diff --git a/codex-rs/app-server/BUILD.bazel b/codex-rs/app-server/BUILD.bazel
index 872f533be..d2d3f42a1 100644
--- a/codex-rs/app-server/BUILD.bazel
+++ b/codex-rs/app-server/BUILD.bazel
@@ -4,5 +4,9 @@ codex_rust_crate(
     name = "app-server",
     crate_name = "codex_app_server",
     integration_test_timeout = "long",
+    test_shard_counts = {
+        "app-server-all-test": 8,
+        "app-server-unit-tests": 8,
+    },
     test_tags = ["no-sandbox"],
 )
diff --git a/codex-rs/core/BUILD.bazel b/codex-rs/core/BUILD.bazel
index 434dc1f6a..dd52bce43 100644
--- a/codex-rs/core/BUILD.bazel
+++ b/codex-rs/core/BUILD.bazel
@@ -47,6 +47,10 @@ codex_rust_crate(
         # succeeds without this workaround.
         "//:AGENTS.md",
     ],
+    test_shard_counts = {
+        "core-all-test": 8,
+        "core-unit-tests": 8,
+    },
     test_tags = ["no-sandbox"],
     unit_test_timeout = "long",
     extra_binaries = [
diff --git a/codex-rs/tui/BUILD.bazel b/codex-rs/tui/BUILD.bazel
index 33cc13343..d0aab0805 100644
--- a/codex-rs/tui/BUILD.bazel
+++ b/codex-rs/tui/BUILD.bazel
@@ -24,4 +24,7 @@ codex_rust_crate(
         "//codex-rs/cli:codex",
     ],
     rustc_flags_extra = MACOS_WEBRTC_RUSTC_LINK_FLAGS,
+    test_shard_counts = {
+        "tui-unit-tests": 8,
+    },
 )
diff --git a/defs.bzl b/defs.bzl
index 6972b65a6..53114a577 100644
--- a/defs.bzl
+++ b/defs.bzl
@@ -140,6 +140,7 @@ def codex_rust_crate(
         integration_test_args = [],
         integration_test_timeout = None,
         test_data_extra = [],
+        test_shard_counts = {},
         test_tags = [],
         unit_test_timeout = None,
         extra_binaries = []):
@@ -174,6 +175,11 @@ def codex_rust_crate(
         integration_test_timeout: Optional Bazel timeout for integration test
             targets generated from `tests/*.rs`.
         test_data_extra: Extra runtime data for tests.
+        test_shard_counts: Mapping from generated test target name to Bazel
+            shard count. Matching tests use native Bazel sharding on the
+            original test label, while rules_rust assigns each Rust test case
+            to a stable bucket by hashing the test name. Matching tests are
+            also marked flaky, which gives them Bazel's default three attempts.
         test_tags: Tags applied to unit + integration test targets.
             Typically used to disable the sandbox, but see https://bazel.build/reference/be/common-definitions#common.tags
         unit_test_timeout: Optional Bazel timeout for the unit-test target
@@ -246,7 +252,13 @@ def codex_rust_crate(
             visibility = ["//visibility:public"],
         )
 
+        unit_test_name = name + "-unit-tests"
         unit_test_binary = name + "-unit-tests-bin"
+        unit_test_shard_count = _test_shard_count(test_shard_counts, unit_test_name)
+        unit_test_binary_kwargs = {}
+        if unit_test_shard_count:
+            unit_test_binary_kwargs["experimental_enable_sharding"] = True
+
         rust_test(
             name = unit_test_binary,
             crate = name,
@@ -265,14 +277,18 @@ def codex_rust_crate(
             rustc_env = rustc_env,
             data = test_data_extra,
             tags = test_tags + ["manual"],
+            **unit_test_binary_kwargs
         )
 
         unit_test_kwargs = {}
         if unit_test_timeout:
             unit_test_kwargs["timeout"] = unit_test_timeout
+        if unit_test_shard_count:
+            unit_test_kwargs["shard_count"] = unit_test_shard_count
+            unit_test_kwargs["flaky"] = True
 
         workspace_root_test(
-            name = name + "-unit-tests",
+            name = unit_test_name,
             env = test_env,
             test_bin = ":" + unit_test_binary,
             workspace_root_marker = "//codex-rs/utils/cargo-bin:repo_root.marker",
@@ -318,6 +334,14 @@ def codex_rust_crate(
         if not test_name.endswith("-test"):
             test_name += "-test"
 
+        test_kwargs = {}
+        test_kwargs.update(integration_test_kwargs)
+        test_shard_count = _test_shard_count(test_shard_counts, test_name)
+        if test_shard_count:
+            test_kwargs["experimental_enable_sharding"] = True
+            test_kwargs["shard_count"] = test_shard_count
+            test_kwargs["flaky"] = True
+
         rust_test(
             name = test_name,
             crate_name = test_crate_name,
@@ -339,5 +363,15 @@ def codex_rust_crate(
             # execute from the repo root and can misplace integration snapshots.
             env = cargo_env,
             tags = test_tags,
-            **integration_test_kwargs
+            **test_kwargs
         )
+
+def _test_shard_count(test_shard_counts, test_name):
+    shard_count = test_shard_counts.get(test_name)
+    if shard_count == None:
+        return None
+
+    if shard_count < 1:
+        fail("test_shard_counts[{}] must be a positive integer".format(test_name))
+
+    return shard_count