xai-org/grok-build · error

JSON object must have a "type" field (e.g., {"type": "acp",

Error message

JSON object must have a "type" field (e.g., {"type": "acp", "content": [...]})

What it means

When the prompt JSON is an object, parse_prompt_json requires a string "type" field to select the format. If the object has no "type" key or it is not a string, this bail message is returned verbatim, including the expected wrapper example. It exists to guide users to the typed wrapper format {"type":"acp","content":[...]}.

Source

Thrown at crates/codegen/xai-grok-pager/src/headless/cli.rs:103

        match self {
            Self::Text(text) => vec![acp::ContentBlock::Text(acp::TextContent::new(text))],
            Self::Blocks(blocks) => blocks,
        }
    }
}

/// Parse ACP content blocks from an array (`[...]`) or typed wrapper (`{"type":"acp","content":[...]}`).
fn parse_prompt_json(json_str: &str) -> anyhow::Result<Vec<acp::ContentBlock>> {
    let value: serde_json::Value =
        serde_json::from_str(json_str).map_err(|e| anyhow::anyhow!("Invalid JSON: {e}"))?;

    let blocks: Vec<acp::ContentBlock> = match value {
        serde_json::Value::Array(_) => serde_json::from_value(value)
            .map_err(|e| anyhow::anyhow!("Invalid ACP content blocks: {e}"))?,

        serde_json::Value::Object(ref map) => {
            let format_type = map.get("type").and_then(|v| v.as_str()).ok_or_else(|| {
                anyhow::anyhow!(
                    "JSON object must have a \"type\" field \
                         (e.g., {{\"type\": \"acp\", \"content\": [...]}})"
                )
            })?;
            let content = map
                .get("content")
                .ok_or_else(|| anyhow::anyhow!("JSON object must have a \"content\" field"))?;

            match format_type {
                "acp" => serde_json::from_value(content.clone()).map_err(|e| {
                    anyhow::anyhow!("Invalid ACP content blocks in \"content\": {e}")
                })?,
                other => anyhow::bail!(
                    "Unsupported prompt format type: \"{other}\". Supported types: \"acp\""
                ),
            }
        }

View on GitHub (pinned to bc7f02eddd)

Solutions

  1. Add a string "type" field with value "acp" to the wrapper object.
  2. Or pass the blocks as a top-level JSON array instead of an object.
  3. Quote the type value: JSON requires "type":"acp", not type:acp.
  4. Confirm the top-level object is the prompt wrapper, not the content block itself (a block object should be inside the content array).

Example fix

// before
{"content":[{"type":"text","text":"hi"}]}
// after
{"type":"acp","content":[{"type":"text","text":"hi"}]}
Defensive patterns

Strategy: validation

Validate before calling

fn wrapper_has_type(s: &str) -> bool {
    serde_json::from_str::<serde_json::Value>(s)
        .ok()
        .and_then(|v| v.get("type").and_then(|t| t.as_str()).map(|_| true))
        .unwrap_or(false)
}

Type guard

fn is_acp_wrapper(v: &serde_json::Value) -> bool {
    v.as_object()
        .and_then(|o| o.get("type").and_then(|t| t.as_str()))
        .map(|t| t == "acp")
        .unwrap_or(false)
}

Try / catch

match HeadlessPrompt::from_json(&raw) {
    Ok(p) => p,
    Err(e) if e.to_string().contains("must have a \"type\" field") => {
        eprintln!("wrap the object as {\"type\":\"acp\",\"content\":[...]}");
        std::process::exit(2);
    }
    Err(e) => return Err(e),
}

Prevention

When it happens

Trigger: Calling from_json with a JSON object like {"content":[...]}, {"prompt":"..."}, or {"type":123} — i.e. an object whose "type" key is absent or non-string.

Common situations: Wrapping blocks in an object but forgetting the discriminator; passing a plain config-like object as a prompt; older tooling emitting a wrapper without the "type" key.

Understand the failure class

Background: "Missing required field" and "field is required" errors: why libraries reject payloads that omit mandatory fields — this error's family across 20 libraries.

Related errors

AI-assisted analysis of xai-org/grok-build@bc7f02eddd (2026-08-31). Data as JSON: /api/errors/adf751b59f453db5. Report an issue: GitHub.