{text}\n"))
+/// Fails without writing when the existing file can't be read or parsed
+/// (`InvalidData` for a parse failure), and otherwise when the meta dir
+/// or the file can't be written.
+pub fn set_limits(name: &str, limits: &AgentLimits) -> std::io::Result<()> {
+    set_limits_at(&resource_limits_path(), name, limits)
 }
 
-/// Set one agent's overrides and persist. An entry with both fields
-/// unset is removed rather than stored, so "reset to hive defaults" and
-/// "never configured" are the same state on disk.
-///
-/// # Errors
-///
-/// Propagates whatever the `write` function fails with. An unreadable or malformed
-/// existing file is *not* an error — the `read` function degrades to an empty map,
-/// so this call rewrites the file from scratch.
-pub fn set_limits(name: &str, limits: &AgentLimits) -> std::io::Result<()> {
-    let mut current = read();
+fn set_limits_at(path: &Path, name: &str, limits: &AgentLimits) -> std::io::Result<()> {
+    let mut current: BTreeMap<String, AgentLimits> = super::read_map(path)?;
     if limits.is_empty() {
         current.remove(name);
     } else {
         current.insert(name.to_owned(), limits.clone());
     }
-    write(&current)
+    super::write_map(path, &current)
 }
 
 /// Validate a systemd `CPUQuota=` value. Percentages only, and values
@@ -390,11 +382,59 @@ mod tests {
         assert!(!limits(Some("400%"), None).is_empty());
     }
 
+    const TRUNCATED: &str =
+        "{\n  \"sock\": { \"cpu_quota\": \"50%\", \"memory_max\": \"1G\" },\n  \"iris\": { \"mem";
+
+    fn corrupt_file() -> (tempfile::TempDir, PathBuf) {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(RESOURCE_LIMITS_FILE);
+        std::fs::write(&path, TRUNCATED).expect("seed");
+        (dir, path)
+    }
+
     #[test]
-    fn malformed_json_reads_as_empty_map() {
-        let parsed: BTreeMap<String, AgentLimits> =
-            serde_json::from_str("{ not json").unwrap_or_default();
-        assert!(parsed.is_empty());
+    fn effective_of_a_corrupt_file_is_an_error() {
+        let (_dir, path) = corrupt_file();
+        let err = effective(&path, "sock", HIVE_CPU, HIVE_MEM)
+            .expect_err("a corrupt file must not resolve to hive defaults");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+    }
+
+    #[test]
+    fn set_limits_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        let err = set_limits_at(&path, "ruth", &limits(Some("400%"), None))
+            .expect_err("a corrupt file must not be overwritten");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn missing_file_resolves_to_hive_defaults() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(RESOURCE_LIMITS_FILE);
+        let (cpu, mem) = effective(&path, "sock", HIVE_CPU, HIVE_MEM).expect("missing file");
+        assert_eq!((cpu.as_str(), mem.as_str()), (HIVE_CPU, HIVE_MEM));
+    }
+
+    #[test]
+    fn set_limits_keeps_other_agents_and_leaves_no_temp_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(RESOURCE_LIMITS_FILE);
+        set_limits_at(&path, "sock", &limits(None, Some("8G"))).expect("set on missing file");
+        set_limits_at(&path, "iris", &limits(Some("50%"), None)).expect("set");
+        let (cpu, mem) = effective(&path, "sock", HIVE_CPU, HIVE_MEM).expect("read");
+        assert_eq!((cpu.as_str(), mem.as_str()), (HIVE_CPU, "8G"));
+        let (cpu, _) = effective(&path, "iris", HIVE_CPU, HIVE_MEM).expect("read");
+        assert_eq!(cpu, "50%");
+        let entries: Vec<_> = std::fs::read_dir(dir.path())
+            .expect("read_dir")
+            .map(|e| e.expect("entry").file_name())
+            .collect();
+        assert_eq!(
+            entries,
+            vec![std::ffi::OsString::from(RESOURCE_LIMITS_FILE)]
+        );
     }
 
     /// Absent fields must deserialize to `None`, not fail — an entry

hive-c0re/src/agent_config/tool_groups.rs

@@ -88,7 +88,7 @@ fn set_groups_at(path: &Path, name: &str, groups: &[String]) -> anyhow::Result<(
     if !groups.is_empty() {
         validate_groups(groups)?;
     }
-    let mut current = super::read_map(path)?;
+    let mut current: BTreeMap<String, Vec<String>> = super::read_map(path)?;
     if groups.is_empty() {
         current.remove(name);
     } else {
@@ -104,7 +104,7 @@ pub fn remove_agent(name: &str) -> std::io::Result<()> {
 }
 
 fn remove_agent_at(path: &Path, name: &str) -> std::io::Result<()> {
-    let mut current = super::read_map(path)?;
+    let mut current: BTreeMap<String, Vec<String>> = super::read_map(path)?;
     if current.remove(name).is_some() {
         super::write_map(path, &current)?;
     }
@@ -151,7 +151,8 @@ mod tests {
         let path = dir.path().join(TOOL_GROUPS_FILE);
         set_groups_at(&path, "alice", &["inbox".to_owned()]).expect("set on missing file");
         set_groups_at(&path, "ruth", &["messaging".to_owned()]).expect("set");
-        let map = crate::agent_config::read_map(&path).expect("read");
+        let map: BTreeMap<String, Vec<String>> =
+            crate::agent_config::read_map(&path).expect("read");
         assert_eq!(map["alice"], vec!["inbox".to_owned()]);
         assert_eq!(map["ruth"], vec!["messaging".to_owned()]);
     }

hive-c0re/src/agent_config/topology.rs

@@ -17,7 +17,7 @@
 //! `lifecycle::set_nspawn_flags`.
 
 use std::collections::BTreeSet;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use serde::Deserialize;
 
@@ -28,7 +28,7 @@ pub fn topology_path() -> PathBuf {
     crate::paths::meta_root().join(TOPOLOGY_FILE)
 }
 
-/// On-disk shapes [`read`] accepts. The array is what [`write()`] emits; the
+/// On-disk shapes [`read_from`] accepts. The array is what [`reconcile`] writes; the
 /// map is the legacy `name → parent | null` format, kept readable so a
 /// hive that upgrades across this change keeps its roster instead of
 /// blanking it until the next `reconcile` pass — and a blank roster is not
@@ -42,33 +42,51 @@ enum OnDisk {
     WithParents(std::collections::BTreeMap<String, Option<String>>),
 }
 
-/// Snapshot of the agent roster. Read on every `container_view::build_all`
-/// and every `render_flake` call. The file is small (one line per agent),
-/// so we re-read rather than caching — keeps the source of truth on disk.
-///
-/// Returns an empty set when the file is absent or unparsable. Safe
-/// degradation for fresh installs that haven't run through
-/// `meta::sync_agents` yet.
-#[must_use]
-pub fn read() -> BTreeSet<String> {
-    let path = topology_path();
-    let Ok(raw) = std::fs::read_to_string(&path) else {
-        return BTreeSet::new();
-    };
-    match serde_json::from_str::<OnDisk>(&raw) {
-        Ok(OnDisk::Roster(names)) => names,
-        Ok(OnDisk::WithParents(map)) => map.into_keys().collect(),
-        Err(_) => BTreeSet::new(),
+/// A missing file is the empty roster (a fresh install that hasn't run
+/// through `meta::sync_agents` yet).
+impl Default for OnDisk {
+    fn default() -> Self {
+        Self::Roster(BTreeSet::new())
     }
 }
 
+/// Snapshot of the agent roster in the file at `path` (normally
+/// [`topology_path`]). The file is small (one line per agent), so it is
+/// re-read rather than cached — keeps the source of truth on disk.
+///
+/// An absent file is the empty set. A file that exists but can't be read
+/// or parsed is an error, so [`reconcile`] never writes over it.
+fn read_from(path: &Path) -> std::io::Result<BTreeSet<String>> {
+    Ok(match super::read_map::<OnDisk>(path)? {
+        OnDisk::Roster(names) => names,
+        OnDisk::WithParents(map) => map.into_keys().collect(),
+    })
+}
+
 /// Every agent the roster knows about, in name order. This is the set
 /// a [`hive_sh4re::permissions::Capability::ManageRootAgent`] holder gets
 /// bind-mounted, and it is deliberately unfiltered: that capability means
 /// "may manage any agent", so the set is all of them.
+///
+/// An unreadable roster is logged and yields no agents, so the holder
+/// starts without cross-agent mounts rather than failing to start.
 #[must_use]
 pub fn all_agents() -> Vec<String> {
-    all_agents_in(&read())
+    all_agents_at(&topology_path())
+}
+
+fn all_agents_at(path: &Path) -> Vec<String> {
+    match read_from(path) {
+        Ok(topo) => all_agents_in(&topo),
+        Err(e) => {
+            tracing::error!(
+                path = %path.display(),
+                error = ?e,
+                "topology unreadable — no cross-agent mounts"
+            );
+            Vec::new()
+        }
+    }
 }
 
 /// Pure form of [`all_agents`] for unit tests.
@@ -77,33 +95,27 @@ pub fn all_agents_in(topo: &BTreeSet<String>) -> Vec<String> {
     topo.iter().cloned().collect()
 }
 
-/// Persist the roster. Sorted JSON output (`BTreeSet` iterates in key
-/// order) keeps git diffs minimal across re-writes. Best-effort —
-/// returns an `io::Error` so callers can decide whether a failure
-/// should abort their op (`sync_agents`) or just log.
-pub fn write(topology: &BTreeSet<String>) -> std::io::Result<()> {
-    let path = topology_path();
-    if let Some(parent) = path.parent() {
-        std::fs::create_dir_all(parent)?;
-    }
-    let text = serde_json::to_string_pretty(topology)
-        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
-    std::fs::write(&path, format!("{text}\n"))
-}
-
 /// Reconcile `topology.json` against the current agent set. Adds any agent
 /// missing from the file; removes entries for agents no longer present.
 /// Returns true when the file changed and should be re-committed by the
-/// caller.
+/// caller. The write is atomic, as a sorted array (`BTreeSet` iterates in
+/// key order) to keep git diffs minimal.
 ///
 /// `pending` lists agents that have a provisioned proposed config repo
 /// but no container yet (provisioned, not yet spawned). They are KEPT
 /// (not dropped) so an agent that exists on disk but has never booted is
 /// still a name the hive knows about.
+///
+/// Fails without writing when the existing file can't be read or parsed:
+/// rebuilding it from the live set alone would drop every pending name.
 pub fn reconcile(agent_names: &[String], pending: &[String]) -> std::io::Result<bool> {
-    let (next, changed) = apply_reconcile(&read(), agent_names, pending);
+    reconcile_at(&topology_path(), agent_names, pending)
+}
+
+fn reconcile_at(path: &Path, agent_names: &[String], pending: &[String]) -> std::io::Result<bool> {
+    let (next, changed) = apply_reconcile(&read_from(path)?, agent_names, pending);
     if changed {
-        write(&next)?;
+        super::write_map(path, &next)?;
     }
     Ok(changed)
 }
@@ -138,7 +150,10 @@ pub fn apply_reconcile(
 
 #[cfg(test)]
 mod tests {
-    use super::{BTreeSet, OnDisk, all_agents_in, apply_reconcile};
+    use super::{
+        BTreeSet, OnDisk, TOPOLOGY_FILE, all_agents_at, all_agents_in, apply_reconcile, read_from,
+        reconcile_at,
+    };
 
     fn roster_three() -> BTreeSet<String> {
         ["alice", "bob", "carol"]
@@ -228,4 +243,44 @@ mod tests {
         };
         assert_eq!(names, roster_three());
     }
+
+    const TRUNCATED: &str = "[\n  \"alice\",\n  \"bob\",\n  \"do";
+
+    fn corrupt_file() -> (tempfile::TempDir, std::path::PathBuf) {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(TOPOLOGY_FILE);
+        std::fs::write(&path, TRUNCATED).expect("seed");
+        (dir, path)
+    }
+
+    #[test]
+    fn reconcile_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        let live = vec!["alice".to_owned(), "bob".to_owned()];
+        let err =
+            reconcile_at(&path, &live, &[]).expect_err("a corrupt file must not be rewritten");
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn all_agents_of_a_corrupt_file_is_empty() {
+        let (_dir, path) = corrupt_file();
+        assert!(all_agents_at(&path).is_empty());
+    }
+
+    #[test]
+    fn reconcile_round_trips_through_a_missing_file_and_leaves_no_temp_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(TOPOLOGY_FILE);
+        assert!(read_from(&path).expect("missing file").is_empty());
+        let live = vec!["alice".to_owned(), "bob".to_owned(), "carol".to_owned()];
+        assert!(reconcile_at(&path, &live, &[]).expect("reconcile"));
+        assert_eq!(read_from(&path).expect("read"), roster_three());
+        let entries: Vec<_> = std::fs::read_dir(dir.path())
+            .expect("read_dir")
+            .map(|e| e.expect("entry").file_name())
+            .collect();
+        assert_eq!(entries, vec![std::ffi::OsString::from(TOPOLOGY_FILE)]);
+    }
 }

hive-c0re/src/container_view.rs

@@ -156,8 +156,17 @@ pub async fn build_all(hive: &crate::coordinator::HiveEnv) -> Vec<ContainerView>
     let raw = lifecycle::list().await.unwrap_or_default();
     let locked = read_meta_locked_revs();
     // Read once per scan rather than re-read for every container on every
-    // SSE scan.
-    let limits = crate::resource_limits::read();
+    // SSE scan. An unreadable file shows every row at the hive defaults;
+    // the error log is the only signal, as `ContainerView` has no field
+    // to carry it.
+    let limits = crate::resource_limits::read().unwrap_or_else(|e| {
+        tracing::error!(
+            path = %crate::resource_limits::resource_limits_path().display(),
+            error = ?e,
+            "resource limits unreadable — container rows show hive defaults"
+        );
+        std::collections::BTreeMap::new()
+    });
     let mut out = Vec::new();
     for c in &raw {
         let Some(logical) = c.strip_prefix(AGENT_PREFIX) else {

hive-c0re/src/lifecycle/host_config.rs

@@ -24,8 +24,12 @@ pub async fn write_dropins(name: &str, hive: &HiveEnv, paths: &AgentPaths) -> Re
     validate(name)?;
     let container = container_name(name);
     set_nspawn_flags(&container, &paths.agent, &paths.claude, &paths.notes).await?;
-    let (cpu_quota, memory_max) =
-        crate::resource_limits::effective(name, &hive.agent_cpu_quota, &hive.agent_memory_max);
+    if let Some((cpu_quota, memory_max)) = limits_to_apply(
+        name,
+        &crate::resource_limits::resource_limits_path(),
+        &limits_dropin_path(&container),
+        hive,
+    ) {
         set_resource_limits(
             &container,
             &cpu_quota,
@@ -34,14 +38,63 @@ pub async fn write_dropins(name: &str, hive: &HiveEnv, paths: &AgentPaths) -> Re
             hive.agent_io_weight,
         )
         .await?;
+    }
     systemd_daemon_reload().await
 }
 
+/// The limits drop-in hive-priv's `write_resource_limits` writes for
+/// `container`. Must match that path: a mismatch reads as "no drop-in
+/// yet" in [`limits_to_apply`].
+fn limits_dropin_path(container: &str) -> PathBuf {
+    PathBuf::from(format!(
+        "/run/systemd/system/container@{container}.service.d/hyperhive-limits.conf"
+    ))
+}
+
+/// The `(CPUQuota, MemoryMax)` to write into `agent_name`'s limits
+/// drop-in, or `None` to leave the drop-in at `dropin_path` as it is.
+///
+/// When the overrides file at `limits_path` can't be read, the error is
+/// logged and the agent still starts. The last-applied drop-in is kept,
+/// because the hive defaults can be looser than the agent's own override.
+/// With no drop-in yet, the agent gets the hive defaults: without one it
+/// would run uncapped.
+fn limits_to_apply(
+    agent_name: &str,
+    limits_path: &Path,
+    dropin_path: &Path,
+    hive: &HiveEnv,
+) -> Option<(String, String)> {
+    let hive_cpu = &hive.agent_cpu_quota;
+    let hive_mem = &hive.agent_memory_max;
+    let e = match crate::resource_limits::effective(limits_path, agent_name, hive_cpu, hive_mem) {
+        Ok(limits) => return Some(limits),
+        Err(e) => e,
+    };
+    if dropin_path.exists() {
+        tracing::error!(
+            agent = %agent_name,
+            path = %limits_path.display(),
+            error = ?e,
+            "resource limits unreadable — keeping the last-applied limits drop-in"
+        );
+        None
+    } else {
+        tracing::error!(
+            agent = %agent_name,
+            path = %limits_path.display(),
+            error = ?e,
+            "resource limits unreadable and no limits drop-in yet — applying hive defaults"
+        );
+        Some((hive_cpu.clone(), hive_mem.clone()))
+    }
+}
+
 /// Write a systemd drop-in for `container@<container>.service` that applies
 /// the agent's effective resource caps — its per-agent overrides from
 /// `meta/resource-limits.json` where set, the hive-wide defaults
 /// otherwise. Goes under `/run/systemd/system/...` so it's ephemeral
-/// (regenerated on every spawn / rebuild).
+/// (regenerated on every spawn / rebuild, except as [`limits_to_apply`] says).
 ///
 /// The weights are hive-wide (`services.hyperhive.agentCpuWeight` /
 /// `agentIoWeight`) — unlike the caps they have no per-agent override in
@@ -426,8 +479,9 @@ async fn set_nspawn_flags(
 mod tests {
     use super::{
         BindMount, QUEUE_CLIENT_ID_CREDENTIAL, QUEUE_SECRET_CREDENTIAL, bind_child_agent_dirs,
-        holds_manage_root_agent, queue_agent_credentials,
+        holds_manage_root_agent, limits_to_apply, queue_agent_credentials,
     };
+    use crate::coordinator::HiveEnv;
 
     fn child_binds() -> Vec<BindMount> {
         let mut binds = Vec::new();
@@ -561,4 +615,48 @@ mod tests {
         assert!(holds_manage_root_agent("ruth", &path));
         assert!(!holds_manage_root_agent("alice", &path));
     }
+
+    const TRUNCATED_LIMITS: &str = "{\n  \"sock\": { \"cpu_quota\": \"50%\", \"mem";
+
+    #[test]
+    fn corrupt_limits_keep_an_existing_dropin() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let limits = dir.path().join("resource-limits.json");
+        let dropin = dir.path().join("hyperhive-limits.conf");
+        std::fs::write(&limits, TRUNCATED_LIMITS).expect("seed limits");
+        std::fs::write(&dropin, "MemoryMax=1G\n").expect("seed drop-in");
+        assert_eq!(
+            limits_to_apply("sock", &limits, &dropin, &HiveEnv::default()),
+            None
+        );
+        assert_eq!(
+            std::fs::read(&limits).expect("read"),
+            TRUNCATED_LIMITS.as_bytes()
+        );
+    }
+
+    #[test]
+    fn corrupt_limits_without_a_dropin_apply_hive_defaults() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let limits = dir.path().join("resource-limits.json");
+        std::fs::write(&limits, TRUNCATED_LIMITS).expect("seed limits");
+        let applied = limits_to_apply(
+            "sock",
+            &limits,
+            &dir.path().join("absent.conf"),
+            &HiveEnv::default(),
+        );
+        assert_eq!(applied, Some(("200%".to_owned(), "4G".to_owned())));
+    }
+
+    #[test]
+    fn readable_limits_are_applied_over_an_existing_dropin() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let limits = dir.path().join("resource-limits.json");
+        let dropin = dir.path().join("hyperhive-limits.conf");
+        std::fs::write(&limits, r#"{"sock": {"memory_max": "1G"}}"#).expect("seed limits");
+        std::fs::write(&dropin, "MemoryMax=8G\n").expect("seed drop-in");
+        let applied = limits_to_apply("sock", &limits, &dropin, &HiveEnv::default());
+        assert_eq!(applied, Some(("200%".to_owned(), "1G".to_owned())));
+    }
 }

hive-c0re/src/meta.rs

@@ -1318,10 +1318,12 @@ where
     );
     // Propagated, never read as empty: an agent with no tool-groups entry
     // renders `toolGroups = null` and gets `AGENT_DEFAULT`, which fails
-    // open for any agent whose explicit entry is narrower.
+    // open for any agent whose explicit entry is narrower. Likewise an
+    // agent with no resource-limits entry gets the hive `memoryMaxBytes`,
+    // a larger heap ceiling than a tighter override.
     let tool_groups_map = crate::tool_groups::read()?;
     let capabilities_map = crate::capabilities::read()?;
-    let resource_limits_map = crate::resource_limits::read();
+    let resource_limits_map = crate::resource_limits::read()?;
     for spec in agents {
         // Emit `toolGroups = "group1,group2"` when the operator has
         // explicitly configured groups for this agent. Absent entry = null

hive-c0re/src/server.rs

@@ -506,10 +506,11 @@ async fn handle_set_resource_limits(
     crate::lifecycle::write_dropins(name.as_str(), &hive, &paths).await?;
 
     let (cpu, mem) = crate::resource_limits::effective(
+        &crate::resource_limits::resource_limits_path(),
         name.as_str(),
         &hive.agent_cpu_quota,
         &hive.agent_memory_max,
-    );
+    )?;
     Ok(HostResponse::messages(vec![format!(
         "{name}: CPUQuota={cpu} MemoryMax={mem} — the cgroup cap itself is live now (restart \
          the container if it's running and needs the new cap immediately), but the derived \