openai/codex · error · anyhow::Error

MCP tool arguments must be a JSON object, got {other}

Error message

MCP tool arguments must be a JSON object, got {other}

What it means

Error "MCP tool arguments must be a JSON object, got {other}" thrown in openai/codex.

Source

Thrown at codex-rs/rmcp-client/src/rmcp_client.rs:782

                .boxed()
            })
            .await?;
        self.persist_oauth_tokens().await;
        Ok(result)
    }

    pub async fn call_tool(
        &self,
        name: String,
        arguments: Option<serde_json::Value>,
        meta: Option<serde_json::Value>,
        timeout: Option<Duration>,
    ) -> Result<CallToolResult> {
        self.refresh_oauth_if_needed().await?;
        let arguments = match arguments {
            Some(Value::Object(map)) => Some(map),
            Some(other) => {
                return Err(anyhow!(
                    "MCP tool arguments must be a JSON object, got {other}"
                ));
            }
            None => None,
        };
        let meta = match meta {
            Some(Value::Object(map)) => Some(RequestMetaObject::from(map)),
            Some(other) => {
                return Err(anyhow!(
                    "MCP tool request _meta must be a JSON object, got {other}"
                ));
            }
            None => None,
        };
        let mut rmcp_params = CallToolRequestParams::new(name);
        rmcp_params.arguments = arguments;
        let requested_modern = self.protocol_mode == McpProtocolMode::V20260728;
        let result = self

View on GitHub (pinned to 339751715c)

When it happens

Trigger: Thrown at codex-rs/rmcp-client/src/rmcp_client.rs:782 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of openai/codex@339751715c (2026-08-25). Data as JSON: /api/errors/0f425253b513f7e9. Report an issue: GitHub.