MCP exception codes
MCP Access Exceptions shows an exception category and an exception code for each row. This page lists every category and code that its Category and Exception Code filters offer, with the message that appears when you hover over a code.
The report shows each category’s name and each code’s label.
Each Workload Event: A structured audit record that Aembit generates when application-level traffic flows through Agent Proxy or MCP Identity Gateway.Learn more records the same exception in
application.mcp.exceptionCategory and application.mcp.exceptionCode, as the values in the following tables.
For those fields, see Supported protocols and application fields.
Categories
Section titled “Categories”| Category | exceptionCategory value |
|---|---|
| Access Policy | ACCESS_POLICY |
| AIDR | AIDR |
| Authorization Server | AUTHORIZATION_SERVER |
| Bad Request | BAD_REQUEST |
| Edge API | EDGE_API |
| Elicitation | ELICITATION |
| MCP Client | MCP_CLIENT |
| MCP Gateway | MCP_GATEWAY |
| MCP Server | MCP_SERVER |
| Output Schema | OUTPUT_SCHEMA |
| Tasks | TASKS |
| Tool Filter | TOOL_FILTER |
Exception codes
Section titled “Exception codes”Three codes appear in two categories, with a different message in each: INTERNAL_ERROR, LISTEN_FAILED, and
UNSUPPORTED_PROTOCOL_VERSION.
Read the category to tell them apart.
Three codes have Warning severity: CONFLICTING_RESOURCE_URI, AIDR_UNAVAILABLE_FAIL_OPEN_BYPASS, and
UNSUPPORTED_METHOD.
Every other code has Error severity.
| Category | Exception Code | exceptionCode value | Message |
|---|---|---|---|
| Access Policy | mTLS Over HTTP | MTLS_OVER_HTTP | mTLS is configured, but the server workload accepts only HTTP. |
| Access Policy | Invalid Server Workload URL | INVALID_SERVER_WORKLOAD_URL | Server workload service endpoint is not a valid URL. |
| Access Policy | Missing Service Endpoint | MISSING_SERVICE_ENDPOINT | Server workload has no service endpoint configured. |
| Access Policy | Conflicting Workload Prefix | CONFLICTING_WORKLOAD_PREFIX | Several assigned MCP servers share the same sanitized name prefix. |
| Access Policy | Conflicting Resource URI | CONFLICTING_RESOURCE_URI | Several assigned MCP servers expose the same resource URI; the first one keeps it. |
| Access Policy | Credential Not Applicable | CREDENTIAL_NOT_APPLICABLE | Credential provider configuration cannot be applied to the MCP server request. |
| AIDR | Tool Input Blocked | TOOL_INPUT_BLOCKED | Tool call input was blocked by the AIDR policy. |
| AIDR | Tool Output Blocked | TOOL_OUTPUT_BLOCKED | Tool call output was blocked by the AIDR policy. |
| AIDR | Tool Listing Blocked | TOOL_LISTING_BLOCKED | Tool listing was blocked by the AIDR policy. |
| AIDR | AIDR Unavailable | AIDR_UNAVAILABLE | AIDR service is unavailable: content was blocked (fail-closed). |
| AIDR | AIDR Unavailable Fail-Open Bypass | AIDR_UNAVAILABLE_FAIL_OPEN_BYPASS | AIDR service is unavailable: content was passed through uninspected (fail-open). |
| Authorization Server | Missing Claims | MISSING_CLAIMS | Access token is missing required claims. |
| Authorization Server | Invalid Claims | INVALID_CLAIMS | Access token claims are not accepted. |
| Bad Request | Identifier Not Found | IDENTIFIER_NOT_FOUND | Requested tool, prompt, resource, or task is not on any assigned MCP server. |
| Edge API | No MCP Servers Assigned | NO_ASSIGNED_WORKLOADS | No MCP servers are assigned to this client. |
| Edge API | Missing Assessment Claims | MISSING_ASSESSMENT_CLAIMS | Workload assessment is missing required claims. |
| Edge API | Internal Gateway Error | INTERNAL_ERROR | Aembit Cloud failed to process the request. |
| Edge API | Subscription Listen Failed | LISTEN_FAILED | Notification stream from Aembit Cloud failed. |
| Edge API | Credential Unavailable | CREDENTIAL_UNAVAILABLE | Aembit Cloud could not provide credentials for the MCP server. |
| Elicitation | Elicitation Failed | ELICITATION_FAILED | Elicitation request to the MCP client failed. |
| Elicitation | Input Rounds Exceeded | INPUT_ROUNDS_EXCEEDED | MCP server requested input for more rounds than the gateway allows. |
| MCP Client | Invalid Audience | INVALID_AUDIENCE | Access token audience does not match the gateway. |
| MCP Client | Malformed JSON-RPC Request | MALFORMED_REQUEST | MCP client request is not a valid JSON-RPC message. |
| MCP Client | Invalid HTTP Request | INVALID_HTTP_REQUEST | MCP client request violates the Streamable HTTP transport specification. |
| MCP Client | Host Not Allowed | HOST_NOT_ALLOWED | Request Host or Origin header value is not allowed by the gateway. |
| MCP Client | Unsupported Protocol Version | UNSUPPORTED_PROTOCOL_VERSION | MCP client requested an unsupported protocol version. |
| MCP Client | Missing Session ID | MISSING_SESSION_ID | MCP session ID is missing. |
| MCP Client | Session Unavailable | SESSION_UNAVAILABLE | MCP session was not found or has expired. |
| MCP Client | Client Terminated | CLIENT_TERMINATED | MCP client closed the connection unexpectedly. |
| MCP Gateway | Internal Gateway Error | INTERNAL_ERROR | MCP gateway failed to process the request. |
| MCP Gateway | Unsupported MCP Method | UNSUPPORTED_METHOD | MCP method is not supported by the gateway. |
| MCP Server | Server Unreachable | SERVER_UNREACHABLE | MCP server is unreachable. |
| MCP Server | Unsupported Protocol Version | UNSUPPORTED_PROTOCOL_VERSION | MCP server supports no protocol version compatible with the gateway. |
| MCP Server | HTTP Error | HTTP_ERROR | MCP server returned an HTTP error. |
| MCP Server | Redirect Refused | REDIRECT_REFUSED | MCP server returned a redirect, which the gateway forbids. |
| MCP Server | Internal Destination Refused | INTERNAL_DESTINATION_REFUSED | MCP server address resolves to an internal IP address, which the gateway forbids. |
| MCP Server | Malformed Response | MALFORMED_RESPONSE | MCP server response is not a valid JSON-RPC message. |
| MCP Server | Server JSON-RPC Error | JSONRPC_ERROR | MCP server returned a JSON-RPC error. |
| MCP Server | Timeout | TIMEOUT | MCP server did not respond in time. |
| MCP Server | Capability Not Declared | CAPABILITY_NOT_DECLARED | MCP server does not declare the capability the method requires. |
| MCP Server | Capability Not Implemented | CAPABILITY_NOT_IMPLEMENTED | MCP server declares the capability but does not implement this method. |
| MCP Server | Tool Execution Failure | TOOL_EXECUTION_FAILURE | Tool call returned an execution error. |
| MCP Server | Server Listen Failed | LISTEN_FAILED | Notification stream from the MCP server failed. |
| MCP Server | Other Server Error | OTHER | MCP server request failed. |
| Output Schema | Schema Violation | SCHEMA_VIOLATION | Tool result does not match the declared output schema. |
| Tasks | Tasks Not Declared | TASKS_NOT_DECLARED | MCP client called a task method without declaring the tasks capability. |
| Tasks | Unexpected Task Result | UNEXPECTED_TASK_RESULT | MCP server returned a task result, but the MCP client does not support tasks. |
| Tool Filter | Tool Not Invocable | TOOL_NOT_INVOCABLE | Tool call was blocked by the tool filter policy. |
| Tool Filter | Call Filter Failed | CALL_FILTER_FAILED | Tool call was blocked: tool filter policy could not be evaluated. |
| Tool Filter | List Filter Failed | LIST_FILTER_FAILED | Tool listing was blocked: tool filter policy could not be evaluated. |