diff --git a/src/anthropic/lib/__init__.py b/src/anthropic/lib/__init__.py
index 076480d7..f78b48cc 100644
--- a/src/anthropic/lib/__init__.py
+++ b/src/anthropic/lib/__init__.py
@@ -1 +1,2 @@
 from ._files import files_from_dir as files_from_dir, async_files_from_dir as async_files_from_dir
+from ._thinking import strip_thinking_blocks as strip_thinking_blocks
diff --git a/src/anthropic/lib/_thinking.py b/src/anthropic/lib/_thinking.py
new file mode 100644
index 00000000..f6b32c7d
--- /dev/null
+++ b/src/anthropic/lib/_thinking.py
@@ -0,0 +1,82 @@
+"""Utilities for managing extended thinking blocks in conversation history.
+
+Extended thinking blocks carry a ``signature`` field that is valid only for
+the specific conversation context they were created in.  Long multi-turn
+sessions or any operation that reconstructs history from stored messages can
+hit a 400 "Invalid ``signature`` in ``thinking`` block" error because the
+server-side context that produced the signature no longer matches.
+``strip_thinking_blocks`` removes those blocks before the history is replayed,
+providing the only documented recovery path.
+"""
+
+from __future__ import annotations
+
+from typing import Iterable
+
+from ..types.message_param import MessageParam
+
+__all__ = ["strip_thinking_blocks"]
+
+_THINKING_TYPES = frozenset({"thinking", "redacted_thinking"})
+
+
+def _block_type(block: object) -> str:
+    """Return the ``type`` field of a content block regardless of representation.
+
+    Accepts plain dicts (TypedDict values, JSON-decoded payloads) and Pydantic
+    ``BaseModel`` instances (``ThinkingBlock``, ``RedactedThinkingBlock``, …).
+    Returns ``""`` for anything that carries no ``type``.
+    """
+    if isinstance(block, dict):
+        return block.get("type", "")
+    return getattr(block, "type", "")
+
+
+def strip_thinking_blocks(
+    messages: Iterable[MessageParam],
+) -> list[MessageParam]:
+    """Return a copy of *messages* with every thinking and redacted-thinking block removed.
+
+    Use this when replaying conversation history fails with
+    ``"Invalid `signature` in `thinking` block"``: the ``signature`` stored in
+    a ``ThinkingBlock`` is bound to the exact server-side conversation context
+    it was created in and is not guaranteed to remain valid after session
+    expiry, model upgrades, or history reconstruction from storage.
+
+    Only the ``content`` list is modified; string content and all non-thinking
+    blocks are preserved verbatim.  The ``role`` and any other keys on each
+    message dict are copied through unchanged.
+
+    Example::
+
+        try:
+            response = client.messages.create(model=model, messages=history, ...)
+        except anthropic.BadRequestError as exc:
+            if "Invalid `signature` in `thinking` block" in str(exc):
+                history = strip_thinking_blocks(history)
+                response = client.messages.create(model=model, messages=history, ...)
+            else:
+                raise
+
+    Args:
+        messages: The conversation history to clean.  Each entry must be a
+            ``MessageParam``-compatible mapping with at least ``role`` and
+            ``content`` keys.
+
+    Returns:
+        A new ``list[MessageParam]`` in the same order, with thinking and
+        redacted-thinking content blocks omitted from every message.
+    """
+    result: list[MessageParam] = []
+    for msg in messages:
+        content = msg.get("content")  # type: ignore[attr-defined]
+        if isinstance(content, str):
+            # Plain-text content carries no thinking blocks.
+            result.append(dict(msg))  # type: ignore[arg-type]
+            continue
+        filtered: list[object] = [
+            block for block in content  # type: ignore[union-attr]
+            if _block_type(block) not in _THINKING_TYPES
+        ]
+        result.append({**msg, "content": filtered})  # type: ignore[typeddict-item]
+    return result
diff --git a/tests/lib/test_thinking.py b/tests/lib/test_thinking.py
new file mode 100644
index 00000000..425aca42
--- /dev/null
+++ b/tests/lib/test_thinking.py
@@ -0,0 +1,194 @@
+"""Tests for strip_thinking_blocks (:mod:`anthropic.lib._thinking`).
+
+Extended thinking block signatures are tied to the conversation context they
+were created in and may become invalid when replaying stored history.
+``strip_thinking_blocks`` removes those blocks so the history can be re-sent
+without a 400 error.
+"""
+
+from __future__ import annotations
+
+from anthropic.lib._thinking import strip_thinking_blocks
+
+# ---------------------------------------------------------------------------
+# Helpers — build content blocks as plain dicts (the most common representation
+# callers have) and as minimal object stubs (mimicking BaseModel responses).
+# ---------------------------------------------------------------------------
+
+
+def _thinking(text: str = "thinking…") -> dict:
+    return {"type": "thinking", "thinking": text, "signature": "sig_abc"}
+
+
+def _redacted() -> dict:
+    return {"type": "redacted_thinking", "data": "opaque"}
+
+
+def _text(text: str = "hello") -> dict:
+    return {"type": "text", "text": text}
+
+
+def _tool_use() -> dict:
+    return {"type": "tool_use", "id": "tu_1", "name": "my_tool", "input": {}}
+
+
+class _FakeThinkingBlock:
+    """Minimal stub that looks like a ThinkingBlock BaseModel instance."""
+
+    type = "thinking"
+    thinking = "deep thought"
+    signature = "sig_xyz"
+
+
+class _FakeRedactedBlock:
+    type = "redacted_thinking"
+    data = "bytes"
+
+
+class _FakeTextBlock:
+    type = "text"
+    text = "plain text"
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+
+def test_string_content_passes_through_unchanged() -> None:
+    messages = [{"role": "user", "content": "hello"}]
+    result = strip_thinking_blocks(messages)
+    assert result == [{"role": "user", "content": "hello"}]
+
+
+def test_thinking_block_removed_from_assistant() -> None:
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_thinking(), _text("answer")],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    assert len(result[0]["content"]) == 1  # type: ignore[arg-type]
+    assert result[0]["content"][0]["type"] == "text"  # type: ignore[index]
+
+
+def test_redacted_thinking_block_removed() -> None:
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_redacted(), _text("answer")],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    assert len(result[0]["content"]) == 1  # type: ignore[arg-type]
+    assert result[0]["content"][0]["type"] == "text"  # type: ignore[index]
+
+
+def test_mixed_content_only_removes_thinking_types() -> None:
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_thinking(), _text("A"), _tool_use(), _redacted(), _text("B")],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    content = result[0]["content"]  # type: ignore[index]
+    assert len(content) == 3  # type: ignore[arg-type]
+    types = [b["type"] for b in content]  # type: ignore[index]
+    assert types == ["text", "tool_use", "text"]
+
+
+def test_user_messages_without_thinking_unchanged() -> None:
+    messages = [
+        {"role": "user", "content": [_text("what?")]},
+    ]
+    result = strip_thinking_blocks(messages)
+    assert result[0]["content"] == [_text("what?")]  # type: ignore[index]
+
+
+def test_non_thinking_blocks_preserved_verbatim() -> None:
+    blocks = [_text("one"), _tool_use(), _text("two")]
+    messages = [{"role": "assistant", "content": blocks}]
+    result = strip_thinking_blocks(messages)
+    assert result[0]["content"] == blocks  # type: ignore[index]
+
+
+def test_all_thinking_produces_empty_content_list() -> None:
+    # Edge case: a message whose entire content is thinking blocks.
+    messages = [{"role": "assistant", "content": [_thinking(), _redacted()]}]
+    result = strip_thinking_blocks(messages)
+    assert result[0]["content"] == []  # type: ignore[index]
+
+
+def test_empty_messages_list() -> None:
+    assert strip_thinking_blocks([]) == []
+
+
+def test_multiple_messages_processed_independently() -> None:
+    messages = [
+        {"role": "user", "content": "first"},
+        {"role": "assistant", "content": [_thinking(), _text("first answer")]},
+        {"role": "user", "content": "second"},
+        {"role": "assistant", "content": [_thinking(), _text("second answer")]},
+    ]
+    result = strip_thinking_blocks(messages)
+    assert result[0]["content"] == "first"
+    assert result[1]["content"] == [_text("first answer")]  # type: ignore[index]
+    assert result[2]["content"] == "second"
+    assert result[3]["content"] == [_text("second answer")]  # type: ignore[index]
+
+
+def test_basemodel_instances_stripped() -> None:
+    # When blocks are BaseModel instances (as returned by the SDK), the type
+    # attribute is accessed via getattr rather than dict lookup.
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_FakeThinkingBlock(), _FakeTextBlock()],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    content = result[0]["content"]  # type: ignore[index]
+    assert len(content) == 1  # type: ignore[arg-type]
+    assert content[0].type == "text"  # type: ignore[index]
+
+
+def test_redacted_basemodel_instance_stripped() -> None:
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_FakeRedactedBlock(), _FakeTextBlock()],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    assert len(result[0]["content"]) == 1  # type: ignore[arg-type]
+
+
+def test_does_not_mutate_original_messages() -> None:
+    original_content = [_thinking(), _text("answer")]
+    messages = [{"role": "assistant", "content": original_content}]
+    strip_thinking_blocks(messages)
+    # The original list must be unchanged.
+    assert len(original_content) == 2
+    assert original_content[0]["type"] == "thinking"
+
+
+def test_role_and_other_keys_preserved() -> None:
+    messages = [
+        {
+            "role": "assistant",
+            "content": [_thinking(), _text("hi")],
+        }
+    ]
+    result = strip_thinking_blocks(messages)
+    assert result[0]["role"] == "assistant"
+
+
+def test_idempotent() -> None:
+    messages = [
+        {"role": "assistant", "content": [_thinking(), _text("hi")]},
+    ]
+    once = strip_thinking_blocks(messages)
+    twice = strip_thinking_blocks(once)
+    assert once == twice