v1.18.8 strict AjvJsonSchemaValidator rejects MCP servers emitting draft-07 schemas (n8n, Dokploy, any MCP TypeScript SDK server) #39333

Activity

1. github-actions

   assigned

   nexxeln

   on Jul 28, 2026
2. argszero commented on Jul 28, 2026

   

   argszero

   on Jul 28, 2026

   Contributor

   More actions

   Technical analysis from a Contributor:

   I traced the validation path in the opencode codebase.

   How the validation happens

   The AjvJsonSchemaValidator error originates from inside the MCP TypeScript SDK v2.0.0-beta.5 (@modelcontextprotocol/client), not from opencode's own code. The chain is:

   index.ts:78 → new Client(...)          // no custom Ajv passed
   catalog.ts:39 → listTools(client, ...)  // SDK validates all tools here
   catalog.ts:42 → convertTool(tool)       // called per-tool after validation
   

   The ClientOptions (index.ts:39-52) does not pass a custom Ajv instance, so the SDK uses its default AjvJsonSchemaValidator which only accepts 2020-12.

   Possible fix approaches

   1. Patch tool schemas after listing (opencode-side, simplest)

   After listTools returns, strip $schema from each tool's inputSchema before calling convertTool. The schema body is valid 2020-12 in practice (only the $schema URI differs). This would go in catalog.ts around line 43:

   const inputSchema: JSONSchema7 = {
     ...({ ...tool.def.inputSchema, $schema: undefined } as JSONSchema7),
     type: object,
     ...
   }

   Caveat: This approach fails if the validation happens inside listTools() before control returns to opencode. The error message format suggests this is the case — the SDK validates during tool listing, not after.

   2. Pass a custom Ajv instance (opencode-side, more robust)

   Create an Ajv2020 instance configured to accept draft-07 (which is backwards-compatible in most cases), and pass it through ClientOptions. The MCP SDK's ClientOptions type may or may not expose this — needs SDK inspection.

   3. Strip $schema in SDK consumer code (patch the SDK)

   The MCP SDK's validateSchema / AjvJsonSchemaValidator could be configured to strip or ignore the $schema field before validation, since the schemas are de facto 2020-12 compatible (they just carry the wrong URI). This would be a PR against modelcontextprotocol/typescript-sdk.

   4. SDK upstream fix

   The root cause is the MCP TypeScript SDK emitting draft-07 by default (typescript-sdk#2084). If the SDK fixes this, both OpenCode and all affected servers are resolved globally.

   Recommendation

   The most pragmatic fix for opencode is option 2 (custom Ajv with draft-07 support) or, if that's not possible through the current SDK API, a combination of option 3 (patches the bundled SDK via a local patch — opencode already has a patches directory, e.g., patches/@modelcontextprotocol%[email protected]).

   The patches/ directory already contains SDK patches; adding a schema-dialect tolerance patch there would be the lowest-risk fix that doesn't require waiting for an SDK release.
3. rekram1-node commented on Jul 28, 2026

   

   rekram1-node

   on Jul 28, 2026

   Collaborator

   More actions

   looking
4. rekram1-node

   assigned

   rekram1-node

   and unassigned

   nexxeln

   on Jul 28, 2026
5. rekram1-node

   closed this as completedin #39369on Jul 28, 2026
6. rekram1-node commented on Jul 28, 2026

   

   rekram1-node

   on Jul 28, 2026

   Collaborator

   More actions

   we downgraded the sdk and released the fix, latest mcp sdk had some bugs in it so we were forced to downgrade