Large tool arguments (>~30KB) are truncated at the content boundary, causing "Unterminated string in JSON" — affects all tools incl. MCP #42224

Summary

When a tool call carries a real string-literal argument larger than roughly 30KB, the opencode client truncates the tool-call JSON exactly at the end of the embedded content (missing closing quote/brace), and the call fails with:

The arguments provided to the tool are invalid: Invalid input for tool <TOOL>: JSON parsing failed: Text: {…content…}. Error message: Unterminated string in JSON at position N (line 1 column N+1)

The failure position N always equals the length of the sent payload — the string is cut at the content boundary. Small arguments (a few KB) work fine.

Environment

• opencode 1.18.15 and 1.18.18 (bug reproduces on both; upgrade did not fix it)
• Windows 11, PowerShell 5.1, TUI
• Reproduced on multiple tool types: bash (built-in), an MCP run_script tool, and task — i.e. it is not specific to any one tool or to MCP servers

Steps to reproduce

1. Invoke any tool (e.g. bash) with an argument containing a literal string of ~30KB+ (e.g. a multi-line ASCII payload). Note: "a" * 30000-style runtime-generated strings do NOT reproduce it — the payload must physically be large in the tool-call argument.
2. The client rejects the call before it ever reaches the tool/server: JSON parsing failed … Unterminated string in JSON at position N.

Evidence (position ≈ payload length)

Key observations:

• Character set is irrelevant: 100% pure-ASCII payloads containing no quotes, no backslashes, no control characters fail identically. Base85 + JSON-escaped payloads fail identically. The truncation is purely length-based.
• JSON is truncated at the content boundary: position N ≈ exact payload length, i.e. the closing " and } are never emitted.
• Never reached the target: the MCP server reported 0 requests received; all failures are client-side.
• Cross-tool: bash (fully local, no network), MCP tools, and task tool all fail the same way.
• Repro'd across two releases (1.18.15 → 1.18.18).

Expected behavior

Large tool arguments should either be transmitted intact, fail with an explicit size-limit error, or be configurable (e.g. a documented max tool-argument size). Currently there is no way to pass payloads >~30KB to any tool, which blocks legitimate workflows such as executing large scripts through MCP run_script or passing large data blobs.

Impact

• Cannot execute any MCP tool that requires a >~30KB argument (e.g. running a 70KB script via run_script).
• Affects all tool types uniformly, not just MCP.

Happy to provide additional traces (opencode.log excerpts, persisted tool-output files) on request.