{"record":{"id":"5727eadbb1aa740d","repo":"headroomlabs-ai/headroom","slug":"metrics-response-build-error-e","errorCode":null,"errorMessage":"metrics response build error: {e}","messagePattern":"metrics response build error: (.+?)","errorType":"http","errorClass":null,"httpStatus":500,"severity":"warning","filePath":"crates/headroom-proxy/src/observability/prometheus.rs","lineNumber":286,"sourceCode":"    if let Err(e) = encoder.encode(&metric_families, &mut buffer) {\n        tracing::error!(\n            event = \"metrics_encode_failed\",\n            error = %e,\n            \"failed to encode Prometheus metrics scrape\"\n        );\n        return Response::builder()\n            .status(StatusCode::INTERNAL_SERVER_ERROR)\n            .body(Body::from(format!(\"metrics encode error: {e}\")))\n            .expect(\"static error response\");\n    }\n    Response::builder()\n        .status(StatusCode::OK)\n        .header(header::CONTENT_TYPE, encoder.format_type())\n        .body(Body::from(buffer))\n        .unwrap_or_else(|e| {\n            Response::builder()\n                .status(StatusCode::INTERNAL_SERVER_ERROR)\n                .body(Body::from(format!(\"metrics response build error: {e}\")))\n                .expect(\"static error response\")\n        })\n}\n\n#[cfg(test)]\nmod tests {\n    use super::*;\n\n    /// Helper: render the registry to a String for assertions.\n    fn scrape() -> String {\n        let mf = registry().gather();\n        let encoder = TextEncoder::new();\n        let mut buf = Vec::new();\n        encoder.encode(&mf, &mut buf).expect(\"encode\");\n        String::from_utf8(buf).expect(\"utf8\")\n    }\n\n    #[test]","sourceCodeStart":268,"sourceCodeEnd":304,"githubUrl":"https://github.com/headroomlabs-ai/headroom/blob/322425c43bffde1ed0b64fecf3cf5951565dd82b/crates/headroom-proxy/src/observability/prometheus.rs#L268-L304","documentation":"Building the 200 OK /metrics Response object failed after successful encoding (setting status/header/body on http::Response). This is a can't-happen branch — the response is fully static except for the encoded buffer — so it exists only as an unwrap_or_else guard, returning 500 with this message.","triggerScenarios":"http::Response::builder rejecting the header value (encoder.format_type() returning something http::HeaderValue refuses) — effectively only possible via a corrupted/changed encoder format string; otherwise unreachable.","commonSituations":"A prometheus-crate version change to format_type() output that no longer parses as a header value; memory corruption; in practice this should never fire.","solutions":["If it fires, capture the metrics_response_build_error body — it embeds the underlying http::Error.","Check the prometheus crate version's format_type() string (should be 'text/plain; version=0.0.4' or similar) against http::HeaderValue rules.","Downgrade/pin the prometheus crate to a version whose format string is known-good."],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":["Pin the prometheus crate version in Cargo.lock.","Monitor for this message in logs — it's effectively an invariant trip, report it upstream if seen."],"tags":["prometheus","metrics","http","unreachable","rust"],"backgroundTag":null,"analyzedSha":"322425c43bffde1ed0b64fecf3cf5951565dd82b","analyzedAt":"2026-08-15T01:03:05.481Z","schemaVersion":2},"datasetVersion":"2026-08-15T17:31:12.345Z"}