diff --git a/run_agent.py b/run_agent.py
index 973f0d95d72ea..f8cf4682cc452 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -1792,7 +1792,7 @@ def __init__(
             tool_names = sorted(self.valid_tool_names)
             if not self.quiet_mode:
                 print(f"🛠️  Loaded {len(self.tools)} tools: {', '.join(tool_names)}")
-                
+
                 # Show filtering info if applied
                 if enabled_toolsets:
                     print(f"   ✅ Enabled toolsets: {', '.join(enabled_toolsets)}")
@@ -1800,6 +1800,16 @@ def __init__(
                     print(f"   ❌ Disabled toolsets: {', '.join(disabled_toolsets)}")
         elif not self.quiet_mode:
             print("🛠️  No tools loaded (all tools filtered out or unavailable)")
+
+        # Kanban worker/orchestrator lifecycle guidance is session-static:
+        # the dispatcher decides at spawn time whether this process is a
+        # kanban worker (kanban_show tool is present iff HERMES_KANBAN_TASK
+        # is set — see tools/kanban_tools.py:54). Resolving the ~835-token
+        # block once here avoids re-running the membership test + reference
+        # on every system-prompt rebuild (init + each context compression).
+        self._kanban_worker_guidance = (
+            KANBAN_GUIDANCE if "kanban_show" in self.valid_tool_names else ""
+        )
         
         # Check tool requirements
         if self.tools and not self.quiet_mode:
@@ -5783,9 +5793,9 @@ def _build_system_prompt_parts(self, system_message: str = None) -> Dict[str, st
         # Kanban worker/orchestrator lifecycle — only present when the
         # dispatcher spawned this process (kanban_show check_fn gates on
         # HERMES_KANBAN_TASK env var). Normal chat sessions never see
-        # this block.
-        if "kanban_show" in self.valid_tool_names:
-            tool_guidance.append(KANBAN_GUIDANCE)
+        # this block. Resolved once at __init__ (see _kanban_worker_guidance).
+        if self._kanban_worker_guidance:
+            tool_guidance.append(self._kanban_worker_guidance)
         if tool_guidance:
             stable_parts.append(" ".join(tool_guidance))
 
diff --git a/tests/tools/test_kanban_tools.py b/tests/tools/test_kanban_tools.py
index c31ae6f08bb92..d390e9a9a01b4 100644
--- a/tests/tools/test_kanban_tools.py
+++ b/tests/tools/test_kanban_tools.py
@@ -917,6 +917,70 @@ def test_kanban_guidance_prompt_size_bounded(monkeypatch, tmp_path):
     )
 
 
+def test_kanban_guidance_resolved_once_at_init(monkeypatch, tmp_path):
+    """Regression: the kanban-worker classification must be resolved once
+    at __init__ and cached on ``self._kanban_worker_guidance``, not re-checked
+    against ``valid_tool_names`` on every system-prompt rebuild.
+
+    The system prompt is rebuilt on every context-compression event; the
+    KANBAN_GUIDANCE block is session-static (the dispatcher decides worker
+    vs. non-worker once at spawn time), so the membership test belongs in
+    __init__ — not in the hot path. This test asserts the cache exists
+    and is the source of truth.
+    """
+    monkeypatch.setenv("HERMES_KANBAN_TASK", "t_fake")
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    from pathlib import Path as _P
+    monkeypatch.setattr(_P, "home", lambda: tmp_path)
+
+    from run_agent import AIAgent
+    from agent.prompt_builder import KANBAN_GUIDANCE
+
+    a = AIAgent(
+        api_key="test",
+        base_url="https://openrouter.ai/api/v1",
+        quiet_mode=True,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+
+    # 1. Cache attribute exists and equals KANBAN_GUIDANCE for workers.
+    assert hasattr(a, "_kanban_worker_guidance")
+    assert a._kanban_worker_guidance == KANBAN_GUIDANCE
+
+    # 2. The system-prompt build consumes the cache, not the live tool set.
+    #    If we remove ``kanban_show`` from valid_tool_names AFTER init, the
+    #    cached guidance still drives the prompt — proving the membership
+    #    test isn't re-run per build.
+    a.valid_tool_names = a.valid_tool_names - {"kanban_show"}
+    prompt = a._build_system_prompt()
+    assert "Kanban task execution protocol" in prompt
+
+
+def test_kanban_guidance_cache_empty_for_normal_chat(monkeypatch, tmp_path):
+    """Normal chat sessions (no HERMES_KANBAN_TASK) must have an empty
+    ``_kanban_worker_guidance`` cache so we never accidentally inject the
+    worker protocol into a non-worker prompt."""
+    monkeypatch.delenv("HERMES_KANBAN_TASK", raising=False)
+    home = tmp_path / ".hermes"
+    home.mkdir()
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    from pathlib import Path as _P
+    monkeypatch.setattr(_P, "home", lambda: tmp_path)
+
+    from run_agent import AIAgent
+    a = AIAgent(
+        api_key="test",
+        base_url="https://openrouter.ai/api/v1",
+        quiet_mode=True,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+    assert a._kanban_worker_guidance == ""
+
+
 # ---------------------------------------------------------------------------
 # Worker task-ownership enforcement (regression tests for #19534)
 # ---------------------------------------------------------------------------