🐛 Do not mutate the passed library in write_string/write_file - #614
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Middlewares passed via prepend_middleware default to in-place modification and run before the copying default unparse stack, so writing mutated the caller's library (e.g. month "January" became "jan"). Deep-copy the library once upfront unless every middleware in the stack allows in-place modification (the explicit opt-in). Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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write_string/write_filemutated the library passed in whenever a middleware with the defaultallow_inplace_modification=Trueran before the copying default unparse stack — e.g. theprepend_middlewareexample from the docs:Fix: deep-copy the library once before running the stack, unless every middleware allows in-place modification. Guarantee:
write_string/write_filenever modify the passed library, unless every middleware in the unparse stack allows in-place modification (an all-in-place stack is the explicit opt-in). Default stack behavior unchanged; no new parameters.Tests: input unchanged after
write_string/write_filewith an in-place prepended middleware (output still transformed); default write does not mutate; all-in-place stack may mutate; copying-only stack gets no extra upfront copy. Full suite: 2725 passed, 12 skipped.🤖 Generated with Claude Code