Persistent problem at startup of opencode: Missing required parameter: 'input[8].arguments'.

[lewisl]

Description

As in the title. No clue as to what causes it.

opencode loads and appears to work normally. I don't know if this issue affects anything.

the mesage is displayed by the tui in a highlighted message box, so it is opencode-'s message but perhaps it originates with something else. No way to know.

Plugins

mcp Serena

OpenCode version

1.18.4

Steps to reproduce

Start opencode at the commandline of a terminal.

Screenshot and/or share link

No response

Operating System

MacOS 26.5.2

Terminal

Otty

[lewisl]

This may be a possible clarification:
Missing required parameter: 'input[N].arguments'

appears to come from the OpenAI Responses API rejecting a reconstructed conversation item. input[N] is apparently a function_call entry that OpenCode has serialized without the required arguments string.

Serena/MCP seems to trigger the tool-call path where this occurs, but I do not currently have evidence that Serena itself is producing invalid arguments.

After the provider rejects the request, OpenCode does not appear to recover and the session hangs.

So, maybe OpenCode 1.18.4 sometimes replays an MCP tool call without input[N].arguments, then fails to recover from the provider error.

[lewisl]

Here is some better information. but first:

Before even spending time on this: is it possible to have opencode use its builtin lsp support--in this case, clangd-lsp for c++--to navigate code semantically and find symbols, etc? If not, then this is an issue that can't be solved. I'll make a feature request, but it is not a big deal.

If the lsp feature does enable code navigation, then the problem may have to do with requesting incorrect tools to use and nothing to do with Serena. My instance of opencode is not configured with ANY mcp servers, so can't be connecting to Serena. But, the AGENTS.md for a project proposes using lsp for some navigation and search tasks--mostly intended for codex cli or kilo code. opencode was trying to follow the "tip" with wrong tools. I fixed this with an explicit ## OPENCODE section which explicitly said NOT to use Serena--problem gone. But, with a more complex OPENCODE section, using lsp search was proposed for limited cases, but possibly with incorrect tool suggestions. What follows goes into that more.

Update: root-caused to OpenCode's native tool-discovery mechanism, not Serena/MCP.

Original report conflated two separate issues. Splitting them out after further investigation:

Not the cause: AGENTS.md was instructing OpenCode to use Serena MCP (a recommendation written for Codex CLI/Kilo Code compatibility). OpenCode has no Serena MCP server configured (opencode mcp list confirms zero servers registered), so there was nothing to actually fail — the model was just following a stale instruction pointing at a nonexistent tool. Fixed on my end via an OpenCode-specific AGENTS.md override; not an OpenCode bug.

Actual bug: After removing the Serena instruction, a semantic-navigation query ("find uses of X in calls to Y using the initializer_list overload") triggers OpenCode's internal tool_search mechanism, which fails with:

Missing required parameter: 'input[21].arguments'.

Model in use: GPT-5.6 Terra (OpenAI family, via Responses API).

Ruled out as external:

Global npm (npm ls -g --depth=0): only npm/corepack, nothing plugin-related
.opencode/plugin/ (both project and global): empty
Vendored node_modules next to the opencode binary: standard deps only (@ai-sdk, zod, effect, cross-spawn, etc.) — nothing named around tool-search/deferred-tools/BM25
Reproduces identically under opencode --pure (external plugins disabled)

Points to tool_search being native to core (or one of the "built-in plugins" per opencode debug v2 — haven't yet confirmed which).

Why this matters more than a normal failed-tool-call: when the earlier (misconfigured) Serena path failed, the agent recovered gracefully and fell back to ripgrep. This failure does not recover — the session hangs on the malformed call. A tool that's occasionally reached for and fails safely is a minor issue; one that hangs the session is not.

Suspected mechanism: this error string is a known signature of the OpenAI Responses API rejecting a tool call where the argument schema wasn't correctly populated — this exact failure mode (deferred/discovered tool + OpenAI-family model + empty or malformed arguments schema → input[N].arguments rejection) is independently documented in at least one unrelated OpenCode community plugin implementing a similar tool-search pattern, which explicitly calls out that non-Anthropic models need the full parameter schema preserved, not just descriptions. Worth checking whether OpenCode's own tool_search implementation has the same gap for OpenAI-family models specifically.

To reproduce:

Model: GPT-5.6 Terra
Prompt requiring semantic call-site/overload tracing (not a simple grep-able string match)
Observe tool_search invocation in the tool trace with a natural-language query
Session hangs on Missing required parameter: 'input[N].arguments'

[lewisl]

My bad. I RTFM'ed. It is clear that lsp is used for diagnostics after code changes. No navigation.

I installed Serena MCP. Asked a question that looked for override constructor uses across all cpp files. Well, using a combination of Serena, RG and ordinary searching, found everything and documented which overrides were not used. Showed the line numbers of each usage. Not very fast; not clear Serena helped; very thorough.