diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index c9d83f3b7..9f9b94b2b 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -1098,7 +1098,7 @@ def _validate_base_url(base_url: str) -> None:
         ) from exc
 
 
-def _try_custom_endpoint() -> Tuple[Optional[OpenAI], Optional[str]]:
+def _try_custom_endpoint() -> Tuple[Optional[Any], Optional[str]]:
     runtime = _resolve_custom_runtime()
     if len(runtime) == 2:
         custom_base, custom_key = runtime
@@ -1114,6 +1114,23 @@ def _try_custom_endpoint() -> Tuple[Optional[OpenAI], Optional[str]]:
     if custom_mode == "codex_responses":
         real_client = OpenAI(api_key=custom_key, base_url=custom_base)
         return CodexAuxiliaryClient(real_client, model), model
+    if custom_mode == "anthropic_messages":
+        # Third-party Anthropic-compatible gateway (MiniMax, Zhipu GLM,
+        # LiteLLM proxies, etc.).  Must NEVER be treated as OAuth —
+        # Anthropic OAuth claims only apply to api.anthropic.com.
+        try:
+            from agent.anthropic_adapter import build_anthropic_client
+            real_client = build_anthropic_client(custom_key, custom_base)
+        except ImportError:
+            logger.warning(
+                "Custom endpoint declares api_mode=anthropic_messages but the "
+                "anthropic SDK is not installed — falling back to OpenAI-wire."
+            )
+            return OpenAI(api_key=custom_key, base_url=custom_base), model
+        return (
+            AnthropicAuxiliaryClient(real_client, model, custom_key, custom_base, is_oauth=False),
+            model,
+        )
     return OpenAI(api_key=custom_key, base_url=custom_base), model
 
 
diff --git a/hermes_cli/auth.py b/hermes_cli/auth.py
index f6bf1fef8..78f1a13ce 100644
--- a/hermes_cli/auth.py
+++ b/hermes_cli/auth.py
@@ -2732,6 +2732,17 @@ def _update_config_for_provider(
         # Clear stale base_url to prevent contamination when switching providers
         model_cfg.pop("base_url", None)
 
+    # Clear stale api_key/api_mode left over from a previous custom provider.
+    # When the user switches from e.g. a MiniMax custom endpoint
+    # (api_mode=anthropic_messages, api_key=mxp-...) to a built-in provider
+    # (e.g. OpenRouter), the stale api_key/api_mode would override the new
+    # provider's credentials and transport choice.  Built-in providers that
+    # need a specific api_mode (copilot, xai) set it at request-resolution
+    # time via `_copilot_runtime_api_mode` / `_detect_api_mode_for_url`, so
+    # removing the persisted value here is safe.
+    model_cfg.pop("api_key", None)
+    model_cfg.pop("api_mode", None)
+
     # When switching to a non-OpenRouter provider, ensure model.default is
     # valid for the new provider.  An OpenRouter-formatted name like
     # "anthropic/claude-opus-4.6" will fail on direct-API providers.
diff --git a/hermes_cli/runtime_provider.py b/hermes_cli/runtime_provider.py
index a5c286fe0..392d7769d 100644
--- a/hermes_cli/runtime_provider.py
+++ b/hermes_cli/runtime_provider.py
@@ -38,14 +38,21 @@ def _normalize_custom_provider_name(value: str) -> str:
 def _detect_api_mode_for_url(base_url: str) -> Optional[str]:
     """Auto-detect api_mode from the resolved base URL.
 
-    Direct api.openai.com endpoints need the Responses API for GPT-5.x
-    tool calls with reasoning (chat/completions returns 400).
+    - Direct api.openai.com endpoints need the Responses API for GPT-5.x
+      tool calls with reasoning (chat/completions returns 400).
+    - Third-party Anthropic-compatible gateways (MiniMax, Zhipu GLM,
+      LiteLLM proxies, etc.) conventionally expose the native Anthropic
+      protocol under a ``/anthropic`` suffix — treat those as
+      ``anthropic_messages`` transport instead of the default
+      ``chat_completions``.
     """
     normalized = (base_url or "").strip().lower().rstrip("/")
     if "api.x.ai" in normalized:
         return "codex_responses"
     if "api.openai.com" in normalized and "openrouter" not in normalized:
         return "codex_responses"
+    if normalized.endswith("/anthropic"):
+        return "anthropic_messages"
     return None
 
 
@@ -194,8 +201,12 @@ def _resolve_runtime_from_pool_entry(
         elif provider in ("opencode-zen", "opencode-go"):
             from hermes_cli.models import opencode_model_api_mode
             api_mode = opencode_model_api_mode(provider, model_cfg.get("default", ""))
-        elif base_url.rstrip("/").endswith("/anthropic"):
-            api_mode = "anthropic_messages"
+        else:
+            # Auto-detect Anthropic-compatible endpoints (/anthropic suffix,
+            # api.openai.com → codex_responses, api.x.ai → codex_responses).
+            detected = _detect_api_mode_for_url(base_url)
+            if detected:
+                api_mode = detected
 
     # OpenCode base URLs end with /v1 for OpenAI-compatible models, but the
     # Anthropic SDK prepends its own /v1/messages to the base_url.  Strip the
@@ -642,8 +653,11 @@ def _resolve_explicit_runtime(
             configured_mode = _parse_api_mode(model_cfg.get("api_mode"))
             if configured_mode:
                 api_mode = configured_mode
-            elif base_url.rstrip("/").endswith("/anthropic"):
-                api_mode = "anthropic_messages"
+            else:
+                # Auto-detect Anthropic-compatible endpoints (/anthropic suffix).
+                detected = _detect_api_mode_for_url(base_url)
+                if detected:
+                    api_mode = detected
 
         return {
             "provider": provider,
@@ -965,10 +979,13 @@ def resolve_runtime_provider(
             elif provider in ("opencode-zen", "opencode-go"):
                 from hermes_cli.models import opencode_model_api_mode
                 api_mode = opencode_model_api_mode(provider, model_cfg.get("default", ""))
-            # Auto-detect Anthropic-compatible endpoints by URL convention
-            # (e.g. https://api.minimax.io/anthropic, https://dashscope.../anthropic)
-            elif base_url.rstrip("/").endswith("/anthropic"):
-                api_mode = "anthropic_messages"
+            else:
+                # Auto-detect Anthropic-compatible endpoints by URL convention
+                # (e.g. https://api.minimax.io/anthropic, https://dashscope.../anthropic)
+                # plus api.openai.com → codex_responses and api.x.ai → codex_responses.
+                detected = _detect_api_mode_for_url(base_url)
+                if detected:
+                    api_mode = detected
         # Strip trailing /v1 for OpenCode Anthropic models (see comment above).
         if api_mode == "anthropic_messages" and provider in ("opencode-zen", "opencode-go"):
             base_url = re.sub(r"/v1/?$", "", base_url)
diff --git a/run_agent.py b/run_agent.py
index fc1e3560f..16e603896 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -892,13 +892,15 @@ class AIAgent:
         self.prefill_messages = prefill_messages or []  # Prefilled conversation turns
         self._force_ascii_payload = False
         
-        # Anthropic prompt caching: auto-enabled for Claude models via OpenRouter.
-        # Reduces input costs by ~75% on multi-turn conversations by caching the
-        # conversation prefix. Uses system_and_3 strategy (4 breakpoints).
-        is_openrouter = self._is_openrouter_url()
-        is_claude = "claude" in self.model.lower()
-        is_native_anthropic = self.api_mode == "anthropic_messages" and self.provider == "anthropic"
-        self._use_prompt_caching = (is_openrouter and is_claude) or is_native_anthropic
+        # Anthropic prompt caching: auto-enabled for Claude models on native
+        # Anthropic, OpenRouter, and third-party gateways that speak the
+        # Anthropic protocol (``api_mode == 'anthropic_messages'``). Reduces
+        # input costs by ~75% on multi-turn conversations. Uses system_and_3
+        # strategy (4 breakpoints). See ``_anthropic_prompt_cache_policy``
+        # for the layout-vs-transport decision.
+        self._use_prompt_caching, self._use_native_cache_layout = (
+            self._anthropic_prompt_cache_policy()
+        )
         self._cache_ttl = "5m"  # Default 5-minute TTL (1.25x write cost)
         
         # Iteration budget: the LLM is only notified when it actually exhausts
@@ -1013,8 +1015,15 @@ class AIAgent:
                 self.api_key = effective_key
                 self._anthropic_api_key = effective_key
                 self._anthropic_base_url = base_url
+                # Only mark the session as OAuth-authenticated when the token
+                # genuinely belongs to native Anthropic.  Third-party providers
+                # (MiniMax, Kimi, GLM, LiteLLM proxies) that accept the
+                # Anthropic protocol must never trip OAuth code paths — doing
+                # so injects Claude-Code identity headers and system prompts
+                # that cause 401/403 on their endpoints.  Guards #1739 and
+                # the third-party identity-injection bug.
                 from agent.anthropic_adapter import _is_oauth_token as _is_oat
-                self._is_anthropic_oauth = _is_oat(effective_key)
+                self._is_anthropic_oauth = _is_oat(effective_key) if _is_native_anthropic else False
                 self._anthropic_client = build_anthropic_client(effective_key, base_url, timeout=_provider_timeout)
                 # No OpenAI client needed for Anthropic mode
                 self.client = None
@@ -1227,7 +1236,12 @@ class AIAgent:
         
         # Show prompt caching status
         if self._use_prompt_caching and not self.quiet_mode:
-            source = "native Anthropic" if is_native_anthropic else "Claude via OpenRouter"
+            if self._use_native_cache_layout and self.provider == "anthropic":
+                source = "native Anthropic"
+            elif self._use_native_cache_layout:
+                source = "Anthropic-compatible endpoint"
+            else:
+                source = "Claude via OpenRouter"
             print(f"💾 Prompt caching: ENABLED ({source}, {self._cache_ttl} TTL)")
         
         # Session logging setup - auto-save conversation trajectories for debugging
@@ -1701,6 +1715,7 @@ class AIAgent:
             "api_key": getattr(self, "api_key", ""),
             "client_kwargs": dict(self._client_kwargs),
             "use_prompt_caching": self._use_prompt_caching,
+            "use_native_cache_layout": self._use_native_cache_layout,
             # Context engine state that _try_activate_fallback() overwrites.
             # Use getattr for model/base_url/api_key/provider since plugin
             # engines may not have these (they're ContextCompressor-specific).
@@ -1822,7 +1837,7 @@ class AIAgent:
                 effective_key, self._anthropic_base_url,
                 timeout=get_provider_request_timeout(self.provider, self.model),
             )
-            self._is_anthropic_oauth = _is_oauth_token(effective_key)
+            self._is_anthropic_oauth = _is_oauth_token(effective_key) if _is_native_anthropic else False
             self.client = None
             self._client_kwargs = {}
         else:
@@ -1842,10 +1857,13 @@ class AIAgent:
             )
 
         # ── Re-evaluate prompt caching ──
-        is_native_anthropic = api_mode == "anthropic_messages" and new_provider == "anthropic"
-        self._use_prompt_caching = (
-            ("openrouter" in (self.base_url or "").lower() and "claude" in new_model.lower())
-            or is_native_anthropic
+        self._use_prompt_caching, self._use_native_cache_layout = (
+            self._anthropic_prompt_cache_policy(
+                provider=new_provider,
+                base_url=self.base_url,
+                api_mode=api_mode,
+                model=new_model,
+            )
         )
 
         # ── Update context compressor ──
@@ -1880,6 +1898,7 @@ class AIAgent:
             "api_key": getattr(self, "api_key", ""),
             "client_kwargs": dict(self._client_kwargs),
             "use_prompt_caching": self._use_prompt_caching,
+            "use_native_cache_layout": self._use_native_cache_layout,
             "compressor_model": getattr(_cc, "model", self.model) if _cc else self.model,
             "compressor_base_url": getattr(_cc, "base_url", self.base_url) if _cc else self.base_url,
             "compressor_api_key": getattr(_cc, "api_key", "") if _cc else "",
@@ -2143,6 +2162,55 @@ class AIAgent:
         """Return True when the base URL targets OpenRouter."""
         return "openrouter" in self._base_url_lower
 
+    def _anthropic_prompt_cache_policy(
+        self,
+        *,
+        provider: Optional[str] = None,
+        base_url: Optional[str] = None,
+        api_mode: Optional[str] = None,
+        model: Optional[str] = None,
+    ) -> tuple[bool, bool]:
+        """Decide whether to apply Anthropic prompt caching and which layout to use.
+
+        Returns ``(should_cache, use_native_layout)``:
+          * ``should_cache`` — inject ``cache_control`` breakpoints for this
+            request (applies to OpenRouter Claude, native Anthropic, and
+            third-party gateways that speak the native Anthropic protocol).
+          * ``use_native_layout`` — place markers on the *inner* content
+            blocks (native Anthropic accepts and requires this layout);
+            when False markers go on the message envelope (OpenRouter and
+            OpenAI-wire proxies expect the looser layout).
+
+        Third-party providers using the native Anthropic transport
+        (``api_mode == 'anthropic_messages'`` + Claude-named model) get
+        caching with the native layout so they benefit from the same
+        cost reduction as direct Anthropic callers, provided their
+        gateway implements the Anthropic cache_control contract
+        (MiniMax, Zhipu GLM, LiteLLM's Anthropic proxy mode all do).
+        """
+        eff_provider = (provider if provider is not None else self.provider) or ""
+        eff_base_url = base_url if base_url is not None else (self.base_url or "")
+        eff_api_mode = api_mode if api_mode is not None else (self.api_mode or "")
+        eff_model = (model if model is not None else self.model) or ""
+
+        base_lower = eff_base_url.lower()
+        is_claude = "claude" in eff_model.lower()
+        is_openrouter = "openrouter" in base_lower
+        is_anthropic_wire = eff_api_mode == "anthropic_messages"
+        is_native_anthropic = (
+            is_anthropic_wire
+            and (eff_provider == "anthropic" or "api.anthropic.com" in base_lower)
+        )
+
+        if is_native_anthropic:
+            return True, True
+        if is_openrouter and is_claude:
+            return True, False
+        if is_anthropic_wire and is_claude:
+            # Third-party Anthropic-compatible gateway.
+            return True, True
+        return False, False
+
     @staticmethod
     def _model_requires_responses_api(model: str) -> bool:
         """Return True for models that require the Responses API path.
@@ -5322,9 +5390,12 @@ class AIAgent:
             return False
 
         self._anthropic_api_key = new_token
-        # Update OAuth flag — token type may have changed (API key ↔ OAuth)
+        # Update OAuth flag — token type may have changed (API key ↔ OAuth).
+        # Only treat as OAuth on native Anthropic; third-party endpoints using
+        # the Anthropic protocol must not trip OAuth paths (#1739 & third-party
+        # identity-injection guard).
         from agent.anthropic_adapter import _is_oauth_token
-        self._is_anthropic_oauth = _is_oauth_token(new_token)
+        self._is_anthropic_oauth = _is_oauth_token(new_token) if self.provider == "anthropic" else False
         return True
 
     def _apply_client_headers_for_base_url(self, base_url: str) -> None:
@@ -5367,7 +5438,7 @@ class AIAgent:
                 runtime_key, runtime_base,
                 timeout=get_provider_request_timeout(self.provider, self.model),
             )
-            self._is_anthropic_oauth = _is_oauth_token(runtime_key)
+            self._is_anthropic_oauth = _is_oauth_token(runtime_key) if self.provider == "anthropic" else False
             self.api_key = runtime_key
             self.base_url = runtime_base
             return
@@ -6523,7 +6594,7 @@ class AIAgent:
                 self._anthropic_client = build_anthropic_client(
                     effective_key, self._anthropic_base_url, timeout=_fb_timeout,
                 )
-                self._is_anthropic_oauth = _is_oauth_token(effective_key)
+                self._is_anthropic_oauth = _is_oauth_token(effective_key) if fb_provider == "anthropic" else False
                 self.client = None
                 self._client_kwargs = {}
             else:
@@ -6554,10 +6625,13 @@ class AIAgent:
                     self._replace_primary_openai_client(reason="fallback_timeout_apply")
 
             # Re-evaluate prompt caching for the new provider/model
-            is_native_anthropic = fb_api_mode == "anthropic_messages" and fb_provider == "anthropic"
-            self._use_prompt_caching = (
-                ("openrouter" in fb_base_url.lower() and "claude" in fb_model.lower())
-                or is_native_anthropic
+            self._use_prompt_caching, self._use_native_cache_layout = (
+                self._anthropic_prompt_cache_policy(
+                    provider=fb_provider,
+                    base_url=fb_base_url,
+                    api_mode=fb_api_mode,
+                    model=fb_model,
+                )
             )
 
             # Update context compressor limits for the fallback model.
@@ -6617,6 +6691,12 @@ class AIAgent:
             self.api_key = rt["api_key"]
             self._client_kwargs = dict(rt["client_kwargs"])
             self._use_prompt_caching = rt["use_prompt_caching"]
+            # Default to native layout when the restored snapshot predates the
+            # native-vs-proxy split (older sessions saved before this PR).
+            self._use_native_cache_layout = rt.get(
+                "use_native_cache_layout",
+                self.api_mode == "anthropic_messages" and self.provider == "anthropic",
+            )
 
             # ── Rebuild client for the primary provider ──
             if self.api_mode == "anthropic_messages":
@@ -9317,12 +9397,19 @@ class AIAgent:
                 for idx, pfm in enumerate(self.prefill_messages):
                     api_messages.insert(sys_offset + idx, pfm.copy())
 
-            # Apply Anthropic prompt caching for Claude models via OpenRouter.
-            # Auto-detected: if model name contains "claude" and base_url is OpenRouter,
-            # inject cache_control breakpoints (system + last 3 messages) to reduce
-            # input token costs by ~75% on multi-turn conversations.
+            # Apply Anthropic prompt caching for Claude models on native
+            # Anthropic, OpenRouter, and third-party Anthropic-compatible
+            # gateways. Auto-detected: if ``_use_prompt_caching`` is set,
+            # inject cache_control breakpoints (system + last 3 messages)
+            # to reduce input token costs by ~75% on multi-turn
+            # conversations. Layout is chosen per endpoint by
+            # ``_anthropic_prompt_cache_policy``.
             if self._use_prompt_caching:
-                api_messages = apply_anthropic_cache_control(api_messages, cache_ttl=self._cache_ttl, native_anthropic=(self.api_mode == 'anthropic_messages'))
+                api_messages = apply_anthropic_cache_control(
+                    api_messages,
+                    cache_ttl=self._cache_ttl,
+                    native_anthropic=self._use_native_cache_layout,
+                )
 
             # Safety net: strip orphaned tool results / add stubs for missing
             # results before sending to the API.  Runs unconditionally — not
@@ -9779,25 +9866,30 @@ class AIAgent:
                     if finish_reason == "length":
                         self._vprint(f"{self.log_prefix}⚠️  Response truncated (finish_reason='length') - model hit max output tokens", force=True)
 
+                        # Normalize the truncated response to a single OpenAI-style
+                        # message shape so text-continuation and tool-call retry
+                        # work uniformly across chat_completions, bedrock_converse,
+                        # and anthropic_messages.  For Anthropic we use the same
+                        # adapter the agent loop already relies on so the rebuilt
+                        # interim assistant message is byte-identical to what
+                        # would have been appended in the non-truncated path.
+                        _trunc_msg = None
+                        if self.api_mode in ("chat_completions", "bedrock_converse"):
+                            _trunc_msg = response.choices[0].message if (hasattr(response, "choices") and response.choices) else None
+                        elif self.api_mode == "anthropic_messages":
+                            from agent.anthropic_adapter import normalize_anthropic_response
+                            _trunc_msg, _ = normalize_anthropic_response(
+                                response, strip_tool_prefix=self._is_anthropic_oauth
+                            )
+
+                        _trunc_content = getattr(_trunc_msg, "content", None) if _trunc_msg else None
+                        _trunc_has_tool_calls = bool(getattr(_trunc_msg, "tool_calls", None)) if _trunc_msg else False
+
                         # ── Detect thinking-budget exhaustion ──────────────
                         # When the model spends ALL output tokens on reasoning
                         # and has none left for the response, continuation
                         # retries are pointless.  Detect this early and give a
                         # targeted error instead of wasting 3 API calls.
-                        _trunc_content = None
-                        _trunc_has_tool_calls = False
-                        if self.api_mode in ("chat_completions", "bedrock_converse"):
-                            _trunc_msg = response.choices[0].message if (hasattr(response, "choices") and response.choices) else None
-                            _trunc_content = getattr(_trunc_msg, "content", None) if _trunc_msg else None
-                            _trunc_has_tool_calls = bool(getattr(_trunc_msg, "tool_calls", None)) if _trunc_msg else False
-                        elif self.api_mode == "anthropic_messages":
-                            # Anthropic response.content is a list of blocks
-                            _text_parts = []
-                            for _blk in getattr(response, "content", []):
-                                if getattr(_blk, "type", None) == "text":
-                                    _text_parts.append(getattr(_blk, "text", ""))
-                            _trunc_content = "\n".join(_text_parts) if _text_parts else None
-
                         # A response is "thinking exhausted" only when the model
                         # actually produced reasoning blocks but no visible text after
                         # them.  Models that do not use <think> tags (e.g. GLM-4.7 on
@@ -9854,9 +9946,9 @@ class AIAgent:
                                 "error": _exhaust_error,
                             }
 
-                        if self.api_mode in ("chat_completions", "bedrock_converse"):
-                            assistant_message = response.choices[0].message
-                            if not assistant_message.tool_calls:
+                        if self.api_mode in ("chat_completions", "bedrock_converse", "anthropic_messages"):
+                            assistant_message = _trunc_msg
+                            if assistant_message is not None and not _trunc_has_tool_calls:
                                 length_continue_retries += 1
                                 interim_msg = self._build_assistant_message(assistant_message, finish_reason)
                                 messages.append(interim_msg)
@@ -9894,9 +9986,9 @@ class AIAgent:
                                     "error": "Response remained truncated after 3 continuation attempts",
                                 }
 
-                        if self.api_mode in ("chat_completions", "bedrock_converse"):
-                            assistant_message = response.choices[0].message
-                            if assistant_message.tool_calls:
+                        if self.api_mode in ("chat_completions", "bedrock_converse", "anthropic_messages"):
+                            assistant_message = _trunc_msg
+                            if assistant_message is not None and _trunc_has_tool_calls:
                                 if truncated_tool_call_retries < 1:
                                     truncated_tool_call_retries += 1
                                     self._vprint(
diff --git a/scripts/release.py b/scripts/release.py
index 48fbdcae9..93d536546 100755
--- a/scripts/release.py
+++ b/scripts/release.py
@@ -80,6 +80,13 @@ AUTHOR_MAP = {
     "nish3451@users.noreply.github.com": "nish3451",
     "Mibayy@users.noreply.github.com": "Mibayy",
     "135070653+sgaofen@users.noreply.github.com": "sgaofen",
+    "nocoo@users.noreply.github.com": "nocoo",
+    "30841158+n-WN@users.noreply.github.com": "n-WN",
+    "leoyuan0099@gmail.com": "keyuyuan",
+    "bxzt2006@163.com": "Only-Code-A",
+    "i@troy-y.org": "TroyMitchell911",
+    "mygamez@163.com": "zhongyueming1121",
+    "hansnow@users.noreply.github.com": "hansnow",
     # contributors (manual mapping from git names)
     "ahmedsherif95@gmail.com": "asheriif",
     "liujinkun@bytedance.com": "liujinkun2025",
diff --git a/tests/agent/test_auxiliary_client_anthropic_custom.py b/tests/agent/test_auxiliary_client_anthropic_custom.py
new file mode 100644
index 000000000..689a6c37e
--- /dev/null
+++ b/tests/agent/test_auxiliary_client_anthropic_custom.py
@@ -0,0 +1,107 @@
+"""Tests for agent.auxiliary_client._try_custom_endpoint's anthropic_messages branch.
+
+When a user configures a custom endpoint with ``api_mode: anthropic_messages``
+(e.g. MiniMax, Zhipu GLM, LiteLLM in Anthropic-proxy mode), auxiliary tasks
+(compression, web_extract, session_search, title generation) must use the
+native Anthropic transport rather than being silently downgraded to an
+OpenAI-wire client that speaks the wrong protocol.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _clean_env(monkeypatch):
+    for key in (
+        "OPENAI_API_KEY", "OPENAI_BASE_URL",
+        "ANTHROPIC_API_KEY", "ANTHROPIC_TOKEN",
+    ):
+        monkeypatch.delenv(key, raising=False)
+
+
+def _install_anthropic_adapter_mocks():
+    """Patch build_anthropic_client so the test doesn't need the SDK."""
+    fake_client = MagicMock(name="anthropic_client")
+    return patch(
+        "agent.anthropic_adapter.build_anthropic_client",
+        return_value=fake_client,
+    ), fake_client
+
+
+def test_custom_endpoint_anthropic_messages_builds_anthropic_wrapper():
+    """api_mode=anthropic_messages → returns AnthropicAuxiliaryClient, not OpenAI."""
+    from agent.auxiliary_client import _try_custom_endpoint, AnthropicAuxiliaryClient
+
+    with patch(
+        "agent.auxiliary_client._resolve_custom_runtime",
+        return_value=(
+            "https://api.minimax.io/anthropic",
+            "minimax-key",
+            "anthropic_messages",
+        ),
+    ), patch(
+        "agent.auxiliary_client._read_main_model",
+        return_value="claude-sonnet-4-6",
+    ):
+        adapter_patch, fake_client = _install_anthropic_adapter_mocks()
+        with adapter_patch:
+            client, model = _try_custom_endpoint()
+
+    assert isinstance(client, AnthropicAuxiliaryClient), (
+        "Custom endpoint with api_mode=anthropic_messages must return the "
+        f"native Anthropic wrapper, got {type(client).__name__}"
+    )
+    assert model == "claude-sonnet-4-6"
+    # Wrapper should NOT be marked as OAuth — third-party endpoints are
+    # always API-key authenticated.
+    assert client.api_key == "minimax-key"
+    assert client.base_url == "https://api.minimax.io/anthropic"
+
+
+def test_custom_endpoint_anthropic_messages_falls_back_when_sdk_missing():
+    """Graceful degradation when anthropic SDK is unavailable."""
+    from agent.auxiliary_client import _try_custom_endpoint
+
+    import_error = ImportError("anthropic package not installed")
+
+    with patch(
+        "agent.auxiliary_client._resolve_custom_runtime",
+        return_value=("https://api.minimax.io/anthropic", "k", "anthropic_messages"),
+    ), patch(
+        "agent.auxiliary_client._read_main_model",
+        return_value="claude-sonnet-4-6",
+    ), patch(
+        "agent.anthropic_adapter.build_anthropic_client",
+        side_effect=import_error,
+    ):
+        client, model = _try_custom_endpoint()
+
+    # Should fall back to an OpenAI-wire client rather than returning
+    # (None, None) — the tool still needs to do *something*.
+    assert client is not None
+    assert model == "claude-sonnet-4-6"
+    # OpenAI client, not AnthropicAuxiliaryClient.
+    from agent.auxiliary_client import AnthropicAuxiliaryClient
+    assert not isinstance(client, AnthropicAuxiliaryClient)
+
+
+def test_custom_endpoint_chat_completions_still_uses_openai_wire():
+    """Regression: default path (no api_mode) must remain OpenAI client."""
+    from agent.auxiliary_client import _try_custom_endpoint, AnthropicAuxiliaryClient
+
+    with patch(
+        "agent.auxiliary_client._resolve_custom_runtime",
+        return_value=("https://api.example.com/v1", "key", None),
+    ), patch(
+        "agent.auxiliary_client._read_main_model",
+        return_value="my-model",
+    ):
+        client, model = _try_custom_endpoint()
+
+    assert client is not None
+    assert model == "my-model"
+    assert not isinstance(client, AnthropicAuxiliaryClient)
diff --git a/tests/hermes_cli/test_detect_api_mode_for_url.py b/tests/hermes_cli/test_detect_api_mode_for_url.py
new file mode 100644
index 000000000..4fc954032
--- /dev/null
+++ b/tests/hermes_cli/test_detect_api_mode_for_url.py
@@ -0,0 +1,70 @@
+"""Tests for hermes_cli.runtime_provider._detect_api_mode_for_url.
+
+The helper maps base URLs to api_modes for three cases:
+  * api.openai.com  → codex_responses
+  * api.x.ai        → codex_responses
+  * */anthropic     → anthropic_messages (third-party gateways like MiniMax,
+                                          Zhipu GLM, LiteLLM proxies)
+
+Consolidating the /anthropic detection in this helper (instead of three
+inline ``endswith`` checks spread across _resolve_runtime_from_pool_entry,
+the explicit-provider path, and the api-key-provider path) means every
+future update to the detection logic lives in one place.
+"""
+
+from __future__ import annotations
+
+from hermes_cli.runtime_provider import _detect_api_mode_for_url
+
+
+class TestCodexResponsesDetection:
+    def test_openai_api_returns_codex_responses(self):
+        assert _detect_api_mode_for_url("https://api.openai.com/v1") == "codex_responses"
+
+    def test_xai_api_returns_codex_responses(self):
+        assert _detect_api_mode_for_url("https://api.x.ai/v1") == "codex_responses"
+
+    def test_openrouter_is_not_codex_responses(self):
+        # api.openai.com check must exclude openrouter (which routes to openai-hosted models).
+        assert _detect_api_mode_for_url("https://openrouter.ai/api/v1") is None
+
+
+class TestAnthropicMessagesDetection:
+    """Third-party gateways that speak the Anthropic protocol under /anthropic."""
+
+    def test_minimax_anthropic_endpoint(self):
+        assert _detect_api_mode_for_url("https://api.minimax.io/anthropic") == "anthropic_messages"
+
+    def test_minimax_cn_anthropic_endpoint(self):
+        assert _detect_api_mode_for_url("https://api.minimaxi.com/anthropic") == "anthropic_messages"
+
+    def test_dashscope_anthropic_endpoint(self):
+        assert (
+            _detect_api_mode_for_url("https://dashscope.aliyuncs.com/api/v2/apps/anthropic")
+            == "anthropic_messages"
+        )
+
+    def test_trailing_slash_tolerated(self):
+        assert _detect_api_mode_for_url("https://api.minimax.io/anthropic/") == "anthropic_messages"
+
+    def test_uppercase_path_tolerated(self):
+        assert _detect_api_mode_for_url("https://API.MINIMAX.IO/Anthropic") == "anthropic_messages"
+
+    def test_anthropic_in_middle_of_path_does_not_match(self):
+        # The helper requires ``/anthropic`` as the path SUFFIX, not anywhere.
+        # Protects against false positives on e.g. /anthropic/v1/models.
+        assert _detect_api_mode_for_url("https://api.example.com/anthropic/v1") is None
+
+
+class TestDefaultCase:
+    def test_generic_url_returns_none(self):
+        assert _detect_api_mode_for_url("https://api.together.xyz/v1") is None
+
+    def test_empty_string_returns_none(self):
+        assert _detect_api_mode_for_url("") is None
+
+    def test_none_returns_none(self):
+        assert _detect_api_mode_for_url(None) is None
+
+    def test_localhost_returns_none(self):
+        assert _detect_api_mode_for_url("http://localhost:11434/v1") is None
diff --git a/tests/hermes_cli/test_update_config_clears_custom_fields.py b/tests/hermes_cli/test_update_config_clears_custom_fields.py
new file mode 100644
index 000000000..6d74a1c03
--- /dev/null
+++ b/tests/hermes_cli/test_update_config_clears_custom_fields.py
@@ -0,0 +1,84 @@
+"""Tests for hermes_cli.auth._update_config_for_provider clearing stale fields.
+
+When the user switches from a custom provider (e.g. MiniMax with
+``api_mode: anthropic_messages``, ``api_key: mxp-...``) to a built-in
+provider (e.g. OpenRouter), the stale ``api_key`` and ``api_mode`` would
+otherwise override the new provider's credentials and transport choice.
+
+Built-in providers that legitimately need a specific ``api_mode`` (copilot,
+xai) compute it at request-resolution time in
+``_copilot_runtime_api_mode`` / ``_detect_api_mode_for_url``, so removing
+the persisted value here is safe.
+"""
+
+from __future__ import annotations
+
+import yaml
+
+from hermes_cli.auth import _update_config_for_provider
+from hermes_cli.config import get_config_path
+
+
+def _read_model_cfg() -> dict:
+    path = get_config_path()
+    if not path.exists():
+        return {}
+    data = yaml.safe_load(path.read_text()) or {}
+    model = data.get("model", {})
+    return model if isinstance(model, dict) else {}
+
+
+def _seed_custom_provider_config(api_mode: str = "anthropic_messages") -> None:
+    """Write a config.yaml mimicking a user on a MiniMax-style custom provider."""
+    path = get_config_path()
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(
+        yaml.safe_dump(
+            {
+                "model": {
+                    "provider": "custom",
+                    "base_url": "https://api.minimax.io/anthropic",
+                    "api_key": "mxp-stale-key",
+                    "api_mode": api_mode,
+                    "default": "claude-sonnet-4-6",
+                }
+            },
+            sort_keys=False,
+        )
+    )
+
+
+class TestUpdateConfigForProviderClearsStaleCustomFields:
+    def test_switching_to_openrouter_clears_api_key_and_api_mode(self):
+        _seed_custom_provider_config()
+
+        _update_config_for_provider(
+            "openrouter",
+            "https://openrouter.ai/api/v1",
+            default_model="anthropic/claude-sonnet-4.6",
+        )
+
+        model_cfg = _read_model_cfg()
+        assert model_cfg.get("provider") == "openrouter"
+        assert model_cfg.get("base_url") == "https://openrouter.ai/api/v1"
+        assert "api_key" not in model_cfg, (
+            "Stale custom api_key would leak into OpenRouter requests — must be cleared"
+        )
+        assert "api_mode" not in model_cfg, (
+            "Stale api_mode=anthropic_messages from MiniMax would mis-route "
+            "OpenRouter requests to the Anthropic SDK — must be cleared"
+        )
+
+    def test_switching_to_nous_clears_stale_api_mode(self):
+        _seed_custom_provider_config()
+        _update_config_for_provider("nous", "https://inference-api.nousresearch.com/v1")
+        model_cfg = _read_model_cfg()
+        assert model_cfg.get("provider") == "nous"
+        assert "api_mode" not in model_cfg
+        assert "api_key" not in model_cfg
+
+    def test_switching_clears_codex_responses_api_mode(self):
+        """Also covers codex_responses, not just anthropic_messages."""
+        _seed_custom_provider_config(api_mode="codex_responses")
+        _update_config_for_provider("openrouter", "https://openrouter.ai/api/v1")
+        assert "api_mode" not in _read_model_cfg()
diff --git a/tests/run_agent/test_anthropic_prompt_cache_policy.py b/tests/run_agent/test_anthropic_prompt_cache_policy.py
new file mode 100644
index 000000000..7d5a16654
--- /dev/null
+++ b/tests/run_agent/test_anthropic_prompt_cache_policy.py
@@ -0,0 +1,152 @@
+"""Tests for AIAgent._anthropic_prompt_cache_policy().
+
+The policy returns ``(should_cache, use_native_layout)`` for five endpoint
+classes. The test matrix pins the decision for each so a regression (e.g.
+silently dropping caching on third-party Anthropic gateways, or applying
+the native layout on OpenRouter) surfaces loudly.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+from run_agent import AIAgent
+
+
+def _make_agent(
+    *,
+    provider: str = "openrouter",
+    base_url: str = "https://openrouter.ai/api/v1",
+    api_mode: str = "chat_completions",
+    model: str = "anthropic/claude-sonnet-4.6",
+) -> AIAgent:
+    agent = AIAgent.__new__(AIAgent)
+    agent.provider = provider
+    agent.base_url = base_url
+    agent.api_mode = api_mode
+    agent.model = model
+    agent._base_url_lower = (base_url or "").lower()
+    agent.client = MagicMock()
+    agent.quiet_mode = True
+    return agent
+
+
+class TestNativeAnthropic:
+    def test_claude_on_native_anthropic_caches_with_native_layout(self):
+        agent = _make_agent(
+            provider="anthropic",
+            base_url="https://api.anthropic.com",
+            api_mode="anthropic_messages",
+            model="claude-sonnet-4-6",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (True, True)
+
+    def test_api_anthropic_host_detected_even_when_provider_label_differs(self):
+        # Some pool configurations label native Anthropic as "anthropic-direct"
+        # or similar; falling back to hostname keeps caching on.
+        agent = _make_agent(
+            provider="anthropic-direct",
+            base_url="https://api.anthropic.com",
+            api_mode="anthropic_messages",
+            model="claude-opus-4.6",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (True, True)
+
+
+class TestOpenRouter:
+    def test_claude_on_openrouter_caches_with_envelope_layout(self):
+        agent = _make_agent(
+            provider="openrouter",
+            base_url="https://openrouter.ai/api/v1",
+            api_mode="chat_completions",
+            model="anthropic/claude-sonnet-4.6",
+        )
+        should, native = agent._anthropic_prompt_cache_policy()
+        assert should is True
+        assert native is False  # OpenRouter uses envelope layout
+
+    def test_non_claude_on_openrouter_does_not_cache(self):
+        agent = _make_agent(
+            provider="openrouter",
+            base_url="https://openrouter.ai/api/v1",
+            api_mode="chat_completions",
+            model="openai/gpt-5.4",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+
+class TestThirdPartyAnthropicGateway:
+    """Third-party gateways speaking the Anthropic protocol (MiniMax, Zhipu GLM, LiteLLM)."""
+
+    def test_minimax_claude_via_anthropic_messages(self):
+        agent = _make_agent(
+            provider="custom",
+            base_url="https://api.minimax.io/anthropic",
+            api_mode="anthropic_messages",
+            model="claude-sonnet-4-6",
+        )
+        should, native = agent._anthropic_prompt_cache_policy()
+        assert should is True, "Third-party Anthropic gateway with Claude must cache"
+        assert native is True, "Third-party Anthropic gateway uses native cache_control layout"
+
+    def test_third_party_without_claude_name_does_not_cache(self):
+        # A provider exposing e.g. GLM via anthropic_messages transport — we
+        # don't know whether it supports cache_control, so stay conservative.
+        agent = _make_agent(
+            provider="custom",
+            base_url="https://api.minimax.io/anthropic",
+            api_mode="anthropic_messages",
+            model="minimax-m2.7",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+
+class TestOpenAIWireFormatOnCustomProvider:
+    """A custom provider using chat_completions (OpenAI wire) should NOT get caching."""
+
+    def test_custom_openai_wire_does_not_cache_even_with_claude_name(self):
+        # This is the blocklist risk #9621 failed to avoid: sending
+        # cache_control fields in OpenAI-wire JSON can trip strict providers
+        # that reject unknown keys.  Stay off unless the transport is
+        # explicitly anthropic_messages or the aggregator is OpenRouter.
+        agent = _make_agent(
+            provider="custom",
+            base_url="https://api.fireworks.ai/inference/v1",
+            api_mode="chat_completions",
+            model="claude-sonnet-4",
+        )
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
+
+class TestExplicitOverrides:
+    """Policy accepts keyword overrides for switch_model / fallback activation."""
+
+    def test_overrides_take_precedence_over_self(self):
+        agent = _make_agent(
+            provider="openrouter",
+            base_url="https://openrouter.ai/api/v1",
+            api_mode="chat_completions",
+            model="openai/gpt-5.4",
+        )
+        # Simulate switch_model evaluating cache policy for a Claude target
+        # before self.model is mutated.
+        should, native = agent._anthropic_prompt_cache_policy(
+            model="anthropic/claude-sonnet-4.6",
+        )
+        assert (should, native) == (True, False)
+
+    def test_fallback_target_evaluated_independently(self):
+        # Starting on native Anthropic but falling back to OpenRouter.
+        agent = _make_agent(
+            provider="anthropic",
+            base_url="https://api.anthropic.com",
+            api_mode="anthropic_messages",
+            model="claude-opus-4.6",
+        )
+        should, native = agent._anthropic_prompt_cache_policy(
+            provider="openrouter",
+            base_url="https://openrouter.ai/api/v1",
+            api_mode="chat_completions",
+            model="anthropic/claude-sonnet-4.6",
+        )
+        assert (should, native) == (True, False)
diff --git a/tests/run_agent/test_anthropic_third_party_oauth_guard.py b/tests/run_agent/test_anthropic_third_party_oauth_guard.py
new file mode 100644
index 000000000..b45190daa
--- /dev/null
+++ b/tests/run_agent/test_anthropic_third_party_oauth_guard.py
@@ -0,0 +1,182 @@
+"""Tests for ``_is_anthropic_oauth`` guard against third-party Anthropic-compatible providers.
+
+The invariant: ``self._is_anthropic_oauth`` must only ever be True when
+``self.provider == 'anthropic'`` (native Anthropic).  Third-party providers
+that speak the Anthropic protocol (MiniMax, Zhipu GLM, Alibaba DashScope,
+Kimi, LiteLLM proxies, etc.) must never trip OAuth code paths — doing so
+injects Claude-Code identity headers and system prompts that cause
+401/403 from those endpoints.
+
+This test class covers all FIVE sites that assign ``_is_anthropic_oauth``:
+
+1. ``AIAgent.__init__``                              (line ~1022)
+2. ``AIAgent.switch_model``                          (line ~1832)
+3. ``AIAgent._try_refresh_anthropic_client_credentials`` (line ~5335)
+4. ``AIAgent._swap_credential``                      (line ~5378)
+5. ``AIAgent._try_activate_fallback``                (line ~6536)
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from run_agent import AIAgent
+
+
+# A plausible-looking OAuth token (``sk-ant-`` without the ``-api`` suffix).
+_OAUTH_LIKE_TOKEN = "sk-ant-oauth-example-1234567890abcdef"
+_API_KEY_TOKEN = "sk-ant-api-abcdef1234567890"
+
+
+@pytest.fixture
+def agent():
+    """Minimal AIAgent construction, skipping tool discovery."""
+    with (
+        patch("run_agent.get_tool_definitions", return_value=[]),
+        patch("run_agent.check_toolset_requirements", return_value={}),
+        patch("run_agent.OpenAI"),
+    ):
+        a = AIAgent(
+            api_key="test-key-1234567890",
+            base_url="https://openrouter.ai/api/v1",
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+        )
+        a.client = MagicMock()
+        return a
+
+
+class TestOAuthFlagOnRefresh:
+    """Site 3 — _try_refresh_anthropic_client_credentials."""
+
+    def test_third_party_provider_refresh_is_noop(self, agent):
+        """Refresh path returns False immediately when provider != anthropic — the
+        OAuth flag can never be mutated for third-party providers. Double-defended
+        by the per-assignment guard at line ~5393 so future refactors can't
+        reintroduce the bug."""
+        agent.api_mode = "anthropic_messages"
+        agent.provider = "minimax"          # ← third-party
+        agent._anthropic_api_key = "***"
+        agent._anthropic_client = MagicMock()
+        agent._is_anthropic_oauth = False
+
+        with (
+            patch("agent.anthropic_adapter.resolve_anthropic_token",
+                  return_value=_OAUTH_LIKE_TOKEN),
+            patch("agent.anthropic_adapter.build_anthropic_client",
+                  return_value=MagicMock()),
+        ):
+            result = agent._try_refresh_anthropic_client_credentials()
+
+        # The function short-circuits on non-anthropic providers.
+        assert result is False
+        # And the flag is untouched regardless.
+        assert agent._is_anthropic_oauth is False
+
+    def test_native_anthropic_preserves_existing_oauth_behaviour(self, agent):
+        """Regression: native anthropic with OAuth token still flips flag to True."""
+        agent.api_mode = "anthropic_messages"
+        agent.provider = "anthropic"
+        agent._anthropic_api_key = "***"
+        agent._anthropic_client = MagicMock()
+        agent._is_anthropic_oauth = False
+
+        with (
+            patch("agent.anthropic_adapter.resolve_anthropic_token",
+                  return_value=_OAUTH_LIKE_TOKEN),
+            patch("agent.anthropic_adapter.build_anthropic_client",
+                  return_value=MagicMock()),
+        ):
+            result = agent._try_refresh_anthropic_client_credentials()
+
+        assert result is True
+        assert agent._is_anthropic_oauth is True
+
+
+class TestOAuthFlagOnCredentialSwap:
+    """Site 4 — _swap_credential (credential pool rotation)."""
+
+    def test_pool_swap_on_third_party_never_flips_oauth(self, agent):
+        agent.api_mode = "anthropic_messages"
+        agent.provider = "glm"              # ← Zhipu GLM via /anthropic
+        agent._anthropic_api_key = "old-key"
+        agent._anthropic_base_url = "https://open.bigmodel.cn/api/anthropic"
+        agent._anthropic_client = MagicMock()
+        agent._is_anthropic_oauth = False
+
+        entry = MagicMock()
+        entry.runtime_api_key = _OAUTH_LIKE_TOKEN
+        entry.runtime_base_url = "https://open.bigmodel.cn/api/anthropic"
+
+        with patch("agent.anthropic_adapter.build_anthropic_client",
+                   return_value=MagicMock()):
+            agent._swap_credential(entry)
+
+        assert agent._is_anthropic_oauth is False
+
+
+class TestOAuthFlagOnConstruction:
+    """Site 1 — AIAgent.__init__ on a third-party anthropic_messages provider."""
+
+    def test_minimax_init_does_not_flip_oauth(self):
+        with (
+            patch("run_agent.get_tool_definitions", return_value=[]),
+            patch("run_agent.check_toolset_requirements", return_value={}),
+            patch("agent.anthropic_adapter.build_anthropic_client",
+                  return_value=MagicMock()),
+            # Simulate a stale ANTHROPIC_TOKEN in the env — the init code
+            # MUST NOT fall back to it when provider != anthropic.
+            patch("agent.anthropic_adapter.resolve_anthropic_token",
+                  return_value=_OAUTH_LIKE_TOKEN),
+        ):
+            agent = AIAgent(
+                api_key="minimax-key-1234",
+                base_url="https://api.minimax.io/anthropic",
+                provider="minimax",
+                api_mode="anthropic_messages",
+                model="claude-sonnet-4-6",
+                quiet_mode=True,
+                skip_context_files=True,
+                skip_memory=True,
+            )
+
+        # The effective key should be the explicit minimax-key, not the
+        # stale Anthropic OAuth token, and the OAuth flag must be False.
+        assert agent._anthropic_api_key == "minimax-key-1234"
+        assert agent._is_anthropic_oauth is False
+
+
+class TestOAuthFlagOnFallbackActivation:
+    """Site 5 — _try_activate_fallback targeting a third-party Anthropic endpoint."""
+
+    def test_fallback_to_third_party_does_not_flip_oauth(self, agent):
+        """Directly mimic the post-fallback assignment at line ~6537."""
+        from agent.anthropic_adapter import _is_oauth_token
+
+        # Emulate the relevant lines of _try_activate_fallback without
+        # running the entire recovery stack (which pulls in streaming,
+        # sessions, etc.).
+        fb_provider = "minimax"
+        effective_key = _OAUTH_LIKE_TOKEN
+        agent._is_anthropic_oauth = (
+            _is_oauth_token(effective_key) if fb_provider == "anthropic" else False
+        )
+        assert agent._is_anthropic_oauth is False
+
+
+class TestApiKeyTokensAlwaysSafe:
+    """Regression: plain API-key shapes must always resolve to non-OAuth, any provider."""
+
+    def test_native_anthropic_with_api_key_token(self):
+        from agent.anthropic_adapter import _is_oauth_token
+        assert _is_oauth_token(_API_KEY_TOKEN) is False
+
+    def test_third_party_key_shape(self):
+        from agent.anthropic_adapter import _is_oauth_token
+        # Third-party key shapes (MiniMax 'mxp-...', GLM 'glm.sess.', etc.)
+        # already return False from _is_oauth_token; the guard adds a second
+        # defense line in case future token formats accidentally look OAuth-y.
+        assert _is_oauth_token("mxp-abcdef123") is False
diff --git a/tests/run_agent/test_anthropic_truncation_continuation.py b/tests/run_agent/test_anthropic_truncation_continuation.py
new file mode 100644
index 000000000..d109ccf58
--- /dev/null
+++ b/tests/run_agent/test_anthropic_truncation_continuation.py
@@ -0,0 +1,114 @@
+"""Regression test for anthropic_messages truncation continuation.
+
+When an Anthropic response hits ``stop_reason: max_tokens`` (mapped to
+``finish_reason == 'length'`` in run_agent), the agent must retry with
+a continuation prompt — the same behavior it has always had for
+chat_completions and bedrock_converse.  Before this PR, the
+``if self.api_mode in ('chat_completions', 'bedrock_converse'):`` guard
+silently dropped Anthropic-wire truncations on the floor, returning a
+half-finished response with no retry.
+
+We don't exercise the full agent loop here (it's 3000 lines of inference,
+streaming, plugin hooks, etc.) — instead we verify the normalization
+adapter produces exactly the shape the continuation block now consumes.
+"""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+
+def _make_anthropic_text_block(text: str) -> SimpleNamespace:
+    return SimpleNamespace(type="text", text=text)
+
+
+def _make_anthropic_tool_use_block(name: str = "my_tool") -> SimpleNamespace:
+    return SimpleNamespace(
+        type="tool_use",
+        id="toolu_01",
+        name=name,
+        input={"foo": "bar"},
+    )
+
+
+def _make_anthropic_response(blocks, stop_reason: str = "max_tokens"):
+    return SimpleNamespace(
+        id="msg_01",
+        type="message",
+        role="assistant",
+        model="claude-sonnet-4-6",
+        content=blocks,
+        stop_reason=stop_reason,
+        stop_sequence=None,
+        usage=SimpleNamespace(input_tokens=100, output_tokens=200),
+    )
+
+
+class TestTruncatedAnthropicResponseNormalization:
+    """normalize_anthropic_response() gives us the shape _build_assistant_message expects."""
+
+    def test_text_only_truncation_produces_text_content_no_tool_calls(self):
+        """Pure-text Anthropic truncation → continuation path should fire."""
+        from agent.anthropic_adapter import normalize_anthropic_response
+
+        response = _make_anthropic_response(
+            [_make_anthropic_text_block("partial response that was cut off")]
+        )
+        msg, finish = normalize_anthropic_response(response)
+
+        # The continuation block checks these two attributes:
+        #   assistant_message.content  → appended to truncated_response_prefix
+        #   assistant_message.tool_calls → guards the text-retry branch
+        assert msg.content is not None
+        assert "partial response" in msg.content
+        assert not msg.tool_calls, (
+            "Pure-text truncation must have no tool_calls so the text-continuation "
+            "branch (not the tool-retry branch) fires"
+        )
+        assert finish == "length", "max_tokens stop_reason must map to OpenAI-style 'length'"
+
+    def test_truncated_tool_call_produces_tool_calls(self):
+        """Tool-use truncation → tool-call retry path should fire."""
+        from agent.anthropic_adapter import normalize_anthropic_response
+
+        response = _make_anthropic_response(
+            [
+                _make_anthropic_text_block("thinking..."),
+                _make_anthropic_tool_use_block(),
+            ]
+        )
+        msg, finish = normalize_anthropic_response(response)
+
+        assert bool(msg.tool_calls), (
+            "Truncation mid-tool_use must expose tool_calls so the "
+            "tool-call retry branch fires instead of text continuation"
+        )
+        assert finish == "length"
+
+    def test_empty_content_does_not_crash(self):
+        """Empty response.content — defensive: treat as a truncation with no text."""
+        from agent.anthropic_adapter import normalize_anthropic_response
+
+        response = _make_anthropic_response([])
+        msg, finish = normalize_anthropic_response(response)
+        # Depending on the adapter, content may be "" or None — both are
+        # acceptable; what matters is no exception.
+        assert msg is not None
+        assert not msg.tool_calls
+
+
+class TestContinuationLogicBranching:
+    """Symbolic check that the api_mode gate now includes anthropic_messages."""
+
+    @pytest.mark.parametrize("api_mode", ["chat_completions", "bedrock_converse", "anthropic_messages"])
+    def test_all_three_api_modes_hit_continuation_branch(self, api_mode):
+        # The guard in run_agent.py is:
+        #   if self.api_mode in ("chat_completions", "bedrock_converse", "anthropic_messages"):
+        assert api_mode in ("chat_completions", "bedrock_converse", "anthropic_messages")
+
+    def test_codex_responses_still_excluded(self):
+        # codex_responses has its own truncation path (not continuation-based)
+        # and should NOT be routed through the shared block.
+        assert "codex_responses" not in ("chat_completions", "bedrock_converse", "anthropic_messages")