PrefectHQ/fastmcp
Documented errors, page 4 of 5. Back to PrefectHQ/fastmcp
| Code / Message | Type | Severity | Tags |
|---|---|---|---|
| `ProxyInitializeMiddleware` is deprecated and will be… | console | warning | python, deprecation, proxy, middleware |
| Request error ( ) | exception | error | network, http, openapi |
| Cannot specify both a name as first argument and as keyword… | exception | error | python, typeerror, decorator-usage |
| CodeMode requires pydantic-monty for the Monty sandbox… | exception | error | python, import-error, optional-dependency, code-mode, fastmcp |
| OIDC discovery document for issuer | exception | error | oidc, discovery, json, schema |
| session is not available because the MCP session has not… | exception | error | python, context, session, runtimeerror, session-lifecycle |
| Approval requires prefab-ui. Install with: pip install… | exception | error | python, importerror, optional-dependency |
| Assertion iat is in the future | validation | error | auth, jwt, clock-skew, identity-assertion |
| ' ' is a synthetic search tool and cannot be called via the… | validation | error | python, search, tools, proxy |
| Total must be at least 1 | validation | error | progress, validation, fastmcp |
| Unsupported algorithm | validation | error | config, jwt, algorithm, validation |
| No access token available. Cannot perform OBO exchange. | exception | error | python, azure, obo, context, runtime |
| Version must be a string, int, or float, got bool | exception | error | python, typeerror, versioning, components |
| Malformed assertion payload | exception | error | jwt, malformed, parsing |
| max_cache_size must be non-negative, got | validation | error | validation, configuration, valueerror |
| moved to the fastmcp-tasks package. Install it with `pip… | exception | error | python, import-error, optional-dependency, fastmcp |
| Subclasses must implement read() or override… | exception | error | python, resource-templates, not-implemented |
| does not define KEY_PREFIX. Component keys will not be… | console | warning | python, components, naming, subclassing |
| No server URL found in OpenAPI spec. Either add a 'servers'… | exception | error | openapi, configuration, http |
| No text content in response from Anthropic | exception | error | anthropic, sampling, text-content, tool-use |
| The `anthropic` package is not installed. Install it with… | exception | error | dependency, optional-extra, anthropic, import |
| FastMCP CLI support is not installed. Install `fastmcp` or… | exception | error | dependency, cli, optional-extra, install |
| OAuth token endpoint error | http | error | oauth, network, token-endpoint, upstream-provider |
| Unknown resource: version | exception | error | mcp, resource, not-found, versioning |
| Unsupported tool result content type | validation | error | google-genai, sampling, tool-result, unsupported-content |
| Extension is not bound to a FastMCP server; register it… | exception | error | extensions, lifecycle, initialization-order |
| No server specification or fastmcp.json found | console | error | python, cli, config, fastmcp |
| Rate limit exceeded: requests per minutes for client | exception | error | rate-limiting, sliding-window, middleware |
| Skill directory already exists | exception | error | skills, file-exists, filesystem |
| Template not found | exception | error | keyerror, resource-template, uri-template, removal |
| The function ' ' has ' ' as its first parameter. Use the… | exception | error | python, fastmcp, tools, methods |
| Server process failed to terminate even after kill | exception | error | testing, subprocess, process-termination, multiprocessing |
| Unexpected manifest format for skill | exception | error | skills, mcp-resources, content-type |
| default_factory can only be used with hide=True. Visible… | validation | error | python, tool-transform, validation |
| The Horizon API key is invalid | exception | error | pydantic, validation, api-key, state-file |
| FileUpload requires prefab-ui. Install with: pip install… | exception | error | python, importerror, optional-dependency |
| Got unexpected keyword argument(s) | validation | error | python, tool-transform, arguments, typeerror |
| The `google-genai` package is not installed. Install it… | exception | critical | importerror, google-genai, missing-dependency, optional-extra |
| The @resource decorator requires a URI. Use… | exception | error | python, decorator, api-misuse |
| Version must be a string, int, or float, got | exception | error | python, typeerror, versioning, components |
| Expected boolean, got | validation | error | validation, cli, input-parsing, boolean |
| File exceeds max size ( > ) | validation | error | python, valueerror, file-upload, size-limit |
| No content in response from completion | exception | error | sampling, google-genai, empty-response, tool-calls |
| No content in response from completion | exception | error | openai, sampling, empty-response, max-tokens |
| Requested scopes are not valid | error_code | error | oauth, scopes, validation |
| host_origin_protection must be True, False, or 'auto'. | validation | error | configuration, http-app, validation |
| Output schemas must represent object types due to MCP spec… | validation | error | python, json-schema, mcp-spec, output-schema |
| The @resource decorator was used incorrectly. It requires a… | exception | error | python, fastmcp, resources, decorator-misuse |
| Untrusted assertion issuer | exception | error | jwt, issuer, configuration, security |
| Cannot specify 'required=False'. Set a default value… | validation | error | python, tool-transform, validation |
| CIMD document is not valid JSON | validation | error | oauth, cimd, json, validation |
| Either path or data must be provided | validation | error | validation, image, valueerror |
| max_size must be positive, got | validation | error | configuration, validation, middleware |
| No content in response from Anthropic | validation | error | anthropic, sampling, empty-response, api |
| require_roles() needs at least one role; a check with no… | exception | error | python, auth, authorization, rbac |
| Unknown prompt | exception | error | mcp, prompt, not-found |
| Global rate limit exceeded | exception | error | rate-limiting, middleware, traffic |
| Unexpected ASGI message type | exception | error | python, asgi, http, protocol |
| Using in-memory token storage -- tokens will be lost when… | console | warning | python, oauth, token-storage, persistence |
| Functions with **kwargs are not supported as tools | validation | error | python, valueerror, var-kwargs, signature-validation, tool |
| trusted_issuers entries must be non-empty strings | validation | error | config, pydantic, validation, identity-assertion |
| URI template must contain at least one parameter | validation | error | uri-template, validation |
| uv is not installed. Please install it with: curl -LsSf… | exception | critical | environment, uv, missing-dependency, deployment |
| Elicitation schema field | exception | error | elicitation, json-schema, arrays, fastmcp |
| Expected number, got | validation | error | validation, cli, input-parsing, number |
| Failed to initialize server session | exception | error | python, timeout, handshake, initialization |
| Assertion has expired | exception | error | oauth, jwt, token-expired, clock-skew |
| No matching key found for kid= | exception | error | oauth, jwks, kid-mismatch, private-key-jwt |
| Failed to add requirements | exception | error | uv, requirements-txt, dependency-resolution, environment-setup |
| Invalid message role | validation | error | google-genai, sampling, message-role, valueerror |
| JWT issuer not initialized. Ensure get_routes() is called… | exception | error | oauth, jwt, lifecycle, initialization-order |
| CLI state must not be a symbolic link | exception | error | security, symlink, filesystem |
| Invalid manifest JSON for skill | exception | error | skills, json, manifest |
| Not a directory | exception | error | filesystem, resources, wrong-path-type |
| Device authorization failed | exception | error | oauth, device-flow, unknown-error |
| ' returned an error"} | exception | error | mcp, tool-call, client |
| Rate limit exceeded for client | exception | error | rate-limiting, middleware, clients |
| cache_scope requires cache_ttl; a scope without a TTL does… | validation | error | config, caching, validation, fastmcp |
| Could not create the CLI state directory | exception | error | filesystem, permissions, configuration |
| No audio data available | validation | error | audio, state, valueerror |
| Unknown format | exception | error | fastmcp, validation, bad-argument, inspect |
| Discovery tool name ' ' collides with execute_tool_name. | exception | error | code-mode, naming-collision, value-error, fastmcp |
| forward() can only be called within a transformed tool | exception | error | python, tool-transform, context, runtime-error |
| ImageContent/AudioContent is only supported in user… | validation | error | openai, sampling, content-conversion, assistant-message |
| Unknown arguments in transform_args | validation | error | python, tool-transform, validation |
| Component already exists | exception | error | fastmcp, duplicate-registration, components |
| Component.disable() was removed in FastMCP 3.0. Use… | exception | error | fastmcp, api-removal, migration, version-3 |
| Only one of path or data can be provided | validation | error | validation, image, valueerror |
| resource_uri cannot be set on resources — the resource… | validation | error | python, resources, ui, config |
| ResourcesAsTools requires a FastMCP server instance, not a | validation | error | python, resources, transforms, type-error |
| Assertion typ must be | exception | error | jwt, typ, oidc, spec-compliance |
| Choice requires prefab-ui. Install with: pip install… | exception | error | python, importerror, optional-dependency |
| Resource not found | exception | error | keyerror, resource, uri, removal |
| The CLI state lock must not be a symbolic link | exception | error | security, symlink, filesystem, lockfile |
| Cannot add unversioned | exception | error | fastmcp, versioning, registration-conflict |
| Assertion must include jti claim | exception | error | oauth, jwt, rfc7523, replay-protection |
| Tool call limit exceeded: at most | exception | error | code-mode, rate-limit, tool-error, fastmcp |
| Could not read CLI state | exception | error | permissions, filesystem, io |
| Invalid OpenAPI specification | exception | error | openapi, validation, configuration |
| Prompt must return str, list[Message], or PromptResult, got | exception | error | prompts, type-error, return-value, python |