fix(scoring): parse underscores in the fractional part of upstream constants

src/scoring/model.ts

@@ -153,10 +153,12 @@ export async function getOrCreateScoringModelSnapshot(env: Env): Promise<Scoring
 export function parsePythonNumberConstants(source: string, options: { knownOnly?: boolean } = { knownOnly: true }): Record<string, number> {
   const constants: Record<string, number> = {};
   for (const line of source.split("\n")) {
-    // Match Python numeric literals including underscore separators (1_000_000), floats, and exponents
-    // (1e-9, 5.8e1). The previous /[-+]?\d+(?:\.\d+)?/ stopped at `_`/`e`, truncating 1_000_000 -> 1 and
-    // 1e-9 -> 1, which silently misparsed any such upstream constant and polluted the unmodeled list (#810).
-    const match = line.match(/^([A-Z][A-Z0-9_]+)\s*=\s*([-+]?(?:\d[\d_]*\.?\d*|\.\d+)(?:[eE][-+]?\d+)?)/);
+    // Match Python numeric literals including underscore separators (1_000_000, 0.000_001), floats, and
+    // exponents (1e-9, 5.8e1). PEP 515 allows underscores between digits in ANY part of the literal, so the
+    // fractional digits accept `_` too — the previous frac classes (`\d*`/`\.\d+`) stopped at the first
+    // fractional underscore, truncating 0.000_001 -> 0. The earlier /[-+]?\d+(?:\.\d+)?/ also stopped at
+    // `_`/`e` (1_000_000 -> 1, 1e-9 -> 1). Both silently misparsed upstream constants (#810).
+    const match = line.match(/^([A-Z][A-Z0-9_]+)\s*=\s*([-+]?(?:\d[\d_]*\.?[\d_]*|\.\d[\d_]*)(?:[eE][-+]?\d+)?)/);
     if (!match) continue;
     const name = match[1]!;
     const raw = match[2]!;

test/unit/scoring.test.ts

@@ -86,6 +86,21 @@ OSS_EMISSION_SHARE = 0.90
     expect(parsed.OSS_EMISSION_SHARE).toBe(0.9);
   });
 
+  it("parses underscores in the FRACTIONAL part (PEP 515) without truncating to zero", () => {
+    const parsed = parsePythonNumberConstants(
+      `
+SMALL_RATE = 0.000_001
+PRECISE_SCALE = 3.14_15
+MIXED_GROUPED = 1_000.000_5
+`,
+      { knownOnly: false },
+    );
+    // Frac classes were \\d*/\\.\\d+ (no `_`), so 0.000_001 stopped at "0.000" -> 0 (a divide/multiply hazard).
+    expect(parsed.SMALL_RATE).toBe(0.000001);
+    expect(parsed.PRECISE_SCALE).toBe(3.1415);
+    expect(parsed.MIXED_GROUPED).toBe(1000.0005);
+  });
+
   it("flags only scoring snapshots older than the freshness window as stale (#810)", () => {
     const now = Date.parse("2026-06-21T12:00:00.000Z");
     const justFresh = new Date(now - SCORING_SNAPSHOT_STALE_MS + 60_000).toISOString();