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[MCP] Schema validation fails for tools with multiple discriminated-union/nested-object parameters (e.g. obsidian-mcp-server patch_note, append_to_note) #26277
Description
Description
Environment
- OpenCode: latest (oh-my-opencode v3.17.15)
- MCP server: [email protected] (cyanheads/obsidian-mcp-server, npm)
- OS: Windows
Steps to reproduce
- Configure obsidian-mcp-server as an MCP server (stdio transport)
- Call
obsidian_patch_notewith valid nested parameters:- target: { type: "path", path: "SomeFile.md" }
- section: { type: "heading", target: "SectionName" }
- operation: "append"
- content: "new content"
- Call
obsidian_append_to_notewith:- target: { type: "path", path: "SomeFile.md" }
- section: { type: "heading", target: "SectionName" } (optional)
- content: "new content"
Expected behavior
Both calls should succeed with properly structured nested parameters matching the server's declared input schema.
Actual behavior
Both calls fail with -32602 Invalid params. The error messages demand flat top-level fields that are actually nested inside discriminated union objects, e.g.:
required property 'path'(path is insidetarget, not top-level)required property 'sectionTargeted'(this is an output field, not an input field)
Which tools are affected
obsidian_patch_note— hastarget(discriminated union) +section(nested object)obsidian_append_to_note— hastarget(discriminated union) + optionalsection(nested object)
Which tools work correctly
obsidian_replace_in_note— all flat parametersobsidian_write_note— single nested targetobsidian_get_note— single nested targetobsidian_list_notes— all flat parameters
Hypothesis
OpenCode's host layer is incorrectly flattening Zod discriminated-union schemas when a tool has two or more nested-object parameters, then validating calls against the flattened (incorrect) schema instead of the server's actual schema.
Server schema (for reference)
From [email protected] source (obsidian-patch-note.tool.js):
input: z.object({
target: TargetSchema, // discriminated union: {type:"path",path} | {type:"active"} | {type:"periodic",...}
section: SectionSchema, // nested object: {type: "heading"|"block"|"frontmatter", target: string}
operation: z.enum('append', 'prepend', 'replace'),
content: z.string(),
})
Workaround
Using obsidian_replace_in_note (literal/regex search-replace) in place of obsidian_patch_note and obsidian_append_to_note works correctly.
Plugins
No response
OpenCode version
No response
Steps to reproduce
No response
Screenshot and/or share link
No response
Operating System
No response
Terminal
No response
Activity
cyanheads commentedon May 8, 2026
Hi there, maintainer of obsidian-mcp-server here — wanted to share a few findings that might help narrow this down.
Schema dump. Here's the actual
inputSchemathe server emits forobsidian_patch_note(descriptions and a couple of optional fields elided for brevity), in case it's useful for comparison against what opencode is validating against:{ "$schema": "http://json-schema.org/draft-07/schema#", "type": "object", "properties": { "target": { "oneOf": [ {"type": "object", "properties": {"type": {"type": "string", "const": "path"}, "path": {"type": "string", "minLength": 1}}, "required": ["type", "path"]}, {"type": "object", "properties": {"type": {"type": "string", "const": "active"}}, "required": ["type"]}, {"type": "object", "properties": {"type": {"type": "string", "const": "periodic"}, "period": {"type": "string", "enum": ["daily","weekly","monthly","quarterly","yearly"]}}, "required": ["type", "period"]} ] }, "section": {"type": "object", "properties": {"type": {"type": "string", "enum": ["heading","block","frontmatter"]}, "target": {"type": "string", "minLength": 1}}, "required": ["type", "target"]}, "operation": {"type": "string", "enum": ["append","prepend","replace"]}, "content": {"type": "string"} }, "required": ["target", "section", "operation", "content"] }It's standard JSON Schema —
oneOfwith aconstdiscriminator and nestedproperties. Both shapes are core features in draft-07 and JSON Schema 2020-12 (the MCP spec default when no$schemais set).One detail that might point at the cause. The reporter mentions a
required property 'sectionTargeted'error forobsidian_append_to_note. That field only exists in the tool'soutputSchema, not its input — so the validator may be reading from the wrong schema (or a merged one) for tools with multiple nested-object parameters. That lines up with @r007b34r's flattening hypothesis.Happy to provide full schema dumps for any of the affected tools or help test a fix against the live server.
github-actions commentedon Jul 9, 2026
To stay organized issues are automatically closed after 60 days of no activity. If the issue is still relevant please open a new one.