MCP HTTP headers helper returned an invalid header value

Error message

MCP HTTP headers helper returned an invalid header value

What it means

Error "MCP HTTP headers helper returned an invalid header value" thrown in openai/codex.

Source

Thrown at codex-rs/rmcp-client/src/http_headers.rs:401

                | "origin"
                | "proxy-connection"
                | "referer"
                | "te"
                | "trailer"
                | "transfer-encoding"
                | "upgrade"
        ) {
            return Err(anyhow!(
                "MCP HTTP headers helper returned a reserved header"
            ));
        }
        if parsed.contains_key(&name) {
            return Err(anyhow!(
                "MCP HTTP headers helper returned duplicate header names"
            ));
        }
        let value = HeaderValue::from_str(&value)
            .map_err(|_| anyhow!("MCP HTTP headers helper returned an invalid header value"))?;
        parsed.insert(name, value);
    }
    Ok(parsed)
}

#[cfg(test)]
#[path = "http_headers_tests.rs"]
mod tests;

View on GitHub (pinned to 339751715c)

When it happens

Trigger: Thrown at codex-rs/rmcp-client/src/http_headers.rs:401 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/0e5c2c5d12688db8. Report an issue: GitHub.