diff --git a/script/check_idf_py_equivalence.py b/script/check_idf_py_equivalence.py
index fa349e223a..ff7c58a5a6 100755
--- a/script/check_idf_py_equivalence.py
+++ b/script/check_idf_py_equivalence.py
@@ -15,6 +15,7 @@ from __future__ import annotations
 import argparse
 import hashlib
 import json
+import os
 from pathlib import Path
 import re
 import subprocess
@@ -106,6 +107,9 @@ def check(build_path: Path) -> list[str]:
         (build_path / "build" / "project_description.json").read_text(encoding="utf-8")
     )
     name, version = _setup_core(build_path, description)
+    # ESP-IDF's openthread stamps the configure time into its compile flags;
+    # pin it before the env is cached so both configures get the same value.
+    os.environ["SOURCE_DATE_EPOCH"] = "0"
     env = toolchain._get_idf_env(version)
     python = toolchain._get_idf_tool("python")
     idf_py = toolchain._get_idf_path(version) / "tools" / "idf.py"
diff --git a/tests/script/test_check_idf_py_equivalence.py b/tests/script/test_check_idf_py_equivalence.py
index f8135772d0..6ded9a3a81 100644
--- a/tests/script/test_check_idf_py_equivalence.py
+++ b/tests/script/test_check_idf_py_equivalence.py
@@ -2,6 +2,7 @@
 
 from collections.abc import Callable, Iterator
 import json
+import os
 from pathlib import Path
 import subprocess
 import sys
@@ -55,22 +56,32 @@ def _run_check(
     side_effect: Callable[[list[str]], None] = lambda cmd: None,
     rc: int = 0,
     esphome_rcs: tuple[int, int] = (0, 0),
+    envs: list[dict[str, str]] | None = None,
 ) -> tuple[list[str], list[list[str]]]:
-    """Run check() with idf.py replaced by ``side_effect``; return problems, calls."""
+    """Run check() with idf.py replaced by ``side_effect``; return problems, calls.
+
+    ``envs`` collects the env each idf.py call receives.
+    """
     calls: list[list[str]] = []
 
     def run(cmd: list[str], **kwargs: object) -> subprocess.CompletedProcess:
         calls.append(cmd)
+        if envs is not None:
+            envs.append(kwargs["env"])
         side_effect(cmd)
         return subprocess.CompletedProcess(cmd, rc, "out\n", "err\n")
 
     with (
-        patch.object(toolchain, "_get_idf_env", return_value={}),
+        # Snapshot at call time like the real cached env.
+        patch.object(
+            toolchain, "_get_idf_env", side_effect=lambda *_: dict(os.environ)
+        ),
         patch.object(toolchain, "_get_idf_tool", return_value="/py"),
         patch.object(toolchain, "_get_idf_path", return_value=Path("/idf")),
         patch.object(toolchain, "run_reconfigure", return_value=esphome_rcs[0]),
         patch.object(toolchain, "_run_ninja", return_value=esphome_rcs[1]),
         patch.object(guard.subprocess, "run", side_effect=run),
+        patch.dict(os.environ),
     ):
         return guard.check(tree), calls
 
@@ -86,6 +97,13 @@ def test_check_passes_when_idf_py_changes_nothing(tmp_path: Path) -> None:
     ]
 
 
+def test_check_pins_source_date_epoch(tmp_path: Path) -> None:
+    """ESP-IDF's openthread bakes the configure time into its compile flags."""
+    envs: list[dict[str, str]] = []
+    _run_check(_make_tree(tmp_path), envs=envs)
+    assert [env.get("SOURCE_DATE_EPOCH") for env in envs] == ["0", "0"]
+
+
 def test_check_reports_changed_files_and_rebuilt_outputs(tmp_path: Path) -> None:
     tree = _make_tree(tmp_path)
     build = tree / "build"