Parallel tool calls corrupt arguments JSON: "JSON parsing failed" with merged/truncated text #52789

Description

Version: 1.18.34 (binary at ~/.opencode/bin/opencode)

Describe the bug

When an assistant message contains multiple tool calls in parallel (especially mixing an MCP tool call with a built-in tool like bash), the arguments JSON delivered to the tool is sometimes corrupted in two distinct ways:

1. Merged arguments — the arguments of one tool call are nested inside a parameter of another tool call.
2. Truncated arguments — the JSON string is cut mid-value, failing with Unexpected EOF.

The error surfaced to the model is:

Invalid input for tool <name>: JSON parsing failed: Text: <corrupted text>.
Error message: JSON Parse error: ...

Evidence from a real session (reproduced 5+ times)

Case 1 — merged: a parallel list_tasks (MCP) + bash call delivered this text to list_tasks (its fields param received the bash args object):

{"fields": {"command": "grep -rn \"attachment\\|anexo\" apps/api/src/routes/attachments.ts | head -5; echo \"---\"; ls apps/api/src/routes/ | grep -i attach", "workdir": "/home/jairo/Projetos/azyboard"}}

Error: JSON Parse error: Expected '}'

Case 2 — truncated: a parallel list_tasks (MCP) + other tool call delivered:

{"includeDescriptions":

Error: JSON Parse error: Unexpected EOF

Observed pattern

• Serial (non-parallel) tool calls never failed in the session.
• Parallel calls with two MCP tools also worked.
• Failures concentrated on MCP tool call + built-in tool call in the same assistant message.
• The corrupted text always starts with the first tool's parameter key, then splices in the second tool's arguments object.

Expected behavior

Each tool call in a parallel set receives its own, intact arguments JSON.

Impact

MCP tools with medium-sized argument payloads fail intermittently; the model must retry serially. Users of MCP servers see "tool errors" caused purely by the harness layer. As a mitigation on our side we reduced tool schema required-fields (less JSON per call), but the merging/truncation itself is in the harness.

Additional context

• The error strings JSON parsing failed: Text: ${h} and Invalid input for tool '${YZ}': ${String(OW)} are present in the compiled binary.
• Reproduced while using a local stdio MCP server (bun run apps/mcp/src/index.ts) with @modelcontextprotocol/sdk.

Plugins

none

OpenCode version

1.18.34

Steps to reproduce

1. Configure a local stdio MCP server in opencode.json (e.g. {"mcp": {"my-server": {"type": "local", "command": ["bun", "run", "server.ts"]}}}) exposing tools with medium-sized JSON schemas (the Azy Board list_tasks tool has ~14 optional params).
2. Start a session and, in a single assistant message, issue one MCP tool call in parallel with one built-in tool call (e.g. bash), with non-trivial arguments on both. Example pair that failed repeatedly:
   • my-server.list_tasks with {"fields": ["id","title","status","columnId"], "includeDescriptions": true}
   • bash with {"command": "grep -rn \"foo\\|bar\" apps/", "workdir": "/repo"}
3. Repeat step 2 several times (intermittent, ~30–50% of parallel batches in my session; never happened with serial calls).
4. Observe the tool error Invalid input for tool <name>: JSON parsing failed: Text: ... where the Text: shows either:
   • the bash args object nested under the MCP tool's first parameter key (merged calls), e.g.
     {"fields": {"command": "grep ...", "workdir": "..."}} → JSON Parse error: Expected '}'
   • or the MCP tool's JSON cut mid-value (truncated call), e.g.
     {"includeDescriptions": → JSON Parse error: Unexpected EOF

Reproducibility: serial tool calls (one per message) never failed in a ~2h session; parallel MCP+MCP calls also worked. The corruption appears specifically when a MCP tool call and a built-in tool call share the same assistant message.

Screenshot and/or share link

No response

Operating System

Linux

Terminal

Konsole