diff --git a/backend/apps/agents/manager/run/client_pool.py b/backend/apps/agents/manager/run/client_pool.py
index 1f281527..3559ff84 100644
--- a/backend/apps/agents/manager/run/client_pool.py
+++ b/backend/apps/agents/manager/run/client_pool.py
@@ -24,7 +24,7 @@ from backend.apps.agents.core.models import AgentSession
 logger = logging.getLogger(__name__)
 
 # Options entries that are per-turn or non-serializable; everything else is boot-frozen and hashed.
-P_NON_BOOT_KEYS = frozenset({"can_use_tool", "stderr", "hooks", "resume", "fork_session"})
+NON_BOOT_KEYS = frozenset({"can_use_tool", "stderr", "hooks", "resume", "fork_session"})
 
 
 def persistent_client_enabled() -> bool:
@@ -48,7 +48,7 @@ def boot_fingerprint(options_kwargs: Dict, session: AgentSession) -> str:
     (needs_fresh_session / needs_fork / fork) already forces a respawn through force_respawn. Hashing
     it bought nothing and cost a full CLI respawn on EVERY turn once a session crossed
     compact_threshold_pct, measured at +1.0s TTFT per turn."""
-    frozen = {k: v for k, v in options_kwargs.items() if k not in P_NON_BOOT_KEYS}
+    frozen = {k: v for k, v in options_kwargs.items() if k not in NON_BOOT_KEYS}
     frozen["p_branch"] = session.active_branch_id
     # Pool diagnostics (OPENSWARM_POOL_DIAG=1): on a respawn, names WHICH boot field drifted; the tool for debugging respawn churn (e.g. the thinking short/long-prompt flip) in the field.
     if os.environ.get("OPENSWARM_POOL_DIAG") == "1":
diff --git a/backend/tests/test_boot_fingerprint_is_deterministic.py b/backend/tests/test_boot_fingerprint_is_deterministic.py
new file mode 100644
index 00000000..a8a332dc
--- /dev/null
+++ b/backend/tests/test_boot_fingerprint_is_deterministic.py
@@ -0,0 +1,112 @@
+"""The boot fingerprint must be a function of CONFIG, never of object identity (ENG-179).
+
+`boot_fingerprint` serialises with `json.dumps(..., default=str)`. Anything not JSON-serialisable
+therefore stringifies to its repr, and a default repr carries a memory address:
+
+    <backend.foo.Thing object at 0x10b155310>
+
+Two equivalent instances then hash differently, so a session whose config never changed still fails
+the pool check and pays a full CLI respawn (measured elsewhere at 0.58s). Today every such value is
+excluded by `NON_BOOT_KEYS`, so the bug is latent, not live. It becomes live the moment someone
+adds a callable, a client, or a dataclass-without-repr to the options and does not think about that
+frozenset, which is exactly the kind of thing nobody thinks about.
+
+This is the seal: the hashed blob may not contain an object address, and equivalent configs must
+hash equal. Both are checked against the REAL options shape rather than a toy.
+
+Run:
+    backend/.venv/bin/python -m pytest backend/tests/test_boot_fingerprint_is_deterministic.py -v
+"""
+
+import json
+import re
+from typing import Any, Dict
+
+from backend.apps.agents.core.models import AgentSession
+from backend.apps.agents.manager.run.client_pool import NON_BOOT_KEYS, boot_fingerprint
+
+# The address pattern a default repr leaves behind.
+P_ADDRESS = re.compile(r" at 0x[0-9a-f]+", re.I)
+
+
+class P_Opaque:
+    """No __repr__, so str() yields an address. Stands in for a callback, client or hook."""
+
+    def __init__(self, value: int) -> None:
+        self.value = value
+
+
+def p_session() -> AgentSession:
+    return AgentSession(name="probe", model="opus", cwd="/tmp")
+
+
+def p_realistic_options() -> Dict[str, Any]:
+    """The shape RunOptions actually builds, opaque values included."""
+    return {
+        "model": "cc/claude-opus-4-5",
+        "max_buffer_size": 64 * 1024 * 1024,
+        "permission_mode": "default",
+        "allowed_tools": ["Read", "Bash", "Edit"],
+        "disallowed_tools": ["Cron"],
+        "include_partial_messages": True,
+        "env": {"CLAUDE_CODE_DISABLE_AUTO_MEMORY": "1"},
+        "mcp_servers": {"openswarm-core": {"env": {"OSW_MCP_MODULES": "ui,memory"}}},
+        "tools": ["Read", "Write"],
+        # The excluded ones, present exactly as the real dict has them.
+        "can_use_tool": P_Opaque(1),
+        "stderr": P_Opaque(2),
+        "hooks": {"PreToolUse": [P_Opaque(3)]},
+        "resume": "abc",
+        "fork_session": False,
+    }
+
+
+def test_equivalent_configs_hash_equal_with_fresh_opaque_instances() -> None:
+    """The bug, stated directly: same config, new callback objects, must be the same fingerprint."""
+    s = p_session()
+    a, b = p_realistic_options(), p_realistic_options()
+    assert a["can_use_tool"] is not b["can_use_tool"], "the fixture must use DISTINCT instances"
+    assert boot_fingerprint(a, s) == boot_fingerprint(b, s), (
+        "identical config hashed differently because an opaque value reached the blob; every turn "
+        "would pay a CLI respawn"
+    )
+
+
+def test_the_hashed_blob_contains_no_object_address() -> None:
+    """Directly assert the property, so a future non-serialisable option is caught at the source."""
+    s = p_session()
+    opts = p_realistic_options()
+    frozen = {k: v for k, v in opts.items() if k not in NON_BOOT_KEYS}
+    frozen["p_branch"] = s.active_branch_id
+    blob = json.dumps(frozen, sort_keys=True, default=str)
+    found = P_ADDRESS.findall(blob)
+    assert not found, (
+        f"the fingerprint blob carries {len(found)} object address(es) {found[:3]}; that value is "
+        "identity-dependent, so equivalent configs will hash differently and respawn the CLI"
+    )
+
+
+def test_a_real_config_change_still_changes_the_fingerprint() -> None:
+    """The other direction: a fingerprint that never changes would silently reuse a stale CLI."""
+    s = p_session()
+    base = p_realistic_options()
+    for key, changed in (
+        ("model", "cc/claude-haiku-4-5"),
+        ("allowed_tools", ["Read"]),
+        ("mcp_servers", {"openswarm-core": {"env": {"OSW_MCP_MODULES": "ui"}}}),
+        ("env", {"CLAUDE_CODE_DISABLE_AUTO_MEMORY": "0"}),
+    ):
+        other = p_realistic_options()
+        other[key] = changed
+        assert boot_fingerprint(base, s) != boot_fingerprint(other, s), (
+            f"changing {key} did not change the fingerprint, so a stale CLI would be reused"
+        )
+
+
+def test_the_excluded_set_is_what_keeps_it_clean() -> None:
+    """Names the actual mechanism, so deleting an entry from NON_BOOT_KEYS fails here loudly."""
+    for key in ("can_use_tool", "stderr", "hooks"):
+        assert key in NON_BOOT_KEYS, (
+            f"{key} holds a non-serialisable value; excluding it is the only reason the blob has no "
+            "object addresses"
+        )