diff --git a/features/mocks/mock_mcp_transport.py b/features/mocks/mock_mcp_transport.py
index e40409e7a..759d8b662 100644
--- a/features/mocks/mock_mcp_transport.py
+++ b/features/mocks/mock_mcp_transport.py
@@ -51,7 +51,9 @@ class MockMCPTransport(MCPTransport):
             if tool_name in self._invoke_errors:
                 return {
                     "isError": True,
-                    "error": self._invoke_errors[tool_name],
+                    "content": [
+                        {"type": "text", "text": self._invoke_errors[tool_name]}
+                    ],
                 }
 
             if tool_name in self._invoke_results:
diff --git a/features/steps/plan_namespaced_name_tdd_steps.py b/features/steps/plan_namespaced_name_tdd_steps.py
index 6728627fa..cf9bd8295 100644
--- a/features/steps/plan_namespaced_name_tdd_steps.py
+++ b/features/steps/plan_namespaced_name_tdd_steps.py
@@ -8,7 +8,9 @@ from cleveragents.domain.models.core.plan import NamespacedName
 
 
 @when('I parse the namespaced name "{full_name}" expecting an error')
-def step_when_parse_namespaced_name_expecting_error(context: Context, full_name: str) -> None:
+def step_when_parse_namespaced_name_expecting_error(
+    context: Context, full_name: str
+) -> None:
     """Parse a namespaced name string and capture any error."""
     try:
         context.namespaced_name = NamespacedName.parse(full_name)
@@ -18,8 +20,12 @@ def step_when_parse_namespaced_name_expecting_error(context: Context, full_name:
         context.namespaced_name = None
 
 
-@when('I construct a NamespacedName with namespace "{namespace}" and name "{name}" expecting an error')
-def step_when_construct_namespaced_name_expecting_error(context: Context, namespace: str, name: str) -> None:
+@when(
+    'I construct a NamespacedName with namespace "{namespace}" and name "{name}" expecting an error'
+)
+def step_when_construct_namespaced_name_expecting_error(
+    context: Context, namespace: str, name: str
+) -> None:
     """Create a NamespacedName and capture any error."""
     try:
         context.namespaced_name = NamespacedName(namespace=namespace, name=name)
diff --git a/features/steps/tdd_mcp_error_content_key_steps.py b/features/steps/tdd_mcp_error_content_key_steps.py
new file mode 100644
index 000000000..dfdbd4c86
--- /dev/null
+++ b/features/steps/tdd_mcp_error_content_key_steps.py
@@ -0,0 +1,71 @@
+"""Step definitions for features/tdd_mcp_error_content_key.feature.
+
+TDD issue-capture scenario for #2158: MCPToolAdapter.invoke() must extract
+error messages from content[0].text per MCP 1.4.0 protocol.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from behave import given, then
+from behave.runner import Context
+
+from cleveragents.mcp.adapter import MCPServerConfig, MCPToolAdapter
+from features.mocks.mock_mcp_transport import MockMCPTransport
+
+
+class _MCP14ErrorTransport(MockMCPTransport):
+    """Mock transport that returns MCP 1.4.0-compliant error responses.
+
+    Returns ``{"isError": True, "content": [{"type": "text", "text": "..."}]}``
+    instead of the non-standard ``{"isError": True, "error": "..."}`` format.
+    """
+
+    def __init__(self, tool_name: str, error_text: str) -> None:
+        tools = [
+            {
+                "name": tool_name,
+                "description": f"Mock tool {tool_name}",
+                "inputSchema": {},
+            }
+        ]
+        super().__init__(tools=tools)
+        self._mcp14_error_tool = tool_name
+        self._mcp14_error_text = error_text
+
+    def call(self, method: str, params: dict[str, Any]) -> dict[str, Any]:
+        if method == "tools/call":
+            tool_name = params.get("name", "")
+            if tool_name == self._mcp14_error_tool:
+                return {
+                    "isError": True,
+                    "content": [{"type": "text", "text": self._mcp14_error_text}],
+                }
+        return super().call(method, params)
+
+
+@given(
+    'a connected MCP adapter with a tool returning MCP 1.4.0 content-based error "{tool_name}"'
+)
+def step_mcp14_error_tool(context: Context, tool_name: str) -> None:
+    """Set up a connected MCP adapter whose tool returns a MCP 1.4.0 content-based error."""
+    transport = _MCP14ErrorTransport(
+        tool_name=tool_name,
+        error_text="File not found: /path/to/file",
+    )
+    config = MCPServerConfig(name="test-server", transport="stdio", command="echo")
+    context.mcp_adapter = MCPToolAdapter(config=config, transport=transport)
+    context.mcp_adapter.connect()
+    context.mcp_adapter.discover_tools()
+
+
+@then('the invocation error should contain "{expected}"')
+def step_invoke_error_contains(context: Context, expected: str) -> None:
+    """Assert the invocation error message contains the expected substring."""
+    result = context.mcp_invoke_result
+    assert result is not None, "No invocation result found"
+    assert result.error is not None, "Expected an error but got None"
+    assert expected in result.error, (
+        f"Expected error to contain {expected!r}, got: {result.error!r}"
+    )
diff --git a/features/tdd_mcp_error_content_key.feature b/features/tdd_mcp_error_content_key.feature
new file mode 100644
index 000000000..ee35b04e9
--- /dev/null
+++ b/features/tdd_mcp_error_content_key.feature
@@ -0,0 +1,11 @@
+@tdd_issue @tdd_issue_2158
+Feature: TDD Issue #2158 — MCPToolAdapter.invoke() error extraction uses non-standard key
+  MCPToolAdapter.invoke() currently reads error messages from result.get('error', 'unknown error'),
+  but the MCP 1.4.0 protocol returns errors in content[0].text.  This means every error from a
+  real MCP 1.4.0 server is silently replaced with "unknown error".
+
+  Scenario: invoke() extracts error message from MCP 1.4.0 content array
+    Given a connected MCP adapter with a tool returning MCP 1.4.0 content-based error "read_file"
+    When I invoke "read_file" with arguments {"path": "/path/to/file"}
+    Then the invocation should fail
+    And the invocation error should contain "File not found: /path/to/file"
diff --git a/src/cleveragents/mcp/adapter.py b/src/cleveragents/mcp/adapter.py
index bc36e85c8..5d20c5438 100644
--- a/src/cleveragents/mcp/adapter.py
+++ b/src/cleveragents/mcp/adapter.py
@@ -514,9 +514,14 @@ class MCPToolAdapter:
             elapsed = (time.monotonic() - start) * 1000
 
             if result.get("isError"):
+                content = result.get("content", [])
+                if content and isinstance(content, list) and len(content) > 0:
+                    error_text = content[0].get("text", "unknown error")
+                else:
+                    error_text = "unknown error"
                 return MCPToolResult(
                     success=False,
-                    error=f"MCP server error: {result.get('error', 'unknown error')}",
+                    error=f"MCP server error: {error_text}",
                     duration_ms=elapsed,
                 )