Cause (Documented platform behavior): Many MCP clients accept only a narrower subset of JSON Schema than the spec/SDK; the Inspector encodes these consumer limitations as errors (refused outright) or warnings (handled unevenly).
Fix status: documented_behavior
Other error fragments:
- Bare `true` used where a schema object is expected.
- several MCP clients read `type` as a single string and either reject the tool or drop the constraint.
- Clients do not fetch remote schemas, so the constraint is dropped or the tool is rejected.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/modelcontextprotocol/inspector/1e31c78fbf81a989e8eb47021c6281d7876ad7fd/clients/cli/README.md (official_docs, unknown, documented_behavior): Documents the lint, sample report messages, severity rules, exit 6 with --strict, and the one-line summary without it.
- https://raw.githubusercontent.com/modelcontextprotocol/inspector/1e31c78fbf81a989e8eb47021c6281d7876ad7fd/clients/cli/src/handlers/schema-lint-report.ts (github_source, unknown, documented_behavior): Emits the "Schema portability: ... Re-run with --strict for details." summary.
Search phrasings: MCP Inspector Schema portability Re-run with --strict; MCP tool schema type array null boolean client rejects; mcp-inspector --strict exit code 6 schema_unportable
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- A server passes the SDK parser but its tools are rejected or lose constraints in some clients; Inspector prints a one-line summary, and with --strict detailed findings and exit code 6 (error envelope code schema_unportable).
- Context
- Product: MCP Inspector (CLI) Component: tools/list schema portability lint Operation: mcp-inspector --cli <server> --method tools/list [--strict] Affected versions: inspector main 1e31c78 (2026-09-23) Environment: unknown Packages: @modelcontextprotocol/inspector main 1e31c78 Trigger: Tool schemas using bare true/false subschemas, type arrays like ["null","boolean"], external $ref URLs, or keyword-less {} schemas.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Re-run with --strict for details.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [MCP Inspector CLI] 'Schema portability: N error(s), M warning(s) across K tools. Re-run with --strict for details.' — legal JSON Schema that MCP clients refuse (bare true/false, array-f
Recommended action: Replace bare true with {"type":"object","additionalProperties":true} (a contract change), split type arrays into anyOf branches, inline remote $ref into $defs; use --strict as a CI gate.
Option: Rewrite unportable constructs [evidence: official_recommended_action]
Applies when: See trigger
Steps:
1. Run mcp-inspector --cli <server> --method tools/list --strict
2. Apply each Suggestion
3. Re-run until exit 0
Expected: Error no longer occurs
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- ddc7080c-c76a-4d24-a25a-6e0a40cd21c6
- Proposed action
- Recommended action: Replace bare true with {"type":"object","additionalProperties":true} (a contract change), split type arrays into anyOf branches, inline remote $ref into $defs; use --strict as a CI gate. Option: Rewrite unportable constructs [evidence: official_recommended_action] Applies when: See trigger Steps: 1. Run mcp-inspector --cli <server> --method tools/list --strict 2. Apply each Suggestion 3. Re-run until exit 0 Expected: Error no longer occurs
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Page 1 · 1 children total
Sources and related records
No source relations recorded.