diff --git a/CHANGELOG.md b/CHANGELOG.md
index 97944c7e..0743df6e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@
 
 - **No more false "No response from provider" after a completed answer that used tools.** When an assistant turn ended with a real final answer delivered as structured content following tool calls (common with reasoning models and multi-tool turns), the completion detector could miss the final text and render a spurious "No response from provider" card under the finished reply. Final-answer detection now understands structured content parts (text after tool-use boundaries), while genuine empty completions still correctly show the no-response notice, and a real terminal provider error still surfaces as an error (never masked as a final answer). Thanks @jtstothard. (#5990, #5686)
 
+- **Custom/proxy providers keep the full vendor namespace when you deliberately pick a non-default model (follow-up to the earlier #5979 fix).** The initial fix preserved `x-ai/grok-4.5` when it was your configured default or already in a warm model catalog, but a model you *selected in the picker* that wasn't your default (e.g. `x-ai/grok-composer-2.5-fast`) was still truncated to the bare `grok-composer-2.5-fast` when the models catalog was cold at send time — the normal state right after a restart, a settings save, or a cache expiry. The proxy then rejected the bare id with `HTTP 400`. For a `custom` / `custom:<slug>` proxy the resolver now remembers that you deliberately selected the model this session and preserves the full `vendor/model` id on a cold catalog, while a stale cross-provider leftover you didn't pick still has its redundant prefix stripped so it keeps routing. The send path also warms model-catalog provenance from the durable on-disk cache first (network-free, non-blocking, never triggers a live probe). Thanks @b3nw for the detailed follow-up and the CLI-vs-WebUI comparison that localized it. (#5979)
+
 - **Cron unread ("Tasks") dots are now scoped to the active profile — no more ghost dots after switching.** In multi-profile setups, the session-sidebar unread dot and the Tasks badge could keep showing a cron completion from a *different* profile after you switched, and the count could resurrect from a stale poll. Cron completion markers now carry their source + owning profile, are cleared for the profile you switch away from (including legacy/untagged markers), and a previous-profile poll response can no longer restore a stale unread. Non-cron completion unread and the current profile's own cron unread are preserved; default ↔ renamed-root aliases are treated as the same profile so a root cron marker is neither wrongly cleared nor wrongly kept. Thanks @jbbottoms. (#5975, #5960)
 
 - **The Docker "manual update" banner instruction is now localized.** On Docker/no-git deployments the update banner shows the manual update command; that guidance string was hardcoded in English. It now goes through the i18n system, so it renders in the active language (English fallback where a translation isn't present yet). Thanks @jbbottoms. (#5973, #5959)
diff --git a/api/config.py b/api/config.py
index 8e6ac076..7bba9db8 100644
--- a/api/config.py
+++ b/api/config.py
@@ -2560,7 +2560,7 @@ def _get_provider_cfg(provider_id) -> dict:
     return provider_cfg if isinstance(provider_cfg, dict) else {}
 
 
-def resolve_model_provider(model_id: str) -> tuple:
+def resolve_model_provider(model_id: str, *, explicitly_picked: bool = False) -> tuple:
     """Resolve model name, provider, and base_url for AIAgent.
 
     Model IDs from the dropdown can be in several formats:
@@ -2580,6 +2580,16 @@ def resolve_model_provider(model_id: str) -> tuple:
     provider.
 
     Returns (model, provider, base_url) where provider and base_url may be None.
+
+    ``explicitly_picked``: True when the caller knows the user DELIBERATELY
+    selected ``model_id`` this session (persisted from an ``explicit_model_pick``
+    UI action), as opposed to it being a stale session leftover. Used ONLY for
+    the custom-proxy COLD-catalog decision (#5979): with no provenance available,
+    a deliberately-picked ``vendor/model`` is preserved verbatim (the user chose
+    it, the proxy routes on it), while an UNMARKED id (a stale cross-provider
+    leftover, e.g. #433's ``openai/gpt-5.4`` on a bare-only relay) still gets the
+    legacy redundant-prefix strip so it keeps routing when cold. Warm provenance
+    (endpoint-advertised ids) always takes precedence over this flag.
     """
     config_provider = None
     config_base_url = None
@@ -2898,13 +2908,30 @@ def resolve_model_provider(model_id: str) -> tuple:
                     return model_id, config_provider, config_base_url
                 # (3) Provenance genuinely unavailable (cold/unbuilt or
                 #     fingerprint-mismatched catalog AND not config-declared).
-                #     Fall back to the LEGACY family heuristic so this narrow edge
-                #     is never WORSE than the pre-fix behaviour: a first-party
-                #     redundant prefix still strips (#433 relay works cold), while
-                #     an intrinsic/unknown prefix (bedrock/opus-4-6 #3872,
-                #     zai-org/GLM-5.1 #548) is preserved. The #5979 active-user
-                #     path can't reach here — a selected id is either config-
-                #     declared or in the catalog the dropdown was built from.
+                #     Distinguish a DELIBERATE selection from a stale leftover:
+                #
+                #     * explicitly_picked → PRESERVE verbatim. The user chose this
+                #       exact ``vendor/model`` in the UI this session; the proxy
+                #       routes on it. A wrong strip destroys a namespace the proxy
+                #       needs (recurs every turn, unrepairable short of declaring
+                #       every model in config) — this is b3nw's #5979 case: a
+                #       non-default pick on a custom:<slug> proxy, cold catalog.
+                #     * NOT explicitly_picked → legacy redundant-prefix strip. An
+                #       unmarked id here is a stale cross-provider leftover (the
+                #       user switched providers and the old session model lingers,
+                #       e.g. #433's ``openai/gpt-5.4`` on a relay that only serves
+                #       bare ``gpt-5.4``); stripping keeps it routing while cold.
+                #
+                #     Warm provenance (case 2, endpoint-advertised ids) always
+                #     wins over this flag; the send path also warms provenance
+                #     network-free from the disk cache first
+                #     (warm_models_catalog_provenance_if_cold), so this branch is
+                #     reached only in the narrow no-disk-cache window. The flag
+                #     removes the data-driven flaw where a model graduating into
+                #     the static first-party catalog silently flipped routing
+                #     (exactly how #5979 regressed).
+                if explicitly_picked:
+                    return model_id, config_provider, config_base_url
                 if prefix in _PROVIDER_MODELS and _is_first_party_model(prefix, bare):
                     return bare, config_provider, config_base_url
                 return model_id, config_provider, config_base_url
@@ -8162,6 +8189,82 @@ def _models_cache_file_age_seconds(cache_path: Path, now: float) -> float | None
         return None
 
 
+def warm_models_catalog_provenance_if_cold() -> None:
+    """Best-effort, NON-BLOCKING, disk-only publish of catalog provenance.
+
+    The send path (``api/streaming.py``) resolves the wire model via
+    ``resolve_model_provider`` without ever building the models catalog, and the
+    #1855 chat/start fast path deliberately skips the catalog when a session
+    already carries a persisted model+provider — so "cold at send" is the
+    designed behaviour after any process restart, memory-TTL expiry, or cache
+    invalidation, not a rare race. In that state the custom-proxy provenance
+    signal (``_endpoint_advertised_model_ids``) is ``None`` and resolution falls
+    to the cold-preserve default; this helper restores the endpoint-advertised
+    signal from the durable disk cache so the #433 bare-only-strip stays exact.
+
+    Deliberately does NOT call ``get_available_models(prefer_cache=True)``: even
+    in prefer-cache mode that acquires ``_available_models_cache_lock`` and can
+    block up to ~60s waiting on an in-flight rebuild (unbounded in synchronous
+    rebuild mode) — unacceptable on the send hot path. Instead this:
+      * tries the cache lock NON-BLOCKING and returns immediately if it's busy
+        (a concurrent rebuild will publish provenance itself);
+      * reads ONLY the on-disk cache (no network, no live probe, no rebuild);
+      * publishes the snapshot + source fingerprint via the same globals the
+        real publish sites use, then ``_sync_models_cache_provenance()``.
+    Publishing the fingerprint from the CURRENT runtime is correct: the disk
+    cache is validated by schema/version/source-fingerprint on load
+    (``_is_loadable_disk_cache``), so a load success means it belongs to this
+    profile. Callers must not hold ``_cfg_lock`` (this reads config for the
+    fingerprint); the send worker satisfies that.
+
+    Profile isolation: the fast no-op is taken ONLY when the resident provenance
+    fingerprint matches the CURRENT profile's runtime fingerprint. The catalog
+    globals are process-wide, so a concurrently-active profile B could have left
+    its own (or a stale) provenance resident; an unconditional non-``None``
+    early return would let B's catalog block profile A from loading A's own valid
+    disk cache (A would then resolve against B's advertised ids). Comparing the
+    published fingerprint to the current one before short-circuiting closes that
+    hole — a mismatch falls through to load THIS profile's disk snapshot.
+    """
+    global _available_models_cache, _available_models_cache_ts
+    global _available_models_cache_source_fingerprint
+
+    def _provenance_is_current() -> bool:
+        prov = _models_cache_provenance
+        if prov is None:
+            return False
+        try:
+            return prov[1] == _models_cache_source_fingerprint()
+        except Exception:
+            return False
+
+    if _provenance_is_current():
+        return  # already warm for THIS profile — one global read, no work
+    got = _available_models_cache_lock.acquire(blocking=False)
+    if not got:
+        return  # a concurrent build/publish holds the lock; it will publish
+    try:
+        if _provenance_is_current():
+            return  # published for this profile while we waited for the lock
+        try:
+            disk_groups = _load_models_cache_from_disk()
+        except Exception:
+            disk_groups = None
+        if disk_groups is None:
+            return  # no durable cache for this profile → stay cold, preserve verbatim
+        _available_models_cache = disk_groups
+        _available_models_cache_ts = time.monotonic()
+        try:
+            _available_models_cache_source_fingerprint = _models_cache_source_fingerprint()
+        except Exception:
+            _available_models_cache_source_fingerprint = None
+        _sync_models_cache_provenance()
+    except Exception:
+        logger.debug("models catalog provenance warm failed", exc_info=True)
+    finally:
+        _available_models_cache_lock.release()
+
+
 def get_available_models_for_session_visit() -> dict:
     """Return /api/models with a short session-visit freshness horizon.
 
diff --git a/api/models.py b/api/models.py
index cd78fa13..8ecbbc0c 100644
--- a/api/models.py
+++ b/api/models.py
@@ -1081,6 +1081,21 @@ def _parse_nonnegative_int(value):
     return parsed if parsed >= 0 else None
 
 
+def model_explicit_pick_signature(model, model_provider) -> str:
+    """Stable signature of a (model, provider) selection for #5979 explicit-pick
+    provenance. The persisted ``Session.model_explicit_pick_signature`` is set to
+    this when the user deliberately picks a model; the streaming resolver only
+    treats a selection as deliberate when the CURRENT routing context produces
+    the same signature. Any model/provider change (chat-start, session-update,
+    normalization, provider repair) yields a different signature and thus
+    invalidates the stale pick — so a #433 first-party leftover is never wrongly
+    preserved. Uses \\x1f (unit separator) so it can't collide with model ids.
+    """
+    _m = str(model or "").strip()
+    _p = str(model_provider or "").strip().lower()
+    return f"{_m}\x1f{_p}"
+
+
 class Session:
     def __init__(self, session_id: str=None, title: str='Untitled',
                  workspace=str(DEFAULT_WORKSPACE), model=DEFAULT_MODEL,
@@ -1136,6 +1151,16 @@ class Session:
         self.workspace = str(Path(workspace).expanduser().resolve())
         self.model = model
         self.model_provider = str(model_provider).strip().lower() if model_provider else None
+        # #5979: signature of the model the user DELIBERATELY picked this session
+        # (``"<model>\x1f<provider>"``), or None. Used by the streaming resolver
+        # to preserve a custom-proxy vendor namespace on a COLD catalog ONLY when
+        # the current routing context still matches what was picked. Storing a
+        # SIGNATURE (not a bare bool) means any later model/provider change — via
+        # /api/chat/start, /api/session/update, normalization, or provider repair
+        # — automatically invalidates the pick (the signatures no longer match),
+        # so a stale first-party leftover (#433) is never wrongly preserved.
+        # Restored from persisted metadata on load (arrives via **kwargs).
+        self.model_explicit_pick_signature = kwargs.get('model_explicit_pick_signature') or None
         self.messages = messages or []
         self.tool_calls = tool_calls or []
         self.created_at = created_at or time.time()
@@ -1255,7 +1280,7 @@ class Session:
         # without parsing the full messages array (which may be 400KB+).
         # Fields are listed in the order they should appear in the JSON file.
         METADATA_FIELDS = [
-            'session_id', 'title', 'workspace', 'model', 'model_provider', 'created_at', 'updated_at',
+            'session_id', 'title', 'workspace', 'model', 'model_provider', 'model_explicit_pick_signature', 'created_at', 'updated_at',
             'pinned', 'archived', 'project_id', 'profile',
             'input_tokens', 'output_tokens', 'estimated_cost',
             'cache_read_tokens', 'cache_write_tokens',
diff --git a/api/routes.py b/api/routes.py
index 999b8caa..9d4685c1 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -21680,6 +21680,20 @@ def _handle_chat_start(handler, body, diag=None):
             profile_config=_pp_cfg,
             explicit_model_pick=explicit_model_pick,
         )
+        # #5979: record a SIGNATURE of the deliberately-picked model+provider so
+        # the streaming resolver can preserve a custom-proxy vendor namespace on a
+        # cold catalog — but ONLY while the routing context still matches. On a
+        # fresh explicit pick, stamp the signature of the resolved model+provider;
+        # otherwise leave any prior signature in place (it self-invalidates when
+        # the model/provider changes, since the streaming side recomputes and
+        # compares). This survives same-model follow-up sends (the onchange marker
+        # is one-shot) yet can't outlive a real switch.
+        try:
+            if explicit_model_pick:
+                from api.models import model_explicit_pick_signature as _mk_sig
+                s.model_explicit_pick_signature = _mk_sig(model, model_provider)
+        except Exception:
+            pass
         catalog_profile_provider = _pp_provider
         if catalog_profile_provider is None and isinstance(_pp_cfg, dict):
             profile_model_config = _pp_cfg.get("model") or {}
diff --git a/api/streaming.py b/api/streaming.py
index 4bc663e1..cdc0e5d8 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -37,6 +37,7 @@ from api.config import (
     resolve_model_provider,
     resolve_custom_provider_connection,
     model_with_provider_context,
+    warm_models_catalog_provenance_if_cold,
     load_settings,
     parse_reasoning_effort,
     coerce_reasoning_effort_for_model,
@@ -7890,9 +7891,46 @@ def _run_agent_streaming(
             # Initialize SessionDB so session_search works in WebUI sessions
             _state_db_path = (Path(_profile_home) / "state.db") if _profile_home else None
             _session_db = _build_session_db_for_stream(_state_db_path)
-            resolved_model, resolved_provider, resolved_base_url = resolve_model_provider(
-                model_with_provider_context(model, provider_context)
-            )
+            # #5979: publish catalog provenance from the durable disk cache when
+            # memory is cold, so the custom-proxy resolver below sees the
+            # endpoint-advertised model ids (non-blocking, disk-only, never
+            # live-rebuilds). Both the warm and the resolve read profile-keyed
+            # config (cache path + source fingerprint via get_active_profile_name),
+            # but this streaming worker is a separate thread that does NOT inherit
+            # the HTTP handler's request-profile TLS — without binding it, a cold
+            # send from a NAMED profile would resolve against the DEFAULT profile's
+            # config and route to the wrong provider/base_url. Bind the captured
+            # owning-session profile across warm + resolve so both see the right
+            # profile (no-op for the default/root profile).
+            from api import profiles as profiles_api
+            # #5979: treat this send as a deliberate pick ONLY when the persisted
+            # explicit-pick signature matches the CURRENT model+provider routing
+            # context. Storing/comparing a signature (not a bare bool) means a
+            # later model/provider change via /api/chat/start, /api/session/update,
+            # normalization, or provider repair automatically invalidates a stale
+            # pick — so a #433 first-party leftover is never wrongly preserved on
+            # a cold catalog. Only affects the cold custom-proxy branch; warm
+            # endpoint-advertised provenance always wins over this flag.
+            from api.models import model_explicit_pick_signature as _mk_sig
+            _picked_sig = getattr(s, "model_explicit_pick_signature", None)
+            # Compare against the session's persisted model+provider — the exact
+            # fields /api/chat/start stamped the signature from (it persists the
+            # resolved model+provider onto the session before dispatch). Falls
+            # back to the worker's model/provider_context if the session fields
+            # are unset. A mismatch (any later model/provider change) yields a
+            # different signature → treated as NOT a deliberate pick.
+            _sig_model = getattr(s, "model", None) or model
+            _sig_provider = getattr(s, "model_provider", None) or provider_context
+            _current_sig = _mk_sig(_sig_model, _sig_provider)
+            _explicitly_picked = bool(_picked_sig) and _picked_sig == _current_sig
+            with profiles_api.profile_scope_for_detached_worker(
+                _resolved_profile_name, "model resolution", logger_override=logger
+            ):
+                warm_models_catalog_provenance_if_cold()
+                resolved_model, resolved_provider, resolved_base_url = resolve_model_provider(
+                    model_with_provider_context(model, provider_context),
+                    explicitly_picked=_explicitly_picked,
+                )
             configured_base_url = resolved_base_url
 
             # Resolve API key via Hermes runtime provider (matches gateway behaviour).
diff --git a/tests/test_issue1897_profile_switch_agent_cache.py b/tests/test_issue1897_profile_switch_agent_cache.py
index 97190f66..7c6782f7 100644
--- a/tests/test_issue1897_profile_switch_agent_cache.py
+++ b/tests/test_issue1897_profile_switch_agent_cache.py
@@ -156,7 +156,7 @@ def test_same_session_profile_switch_rebuilds_agent_under_new_soul_home(tmp_path
     monkeypatch.setattr(
         streaming,
         "resolve_model_provider",
-        lambda _model: ("test-model", "test-provider", None),
+        lambda _model, **_kw: ("test-model", "test-provider", None),
     )
     monkeypatch.setattr(streaming, "_maybe_schedule_title_refresh", lambda *args, **kwargs: None)
     monkeypatch.setattr(profiles, "get_hermes_home_for_profile", home_for_profile)
diff --git a/tests/test_issue2965_streaming_sessiondb_profile_home.py b/tests/test_issue2965_streaming_sessiondb_profile_home.py
index 7988792c..520eca3e 100644
--- a/tests/test_issue2965_streaming_sessiondb_profile_home.py
+++ b/tests/test_issue2965_streaming_sessiondb_profile_home.py
@@ -129,7 +129,7 @@ def test_streaming_sessiondb_uses_session_profile_state_db(tmp_path, monkeypatch
     monkeypatch.setattr(
         streaming,
         "resolve_model_provider",
-        lambda _model: ("test-model", "test-provider", None),
+        lambda _model, **_kw: ("test-model", "test-provider", None),
     )
     monkeypatch.setattr(streaming, "_maybe_schedule_title_refresh", lambda *args, **kwargs: None)
     monkeypatch.setattr(profiles, "get_hermes_home_for_profile", lambda _profile: profile_home)
diff --git a/tests/test_model_resolver.py b/tests/test_model_resolver.py
index 5c67d1e8..165e4d3c 100644
--- a/tests/test_model_resolver.py
+++ b/tests/test_model_resolver.py
@@ -7,7 +7,8 @@ import pytest
 import api.config as config
 
 
-def _resolve_with_config(model_id, provider=None, base_url=None, default=None, custom_providers=None):
+def _resolve_with_config(model_id, provider=None, base_url=None, default=None, custom_providers=None,
+                         explicitly_picked=False):
     """Helper: temporarily set config.cfg model/custom provider sections, call resolve, restore."""
     old_cfg = dict(config.cfg)
     model_cfg = {}
@@ -21,14 +22,14 @@ def _resolve_with_config(model_id, provider=None, base_url=None, default=None, c
     if custom_providers is not None:
         config.cfg['custom_providers'] = custom_providers
     try:
-        return config.resolve_model_provider(model_id)
+        return config.resolve_model_provider(model_id, explicitly_picked=explicitly_picked)
     finally:
         config.cfg.clear()
         config.cfg.update(old_cfg)
 
 
 def _resolve_with_catalog(model_id, advertised_ids, *, provider=None, base_url=None,
-                          provider_id='custom', default=None):
+                          provider_id='custom', default=None, explicitly_picked=False):
     """Resolve with a seeded models-catalog snapshot (#5979 provenance).
 
     ``advertised_ids`` is the list of model ids the endpoint's own group
@@ -57,7 +58,8 @@ def _resolve_with_catalog(model_id, advertised_ids, *, provider=None, base_url=N
     config._advertised_model_ids_memo = None  # force recompute against the seeded snapshot
     config._sync_models_cache_provenance()  # publish the atomic (snapshot, fingerprint) pair
     try:
-        return _resolve_with_config(model_id, provider=provider, base_url=base_url, default=default)
+        return _resolve_with_config(model_id, provider=provider, base_url=base_url, default=default,
+                                    explicitly_picked=explicitly_picked)
     finally:
         config._available_models_cache = old_cache
         config._advertised_model_ids_memo = old_memo
@@ -300,28 +302,36 @@ def test_named_custom_slug_preserves_advertised_full_id_5979():
     assert model == 'x-ai/grok-4.5', f"full id must be preserved for custom:slug, got {model!r}"
 
 
-def test_custom_remote_cold_catalog_falls_back_to_legacy_heuristic_5979():
-    """#5979 tri-state: with a COLD/unbuilt catalog AND no config declaration
-    (no provenance at all), resolution falls back to the LEGACY family heuristic
-    so this narrow edge is never worse than the pre-fix behaviour.
+def test_custom_remote_cold_catalog_explicit_pick_preserves_5979():
+    """#5979 (reopened): with a COLD catalog and no config declaration, an
+    EXPLICITLY-PICKED custom-proxy id is preserved verbatim (the user chose it;
+    the proxy routes on it), while an UNMARKED id gets the legacy strip.
 
-    A first-party redundant prefix still strips (the #433 relay keeps working
-    cold), while an intrinsic/unknown prefix is preserved. The #5979 active-user
-    path never reaches this branch — a selected id is either config-declared
-    (see the config-declared test) or in the catalog the dropdown was built from.
+    This is the persisted-explicit-pick resolution (Codex's mechanism, Nathan's
+    call): the cold decision is no longer an unconditional preserve — it is
+    gated on whether the user deliberately selected the model this session.
     """
-    # first-party redundant prefix → strip (matches pre-fix / #433 cold path)
-    model_a, _, _ = _resolve_with_catalog(
+    # explicitly picked → PRESERVE even though grok graduated into first-party x-ai
+    picked, _, _ = _resolve_with_catalog(
         'openai/gpt-5.4', advertised_ids=None,
         provider='custom', base_url='https://router.example.com/v1',
+        explicitly_picked=True,
     )
-    assert model_a == 'gpt-5.4', f"cold first-party prefix must strip (legacy), got {model_a!r}"
-    # intrinsic/unknown prefix → preserve
-    model_b, _, _ = _resolve_with_catalog(
+    assert picked == 'openai/gpt-5.4', f"explicit pick must preserve cold, got {picked!r}"
+    # NOT picked (stale leftover) → legacy strip keeps #433 relay routing cold
+    stale, _, _ = _resolve_with_catalog(
+        'openai/gpt-5.4', advertised_ids=None,
+        provider='custom', base_url='https://router.example.com/v1',
+        explicitly_picked=False,
+    )
+    assert stale == 'gpt-5.4', f"unmarked stale id must strip cold (legacy #433), got {stale!r}"
+    # intrinsic/unknown prefix → preserve regardless (no first-party family match)
+    unknown, _, _ = _resolve_with_catalog(
         'zai-org/GLM-5.1', advertised_ids=None,
         provider='custom', base_url='https://api.deepinfra.com/v1/openai',
+        explicitly_picked=False,
     )
-    assert model_b == 'zai-org/GLM-5.1', f"cold unknown prefix must preserve, got {model_b!r}"
+    assert unknown == 'zai-org/GLM-5.1', f"cold unknown prefix must preserve, got {unknown!r}"
 
 
 def test_custom_remote_config_declared_full_id_preserved_cold_5979():
@@ -405,36 +415,40 @@ def test_custom_remote_foreign_profile_catalog_ignored_5979():
     """Profile-isolation fail-safe: when the catalog snapshot's source
     fingerprint does NOT match the current runtime (a concurrently-active
     foreign profile published it), that snapshot is NOT trusted for provenance —
-    resolution falls back to the legacy family heuristic instead of stripping
-    against another profile's catalog.
+    the id is preserved verbatim instead of stripped against another profile's
+    catalog.
 
-    Proof id: ``zai-org/GLM-5.1``. The foreign snapshot advertises a bare
-    ``GLM-5.1`` (which, if trusted, would strip the prefix), but ``zai-org`` is
-    NOT a first-party provider, so the legacy fallback preserves the full id.
-    A result of ``zai-org/GLM-5.1`` therefore proves the foreign catalog was
-    ignored (a trusted-catalog strip would have returned ``GLM-5.1``).
+    Proof id: ``openai/gpt-5.4`` with the FOREIGN catalog advertising ONLY the
+    bare ``gpt-5.4``, resolved as an EXPLICIT pick. If the foreign catalog were
+    (wrongly) trusted, the bare-only-advertised rule (warm provenance, which
+    wins over the pick flag) would strip → ``gpt-5.4``. Because the fingerprint
+    mismatches, the snapshot is ignored and resolution takes the cold branch;
+    with the explicit-pick flag set that preserves → ``openai/gpt-5.4``. The two
+    outcomes genuinely diverge, so ``openai/gpt-5.4`` proves the foreign catalog
+    was ignored.
     """
     old_cache = config._available_models_cache
     old_memo = config._advertised_model_ids_memo
     old_fp = config._available_models_cache_source_fingerprint
     old_prov = config._models_cache_provenance
     config._available_models_cache = {
-        'groups': [{'provider_id': 'custom', 'models': [{'id': 'GLM-5.1', 'label': 'GLM-5.1'}]}]
+        'groups': [{'provider_id': 'custom', 'models': [{'id': 'gpt-5.4', 'label': 'gpt-5.4'}]}]
     }
     config._available_models_cache_source_fingerprint = {'config_yaml': {'path': '/some/other/profile'}}
     config._advertised_model_ids_memo = None
     config._sync_models_cache_provenance()
     try:
         model, _, _ = _resolve_with_config(
-            'zai-org/GLM-5.1', provider='custom', base_url='https://relay.example/v1',
+            'openai/gpt-5.4', provider='custom', base_url='https://relay.example/v1',
+            explicitly_picked=True,
         )
     finally:
         config._available_models_cache = old_cache
         config._advertised_model_ids_memo = old_memo
         config._available_models_cache_source_fingerprint = old_fp
         config._models_cache_provenance = old_prov
-    assert model == 'zai-org/GLM-5.1', (
-        f"foreign-profile catalog must be ignored (legacy fallback preserves), got {model!r}"
+    assert model == 'openai/gpt-5.4', (
+        f"foreign-profile catalog must be ignored (cold explicit-pick preserve), got {model!r}"
     )
 
 
@@ -484,6 +498,242 @@ def test_resolver_provenance_read_does_not_block_on_cache_lock_5979():
         config.invalidate_models_cache()
 
 
+def test_b3nw_cold_nondeclared_custom_slug_preserves_full_id_5979():
+    """#5979 (reopened, b3nw's exact case): a NON-declared model actively
+    selected on a ``custom:<slug>`` proxy is preserved on a COLD catalog.
+
+    b3nw's config: ``provider: custom:llm-proxy``, ``model.default: x-ai/grok-4.5``,
+    NO ``models[]`` allowlist. He picks ``x-ai/grok-composer-2.5-fast`` in the UI
+    (not his default). On a cold send the old code stripped it to
+    ``grok-composer-2.5-fast`` via the legacy family heuristic (the model had
+    graduated into the x-ai first-party catalog), and his proxy 400'd. The flip
+    to cold-preserve fixes it. grok-composer-2.5-fast is stubbed into
+    ``_PROVIDER_MODELS['x-ai']`` (as test_custom_remote_preserves_advertised_full_id_5979
+    does) so the first-party trigger is reproduced deterministically regardless
+    of the agent catalog version.
+    """
+    mid = 'x-ai/grok-composer-2.5-fast'
+    bare = 'grok-composer-2.5-fast'
+    xai = list(config._PROVIDER_MODELS.get('x-ai') or [])
+    had = any(isinstance(m, dict) and m.get('id') == bare for m in xai)
+    old_xai = config._PROVIDER_MODELS.get('x-ai')
+    old_cfg = dict(config.cfg)
+    old_prov = config._models_cache_provenance
+    old_cache = config._available_models_cache
+    if not had:
+        config._PROVIDER_MODELS['x-ai'] = xai + [{'id': bare, 'label': 'Grok Composer 2.5 Fast'}]
+    config.cfg.clear()
+    config.cfg.update({
+        'model': {'default': 'x-ai/grok-4.5', 'provider': 'custom:llm-proxy',
+                  'base_url': 'https://proxy.example/v1'},
+        'custom_providers': [{'name': 'llm-proxy', 'base_url': 'https://proxy.example/v1',
+                              'key_env': 'LLM_PROXY_API_KEY'}],
+    })
+    config._available_models_cache = None  # COLD
+    config._advertised_model_ids_memo = None
+    config._sync_models_cache_provenance()
+    try:
+        assert config._is_first_party_model('x-ai', bare), "precondition: stub failed"
+        model, provider, _ = config.resolve_model_provider(mid, explicitly_picked=True)
+    finally:
+        if not had:
+            if old_xai is None:
+                config._PROVIDER_MODELS.pop('x-ai', None)
+            else:
+                config._PROVIDER_MODELS['x-ai'] = old_xai
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+        config._models_cache_provenance = old_prov
+        config._available_models_cache = old_cache
+    assert model == mid, f"b3nw's non-declared cold custom-proxy pick must preserve, got {model!r}"
+    assert provider == 'custom:llm-proxy'
+
+
+def test_custom_slug_cold_stale_not_picked_still_strips_5979():
+    """Companion to b3nw's case: the SAME cold custom:<slug> shape, but the id is
+    NOT explicitly picked (a stale first-party leftover), still gets the legacy
+    strip — so #433-style stale sessions keep routing when cold and only
+    deliberate picks are preserved verbatim.
+    """
+    mid = 'x-ai/grok-composer-2.5-fast'
+    bare = 'grok-composer-2.5-fast'
+    xai = list(config._PROVIDER_MODELS.get('x-ai') or [])
+    had = any(isinstance(m, dict) and m.get('id') == bare for m in xai)
+    old_xai = config._PROVIDER_MODELS.get('x-ai')
+    old_cfg = dict(config.cfg)
+    old_prov = config._models_cache_provenance
+    old_cache = config._available_models_cache
+    if not had:
+        config._PROVIDER_MODELS['x-ai'] = xai + [{'id': bare, 'label': 'Grok Composer 2.5 Fast'}]
+    config.cfg.clear()
+    config.cfg.update({
+        'model': {'default': 'x-ai/grok-4.5', 'provider': 'custom:llm-proxy',
+                  'base_url': 'https://proxy.example/v1'},
+        'custom_providers': [{'name': 'llm-proxy', 'base_url': 'https://proxy.example/v1',
+                              'key_env': 'LLM_PROXY_API_KEY'}],
+    })
+    config._available_models_cache = None  # COLD
+    config._advertised_model_ids_memo = None
+    config._sync_models_cache_provenance()
+    try:
+        model, _, _ = config.resolve_model_provider(mid, explicitly_picked=False)
+    finally:
+        if not had:
+            if old_xai is None:
+                config._PROVIDER_MODELS.pop('x-ai', None)
+            else:
+                config._PROVIDER_MODELS['x-ai'] = old_xai
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+        config._models_cache_provenance = old_prov
+        config._available_models_cache = old_cache
+    assert model == bare, f"unmarked stale cold custom:slug id must strip (legacy), got {model!r}"
+
+
+def test_warm_models_catalog_provenance_if_cold_publishes_from_disk_5979():
+    """The send-path warm helper publishes provenance from a valid disk cache
+    when memory is cold — restoring the endpoint-advertised signal so #433
+    strips and #5979 preserves — WITHOUT a live rebuild.
+    """
+    old_cfg = dict(config.cfg)
+    config.cfg.clear()
+    config.cfg.update({
+        'model': {'default': 'x-ai/grok-4.5', 'provider': 'custom:llm-proxy',
+                  'base_url': 'https://proxy.example/v1'},
+        'custom_providers': [{'name': 'llm-proxy', 'base_url': 'https://proxy.example/v1',
+                              'key_env': 'LLM_PROXY_API_KEY',
+                              'models': ['x-ai/grok-4.5', 'x-ai/grok-composer-2.5-fast']}],
+    })
+    try:
+        # Build + persist to disk, then simulate a cold memory cache (restart).
+        config.invalidate_models_cache()
+        config.get_available_models()  # publishes to memory + disk
+        assert config._models_cache_provenance is not None
+        # Simulate cold memory but valid disk cache (do NOT delete disk).
+        config._available_models_cache = None
+        config._advertised_model_ids_memo = None
+        config._sync_models_cache_provenance()
+        assert config._models_cache_provenance is None, "precondition: memory cold"
+        # The warm helper should republish provenance from disk (network-free).
+        config.warm_models_catalog_provenance_if_cold()
+        assert config._models_cache_provenance is not None, (
+            "warm helper must publish provenance from the disk cache"
+        )
+        adv = config._endpoint_advertised_model_ids('custom:llm-proxy')
+        assert adv and 'x-ai/grok-composer-2.5-fast' in adv, (
+            f"warmed provenance must carry the endpoint-advertised ids, got {adv!r}"
+        )
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+        config.invalidate_models_cache()
+
+
+def test_warm_models_catalog_provenance_noop_when_already_warm_5979():
+    """The warm helper is a cheap no-op when provenance for the CURRENT profile
+    is already published — it must not reload or mutate a warm catalog.
+    """
+    # Use the CURRENT runtime fingerprint so the profile-match fast path is hit.
+    current_fp = config._models_cache_source_fingerprint()
+    sentinel = ({'groups': []}, current_fp)
+    old_prov = config._models_cache_provenance
+    config._models_cache_provenance = sentinel
+    try:
+        config.warm_models_catalog_provenance_if_cold()
+        assert config._models_cache_provenance is sentinel, (
+            "warm helper must not touch already-warm current-profile provenance"
+        )
+    finally:
+        config._models_cache_provenance = old_prov
+
+
+def test_warm_models_catalog_provenance_ignores_foreign_profile_resident_5979(monkeypatch):
+    """Profile isolation (Codex finding): a FOREIGN profile's resident provenance
+    must NOT satisfy the fast no-op. The helper falls through, loads THIS
+    profile's own disk snapshot, and republishes provenance stamped with the
+    current runtime fingerprint — so a concurrently-active profile's catalog can
+    never block this profile from warming its own.
+    """
+    foreign = ({'groups': [{'provider_id': 'custom', 'models': [{'id': 'gpt-5.4'}]}]},
+               {'config_yaml': {'path': '/some/other/profile'}})
+    my_disk = {'groups': [{'provider_id': 'custom:llm-proxy',
+                           'models': [{'id': 'x-ai/grok-composer-2.5-fast'}]}]}
+    old_prov = config._models_cache_provenance
+    old_cache = config._available_models_cache
+    old_fp = config._available_models_cache_source_fingerprint
+    old_memo = config._advertised_model_ids_memo
+    config._models_cache_provenance = foreign
+    monkeypatch.setattr(config, '_load_models_cache_from_disk', lambda: my_disk)
+    try:
+        config.warm_models_catalog_provenance_if_cold()
+        prov = config._models_cache_provenance
+        assert prov is not None and prov[0] is my_disk, (
+            "warm must replace foreign provenance with THIS profile's disk snapshot"
+        )
+        assert prov[1] == config._models_cache_source_fingerprint(), (
+            "republished provenance must carry the current profile's fingerprint"
+        )
+    finally:
+        config._models_cache_provenance = old_prov
+        config._available_models_cache = old_cache
+        config._available_models_cache_source_fingerprint = old_fp
+        config._advertised_model_ids_memo = old_memo
+
+
+def test_warm_models_catalog_provenance_never_live_rebuilds_5979(monkeypatch):
+    """The warm helper must NEVER call get_available_models (which can acquire
+    the cache lock and block on / trigger a rebuild). It reads the disk cache
+    directly, so a live provider probe is structurally impossible on the send
+    path.
+    """
+    old_prov = config._models_cache_provenance
+    config._models_cache_provenance = None
+    called = {'get_available_models': False}
+    def _boom(*a, **k):
+        called['get_available_models'] = True
+        raise AssertionError("warm must not call get_available_models")
+    monkeypatch.setattr(config, 'get_available_models', _boom)
+    try:
+        config.warm_models_catalog_provenance_if_cold()  # must not raise / call the above
+    finally:
+        config._models_cache_provenance = old_prov
+    assert called['get_available_models'] is False, (
+        "warm helper must read disk directly, never via get_available_models"
+    )
+
+
+def test_warm_models_catalog_provenance_nonblocking_when_lock_held_5979():
+    """The warm helper must NOT block on the models-cache lock. It tries the
+    lock non-blocking and returns immediately when a concurrent build/publish
+    holds it, so a send can never hang up to 60s behind a rebuild.
+    """
+    import threading
+    import time as _time
+    old_prov = config._models_cache_provenance
+    config._models_cache_provenance = None
+    got = config._available_models_cache_lock.acquire(blocking=False)
+    assert got, "precondition: could not take cache lock"
+    try:
+        result = {}
+        def _worker():
+            t0 = _time.time()
+            config.warm_models_catalog_provenance_if_cold()
+            result['elapsed'] = _time.time() - t0
+        th = threading.Thread(target=_worker)
+        th.start()
+        th.join(timeout=5)
+        blocked = th.is_alive()
+    finally:
+        config._available_models_cache_lock.release()
+        config._models_cache_provenance = old_prov
+    if blocked:
+        th.join(timeout=5)
+    assert not blocked, "warm helper blocked on a held cache lock (must be non-blocking)"
+    assert result.get('elapsed', 99) < 2.0, (
+        f"warm helper should return promptly when lock held, took {result.get('elapsed')!r}s"
+    )
+
+
 def test_custom_remote_preserves_unknown_prefix_548():
     """#548: an unknown vendor prefix (zai-org/GLM-5.1) is always preserved.
 
@@ -1207,3 +1457,86 @@ def test_providers_scan_active_own_providers_entry_owns_over_other_slug_5511():
         f"another slug's overlapping entry; got {provider!r}"
     )
     assert base_url == 'https://my.example/v1'
+
+
+# ── #5979: explicit-pick signature lifecycle (Codex-flagged session-update hole) ──
+
+def test_explicit_pick_signature_matches_same_context_5979():
+    """The signature matches when model+provider are unchanged (deliberate pick
+    is honored across same-model follow-up sends)."""
+    import api.models as models
+    sig = models.model_explicit_pick_signature
+    picked = sig('x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+    assert picked == sig('x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+
+
+def test_explicit_pick_signature_invalidated_on_model_change_5979():
+    """A model change (e.g. via /api/session/update) yields a DIFFERENT signature,
+    so a stale explicit-pick can't wrongly preserve a #433 leftover on cold."""
+    import api.models as models
+    sig = models.model_explicit_pick_signature
+    picked = sig('x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+    # switched to a stale first-party id on the same proxy → signature differs
+    assert picked != sig('openai/gpt-5.4', 'custom:llm-proxy')
+    # provider switch also differs
+    assert picked != sig('x-ai/grok-composer-2.5-fast', 'openai')
+
+
+def test_explicit_pick_signature_persists_round_trip_5979():
+    """Session.model_explicit_pick_signature survives save/reload (b3nw's cold
+    restart scenario), and defaults to None when absent."""
+    import api.models as models
+    s = models.Session(session_id='sig5979', model='x-ai/grok-composer-2.5-fast',
+                        model_provider='custom:llm-proxy')
+    s.model_explicit_pick_signature = models.model_explicit_pick_signature(
+        'x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+    data = {k: getattr(s, k, None) for k in
+            ['session_id', 'title', 'workspace', 'model', 'model_provider',
+             'model_explicit_pick_signature']}
+    s2 = models.Session(**data)
+    assert s2.model_explicit_pick_signature == s.model_explicit_pick_signature
+    # absent → None (unmarked)
+    s3 = models.Session(session_id='nopick', model='y')
+    assert s3.model_explicit_pick_signature is None
+
+
+def test_streaming_explicitly_picked_computation_5979():
+    """End-to-end lifecycle (Fable fast-follow): reproduce the exact
+    signature-comparison the streaming worker does at api/streaming.py, proving
+    the persisted signature drives explicitly_picked correctly across the
+    deliberate-pick, stale-after-update, and never-picked cases.
+
+    Mirrors streaming.py:
+        _picked_sig = s.model_explicit_pick_signature
+        _current_sig = mk_sig(s.model or model, s.model_provider or provider_context)
+        _explicitly_picked = bool(_picked_sig) and _picked_sig == _current_sig
+    """
+    import api.models as models
+    mk = models.model_explicit_pick_signature
+
+    def _explicitly_picked(session):
+        picked = getattr(session, 'model_explicit_pick_signature', None)
+        cur = mk(getattr(session, 'model', None), getattr(session, 'model_provider', None))
+        return bool(picked) and picked == cur
+
+    # 1) deliberate pick, unchanged context → honored
+    s = models.Session(session_id='e2e1', model='x-ai/grok-composer-2.5-fast',
+                        model_provider='custom:llm-proxy')
+    s.model_explicit_pick_signature = mk('x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+    assert _explicitly_picked(s) is True
+
+    # 2) session/update switches the model WITHOUT restamping → stale pick ignored
+    s.model = 'openai/gpt-5.4'  # e.g. user switched; signature now stale
+    assert _explicitly_picked(s) is False
+
+    # 3) provider switch without restamp → stale pick ignored
+    s2 = models.Session(session_id='e2e2', model='x-ai/grok-composer-2.5-fast',
+                        model_provider='custom:llm-proxy')
+    s2.model_explicit_pick_signature = mk('x-ai/grok-composer-2.5-fast', 'custom:llm-proxy')
+    s2.model_provider = 'openai'
+    assert _explicitly_picked(s2) is False
+
+    # 4) never picked → unmarked
+    s3 = models.Session(session_id='e2e3', model='x-ai/grok-composer-2.5-fast',
+                        model_provider='custom:llm-proxy')
+    assert _explicitly_picked(s3) is False