diff --git a/scripts/detect-breaking-changes b/scripts/detect-breaking-changes
index bfb8d52858..cb80da6998 100755
--- a/scripts/detect-breaking-changes
+++ b/scripts/detect-breaking-changes
@@ -1,24 +1,30 @@
 #!/usr/bin/env bash
 
-set -e
+set -euo pipefail
 
 cd "$(dirname "$0")/.."
+base=$(git rev-parse --verify --end-of-options "${1:?expected a base revision}^{commit}")
 
-echo "==> Detecting breaking changes"
-
-TEST_PATHS=(
-	tests/api_resources
-	tests/test_client.py
-	tests/test_response.py
-	tests/test_legacy_response.py
-)
+# Keep today's callers Pyright-clean before they become tomorrow's baseline.
+echo '==> Checking current callers against the candidate SDK'
+uv run --locked --all-extras scripts/run-pyright
 
-for PATHSPEC in "${TEST_PATHS[@]}"; do
-    # Try to check out previous versions of the test files
-    # with the current SDK.
-    git checkout "$1" -- "${PATHSPEC}" 2>/dev/null || true
-done
+# If the tests are identical, the first run already checked the base callers.
+# A change confined to tests and this script has no new SDK to compare.
+test_changes=$(git diff --name-only "$base" -- tests)
+other_changes=$(git diff --name-only "$base" -- . ':(exclude)tests' ':(exclude)scripts/detect-breaking-changes')
+if [[ -z "$test_changes" || -z "$other_changes" ]]; then
+    exit 0
+fi
 
-# Check unchanged previous-version callers with Pyright. Repository development
-# uses ty; this isolated gate protects consumers who use another checker.
-uv run --locked --all-extras scripts/run-pyright
+# Check the complete base test tree, including its helpers, against today's
+# src and checker configuration. Leave the working tree and Git index intact.
+echo '==> Checking previous callers against the candidate SDK'
+snapshot=$(mktemp -d)
+trap 'rm -rf -- "$snapshot"' EXIT
+git archive "$base" tests | tar -x -C "$snapshot"
+ln -s "$PWD/src" "$snapshot/src"
+mkdir "$snapshot/scripts"
+config=scripts/pyrightconfig.breaking-changes.json
+cp "$config" "$snapshot/$config"
+PYRIGHT_PROJECT="$snapshot/$config" uv run --locked --all-extras scripts/run-pyright
diff --git a/tests/test_client.py b/tests/test_client.py
index b97d53574f..8626b7b54f 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -1120,8 +1120,8 @@ def test_binary_content_upload_with_body_is_deprecated(self, respx2_mock: MockRo
     @pytest.mark.parametrize("client", [False], indirect=True)
     def test_bare_container_response(self, respx2_mock: MockRouter, client: OpenAI) -> None:
         class Model(BaseModel):
-            metadata: dict
-            items: list
+            metadata: dict  # pyright: ignore[reportMissingTypeArgument]
+            items: list  # pyright: ignore[reportMissingTypeArgument]
 
         data = {"metadata": {"key": "value"}, "items": [1, "two"]}
         respx2_mock.get("/foo").mock(return_value=httpx2.Response(200, json=data))
@@ -2510,8 +2510,8 @@ async def test_binary_content_upload_with_body_is_deprecated(
     @pytest.mark.parametrize("async_client", [False], indirect=True)
     async def test_bare_container_response(self, respx2_mock: MockRouter, async_client: AsyncOpenAI) -> None:
         class Model(BaseModel):
-            metadata: dict
-            items: list
+            metadata: dict  # pyright: ignore[reportMissingTypeArgument]
+            items: list  # pyright: ignore[reportMissingTypeArgument]
 
         data = {"metadata": {"key": "value"}, "items": [1, "two"]}
         respx2_mock.get("/foo").mock(return_value=httpx2.Response(200, json=data))
diff --git a/tests/test_consumer_compatibility.py b/tests/test_consumer_compatibility.py
new file mode 100644
index 0000000000..444f9236cb
--- /dev/null
+++ b/tests/test_consumer_compatibility.py
@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.skipif(os.name != "posix", reason="The compatibility gate runs in Bash on CI")
+@pytest.mark.parametrize("also_change", [None, "src/model.py", "pyproject.toml"])
+def test_compatibility_gate_selects_callers(tmp_path: Path, also_change: str | None) -> None:
+    scripts = tmp_path / "scripts"
+    scripts.mkdir()
+    gate = scripts / "detect-breaking-changes"
+    shutil.copy(Path(__file__).resolve().parents[1] / "scripts" / gate.name, gate)
+    (scripts / "pyrightconfig.breaking-changes.json").write_text("{}")
+    (tmp_path / "tests").mkdir()
+    (tmp_path / "src").mkdir()
+    (tmp_path / "src/model.py").write_text("original SDK\n")
+    caller = tmp_path / "tests/caller.py"
+    caller.write_text("previous caller\n")
+
+    # Record the checker inputs to exercise the real Git and snapshot routing.
+    uv = tmp_path / "uv"
+    uv.write_text(
+        "#!/bin/sh\nset -eu\nconfig=${PYRIGHT_PROJECT:-scripts/pyrightconfig.breaking-changes.json}\n"
+        'root=$(dirname "$config")/..\ncat "$root/tests/caller.py" "$root/src/model.py"\n'
+    )
+    uv.chmod(0o755)
+    subprocess.run(["git", "init", "--quiet"], cwd=tmp_path, check=True)
+    subprocess.run(["git", "add", "."], cwd=tmp_path, check=True)
+    subprocess.run(
+        ["git", "-c", "user.name=Test", "-c", "user.email=test@example.org", "commit", "--no-gpg-sign", "-qm", "base"],
+        cwd=tmp_path,
+        check=True,
+    )
+
+    caller.write_text("current caller\n")
+    if also_change is not None:
+        (tmp_path / also_change).write_text("changed input\n")
+        subprocess.run(["git", "add", also_change], cwd=tmp_path, check=True)
+    result = subprocess.run(
+        [str(gate), "HEAD"],
+        cwd=tmp_path,
+        env={**os.environ, "PATH": f"{tmp_path}{os.pathsep}{os.environ['PATH']}"},
+        text=True,
+        capture_output=True,
+        check=True,
+    )
+    sdk = "changed input" if also_change == "src/model.py" else "original SDK"
+    expected = ["current caller", sdk]
+    if also_change is not None:
+        expected.extend(["previous caller", sdk])
+    assert [line for line in result.stdout.splitlines() if not line.startswith("==>")] == expected
+    assert caller.read_text() == "current caller\n"