What's wrong: The base file (as of PR #2157, commit 3355132) ends with the byte sequence ...pull/2157\n \n — the blank separator line after the last entry carries two trailing spaces. This PR's diff shows that line changing from - to + (a bare empty line) before the new entry is appended:

   pr-link: https://github.com/SemiAnalysisAI/InferenceX/pull/2157
-  
+
+- config-keys:
+    - qwen3.5-fp4-gb300-dynamo-sglang
...

Why it's forbidden: AGENTS.md is explicit and unambiguous on this point: "Never delete or modify whitespace in perf-changelog.yaml - CI depends on exact whitespace (including trailing spaces on blank separator lines). Altering it breaks CI." This diff does precisely the prohibited operation.

Why it will actually fail CI (not just a style nit): utils/validate_perf_changelog.py::validate_raw_change() is invoked by the check-changelog job in .github/workflows/run-sweep.yml (--base-ref origin/${{ github.base_ref }}). For any PR that only appends entries, it enforces:

if not head_raw.startswith(base_raw):
    raise ChangelogValidationError(
        "appended entries changed historical perf-changelog.yaml bytes; "
        "restore the base file byte-for-byte and append at the end"
    )

Step-by-step proof (verified directly against the repo history):

1. git show 9af6bf5^:perf-changelog.yaml | tail -c 20 → b'renceX/pull/2157\n \n' — base ends with \n \n (two trailing spaces before EOF).
2. git show 9af6bf5:perf-changelog.yaml (this PR's head) at the same offset reads ...pull/2157\n\n- config-keys:... — the trailing spaces are gone.
3. A direct byte comparison (head_raw.startswith(base_raw)) confirms this is False: the first divergence is at byte offset 255901, where base has pull/2157\n \n and head has pull/2157\n\n- config-.
4. Because additions is nonzero for this PR (a new - config-keys: block was appended), validate_raw_change() takes the if not head_raw.startswith(base_raw) branch and raises ChangelogValidationError, which fails the check-changelog job — and downstream jobs (needs: check-changelog) gate on its success.

The fix: Keep the original last line of the base file exactly as-is ( — two trailing spaces, no more, no less) and append the new entry as a new block after it, so head_raw starts with base_raw byte-for-byte. Concretely, restore the line to before the new - config-keys: block rather than collapsing it to an empty line.