fix: preserve tool argument descriptions in schema conversion

[Hemanshu-Upadhyay]

Issue for this PR

Closes #4357

Type of change

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

What does this PR do?

Custom tools using .describe() on Zod schema properties lose those descriptions before they reach the LLM. This happens because session/prompt.ts uses z.toJSONSchema() which strips description metadata from schema properties.

The fix swaps it for zodToJsonSchema() from the zod-to-json-schema package, which correctly preserves .describe() metadata. This is the same function already used in server/routes/experimental.ts for the same purpose, and zod-to-json-schema is already a dependency.

How did you verify your code works?

• Full typecheck passes (bun turbo typecheck — all 12 packages pass)
• bun dev starts without issues
• Verified zodToJsonSchema() output includes description fields for properties with .describe(), while z.toJSONSchema() drops them

Screenshots / recordings

Not a UI change.

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 my search, I found a potentially related PR:

Related PR Found:

• PR fix(opencode): preserve custom tool arg descriptions in LLM prompts #13417: "fix(opencode): preserve custom tool arg descriptions in LLM prompts"
  • fix(opencode): preserve custom tool arg descriptions in LLM prompts #13417

This PR appears to address the same issue—preserving custom tool argument descriptions. It may be worth checking if it was already merged or closed, and whether the current PR (#14476) is a follow-up, alternative approach, or if they're addressing different aspects of the same problem.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[jbmml]

Heads up — switching from z.toJSONSchema() to zodToJsonSchema() would introduce a regression on the current codebase. zod-to-json-schema@3.24.5 is incompatible with Zod v4 — it returns {"type":"string"} for object schemas, losing all properties and descriptions. I verified this directly.

The actual root cause for plugin tools losing descriptions is cross-Zod-instance metadata loss (Zod v4 stores .describe() in a per-instance globalRegistry). PR #15957 has the correct fix for this.