[bug] Tool params declared as oneOf (no top-level type) are serialized to a string, so oneOf validation always fails with matched 0

[RGarvel]

Summary

Tool parameters whose schema uses oneOf (no top-level type) reach the tool's validator as a string instead of an object, so they fail validation with … must match exactly one oneOf branch (matched 0) (or must be an object). Explicit type: "object" parameters work fine. This breaks several built-in tools deterministically — cordis_define's plugin parameter (and the dynamic-Cordis probe it gates) has been unusable.

Root-cause chain (paths/line numbers from the v29b22c5 install; re-check against the repo):

1. dsh-tool-cordis/lib/index.js:7189-7222 — cordis_define declares parameters.plugin as oneOf: [ {type:"object", …}, {type:"object", …} ] with no top-level type.
2. dsh-tools/lib/types/schema.js:130-149 — oneOf compiles correctly to a node.oneOf node (still no top-level type).
3. dsh-tools/lib/types/index.js:664-675 (schemaOf) — the compiled parameters are projected to the model as-is (no lowering).
4. dsh-llm-deepseek/lib/index.js:226-234 — parameters: tool.parameters is sent verbatim, so the wire schema for plugin is a bare oneOf.
5. DeepSeek function-calling, given a oneOf union with no concrete type, emits the value as a JSON string — e.g. wire arguments become {"plugin":"{\"kind\":\"new\",\"idPrefix\":\"x\"}", …}.
6. dsh-agent-loop/lib/index.js:145-152 — parseArguments only does a top-level JSON.parse (raw ? JSON.parse(raw) : {}), so args.plugin arrives as that nested string.
7. dsh-tools/lib/types/json-schema.js:440-441 (oneOf) / :472-475 (object) — the string matches 0 object branches → matched 0; a concrete type:"object" node would instead report must be an object.

Repro

Call cordis_define with plugin: { kind: "new", idPrefix: "test" } → always matched 0. Change plugin to an explicit type: "object" declaration → succeeds.

Suggested fixes (by cost/benefit)

1. Stop modeling union-object parameters as oneOf in built-ins — use a single type: "object" with a kind discriminator (enum) and optional idPrefix/pluginId, which the provider parses as an object unambiguously.
2. Add tolerance in the argument layer: for a oneOf-typed parameter, if the value is a string that parses to an object, parse it before validation.
3. Lower "all-object-branch oneOf" into an explicit object hint at the schema-serialization boundary.

Impact: cordis_define/cordis_inspect tooling and any tool or MCP binding that uses a oneOf (or type:"json"-only) parameter is non-functional on the DeepSeek provider.

[MoTingFeng]

补充一个同一根因的另一症状变体（实测遇到，供参考）：

除上述「嵌套 string 透传」外，当顶层 arguments 整体偶发不是合法 JSON（模型输出截断/格式瑕疵）时，parseArguments 的 catch { return raw } 会把整段原始字符串当作 args 返回，随后被 validateArgs 当对象遍历 → 因字符串无任何声明属性而报第一个 required 缺失，产生误导性错误：

invalid arguments: missing required property "description"

误导点：

• 报错字段与真实问题无关——例如 read 工具根本没有 description 参数，触发时却报缺 description；
• 低概率偶发、重试即成功（模型重新生成完整 JSON 后解析正常）；
• run_code 等以程序内字面量对象调用工具的路径永不触发（绕过模型 JSON 生成）。

这与 #5512 第 6 步同一行代码（parseArguments 只做顶层 JSON.parse 且失败静默降级）相关，建议修复时一并覆盖：parseArguments 对无法解析的输入返回显式失败（typed error / 标记），或调用方确认解析结果为 plain object 再交 schema 校验，避免误导性缺字段报错。

（脱敏：无环境特定路径与个人数据）