diff --git a/src/openhuman/agent/harness/definition.rs b/src/openhuman/agent/harness/definition.rs
index 944e56954..8106c5a4e 100644
--- a/src/openhuman/agent/harness/definition.rs
+++ b/src/openhuman/agent/harness/definition.rs
@@ -312,6 +312,57 @@ impl AgentTier {
     }
 }
 
+impl std::fmt::Display for AgentTier {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// Single source of truth for the spawn-hierarchy rule: is a `parent`-tier
+/// agent allowed to delegate to a `child`-tier agent?
+///
+/// Returns `Ok(())` for the legal handoffs and `Err(reason)` for the three
+/// forbidden shapes, where `reason` is a tier-only human-readable explanation
+/// (no agent ids — callers prepend their own context):
+///
+/// - `Worker → *` — workers are leaf executors and must not spawn anything.
+/// - `Chat → Chat` — the chat tier is a leaf in its own dimension; cloning it
+///   defeats the fast-path and risks unbounded `chat → chat → …` chains.
+/// - `Reasoning → Reasoning` — reasoning agents compose downward into workers,
+///   not into each other (a depth-blowing recursion of slow models).
+///
+/// Note this forbids same-tier and worker-as-parent hops, **not** upward hops:
+/// `reasoning → chat` is a real, intentional builtin edge (the `subconscious`
+/// reasoner can hand a follow-up back to the `orchestrator` chat agent), so it
+/// must stay legal. The harness'es `MAX_SPAWN_DEPTH` cap bounds chain length
+/// independently of tier direction.
+///
+/// This is the static authoring rule the loader walks over declared `subagents`
+/// pairs at boot (see
+/// [`crate::openhuman::agent_registry::agents::validate_tier_hierarchy`]). The
+/// runtime spawn gate (`run_subagent`) reuses it as defense-in-depth, but
+/// deliberately exempts worker *parents* — at runtime a worker only reaches the
+/// spawn chokepoint via the documented collapsed `delegate_to_integrations_agent`
+/// path (→ `integrations_agent`, itself a worker), which the loader intentionally
+/// leaves untouched.
+pub fn validate_tier_transition(parent: AgentTier, child: AgentTier) -> Result<(), String> {
+    match (parent, child) {
+        (AgentTier::Worker, _) => Err(format!(
+            "a `worker` tier agent must not spawn `{}` — workers are leaf executors",
+            child.as_str()
+        )),
+        (AgentTier::Chat, AgentTier::Chat) => Err(
+            "the chat tier is a leaf in its own dimension — hand off to a `reasoning` or \
+             `worker` agent instead"
+                .to_string(),
+        ),
+        (AgentTier::Reasoning, AgentTier::Reasoning) => {
+            Err("reasoning agents compose downward into workers, not into each other".to_string())
+        }
+        _ => Ok(()),
+    }
+}
+
 // ─────────────────────────────────────────────────────────────────────────────
 // Subagent delegation entries
 // ─────────────────────────────────────────────────────────────────────────────
diff --git a/src/openhuman/agent/harness/definition_tests.rs b/src/openhuman/agent/harness/definition_tests.rs
index ddf3c8441..fe4a0c5da 100644
--- a/src/openhuman/agent/harness/definition_tests.rs
+++ b/src/openhuman/agent/harness/definition_tests.rs
@@ -281,3 +281,61 @@ max_iterations = 1
     assert_eq!(def.iteration_policy, IterationPolicy::Strict);
     assert_eq!(def.effective_max_iterations(), 1);
 }
+
+// ── Spawn-hierarchy tier rule (validate_tier_transition) ────────────────────
+// Single source of truth shared by the boot loader walk and the runtime
+// spawn gate in `run_subagent` (issue #4098).
+
+#[test]
+fn tier_transition_allows_legal_descending_handoffs() {
+    // chat → reasoning, chat → worker, reasoning → worker are the only
+    // legal hops; each strictly descends the hierarchy.
+    assert!(validate_tier_transition(AgentTier::Chat, AgentTier::Reasoning).is_ok());
+    assert!(validate_tier_transition(AgentTier::Chat, AgentTier::Worker).is_ok());
+    assert!(validate_tier_transition(AgentTier::Reasoning, AgentTier::Worker).is_ok());
+}
+
+#[test]
+fn tier_transition_rejects_chat_to_chat() {
+    let err = validate_tier_transition(AgentTier::Chat, AgentTier::Chat)
+        .expect_err("chat→chat must be rejected");
+    // Wording must carry the substrings the loader diagnostics rely on.
+    assert!(err.contains("chat") && err.contains("leaf"), "got: {err}");
+}
+
+#[test]
+fn tier_transition_rejects_reasoning_to_reasoning() {
+    let err = validate_tier_transition(AgentTier::Reasoning, AgentTier::Reasoning)
+        .expect_err("reasoning→reasoning must be rejected");
+    assert!(err.contains("reasoning"), "got: {err}");
+}
+
+#[test]
+fn tier_transition_allows_upward_reasoning_to_chat() {
+    // Upward delegation is intentionally legal: the `subconscious` reasoner
+    // hands follow-ups back to the `orchestrator` chat agent. Only same-tier
+    // and worker-as-parent hops are forbidden.
+    assert!(validate_tier_transition(AgentTier::Reasoning, AgentTier::Chat).is_ok());
+}
+
+#[test]
+fn tier_transition_rejects_worker_as_parent() {
+    // A worker is a leaf executor — it may not spawn any tier, including
+    // another worker.
+    for child in [AgentTier::Chat, AgentTier::Reasoning, AgentTier::Worker] {
+        let err = validate_tier_transition(AgentTier::Worker, child)
+            .expect_err("worker must not spawn any tier");
+        assert!(
+            err.contains("worker") && err.contains("leaf"),
+            "worker→{} reason should call out the worker-leaf rule, got: {err}",
+            child.as_str()
+        );
+    }
+}
+
+#[test]
+fn tier_display_matches_as_str() {
+    assert_eq!(AgentTier::Chat.to_string(), "chat");
+    assert_eq!(AgentTier::Reasoning.to_string(), "reasoning");
+    assert_eq!(AgentTier::Worker.to_string(), "worker");
+}
diff --git a/src/openhuman/agent/harness/subagent_runner/ops/runner.rs b/src/openhuman/agent/harness/subagent_runner/ops/runner.rs
index e72c3df57..957433eac 100644
--- a/src/openhuman/agent/harness/subagent_runner/ops/runner.rs
+++ b/src/openhuman/agent/harness/subagent_runner/ops/runner.rs
@@ -11,7 +11,8 @@ use std::sync::Arc;
 use std::time::Instant;
 
 use crate::openhuman::agent::harness::definition::{
-    AgentDefinition, IterationPolicy, PromptSource,
+    validate_tier_transition, AgentDefinition, AgentDefinitionRegistry, AgentTier, IterationPolicy,
+    PromptSource,
 };
 use crate::openhuman::agent::harness::fork_context::{current_parent, ParentExecutionContext};
 use crate::openhuman::agent::harness::subagent_runner::extract_tool::ExtractFromResultTool;
@@ -37,6 +38,58 @@ use super::provider::{
     resolve_subagent_provider, user_is_signed_in_to_composio, LazyToolkitResolver,
 };
 
+/// Runtime spawn-hierarchy gate decision for one delegation hop.
+///
+/// `parent_def` is the resolved parent agent definition (looked up from the
+/// global registry by its definition id) or `None` when the parent can't be
+/// resolved — e.g. a dynamically-named agent (model-council juror) or a custom
+/// agent absent from the registry, or any context where the registry isn't
+/// initialised. A `None` parent yields `Ok(())`: we skip rather than mask, the
+/// same defensive posture the loader takes for unknown child ids.
+///
+/// A **worker** parent is also exempted. At runtime a worker only reaches the
+/// spawn chokepoint via the documented collapsed `delegate_to_integrations_agent`
+/// path (→ `integrations_agent`, itself a worker) — a shape the loader
+/// intentionally leaves untouched. Re-denying it here would turn valid custom
+/// worker agents that use `{ skills = "*" }` into runtime failures. The
+/// worker-leaf authoring rule stays enforced statically at boot, and the
+/// per-parent allowlist gate blocks any other worker spawn.
+///
+/// For chat / reasoning parents the hop is checked against
+/// [`validate_tier_transition`] (the single source of truth shared with the
+/// boot loader walk); a forbidden hop is logged and becomes a
+/// [`SubagentRunError::TierViolation`]. Logging lives here (rather than at the
+/// call site) so the deny path is exercised by this fn's unit tests.
+pub(crate) fn tier_gate_decision(
+    parent_def: Option<&AgentDefinition>,
+    child: &AgentDefinition,
+    parent_agent_id: &str,
+    task_id: &str,
+) -> Result<(), SubagentRunError> {
+    let Some(parent_def) = parent_def else {
+        return Ok(());
+    };
+    if parent_def.agent_tier == AgentTier::Worker {
+        return Ok(());
+    }
+    if let Err(reason) = validate_tier_transition(parent_def.agent_tier, child.agent_tier) {
+        tracing::warn!(
+            parent_agent = %parent_agent_id,
+            parent_tier = %parent_def.agent_tier,
+            child_agent = %child.id,
+            child_tier = %child.agent_tier,
+            task_id = %task_id,
+            "[subagent_runner] blocked tier-violating delegation: {reason}"
+        );
+        return Err(SubagentRunError::TierViolation {
+            parent_tier: parent_def.agent_tier,
+            child_tier: child.agent_tier,
+            reason,
+        });
+    }
+    Ok(())
+}
+
 /// Run a sub-agent based on its definition and a task prompt.
 ///
 /// This is the primary entry point for agent delegation. It performs the following:
@@ -90,6 +143,17 @@ pub async fn run_subagent(
             });
         }
 
+        // Runtime spawn-hierarchy (tier) gate — defense-in-depth alongside the
+        // depth gate above. The loader validates *declared* `subagents` pairs
+        // statically at boot (`validate_tier_hierarchy`), but dynamic, custom,
+        // or model-chosen spawns reach this chokepoint without ever passing
+        // through that walk. Resolve the parent's tier from the registry by its
+        // definition id; `tier_gate_decision` rejects (and logs) any forbidden
+        // chat/reasoning hop while exempting unresolved + worker parents.
+        let parent_def =
+            AgentDefinitionRegistry::global().and_then(|reg| reg.get(&parent.agent_definition_id));
+        tier_gate_decision(parent_def, definition, &parent.agent_definition_id, &task_id)?;
+
         tracing::info!(
             agent_id = %definition.id,
             task_id = %task_id,
diff --git a/src/openhuman/agent/harness/subagent_runner/ops_tests.rs b/src/openhuman/agent/harness/subagent_runner/ops_tests.rs
index 9674cb595..ab079a7a9 100644
--- a/src/openhuman/agent/harness/subagent_runner/ops_tests.rs
+++ b/src/openhuman/agent/harness/subagent_runner/ops_tests.rs
@@ -1542,3 +1542,99 @@ fn prefix_tier_refuses_ambiguous_and_short_slugs() {
     // Short slug below the length gate never engages the prefix tier.
     assert!(resolver.resolve("NOTION").is_none());
 }
+
+// ── Runtime spawn-hierarchy (tier) gate (issue #4098) ───────────────────────
+// `tier_gate_decision` is the pure decision the runtime gate in `run_subagent`
+// applies to each delegation hop. Tested directly so the deny/allow/skip
+// table is covered without standing up a global registry or a live spawn.
+
+// Thin wrapper to call the gate with throwaway log-context ids.
+fn gate(parent: Option<&AgentDefinition>, child: &AgentDefinition) -> Result<(), SubagentRunError> {
+    super::runner::tier_gate_decision(parent, child, "parent-agent", "task-1")
+}
+
+#[test]
+fn tier_gate_skips_when_parent_unresolved() {
+    use crate::openhuman::agent::harness::definition::AgentTier;
+    // No resolvable parent definition (e.g. registry uninitialised, or a
+    // dynamically-named model-council juror / custom agent absent from it) →
+    // skip rather than mask. Even a would-be-illegal child tier passes, because
+    // we have no parent tier to judge against.
+    let mut child = make_def_named_tools(&[]);
+    child.agent_tier = AgentTier::Chat;
+    assert!(gate(None, &child).is_ok());
+}
+
+#[test]
+fn tier_gate_allows_legal_descending_hops() {
+    use crate::openhuman::agent::harness::definition::AgentTier;
+    let mut parent = make_def_named_tools(&[]);
+    let mut child = make_def_named_tools(&[]);
+
+    // chat → worker
+    parent.agent_tier = AgentTier::Chat;
+    child.agent_tier = AgentTier::Worker;
+    assert!(gate(Some(&parent), &child).is_ok());
+
+    // chat → reasoning
+    child.agent_tier = AgentTier::Reasoning;
+    assert!(gate(Some(&parent), &child).is_ok());
+
+    // reasoning → worker
+    parent.agent_tier = AgentTier::Reasoning;
+    child.agent_tier = AgentTier::Worker;
+    assert!(gate(Some(&parent), &child).is_ok());
+}
+
+#[test]
+fn tier_gate_allows_worker_parent_for_collapsed_integration() {
+    use crate::openhuman::agent::harness::definition::AgentTier;
+    // A worker only reaches the runtime spawn chokepoint via the documented
+    // collapsed `delegate_to_integrations_agent` path (→ `integrations_agent`,
+    // itself a worker). The gate must NOT re-deny that — the worker-leaf rule
+    // is a static boot-time authoring constraint, not a runtime one. Regression
+    // for the wildcard-integration case (CodeRabbit P2 on PR #4102).
+    let mut parent = make_def_named_tools(&[]);
+    let child = make_def_named_tools(&[]); // worker by default
+    parent.agent_tier = AgentTier::Worker;
+    assert!(gate(Some(&parent), &child).is_ok());
+}
+
+#[test]
+fn tier_gate_denies_chat_to_chat() {
+    use crate::openhuman::agent::harness::definition::AgentTier;
+    let mut parent = make_def_named_tools(&[]);
+    let mut child = make_def_named_tools(&[]);
+    parent.agent_tier = AgentTier::Chat;
+    child.agent_tier = AgentTier::Chat;
+
+    let err =
+        gate(Some(&parent), &child).expect_err("chat→chat must be denied at the runtime gate");
+    match err {
+        SubagentRunError::TierViolation {
+            parent_tier,
+            child_tier,
+            reason,
+        } => {
+            assert_eq!(parent_tier, AgentTier::Chat);
+            assert_eq!(child_tier, AgentTier::Chat);
+            assert!(
+                reason.contains("chat") && reason.contains("leaf"),
+                "got: {reason}"
+            );
+        }
+        other => panic!("expected TierViolation, got: {other:?}"),
+    }
+}
+
+#[test]
+fn tier_gate_allows_upward_reasoning_to_chat() {
+    use crate::openhuman::agent::harness::definition::AgentTier;
+    // Upward delegation is intentionally legal (subconscious reasoner →
+    // orchestrator chat). The gate must not deny it.
+    let mut parent = make_def_named_tools(&[]);
+    let mut child = make_def_named_tools(&[]);
+    parent.agent_tier = AgentTier::Reasoning;
+    child.agent_tier = AgentTier::Chat;
+    assert!(gate(Some(&parent), &child).is_ok());
+}
diff --git a/src/openhuman/agent/harness/subagent_runner/types.rs b/src/openhuman/agent/harness/subagent_runner/types.rs
index c67f45d2a..f29073a55 100644
--- a/src/openhuman/agent/harness/subagent_runner/types.rs
+++ b/src/openhuman/agent/harness/subagent_runner/types.rs
@@ -7,6 +7,7 @@ use std::path::PathBuf;
 use std::time::Duration;
 use thiserror::Error;
 
+use crate::openhuman::agent::harness::definition::AgentTier;
 use crate::openhuman::inference::provider::ChatMessage;
 
 /// Per-spawn options that override or augment what the
@@ -178,6 +179,16 @@ pub enum SubagentRunError {
         max_depth: usize,
     },
 
+    #[error(
+        "delegation blocked by the spawn-hierarchy gate: a `{parent_tier}` agent may not \
+         delegate to a `{child_tier}` agent — {reason}"
+    )]
+    TierViolation {
+        parent_tier: AgentTier,
+        child_tier: AgentTier,
+        reason: String,
+    },
+
     #[error("sub-agent exceeded maximum iterations ({0})")]
     MaxIterationsExceeded(usize),
 }
diff --git a/src/openhuman/agent_registry/agents/loader.rs b/src/openhuman/agent_registry/agents/loader.rs
index 58b4390cb..65b442813 100644
--- a/src/openhuman/agent_registry/agents/loader.rs
+++ b/src/openhuman/agent_registry/agents/loader.rs
@@ -35,7 +35,8 @@
 //! collision.
 
 use crate::openhuman::agent::harness::definition::{
-    AgentDefinition, AgentTier, DefinitionSource, PromptBuilder, PromptSource, SubagentEntry,
+    validate_tier_transition, AgentDefinition, AgentTier, DefinitionSource, PromptBuilder,
+    PromptSource, SubagentEntry,
 };
 use anyhow::{Context, Result};
 use std::collections::HashMap;
@@ -305,22 +306,18 @@ pub fn validate_tier_hierarchy(defs: &[AgentDefinition]) -> Result<()> {
             // Same-tier delegation is forbidden for chat and reasoning.
             // (Chat→Chat would defeat the whole point of the fast tier;
             // Reasoning→Reasoning produces a depth-blowing recursion of
-            // slow models.)
-            match (def.agent_tier, child_tier) {
-                (AgentTier::Chat, AgentTier::Chat) => anyhow::bail!(
-                    "agent `{parent}` (chat) lists `{child}` (chat) in subagents — the chat tier \
-                     is a leaf in its own dimension. Hand off to a `reasoning` or `worker` agent \
-                     instead.",
+            // slow models.) The pair-rule lives in `validate_tier_transition`
+            // (the single source of truth shared with the runtime spawn gate
+            // in `run_subagent`); here we wrap its reason with the offending
+            // agent ids + tiers for a boot-time-friendly diagnostic.
+            if let Err(reason) = validate_tier_transition(def.agent_tier, child_tier) {
+                anyhow::bail!(
+                    "agent `{parent}` ({ptier}) lists `{child}` ({ctier}) in subagents — {reason}",
                     parent = def.id,
+                    ptier = def.agent_tier.as_str(),
                     child = child_id,
-                ),
-                (AgentTier::Reasoning, AgentTier::Reasoning) => anyhow::bail!(
-                    "agent `{parent}` (reasoning) lists `{child}` (reasoning) in subagents — \
-                     reasoning agents compose downward into workers, not into each other.",
-                    parent = def.id,
-                    child = child_id,
-                ),
-                _ => {}
+                    ctier = child_tier.as_str(),
+                );
             }
         }
     }
diff --git a/src/openhuman/agent_registry/agents/orchestrator/prompt.md b/src/openhuman/agent_registry/agents/orchestrator/prompt.md
index e2260f50a..6be9b0255 100644
--- a/src/openhuman/agent_registry/agents/orchestrator/prompt.md
+++ b/src/openhuman/agent_registry/agents/orchestrator/prompt.md
@@ -59,7 +59,7 @@ You can open and operate native apps on this machine, but you do it by **delegat
 
 - **You are the chat tier.** You run on a fast UX-focused model (TTFT > deep reasoning). When a task needs sustained multi-step thinking — planning across many steps, comparing several non-obvious options, untangling ambiguous requirements — **delegate to the reasoning tier (`delegate_plan`)** rather than reasoning through it yourself. Your job at that point is to brief the planner well and synthesise its output back to the user.
 - **Never spawn yourself** — You cannot delegate to another chat-tier agent (Orchestrator or otherwise). The chat tier is a leaf in its own dimension.
-- **Spawn hierarchy (hard rule).** Allowed handoffs from here: `chat → worker` (fast path) or `chat → reasoning → worker` (deep path). Never `chat → chat` and never `chat → reasoning → reasoning`. The loader rejects same-tier delegation at boot; a runtime depth gate capping chains at 3 hops is a planned follow-up — until it lands, this rule is enforced by you, by the planner's matching rule, and by the static loader check.
+- **Spawn hierarchy (hard rule).** Allowed handoffs from here: `chat → worker` (fast path) or `chat → reasoning → worker` (deep path). Never `chat → chat` and never `chat → reasoning → reasoning`. This is enforced in depth: the loader rejects same-tier delegation at boot, and the spawn chokepoint denies any tier-violating or over-deep spawn at runtime (a depth gate caps chains at 3 hops and a tier gate rejects the forbidden hops). Those gates are a safety net, not a license to mis-route — still follow the hierarchy yourself, as does the planner's matching rule.
 - **Minimise sub-agents** — Use the fewest agents necessary. Simple questions don't need a DAG.
 - **Direct-first always** — First try direct reply or direct tools; delegate only when required by task complexity/capability gaps.
 - **Context is expensive** — Pass only relevant context to sub-agents, not everything.