diff --git a/.github/blob-size-allowlist.txt b/.github/blob-size-allowlist.txt
new file mode 100644
index 000000000..4c9462e8e
--- /dev/null
+++ b/.github/blob-size-allowlist.txt
@@ -0,0 +1,8 @@
+# Paths are matched exactly, relative to the repository root.
+# Keep this list short and limited to intentional large checked-in assets.
+
+.github/codex-cli-splash.png
+MODULE.bazel.lock
+codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json
+codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json
+codex-rs/tui/tests/fixtures/oss-story.jsonl
diff --git a/.github/workflows/blob-size-policy.yml b/.github/workflows/blob-size-policy.yml
new file mode 100644
index 000000000..441775c0e
--- /dev/null
+++ b/.github/workflows/blob-size-policy.yml
@@ -0,0 +1,29 @@
+name: blob-size-policy
+
+on:
+  pull_request: {}
+
+jobs:
+  check:
+    name: Blob size policy
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Determine PR comparison range
+        id: range
+        shell: bash
+        run: |
+          set -euo pipefail
+          echo "base=$(git rev-parse HEAD^1)" >> "$GITHUB_OUTPUT"
+          echo "head=$(git rev-parse HEAD^2)" >> "$GITHUB_OUTPUT"
+
+      - name: Check changed blob sizes
+        run: |
+          python3 scripts/check_blob_size.py \
+            --base "${{ steps.range.outputs.base }}" \
+            --head "${{ steps.range.outputs.head }}" \
+            --max-bytes 512000 \
+            --allowlist .github/blob-size-allowlist.txt
diff --git a/scripts/check_blob_size.py b/scripts/check_blob_size.py
new file mode 100755
index 000000000..455145f18
--- /dev/null
+++ b/scripts/check_blob_size.py
@@ -0,0 +1,193 @@
+#!/usr/bin/env python3
+
+from __future__ import annotations
+
+import argparse
+import os
+import subprocess
+import sys
+from dataclasses import dataclass
+from pathlib import Path
+
+
+DEFAULT_MAX_BYTES = 500 * 1024
+
+
+@dataclass(frozen=True)
+class ChangedBlob:
+    path: str
+    size_bytes: int
+    is_allowlisted: bool
+    is_binary: bool
+
+
+def run_git(*args: str) -> str:
+    result = subprocess.run(
+        ["git", *args],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return result.stdout
+
+
+def load_allowlist(path: Path) -> set[str]:
+    allowlist: set[str] = set()
+    for raw_line in path.read_text(encoding="utf-8").splitlines():
+        line = raw_line.split("#", 1)[0].strip()
+        if line:
+            allowlist.add(line)
+    return allowlist
+
+
+def get_changed_paths(base: str, head: str) -> list[str]:
+    output = run_git(
+        "diff",
+        "--name-only",
+        "--diff-filter=AM",
+        "--no-renames",
+        "-z",
+        base,
+        head,
+    )
+    return [path for path in output.split("\0") if path]
+
+
+def is_binary_change(base: str, head: str, path: str) -> bool:
+    output = run_git(
+        "diff",
+        "--numstat",
+        "--diff-filter=AM",
+        "--no-renames",
+        base,
+        head,
+        "--",
+        path,
+    ).strip()
+    if not output:
+        return False
+
+    added, deleted, _ = output.split("\t", 2)
+    return added == "-" and deleted == "-"
+
+
+def blob_size(commit: str, path: str) -> int:
+    return int(run_git("cat-file", "-s", f"{commit}:{path}").strip())
+
+
+def collect_changed_blobs(base: str, head: str, allowlist: set[str]) -> list[ChangedBlob]:
+    blobs: list[ChangedBlob] = []
+    for path in get_changed_paths(base, head):
+        blobs.append(
+            ChangedBlob(
+                path=path,
+                size_bytes=blob_size(head, path),
+                is_allowlisted=path in allowlist,
+                is_binary=is_binary_change(base, head, path),
+            )
+        )
+    return blobs
+
+
+def format_kib(size_bytes: int) -> str:
+    return f"{size_bytes / 1024:.1f} KiB"
+
+
+def write_step_summary(
+    max_bytes: int,
+    blobs: list[ChangedBlob],
+    violations: list[ChangedBlob],
+) -> None:
+    summary_path = os.environ.get("GITHUB_STEP_SUMMARY")
+    if not summary_path:
+        return
+
+    lines = [
+        "## Blob Size Policy",
+        "",
+        f"Default max: `{max_bytes}` bytes ({format_kib(max_bytes)})",
+        f"Changed files checked: `{len(blobs)}`",
+        f"Violations: `{len(violations)}`",
+        "",
+    ]
+
+    if blobs:
+        lines.extend(
+            [
+                "| Path | Kind | Size | Status |",
+                "| --- | --- | ---: | --- |",
+            ]
+        )
+        for blob in blobs:
+            status = "allowlisted" if blob.is_allowlisted else "ok"
+            if blob in violations:
+                status = "blocked"
+            kind = "binary" if blob.is_binary else "non-binary"
+            lines.append(
+                f"| `{blob.path}` | {kind} | `{blob.size_bytes}` bytes ({format_kib(blob.size_bytes)}) | {status} |"
+            )
+    else:
+        lines.append("No changed files were detected.")
+
+    lines.append("")
+    Path(summary_path).write_text("\n".join(lines), encoding="utf-8")
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(
+        description="Fail if changed blobs exceed the configured size budget."
+    )
+    parser.add_argument("--base", required=True, help="Base git revision to diff against.")
+    parser.add_argument("--head", required=True, help="Head git revision to inspect.")
+    parser.add_argument(
+        "--max-bytes",
+        type=int,
+        default=DEFAULT_MAX_BYTES,
+        help=f"Maximum allowed blob size in bytes. Default: {DEFAULT_MAX_BYTES}.",
+    )
+    parser.add_argument(
+        "--allowlist",
+        type=Path,
+        required=True,
+        help="Path to the newline-delimited allowlist file.",
+    )
+    args = parser.parse_args()
+
+    allowlist = load_allowlist(args.allowlist)
+    blobs = collect_changed_blobs(args.base, args.head, allowlist)
+    violations = [
+        blob for blob in blobs if blob.size_bytes > args.max_bytes and not blob.is_allowlisted
+    ]
+
+    write_step_summary(args.max_bytes, blobs, violations)
+
+    if not blobs:
+        print("No changed files were detected.")
+        return 0
+
+    print(f"Checked {len(blobs)} changed file(s) against the {args.max_bytes}-byte limit.")
+    for blob in blobs:
+        status = "allowlisted" if blob.is_allowlisted else "ok"
+        if blob in violations:
+            status = "blocked"
+        kind = "binary" if blob.is_binary else "non-binary"
+        print(
+            f"- {blob.path}: {blob.size_bytes} bytes ({format_kib(blob.size_bytes)}) [{kind}, {status}]"
+        )
+
+    if violations:
+        print("\nFile(s) exceed the configured limit:")
+        for blob in violations:
+            print(f"- {blob.path}: {blob.size_bytes} bytes > {args.max_bytes} bytes")
+        print(
+            "\nIf one of these is a real checked-in asset we want to keep, add its "
+            "repo-relative path to .github/blob-size-allowlist.txt. Otherwise, "
+            "shrink it or keep it out of git."
+        )
+        return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())