PrefectHQ/fastmcp
Documented errors, page 3 of 5. Back to PrefectHQ/fastmcp
| Code / Message | Type | Severity | Tags |
|---|---|---|---|
| Client is not connected. Use the 'async with client:'… | exception | error | python, async, lifecycle, client-session |
| GenerativeUI requires prefab-ui. Install with: pip install… | exception | error | dependency, optional-extra, import-error, prefab-ui |
| OAuth callback timed out after | exception | error | oauth, timeout, callback, browser |
| [ ] Reached auto-pagination limit ( pages) for… | exception | error | pagination, client, mcp, runtime-error |
| Tool not found | exception | error | keyerror, tool, removal, provider |
| CIMD document must have jwks_uri or jwks for private_key_jwt | exception | error | oauth, cimd, jwt, private-key-jwt, validation |
| Elicitation expected an empty response, but received | exception | error | elicitation, deprecated-api, fastmcp, mcp-client |
| Expected JSON , got | validation | error | cli, json-parsing, validation |
| Subclasses must implement read() | exception | error | python, resources, not-implemented |
| Cannot add versioned | exception | error | fastmcp, versioning, registration-conflict |
| Unsupported content type for OpenAI | validation | error | openai, sampling, content-conversion, tool-calls |
| Unsupported JWK key type | validation | error | jwk, cimd, jwt, unsupported-key-type |
| Accessing ` . ` is deprecated; MCP SDK v2 renamed this… | console | warning | python, deprecation, mcp-sdk-v2, naming |
| Assertion is missing resource claim | validation | error | auth, oauth, resource-indicators, identity-assertion |
| Cannot compose Lifespan with | validation | error | lifespan, type-error, composition |
| mode must be 'legacy', 'auto', or one of | validation | error | validation, configuration, arguments, client |
| Azure AD B2C does not support the On-Behalf-Of (OBO) flow… | exception | error | python, azure, azure-ad-b2c, obo, unsupported-feature |
| EntraOBOToken requires an AzureProvider as the auth… | exception | error | python, azure, obo, provider-mismatch, configuration |
| MultiAuth requires at least a server or one verifier | validation | error | python, configuration, multiauth, auth |
| No resource data available | validation | error | python, resources, value-error |
| URI parameters must be a subset of the function arguments | validation | error | uri-template, validation, signature |
| Model returned only thinking/reasoning content with no… | exception | error | sampling, google-genai, empty-response |
| session_id is not available because no session exists. This… | exception | error | python, context, session, runtimeerror, background-tasks |
| At least one of 'tools', 'include_tags', or 'exclude_tags'… | validation | error | config, validation, pydantic, mcp |
| Server session was closed unexpectedly | exception | error | python, network, transport, server-crash |
| Multiple arguments would be mapped to the same names | validation | error | python, tool-transform, validation |
| No candidate in response from completion. | exception | error | sampling, google-genai, empty-response |
| The Horizon host changed during login | exception | error | cli, state-file, host-mismatch |
| Assertion lifetime too long | validation | error | auth, jwt, token-lifetime, identity-assertion |
| Cannot pass both 'metadata' and individual parameters to… | validation | error | python, resources, api-misuse |
| Functions with positional-only parameters are not supported… | validation | error | python, valueerror, positional-only, signature-validation, tool |
| Invalid manifest format for skill | exception | error | skills, manifest, schema-validation |
METHOD_NOT_FOUND Method is not available at protocol version . | error_code | error | protocol-version, extensions, method-not-found |
| Tool : timeout cannot be enforced when run_in_thread=False… | validation | error | python, timeout, concurrency, sync-async |
| Authorization failed | exception | error | oauth, http, auth, testing |
| Subclasses must implement verify_token | exception | error | python, not-implemented, auth, subclass-contract |
| The 'verify' parameter is only supported for HTTP… | validation | error | tls, ssl, validation, transport, arguments |
| Authorization failed for resource | exception | error | auth, middleware, access-control, custom-auth |
| Upstream request timed out, please retry | exception | warning | timeout, network, tools, retry |
| server/discover handler returned | validation | error | discover, low-level, return-type |
| Expected Resource, ResourceTemplate, or @resource-decorated… | exception | error | python, fastmcp, resources, type-error |
| Failed to pin Python version | exception | error | uv, python-version, subprocess, environment-setup |
invalid_client Client ID is required | error_code | error | oauth, authorize, client-id, validation |
| Missing required OIDC endpoints | validation | error | python, oauth, openid-connect, discovery, configuration |
| No MCP servers defined in the config | exception | error | config, file-io, validation, mcp |
| OAuth authorization failed | exception | error | oauth, auth, authorization, testing |
| Error reading resource | exception | error | mcp, resource, unhandled-exception, python |
| Invalid first argument | exception | error | prompts, decorator, typeerror, arguments |
| Unsupported algorithm | validation | error | config, jwt, algorithm, validation |
| Could not write CLI state | exception | error | filesystem, io, atomic-write, state |
| Authorization failed for resource | exception | error | auth, oauth, scopes, insufficient-scope |
| Failed to add editable packages | exception | error | uv, editable-install, packaging, environment-setup |
| identity_assertion.trusted_issuers must not be empty | validation | error | config, pydantic, validation, identity-assertion |
| Unknown resource | exception | error | mcp, resource, not-found, uri |
| Cannot specify 'algorithm' when providing a custom… | validation | error | oidc, configuration, conflicting-options |
| CIMD client_id mismatch: document says | validation | error | oauth, cimd, validation, client-id-mismatch |
| CIMD redirect_uris must be non-empty strings | validation | error | pydantic, cimd, validation, redirect-uri |
| Server did not start listening on | exception | error | network, timeout, testing, asyncio |
| Elicitation schema field | exception | error | elicitation, json-schema, union-types, fastmcp |
| HTTP fetching | http | error | network, http, ssrf, status-code |
| Path must be absolute | validation | error | filesystem, pydantic, validation |
| Subclasses must implement run() | exception | error | python, notimplementederror, abstract-method, tool, subclass |
| The device authorization request expired | exception | error | oauth, device-flow, timeout |
| Dict response_type cannot be empty. | exception | error | elicitation, schema, fastmcp, python |
| Invalid boolean value for | validation | error | python, resource-templates, value-error, uri-parameters |
| Invalid timeout type | validation | error | python, type-error, timeout, configuration |
| OAuth callback handler could not be started | exception | error | oauth, callback, port-in-use, server |
| Response too large: exceeded | exception | error | network, ssrf, size-limit, streaming |
invalid_scope invalid_scope: Requested scopes exceed those authorized by the refresh token. | error_code | error | oauth, scopes, refresh-token |
| The function ' ' has ' ' as its first parameter. Use the… | exception | error | python, fastmcp, resources, methods |
| Tool version not found | exception | error | keyerror, tool, versioning, removal |
| Assertion sub claim must be | exception | error | oauth, jwt, rfc7523, validation |
| Elicitation schema must be an object schema, got type | exception | error | elicitation, json-schema, schema-validation, fastmcp |
| messages[ ] must be Message, got . Use Message( ) to wrap… | exception | error | prompts, type-error, validation, python |
unauthorized_client unauthorized_client: Client | error_code | error | oauth, authorization, unauthorized-client |
| Error reading file | exception | error | filesystem, io, resources |
| Assertion replay detected: jti | exception | error | oauth, jwt, replay-attack, security |
| Cannot specify name both as first argument and keyword | exception | error | prompts, decorator, arguments, typeerror |
| Can not apply name to non-object schema | exception | error | json-schema, validation, pydantic, bad-argument |
| Either name or uri must be provided | exception | error | python, resources, value-error |
| Error calling tool | exception | error | tools, error-masking, debugging |
| Failed to reload module | exception | error | python, reload, hot-reload, module-loading |
| Unsupported token_endpoint_auth_method | validation | error | oauth, configuration, auth-method |
| Failed to add dependencies | exception | error | uv, dependency-resolution, pip, environment-setup |
| Invalid transport | validation | error | testing, transport, sse, configuration |
| Error building request for | exception | error | openapi, request-building, validation |
| Failed to extract key ID from token | exception | error | jwt, jwks, parsing, base64 |
| Functions with *args are not supported as tools | validation | error | python, valueerror, var-args, signature-validation, tool |
| PromptsAsTools requires a FastMCP server instance, not a | validation | error | python, prompts, transforms, type-error |
-32000 Tool ' ' execution timed out after s | error_code | error | timeout, mcp, execution, concurrency |
| Assertion exp too far in future | validation | error | auth, jwt, token-lifetime, identity-assertion |
| Query parameters must be optional function parameters with… | validation | error | uri-template, query-params, validation |
| requires the `azure` extra. Install with: pip install… | exception | error | python, azure, missing-dependency, importerror |
| Required function arguments | validation | error | uri-template, validation, signature |
| The function ' ' has ' ' as its first parameter. Use the… | exception | error | python, methods, decorators, prompts |
| Component.enable() was removed in FastMCP 3.0. Use… | exception | error | fastmcp, api-removal, migration, version-3 |
| mcp_resource() got unexpected keyword argument(s) | exception | error | python, decorator, type-error, fastmcp |
| The device authorization request was denied | exception | error | oauth, device-flow, user-denied |
| Either content or structured_content must be provided | validation | error | python, valueerror, tool-result, empty-response, mcp |
| Could not lock CLI state | exception | error | lockfile, concurrency, filesystem |