subagents: fold hive-extra-mcp into hive-agent-sock per mara's rescoping

CLAUDE.md

@@ -123,15 +123,18 @@ hand-maintained per-file tree drifts out of sync with the code.
   + manager socket (`/run/hive/mcp.sock`), the protocol an agent's harness
   speaks to `hive-c0re`. Same split rationale as the two above; the shared
   payload types it references stay in `hive-sh4re`.
-- **`hive-agent-sock/`** — wire types for the *in-agent* socket, served by
+- **`hive-agent-sock/`** — what's common across the in-container "agent
-  the harness to in-container producers — matrix / bash MCP daemons and
+  plugins" (matrix/bash/subagent MCP daemons, `forge_notify`, any
-  `forge_notify` are the built-in ones, but any user-configured MCP server
+  user-configured MCP server). Wire types for the *in-agent* socket, served
-  can push todos here too, nothing restricts the `subsystem` set. Carries
+  by the harness to those producers — nothing restricts the `subsystem` set
-  the loose-ends-v2 **todo** ops plus
+  on the todo ops, any producer can push its own. Carries the loose-ends-v2
-  harness-local reminders. ⚠️ Distinct from
+  **todo** ops plus harness-local reminders. ⚠️ Distinct from
   `hive-core-agent-sock` above: **this socket never leaves the container**
   and `hive-c0re` is not in the path at all — no broker round-trip, no
-  long-poll, no marker files.
+  long-poll, no marker files. Also carries `extra_mcp`: the
+  `hyperhive.extraMcpServers` spec + parsing, shared by `hive-agent` and
+  `hive-subagent-mcp` — not a socket protocol, just the natural home since
+  both already depend on this crate.
 - **`hive-types/`** — zero-dependency (bar serde) leaf crate holding the
   foundational newtypes, chiefly `Ident` (1–63 chars of `[a-z0-9-]`,
   constructed only via the validating parser). Lets every wire-type crate

Cargo.lock

@@ -1705,6 +1705,8 @@ version = "0.1.0"
 dependencies = [
  "hive-sh4re",
  "serde",
+ "serde_json",
+ "tracing",
 ]
 
 [[package]]

docs/tools/subagent.md

@@ -54,3 +54,21 @@ agent that needs a stable or non-default port.
 
 Own systemd unit, defined alongside the other per-agent MCP daemons in
 `nix/agent-modules/mcp.nix`.
+
+## MCP servers available to a subagent
+
+A subagent runs with `--strict-mcp-config` and no `--mcp-config` by
+default — zero MCP servers, full stop; it falls back to claude's own
+native tools (`Bash`, `WebFetch`, etc.), not the parent's `mcp__bash__*` /
+`mcp__hyperhive__*` surface. Nothing implicit reaches it: the built-in
+hyperhive surface (todos/messaging) isn't an `extraMcpServers` entry at
+all, and the automatically injected `bash`/`subagent` entries default to excluded
+too (a subagent can't spawn hive-bash tasks or its own nested subagents
+unless an operator opts them in explicitly, same as anything else).
+
+Set `hyperhive.extraMcpServers.<name>.availableToSubagents = true` on a
+specific entry to hand that one server to subagents as well — useful for,
+say, a read-only lookup or scraper MCP a subagent's bounded, single-batch
+task might need. `hive-subagent-mcp`'s `mcp_config` module renders the
+opted-in subset into its own `--mcp-config` file per turn; an entry left
+at the default `false` never appears there.

docs/turn-loop/mcp.md

@@ -13,7 +13,10 @@ entry (`type = "stdio" | "http"`, default `"stdio"`): `matrix` stays a
 stdio bridge, `bash` runs its own persistent streamable-http listener
 (`hive-bash-daemon`) — same reasoning as the built-in surface. The
 server name is `hyperhive`, so the tools land in claude as
-`mcp__hyperhive__<tool>`.
+`mcp__hyperhive__<tool>`. Each entry also has an `availableToSubagents`
+toggle (default `false`) — see
+[`docs/tools/subagent.md`](../tools/subagent.md#mcp-servers-available-to-a-subagent)
+for what reaches a subagent's own `--mcp-config` and what doesn't.
 
 Tool groups (`HIVE_TOOL_GROUPS`) gate tool access. The default
 preset (`AGENT_DEFAULT`) includes `messaging`, `meta`, `inbox`, and

hive-agent-sock/Cargo.toml

@@ -9,4 +9,6 @@ workspace = true
 
 [dependencies]
 serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
 hive-sh4re.workspace = true

hive-agent-sock/README.md

@@ -1,14 +1,15 @@
 # hive-agent-sock
 
-Wire types for the **in-agent socket** — the one the `hive-agent` harness serves
+What's common across the in-container "agent plugins" — the matrix / bash /
-_inside_ the container to its local producers. The matrix / bash MCP daemons
+subagent MCP daemons and `forge_notify` today, plus any user-configured MCP
-and `forge_notify` are the _built-in_ producers that ship today, but any
+server declared in an agent's `agent.nix`. Two things live here:
-user-configured MCP server declared in an agent's `agent.nix` can dial this
-socket and push its own todos with an arbitrary `subsystem` marker — the
-wire format doesn't restrict the set. Unlike the host-served sockets, this
-one **never leaves the container**.
 
-## What it carries
+## Wire types for the in-agent socket
+
+The socket the `hive-agent` harness serves _inside_ the container to those
+local producers. Any producer can dial it and push its own todos with an
+arbitrary `subsystem` marker — the wire format doesn't restrict the set.
+Unlike the host-served sockets, this one **never leaves the container**.
 
 - the loose-ends-v2 **todo** op family
 - the harness-local **reminder** op family
@@ -16,14 +17,23 @@ one **never leaves the container**.
 More in-agent request families may be added over time — the socket is
 deliberately named for the agent, not for the todos.
 
-## Why in-container
+**Why in-container**: the harness owns the todo + reminder stores locally and
+signals its own turn loop directly, so `hive-c0re` is not in either path: no
+broker round-trip, no long-poll, no marker files. That locality is the
+point — it's also what lets these stores travel with the agent for hive
+portability.
 
-The harness owns the todo + reminder stores locally and signals its own turn
+**Not to be confused with `hive-core-agent-sock`**: that crate is the
-loop directly, so `hive-c0re` is not in either path: no broker round-trip, no
+_host_-served core↔agent protocol on `/run/hive/mcp.sock` (the harness
-long-poll, no marker files. That locality is the point — it's also what lets
+talking out to `hive-c0re`). This socket is purely intra-container.
-these stores travel with the agent for hive portability.
 
-## Not to be confused with `hive-core-agent-sock`
+## `extra_mcp`: the `hyperhive.extraMcpServers` spec
 
-That crate is the _host_-served core↔agent protocol on `/run/hive/mcp.sock`
+Not a socket protocol — a shared config type. Parses
-(the harness talking out to `hive-c0re`). This socket is purely intra-container.
+`/etc/hyperhive/extra-mcp.json` (the nix-rendered `hyperhive.extraMcpServers`
+option) and renders each entry into the shape claude expects in a
+`--mcp-config` blob. Shared by `hive-agent` (the main session's servers) and
+`hive-subagent-mcp` (the subagent-eligible subset, gated on
+`availableToSubagents`) — both already depend on this crate for the socket
+types above, so a new plugin-facing shared type belongs here rather than in
+a crate of its own.

hive-agent-sock/src/extra_mcp.rs

@@ -0,0 +1,186 @@
+//! Shared spec for `hyperhive.extraMcpServers` entries: the on-disk JSON
+//! shape the nix module (`nix/agent-modules/mcp.nix`) renders to
+//! `/etc/hyperhive/extra-mcp.json`, plus the parsing and per-entry
+//! JSON-rendering logic every consumer needs. Lives in `hive-agent-sock`
+//! rather than a crate of its own: this crate is already what's common
+//! across the in-container "agent plugins" (matrix/bash/subagent MCP
+//! daemons, `forge_notify`) — the todo-socket wire types above, and now
+//! this — so a new plugin-facing shared type belongs here rather than in
+//! a fresh single-purpose crate.
+
+use std::collections::BTreeMap;
+use std::path::Path;
+
+/// Where the NixOS module writes the per-agent extra-MCP spec. Each entry
+/// becomes an additional `mcpServers.<key>` block in a rendered claude
+/// `--mcp-config`.
+pub const EXTRA_MCP_PATH: &str = "/etc/hyperhive/extra-mcp.json";
+
+/// An extra MCP server declared via `hyperhive.extraMcpServers`. Two
+/// transports: `Stdio` (the consumer spawns `command` fresh each turn, talks
+/// JSON-RPC over its stdin/stdout) and `Http` (point claude at a long-lived
+/// streamable-http `url` instead — no per-turn spawn, no re-registration
+/// race). Internally tagged on the nix-rendered `type` field; unrecognised
+/// fields for the inactive variant (e.g. `command` on an `Http` entry) are
+/// ignored by serde's default struct deserialization.
+#[derive(Debug, Clone, serde::Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ExtraMcpServer {
+    Stdio {
+        command: String,
+        #[serde(default)]
+        args: Vec<String>,
+        #[serde(default)]
+        env: BTreeMap<String, String>,
+        #[serde(default = "default_allowed_tools", rename = "allowedTools")]
+        allowed_tools: Vec<String>,
+        #[serde(default, rename = "availableToSubagents")]
+        available_to_subagents: bool,
+    },
+    Http {
+        url: String,
+        #[serde(default = "default_allowed_tools", rename = "allowedTools")]
+        allowed_tools: Vec<String>,
+        #[serde(default, rename = "availableToSubagents")]
+        available_to_subagents: bool,
+    },
+}
+
+impl ExtraMcpServer {
+    #[must_use]
+    pub fn allowed_tools(&self) -> &[String] {
+        match self {
+            Self::Stdio { allowed_tools, .. } | Self::Http { allowed_tools, .. } => allowed_tools,
+        }
+    }
+
+    /// Whether this entry is opted in to subagent sessions
+    /// (`hyperhive.extraMcpServers.<name>.availableToSubagents`, default
+    /// `false`). Subagents run `--strict-mcp-config` with no discovery, so
+    /// this flag is the only path an entry reaches a subagent's own
+    /// `--mcp-config` at all — see `hive-subagent-mcp`'s `mcp_config`
+    /// module.
+    #[must_use]
+    pub fn available_to_subagents(&self) -> bool {
+        match self {
+            Self::Stdio {
+                available_to_subagents,
+                ..
+            }
+            | Self::Http {
+                available_to_subagents,
+                ..
+            } => *available_to_subagents,
+        }
+    }
+
+    /// Render this entry into the shape claude expects under
+    /// `mcpServers.<name>` in a `--mcp-config` blob. `state_dir` is injected
+    /// as `HYPERHIVE_STATE_DIR` into a stdio entry's env when the entry
+    /// doesn't already set it, so an extra stdio MCP server can resolve the
+    /// agent's durable state dir without its author hard-coding it; an http
+    /// entry has no child-process env to inject into (the daemon behind the
+    /// URL resolves its own state dir independently).
+    #[must_use]
+    pub fn to_json_entry(&self, state_dir: &Path) -> serde_json::Value {
+        match self {
+            Self::Stdio {
+                command, args, env, ..
+            } => {
+                let mut env = env.clone();
+                env.entry("HYPERHIVE_STATE_DIR".to_owned())
+                    .or_insert_with(|| state_dir.display().to_string());
+                serde_json::json!({ "command": command, "args": args, "env": env })
+            }
+            Self::Http { url, .. } => serde_json::json!({ "type": "http", "url": url }),
+        }
+    }
+}
+
+fn default_allowed_tools() -> Vec<String> {
+    vec!["*".to_owned()]
+}
+
+/// Read + parse the extra-MCP spec from [`EXTRA_MCP_PATH`]. Returns an empty
+/// map when the file is missing or unparsable (the agent has none
+/// configured, or the file is malformed — both cases degrade to "no extra
+/// servers").
+#[must_use]
+pub fn load_extra_mcp() -> BTreeMap<String, ExtraMcpServer> {
+    let Ok(raw) = std::fs::read_to_string(EXTRA_MCP_PATH) else {
+        return BTreeMap::new();
+    };
+    serde_json::from_str(&raw).unwrap_or_else(|e| {
+        tracing::warn!(
+            path = EXTRA_MCP_PATH,
+            error = ?e,
+            "extra-mcp spec parse failed; ignoring",
+        );
+        BTreeMap::new()
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn available_to_subagents_defaults_false() {
+        let stdio: ExtraMcpServer =
+            serde_json::from_value(serde_json::json!({"type": "stdio", "command": "/bin/foo"}))
+                .unwrap();
+        assert!(!stdio.available_to_subagents());
+
+        let http: ExtraMcpServer = serde_json::from_value(
+            serde_json::json!({"type": "http", "url": "http://127.0.0.1:1/mcp"}),
+        )
+        .unwrap();
+        assert!(!http.available_to_subagents());
+    }
+
+    #[test]
+    fn available_to_subagents_true_is_read() {
+        let stdio: ExtraMcpServer = serde_json::from_value(serde_json::json!({
+            "type": "stdio",
+            "command": "/bin/foo",
+            "availableToSubagents": true,
+        }))
+        .unwrap();
+        assert!(stdio.available_to_subagents());
+    }
+
+    #[test]
+    fn to_json_entry_injects_state_dir_when_absent() {
+        let stdio: ExtraMcpServer = serde_json::from_value(serde_json::json!({
+            "type": "stdio",
+            "command": "/bin/foo",
+        }))
+        .unwrap();
+        let entry = stdio.to_json_entry(Path::new("/agents/x/state"));
+        assert_eq!(entry["env"]["HYPERHIVE_STATE_DIR"], "/agents/x/state");
+    }
+
+    #[test]
+    fn to_json_entry_respects_explicit_state_dir() {
+        let stdio: ExtraMcpServer = serde_json::from_value(serde_json::json!({
+            "type": "stdio",
+            "command": "/bin/foo",
+            "env": {"HYPERHIVE_STATE_DIR": "/custom"},
+        }))
+        .unwrap();
+        let entry = stdio.to_json_entry(Path::new("/agents/x/state"));
+        assert_eq!(entry["env"]["HYPERHIVE_STATE_DIR"], "/custom");
+    }
+
+    #[test]
+    fn http_entry_has_no_env() {
+        let http: ExtraMcpServer = serde_json::from_value(serde_json::json!({
+            "type": "http",
+            "url": "http://127.0.0.1:1/mcp",
+        }))
+        .unwrap();
+        let entry = http.to_json_entry(Path::new("/agents/x/state"));
+        assert_eq!(entry["url"], "http://127.0.0.1:1/mcp");
+        assert!(entry.get("env").is_none());
+    }
+}

hive-agent-sock/src/lib.rs

@@ -1,23 +1,31 @@
-//! Wire types for the *in-agent* socket, served by the hive-agent harness
+//! Rescoped beyond its original name: what's common across the in-container
-//! to the in-container producers. Matrix / bash MCP daemons and
+//! "agent plugins" (matrix/bash/subagent MCP daemons, `forge_notify`, and any
-//! `forge_notify` are the built-in ones today, but `subsystem` on the todo
+//! user-configured MCP server) — not just the in-agent socket wire types.
-//! ops below is a plain string, not a closed set: any user-configured MCP
+//!
-//! server declared in an agent's `agent.nix` can dial this socket and push
+//! **Wire types for the *in-agent* socket**, served by the hive-agent harness
-//! its own todos the same way. Carries the loose-ends-v2 *todo* op family
+//! to those producers. `subsystem` on the todo ops below is a plain string,
-//! plus the harness-local *reminder* op family; more in-agent request
+//! not a closed set: any user-configured MCP server declared in an agent's
-//! families may be added over time (the socket is deliberately named for
+//! `agent.nix` can dial this socket and push its own todos the same way.
-//! the agent, not the todos).
+//! Carries the loose-ends-v2 *todo* op family plus the harness-local
+//! *reminder* op family; more in-agent request families may be added over
+//! time (the socket is deliberately named for the agent, not the todos).
 //!
 //! Distinct from `hive-core-agent-sock`, the *host*-served core↔agent
 //! protocol on `/run/hive/mcp.sock`: this socket never leaves the
 //! container. The harness owns the todo + reminder stores locally and
 //! signals its own turn loop directly, so hive-c0re is not in either path —
 //! no broker round-trip, no long-poll, no marker files.
+//!
+//! **`extra_mcp`**: the `hyperhive.extraMcpServers` spec + parsing, shared by
+//! `hive-agent` (the main session's `--mcp-config`) and `hive-subagent-mcp`
+//! (the subagent-eligible subset) — not a socket protocol at all, but this
+//! crate is the natural home since both consumers already depend on it.
 
 use serde::{Deserialize, Serialize};
 
 use hive_sh4re::inbox::LooseEnd;
 
+pub mod extra_mcp;
 pub mod paths;
 
 /// In-container path of the harness-served in-agent socket. The harness

hive-agent/src/mcp_config.rs

@@ -184,7 +184,7 @@ pub fn allowed_mcp_tools(groups: &[hive_sh4re::permissions::ToolGroup]) -> Vec<S
     // pattern list to `mcp__<server>__<pattern>` so claude can call
     // them without per-tool operator approval. `["*"]` (the default)
     // expands to `mcp__<server>__*` — every tool from that server.
-    for (server, spec) in load_extra_mcp() {
+    for (server, spec) in hive_agent_sock::extra_mcp::load_extra_mcp() {
         if server == SERVER_NAME || !extra_server_enabled(&server, groups) {
             continue;
         }
@@ -241,70 +241,6 @@ pub fn builtin_tools_arg() -> String {
     tools.join(",")
 }
 
-/// Where the NixOS module writes the per-agent extra-MCP spec (see
-/// `nix/agent-modules/mcp.nix`). Each entry becomes an additional
-/// `mcpServers.<key>` block in the rendered claude config + a
-/// `mcp__<key>__<tool>` pattern in `--allowedTools`.
-const EXTRA_MCP_PATH: &str = "/etc/hyperhive/extra-mcp.json";
-
-/// An extra MCP server declared via `hyperhive.extraMcpServers`. Two
-/// transports: `Stdio` (claude spawns `command` fresh each turn, talks
-/// JSON-RPC over its stdin/stdout) and `Http` (claude points at a
-/// long-lived streamable-http `url` instead — no per-turn spawn, no
-/// re-registration race, same shape as the built-in hyperhive surface).
-/// Internally tagged on the nix-rendered `type` field; unrecognised
-/// fields for the inactive variant (e.g. `command` on an `Http` entry)
-/// are ignored by serde's default struct deserialization.
-#[derive(Debug, serde::Deserialize)]
-#[serde(tag = "type", rename_all = "snake_case")]
-enum ExtraMcpServer {
-    Stdio {
-        command: String,
-        #[serde(default)]
-        args: Vec<String>,
-        #[serde(default)]
-        env: std::collections::BTreeMap<String, String>,
-        #[serde(default = "default_allowed_tools")]
-        #[serde(rename = "allowedTools")]
-        allowed_tools: Vec<String>,
-    },
-    Http {
-        url: String,
-        #[serde(default = "default_allowed_tools")]
-        #[serde(rename = "allowedTools")]
-        allowed_tools: Vec<String>,
-    },
-}
-
-impl ExtraMcpServer {
-    fn allowed_tools(&self) -> &[String] {
-        match self {
-            Self::Stdio { allowed_tools, .. } | Self::Http { allowed_tools, .. } => allowed_tools,
-        }
-    }
-}
-
-fn default_allowed_tools() -> Vec<String> {
-    vec!["*".to_owned()]
-}
-
-/// Read + parse the extra-MCP spec. Returns an empty map when
-/// the file is missing or unparsable (the agent has none configured,
-/// or the file is malformed — both cases degrade to "no extra servers").
-fn load_extra_mcp() -> std::collections::BTreeMap<String, ExtraMcpServer> {
-    let Ok(raw) = std::fs::read_to_string(EXTRA_MCP_PATH) else {
-        return std::collections::BTreeMap::new();
-    };
-    serde_json::from_str(&raw).unwrap_or_else(|e| {
-        tracing::warn!(
-            path = EXTRA_MCP_PATH,
-            error = ?e,
-            "extra-mcp spec parse failed; ignoring",
-        );
-        std::collections::BTreeMap::new()
-    })
-}
-
 /// Render the MCP config blob claude reads from `--mcp-config <path>`.
 /// The built-in `hyperhive` surface is an HTTP entry pointing at the
 /// persistent `hive-mcp-http` daemon (see [`DEFAULT_MCP_HTTP_PORT`]); there
@@ -364,7 +300,7 @@ fn build_mcp_servers() -> serde_json::Map<String, serde_json::Value> {
     // agent isn't entitled to must not even appear in the MCP config, or the
     // agent could call it directly (there is no later enforcement point).
     let groups = effective_tool_groups();
-    for (name, spec) in load_extra_mcp() {
+    for (name, spec) in hive_agent_sock::extra_mcp::load_extra_mcp() {
         if name == SERVER_NAME {
             tracing::warn!(
                 "extra MCP server name `{SERVER_NAME}` collides with the built-in surface; ignoring",
@@ -378,20 +314,7 @@ fn build_mcp_servers() -> serde_json::Map<String, serde_json::Value> {
             );
             continue;
         }
-        let entry = match spec {
+        servers.insert(name, spec.to_json_entry(&state_dir));
-            ExtraMcpServer::Stdio {
-                command,
-                args,
-                mut env,
-                ..
-            } => {
-                env.entry("HYPERHIVE_STATE_DIR".to_owned())
-                    .or_insert_with(|| state_dir.display().to_string());
-                serde_json::json!({ "command": command, "args": args, "env": env })
-            }
-            ExtraMcpServer::Http { url, .. } => serde_json::json!({ "type": "http", "url": url }),
-        };
-        servers.insert(name, entry);
     }
     servers
 }

hive-subagent-mcp/src/lib.rs

@@ -10,5 +10,6 @@
 //! in-memory `name -> Cancel` map, live only as long as the process is.
 
 pub mod mcp;
+pub mod mcp_config;
 pub mod paths;
 pub mod session;

hive-subagent-mcp/src/mcp_config.rs

@@ -0,0 +1,57 @@
+//! Builds a subagent's own filtered `--mcp-config`: only
+//! `hyperhive.extraMcpServers` entries with `availableToSubagents = true`
+//! (see `hive_agent_sock::extra_mcp::ExtraMcpServer::available_to_subagents`) ever reach
+//! a subagent's claude invocation. Everything else — the built-in hyperhive
+//! surface (todos/messaging), the automatically injected `bash` and `subagent`
+//! entries — stays unreachable by construction: none of those default to
+//! opted in, and the built-in surface isn't an `extraMcpServers` entry at
+//! all, so there's no name for an operator to opt it in under even if they
+//! wanted to.
+
+use std::path::PathBuf;
+
+/// Filename the rendered config lives at, under [`crate::paths::harness_dir`].
+const CONFIG_FILE: &str = "subagent-mcp-config.json";
+
+/// Render the subagent-eligible extra-MCP servers to a `--mcp-config` file,
+/// returning its path — or `None` when no entry opts in (or the render/write
+/// fails), so [`hive_claude::Config::mcp_config`] stays unset and the
+/// subagent gets literally zero MCP servers, the same default as before this
+/// toggle existed. Re-rendered on every call (cheap: a filter plus a small
+/// file write) rather than cached once at daemon startup, so a config change
+/// takes effect on this subagent's next `start`/`continue` without needing
+/// the daemon itself restarted.
+#[must_use]
+pub fn build() -> Option<PathBuf> {
+    let state_dir = crate::paths::state_dir();
+    let servers: serde_json::Map<String, serde_json::Value> =
+        hive_agent_sock::extra_mcp::load_extra_mcp()
+            .into_iter()
+            .filter(|(_, spec)| spec.available_to_subagents())
+            .map(|(name, spec)| (name, spec.to_json_entry(&state_dir)))
+            .collect();
+    if servers.is_empty() {
+        return None;
+    }
+    let body = serde_json::to_string_pretty(&serde_json::json!({ "mcpServers": servers }))
+        .unwrap_or_else(|_| "{}".into());
+    let dir = crate::paths::harness_dir();
+    if let Err(e) = std::fs::create_dir_all(&dir) {
+        tracing::warn!(
+            error = ?e,
+            dir = %dir.display(),
+            "subagent mcp-config: harness dir create failed; subagent gets zero extra MCP servers this turn",
+        );
+        return None;
+    }
+    let path = dir.join(CONFIG_FILE);
+    if let Err(e) = std::fs::write(&path, body) {
+        tracing::warn!(
+            error = ?e,
+            path = %path.display(),
+            "subagent mcp-config: write failed; subagent gets zero extra MCP servers this turn",
+        );
+        return None;
+    }
+    Some(path)
+}

hive-subagent-mcp/src/paths.rs

@@ -1,4 +1,4 @@
-//! The one per-agent path this daemon needs.
+//! The per-agent paths this daemon needs.
 
 use std::path::PathBuf;
 
@@ -13,3 +13,28 @@ pub fn agent_socket() -> PathBuf {
         PathBuf::from,
     )
 }
+
+/// Durable state directory for the current agent. Same resolution as
+/// `hive-agent`'s own `paths::state_dir` — reads `HYPERHIVE_STATE_DIR`
+/// (always injected via `systemd.globalEnvironment` by the meta flake),
+/// falling back to the `HIVE_LABEL`-derived pattern for dev/test
+/// environments where the env var may not be set. Copied rather than shared
+/// with `hive-agent` (a binary crate with no lib target) — a five-line env
+/// read isn't worth a crate dependency to dedupe.
+#[must_use]
+pub fn state_dir() -> PathBuf {
+    if let Some(p) = std::env::var_os("HYPERHIVE_STATE_DIR") {
+        return PathBuf::from(p);
+    }
+    let label = std::env::var("HIVE_LABEL").unwrap_or_default();
+    PathBuf::from(format!("/agents/{label}/state"))
+}
+
+/// Harness-internal scratch dir this daemon writes its own generated
+/// subagent `--mcp-config` file into (see `crate::mcp_config`). Delegates to
+/// the shared resolver so this and `hive-agent`'s own harness dir always
+/// agree on the same path.
+#[must_use]
+pub fn harness_dir() -> PathBuf {
+    hive_agent_sock::paths::harness_dir()
+}

hive-subagent-mcp/src/session.rs

@@ -185,8 +185,14 @@ fn subagent_otel_attrs(name: &str) -> String {
 /// instructions. `dir`, when given, becomes `Config::cwd` (e.g. a worktree
 /// the caller already prepared); `None` inherits this daemon's own working
 /// directory, same as before this field existed. Always
-/// `--dangerously-skip-permissions --strict-mcp-config` (no `--mcp-config`
+/// `--dangerously-skip-permissions --strict-mcp-config` — the safety
-/// override — a safety property, not a knob).
+/// property is `strict_mcp_config: true` with no ambient MCP discovery, not
+/// an unconditional absence of `--mcp-config`: a subagent gets exactly the
+/// `hyperhive.extraMcpServers` entries an operator has explicitly opted in
+/// via `availableToSubagents = true` (`crate::mcp_config::build`), nothing
+/// implicit and nothing more. With no entry opted in — the default — that
+/// resolves to `None` and the invocation is unchanged from before this
+/// toggle existed: zero MCP servers, full stop.
 fn build_config(
     name: &str,
     model: Option<String>,
@@ -201,6 +207,7 @@ fn build_config(
     Config {
         model,
         cwd: dir.map(PathBuf::from),
+        mcp_config: crate::mcp_config::build(),
         strict_mcp_config: true,
         extra_args,
         env: vec![(

nix/agent-modules/mcp.nix

@@ -124,6 +124,27 @@ in
               `mcp__<server-key>__` at build time.
             '';
           };
+          availableToSubagents = lib.mkOption {
+            type = lib.types.bool;
+            default = false;
+            description = ''
+              Whether a claude subagent (`mcp__subagent__start`/`continue`,
+              served by `hive-subagent-daemon`) also gets this MCP server.
+              Default `false`: a subagent runs `--strict-mcp-config` with no
+              ambient MCP discovery, so this is the *only* path an
+              `extraMcpServers` entry reaches a subagent's own claude
+              invocation at all — leaving it unset keeps a subagent exactly
+              as narrow as before this option existed (its native built-in
+              tools only, zero MCP servers).
+
+              The automatically injected `bash` and `subagent` entries below stay at
+              this default — a subagent never gets hive-bash (it already has
+              claude's native `Bash` tool instead) or the ability to spawn
+              its own nested subagents, unless an operator opts a
+              *different* entry in explicitly. Set `true` on an entry (e.g.
+              a scraper or read-only lookup MCP) to hand it to subagents too.
+            '';
+          };
         };
       }
     );