diff --git a/package.json b/package.json
index e54d6fc..7346bc7 100644
--- a/package.json
+++ b/package.json
@@ -36,6 +36,7 @@
     "artifact-bundle-verifier-smoke": "tsx scripts/artifact-bundle-verifier-smoke.ts",
     "artifact-redaction-smoke": "tsx scripts/artifact-redaction-smoke.ts",
     "artifact-patch-git-apply-smoke": "tsx scripts/artifact-patch-git-apply-smoke.ts",
+    "mounted-workspace-diff-smoke": "tsx scripts/mounted-workspace-diff-smoke.ts",
     "policy-validation-smoke": "tsx scripts/policy-validation-smoke.ts",
     "workspace-policy-smoke": "tsx scripts/workspace-policy-smoke.ts",
     "artifact-contract-smoke": "tsx scripts/artifact-contract-smoke.ts",
diff --git a/packages/runtime-playground/src/artifact-bundle-builder.ts b/packages/runtime-playground/src/artifact-bundle-builder.ts
index 44f061b..46dbc56 100644
--- a/packages/runtime-playground/src/artifact-bundle-builder.ts
+++ b/packages/runtime-playground/src/artifact-bundle-builder.ts
@@ -115,7 +115,7 @@ export class ArtifactBundleBuilder {
         .map((ref) => artifactManifestFile(join(source.artifactRoot, ref.path), "runtime-snapshot", "application/json")),
     )
     const capturedMounts = await source.captureMountedFiles(filesDirectory, redactor)
-    const { mountDiffs, changedFiles, patch } = await source.captureMountDiffs(filesDirectory, redactor)
+    const { mountDiffs, changedFiles, patch, diagnostics: mountDiffDiagnostics } = await source.captureMountDiffs(filesDirectory, redactor)
     const changedFilesJson = redactor.redact("files/changed-files.json", `${JSON.stringify(changedFiles, null, 2)}\n`)
     const redactedPatch = redactor.redact("files/patch.diff", patch)
     const contentDigest = artifactContentDigest(changedFilesJson, redactedPatch)
@@ -143,6 +143,13 @@ export class ArtifactBundleBuilder {
     }
     source.recordArtifactsCollected(bundleId, createdAt, spec)
     const diagnostics = buildArtifactDiagnostics(source.observations)
+    diagnostics.diagnostics.push(...mountDiffDiagnostics)
+    diagnostics.summary.total = diagnostics.diagnostics.length
+    diagnostics.summary.error = diagnostics.diagnostics.filter((diagnostic) => diagnostic.severity === "error").length
+    diagnostics.summary.warning = diagnostics.diagnostics.filter((diagnostic) => diagnostic.severity === "warning").length
+    diagnostics.summary.notice = diagnostics.diagnostics.filter((diagnostic) => diagnostic.severity === "notice").length
+    diagnostics.summary.info = diagnostics.diagnostics.filter((diagnostic) => diagnostic.severity === "info").length
+    diagnostics.status = diagnostics.summary.total > 0 ? "reported" : "clean"
     const testResults = buildTestResults()
     const review = buildArtifactReview({
       artifactId: bundleId,
diff --git a/packages/runtime-playground/src/artifacts.ts b/packages/runtime-playground/src/artifacts.ts
index 99f5ee2..f4f00ef 100644
--- a/packages/runtime-playground/src/artifacts.ts
+++ b/packages/runtime-playground/src/artifacts.ts
@@ -54,9 +54,12 @@ export interface MountDiff {
   mountIndex: number
   source: string
   target: string
-  baselineSource: string
+  baselineSource?: string
   artifactPath: string
   changed: boolean
+  status: "changed" | "unchanged" | "skipped" | "failed"
+  reason?: string
+  error?: string
 }
 
 export interface ChangedFile {
@@ -82,6 +85,7 @@ export interface MountDiffsResult {
   mountDiffs: MountDiff[]
   changedFiles: CanonicalChangedFiles
   patch: string
+  diagnostics: ArtifactDiagnostic[]
 }
 
 interface RedactionResult {
diff --git a/packages/runtime-playground/src/mounted-artifact-capture.ts b/packages/runtime-playground/src/mounted-artifact-capture.ts
index 6220240..3979346 100644
--- a/packages/runtime-playground/src/mounted-artifact-capture.ts
+++ b/packages/runtime-playground/src/mounted-artifact-capture.ts
@@ -1,7 +1,7 @@
 import { createHash } from "node:crypto"
 import { copyFile, mkdir, readdir, readFile, stat, writeFile } from "node:fs/promises"
 import { basename, dirname, join } from "node:path"
-import type { MountSpec } from "@chubes4/wp-codebox-core"
+import type { ArtifactDiagnostic, MountSpec } from "@chubes4/wp-codebox-core"
 import {
   MAX_CAPTURED_MOUNT_FILE_BYTES,
   MAX_CAPTURED_MOUNT_FILES,
@@ -52,15 +52,60 @@ export async function captureMountDiffs(artifactRoot: string, filesDirectory: st
   const diffs: MountDiff[] = []
   const changedFiles: ChangedFile[] = []
   const patches: string[] = []
+  const diagnostics: ArtifactDiagnostic[] = []
 
   for (const [mountIndex, mount] of mounts.entries()) {
     const baselineSource = typeof mount.metadata?.baselineSource === "string" ? mount.metadata.baselineSource : ""
-    if (mount.mode !== "readwrite" || !baselineSource) {
+    if (mount.mode !== "readwrite") {
       continue
     }
 
-    const diff = await directoryDiff(baselineSource, mount.source, mount.target)
     const artifactPath = `files/diffs/mount-${mountIndex}.patch`
+    if (!baselineSource) {
+      await writeFile(join(artifactRoot, artifactPath), "")
+      diffs.push({
+        mountIndex,
+        source: mount.source,
+        target: mount.target,
+        artifactPath,
+        changed: false,
+        status: "skipped",
+        reason: "missing-baseline-source",
+      })
+      continue
+    }
+
+    let diff: Awaited<ReturnType<typeof directoryDiff>>
+    try {
+      diff = await directoryDiff(baselineSource, mount.source, mount.target)
+    } catch (error) {
+      const message = error instanceof Error ? error.message : String(error)
+      await writeFile(join(artifactRoot, artifactPath), "")
+      diffs.push({
+        mountIndex,
+        source: mount.source,
+        target: mount.target,
+        baselineSource,
+        artifactPath,
+        changed: false,
+        status: "failed",
+        reason: "diff-extraction-failed",
+        error: message,
+      })
+      diagnostics.push({
+        id: `mount-${mountIndex}-diff-extraction-failed`,
+        type: "mount-diff-extraction-failed",
+        severity: "error",
+        message: `Failed to compare mounted workspace ${mount.target} against its baseline: ${message}`,
+        category: "artifact-capture",
+        source: mount.source,
+        path: mount.target,
+        refs: [{ path: artifactPath, kind: "diff" }],
+        details: { mountIndex, baselineSource, target: mount.target },
+      })
+      continue
+    }
+
     await writeFile(join(artifactRoot, artifactPath), redactor.redact(artifactPath, diff.patch))
     diffs.push({
       mountIndex,
@@ -69,6 +114,7 @@ export async function captureMountDiffs(artifactRoot: string, filesDirectory: st
       baselineSource,
       artifactPath,
       changed: diff.patch.trim().length > 0,
+      status: diff.patch.trim().length > 0 ? "changed" : "unchanged",
     })
     patches.push(diff.patch)
     changedFiles.push(
@@ -88,6 +134,7 @@ export async function captureMountDiffs(artifactRoot: string, filesDirectory: st
       files: changedFiles,
     },
     patch: patches.filter((patch) => patch.length > 0).join("\n"),
+    diagnostics,
   }
 }
 
diff --git a/scripts/mounted-workspace-diff-smoke.ts b/scripts/mounted-workspace-diff-smoke.ts
new file mode 100644
index 0000000..512f248
--- /dev/null
+++ b/scripts/mounted-workspace-diff-smoke.ts
@@ -0,0 +1,78 @@
+import assert from "node:assert/strict"
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+import type { MountSpec } from "@chubes4/wp-codebox-core"
+import { ArtifactRedactor } from "../packages/runtime-playground/src/artifacts.js"
+import { captureMountDiffs } from "../packages/runtime-playground/src/mounted-artifact-capture.js"
+
+const root = await mkdtemp(join(tmpdir(), "wp-codebox-mounted-diff-smoke-"))
+
+try {
+  const artifactRoot = join(root, "artifacts")
+  const filesDirectory = join(artifactRoot, "files")
+  const baseline = join(root, "baseline")
+  const workspace = join(root, "workspace")
+  await mkdir(filesDirectory, { recursive: true })
+  await mkdir(baseline, { recursive: true })
+  await mkdir(workspace, { recursive: true })
+
+  await writeFile(join(baseline, "plugin.php"), "<?php\n// before\n")
+  await writeFile(join(baseline, "delete-me.txt"), "remove me\n")
+
+  // Simulates workspace_edit mutations against the mounted workspace copy.
+  await writeFile(join(workspace, "plugin.php"), "<?php\n// after\n")
+
+  // Simulates workspace_write creating a new file in the mounted workspace copy.
+  await writeFile(join(workspace, "generated.txt"), "cooked\n")
+
+  const mounts: MountSpec[] = [
+    {
+      type: "directory",
+      source: workspace,
+      target: "/workspace/plugin",
+      mode: "readwrite",
+      metadata: {
+        kind: "recipe-workspace",
+        sourceMode: "repo-backed",
+        baselineSource: baseline,
+        workspaceRef: "wp-codebox-fixture",
+      },
+    },
+  ]
+
+  const result = await captureMountDiffs(artifactRoot, filesDirectory, mounts, new ArtifactRedactor())
+  const changed = new Map(result.changedFiles.files.map((file) => [file.relativePath, file]))
+
+  assert.equal(result.diagnostics.length, 0)
+  assert.equal(result.mountDiffs.length, 1)
+  assert.equal(result.mountDiffs[0].status, "changed")
+  assert.equal(result.mountDiffs[0].changed, true)
+  assert.equal(changed.get("generated.txt")?.status, "added")
+  assert.equal(changed.get("plugin.php")?.status, "modified")
+  assert.equal(changed.get("delete-me.txt")?.status, "deleted")
+  assert.match(result.patch, /diff --git a\/workspace\/plugin\/generated\.txt b\/workspace\/plugin\/generated\.txt/)
+  assert.match(result.patch, /\+cooked/)
+  assert.match(result.patch, /diff --git a\/workspace\/plugin\/plugin\.php b\/workspace\/plugin\/plugin\.php/)
+  assert.match(result.patch, /\+\/\/ after/)
+  assert.match(result.patch, /deleted file mode 100644/)
+
+  const mountPatch = await readFile(join(artifactRoot, result.mountDiffs[0].artifactPath), "utf8")
+  assert.equal(mountPatch, result.patch)
+
+  const missingBaselineResult = await captureMountDiffs(artifactRoot, filesDirectory, [
+    {
+      type: "directory",
+      source: workspace,
+      target: "/workspace/untracked",
+      mode: "readwrite",
+      metadata: { sourceMode: "repo-backed" },
+    },
+  ], new ArtifactRedactor())
+  assert.equal(missingBaselineResult.patch, "")
+  assert.equal(missingBaselineResult.changedFiles.files.length, 0)
+  assert.equal(missingBaselineResult.mountDiffs[0].status, "skipped")
+  assert.equal(missingBaselineResult.mountDiffs[0].reason, "missing-baseline-source")
+} finally {
+  await rm(root, { recursive: true, force: true })
+}