diff --git a/frontier_agent/infra/config.py b/frontier_agent/infra/config.py
index 4c94249b..7c6b1d2f 100644
--- a/frontier_agent/infra/config.py
+++ b/frontier_agent/infra/config.py
@@ -134,7 +134,7 @@ class FrontierAgentConfig(BaseSettings):
     openai_model: str = "gpt-4o"
     openai_base_url: str = "https://api.openai.com/v1"
     anthropic_api_key: str = ""
-    anthropic_model: str = "claude-sonnet-4-20250514"
+    anthropic_model: str = "claude-sonnet-5-5"
     anthropic_base_url: str = ""
     anthropic_thinking: bool = False
     anthropic_thinking_type: str = "adaptive"
diff --git a/frontier_agent/infra/llm_adapter.py b/frontier_agent/infra/llm_adapter.py
index 70e7ca50..87cdf719 100644
--- a/frontier_agent/infra/llm_adapter.py
+++ b/frontier_agent/infra/llm_adapter.py
@@ -5,6 +5,7 @@
 import asyncio
 import logging
 import random
+import re
 import time
 from collections.abc import AsyncIterator
 from typing import Any, TypeGuard
@@ -365,7 +366,26 @@ async def stream(
             raise last_error from fb_err
 
 
-def _anthropic_thinking_kwarg(config: Any, max_tokens: int) -> dict[str, Any] | None:
+# Claude 5 models and Opus 4.7+ reject non-default sampling parameters (temperature) with a 400.
+# Not anchored, so OpenRouter (``anthropic/claude-sonnet-5.5``) and Bedrock
+# (``global.anthropic.claude-sonnet-5-5``) ids match too.
+_ANTHROPIC_NO_SAMPLING_MODEL = re.compile(
+    r"claude-(?:(?:opus|sonnet|haiku|fable|mythos)-5|opus-4[-.][78])"
+)
+
+
+def _anthropic_temperature(
+    model: str, thinking: dict[str, Any] | None, temperature: float = 0.3,
+) -> float | None:
+    """Sampling temperature for the Anthropic client, ``None`` to omit it."""
+    if thinking or _ANTHROPIC_NO_SAMPLING_MODEL.search(model):
+        return None
+    return temperature
+
+
+def _anthropic_thinking_kwarg(
+    config: Any, max_tokens: int, model: str,
+) -> dict[str, Any] | None:
     """Anthropic extended-thinking request kwarg, or ``None`` when off.
 
     ``ANTHROPIC_THINKING`` on → the shape selected by ``anthropic_thinking_type``
@@ -382,7 +402,9 @@ def _anthropic_thinking_kwarg(config: Any, max_tokens: int) -> dict[str, Any] |
     if not getattr(config, "anthropic_thinking", False):
         return None
     ttype = str(getattr(config, "anthropic_thinking_type", "adaptive") or "adaptive").strip().lower()
-    if ttype == "enabled":
+    # Claude 5 and Opus 4.7+ require adaptive thinking; migrate legacy budgets
+    # using the resolved model so role and fallback overrides stay compatible.
+    if ttype == "enabled" and not _ANTHROPIC_NO_SAMPLING_MODEL.search(model):
         budget = int(getattr(config, "anthropic_thinking_budget", 8192) or 8192)
         budget = max(1024, min(budget, int(max_tokens) - 1))
         return {"type": "enabled", "budget_tokens": budget}
@@ -432,13 +454,14 @@ def _create_single_provider(config: FrontierAgentConfig, model_override: str | N
         from frontier_agent.infra.anthropic_client import AnthropicClient
         # Opt-in extended thinking (ANTHROPIC_THINKING): standard enabled+budget
         # thinking + drop temperature (Anthropic 400s on the combo). Flag off →
-        # prior kwargs preserved exactly (temperature=0.3, no thinking).
-        thinking = _anthropic_thinking_kwarg(config, max_tokens)
+        # prior kwargs preserved exactly (temperature=0.3 except on Claude 5 models, no thinking).
+        model = model_override or config.anthropic_model
+        thinking = _anthropic_thinking_kwarg(config, max_tokens, model)
         return AnthropicClient(
-            model=model_override or config.anthropic_model,
+            model=model,
             api_key=config.anthropic_api_key,
             base_url=config.anthropic_base_url or None,
-            temperature=None if thinking else 0.3,
+            temperature=_anthropic_temperature(model, thinking),
             max_tokens=max_tokens,
             thinking=thinking,
             effort=_anthropic_effort_kwarg(config, thinking),
@@ -619,16 +642,17 @@ def _create_overridden_provider(
         # Opt-in extended thinking (see sibling branch above): standard
         # enabled+budget thinking + drop temperature. Flag off → prior kwargs
         # preserved exactly.
-        thinking = _anthropic_thinking_kwarg(config, max_tokens)
+        anthropic_model = model if (provider and model) else _resolve_model_name(
+            config, resolved_provider, model,
+        )
+        thinking = _anthropic_thinking_kwarg(config, max_tokens, anthropic_model)
         return AnthropicClient(
-            model=model if (provider and model) else _resolve_model_name(
-                config, resolved_provider, model,
-            ),
+            model=anthropic_model,
             api_key=api_key if api_key is not None else config.anthropic_api_key,
             # Honour a per-entry base_url (fallback-chain / override) — fall back
             # to the configured default only when the caller didn't pass one.
             base_url=(base_url if base_url is not None else config.anthropic_base_url) or None,
-            temperature=None if thinking else temperature,
+            temperature=_anthropic_temperature(anthropic_model, thinking, temperature),
             max_tokens=max_tokens,
             thinking=thinking,
             effort=_anthropic_effort_kwarg(config, thinking),
diff --git a/tests/test_anthropic_default_model.py b/tests/test_anthropic_default_model.py
new file mode 100644
index 00000000..0736476d
--- /dev/null
+++ b/tests/test_anthropic_default_model.py
@@ -0,0 +1,84 @@
+import pytest
+
+from frontier_agent.infra.config import FrontierAgentConfig
+from frontier_agent.infra.llm_adapter import _anthropic_temperature
+
+# claude-sonnet-4-20250514 was retired by Anthropic on 2026-06-15.
+RETIRED_MODEL = "claude-sonnet-4-20250514"
+
+
+def test_default_anthropic_model_is_not_retired():
+    assert FrontierAgentConfig.model_fields["anthropic_model"].default != RETIRED_MODEL
+
+
+def test_default_anthropic_model_gets_no_temperature():
+    default = FrontierAgentConfig.model_fields["anthropic_model"].default
+
+    assert _anthropic_temperature(default, None) is None
+
+
+def test_older_models_keep_the_temperature():
+    assert _anthropic_temperature("claude-sonnet-4-6", None) == 0.3
+
+
+def test_thinking_drops_the_temperature():
+    assert _anthropic_temperature("claude-sonnet-4-6", {"type": "adaptive"}) is None
+
+
+def test_prefixed_claude_5_ids_get_no_temperature():
+    assert _anthropic_temperature("anthropic/claude-sonnet-5.5", None) is None
+    assert _anthropic_temperature("global.anthropic.claude-sonnet-5-5", None) is None
+
+
+def test_opus_4_7_and_4_8_get_no_temperature():
+    assert _anthropic_temperature("claude-opus-4-7", None) is None
+    assert _anthropic_temperature("claude-opus-4-8", None) is None
+    assert _anthropic_temperature("claude-opus-4-6", None) == 0.3
+
+
+def test_role_temperature_is_kept_for_older_models():
+    assert _anthropic_temperature("claude-sonnet-4-6", None, 0.7) == 0.7
+    assert _anthropic_temperature("claude-sonnet-5-5", None, 0.7) is None
+
+
+def test_overridden_provider_omits_temperature_for_claude_5(monkeypatch):
+    import frontier_agent.infra.anthropic_client as anthropic_client
+    from frontier_agent.infra.llm_adapter import _create_overridden_provider
+
+    captured = {}
+
+    class FakeClient:
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+
+    monkeypatch.setattr(anthropic_client, "AnthropicClient", FakeClient)
+    config = FrontierAgentConfig(llm_provider="anthropic", anthropic_api_key="test-key")
+
+    _create_overridden_provider(config, temperature=0.7)
+    assert captured["model"] == config.anthropic_model
+    assert captured["temperature"] is None
+
+    _create_overridden_provider(config, "claude-sonnet-4-6", temperature=0.7)
+    assert captured["temperature"] == 0.7
+
+
+@pytest.mark.parametrize("factory_name", ["_create_single_provider", "_create_overridden_provider"])
+@pytest.mark.parametrize("model", [None, "claude-sonnet-4-20250514", "claude-sonnet-5-5"])
+def test_manual_thinking_migration_on_wire(factory_name, model):
+    from frontier_agent.infra import llm_adapter
+
+    config = FrontierAgentConfig(
+        _env_file=None, llm_provider="anthropic", anthropic_api_key="test-key",
+        anthropic_model="claude-sonnet-5-5", anthropic_thinking=True,
+        anthropic_thinking_type="enabled", anthropic_thinking_budget=2048,
+    )
+    client = getattr(llm_adapter, factory_name)(config, model)
+    payload = client._build_kwargs(
+        [{"role": "user", "content": "hello"}], tools=None, temperature=None,
+        max_tokens=None, extra_headers=None, timeout=None,
+    )
+    if model == "claude-sonnet-4-20250514":
+        assert payload["thinking"] == {"type": "enabled", "budget_tokens": 2048}
+    else:
+        assert payload["thinking"] == {"type": "adaptive", "display": "summarized"}
+    assert payload["model"] == (model or config.anthropic_model)