diff --git a/litellm/llms/bedrock/claude_platform/common_utils.py b/litellm/llms/bedrock/claude_platform/common_utils.py
index fb7f2185ec5..d9237972b84 100644
--- a/litellm/llms/bedrock/claude_platform/common_utils.py
+++ b/litellm/llms/bedrock/claude_platform/common_utils.py
@@ -10,6 +10,16 @@ from litellm.secret_managers.main import get_secret_str
 CLAUDE_PLATFORM_SERVICE_NAME: Final = "aws-external-anthropic"
 CLAUDE_PLATFORM_BEDROCK_ROUTE: Final = "claude_platform/"
 
+# Aliases litellm accepts for the workspace identifier; all of them are
+# consumed only as the `anthropic-workspace-id` HTTP header and must be
+# stripped from request-body params before transformation (#29272).
+_WORKSPACE_ID_PARAM_KEYS: Tuple[str, ...] = (
+    "workspace_id",
+    "aws_workspace_id",
+    "anthropic-workspace-id",
+    "anthropic_workspace_id",
+)
+
 
 def strip_claude_platform_route(model: str) -> str:
     if model.startswith(CLAUDE_PLATFORM_BEDROCK_ROUTE):
@@ -42,6 +52,20 @@ class BedrockClaudePlatformMixin(BaseAWSLLM):
             return str(workspace_id)
         return get_secret_str("ANTHROPIC_AWS_WORKSPACE_ID") or get_secret_str("ANTHROPIC_WORKSPACE_ID")
 
+    @staticmethod
+    def _pop_workspace_id_params(optional_params: dict, litellm_params: dict) -> None:
+        """Strip every workspace_id alias from both param dicts.
+
+        ``workspace_id`` (and its aliases) is consumed only as the
+        ``anthropic-workspace-id`` HTTP header. If we leave the keys in
+        ``optional_params``, the inherited ``AnthropicConfig.transform_request``
+        serializes them into the JSON body, and Anthropic's ``/v1/messages``
+        rejects unknown top-level fields (#29272).
+        """
+        for key in _WORKSPACE_ID_PARAM_KEYS:
+            optional_params.pop(key, None)
+            litellm_params.pop(key, None)
+
     def _get_required_aws_region_name(self, optional_params: dict) -> str:
         aws_region_name: Final = (
             optional_params.get("aws_region_name")
diff --git a/litellm/llms/bedrock/claude_platform/transformation.py b/litellm/llms/bedrock/claude_platform/transformation.py
index a57f309b605..4a831dc1a39 100644
--- a/litellm/llms/bedrock/claude_platform/transformation.py
+++ b/litellm/llms/bedrock/claude_platform/transformation.py
@@ -41,6 +41,13 @@ class BedrockClaudePlatformConfig(BedrockClaudePlatformMixin, AnthropicConfig):
                 model=model,
             )
 
+        # Drop workspace_id alias keys from optional_params/litellm_params now
+        # that we've turned the value into the `anthropic-workspace-id` header.
+        # Without this they survive into `AnthropicConfig.transform_request` and
+        # ship as a top-level field in the JSON body, which Anthropic's
+        # /v1/messages rejects (#29272).
+        self._pop_workspace_id_params(optional_params, litellm_params)
+
         api_key = api_key or get_secret_str("ANTHROPIC_AWS_API_KEY")
         anthropic_headers: Final = self.get_anthropic_headers(
             api_key=api_key,
diff --git a/tests/test_litellm/llms/bedrock/test_claude_platform_provider.py b/tests/test_litellm/llms/bedrock/test_claude_platform_provider.py
index dbded8e0a2e..0abc2af9441 100644
--- a/tests/test_litellm/llms/bedrock/test_claude_platform_provider.py
+++ b/tests/test_litellm/llms/bedrock/test_claude_platform_provider.py
@@ -362,3 +362,61 @@ def test_sigv4_no_duplicate_content_type_when_caller_sets_lowercase():
         "Duplicate keys produce 'application/json, application/json' in the "
         "SigV4 canonical string and cause a 401."
     )
+
+
+def test_claude_platform_strips_workspace_id_aliases_from_request_body():
+    """Regression for #29272.
+
+    `workspace_id` (and its aliases `aws_workspace_id`, `anthropic-workspace-id`,
+    `anthropic_workspace_id`) is consumed only as the
+    `anthropic-workspace-id` header. They were left in `optional_params`, so
+    the inherited `AnthropicConfig.transform_request` serialized them as
+    top-level fields of the JSON body and Anthropic's `/v1/messages`
+    rejected the request with `unknown field`.
+
+    Verify every alias is popped from both `optional_params` and
+    `litellm_params` once `validate_environment` runs.
+    """
+    from litellm.llms.bedrock.claude_platform.transformation import (
+        BedrockClaudePlatformConfig,
+    )
+
+    config = BedrockClaudePlatformConfig()
+
+    optional_params = {
+        "workspace_id": "wrkspc_a",
+        "aws_workspace_id": "wrkspc_b",
+        "anthropic-workspace-id": "wrkspc_c",
+        "anthropic_workspace_id": "wrkspc_d",
+        "max_tokens": 1024,  # must survive
+    }
+    litellm_params = {
+        "workspace_id": "wrkspc_e",
+        "anthropic_workspace_id": "wrkspc_f",
+        "metadata": {"trace_id": "abc"},  # must survive
+    }
+
+    headers = config.validate_environment(
+        api_key="fake-platform-key",
+        headers={},
+        model="claude-sonnet-4-6",
+        messages=[{"role": "user", "content": "hello"}],
+        optional_params=optional_params,
+        litellm_params=litellm_params,
+    )
+
+    assert headers["anthropic-workspace-id"] == "wrkspc_a"
+
+    # No alias survives into params that downstream transformers serialize.
+    for key in (
+        "workspace_id",
+        "aws_workspace_id",
+        "anthropic-workspace-id",
+        "anthropic_workspace_id",
+    ):
+        assert key not in optional_params, f"{key} leaked into optional_params"
+        assert key not in litellm_params, f"{key} leaked into litellm_params"
+
+    # Unrelated params untouched.
+    assert optional_params["max_tokens"] == 1024
+    assert litellm_params["metadata"] == {"trace_id": "abc"}