DeepSeek tool calls can use malformed argument JSON

[HNGM-HP]

Description

DeepSeek models can produce malformed tool-call arguments in some cases, for example optional placeholders (null, "", {}, []), arrays encoded as strings, decorated paths, or partial paired parameters.

This is related to DeepSeek tool-call formatting, but narrower than #24566. That issue is about raw DSML/XML-style tool calls not being parsed by a provider. This issue is about prompt-level mitigation for argument JSON shape when OpenCode is already using normal tool calls.

OpenCode already has provider-specific prompt sections for some models. A small DeepSeek-specific prompt section can make the expected tool-call JSON shape explicit without changing behavior for other providers.

Plugins

None

OpenCode version

Current dev branch

Steps to reproduce

1. Use a DeepSeek model with tools enabled.
2. Ask it to call tools with array, object, path, or optional parameters.
3. Observe that some calls may use invalid argument shapes, such as arrays as strings, decorated paths, placeholder optional values, or only one side of a paired parameter.

Screenshot and/or share link

No screenshot. This is a tool-call argument formatting issue.

Operating System

Ubuntu 24.04

Terminal

Not terminal-specific

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• No issue template was used. Please use one of the three templates: Bug Report, Feature Request, or Question. If this is a feature request, the title should start with [FEATURE]: and a verification checkbox is required. If it's a bug report, steps to reproduce and system info are expected.

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• OpenCode can not support DeepSeek tool calling method: DSML(DeepSeek Makeup Language) #24566: DeepSeek DSML (XML-style) tool call format causing tool call failures — also about DeepSeek producing malformed/unsupported tool-call argument formats

If you believe this was flagged incorrectly, please let a maintainer know.

[HNGM-HP]

Thanks for the duplicate pointer. I updated the issue with the bug template and added a note on the distinction from #24566: this PR is a prompt-level mitigation for malformed JSON argument shapes, not provider-side DSML parsing support.

[github-actions]

Thanks for updating the issue! It now includes a clear description, steps to reproduce, and system information. Everything looks good — appreciate you taking the time to fill it out properly.