Janet eval scope & source-to-source failure: `(eval '(core-inc 1))` fails with "unknown symbol core-inc" even when the file does `(use ./core)`. Janet's `eval` runs in the default environment and doesn't see `use`-imported symbols. NEVER emit Janet source strings for eval. Instead emit Janet data structures where function VALUES are embedded directly: `[core-inc 1]` not `"(core-inc 1)"`. Use a `core-fn-values` table to resolve names at compile time. The `(parse s)` function returns `[symbol, error-position]` — must use parser pipeline for proper eval: `parser/new→consume→eof→produce→eval`. Without `parser/eof`, numbers/literals return nil from produce. Janet's `eval` also doesn't support special forms natively in data structures — `(eval [if true 1 2])` fails with "unknown symbol if". Must parse strings, not eval tuples.
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When you need to mutate a local with `set`, use `(var x nil)` not `(def x nil)`. `def` creates constants — `(set ns-name ...)` on a `def` fails with "cannot set constant". This hit us in loader.janet ns-name extraction loop.
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core-renames MUST match actual function names defined in core.janet. `"-"` maps to `core-sub` NOT `core--`. `"not"` maps to `core-not` (defined as `(defn core-not ...)`). Missing entries cause silent nil returns. When adding to core-renames, grep core.janet for the actual `(defn core-XXX ...)` or `(def core-XXX ...)` name. Then add matching entry to core-fn-values.