diff --git a/.gitea/scripts/pr-diff-guard.py b/.gitea/scripts/pr-diff-guard.py
new file mode 100644
index 00000000..28657773
--- /dev/null
+++ b/.gitea/scripts/pr-diff-guard.py
@@ -0,0 +1,141 @@
+#!/usr/bin/env python3
+"""PR diff-size / destructive-diff guard.
+
+Implements core#2875: block stale branches whose head has drifted into a
+massive destructive diff against current main (e.g., PR #1100: 481 files
+changed, ~55k deletions). The guard runs on every PR and fails loudly when
+any of the configured thresholds are exceeded.
+
+The check compares the PR head against the merge base of the target branch,
+so rebasing a stale branch to a clean, narrow diff will clear the guard.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+
+
+PROTECTED_PATHS = (
+    ".gitea/workflows/",
+    ".gitea/scripts/",
+    "tests/e2e/",
+    "workspace-server/internal/handlers/",
+    "workspace-server/internal/provisioner/",
+    "workspace-server/internal/middleware/",
+    "canvas/src/",
+)
+
+DEFAULT_MAX_CHANGED_FILES = int(os.environ.get("DIFFGUARD_MAX_CHANGED_FILES", "100"))
+DEFAULT_MAX_DELETIONS = int(os.environ.get("DIFFGUARD_MAX_DELETIONS", "5000"))
+DEFAULT_MAX_INSERTIONS = int(os.environ.get("DIFFGUARD_MAX_INSERTIONS", "10000"))
+
+
+def git(*args: str) -> str:
+    result = subprocess.run(
+        ["git", *args],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    return result.stdout
+
+
+def main() -> int:
+    base_ref = os.environ.get("PR_BASE_REF", os.environ.get("GITHUB_BASE_REF", "main"))
+    head_sha = os.environ.get("PR_HEAD_SHA", os.environ.get("GITHUB_SHA", ""))
+
+    if not head_sha:
+        # In a pull_request workflow, GITHUB_SHA is the merge commit. Use the
+        # PR head ref instead when available.
+        head_sha = os.environ.get("GITHUB_EVENT_PULL_REQUEST_HEAD_SHA", "HEAD")
+    if not head_sha:
+        head_sha = "HEAD"
+
+    # Ensure base ref is available.
+    try:
+        git("rev-parse", f"origin/{base_ref}")
+    except subprocess.CalledProcessError:
+        git("fetch", "origin", base_ref)
+
+    # Find merge base so the diff reflects only what the PR added, not main
+    # drift since the branch was created. If no merge base exists (e.g.,
+    # unrelated-history branch), fall back to the base ref itself — the guard
+    # should still catch a massive destructive diff.
+    try:
+        merge_base = git("merge-base", f"origin/{base_ref}", head_sha).strip()
+    except subprocess.CalledProcessError:
+        print(f"::warning::no merge base with origin/{base_ref}; falling back to direct diff")
+        merge_base = f"origin/{base_ref}"
+
+    # Diff stat.
+    numstat = git("diff", "--numstat", f"{merge_base}..{head_sha}").strip()
+    changed_files = 0
+    insertions = 0
+    deletions = 0
+    for line in numstat.splitlines():
+        parts = line.split()
+        if len(parts) < 3:
+            continue
+        add, rem = parts[0], parts[1]
+        if add == "-" or rem == "-":
+            continue  # binary
+        insertions += int(add)
+        deletions += int(rem)
+        changed_files += 1
+
+    # Deleted files and protected-path deletions.
+    name_status = git("diff", "--name-status", f"{merge_base}..{head_sha}").strip()
+    deleted_files: list[str] = []
+    protected_deletions: list[str] = []
+    for line in name_status.splitlines():
+        if not line:
+            continue
+        status, path = line.split("\t", 1)
+        if status.startswith("D"):
+            deleted_files.append(path)
+            if any(path.startswith(p) for p in PROTECTED_PATHS):
+                protected_deletions.append(path)
+
+    # Evaluate thresholds.
+    failures: list[str] = []
+    if changed_files > DEFAULT_MAX_CHANGED_FILES:
+        failures.append(
+            f"changed files ({changed_files}) exceeds threshold ({DEFAULT_MAX_CHANGED_FILES})"
+        )
+    if insertions > DEFAULT_MAX_INSERTIONS:
+        failures.append(
+            f"insertions (+{insertions}) exceeds threshold ({DEFAULT_MAX_INSERTIONS})"
+        )
+    if deletions > DEFAULT_MAX_DELETIONS:
+        failures.append(
+            f"deletions (-{deletions}) exceeds threshold ({DEFAULT_MAX_DELETIONS})"
+        )
+    if protected_deletions:
+        failures.append(
+            f"deleted {len(protected_deletions)} protected path(s): "
+            + ", ".join(protected_deletions[:10])
+        )
+
+    # Report.
+    print(f"Diff guard: {changed_files} files changed, +{insertions}/-{deletions} lines")
+    print(f"Deleted files: {len(deleted_files)}")
+    if protected_deletions:
+        print(f"Protected-path deletions: {len(protected_deletions)}")
+
+    if failures:
+        print("::error::PR diff guard failed:")
+        for f in failures:
+            print(f"  - {f}")
+        print(
+            "If this diff is intentional, split the PR or request a threshold override from the PM."
+        )
+        return 1
+
+    print("Diff guard passed.")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.gitea/workflows/pr-diff-guard.yml b/.gitea/workflows/pr-diff-guard.yml
new file mode 100644
index 00000000..8c2d479d
--- /dev/null
+++ b/.gitea/workflows/pr-diff-guard.yml
@@ -0,0 +1,33 @@
+name: PR Diff Guard
+
+# core#2875: block stale PR branches that have drifted into massive,
+# destructive diffs against current main (e.g., PR #1100: 481 files,
+# ~55k deletions). Runs on every PR and fails when configured size or
+# protected-path deletion thresholds are exceeded.
+
+on:
+  pull_request:
+    branches: [main, staging]
+
+permissions:
+  contents: read
+
+jobs:
+  # bp-required: pending #2875
+  # New gate; wiring into branch protection will follow once it has
+  # green history and the thresholds are calibrated.
+  diff-guard:
+    name: PR diff guard
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # Full history is required to compute the merge-base against origin/main.
+          fetch-depth: 0
+
+      - name: Run PR diff guard
+        env:
+          PR_BASE_REF: ${{ github.base_ref }}
+          PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        run: python3 .gitea/scripts/pr-diff-guard.py