diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
new file mode 100644
index 000000000..d19d50da6
--- /dev/null
+++ b/agent/anthropic_adapter.py
@@ -0,0 +1,351 @@
+"""Anthropic Messages API adapter for Hermes Agent.
+
+Translates between Hermes's internal OpenAI-style message format and
+Anthropic's Messages API. Follows the same pattern as the codex_responses
+adapter — all provider-specific logic is isolated here.
+
+Auth supports:
+  - Regular API keys (sk-ant-api*) → x-api-key header
+  - OAuth setup-tokens (sk-ant-oat*) → Bearer auth + beta header
+  - Claude Code credentials (~/.claude/.credentials.json) → Bearer auth
+"""
+
+import json
+import logging
+import os
+from pathlib import Path
+from types import SimpleNamespace
+from typing import Any, Dict, List, Optional, Tuple
+
+try:
+    import anthropic as _anthropic_sdk
+except ImportError:
+    _anthropic_sdk = None  # type: ignore[assignment]
+
+logger = logging.getLogger(__name__)
+
+THINKING_BUDGET = {"xhigh": 32000, "high": 16000, "medium": 8000, "low": 4000}
+
+# Beta headers required for OAuth/subscription auth
+_OAUTH_BETAS = ["oauth-2025-04-20"]
+
+
+def _is_oauth_token(key: str) -> bool:
+    """Check if the key is an OAuth access/setup token (not a regular API key)."""
+    return key.startswith("sk-ant-oat")
+
+
+def build_anthropic_client(api_key: str, base_url: str = None):
+    """Create an Anthropic client, auto-detecting setup-tokens vs API keys.
+
+    Returns an anthropic.Anthropic instance.
+    """
+    if _anthropic_sdk is None:
+        raise ImportError(
+            "The 'anthropic' package is required for the Anthropic provider. "
+            "Install it with: pip install 'anthropic>=0.39.0'"
+        )
+    from httpx import Timeout
+
+    kwargs = {
+        "timeout": Timeout(timeout=900.0, connect=10.0),
+    }
+    if base_url:
+        kwargs["base_url"] = base_url
+
+    if _is_oauth_token(api_key):
+        # OAuth access token / setup-token → Bearer auth + beta header
+        kwargs["auth_token"] = api_key
+        kwargs["default_headers"] = {"anthropic-beta": ",".join(_OAUTH_BETAS)}
+    else:
+        # Regular API key → x-api-key header
+        kwargs["api_key"] = api_key
+
+    return _anthropic_sdk.Anthropic(**kwargs)
+
+
+def read_claude_code_credentials() -> Optional[Dict[str, Any]]:
+    """Read credentials from Claude Code's credential file.
+
+    Returns dict with {accessToken, refreshToken, expiresAt} or None.
+    """
+    cred_path = Path.home() / ".claude" / ".credentials.json"
+    if not cred_path.exists():
+        return None
+
+    try:
+        data = json.loads(cred_path.read_text(encoding="utf-8"))
+        oauth_data = data.get("claudeAiOauth")
+        if not oauth_data or not isinstance(oauth_data, dict):
+            return None
+
+        access_token = oauth_data.get("accessToken", "")
+        if not access_token:
+            return None
+
+        return {
+            "accessToken": access_token,
+            "refreshToken": oauth_data.get("refreshToken", ""),
+            "expiresAt": oauth_data.get("expiresAt", 0),
+        }
+    except (json.JSONDecodeError, OSError, IOError) as e:
+        logger.debug("Failed to read Claude Code credentials: %s", e)
+        return None
+
+
+def is_claude_code_token_valid(creds: Dict[str, Any]) -> bool:
+    """Check if Claude Code credentials have a non-expired access token."""
+    import time
+
+    expires_at = creds.get("expiresAt", 0)
+    if not expires_at:
+        return bool(creds.get("accessToken"))
+
+    # expiresAt is in milliseconds since epoch
+    now_ms = int(time.time() * 1000)
+    # Allow 60 seconds of buffer
+    return now_ms < (expires_at - 60_000)
+
+
+def resolve_anthropic_token() -> Optional[str]:
+    """Resolve an Anthropic token from all available sources.
+
+    Priority:
+      1. ANTHROPIC_API_KEY env var (regular API key)
+      2. ANTHROPIC_TOKEN env var (OAuth/setup token)
+      3. Claude Code credentials (~/.claude/.credentials.json)
+
+    Returns the token string or None.
+    """
+    # 1. Regular API key
+    api_key = os.getenv("ANTHROPIC_API_KEY", "").strip()
+    if api_key:
+        return api_key
+
+    # 2. OAuth/setup token env var
+    token = os.getenv("ANTHROPIC_TOKEN", "").strip()
+    if token:
+        return token
+
+    # Also check CLAUDE_CODE_OAUTH_TOKEN (used by Claude Code for setup-tokens)
+    cc_token = os.getenv("CLAUDE_CODE_OAUTH_TOKEN", "").strip()
+    if cc_token:
+        return cc_token
+
+    # 3. Claude Code credential file
+    creds = read_claude_code_credentials()
+    if creds and is_claude_code_token_valid(creds):
+        logger.debug("Using Claude Code credentials from ~/.claude/.credentials.json")
+        return creds["accessToken"]
+    elif creds:
+        logger.debug("Claude Code credentials expired — run 'claude' to refresh")
+
+    return None
+
+
+# ---------------------------------------------------------------------------
+# Message / tool / response format conversion
+# ---------------------------------------------------------------------------
+
+
+def normalize_model_name(model: str) -> str:
+    """Normalize a model name for the Anthropic API.
+
+    - Strips 'anthropic/' prefix (OpenRouter format)
+    """
+    if model.startswith("anthropic/"):
+        model = model[len("anthropic/"):]
+    return model
+
+
+def convert_tools_to_anthropic(tools: List[Dict]) -> List[Dict]:
+    """Convert OpenAI tool definitions to Anthropic format."""
+    if not tools:
+        return []
+    result = []
+    for t in tools:
+        fn = t.get("function", {})
+        result.append({
+            "name": fn.get("name", ""),
+            "description": fn.get("description", ""),
+            "input_schema": fn.get("parameters", {"type": "object", "properties": {}}),
+        })
+    return result
+
+
+def convert_messages_to_anthropic(
+    messages: List[Dict],
+) -> Tuple[Optional[Any], List[Dict]]:
+    """Convert OpenAI-format messages to Anthropic format.
+
+    Returns (system_prompt, anthropic_messages).
+    System messages are extracted since Anthropic takes them as a separate param.
+    system_prompt is a string or list of content blocks (when cache_control present).
+    """
+    system = None
+    result = []
+
+    for m in messages:
+        role = m.get("role", "user")
+        content = m.get("content", "")
+
+        if role == "system":
+            if isinstance(content, list):
+                # Preserve cache_control markers on content blocks
+                has_cache = any(
+                    p.get("cache_control") for p in content if isinstance(p, dict)
+                )
+                if has_cache:
+                    system = [p for p in content if isinstance(p, dict)]
+                else:
+                    system = "\n".join(
+                        p["text"] for p in content if p.get("type") == "text"
+                    )
+            else:
+                system = content
+            continue
+
+        if role == "assistant":
+            blocks = []
+            if content:
+                text = content if isinstance(content, str) else json.dumps(content)
+                blocks.append({"type": "text", "text": text})
+            for tc in m.get("tool_calls", []):
+                fn = tc.get("function", {})
+                args = fn.get("arguments", "{}")
+                blocks.append({
+                    "type": "tool_use",
+                    "id": tc.get("id", ""),
+                    "name": fn.get("name", ""),
+                    "input": json.loads(args) if isinstance(args, str) else args,
+                })
+            result.append({"role": "assistant", "content": blocks or content})
+            continue
+
+        if role == "tool":
+            tool_result = {
+                "type": "tool_result",
+                "tool_use_id": m.get("tool_call_id", ""),
+                "content": content if isinstance(content, str) else json.dumps(content),
+            }
+            # Merge consecutive tool results into one user message
+            if (
+                result
+                and result[-1]["role"] == "user"
+                and isinstance(result[-1]["content"], list)
+                and result[-1]["content"]
+                and result[-1]["content"][0].get("type") == "tool_result"
+            ):
+                result[-1]["content"].append(tool_result)
+            else:
+                result.append({"role": "user", "content": [tool_result]})
+            continue
+
+        # Regular user message
+        result.append({"role": "user", "content": content})
+
+    # Strip orphaned tool_use blocks (no matching tool_result follows)
+    tool_result_ids = set()
+    for m in result:
+        if m["role"] == "user" and isinstance(m["content"], list):
+            for block in m["content"]:
+                if block.get("type") == "tool_result":
+                    tool_result_ids.add(block.get("tool_use_id"))
+    for m in result:
+        if m["role"] == "assistant" and isinstance(m["content"], list):
+            m["content"] = [
+                b
+                for b in m["content"]
+                if b.get("type") != "tool_use" or b.get("id") in tool_result_ids
+            ]
+            if not m["content"]:
+                m["content"] = [{"type": "text", "text": "(tool call removed)"}]
+
+    return system, result
+
+
+def build_anthropic_kwargs(
+    model: str,
+    messages: List[Dict],
+    tools: Optional[List[Dict]],
+    max_tokens: Optional[int],
+    reasoning_config: Optional[Dict[str, Any]],
+) -> Dict[str, Any]:
+    """Build kwargs for anthropic.messages.create()."""
+    system, anthropic_messages = convert_messages_to_anthropic(messages)
+    anthropic_tools = convert_tools_to_anthropic(tools) if tools else []
+
+    model = normalize_model_name(model)
+    effective_max_tokens = max_tokens or 16384
+
+    kwargs: Dict[str, Any] = {
+        "model": model,
+        "messages": anthropic_messages,
+        "max_tokens": effective_max_tokens,
+    }
+
+    if system:
+        kwargs["system"] = system
+
+    if anthropic_tools:
+        kwargs["tools"] = anthropic_tools
+
+    # Map reasoning_config to Anthropic's thinking parameter
+    if reasoning_config and isinstance(reasoning_config, dict):
+        if reasoning_config.get("enabled") is not False:
+            effort = reasoning_config.get("effort", "medium")
+            budget = THINKING_BUDGET.get(effort, 8000)
+            kwargs["thinking"] = {"type": "enabled", "budget_tokens": budget}
+            kwargs["max_tokens"] = max(effective_max_tokens, budget + 4096)
+
+    return kwargs
+
+
+def normalize_anthropic_response(
+    response,
+) -> Tuple[SimpleNamespace, str]:
+    """Normalize Anthropic response to match the shape expected by AIAgent.
+
+    Returns (assistant_message, finish_reason) where assistant_message has
+    .content, .tool_calls, and .reasoning attributes.
+    """
+    text_parts = []
+    reasoning_parts = []
+    tool_calls = []
+
+    for block in response.content:
+        if block.type == "text":
+            text_parts.append(block.text)
+        elif block.type == "thinking":
+            reasoning_parts.append(block.thinking)
+        elif block.type == "tool_use":
+            tool_calls.append(
+                SimpleNamespace(
+                    id=block.id,
+                    type="function",
+                    function=SimpleNamespace(
+                        name=block.name,
+                        arguments=json.dumps(block.input),
+                    ),
+                )
+            )
+
+    # Map Anthropic stop_reason to OpenAI finish_reason
+    stop_reason_map = {
+        "end_turn": "stop",
+        "tool_use": "tool_calls",
+        "max_tokens": "length",
+        "stop_sequence": "stop",
+    }
+    finish_reason = stop_reason_map.get(response.stop_reason, "stop")
+
+    return (
+        SimpleNamespace(
+            content="\n".join(text_parts) if text_parts else None,
+            tool_calls=tool_calls or None,
+            reasoning="\n\n".join(reasoning_parts) if reasoning_parts else None,
+            reasoning_content=None,
+            reasoning_details=None,
+        ),
+        finish_reason,
+    )
diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 1c6ac271f..a2175bed7 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -51,6 +51,7 @@ _API_KEY_PROVIDER_AUX_MODELS: Dict[str, str] = {
     "kimi-coding": "kimi-k2-turbo-preview",
     "minimax": "MiniMax-M2.5-highspeed",
     "minimax-cn": "MiniMax-M2.5-highspeed",
+    "anthropic": "claude-haiku-4-5-20251001",
 }
 
 # OpenRouter app attribution headers
diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index e8d1e51b4..6ad741998 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -41,6 +41,10 @@ DEFAULT_CONTEXT_LENGTHS = {
     "anthropic/claude-sonnet-4": 200000,
     "anthropic/claude-sonnet-4-20250514": 200000,
     "anthropic/claude-haiku-4.5": 200000,
+    # Bare Anthropic model IDs (for native API provider)
+    "claude-opus-4-20250514": 200000,
+    "claude-sonnet-4-20250514": 200000,
+    "claude-haiku-4-5-20251001": 200000,
     "openai/gpt-4o": 128000,
     "openai/gpt-4-turbo": 128000,
     "openai/gpt-4o-mini": 128000,
diff --git a/hermes_cli/auth.py b/hermes_cli/auth.py
index 1ffa85bdc..c1b083484 100644
--- a/hermes_cli/auth.py
+++ b/hermes_cli/auth.py
@@ -132,6 +132,13 @@ PROVIDER_REGISTRY: Dict[str, ProviderConfig] = {
         api_key_env_vars=("MINIMAX_API_KEY",),
         base_url_env_var="MINIMAX_BASE_URL",
     ),
+    "anthropic": ProviderConfig(
+        id="anthropic",
+        name="Anthropic",
+        auth_type="api_key",
+        inference_base_url="https://api.anthropic.com",
+        api_key_env_vars=("ANTHROPIC_API_KEY", "ANTHROPIC_TOKEN", "CLAUDE_CODE_OAUTH_TOKEN"),
+    ),
     "minimax-cn": ProviderConfig(
         id="minimax-cn",
         name="MiniMax (China)",
@@ -516,6 +523,7 @@ def resolve_provider(
         "glm": "zai", "z-ai": "zai", "z.ai": "zai", "zhipu": "zai",
         "kimi": "kimi-coding", "moonshot": "kimi-coding",
         "minimax-china": "minimax-cn", "minimax_cn": "minimax-cn",
+        "claude": "anthropic", "claude-code": "anthropic",
     }
     normalized = _PROVIDER_ALIASES.get(normalized, normalized)
 
diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 8e25bc2df..7a8653272 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -2035,7 +2035,7 @@ For more help on a command:
     )
     chat_parser.add_argument(
         "--provider",
-        choices=["auto", "openrouter", "nous", "openai-codex", "zai", "kimi-coding", "minimax", "minimax-cn"],
+        choices=["auto", "openrouter", "nous", "openai-codex", "anthropic", "zai", "kimi-coding", "minimax", "minimax-cn"],
         default=None,
         help="Inference provider (default: auto)"
     )
diff --git a/hermes_cli/models.py b/hermes_cli/models.py
index 92dcbf975..f67c5aa28 100644
--- a/hermes_cli/models.py
+++ b/hermes_cli/models.py
@@ -68,6 +68,11 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
         "MiniMax-M2.5-highspeed",
         "MiniMax-M2.1",
     ],
+    "anthropic": [
+        "claude-sonnet-4-20250514",
+        "claude-opus-4-20250514",
+        "claude-haiku-4-5-20251001",
+    ],
 }
 
 _PROVIDER_LABELS = {
@@ -78,6 +83,7 @@ _PROVIDER_LABELS = {
     "kimi-coding": "Kimi / Moonshot",
     "minimax": "MiniMax",
     "minimax-cn": "MiniMax (China)",
+    "anthropic": "Anthropic",
     "custom": "Custom endpoint",
 }
 
@@ -90,6 +96,8 @@ _PROVIDER_ALIASES = {
     "moonshot": "kimi-coding",
     "minimax-china": "minimax-cn",
     "minimax_cn": "minimax-cn",
+    "claude": "anthropic",
+    "claude-code": "anthropic",
 }
 
 
@@ -123,7 +131,7 @@ def list_available_providers() -> list[dict[str, str]]:
     # Canonical providers in display order
     _PROVIDER_ORDER = [
         "openrouter", "nous", "openai-codex",
-        "zai", "kimi-coding", "minimax", "minimax-cn",
+        "zai", "kimi-coding", "minimax", "minimax-cn", "anthropic",
     ]
     # Build reverse alias map
     aliases_for: dict[str, list[str]] = {}
diff --git a/hermes_cli/runtime_provider.py b/hermes_cli/runtime_provider.py
index 4e6910dad..474295ea6 100644
--- a/hermes_cli/runtime_provider.py
+++ b/hermes_cli/runtime_provider.py
@@ -153,6 +153,19 @@ def resolve_runtime_provider(
             "requested_provider": requested_provider,
         }
 
+    # Anthropic (native Messages API)
+    if provider == "anthropic":
+        from agent.anthropic_adapter import resolve_anthropic_token
+        token = resolve_anthropic_token()
+        return {
+            "provider": "anthropic",
+            "api_mode": "anthropic_messages",
+            "base_url": "https://api.anthropic.com",
+            "api_key": token or "",
+            "source": "env",
+            "requested_provider": requested_provider,
+        }
+
     # API-key providers (z.ai/GLM, Kimi, MiniMax, MiniMax-CN)
     pconfig = PROVIDER_REGISTRY.get(provider)
     if pconfig and pconfig.auth_type == "api_key":
diff --git a/hermes_cli/setup.py b/hermes_cli/setup.py
index 02fb3c4b4..5b29b9d24 100644
--- a/hermes_cli/setup.py
+++ b/hermes_cli/setup.py
@@ -626,6 +626,7 @@ def setup_model_provider(config: dict):
         "Kimi / Moonshot (Kimi coding models)",
         "MiniMax (global endpoint)",
         "MiniMax China (mainland China endpoint)",
+        "Anthropic (Claude models — API key or Claude Code subscription)",
     ]
     if keep_label:
         provider_choices.append(keep_label)
@@ -1004,7 +1005,53 @@ def setup_model_provider(config: dict):
         _update_config_for_provider("minimax-cn", pconfig.inference_base_url)
         _set_model_provider(config, "minimax-cn", pconfig.inference_base_url)
 
-    # else: provider_idx == 8 (Keep current) — only shown when a provider already exists
+    elif provider_idx == 8:  # Anthropic
+        selected_provider = "anthropic"
+        print()
+        print_header("Anthropic API Key or Claude Code Credentials")
+        from hermes_cli.auth import PROVIDER_REGISTRY
+        pconfig = PROVIDER_REGISTRY["anthropic"]
+        print_info(f"Provider: {pconfig.name}")
+        print_info("Accepts API keys (sk-ant-api-*) or setup-tokens (sk-ant-oat-*)")
+        print_info("Get an API key at: https://console.anthropic.com/")
+        print_info("Or run 'claude setup-token' to get a setup-token from Claude Code")
+        print()
+
+        # Check for Claude Code credential auto-discovery
+        from agent.anthropic_adapter import read_claude_code_credentials, is_claude_code_token_valid
+        cc_creds = read_claude_code_credentials()
+        if cc_creds and is_claude_code_token_valid(cc_creds):
+            print_success("Found valid Claude Code credentials (~/.claude/.credentials.json)")
+            if not prompt_yes_no("Use Claude Code credentials? (You can also enter an API key)", True):
+                cc_creds = None
+
+        existing_key = get_env_value("ANTHROPIC_API_KEY") or get_env_value("ANTHROPIC_TOKEN")
+        if cc_creds and is_claude_code_token_valid(cc_creds):
+            # Use Claude Code creds — no need to prompt for a key
+            print_success("Using Claude Code subscription credentials")
+        elif existing_key:
+            print_info(f"Current: {existing_key[:12]}... (configured)")
+            if prompt_yes_no("Update key?", False):
+                api_key = prompt("Enter Anthropic API key or setup-token", password=True)
+                if api_key:
+                    save_env_value("ANTHROPIC_API_KEY", api_key)
+                    print_success("Anthropic key saved")
+        else:
+            api_key = prompt("Enter Anthropic API key or setup-token", password=True)
+            if api_key:
+                save_env_value("ANTHROPIC_API_KEY", api_key)
+                print_success("Anthropic key saved")
+            else:
+                print_warning("Skipped - agent won't work without an API key")
+
+        # Clear custom endpoint vars if switching
+        if existing_custom:
+            save_env_value("OPENAI_BASE_URL", "")
+            save_env_value("OPENAI_API_KEY", "")
+        _update_config_for_provider("anthropic", pconfig.inference_base_url)
+        _set_model_provider(config, "anthropic", pconfig.inference_base_url)
+
+    # else: provider_idx == 9 (Keep current) — only shown when a provider already exists
 
     # ── OpenRouter API Key for tools (if not already set) ──
     # Tools (vision, web, MoA) use OpenRouter independently of the main provider.
@@ -1017,6 +1064,7 @@ def setup_model_provider(config: dict):
         "kimi-coding",
         "minimax",
         "minimax-cn",
+        "anthropic",
     ) and not get_env_value("OPENROUTER_API_KEY"):
         print()
         print_header("OpenRouter API Key (for tools)")
@@ -1160,6 +1208,26 @@ def setup_model_provider(config: dict):
                 if custom:
                     _set_default_model(config, custom)
             # else: keep current
+        elif selected_provider == "anthropic":
+            anthropic_models = [
+                "claude-sonnet-4-20250514",
+                "claude-opus-4-20250514",
+                "claude-haiku-4-5-20251001",
+            ]
+            model_choices = list(anthropic_models)
+            model_choices.append("Custom model")
+            model_choices.append(f"Keep current ({current_model})")
+
+            keep_idx = len(model_choices) - 1
+            model_idx = prompt_choice("Select default model:", model_choices, keep_idx)
+
+            if model_idx < len(anthropic_models):
+                _set_default_model(config, anthropic_models[model_idx])
+            elif model_idx == len(anthropic_models):
+                custom = prompt("Enter model name (e.g., claude-sonnet-4-20250514)")
+                if custom:
+                    _set_default_model(config, custom)
+            # else: keep current
         else:
             # Static list for OpenRouter / fallback (from canonical list)
             from hermes_cli.models import model_ids, menu_labels
diff --git a/pyproject.toml b/pyproject.toml
index 876c47f73..fef457e83 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,7 @@ license = { text = "MIT" }
 dependencies = [
   # Core
   "openai",
+  "anthropic>=0.39.0",
   "python-dotenv",
   "fire",
   "httpx",
diff --git a/run_agent.py b/run_agent.py
index 7808435d0..20a05d88b 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -296,13 +296,16 @@ class AIAgent:
         self.base_url = base_url or OPENROUTER_BASE_URL
         provider_name = provider.strip().lower() if isinstance(provider, str) and provider.strip() else None
         self.provider = provider_name or "openrouter"
-        if api_mode in {"chat_completions", "codex_responses"}:
+        if api_mode in {"chat_completions", "codex_responses", "anthropic_messages"}:
             self.api_mode = api_mode
         elif self.provider == "openai-codex":
             self.api_mode = "codex_responses"
         elif (provider_name is None) and "chatgpt.com/backend-api/codex" in self.base_url.lower():
             self.api_mode = "codex_responses"
             self.provider = "openai-codex"
+        elif self.provider == "anthropic" or (provider_name is None and "api.anthropic.com" in self.base_url.lower()):
+            self.api_mode = "anthropic_messages"
+            self.provider = "anthropic"
         else:
             self.api_mode = "chat_completions"
 
@@ -343,7 +346,8 @@ class AIAgent:
         # conversation prefix. Uses system_and_3 strategy (4 breakpoints).
         is_openrouter = "openrouter" in self.base_url.lower()
         is_claude = "claude" in self.model.lower()
-        self._use_prompt_caching = is_openrouter and is_claude
+        is_native_anthropic = self.api_mode == "anthropic_messages"
+        self._use_prompt_caching = (is_openrouter and is_claude) or is_native_anthropic
         self._cache_ttl = "5m"  # Default 5-minute TTL (1.25x write cost)
         
         # Iteration budget pressure: warn the LLM as it approaches max_iterations.
@@ -420,66 +424,84 @@ class AIAgent:
                 ]:
                     logging.getLogger(quiet_logger).setLevel(logging.ERROR)
         
-        # Initialize OpenAI client via centralized provider router.
+        # Initialize LLM client via centralized provider router.
         # The router handles auth resolution, base URL, headers, and
-        # Codex wrapping for all known providers.
+        # Codex/Anthropic wrapping for all known providers.
         # raw_codex=True because the main agent needs direct responses.stream()
         # access for Codex Responses API streaming.
-        if api_key and base_url:
-            # Explicit credentials from CLI/gateway — construct directly.
-            # The runtime provider resolver already handled auth for us.
-            client_kwargs = {"api_key": api_key, "base_url": base_url}
-            effective_base = base_url
-            if "openrouter" in effective_base.lower():
-                client_kwargs["default_headers"] = {
-                    "HTTP-Referer": "https://github.com/NousResearch/hermes-agent",
-                    "X-OpenRouter-Title": "Hermes Agent",
-                    "X-OpenRouter-Categories": "productivity,cli-agent",
-                }
-            elif "api.kimi.com" in effective_base.lower():
-                client_kwargs["default_headers"] = {
-                    "User-Agent": "KimiCLI/1.3",
-                }
+        self._anthropic_client = None
+
+        if self.api_mode == "anthropic_messages":
+            from agent.anthropic_adapter import build_anthropic_client
+            effective_key = api_key or os.getenv("ANTHROPIC_API_KEY", "") or os.getenv("ANTHROPIC_TOKEN", "")
+            if not effective_key:
+                from agent.anthropic_adapter import resolve_anthropic_token
+                effective_key = resolve_anthropic_token() or ""
+            self._anthropic_api_key = effective_key
+            self._anthropic_client = build_anthropic_client(effective_key, base_url if base_url and "anthropic" in base_url else None)
+            # No OpenAI client needed for Anthropic mode
+            self.client = None
+            self._client_kwargs = {}
+            if not self.quiet_mode:
+                print(f"🤖 AI Agent initialized with model: {self.model} (Anthropic native)")
+                if effective_key and len(effective_key) > 12:
+                    print(f"🔑 Using token: {effective_key[:8]}...{effective_key[-4:]}")
         else:
-            # No explicit creds — use the centralized provider router
-            from agent.auxiliary_client import resolve_provider_client
-            _routed_client, _ = resolve_provider_client(
-                self.provider or "auto", model=self.model, raw_codex=True)
-            if _routed_client is not None:
-                client_kwargs = {
-                    "api_key": _routed_client.api_key,
-                    "base_url": str(_routed_client.base_url),
-                }
-                # Preserve any default_headers the router set
-                if hasattr(_routed_client, '_default_headers') and _routed_client._default_headers:
-                    client_kwargs["default_headers"] = dict(_routed_client._default_headers)
-            else:
-                # Final fallback: try raw OpenRouter key
-                client_kwargs = {
-                    "api_key": os.getenv("OPENROUTER_API_KEY", ""),
-                    "base_url": OPENROUTER_BASE_URL,
-                    "default_headers": {
+            if api_key and base_url:
+                # Explicit credentials from CLI/gateway — construct directly.
+                # The runtime provider resolver already handled auth for us.
+                client_kwargs = {"api_key": api_key, "base_url": base_url}
+                effective_base = base_url
+                if "openrouter" in effective_base.lower():
+                    client_kwargs["default_headers"] = {
                         "HTTP-Referer": "https://github.com/NousResearch/hermes-agent",
                         "X-OpenRouter-Title": "Hermes Agent",
                         "X-OpenRouter-Categories": "productivity,cli-agent",
-                    },
-                }
-        
-        self._client_kwargs = client_kwargs  # stored for rebuilding after interrupt
-        try:
-            self.client = OpenAI(**client_kwargs)
-            if not self.quiet_mode:
-                print(f"🤖 AI Agent initialized with model: {self.model}")
-                if base_url:
-                    print(f"🔗 Using custom base URL: {base_url}")
-                # Always show API key info (masked) for debugging auth issues
-                key_used = client_kwargs.get("api_key", "none")
-                if key_used and key_used != "dummy-key" and len(key_used) > 12:
-                    print(f"🔑 Using API key: {key_used[:8]}...{key_used[-4:]}")
+                    }
+                elif "api.kimi.com" in effective_base.lower():
+                    client_kwargs["default_headers"] = {
+                        "User-Agent": "KimiCLI/1.3",
+                    }
+            else:
+                # No explicit creds — use the centralized provider router
+                from agent.auxiliary_client import resolve_provider_client
+                _routed_client, _ = resolve_provider_client(
+                    self.provider or "auto", model=self.model, raw_codex=True)
+                if _routed_client is not None:
+                    client_kwargs = {
+                        "api_key": _routed_client.api_key,
+                        "base_url": str(_routed_client.base_url),
+                    }
+                    # Preserve any default_headers the router set
+                    if hasattr(_routed_client, '_default_headers') and _routed_client._default_headers:
+                        client_kwargs["default_headers"] = dict(_routed_client._default_headers)
                 else:
-                    print(f"⚠️  Warning: API key appears invalid or missing (got: '{key_used[:20] if key_used else 'none'}...')")
-        except Exception as e:
-            raise RuntimeError(f"Failed to initialize OpenAI client: {e}")
+                    # Final fallback: try raw OpenRouter key
+                    client_kwargs = {
+                        "api_key": os.getenv("OPENROUTER_API_KEY", ""),
+                        "base_url": OPENROUTER_BASE_URL,
+                        "default_headers": {
+                            "HTTP-Referer": "https://github.com/NousResearch/hermes-agent",
+                            "X-OpenRouter-Title": "Hermes Agent",
+                            "X-OpenRouter-Categories": "productivity,cli-agent",
+                        },
+                    }
+            
+            self._client_kwargs = client_kwargs  # stored for rebuilding after interrupt
+            try:
+                self.client = OpenAI(**client_kwargs)
+                if not self.quiet_mode:
+                    print(f"🤖 AI Agent initialized with model: {self.model}")
+                    if base_url:
+                        print(f"🔗 Using custom base URL: {base_url}")
+                    # Always show API key info (masked) for debugging auth issues
+                    key_used = client_kwargs.get("api_key", "none")
+                    if key_used and key_used != "dummy-key" and len(key_used) > 12:
+                        print(f"🔑 Using API key: {key_used[:8]}...{key_used[-4:]}")
+                    else:
+                        print(f"⚠️  Warning: API key appears invalid or missing (got: '{key_used[:20] if key_used else 'none'}...')")
+            except Exception as e:
+                raise RuntimeError(f"Failed to initialize OpenAI client: {e}")
         
         # Provider fallback — a single backup model/provider tried when the
         # primary is exhausted (rate-limit, overload, connection failure).
@@ -533,7 +555,8 @@ class AIAgent:
         
         # Show prompt caching status
         if self._use_prompt_caching and not self.quiet_mode:
-            print(f"💾 Prompt caching: ENABLED (Claude via OpenRouter, {self._cache_ttl} TTL)")
+            source = "native Anthropic" if is_native_anthropic else "Claude via OpenRouter"
+            print(f"💾 Prompt caching: ENABLED ({source}, {self._cache_ttl} TTL)")
         
         # Session logging setup - auto-save conversation trajectories for debugging
         self.session_start = datetime.now()
@@ -2233,6 +2256,8 @@ class AIAgent:
             try:
                 if self.api_mode == "codex_responses":
                     result["response"] = self._run_codex_stream(api_kwargs)
+                elif self.api_mode == "anthropic_messages":
+                    result["response"] = self._anthropic_client.messages.create(**api_kwargs)
                 else:
                     result["response"] = self.client.chat.completions.create(**api_kwargs)
             except Exception as e:
@@ -2245,12 +2270,19 @@ class AIAgent:
             if self._interrupt_requested:
                 # Force-close the HTTP connection to stop token generation
                 try:
-                    self.client.close()
+                    if self.api_mode == "anthropic_messages":
+                        self._anthropic_client.close()
+                    else:
+                        self.client.close()
                 except Exception:
                     pass
                 # Rebuild the client for future calls (cheap, no network)
                 try:
-                    self.client = OpenAI(**self._client_kwargs)
+                    if self.api_mode == "anthropic_messages":
+                        from agent.anthropic_adapter import build_anthropic_client
+                        self._anthropic_client = build_anthropic_client(self._anthropic_api_key)
+                    else:
+                        self.client = OpenAI(**self._client_kwargs)
                 except Exception:
                     pass
                 raise InterruptedError("Agent interrupted during API call")
@@ -2336,6 +2368,16 @@ class AIAgent:
 
     def _build_api_kwargs(self, api_messages: list) -> dict:
         """Build the keyword arguments dict for the active API mode."""
+        if self.api_mode == "anthropic_messages":
+            from agent.anthropic_adapter import build_anthropic_kwargs
+            return build_anthropic_kwargs(
+                model=self.model,
+                messages=api_messages,
+                tools=self.tools,
+                max_tokens=None,
+                reasoning_config=self.reasoning_config,
+            )
+
         if self.api_mode == "codex_responses":
             instructions = ""
             payload_messages = api_messages
@@ -3561,6 +3603,17 @@ class AIAgent:
                         elif len(output_items) == 0:
                             response_invalid = True
                             error_details.append("response.output is empty")
+                    elif self.api_mode == "anthropic_messages":
+                        content_blocks = getattr(response, "content", None) if response is not None else None
+                        if response is None:
+                            response_invalid = True
+                            error_details.append("response is None")
+                        elif not isinstance(content_blocks, list):
+                            response_invalid = True
+                            error_details.append("response.content is not a list")
+                        elif len(content_blocks) == 0:
+                            response_invalid = True
+                            error_details.append("response.content is empty")
                     else:
                         if response is None or not hasattr(response, 'choices') or response.choices is None or len(response.choices) == 0:
                             response_invalid = True
@@ -3662,6 +3715,9 @@ class AIAgent:
                             finish_reason = "length"
                         else:
                             finish_reason = "stop"
+                    elif self.api_mode == "anthropic_messages":
+                        stop_reason_map = {"end_turn": "stop", "tool_use": "tool_calls", "max_tokens": "length", "stop_sequence": "stop"}
+                        finish_reason = stop_reason_map.get(response.stop_reason, "stop")
                     else:
                         finish_reason = response.choices[0].finish_reason
 
@@ -3739,7 +3795,7 @@ class AIAgent:
                     
                     # Track actual token usage from response for context management
                     if hasattr(response, 'usage') and response.usage:
-                        if self.api_mode == "codex_responses":
+                        if self.api_mode in ("codex_responses", "anthropic_messages"):
                             prompt_tokens = getattr(response.usage, 'input_tokens', 0) or 0
                             completion_tokens = getattr(response.usage, 'output_tokens', 0) or 0
                             total_tokens = (
@@ -4068,6 +4124,9 @@ class AIAgent:
             try:
                 if self.api_mode == "codex_responses":
                     assistant_message, finish_reason = self._normalize_codex_response(response)
+                elif self.api_mode == "anthropic_messages":
+                    from agent.anthropic_adapter import normalize_anthropic_response
+                    assistant_message, finish_reason = normalize_anthropic_response(response)
                 else:
                     assistant_message = response.choices[0].message
                 
diff --git a/tests/test_anthropic_adapter.py b/tests/test_anthropic_adapter.py
new file mode 100644
index 000000000..54c722f93
--- /dev/null
+++ b/tests/test_anthropic_adapter.py
@@ -0,0 +1,406 @@
+"""Tests for agent/anthropic_adapter.py — Anthropic Messages API adapter."""
+
+import json
+import time
+from types import SimpleNamespace
+from unittest.mock import patch, MagicMock
+
+import pytest
+
+from agent.anthropic_adapter import (
+    _is_oauth_token,
+    build_anthropic_client,
+    build_anthropic_kwargs,
+    convert_messages_to_anthropic,
+    convert_tools_to_anthropic,
+    is_claude_code_token_valid,
+    normalize_anthropic_response,
+    normalize_model_name,
+    read_claude_code_credentials,
+    resolve_anthropic_token,
+)
+
+
+# ---------------------------------------------------------------------------
+# Auth helpers
+# ---------------------------------------------------------------------------
+
+
+class TestIsOAuthToken:
+    def test_setup_token(self):
+        assert _is_oauth_token("sk-ant-oat01-abcdef1234567890") is True
+
+    def test_api_key(self):
+        assert _is_oauth_token("sk-ant-api03-abcdef1234567890") is False
+
+    def test_empty(self):
+        assert _is_oauth_token("") is False
+
+
+class TestBuildAnthropicClient:
+    def test_setup_token_uses_auth_token(self):
+        with patch("agent.anthropic_adapter._anthropic_sdk") as mock_sdk:
+            build_anthropic_client("sk-ant-oat01-" + "x" * 60)
+            kwargs = mock_sdk.Anthropic.call_args[1]
+            assert "auth_token" in kwargs
+            assert "oauth-2025-04-20" in kwargs["default_headers"]["anthropic-beta"]
+            assert "api_key" not in kwargs
+
+    def test_api_key_uses_api_key(self):
+        with patch("agent.anthropic_adapter._anthropic_sdk") as mock_sdk:
+            build_anthropic_client("sk-ant-api03-something")
+            kwargs = mock_sdk.Anthropic.call_args[1]
+            assert kwargs["api_key"] == "sk-ant-api03-something"
+            assert "auth_token" not in kwargs
+
+    def test_custom_base_url(self):
+        with patch("agent.anthropic_adapter._anthropic_sdk") as mock_sdk:
+            build_anthropic_client("sk-ant-api03-x", base_url="https://custom.api.com")
+            kwargs = mock_sdk.Anthropic.call_args[1]
+            assert kwargs["base_url"] == "https://custom.api.com"
+
+
+class TestReadClaudeCodeCredentials:
+    def test_reads_valid_credentials(self, tmp_path, monkeypatch):
+        cred_file = tmp_path / ".claude" / ".credentials.json"
+        cred_file.parent.mkdir(parents=True)
+        cred_file.write_text(json.dumps({
+            "claudeAiOauth": {
+                "accessToken": "sk-ant-oat01-test-token",
+                "refreshToken": "sk-ant-ort01-refresh",
+                "expiresAt": int(time.time() * 1000) + 3600_000,
+            }
+        }))
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        creds = read_claude_code_credentials()
+        assert creds is not None
+        assert creds["accessToken"] == "sk-ant-oat01-test-token"
+        assert creds["refreshToken"] == "sk-ant-ort01-refresh"
+
+    def test_returns_none_for_missing_file(self, tmp_path, monkeypatch):
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        assert read_claude_code_credentials() is None
+
+    def test_returns_none_for_missing_oauth_key(self, tmp_path, monkeypatch):
+        cred_file = tmp_path / ".claude" / ".credentials.json"
+        cred_file.parent.mkdir(parents=True)
+        cred_file.write_text(json.dumps({"someOtherKey": {}}))
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        assert read_claude_code_credentials() is None
+
+    def test_returns_none_for_empty_access_token(self, tmp_path, monkeypatch):
+        cred_file = tmp_path / ".claude" / ".credentials.json"
+        cred_file.parent.mkdir(parents=True)
+        cred_file.write_text(json.dumps({
+            "claudeAiOauth": {"accessToken": "", "refreshToken": "x"}
+        }))
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        assert read_claude_code_credentials() is None
+
+
+class TestIsClaudeCodeTokenValid:
+    def test_valid_token(self):
+        creds = {"accessToken": "tok", "expiresAt": int(time.time() * 1000) + 3600_000}
+        assert is_claude_code_token_valid(creds) is True
+
+    def test_expired_token(self):
+        creds = {"accessToken": "tok", "expiresAt": int(time.time() * 1000) - 3600_000}
+        assert is_claude_code_token_valid(creds) is False
+
+    def test_no_expiry_but_has_token(self):
+        creds = {"accessToken": "tok", "expiresAt": 0}
+        assert is_claude_code_token_valid(creds) is True
+
+
+class TestResolveAnthropicToken:
+    def test_prefers_api_key(self, monkeypatch):
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-ant-api03-mykey")
+        monkeypatch.setenv("ANTHROPIC_TOKEN", "sk-ant-oat01-mytoken")
+        assert resolve_anthropic_token() == "sk-ant-api03-mykey"
+
+    def test_falls_back_to_token(self, monkeypatch):
+        monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
+        monkeypatch.setenv("ANTHROPIC_TOKEN", "sk-ant-oat01-mytoken")
+        assert resolve_anthropic_token() == "sk-ant-oat01-mytoken"
+
+    def test_returns_none_with_no_creds(self, monkeypatch, tmp_path):
+        monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
+        monkeypatch.delenv("ANTHROPIC_TOKEN", raising=False)
+        monkeypatch.delenv("CLAUDE_CODE_OAUTH_TOKEN", raising=False)
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        assert resolve_anthropic_token() is None
+
+
+# ---------------------------------------------------------------------------
+# Model name normalization
+# ---------------------------------------------------------------------------
+
+
+class TestNormalizeModelName:
+    def test_strips_anthropic_prefix(self):
+        assert normalize_model_name("anthropic/claude-sonnet-4-20250514") == "claude-sonnet-4-20250514"
+
+    def test_leaves_bare_name(self):
+        assert normalize_model_name("claude-sonnet-4-20250514") == "claude-sonnet-4-20250514"
+
+
+# ---------------------------------------------------------------------------
+# Tool conversion
+# ---------------------------------------------------------------------------
+
+
+class TestConvertTools:
+    def test_converts_openai_to_anthropic_format(self):
+        tools = [
+            {
+                "type": "function",
+                "function": {
+                    "name": "search",
+                    "description": "Search the web",
+                    "parameters": {
+                        "type": "object",
+                        "properties": {"query": {"type": "string"}},
+                        "required": ["query"],
+                    },
+                },
+            }
+        ]
+        result = convert_tools_to_anthropic(tools)
+        assert len(result) == 1
+        assert result[0]["name"] == "search"
+        assert result[0]["description"] == "Search the web"
+        assert result[0]["input_schema"]["properties"]["query"]["type"] == "string"
+
+    def test_empty_tools(self):
+        assert convert_tools_to_anthropic([]) == []
+        assert convert_tools_to_anthropic(None) == []
+
+
+# ---------------------------------------------------------------------------
+# Message conversion
+# ---------------------------------------------------------------------------
+
+
+class TestConvertMessages:
+    def test_extracts_system_prompt(self):
+        messages = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hello"},
+        ]
+        system, result = convert_messages_to_anthropic(messages)
+        assert system == "You are helpful."
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_converts_tool_calls(self):
+        messages = [
+            {
+                "role": "assistant",
+                "content": "Let me search.",
+                "tool_calls": [
+                    {
+                        "id": "tc_1",
+                        "function": {
+                            "name": "search",
+                            "arguments": '{"query": "test"}',
+                        },
+                    }
+                ],
+            },
+            {"role": "tool", "tool_call_id": "tc_1", "content": "search results"},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        blocks = result[0]["content"]
+        assert blocks[0] == {"type": "text", "text": "Let me search."}
+        assert blocks[1]["type"] == "tool_use"
+        assert blocks[1]["id"] == "tc_1"
+        assert blocks[1]["input"] == {"query": "test"}
+
+    def test_converts_tool_results(self):
+        messages = [
+            {"role": "tool", "tool_call_id": "tc_1", "content": "result data"},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert result[0]["role"] == "user"
+        assert result[0]["content"][0]["type"] == "tool_result"
+        assert result[0]["content"][0]["tool_use_id"] == "tc_1"
+
+    def test_merges_consecutive_tool_results(self):
+        messages = [
+            {"role": "tool", "tool_call_id": "tc_1", "content": "result 1"},
+            {"role": "tool", "tool_call_id": "tc_2", "content": "result 2"},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        assert len(result) == 1
+        assert len(result[0]["content"]) == 2
+
+    def test_strips_orphaned_tool_use(self):
+        messages = [
+            {
+                "role": "assistant",
+                "content": "",
+                "tool_calls": [
+                    {"id": "tc_orphan", "function": {"name": "x", "arguments": "{}"}}
+                ],
+            },
+            {"role": "user", "content": "never mind"},
+        ]
+        _, result = convert_messages_to_anthropic(messages)
+        # tc_orphan has no matching tool_result, should be stripped
+        assistant_blocks = result[0]["content"]
+        assert all(b.get("type") != "tool_use" for b in assistant_blocks)
+
+    def test_system_with_cache_control(self):
+        messages = [
+            {
+                "role": "system",
+                "content": [
+                    {"type": "text", "text": "System prompt", "cache_control": {"type": "ephemeral"}},
+                ],
+            },
+            {"role": "user", "content": "Hi"},
+        ]
+        system, result = convert_messages_to_anthropic(messages)
+        # When cache_control is present, system should be a list of blocks
+        assert isinstance(system, list)
+        assert system[0]["cache_control"] == {"type": "ephemeral"}
+
+
+# ---------------------------------------------------------------------------
+# Build kwargs
+# ---------------------------------------------------------------------------
+
+
+class TestBuildAnthropicKwargs:
+    def test_basic_kwargs(self):
+        messages = [
+            {"role": "system", "content": "Be helpful."},
+            {"role": "user", "content": "Hi"},
+        ]
+        kwargs = build_anthropic_kwargs(
+            model="claude-sonnet-4-20250514",
+            messages=messages,
+            tools=None,
+            max_tokens=4096,
+            reasoning_config=None,
+        )
+        assert kwargs["model"] == "claude-sonnet-4-20250514"
+        assert kwargs["system"] == "Be helpful."
+        assert kwargs["max_tokens"] == 4096
+        assert "tools" not in kwargs
+
+    def test_strips_anthropic_prefix(self):
+        kwargs = build_anthropic_kwargs(
+            model="anthropic/claude-sonnet-4-20250514",
+            messages=[{"role": "user", "content": "Hi"}],
+            tools=None,
+            max_tokens=4096,
+            reasoning_config=None,
+        )
+        assert kwargs["model"] == "claude-sonnet-4-20250514"
+
+    def test_reasoning_config_maps_to_thinking(self):
+        kwargs = build_anthropic_kwargs(
+            model="claude-sonnet-4-20250514",
+            messages=[{"role": "user", "content": "think hard"}],
+            tools=None,
+            max_tokens=4096,
+            reasoning_config={"enabled": True, "effort": "high"},
+        )
+        assert kwargs["thinking"]["type"] == "enabled"
+        assert kwargs["thinking"]["budget_tokens"] == 16000
+        assert kwargs["max_tokens"] >= 16000 + 4096
+
+    def test_reasoning_disabled(self):
+        kwargs = build_anthropic_kwargs(
+            model="claude-sonnet-4-20250514",
+            messages=[{"role": "user", "content": "quick"}],
+            tools=None,
+            max_tokens=4096,
+            reasoning_config={"enabled": False},
+        )
+        assert "thinking" not in kwargs
+
+    def test_default_max_tokens(self):
+        kwargs = build_anthropic_kwargs(
+            model="claude-sonnet-4-20250514",
+            messages=[{"role": "user", "content": "Hi"}],
+            tools=None,
+            max_tokens=None,
+            reasoning_config=None,
+        )
+        assert kwargs["max_tokens"] == 16384
+
+
+# ---------------------------------------------------------------------------
+# Response normalization
+# ---------------------------------------------------------------------------
+
+
+class TestNormalizeResponse:
+    def _make_response(self, content_blocks, stop_reason="end_turn"):
+        resp = SimpleNamespace()
+        resp.content = content_blocks
+        resp.stop_reason = stop_reason
+        resp.usage = SimpleNamespace(input_tokens=100, output_tokens=50)
+        return resp
+
+    def test_text_response(self):
+        block = SimpleNamespace(type="text", text="Hello world")
+        msg, reason = normalize_anthropic_response(self._make_response([block]))
+        assert msg.content == "Hello world"
+        assert reason == "stop"
+        assert msg.tool_calls is None
+
+    def test_tool_use_response(self):
+        blocks = [
+            SimpleNamespace(type="text", text="Searching..."),
+            SimpleNamespace(
+                type="tool_use",
+                id="tc_1",
+                name="search",
+                input={"query": "test"},
+            ),
+        ]
+        msg, reason = normalize_anthropic_response(
+            self._make_response(blocks, "tool_use")
+        )
+        assert msg.content == "Searching..."
+        assert reason == "tool_calls"
+        assert len(msg.tool_calls) == 1
+        assert msg.tool_calls[0].function.name == "search"
+        assert json.loads(msg.tool_calls[0].function.arguments) == {"query": "test"}
+
+    def test_thinking_response(self):
+        blocks = [
+            SimpleNamespace(type="thinking", thinking="Let me reason about this..."),
+            SimpleNamespace(type="text", text="The answer is 42."),
+        ]
+        msg, reason = normalize_anthropic_response(self._make_response(blocks))
+        assert msg.content == "The answer is 42."
+        assert msg.reasoning == "Let me reason about this..."
+
+    def test_stop_reason_mapping(self):
+        block = SimpleNamespace(type="text", text="x")
+        _, r1 = normalize_anthropic_response(
+            self._make_response([block], "end_turn")
+        )
+        _, r2 = normalize_anthropic_response(
+            self._make_response([block], "tool_use")
+        )
+        _, r3 = normalize_anthropic_response(
+            self._make_response([block], "max_tokens")
+        )
+        assert r1 == "stop"
+        assert r2 == "tool_calls"
+        assert r3 == "length"
+
+    def test_no_text_content(self):
+        block = SimpleNamespace(
+            type="tool_use", id="tc_1", name="search", input={"q": "hi"}
+        )
+        msg, reason = normalize_anthropic_response(
+            self._make_response([block], "tool_use")
+        )
+        assert msg.content is None
+        assert len(msg.tool_calls) == 1
diff --git a/tests/test_run_agent.py b/tests/test_run_agent.py
index c789d7352..24172a94c 100644
--- a/tests/test_run_agent.py
+++ b/tests/test_run_agent.py
@@ -281,20 +281,21 @@ class TestMaskApiKey:
 
 class TestInit:
     def test_anthropic_base_url_accepted(self):
-        """Anthropic base URLs should be accepted (OpenAI-compatible endpoint)."""
+        """Anthropic base URLs should route to native Anthropic client."""
         with (
             patch("run_agent.get_tool_definitions", return_value=[]),
             patch("run_agent.check_toolset_requirements", return_value={}),
-            patch("run_agent.OpenAI") as mock_openai,
+            patch("agent.anthropic_adapter._anthropic_sdk") as mock_anthropic,
         ):
-            AIAgent(
+            agent = AIAgent(
                 api_key="test-key-1234567890",
                 base_url="https://api.anthropic.com/v1/",
                 quiet_mode=True,
                 skip_context_files=True,
                 skip_memory=True,
             )
-            mock_openai.assert_called_once()
+            assert agent.api_mode == "anthropic_messages"
+            mock_anthropic.Anthropic.assert_called_once()
 
     def test_prompt_caching_claude_openrouter(self):
         """Claude model via OpenRouter should enable prompt caching."""
@@ -345,6 +346,23 @@ class TestInit:
             )
             assert a._use_prompt_caching is False
 
+    def test_prompt_caching_native_anthropic(self):
+        """Native Anthropic provider should enable prompt caching."""
+        with (
+            patch("run_agent.get_tool_definitions", return_value=[]),
+            patch("run_agent.check_toolset_requirements", return_value={}),
+            patch("agent.anthropic_adapter._anthropic_sdk"),
+        ):
+            a = AIAgent(
+                api_key="test-key-1234567890",
+                base_url="https://api.anthropic.com/v1/",
+                quiet_mode=True,
+                skip_context_files=True,
+                skip_memory=True,
+            )
+            assert a.api_mode == "anthropic_messages"
+            assert a._use_prompt_caching is True
+
     def test_valid_tool_names_populated(self):
         """valid_tool_names should contain names from loaded tools."""
         tools = _make_tool_defs("web_search", "terminal")