fix(opencode): replace zodToJsonSchema with z.toJSONSchema for Zod v4 compat

[jbmml]

Issue for this PR

Fixes #20807
Related: #4357

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The experimental tools endpoint (GET /experimental/tools) uses zodToJsonSchema() from zod-to-json-schema@3.24.5 to convert tool parameter schemas. This library is incompatible with Zod v4 — for an object schema with two described string properties, it returns {"type":"string"} instead of the correct object schema. All properties, descriptions, and required fields are lost.

The fix switches to z.toJSONSchema() (Zod v4 built-in), which is already used in session/prompt.ts for the same purpose and correctly preserves .describe() metadata. The Zod-vs-plain detection also changes from _def (Zod v3 internals) to typeof parse === "function" which works reliably with Zod v4.

I confirmed the original bug reported in #4357 (tool descriptions lost) is already fixed in the prompt path by the Zod v4 migration. This PR fixes the same issue in the experimental API path.

How did you verify your code works?

• Tested both converters directly: z.toJSONSchema() produces correct output with descriptions, zodToJsonSchema() returns garbage {"type":"string"}
• 1808/1808 passing tests unchanged (6 pre-existing skill discovery failures on unmodified code)
• Full turbo typecheck passes across all 13 packages

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found related PRs but they appear to be older attempts addressing similar issues with tool descriptions. However, PR #20809 is the current PR being analyzed, so I need to verify if any of the found PRs are still open and potentially duplicate:

The most relevant related PRs are:

• fix(opencode): missing custom tool arg descriptions #15957 - "fix(opencode): missing custom tool arg descriptions" - Addresses similar tool description issues
• fix(opencode): preserve custom tool arg descriptions in LLM prompts #13417 - "fix(opencode): preserve custom tool arg descriptions in LLM prompts" - Related to preserving descriptions
• fix: preserve tool argument descriptions in schema conversion #14476 - "fix: preserve tool argument descriptions in schema conversion" - Related schema conversion issue
• fix(opencode): preserve tool argument descriptions in schema conversion #19974 - "fix(opencode): preserve tool argument descriptions in schema conversion" - Another schema conversion fix

These appear to be previous attempts at fixing similar issues. The current PR #20809 is a more targeted fix for the experimental tools endpoint using the Zod v4 built-in z.toJSONSchema() method instead of the external zodToJsonSchema library.

No duplicate PRs found

[jbmml]

Re: the bot-flagged related PRs — I investigated all four:

• fix(opencode): preserve custom tool arg descriptions in LLM prompts #13417, fix: preserve tool argument descriptions in schema conversion #14476, fix(opencode): preserve tool argument descriptions in schema conversion #19974 all switch prompt.ts from z.toJSONSchema() to zodToJsonSchema(). This would introduce a regression — zodToJsonSchema from zod-to-json-schema@3.24.5 is incompatible with Zod v4 and produces incorrect output (returns {"type":"string"} for object schemas). I verified this directly.

• fix(opencode): missing custom tool arg descriptions #15957 correctly identifies the real root cause: cross-Zod-instance metadata loss. When plugins load tools from their own node_modules, .describe() metadata is stored in the plugin's z.globalRegistry but not the core's. The rehydration fix in fix(opencode): missing custom tool arg descriptions #15957 is the right approach for the prompt path.

This PR (#20809) fixes a different code path — the experimental HTTP API endpoint, which was the only remaining call site for the broken zodToJsonSchema. These are complementary, not overlapping.

[jbmml]

Quick note for reviewers: this PR is independent of #19916 (plugin cross-instance Zod metadata rehydration).

This fixes a narrower issue — experimental.ts is the only remaining call site using zodToJsonSchema from zod-to-json-schema@3.24.5, which is incompatible with Zod v4 (returns {"type":"string"} for object schemas). The fix switches to z.toJSONSchema() which is native to Zod v4 and produces correct output.

I previously linked this to a broader re-architecture RFC (#20922) which I've since closed — the unification premise didn't hold up under analysis. This fix stands on its own.