diff --git a/scripts/src/platform-infra-pipelines-as-code-source-artifact.test.ts b/scripts/src/platform-infra-pipelines-as-code-source-artifact.test.ts
index 1b1a2b02..6891b0c6 100644
--- a/scripts/src/platform-infra-pipelines-as-code-source-artifact.test.ts
+++ b/scripts/src/platform-infra-pipelines-as-code-source-artifact.test.ts
@@ -11,6 +11,7 @@ import {
   canonicalSha256,
   canonicalizePacPipelineSpec,
   firstPacSourceArtifactDrift,
+  pacSourceArtifactSafeError,
   pacValueEvidence,
   pacSourceArtifactYaml,
   parsePacSourceArtifactOptions,
@@ -284,6 +285,61 @@ describe("source artifact serialization", () => {
     expect(serialized.endsWith("\n\n")).toBe(false);
     expect(Bun.YAML.parse(serialized)).toEqual(manifest);
   });
+
+  test("multi-line script scalar preserves trailing spaces and tabs exactly", () => {
+    const script = [
+      "#!/bin/sh  ",
+      "printf 'mapping-looking scalar: '  ",
+      "metadata: ",
+      "  {}",
+      "printf 'tab follows'\t",
+      "",
+    ].join("\n");
+    const manifest = {
+      apiVersion: "tekton.dev/v1",
+      kind: "Pipeline",
+      metadata: { name: "scalar-round-trip" },
+      spec: { tasks: [{ name: "build", taskSpec: { steps: [{ name: "build", script }] } }] },
+    };
+    const serialized = pacSourceArtifactYaml(manifest);
+    const parsed = Bun.YAML.parse(serialized) as typeof manifest;
+    expect(parsed).toEqual(manifest);
+    expect(parsed.spec.tasks[0]?.taskSpec.steps[0]?.script).toBe(script);
+    expect(Buffer.from(parsed.spec.tasks[0]?.taskSpec.steps[0]?.script ?? "")).toEqual(Buffer.from(script));
+    expect(serialized.endsWith("\n")).toBe(true);
+    expect(serialized.endsWith("\n\n")).toBe(false);
+  });
+
+  test("round-trip failure exposes only secret-safe drift evidence", () => {
+    const secret = "ROUND_TRIP_SCRIPT_SECRET_DO_NOT_PRINT";
+    let reads = 0;
+    const spec: Record<string, unknown> = {};
+    Object.defineProperty(spec, "script", {
+      enumerable: true,
+      get: () => {
+        reads += 1;
+        return reads <= 2 ? secret : "changed-after-serialization";
+      },
+    });
+
+    let thrown: unknown;
+    try {
+      pacSourceArtifactYaml({ apiVersion: "tekton.dev/v1", kind: "Pipeline", spec });
+    } catch (error) {
+      thrown = error;
+    }
+    expect(thrown).toBeInstanceOf(Error);
+    const safe = pacSourceArtifactSafeError(thrown);
+    const serializedError = JSON.stringify({
+      message: thrown instanceof Error ? thrown.message : String(thrown),
+      direct: thrown,
+      safe,
+    });
+    expect(safe.code).toBe("source-artifact-yaml-round-trip-failed");
+    expect(serializedError).not.toContain(secret);
+    expect(serializedError).not.toContain("changed-after-serialization");
+    expect(serializedError).toContain("sha256");
+  });
 });
 
 describe("runtime source-commit selection", () => {
diff --git a/scripts/src/platform-infra-pipelines-as-code-source-artifact.ts b/scripts/src/platform-infra-pipelines-as-code-source-artifact.ts
index bc77a0e4..1f81253d 100644
--- a/scripts/src/platform-infra-pipelines-as-code-source-artifact.ts
+++ b/scripts/src/platform-infra-pipelines-as-code-source-artifact.ts
@@ -1032,12 +1032,33 @@ function parseYamlRecord(text: string, label: string): Record<string, unknown> {
 }
 
 export function pacSourceArtifactYaml(value: Record<string, unknown>): string {
-  const serialized = Bun.YAML.stringify(value, null, 2)
-    .split("\n")
-    .map((line) => line.replace(/[\t ]+$/u, ""))
-    .join("\n")
-    .trim();
-  return `${serialized}\n`;
+  const lines = Bun.YAML.stringify(value, null, 2).split("\n");
+  let blockScalarHeaderIndent: number | null = null;
+  const normalized = lines.map((line) => {
+    const content = line.trim();
+    const indent = line.length - line.trimStart().length;
+    if (blockScalarHeaderIndent !== null) {
+      if (content.length === 0 || indent > blockScalarHeaderIndent) return line;
+      blockScalarHeaderIndent = null;
+    }
+
+    const structuralLine = line.replace(/:[\t ]+$/u, ":");
+    if (/(?:^|:[\t ]+|-[\t ]+)[|>](?:[1-9][+-]?|[+-][1-9]?|[+-])?[\t ]*(?:#.*)?$/u.test(structuralLine)) {
+      blockScalarHeaderIndent = indent;
+    }
+    return structuralLine;
+  }).join("\n");
+  const withSingleEofNewline = `${normalized.replace(/\n+$/u, "")}\n`;
+  const roundTrip = parseYamlRecord(withSingleEofNewline, "serialized source artifact");
+  const roundTripDrift = firstCanonicalDrift(value, roundTrip, "$");
+  if (roundTripDrift !== null) {
+    throw new PacSourceArtifactError("source-artifact-yaml-round-trip-failed", {
+      driftPath: roundTripDrift.path,
+      expectedEvidence: roundTripDrift.expected,
+      actualEvidence: roundTripDrift.actual,
+    });
+  }
+  return withSingleEofNewline;
 }
 
 function requiredParam(binding: PacSourceArtifactBinding, key: string): string {