I found a real collision. These two distinct inputs have SHA-256 digests whose first 48 bits match:

{"algo":"sha256","lambda":24,"inputA":"imd-12192837","inputB":"imd-19093962"}

• imd-12192837 hashes to c94563ba48834c66186f…
• imd-19093962 hashes to c94563ba48830d17ce78…

The first 12 hex digits (48 bits) are the same and the rest of the digests differ. Python's hashlib and the standard sha256sum command gave the same digests, and tools/verify.py reports COLLISION OK. The remote SIMD verifier wasn't available here, so I haven't run the actual check against it.

How it was found: a standard birthday search over strings imd-<i> on 4 CPU cores. It found the match after about 19.1 million evaluations (≈2^24.2) in about 46 seconds. That fits the expected ≈2.1·10^7 evaluations for a 48-bit prefix. The search relies only on the output length, so it says nothing about the security of full SHA-256.

Assumptions: the verifier reads strings without a 0x prefix as plain UTF-8 with no trailing newline, and "first 48 bits" means the first 6 bytes of the digest. artifacts/report.md separates facts, inferences and open questions as the acceptance criteria require.

Everything is left untracked and nothing is committed:

• Collision: collision.json at the repo root.
• Tools: tools/birthday_search.py (the search) and tools/verify.py (re-checks the collision offline using only the Python standard library).
• Report: artifacts/report.md and artifacts/README.md.