diff --git a/agent/agent_init.py b/agent/agent_init.py
index f608e95084..97b3349123 100644
--- a/agent/agent_init.py
+++ b/agent/agent_init.py
@@ -23,6 +23,7 @@ from typing import Any, Callable, Dict, List, Optional
 from urllib.parse import parse_qs, urlparse, urlunparse
 
 from agent.context_compressor import ContextCompressor
+from agent.agent_init_fallback import _fallback_entries, _init_fallback_chain, recompute_init_fallback_api_mode
 from agent.agent_runtime_helpers import _ra
 from agent.iteration_budget import IterationBudget, normalize_budget_warning_ratio
 from agent.memory_manager import StreamingContextScrubber
@@ -899,6 +900,7 @@ def _routed_client_kwargs(agent, fallback_model, _provider_timeout) -> Optional[
             return None
         agent.provider = agent.requested_provider = _fb["provider"]
         agent.model = _fb_model or _fb["model"]
+        recompute_init_fallback_api_mode(agent, _fb_client)
         return _client_kwargs_from_routed(_fb_client, _provider_timeout)
     # A burned credential pool (#119533) is otherwise indistinguishable from missing config,
     # so name it even when no fallback entries are configured.
@@ -1088,38 +1090,6 @@ def _client_kwargs_from_routed(client, timeout) -> Dict[str, Any]:
     return kwargs
 
 
-def _fallback_entries(fallback_model) -> List[Dict[str, Any]]:
-    """Normalize legacy single-dict ``fallback_model`` / list ``fallback_providers``."""
-    if isinstance(fallback_model, dict):
-        fallback_model = [fallback_model]
-    if not isinstance(fallback_model, list):
-        return []
-    return [
-        f for f in fallback_model if isinstance(f, dict) and f.get("provider") and f.get("model")
-    ]
-
-
-def _init_fallback_chain(agent, fallback_model):
-    # Stable pool-entry identity: OAuth refreshes can replace the token before a failed
-    # request is recovered, so the key value alone can't attribute the failure.
-    from agent.agent_runtime_helpers import sync_credential_pool_entry_id
-    sync_credential_pool_entry_id(agent)
-
-    # Ordered backups tried when the primary is exhausted (legacy single-dict or list).
-    agent._fallback_chain = _fallback_entries(fallback_model)
-    agent._fallback_index = 0
-    agent._fallback_activated = getattr(agent, "_fallback_activated", False)
-    # Legacy attribute kept for backward compat (tests, external callers)
-    agent._fallback_model = agent._fallback_chain[0] if agent._fallback_chain else None
-    chain = agent._fallback_chain
-    if chain and not agent.quiet_mode:
-        labels = [f"{f['model']} ({f['provider']})" for f in chain]
-        if len(chain) == 1:
-            print(f"🔄 Fallback model: {labels[0]}")
-        else:
-            print(f"🔄 Fallback chain ({len(chain)} providers): " + " → ".join(labels))
-
-
 def _load_tools(agent, enabled_toolsets, disabled_toolsets):
     # A feature that left core for a catalog plugin (Home Assistant) is installed for a home that
     # used it, once per process, before discovery so its tools are in this agent's snapshot.
diff --git a/agent/agent_init_fallback.py b/agent/agent_init_fallback.py
new file mode 100644
index 0000000000..157138225c
--- /dev/null
+++ b/agent/agent_init_fallback.py
@@ -0,0 +1,61 @@
+"""Init-time fallback chain: entry normalization, chain setup, and the fallback's api_mode."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Dict, List
+
+logger = logging.getLogger("run_agent")
+
+
+def _fallback_entries(fallback_model) -> List[Dict[str, Any]]:
+    """Normalize legacy single-dict ``fallback_model`` / list ``fallback_providers``."""
+    if isinstance(fallback_model, dict):
+        fallback_model = [fallback_model]
+    if not isinstance(fallback_model, list):
+        return []
+    return [
+        f for f in fallback_model if isinstance(f, dict) and f.get("provider") and f.get("model")
+    ]
+
+
+def _init_fallback_chain(agent, fallback_model):
+    # Stable pool-entry identity: OAuth refreshes can replace the token before a failed
+    # request is recovered, so the key value alone can't attribute the failure.
+    from agent.agent_runtime_helpers import sync_credential_pool_entry_id
+    sync_credential_pool_entry_id(agent)
+
+    # Ordered backups tried when the primary is exhausted (legacy single-dict or list).
+    agent._fallback_chain = _fallback_entries(fallback_model)
+    agent._fallback_index = 0
+    agent._fallback_activated = getattr(agent, "_fallback_activated", False)
+    # Legacy attribute kept for backward compat (tests, external callers)
+    agent._fallback_model = agent._fallback_chain[0] if agent._fallback_chain else None
+    chain = agent._fallback_chain
+    if chain and not agent.quiet_mode:
+        labels = [f"{f['model']} ({f['provider']})" for f in chain]
+        if len(chain) == 1:
+            print(f"🔄 Fallback model: {labels[0]}")
+        else:
+            print(f"🔄 Fallback chain ({len(chain)} providers): " + " → ".join(labels))
+
+
+def recompute_init_fallback_api_mode(agent, fb_client) -> None:
+    """Give an init-time fallback its own api_mode instead of the unreachable primary's.
+
+    A Nous primary configured with ``api_mode: codex_responses`` otherwise leaves a Copilot
+    gpt-5-mini fallback on the Responses API, which silently drops reasoning content (#46527).
+    The per-turn path (``try_activate_fallback``) already recomputes via
+    ``_fallback_api_mode_resolved``; this keeps init in agreement, and runs before the
+    ``_primary_runtime`` snapshot so ``restore_primary_runtime`` replays the recomputed mode.
+    A detection failure keeps the primary's mode and never blocks the fallback itself.
+    """
+    from agent.chat_completion_helpers import _fallback_api_mode_resolved
+    try:
+        agent.api_mode = _fallback_api_mode_resolved(
+            agent, agent.provider, agent.model, str(getattr(fb_client, "base_url", "") or ""))
+    except Exception:
+        logger.debug("Init-time fallback api_mode detection failed for %s", agent.provider, exc_info=True)
+        return
+    if hasattr(agent, "_transport_cache"):
+        agent._transport_cache.clear()
diff --git a/agent/transports/codex.py b/agent/transports/codex.py
index 3cbb0b632b..82c7def219 100644
--- a/agent/transports/codex.py
+++ b/agent/transports/codex.py
@@ -591,7 +591,15 @@ def _reasoning_fields(
     elif enabled:
         if is_github_responses:
             if params.get("github_reasoning_extra") is not None:
-                fields["reasoning"] = params["github_reasoning_extra"]
+                # Request a reasoning summary so the Responses API actually returns
+                # reasoning text. Without ``summary: "auto"`` the GitHub/Copilot
+                # /responses endpoint returns reasoning items with no summary, so Hermes
+                # persists empty reasoning/thinking content for every Copilot model on
+                # the Responses API (gpt-5.4, gpt-5, ...) — the same loss the non-GitHub
+                # branch below already avoids. Verified against api.githubcopilot.com
+                # /responses: the summary is only emitted when summary="auto" is sent.
+                # See #46527.
+                fields["reasoning"] = {**params["github_reasoning_extra"], "summary": "auto"}
         else:
             fields["reasoning"] = {"effort": effort, "summary": "auto"}
             fields["include"] = include
diff --git a/hermes_cli/runtime_provider.py b/hermes_cli/runtime_provider.py
index 2e1f9d2630..74ba470939 100644
--- a/hermes_cli/runtime_provider.py
+++ b/hermes_cli/runtime_provider.py
@@ -224,6 +224,18 @@ def _effective_model(model_cfg: Dict[str, Any], target_model: Optional[str]) ->
 def _copilot_runtime_api_mode(model_cfg: Dict[str, Any], api_key: str, *, target_model: Optional[str] = None) -> str:
     configured_mode = _configured_api_mode("copilot", model_cfg)
     if configured_mode:
+        # A stale/incompatible explicit ``codex_responses`` must not override Copilot's hard
+        # per-model requirement. Copilot serves ``gpt-5-mini`` on chat completions only; a
+        # Responses-API request there silently succeeds but returns no reasoning/thinking
+        # content (#46527). The stale value commonly survives an ordinary flow: the Copilot
+        # OAuth setup writes ``api_mode: codex_responses`` (correct for the gpt-5.4-mini it
+        # initially selects), then the user switches to gpt-5-mini without the mode being
+        # recomputed. The check is the pure-regex Copilot exception — no network call; a
+        # legitimate ``codex_responses`` for GPT-5 variants like gpt-5.4-mini is still honored.
+        model_name = str(_effective_model(model_cfg, target_model)).strip()
+        if (configured_mode == "codex_responses" and model_name
+                and not _models._should_use_copilot_responses_api(model_name)):
+            return "chat_completions"
         return configured_mode
     # Use the model being resolved, not the persisted default: a Claude MoA slot inheriting
     # codex_responses from a GPT-5 default fails with "model ... does not support Responses API".
diff --git a/tests/agent/test_init_fallback_on_exhausted_pool.py b/tests/agent/test_init_fallback_on_exhausted_pool.py
index 53897ce8d0..98bcb53f57 100644
--- a/tests/agent/test_init_fallback_on_exhausted_pool.py
+++ b/tests/agent/test_init_fallback_on_exhausted_pool.py
@@ -131,3 +131,79 @@ def test_init_openrouter_exhausted_without_chain_keeps_generic_error():
                 fallback_model=[{"provider": "openrouter",
                                  "model": "poolside/laguna-s-2.1:free"}],
             )
+
+
+def test_init_fallback_recomputes_api_mode_for_copilot_gpt5_mini():
+    """#46527: Nous primary (api_mode=codex_responses, no creds) -> Copilot gpt-5-mini
+    fallback must end up with api_mode=chat_completions, not codex_responses.
+
+    Copilot's gpt-5-mini is the documented exception that uses Chat Completions,
+    not the Responses API. Before the fix, ``agent.api_mode`` stayed
+    ``"codex_responses"`` after the init-time fallback activated, so every request
+    for the fallback model went out via the Responses API and silently returned no
+    reasoning/thinking content.
+    """
+    fb_client = _mock_client(base_url="https://api.githubcopilot.com")
+
+    def fake_resolve(provider, model=None, raw_codex=False,
+                     explicit_base_url=None, explicit_api_key=None):
+        if provider == "copilot":
+            return fb_client, "gpt-5-mini"
+        return None, None  # primary (nous) has no usable credentials
+
+    with patch("agent.auxiliary_client.resolve_provider_client", side_effect=fake_resolve), \
+         patch("model_tools.get_tool_definitions", return_value=_make_tool_defs()), \
+         patch("model_tools.check_toolset_requirements", return_value={}), \
+         patch("agent.process_bootstrap.OpenAI", return_value=MagicMock()):
+
+        agent = AIAgent(
+            provider="nous",
+            model="gpt-5.4-mini",
+            api_mode="codex_responses",
+            api_key=None,
+            base_url=None,
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+            fallback_model=[{"provider": "copilot", "model": "gpt-5-mini"}],
+        )
+
+        assert agent.provider == "copilot"
+        assert agent.model == "gpt-5-mini"
+        assert agent._fallback_activated is True
+        assert agent.api_mode == "chat_completions"
+
+
+def test_init_fallback_keeps_codex_responses_for_openai_codex():
+    """#46527: Nous primary (api_mode=codex_responses, no creds) -> openai-codex
+    fallback must keep api_mode=codex_responses (openai-codex always uses the
+    Responses API)."""
+    fb_client = _mock_client(base_url="https://chatgpt.com/backend-api/codex")
+
+    def fake_resolve(provider, model=None, raw_codex=False,
+                     explicit_base_url=None, explicit_api_key=None):
+        if provider == "openai-codex":
+            return fb_client, "gpt-5.4-mini"
+        return None, None
+
+    with patch("agent.auxiliary_client.resolve_provider_client", side_effect=fake_resolve), \
+         patch("model_tools.get_tool_definitions", return_value=_make_tool_defs()), \
+         patch("model_tools.check_toolset_requirements", return_value={}), \
+         patch("agent.process_bootstrap.OpenAI", return_value=MagicMock()):
+
+        agent = AIAgent(
+            provider="nous",
+            model="gpt-5.4-mini",
+            api_mode="codex_responses",
+            api_key=None,
+            base_url=None,
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+            fallback_model=[{"provider": "openai-codex", "model": "gpt-5.4-mini"}],
+        )
+
+        assert agent.provider == "openai-codex"
+        assert agent.model == "gpt-5.4-mini"
+        assert agent._fallback_activated is True
+        assert agent.api_mode == "codex_responses"
diff --git a/tests/agent/transports/test_codex_transport.py b/tests/agent/transports/test_codex_transport.py
index 64c1efc009..5ed2d539c2 100644
--- a/tests/agent/transports/test_codex_transport.py
+++ b/tests/agent/transports/test_codex_transport.py
@@ -53,7 +53,7 @@ class TestCodexBuildKwargs:
             reasoning_config=reasoning,
             github_reasoning_extra=ReasoningParamsMixin._github_models_reasoning_extra_body(agent),
         )
-        assert kw.get("reasoning") == ({"effort": expected} if expected else None)
+        assert kw.get("reasoning") == ({"effort": expected, "summary": "auto"} if expected else None)
 
     def test_astra_direct_request_applies_model_contract_after_overrides(self, transport):
         kw = transport.build_kwargs(
diff --git a/tests/agent/transports/test_codex_transport_github_reasoning.py b/tests/agent/transports/test_codex_transport_github_reasoning.py
new file mode 100644
index 0000000000..40f6329bda
--- /dev/null
+++ b/tests/agent/transports/test_codex_transport_github_reasoning.py
@@ -0,0 +1,25 @@
+"""GitHub/Copilot Responses reasoning fields on the Codex transport.
+
+Regression for #46527: the GitHub Responses branch must request ``summary: "auto"`` so
+Copilot returns reasoning text. Without it, reasoning items come back with no summary and
+Hermes persists no reasoning/thinking content.
+"""
+
+import pytest
+
+from agent.transports import get_transport
+
+
+@pytest.fixture
+def transport():
+    import agent.transports.codex  # noqa: F401
+    return get_transport("codex_responses")
+
+
+def test_github_responses_requests_reasoning_summary(transport):
+    kw = transport.build_kwargs(
+        model="gpt-5.4", messages=[{"role": "user", "content": "Hi"}], tools=[],
+        is_github_responses=True,
+        github_reasoning_extra={"effort": "medium"},
+    )
+    assert kw.get("reasoning") == {"effort": "medium", "summary": "auto"}
diff --git a/tests/fixtures/provider_wire_snapshot.json b/tests/fixtures/provider_wire_snapshot.json
index e313644a4f..2835c60042 100644
--- a/tests/fixtures/provider_wire_snapshot.json
+++ b/tests/fixtures/provider_wire_snapshot.json
@@ -2773,7 +2773,8 @@
    "model": "gpt-5.5",
    "parallel_tool_calls": true,
    "reasoning": {
-    "effort": "high"
+    "effort": "high",
+    "summary": "auto"
    },
    "store": false,
    "timeout": 1800.0,
@@ -2877,7 +2878,8 @@
    "model": "gpt-5.5",
    "parallel_tool_calls": true,
    "reasoning": {
-    "effort": "medium"
+    "effort": "medium",
+    "summary": "auto"
    },
    "store": false,
    "timeout": 1800.0,
diff --git a/tests/hermes_cli/test_copilot_runtime_api_mode_stale.py b/tests/hermes_cli/test_copilot_runtime_api_mode_stale.py
new file mode 100644
index 0000000000..9302c65693
--- /dev/null
+++ b/tests/hermes_cli/test_copilot_runtime_api_mode_stale.py
@@ -0,0 +1,53 @@
+"""Regression tests for #46527: Copilot primary api_mode resolution must not
+honor a stale/incompatible explicit ``api_mode``.
+
+The gateway resolves and snapshots the primary provider/model at startup via
+``resolve_runtime_provider`` -> ``_copilot_runtime_api_mode``. When the config
+carries a stale ``api_mode: codex_responses`` (e.g. the Copilot OAuth setup
+wrote it for the ``gpt-5.4-mini`` it initially selects, then the user switched
+to ``gpt-5-mini``) the old code honored the stale ``codex_responses`` verbatim.
+That dispatches gpt-5-mini through the Responses API — which Copilot serves on
+chat completions only — and the response comes back with no reasoning/thinking
+content (the symptom in #46527).
+"""
+from hermes_cli.runtime_provider import _copilot_runtime_api_mode
+
+
+def test_stale_codex_responses_vetoed_for_gpt5_mini():
+    """gpt-5-mini + stale api_mode=codex_responses must resolve to chat_completions."""
+    cfg = {"provider": "copilot", "default": "gpt-5-mini", "api_mode": "codex_responses"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x") == "chat_completions"
+
+
+def test_explicit_codex_responses_kept_for_gpt5_variant():
+    """gpt-5.4-mini legitimately uses the Responses API on Copilot — an explicit
+    codex_responses must still be honored (no over-correction)."""
+    cfg = {"provider": "copilot", "default": "gpt-5.4-mini", "api_mode": "codex_responses"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x") == "codex_responses"
+
+
+def test_no_explicit_mode_derives_chat_completions_for_gpt5_mini():
+    """Without an explicit api_mode, gpt-5-mini derives chat_completions (unchanged)."""
+    cfg = {"provider": "copilot", "default": "gpt-5-mini"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x") == "chat_completions"
+
+
+def test_explicit_chat_completions_unchanged():
+    """An explicit chat_completions stays chat_completions for gpt-5-mini."""
+    cfg = {"provider": "copilot", "default": "gpt-5-mini", "api_mode": "chat_completions"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x") == "chat_completions"
+
+
+def test_stale_mode_from_different_provider_not_honored():
+    """When the config's provider doesn't match copilot, the explicit mode is
+    not honored at all (existing _provider_supports_explicit_api_mode guard),
+    so gpt-5-mini still derives chat_completions."""
+    cfg = {"provider": "nous", "default": "gpt-5-mini", "api_mode": "codex_responses"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x") == "chat_completions"
+
+
+def test_target_model_beats_stale_default_mode():
+    """A /model switch to gpt-5-mini must not inherit a stale codex_responses
+    recorded for the persisted default."""
+    cfg = {"provider": "copilot", "default": "gpt-5.4-mini", "api_mode": "codex_responses"}
+    assert _copilot_runtime_api_mode(cfg, api_key="x", target_model="gpt-5-mini") == "chat_completions"