{"record":{"id":"4d6868ad017aa1a9","repo":"vllm-project/vllm","slug":"engine-start-index-does-not-fit-usize","errorCode":null,"errorMessage":"engine start index does not fit usize","messagePattern":"engine start index does not fit usize","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"rust/src/server/src/config.rs","lineNumber":269,"sourceCode":"        match &self.transport_mode {\n            TransportMode::HandshakeOwner { engine_count, .. } => {\n                if *engine_count != self.data_parallel_size {\n                    bail!(\n                        \"managed frontend engine count ({engine_count}) must equal data parallel size ({})\",\n                        self.data_parallel_size\n                    );\n                }\n            }\n            TransportMode::Bootstrapped {\n                engine_start_index,\n                engine_count,\n                ..\n            } => {\n                if *engine_count == 0 {\n                    bail!(\"engine count must be at least 1\");\n                }\n                let engine_start_index = usize::try_from(*engine_start_index)\n                    .map_err(|_| anyhow::anyhow!(\"engine start index does not fit usize\"))?;\n                let engine_end_index =\n                    engine_start_index.checked_add(*engine_count).ok_or_else(|| {\n                        anyhow::anyhow!(\"engine start index + engine count overflows\")\n                    })?;\n                if engine_end_index > self.data_parallel_size {\n                    bail!(\n                        \"connected engine range [{engine_start_index}, {engine_end_index}) exceeds data parallel size {}\",\n                        self.data_parallel_size\n                    );\n                }\n            }\n        }\n\n        Ok(())\n    }\n\n    /// Return the number of engines implied by the configured transport mode.\n    pub fn engine_count(&self) -> usize {","sourceCodeStart":251,"sourceCodeEnd":287,"githubUrl":"https://github.com/vllm-project/vllm/blob/c794754062d49a8fdb63ab3c5215b488b865030c/rust/src/server/src/config.rs#L251-L287","documentation":"Thrown by Config::validate() when a Bootstrapped transport's engine_start_index cannot be converted to usize via usize::try_from. On the Unix targets this crate supports, that means the index is negative. The start index identifies the first engine this frontend serves within the data-parallel pool.","triggerScenarios":"Passing a negative engine start index in bootstrapped mode (e.g. -1), typically from an arithmetic error when computing which slice of engines a frontend should own.","commonSituations":"A scheduler computing start = shard_id * engines_per_shard - offset that underflows; int-typed config parsing accepting negatives; off-by-one in loop bounds for engine assignment.","solutions":["Pass a non-negative start index: the first engine of your slice within [0, data_parallel_size).","Add a max(0, computed) clamp or debug-assert in the component that computes the slice.","Check the type of the field feeding engine_start_index — signed integers accept values the transport cannot."],"exampleFix":"# before\nengine_start_index = -1\n\n# after\nengine_start_index = 0","handlingStrategy":"validation","validationCode":"let start: i64 = assignment.start_index;\nassert!(start >= 0, \"engine start index must be non-negative\");","typeGuard":"fn is_valid_start_index(start: &i64) -> bool {\n    usize::try_from(*start).is_ok()\n}","tryCatchPattern":null,"preventionTips":["Debug-assert slice arithmetic in the scheduler before assignment.","Prefer usize end-to-end for engine indices to make negatives unrepresentable.","Clamp computed starts with .max(0)."],"tags":["configuration","transport","bootstrap","validation","rust","vllm","startup"],"backgroundTag":null,"analyzedSha":"c794754062d49a8fdb63ab3c5215b488b865030c","analyzedAt":"2026-08-14T21:17:39.825Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}