diff --git a/hive-c0re/src/agent_config/capabilities.rs b/hive-c0re/src/agent_config/capabilities.rs
index 9e077798..90e9f57b 100644
--- a/hive-c0re/src/agent_config/capabilities.rs
+++ b/hive-c0re/src/agent_config/capabilities.rs
@@ -22,7 +22,7 @@
 
 use hive_sh4re::permissions::Capability;
 use std::collections::BTreeMap;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 const CAPABILITIES_FILE: &str = "capabilities.json";
 
@@ -61,50 +61,35 @@ fn prune_unknown(map: &mut BTreeMap<String, Vec<String>>) -> bool {
     dropped_any
 }
 
-/// Read the per-agent capability map. Returns an empty map when the
-/// file is absent or unparsable — callers treat a missing entry as
-/// "no extra capabilities". Any entry that isn't a recognised
-/// [`Capability`] is dropped (with a `warn!`) from what's returned —
-/// a stale or typo'd name is never honoured — but `read` itself never
-/// writes; the on-disk file only gets repaired the next time something
-/// calls `write`/`set_caps` anyway.
-#[must_use]
-pub fn read() -> BTreeMap<String, Vec<String>> {
-    let path = capabilities_path();
-    let Ok(raw) = std::fs::read_to_string(&path) else {
-        return BTreeMap::new();
-    };
-    let mut map: BTreeMap<String, Vec<String>> = serde_json::from_str(&raw).unwrap_or_default();
+/// Read the per-agent capability map. An absent file is the empty map —
+/// callers treat a missing entry as "no extra capabilities". A file
+/// that exists but can't be read or parsed is an error. Any entry that
+/// isn't a recognised [`Capability`] is dropped (with a `warn!`) from
+/// what's returned — a stale or typo'd name is never honoured — but
+/// `read` itself never writes; the on-disk file only gets repaired the
+/// next time something calls `set_caps`/`remove_agent` anyway.
+pub fn read() -> std::io::Result<BTreeMap<String, Vec<String>>> {
+    read_from(&capabilities_path())
+}
+
+fn read_from(path: &Path) -> std::io::Result<BTreeMap<String, Vec<String>>> {
+    let mut map = super::read_map(path)?;
     prune_unknown(&mut map);
-    map
+    Ok(map)
 }
 
 /// Look up the configured capabilities for one agent. Returns an empty
-/// vec when the agent has no entry.
-#[must_use]
-pub fn caps_for(name: &str) -> Vec<String> {
-    read().get(name).cloned().unwrap_or_default()
+/// vec when the agent has no entry. Errors as [`read`] does.
+pub fn caps_for(name: &str) -> std::io::Result<Vec<String>> {
+    Ok(read()?.get(name).cloned().unwrap_or_default())
 }
 
-/// Check whether an agent holds a specific capability.
-#[must_use]
-pub fn has_cap(name: &str, cap: hive_sh4re::permissions::Capability) -> bool {
-    caps_for(name)
+/// Check whether an agent holds a specific capability. Errors as
+/// [`read`] does.
+pub fn has_cap(name: &str, cap: hive_sh4re::permissions::Capability) -> std::io::Result<bool> {
+    Ok(caps_for(name)?
         .iter()
-        .any(|s| s.eq_ignore_ascii_case(<&str>::from(cap)))
-}
-
-/// Persist the full capability map. Sorted JSON output keeps diffs
-/// minimal. Best-effort — returns `io::Error` so callers decide
-/// whether to abort or log.
-pub fn write(map: &BTreeMap<String, Vec<String>>) -> std::io::Result<()> {
-    let path = capabilities_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"))
+        .any(|s| s.eq_ignore_ascii_case(<&str>::from(cap))))
 }
 
 /// Filter `caps` down to recognised [`Capability`] names (warning per
@@ -137,20 +122,30 @@ fn apply_known_caps(current: &mut BTreeMap<String, Vec<String>>, name: &str, cap
 /// rather than written — an unknown grant should never look like it
 /// took effect. An empty `caps` vec, or one that becomes empty after
 /// dropping unknown names, removes the entry (agent has no
-/// capabilities).
+/// capabilities). Fails without writing if the existing file can't be
+/// read or parsed.
 pub fn set_caps(name: &str, caps: &[String]) -> std::io::Result<()> {
-    let mut current = read();
+    set_caps_at(&capabilities_path(), name, caps)
+}
+
+fn set_caps_at(path: &Path, name: &str, caps: &[String]) -> std::io::Result<()> {
+    let mut current = read_from(path)?;
     apply_known_caps(&mut current, name, caps);
-    write(&current)
+    super::write_map(path, &current)
 }
 
 /// Remove an agent from the capability map entirely. Called by
 /// `meta::sync_agents` when an agent is deprovisioned so stale entries
-/// don't accumulate. No-op if the agent has no entry.
+/// don't accumulate. No-op if the agent has no entry. Fails without
+/// writing if the existing file can't be read or parsed.
 pub fn remove_agent(name: &str) -> std::io::Result<()> {
-    let mut current = read();
+    remove_agent_at(&capabilities_path(), name)
+}
+
+fn remove_agent_at(path: &Path, name: &str) -> std::io::Result<()> {
+    let mut current = read_from(path)?;
     if current.remove(name).is_some() {
-        write(&current)?;
+        super::write_map(path, &current)?;
     }
     Ok(())
 }
@@ -159,11 +154,51 @@ pub fn remove_agent(name: &str) -> std::io::Result<()> {
 mod tests {
     use super::*;
 
-    // `read`/`write`/`set_caps` shell out to `crate::paths::meta_root`,
+    // `read`/`set_caps`/`remove_agent` shell out to `crate::paths::meta_root`,
     // which is hardcoded to `/var/lib/hyperhive` (no test override) — so
     // these tests pin the pure decision logic (`prune_unknown`,
-    // `apply_known_caps`) directly against in-memory maps rather than
-    // round-tripping through the real capabilities file.
+    // `apply_known_caps`) against in-memory maps, and the file handling
+    // through the path-taking `*_at` / `read_from` variants.
+
+    const TRUNCATED: &str = "{\n  \"atlas\": [\"manage_root_ag";
+
+    fn corrupt_file() -> (tempfile::TempDir, std::path::PathBuf) {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(CAPABILITIES_FILE);
+        std::fs::write(&path, TRUNCATED).expect("seed");
+        (dir, path)
+    }
+
+    #[test]
+    fn read_of_a_corrupt_file_is_an_error() {
+        let (_dir, path) = corrupt_file();
+        read_from(&path).expect_err("truncated JSON must not read as a map");
+    }
+
+    #[test]
+    fn set_caps_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        set_caps_at(&path, "ruth", &["manage_root_agent".to_owned()])
+            .expect_err("a corrupt file must not be overwritten");
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn remove_agent_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        remove_agent_at(&path, "atlas").expect_err("a corrupt file must not be overwritten");
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn set_caps_round_trips_through_a_missing_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(CAPABILITIES_FILE);
+        assert!(read_from(&path).expect("missing file is empty").is_empty());
+        set_caps_at(&path, "ruth", &["manage_root_agent".to_owned()]).expect("set");
+        let map = read_from(&path).expect("read");
+        assert_eq!(map["ruth"], vec!["manage_root_agent".to_owned()]);
+    }
 
     #[test]
     fn prune_unknown_keeps_known_name() {
diff --git a/hive-c0re/src/agent_config/mod.rs b/hive-c0re/src/agent_config/mod.rs
index 8fc2807b..249077bd 100644
--- a/hive-c0re/src/agent_config/mod.rs
+++ b/hive-c0re/src/agent_config/mod.rs
@@ -13,3 +13,91 @@ pub mod limits;
 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>>> {
+    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) => {
+            return Err(io::Error::new(
+                e.kind(),
+                format!("read {}: {e}", path.display()),
+            ));
+        }
+    };
+    serde_json::from_str(&raw).map_err(|e| {
+        io::Error::new(
+            io::ErrorKind::InvalidData,
+            format!("parse {}: {e}", path.display()),
+        )
+    })
+}
+
+/// Serialise `map` 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)
+        .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(
+            io::ErrorKind::InvalidInput,
+            format!("{} has no parent directory or file name", path.display()),
+        ));
+    };
+    std::fs::create_dir_all(dir)?;
+    let tmp = dir.join(format!(".{}.tmp", name.to_string_lossy()));
+    let mut file = std::fs::File::create(&tmp)?;
+    file.write_all(format!("{text}\n").as_bytes())?;
+    file.sync_all()?;
+    drop(file);
+    std::fs::rename(&tmp, path)?;
+    std::fs::File::open(dir)?.sync_all()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn write_map_round_trips_and_leaves_no_temp_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("map.json");
+        let map = BTreeMap::from([("alice".to_owned(), vec!["messaging".to_owned()])]);
+        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);
+        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("map.json")]);
+    }
+
+    #[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");
+        assert!(map.is_empty());
+    }
+
+    #[test]
+    fn read_map_of_a_truncated_file_is_an_error() {
+        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");
+        assert_eq!(err.kind(), io::ErrorKind::InvalidData);
+    }
+}
diff --git a/hive-c0re/src/agent_config/tool_groups.rs b/hive-c0re/src/agent_config/tool_groups.rs
index 8c93c64b..04d5b9ec 100644
--- a/hive-c0re/src/agent_config/tool_groups.rs
+++ b/hive-c0re/src/agent_config/tool_groups.rs
@@ -22,7 +22,7 @@
 //! that the operator uses to grant/revoke tool groups per agent.
 
 use std::collections::BTreeMap;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use anyhow::Context as _;
 
@@ -33,37 +33,18 @@ pub fn tool_groups_path() -> PathBuf {
     crate::paths::meta_root().join(TOOL_GROUPS_FILE)
 }
 
-/// Read the per-agent tool-group map. Returns an empty map when the
-/// file is absent or unparsable — callers treat a missing entry as
-/// "use role default".
-#[must_use]
-pub fn read() -> BTreeMap<String, Vec<String>> {
-    let path = tool_groups_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 tool-group map. An absent file is the empty map —
+/// callers treat a missing entry as "use role default". A file that
+/// exists but can't be read or parsed is an error.
+pub fn read() -> std::io::Result<BTreeMap<String, Vec<String>>> {
+    super::read_map(&tool_groups_path())
 }
 
 /// Look up the configured tool groups for one agent. Returns an empty
 /// vec when the agent has no entry — callers should treat this as
-/// "use the harness role default."
-#[must_use]
-pub fn groups_for(name: &str) -> Vec<String> {
-    read().get(name).cloned().unwrap_or_default()
-}
-
-/// Persist the full tool-groups map. Sorted JSON output keeps diffs
-/// minimal. Use `set_groups` (or `remove_agent`) from outside this
-/// module — they go through the validated write path.
-fn write(map: &BTreeMap<String, Vec<String>>) -> std::io::Result<()> {
-    let path = tool_groups_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"))
+/// "use the harness role default." Errors as [`read`] does.
+pub fn groups_for(name: &str) -> std::io::Result<Vec<String>> {
+    Ok(read()?.get(name).cloned().unwrap_or_default())
 }
 
 /// Validate a slice of group name strings against `ToolGroup::ALL`.
@@ -96,25 +77,76 @@ pub fn validate_groups(groups: &[String]) -> anyhow::Result<()> {
 
 /// Set the tool groups for one agent and persist the map. An empty
 /// `groups` vec removes the entry (agent reverts to role default).
-/// Returns an error if any name is not in `ToolGroup::ALL`.
+/// Returns an error if any name is not in `ToolGroup::ALL`, or if the
+/// existing file can't be read or parsed — the file is then left
+/// untouched.
 pub fn set_groups(name: &str, groups: &[String]) -> anyhow::Result<()> {
+    set_groups_at(&tool_groups_path(), name, groups)
+}
+
+fn set_groups_at(path: &Path, name: &str, groups: &[String]) -> anyhow::Result<()> {
     if !groups.is_empty() {
         validate_groups(groups)?;
     }
-    let mut current = read();
+    let mut current = super::read_map(path)?;
     if groups.is_empty() {
         current.remove(name);
     } else {
         current.insert(name.to_owned(), groups.to_vec());
     }
-    write(&current).with_context(|| format!("write tool-groups for {name}"))
+    super::write_map(path, &current).with_context(|| format!("write tool-groups for {name}"))
 }
 
 /// Drop the entry for an agent that is being destroyed. Idempotent.
+/// Fails without writing if the existing file can't be read or parsed.
 pub fn remove_agent(name: &str) -> std::io::Result<()> {
-    let mut current = read();
+    remove_agent_at(&tool_groups_path(), name)
+}
+
+fn remove_agent_at(path: &Path, name: &str) -> std::io::Result<()> {
+    let mut current = super::read_map(path)?;
     if current.remove(name).is_some() {
-        write(&current)?;
+        super::write_map(path, &current)?;
     }
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TRUNCATED: &str = "{\n  \"alice\": [\"messaging\", \"meta\"],\n  \"bob\": [\"mess";
+
+    fn corrupt_file() -> (tempfile::TempDir, PathBuf) {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join(TOOL_GROUPS_FILE);
+        std::fs::write(&path, TRUNCATED).expect("seed");
+        (dir, path)
+    }
+
+    #[test]
+    fn set_groups_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        set_groups_at(&path, "ruth", &["messaging".to_owned()])
+            .expect_err("a corrupt file must not be overwritten");
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn remove_agent_leaves_a_corrupt_file_untouched() {
+        let (_dir, path) = corrupt_file();
+        remove_agent_at(&path, "alice").expect_err("a corrupt file must not be overwritten");
+        assert_eq!(std::fs::read(&path).expect("read"), TRUNCATED.as_bytes());
+    }
+
+    #[test]
+    fn set_groups_keeps_other_agents_entries() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        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");
+        assert_eq!(map["alice"], vec!["inbox".to_owned()]);
+        assert_eq!(map["ruth"], vec!["messaging".to_owned()]);
+    }
+}
diff --git a/hive-c0re/src/coordinator.rs b/hive-c0re/src/coordinator.rs
index 35da2088..06c48abe 100644
--- a/hive-c0re/src/coordinator.rs
+++ b/hive-c0re/src/coordinator.rs
@@ -643,7 +643,15 @@ impl Coordinator {
             .iter()
             .map(|c| (<&str>::from(*c), c.description()))
             .collect();
-        let assignments = crate::capabilities::read();
+        // An empty snapshot would show every agent as having lost its
+        // capabilities; emit nothing and let the HTTP refetch report it.
+        let assignments = match crate::capabilities::read() {
+            Ok(map) => map,
+            Err(e) => {
+                tracing::warn!(error = ?e, "capabilities snapshot skipped");
+                return;
+            }
+        };
         // Best-effort roster (sync path); on a contended cache miss we
         // emit explicit keys only — the HTTP refetch fills the rest in.
         let roster = self.live_container_names_blocking().unwrap_or_default();
@@ -670,7 +678,13 @@ impl Coordinator {
             .iter()
             .map(|g| (<&str>::from(*g), g.description()))
             .collect();
-        let assignments = crate::tool_groups::read();
+        let assignments = match crate::tool_groups::read() {
+            Ok(map) => map,
+            Err(e) => {
+                tracing::warn!(error = ?e, "tool-groups snapshot skipped");
+                return;
+            }
+        };
         let roster = self.live_container_names_blocking().unwrap_or_default();
         let (agents, effective) = crate::dashboard::permissions::roster_and_effective(
             roster,
diff --git a/hive-c0re/src/dashboard/permissions.rs b/hive-c0re/src/dashboard/permissions.rs
index 121324c8..0ec25b05 100644
--- a/hive-c0re/src/dashboard/permissions.rs
+++ b/hive-c0re/src/dashboard/permissions.rs
@@ -42,12 +42,15 @@ pub(super) struct ToolGroupsSnapshot {
 #[utoipa::path(
     get,
     path = "/api/tool-groups",
-    responses((status = 200, description = "tool-group catalogue + assignments", body = ToolGroupsSnapshot)),
+    responses(
+        (status = 200, description = "tool-group catalogue + assignments", body = ToolGroupsSnapshot),
+        (status = 500, description = "tool-groups file unreadable"),
+    ),
     tag = "permissions"
 )]
 pub(super) async fn get_tool_groups(
     State(state): State<AppState>,
-) -> axum::Json<ToolGroupsSnapshot> {
+) -> Result<axum::Json<ToolGroupsSnapshot>, ProblemDetails> {
     let groups = hive_sh4re::permissions::ToolGroup::ALL
         .iter()
         .map(|g| <&str>::from(*g))
@@ -56,7 +59,7 @@ pub(super) async fn get_tool_groups(
         .iter()
         .map(|g| (<&str>::from(*g), g.description()))
         .collect();
-    let assignments = crate::tool_groups::read();
+    let assignments = crate::tool_groups::read().map_err(|e| unreadable(&e))?;
     let roster = state
         .coord
         .containers_snapshot()
@@ -65,13 +68,20 @@ pub(super) async fn get_tool_groups(
         .map(|c| c.name);
     let (agents, effective) =
         roster_and_effective(roster, &assignments, &tool_group_default_names());
-    axum::Json(ToolGroupsSnapshot {
+    Ok(axum::Json(ToolGroupsSnapshot {
         groups,
         descriptions,
         assignments,
         agents,
         effective,
-    })
+    }))
+}
+
+/// A permission file that exists but can't be read or parsed. Surfaced
+/// as a 500 rather than an empty table, which would read as "nobody has
+/// any grants".
+fn unreadable(e: &std::io::Error) -> ProblemDetails {
+    ProblemDetails::from_status_code(StatusCode::INTERNAL_SERVER_ERROR).with_detail(e.to_string())
 }
 
 /// The role-default tool-group names the harness falls back to for an
@@ -198,19 +208,22 @@ pub(super) struct CapabilitiesSnapshot {
 #[utoipa::path(
     get,
     path = "/api/capabilities",
-    responses((status = 200, description = "capability catalogue + assignments", body = CapabilitiesSnapshot)),
+    responses(
+        (status = 200, description = "capability catalogue + assignments", body = CapabilitiesSnapshot),
+        (status = 500, description = "capabilities file unreadable"),
+    ),
     tag = "permissions"
 )]
 pub(super) async fn get_capabilities(
     State(state): State<AppState>,
-) -> axum::Json<CapabilitiesSnapshot> {
+) -> Result<axum::Json<CapabilitiesSnapshot>, ProblemDetails> {
     use hive_sh4re::permissions::Capability;
     let caps = Capability::ALL.iter().map(|c| <&str>::from(*c)).collect();
     let descriptions = Capability::ALL
         .iter()
         .map(|c| (<&str>::from(*c), c.description()))
         .collect();
-    let assignments = crate::capabilities::read();
+    let assignments = crate::capabilities::read().map_err(|e| unreadable(&e))?;
     let roster = state
         .coord
         .containers_snapshot()
@@ -219,13 +232,13 @@ pub(super) async fn get_capabilities(
         .map(|c| c.name);
     // Capability default is "no extra caps" — empty default slice.
     let (agents, effective) = roster_and_effective(roster, &assignments, &[]);
-    axum::Json(CapabilitiesSnapshot {
+    Ok(axum::Json(CapabilitiesSnapshot {
         caps,
         descriptions,
         assignments,
         agents,
         effective,
-    })
+    }))
 }
 
 #[derive(Deserialize, ToSchema)]
@@ -406,12 +419,15 @@ pub(super) struct StalePermsResponse {
 #[utoipa::path(
     get,
     path = "/api/permissions/stale",
-    responses((status = 200, description = "ghost agent names with stale permission entries", body = StalePermsResponse)),
+    responses(
+        (status = 200, description = "ghost agent names with stale permission entries", body = StalePermsResponse),
+        (status = 500, description = "tool-groups/capabilities file unreadable"),
+    ),
     tag = "permissions"
 )]
 pub(super) async fn get_stale_permissions(
     State(state): State<AppState>,
-) -> axum::Json<StalePermsResponse> {
+) -> Result<axum::Json<StalePermsResponse>, ProblemDetails> {
     // Live container names — includes stopped-but-configured containers.
     let live: std::collections::HashSet<String> = state
         .coord
@@ -432,8 +448,8 @@ pub(super) async fn get_stale_permissions(
     // Known = live roster ∪ kept-state names.
     let known: std::collections::HashSet<&String> = live.iter().chain(kept.iter()).collect();
     // Explicit entries in either JSON file.
-    let caps = crate::capabilities::read();
-    let tgs = crate::tool_groups::read();
+    let caps = crate::capabilities::read().map_err(|e| unreadable(&e))?;
+    let tgs = crate::tool_groups::read().map_err(|e| unreadable(&e))?;
     let mut ghost_names: Vec<String> = caps
         .keys()
         .chain(tgs.keys())
@@ -443,7 +459,7 @@ pub(super) async fn get_stale_permissions(
         .into_iter()
         .collect();
     ghost_names.sort();
-    axum::Json(StalePermsResponse { stale: ghost_names })
+    Ok(axum::Json(StalePermsResponse { stale: ghost_names }))
 }
 
 /// Clear all explicit permission entries for a named agent without
diff --git a/hive-c0re/src/lifecycle/host_config.rs b/hive-c0re/src/lifecycle/host_config.rs
index 4bf1da77..e1bb5bc4 100644
--- a/hive-c0re/src/lifecycle/host_config.rs
+++ b/hive-c0re/src/lifecycle/host_config.rs
@@ -339,7 +339,7 @@ async fn set_nspawn_flags(
     // parent field took with it the unconditional grant every
     // agent used to get over its own direct children — so an agent with
     // no capability now sees its own dirs and nothing else.
-    if crate::capabilities::has_cap(agent_name, Capability::ManageRootAgent) {
+    if crate::capabilities::has_cap(agent_name, Capability::ManageRootAgent)? {
         // Skipping self is a no-op, not a narrowing: `agent_notes_dir` is
         // `agent_state_dir/state` and `config_bind_source` is shared, so
         // binding the holder as its own virtual child reproduced the two
diff --git a/hive-c0re/src/meta.rs b/hive-c0re/src/meta.rs
index e510ab40..42fce888 100644
--- a/hive-c0re/src/meta.rs
+++ b/hive-c0re/src/meta.rs
@@ -102,7 +102,7 @@ pub async fn sync_agents(hive: &HiveEnv, agents: &[AgentSpec]) -> Result<()> {
         &hive.context_window_tokens,
         &hive.agent_memory_max,
         agents,
-    );
+    )?;
     let flake_path = dir.join("flake.nix");
     let on_disk = std::fs::read_to_string(&flake_path).unwrap_or_default();
     let initial = !dir.join(".git").exists();
@@ -585,7 +585,7 @@ fn render_flake(
     context_window_tokens: &std::collections::HashMap<String, u64>,
     hive_memory_max: &str,
     agents: &[AgentSpec],
-) -> String {
+) -> Result<String> {
     render_flake_with_lookup(
         hyperhive_flake,
         docs_flake,
@@ -1000,7 +1000,7 @@ fn render_flake_with_lookup<F>(
     hive_memory_max: &str,
     agents: &[AgentSpec],
     lookup: F,
-) -> String
+) -> Result<String>
 where
     F: Fn(&str) -> Vec<&'static str>,
 {
@@ -1316,8 +1316,8 @@ where
       nixosConfigurations = {
 "#,
     );
-    let tool_groups_map = crate::tool_groups::read();
-    let capabilities_map = crate::capabilities::read();
+    let tool_groups_map = crate::tool_groups::read()?;
+    let capabilities_map = crate::capabilities::read()?;
     let resource_limits_map = crate::resource_limits::read();
     for spec in agents {
         // Emit `toolGroups = "group1,group2"` when the operator has
@@ -1370,7 +1370,7 @@ where
         );
     }
     out.push_str("      };\n    };\n}\n");
-    out
+    Ok(out)
 }
 
 /// Return the list of file names that are currently staged (index differs
@@ -1671,7 +1671,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         // nixpkgs is a top-level input with an explicit URL; hyperhive
         // follows it.
         assert!(
@@ -1718,7 +1719,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         assert!(
             !out.contains("hyperhive-docs"),
             "no docs input/source when docs_flake is empty:\n{out}"
@@ -1737,7 +1739,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         assert!(
             out.contains(r#"networking.hostName = "h-${name}";"#),
             "an agent container with no hostname of its own is called `nixos`, \
@@ -1778,7 +1781,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         assert!(
             out.contains(
                 "services.hyperhive.agent.claudeCodePath = \"/nix/store/cccc-claude-code-2.1.220\";"
@@ -1807,7 +1811,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         assert!(
             !out.contains("claudeCodePath"),
             "no claude assignment when unpinned:\n{out}"
@@ -1828,7 +1833,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         assert!(
             out.contains("nixpkgs.follows = \"hyperhive/nixpkgs\""),
             "expected fallback follows:\n{out}"
@@ -1864,7 +1870,8 @@ mod tests {
                 sample_spec("dmatrix", false, 9003),
             ],
             lookup,
-        );
+        )
+        .expect("render flake");
         // bitburner declares nixpkgs → follows emitted.
         assert!(
             out.contains("agent-bitburner.inputs.nixpkgs.follows = \"nixpkgs\""),
@@ -1894,7 +1901,8 @@ mod tests {
             "4G",
             &[sample_spec("alice", false, 9001)],
             |_| Vec::new(),
-        );
+        )
+        .expect("render flake");
         // No agent-side follows when the lookup reports nothing
         // declared — protects agents whose flake.lock can't be read
         // (missing / unparsable) from being broken by a follows on a
@@ -1932,7 +1940,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         unsafe {
             std::env::remove_var("HIVE_FORGE_URL");
         }
@@ -2110,7 +2119,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         unsafe {
             std::env::remove_var("HIVE_FORGE_URL");
             std::env::remove_var("HIVE_MATRIX_URL");
@@ -2147,7 +2157,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         let want = format!(
             "agent-alice.url = \"git+file://{}\"",
             crate::paths::applied_dir("alice").display()
@@ -2193,6 +2204,7 @@ mod tests {
                 "4G",
                 &[sample_spec("alice", false, 9001)],
             )
+            .expect("render flake")
         };
 
         // A leftover peer-CA file + the env var that used to name it. Both
@@ -2286,6 +2298,7 @@ mod tests {
                 "4G",
                 &[sample_spec("alice", false, 9001)],
             )
+            .expect("render flake")
         };
         unsafe {
             std::env::remove_var("HYPERHIVE_OTEL_EXTRA_RESOURCE_ATTRIBUTES");
@@ -2361,6 +2374,7 @@ mod tests {
                 "4G",
                 &[sample_spec("alice", false, 9001)],
             )
+            .expect("render flake")
         };
         unsafe {
             std::env::remove_var("HYPERHIVE_GITHUB_DISABLED");
@@ -2403,7 +2417,8 @@ mod tests {
             &std::collections::HashMap::new(),
             "4G",
             &[sample_spec("alice", false, 9001)],
-        );
+        )
+        .expect("render flake");
         let want_bytes = 4u64 * 1024 * 1024 * 1024;
         assert!(
             out.contains(&format!("memoryMaxBytes = {want_bytes};")),
diff --git a/hive-c0re/src/socket_server/mod.rs b/hive-c0re/src/socket_server/mod.rs
index 9b6c84b4..3777cc3e 100644
--- a/hive-c0re/src/socket_server/mod.rs
+++ b/hive-c0re/src/socket_server/mod.rs
@@ -598,15 +598,14 @@ async fn dispatch_orchestration(req: &Request, agent: &str, coord: &Arc<Coordina
 /// these verbs without any positional/hardcoded privilege. `action` is the
 /// verb phrase for the message.
 fn require_group(agent: &str, group: &str, action: &str) -> Option<Response> {
-    if crate::tool_groups::groups_for(agent)
-        .iter()
-        .any(|g| g == group)
-    {
-        None
-    } else {
-        Some(Response::Err {
+    match crate::tool_groups::groups_for(agent) {
+        Ok(groups) if groups.iter().any(|g| g == group) => None,
+        Ok(_) => Some(Response::Err {
             message: format!("agent `{agent}` cannot {action}: requires the `{group}` tool group"),
-        })
+        }),
+        Err(e) => Some(Response::Err {
+            message: format!("agent `{agent}` cannot {action}: {e}"),
+        }),
     }
 }
 
@@ -699,6 +698,7 @@ fn handle_cancel_loose_end(
 fn check_can_cancel_approval(canceller: &str) -> Result<(), String> {
     const APPROVALS_GROUP: &str = "approvals";
     if crate::tool_groups::groups_for(canceller)
+        .map_err(|e| format!("cancel_loose_end: {e}"))?
         .iter()
         .any(|g| g == APPROVALS_GROUP)
     {
diff --git a/hive-c0re/src/workers/auto_update.rs b/hive-c0re/src/workers/auto_update.rs
index ee5aa1dd..d2899ebc 100644
--- a/hive-c0re/src/workers/auto_update.rs
+++ b/hive-c0re/src/workers/auto_update.rs
@@ -212,11 +212,23 @@ pub async fn ensure_root_agent(coord: &Arc<Coordinator>) -> Result<()> {
 ///
 /// Skips a name that already has an entry: a destroy+recreate under the
 /// same name must not silently reset an operator's chosen group set back
-/// to the default.
+/// to the default. An unreadable file is logged and left alone: ruth has
+/// already been spawned, and seeding stays best-effort like the write
+/// below.
 fn seed_manager_tool_groups() {
-    if !tool_groups::groups_for(MANAGER_NAME).is_empty() {
-        tracing::debug!("manager tool groups already set — leaving as-is");
-        return;
+    match tool_groups::groups_for(MANAGER_NAME) {
+        Ok(groups) if groups.is_empty() => {}
+        Ok(_) => {
+            tracing::debug!("manager tool groups already set — leaving as-is");
+            return;
+        }
+        Err(e) => {
+            tracing::warn!(
+                error = ?e,
+                "tool-groups file unreadable — not seeding ruth's tool groups"
+            );
+            return;
+        }
     }
     let all_groups: Vec<String> = hive_sh4re::permissions::ToolGroup::MANAGER_DEFAULT
         .iter()
@@ -263,7 +275,7 @@ fn should_seed_manager_caps(store_written: bool) -> bool {
 /// entry that has been emptied rather than tombstoning it, so "the manager
 /// has no entry" cannot tell a fresh hive apart from a deliberate revoke.
 /// File existence can: every grant and revoke goes through
-/// `meta::commit_capabilities` → `capabilities::set_caps` → `write`, which
+/// `meta::commit_capabilities` → `capabilities::set_caps`, which
 /// writes the file even when the result is an empty `{}`. So while the file
 /// is absent nobody has ever had a say, and once it exists this is inert
 /// forever — including on the destroy+recreate path, matching