diff --git a/README.md b/README.md
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+++ b/README.md
@@ -82,6 +82,15 @@ triggered when Python types are used Mojo or a Python module needs to interact
 with Mojo code. Many capabilities of Mojo - Python interoperability are fairly
 new, and existing coding agents don't handle them correctly without guidance.
 
+### `mojo-stdlib-contributing`
+
+[This skill](mojo-stdlib-contributing/SKILL.md) captures patterns and pitfalls
+for contributing to the Mojo standard library, distilled from 30+ reviewed PRs.
+It covers process (GitHub issues before new APIs, draft PRs), assertion
+semantics (`assert_mode="safe"` is intentional), benchmark patterns, testing
+requirements, SIMD/memory safety, and common reviewer feedback. Use when
+modifying code under `mojo/stdlib/`.
+
 ## Examples
 
 Once these skills are installed, you can use them for many common tasks.
diff --git a/mojo-stdlib-contributing/SKILL.md b/mojo-stdlib-contributing/SKILL.md
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@@ -0,0 +1,113 @@
+---
+name: mojo-stdlib-contributing
+description: Patterns and pitfalls for contributing to the Mojo standard library. Use when modifying code under mojo/stdlib/, writing tests or benchmarks for stdlib, or preparing PRs to the modular/modular repository. Distilled from 30+ reviewed PRs.
+---
+
+<!-- EDITORIAL GUIDELINES FOR THIS SKILL FILE
+This file is loaded into an agent's context window as a correction layer for
+pretrained contribution knowledge. Every line costs context. When editing:
+- Be terse. Use tables and inline code over prose where possible.
+- Never duplicate information from the mojo-syntax skill.
+- Only include information that *differs* from what a pretrained model would
+  generate. Don't document things models already get right.
+- Prefer one consolidated code block over multiple small ones.
+- If adding a new section, ask: "Would a model get this wrong?" If not, skip it.
+These same principles apply to any files this skill references.
+-->
+
+Contributing to the Mojo stdlib has non-obvious patterns that differ from typical open-source projects. **Always follow this skill to avoid common rejection reasons.**
+
+**Read before contributing:** `mojo/CONTRIBUTING.md` (accepted/avoided changes, proposal process), `mojo/stdlib/docs/development.md` (building, testing), `mojo/stdlib/docs/style-guide.md` (coding standards), and `AI_TOOL_POLICY.md` (labeling, human-in-the-loop requirements).
+
+## Process
+
+- **New APIs require a GitHub issue first.** Do not implement new methods on existing types without prior consensus. Open the issue, then create a draft PR linking it.
+- **Keep PRs focused.** One logical change per PR. Benchmark utilities and their usage belong in separate PRs.
+- **Always create draft PRs.** Never mark as ready yourself. Ask the human to review the diff, description, and benchmarks before requesting maintainer review.
+- **No changelog entries for internal changes.** Only user-facing behavior changes belong in the changelog.
+
+## Assertion semantics
+
+The default `ASSERT_MODE` is `"safe"` (from `get_defined_string["ASSERT", "safe"]()`).
+
+| Form | Runs by default (`"safe"`) | Runs with `-D ASSERT=all` |
+|---|---|---|
+| `debug_assert[assert_mode="safe"](...)` | Yes | Yes |
+| `debug_assert(...)` / `assert cond, msg` | No | Yes |
+
+**Do NOT downgrade `debug_assert[assert_mode="safe"]`.** These are intentional safety invariants, not performance bugs.
+
+## Optimizations
+
+**Every optimization PR must include both IR evidence and benchmark results.**
+
+Use `std.compile.compile_info` to compare generated IR before and after your change:
+
+```mojo
+from std.compile import compile_info
+
+def my_function(x: SIMD[DType.float32, 4]) -> SIMD[DType.float32, 4]:
+    return x + x
+
+# Inspect optimized LLVM IR ("llvm" for unoptimized, "asm" for assembly)
+comptime info = compile_info[my_function, emission_kind="llvm-opt"]()
+print(info)
+
+# Pattern-match on IR content
+assert "fadd" in compile_info[my_function, emission_kind="llvm-opt"]()
+```
+
+**Workflow:** save IR before your change, apply change, compare. If IR is identical, the optimization is a no-op. Do not submit. If IR differs, run benchmarks to confirm measurable improvement.
+
+- **Do not add manual fast paths that LLVM already optimizes.** E.g., `1 if b < 128 else _utf8_first_byte_sequence_length(b)` produces identical IR to just calling the function.
+- **Verify alignment claims with benchmarks.** Cache-line alignment requires evidence.
+- **Include before/after benchmark numbers in the PR description.**
+
+## Code design
+
+- **Reuse existing stdlib primitives.** Use `_memchr`/`_memchr_impl`, `clamp`, etc. rather than reimplementing. If a new algorithm is better, update the existing primitive.
+- **Implement fast paths on `Span`**, not `List`. Then delegate upward.
+- **Generalize type constraints.** Use trait conformance (e.g. `TrivialRegisterPassable`) rather than hard-coding a single type.
+- **Use move semantics (`^`)**, not `.copy()`. Use `uninit_copy_n`, `uninit_move_n`, `destroy_n` for bulk operations.
+- **Question two-pass patterns.** A "collect then delete" approach may be slower than single-pass. Benchmark to confirm.
+
+## Benchmarks
+
+```mojo
+# CORRECT
+@parameter
+def bench_something(mut b: Bencher) raises:
+    var data = setup_data()  # setup outside hot loop
+    @always_inline
+    def call_fn() unified {read}:
+        var result = black_box(data).some_operation(black_box(arg))
+        keep(result)
+    b.iter(call_fn)
+```
+
+- **Always `black_box()` inputs and `keep()` results.** Otherwise the compiler may optimize away the benchmark.
+- **Setup goes outside `call_fn`.** Use `iter_with_setup` for destructive benchmarks. Setup must not raise.
+- **Data must match the described scenario.** If the docstring says "element in the middle", verify it.
+- **Provide benchmarks for performance PRs**, covering both small and large inputs.
+
+## Testing
+
+- **Add unicode test cases** for any string/byte operation.
+- **Cover both hit and miss paths** in search/contains tests.
+- **Add edge cases:** `start > end`, out-of-bounds indices, empty input, exact-width.
+- **Tests for `Writable` must use `check_write_to`**, not `String(x)` or `repr(x)`.
+
+## SIMD / memory safety
+
+- **Validate pointer arithmetic** before low-level memory access. Clamp `[start, end]` into `[0, len]` before `memcmp`.
+- **Check for negative `pos_in_block`** in reverse SIMD scans with `count_leading_zeros`.
+- **Verify `vectorized_end`** accounts for full SIMD block width to prevent OOB reads.
+- **SIMD test comments must be platform-agnostic.** Write "exercises both SIMD path and scalar tail" instead of assuming 64-byte width.
+
+## Writable trait (stdlib-specific additions to mojo-syntax)
+
+The `mojo-syntax` skill covers basic `Writable` patterns. Additional stdlib-contributing rules:
+
+- **`write_to` and `write_repr_to` must not allocate.** No `repr()`, `String(x)`, or heap-allocating calls inside these methods.
+- **Use `FormatStruct`** from `format._utils` for consistent repr formatting of structured types.
+- **Add an explicit `else` fallback** in enum-style `write_to`.