diff --git a/.github/scripts/fork-changed-packages.py b/.github/scripts/fork-changed-packages.py
index b7cdcf082705..d5eed743e866 100644
--- a/.github/scripts/fork-changed-packages.py
+++ b/.github/scripts/fork-changed-packages.py
@@ -4,29 +4,140 @@
 one per line, on argv. A file belongs to the package whose manifest directory
 is the longest prefix of its path, so a nested crate wins over its parent and
 the directory name never has to equal the package name.
+
+A change to a member's Cargo.toml also selects the workspace packages that
+depend on it, directly or transitively (reverse dependencies). A change to the
+workspace Cargo.toml selects every package. Cargo.lock is not handled here:
+the caller compares the resolve graph and passes the affected members as
+ordinary paths, or prints them itself.
 """
 
 import json
 import os
 import sys
 
-meta = json.load(sys.stdin)
-root = os.getcwd()
-dirs = []
-for package in meta["packages"]:
-    manifest = os.path.relpath(package["manifest_path"], root)
-    dirs.append((os.path.dirname(manifest), package["name"]))
-dirs.sort(key=lambda entry: len(entry[0]), reverse=True)
-
-seen = []
-for argument in sys.argv[1:]:
-    path = os.path.normpath(argument)
+
+def strip_workspace_prefix(path):
+    path = os.path.normpath(path)
     prefix = "codex-rs" + os.sep
+    marker = os.sep + "codex-rs" + os.sep
+    if path == "codex-rs" or path.endswith(os.sep + "codex-rs"):
+        return ""
+    if marker in path:
+        path = path.split(marker, 1)[1]
     if path.startswith(prefix):
-        path = path[len(prefix):]
+        return path[len(prefix):]
+    return path
+
+
+def package_dirs(meta, root):
+    dirs = []
+    for package in meta["packages"]:
+        manifest = package["manifest_path"]
+        if os.path.isabs(manifest):
+            manifest = os.path.relpath(manifest, root)
+        manifest = strip_workspace_prefix(manifest)
+        dirs.append((os.path.dirname(manifest), package["name"]))
+    dirs.sort(key=lambda entry: len(entry[0]), reverse=True)
+    return dirs
+
+
+def owner_of(path, dirs):
+    path = strip_workspace_prefix(path)
     for directory, name in dirs:
         if path == directory or path.startswith(directory + os.sep):
-            if name not in seen:
-                seen.append(name)
-            break
-print("\n".join(seen))
+            return name
+    return None
+
+
+def reverse_closure(meta, names):
+    """Workspace members that depend on `names`, directly or transitively.
+
+    The selection step runs `cargo metadata --no-deps`, whose `resolve` is null,
+    so the only edges available are the declared requirements. A declared
+    dependency names a workspace member when it has a `path` or its `name` is
+    itself a member; registry requirements match neither.
+    """
+    members = set(meta.get("workspace_members", []))
+    member_names = {
+        package["name"]
+        for package in meta["packages"]
+        if package["id"] in members
+    }
+    # name -> packages in the workspace that list it as a normal/dev/build dep
+    dependents = {}
+    for package in meta["packages"]:
+        if package["id"] not in members:
+            continue
+        for dep in package.get("dependencies", []):
+            dep_name = dep.get("name")
+            if dep.get("path") or dep_name in member_names:
+                dependents.setdefault(dep_name, set()).add(package["name"])
+    selected = set(names)
+    frontier = list(names)
+    while frontier:
+        name = frontier.pop()
+        for dependent in dependents.get(name, ()):
+            if dependent not in selected:
+                selected.add(dependent)
+                frontier.append(dependent)
+    return selected
+
+
+def select(meta, paths, root=None):
+    """Return workspace package names selected by `paths`, in first-seen order.
+
+    `paths` are repo-relative. The workspace Cargo.toml selects everything.
+    A member Cargo.toml selects that member and its reverse dependencies.
+    Any other file selects only its owning package.
+    """
+    root = root or os.getcwd()
+    dirs = package_dirs(meta, root)
+    member_names = []
+    for package in meta["packages"]:
+        if package["id"] in set(meta.get("workspace_members", [])):
+            member_names.append(package["name"])
+    manifest_dir = {}
+    for package in meta["packages"]:
+        manifest = package["manifest_path"]
+        if os.path.isabs(manifest):
+            manifest = os.path.relpath(manifest, root)
+        manifest = strip_workspace_prefix(manifest)
+        manifest_dir[package["name"]] = os.path.dirname(manifest)
+
+    seen = []
+
+    def add(name):
+        if name and name not in seen:
+            seen.append(name)
+
+    manifest_hits = []
+    for argument in paths:
+        path = os.path.normpath(argument)
+        stripped = strip_workspace_prefix(path)
+        if stripped in ("Cargo.toml", "./Cargo.toml"):
+            return member_names
+        owner = owner_of(path, dirs)
+        if owner is None:
+            continue
+        if os.path.basename(stripped) == "Cargo.toml" and manifest_dir.get(owner) == os.path.dirname(stripped):
+            manifest_hits.append(owner)
+        else:
+            add(owner)
+    if manifest_hits:
+        closure = reverse_closure(meta, manifest_hits)
+        for name in member_names:
+            if name in closure:
+                add(name)
+        for name in manifest_hits:
+            add(name)
+    return seen
+
+
+def main(argv):
+    meta = json.load(sys.stdin)
+    print("\n".join(select(meta, argv[1:])))
+
+
+if __name__ == "__main__":
+    main(sys.argv)
diff --git a/.github/scripts/fork-lock-affected.py b/.github/scripts/fork-lock-affected.py
new file mode 100644
index 000000000000..07c050923d41
--- /dev/null
+++ b/.github/scripts/fork-lock-affected.py
@@ -0,0 +1,82 @@
+"""Workspace packages whose resolved dependency set changed in Cargo.lock.
+
+Compares two `cargo metadata --format-version 1` documents (the full resolve
+graph, not `--no-deps`). A workspace member is affected when the set of
+package ids reachable from it through normal, dev and build dependencies
+differs between the two resolves. Prints one package name per line.
+
+Usage: fork-lock-affected.py BASE.json HEAD.json
+"""
+
+import json
+import sys
+
+
+# cargo metadata records resolved edges on resolve.nodes[].deps, not on the
+# declared packages[].dependencies list (that one has name/req/path and no
+# package id). None is cargo's kind for a normal dependency.
+KEPT_KINDS = (None, "dev", "build")
+
+
+def edges(meta):
+    """package id -> resolved package ids it depends on (normal, dev, build)."""
+    graph = {}
+    for node in (meta.get("resolve") or {}).get("nodes", []):
+        deps = set()
+        for dep in node.get("deps", []):
+            kinds = [entry.get("kind") for entry in dep.get("dep_kinds", [])]
+            if not kinds or any(kind in KEPT_KINDS for kind in kinds):
+                deps.add(dep["pkg"])
+        graph[node["id"]] = deps
+    return graph
+
+
+def reachable(meta):
+    """name -> frozenset of resolved package ids reachable from that member."""
+    graph = edges(meta)
+    members = [
+        (package["id"], package["name"])
+        for package in meta["packages"]
+        if package["id"] in set(meta.get("workspace_members", []))
+    ]
+    cache = {}
+
+    def walk(package_id, stack):
+        if package_id in cache:
+            return cache[package_id]
+        if package_id in stack or package_id not in graph:
+            return frozenset()
+        stack.add(package_id)
+        found = {package_id}
+        for dep_id in graph[package_id]:
+            found |= walk(dep_id, stack)
+        stack.remove(package_id)
+        cache[package_id] = frozenset(found)
+        return cache[package_id]
+
+    return {name: walk(package_id, set()) for package_id, name in members}
+
+
+def affected(base, head):
+    base_sets = reachable(base)
+    head_sets = reachable(head)
+    names = []
+    for name in head_sets:
+        if base_sets.get(name) != head_sets[name]:
+            names.append(name)
+    for name in base_sets:
+        if name not in head_sets and name not in names:
+            names.append(name)
+    return names
+
+
+def main(argv):
+    with open(argv[1], encoding="utf-8") as handle:
+        base = json.load(handle)
+    with open(argv[2], encoding="utf-8") as handle:
+        head = json.load(handle)
+    print("\n".join(affected(base, head)))
+
+
+if __name__ == "__main__":
+    main(sys.argv)
diff --git a/.github/scripts/test_fork_changed_packages.py b/.github/scripts/test_fork_changed_packages.py
new file mode 100644
index 000000000000..be4773d3fbf8
--- /dev/null
+++ b/.github/scripts/test_fork_changed_packages.py
@@ -0,0 +1,133 @@
+"""Selection semantics for fork-ci's changed-package scripts.
+
+The fixtures are real `cargo metadata --format-version 1` output from a
+three-member scratch workspace (codex-protocol, codex-core, codex-cli), trimmed
+to the packages the graph walk reaches. `nodeps.json` was generated with
+`--no-deps` (resolve is null); `lock-base.json` and `lock-head.json` are full
+resolves whose only difference is the pinned serde version.
+"""
+
+import json
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+
+import importlib.util
+
+
+def load(name, filename):
+    spec = importlib.util.spec_from_file_location(
+        name, os.path.join(os.path.dirname(os.path.abspath(__file__)), filename)
+    )
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+fork_changed_packages = load("fork_changed_packages", "fork-changed-packages.py")
+fork_lock_affected = load("fork_lock_affected", "fork-lock-affected.py")
+
+
+def fixture(name):
+    with open(os.path.join(HERE, "testdata", name), encoding="utf-8") as handle:
+        return json.load(handle)
+
+
+NODEPS = fixture("nodeps.json")
+ROOT = NODEPS["workspace_root"]
+
+
+class SelectTest(unittest.TestCase):
+    def select(self, paths):
+        return fork_changed_packages.select(NODEPS, paths, root=ROOT)
+
+    def test_source_file_selects_its_package(self):
+        self.assertEqual(self.select(["codex-rs/core/src/lib.rs"]), ["codex-core"])
+
+    def test_workspace_manifest_selects_everything(self):
+        self.assertEqual(
+            self.select(["codex-rs/Cargo.toml"]),
+            ["codex-protocol", "codex-core", "codex-cli"],
+        )
+
+    def test_member_manifest_selects_reverse_dependencies(self):
+        selected = self.select(["codex-rs/protocol/Cargo.toml"])
+        self.assertEqual(selected, ["codex-protocol", "codex-core", "codex-cli"])
+
+    def test_member_manifest_without_dependents_selects_itself(self):
+        self.assertEqual(self.select(["codex-rs/cli/Cargo.toml"]), ["codex-cli"])
+
+
+class LockTest(unittest.TestCase):
+    def test_lock_only_change_selects_the_affected_subset(self):
+        base = fixture("lock-base.json")
+        head = fixture("lock-head.json")
+        # serde 1.0.200 -> 1.0.210. codex-core depends on it directly and
+        # codex-cli only through codex-core, so both reachable sets move.
+        self.assertEqual(
+            fork_lock_affected.affected(base, head), ["codex-cli", "codex-core"]
+        )
+
+    def test_identical_resolves_select_nothing(self):
+        head = fixture("lock-head.json")
+        self.assertEqual(fork_lock_affected.affected(head, head), [])
+
+
+CLIPPY_GATE = r"""
+select(.reason == "compiler-message")
+| .message
+| select(.level == "warning" or .level == "error")
+| . as $m
+| ($m.spans[] | select(.is_primary)) as $s
+| select(any($changed[]; . == $s.file_name))
+| "\($s.file_name):\($s.line_start):\($s.column_start): \($m.level): \($m.message)"
+"""
+
+
+class ClippyGateTest(unittest.TestCase):
+    def test_nameless_package_id_in_a_changed_file_fails_the_gate(self):
+        # cargo omits the name from a package id when it equals the directory's
+        # last segment, so codex-home's id has no `#codex-home@` to match on.
+        line = {
+            "reason": "compiler-message",
+            "package_id": "path+file:///work/codex-rs/codex-home#0.1.0",
+            "message": {
+                "level": "warning",
+                "message": "this function has too many arguments",
+                "spans": [
+                    {
+                        "file_name": "codex-home/src/lib.rs",
+                        "line_start": 12,
+                        "column_start": 1,
+                        "is_primary": True,
+                    }
+                ],
+            },
+        }
+        with tempfile.TemporaryDirectory() as directory:
+            clippy = os.path.join(directory, "clippy.json")
+            changed = os.path.join(directory, "changed")
+            with open(clippy, "w", encoding="utf-8") as handle:
+                handle.write(json.dumps(line) + "\n")
+            with open(changed, "w", encoding="utf-8") as handle:
+                handle.write("codex-home/src/lib.rs\n")
+            # The workflow slurps `jq -R .` of the changed-file list, which
+            # yields an array of JSON strings. Slurping the raw file would
+            # yield one string with the newline still in it.
+            quoted = os.path.join(directory, "changed.json")
+            with open(quoted, "w", encoding="utf-8") as handle:
+                subprocess.run(["jq", "-R", ".", changed], check=True, stdout=handle)
+            result = subprocess.run(
+                ["jq", "-r", "--slurpfile", "changed", quoted, CLIPPY_GATE, clippy],
+                check=True, capture_output=True, text=True,
+            )
+        self.assertIn("codex-home/src/lib.rs:12:1: warning:", result.stdout)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/.github/scripts/testdata/lock-base.json b/.github/scripts/testdata/lock-base.json
new file mode 100644
index 000000000000..89dcdfb1d949
--- /dev/null
+++ b/.github/scripts/testdata/lock-base.json
@@ -0,0 +1,315 @@
+{
+  "version": 1,
+  "workspace_members": [
+    "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+    "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+    "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0"
+  ],
+  "workspace_root": "/tmp/fix-ws",
+  "packages": [
+    {
+      "name": "codex-cli",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0",
+      "manifest_path": "/tmp/fix-ws/cli/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-core",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/core",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "codex-protocol",
+          "kind": "dev",
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_cli",
+          "src_path": "/tmp/fix-ws/cli/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-core",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+      "manifest_path": "/tmp/fix-ws/core/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-protocol",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde",
+          "kind": null,
+          "optional": false,
+          "req": "=1.0.200",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_core",
+          "src_path": "/tmp/fix-ws/core/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-protocol",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+      "manifest_path": "/tmp/fix-ws/protocol/Cargo.toml",
+      "dependencies": [],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_protocol",
+          "src_path": "/tmp/fix-ws/protocol/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "serde",
+      "version": "1.0.200",
+      "id": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.200",
+      "manifest_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.200/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "serde_derive",
+          "kind": null,
+          "optional": true,
+          "req": "^1",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde_derive",
+          "kind": "dev",
+          "optional": false,
+          "req": "^1",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde_derive",
+          "kind": null,
+          "optional": false,
+          "req": "=1.0.200",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": "cfg(any())"
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "serde",
+          "src_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.200/src/lib.rs",
+          "edition": "2018",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        },
+        {
+          "kind": [
+            "custom-build"
+          ],
+          "crate_types": [
+            "bin"
+          ],
+          "name": "build-script-build",
+          "src_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.200/build.rs",
+          "edition": "2018",
+          "doc": false,
+          "doctest": false,
+          "test": false
+        }
+      ],
+      "features": {
+        "alloc": [],
+        "default": [
+          "std"
+        ],
+        "derive": [
+          "serde_derive"
+        ],
+        "rc": [],
+        "serde_derive": [
+          "dep:serde_derive"
+        ],
+        "std": [],
+        "unstable": []
+      },
+      "source": "registry+https://github.com/rust-lang/crates.io-index"
+    }
+  ],
+  "resolve": {
+    "nodes": [
+      {
+        "id": "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0",
+        "dependencies": [
+          "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+          "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0"
+        ],
+        "deps": [
+          {
+            "name": "codex_core",
+            "pkg": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          },
+          {
+            "name": "codex_protocol",
+            "pkg": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": "dev",
+                "target": null
+              }
+            ]
+          }
+        ],
+        "features": []
+      },
+      {
+        "id": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+        "dependencies": [
+          "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+          "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.200"
+        ],
+        "deps": [
+          {
+            "name": "codex_protocol",
+            "pkg": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          },
+          {
+            "name": "serde",
+            "pkg": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.200",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          }
+        ],
+        "features": []
+      },
+      {
+        "id": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+        "dependencies": [],
+        "deps": [],
+        "features": []
+      },
+      {
+        "id": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.200",
+        "dependencies": [
+          "registry+https://github.com/rust-lang/crates.io-index#serde_derive@1.0.200"
+        ],
+        "deps": [
+          {
+            "name": "serde_derive",
+            "pkg": "registry+https://github.com/rust-lang/crates.io-index#serde_derive@1.0.200",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": "cfg(any())"
+              }
+            ]
+          }
+        ],
+        "features": [
+          "default",
+          "std"
+        ]
+      }
+    ],
+    "root": null
+  }
+}
diff --git a/.github/scripts/testdata/lock-head.json b/.github/scripts/testdata/lock-head.json
new file mode 100644
index 000000000000..4972628172a0
--- /dev/null
+++ b/.github/scripts/testdata/lock-head.json
@@ -0,0 +1,315 @@
+{
+  "version": 1,
+  "workspace_members": [
+    "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+    "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+    "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0"
+  ],
+  "workspace_root": "/tmp/fix-ws",
+  "packages": [
+    {
+      "name": "codex-cli",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0",
+      "manifest_path": "/tmp/fix-ws/cli/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-core",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/core",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "codex-protocol",
+          "kind": "dev",
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_cli",
+          "src_path": "/tmp/fix-ws/cli/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-core",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+      "manifest_path": "/tmp/fix-ws/core/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-protocol",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde",
+          "kind": null,
+          "optional": false,
+          "req": "=1.0.210",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_core",
+          "src_path": "/tmp/fix-ws/core/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-protocol",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+      "manifest_path": "/tmp/fix-ws/protocol/Cargo.toml",
+      "dependencies": [],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_protocol",
+          "src_path": "/tmp/fix-ws/protocol/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "serde",
+      "version": "1.0.210",
+      "id": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.210",
+      "manifest_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.210/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "serde_derive",
+          "kind": null,
+          "optional": true,
+          "req": "^1",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde_derive",
+          "kind": "dev",
+          "optional": false,
+          "req": "^1",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde_derive",
+          "kind": null,
+          "optional": false,
+          "req": "=1.0.210",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": "cfg(any())"
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "serde",
+          "src_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.210/src/lib.rs",
+          "edition": "2018",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        },
+        {
+          "kind": [
+            "custom-build"
+          ],
+          "crate_types": [
+            "bin"
+          ],
+          "name": "build-script-build",
+          "src_path": "/opt/rust/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.210/build.rs",
+          "edition": "2018",
+          "doc": false,
+          "doctest": false,
+          "test": false
+        }
+      ],
+      "features": {
+        "alloc": [],
+        "default": [
+          "std"
+        ],
+        "derive": [
+          "serde_derive"
+        ],
+        "rc": [],
+        "serde_derive": [
+          "dep:serde_derive"
+        ],
+        "std": [],
+        "unstable": []
+      },
+      "source": "registry+https://github.com/rust-lang/crates.io-index"
+    }
+  ],
+  "resolve": {
+    "nodes": [
+      {
+        "id": "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0",
+        "dependencies": [
+          "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+          "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0"
+        ],
+        "deps": [
+          {
+            "name": "codex_core",
+            "pkg": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          },
+          {
+            "name": "codex_protocol",
+            "pkg": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": "dev",
+                "target": null
+              }
+            ]
+          }
+        ],
+        "features": []
+      },
+      {
+        "id": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+        "dependencies": [
+          "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+          "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.210"
+        ],
+        "deps": [
+          {
+            "name": "codex_protocol",
+            "pkg": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          },
+          {
+            "name": "serde",
+            "pkg": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.210",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": null
+              }
+            ]
+          }
+        ],
+        "features": []
+      },
+      {
+        "id": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+        "dependencies": [],
+        "deps": [],
+        "features": []
+      },
+      {
+        "id": "registry+https://github.com/rust-lang/crates.io-index#serde@1.0.210",
+        "dependencies": [
+          "registry+https://github.com/rust-lang/crates.io-index#serde_derive@1.0.210"
+        ],
+        "deps": [
+          {
+            "name": "serde_derive",
+            "pkg": "registry+https://github.com/rust-lang/crates.io-index#serde_derive@1.0.210",
+            "dep_kinds": [
+              {
+                "kind": null,
+                "target": "cfg(any())"
+              }
+            ]
+          }
+        ],
+        "features": [
+          "default",
+          "std"
+        ]
+      }
+    ],
+    "root": null
+  }
+}
diff --git a/.github/scripts/testdata/nodeps.json b/.github/scripts/testdata/nodeps.json
new file mode 100644
index 000000000000..fe3c7f4f3230
--- /dev/null
+++ b/.github/scripts/testdata/nodeps.json
@@ -0,0 +1,136 @@
+{
+  "version": 1,
+  "workspace_members": [
+    "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+    "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+    "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0"
+  ],
+  "workspace_root": "/tmp/fix-ws",
+  "resolve": null,
+  "packages": [
+    {
+      "name": "codex-protocol",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/protocol#codex-protocol@0.1.0",
+      "manifest_path": "/tmp/fix-ws/protocol/Cargo.toml",
+      "dependencies": [],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_protocol",
+          "src_path": "/tmp/fix-ws/protocol/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-core",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/core#codex-core@0.1.0",
+      "manifest_path": "/tmp/fix-ws/core/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-protocol",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "serde",
+          "kind": null,
+          "optional": false,
+          "req": "=1.0.210",
+          "source": "registry+https://github.com/rust-lang/crates.io-index",
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_core",
+          "src_path": "/tmp/fix-ws/core/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    },
+    {
+      "name": "codex-cli",
+      "version": "0.1.0",
+      "id": "path+file:///tmp/fix-ws/cli#codex-cli@0.1.0",
+      "manifest_path": "/tmp/fix-ws/cli/Cargo.toml",
+      "dependencies": [
+        {
+          "name": "codex-core",
+          "kind": null,
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/core",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        },
+        {
+          "name": "codex-protocol",
+          "kind": "dev",
+          "optional": false,
+          "req": "*",
+          "path": "/tmp/fix-ws/protocol",
+          "source": null,
+          "rename": null,
+          "uses_default_features": true,
+          "features": [],
+          "target": null
+        }
+      ],
+      "targets": [
+        {
+          "kind": [
+            "lib"
+          ],
+          "crate_types": [
+            "lib"
+          ],
+          "name": "codex_cli",
+          "src_path": "/tmp/fix-ws/cli/src/lib.rs",
+          "edition": "2021",
+          "doc": true,
+          "doctest": true,
+          "test": true
+        }
+      ],
+      "features": {},
+      "source": null
+    }
+  ]
+}
diff --git a/.github/workflows/fork-ci.yml b/.github/workflows/fork-ci.yml
index 7e0dac9d6428..1a532e1da686 100644
--- a/.github/workflows/fork-ci.yml
+++ b/.github/workflows/fork-ci.yml
@@ -30,6 +30,20 @@ jobs:
     defaults:
       run:
         working-directory: codex-rs
+    env:
+      # CI-scoped build settings from the cargo build-performance guide.
+      # Job env only: upstream's profiles in Cargo.toml stay untouched.
+      # line-tables-only keeps file/line in backtraces (tests do read them)
+      # while dropping variable debuginfo, which dominates codegen and linking.
+      CARGO_PROFILE_DEV_DEBUG: line-tables-only
+      CARGO_PROFILE_TEST_DEBUG: line-tables-only
+      # CI builds once per crate version; incremental caches never pay back.
+      CARGO_INCREMENTAL: "0"
+      # No RUSTFLAGS. Stable rustc 1.95 on x86_64-unknown-linux-gnu already
+      # links with its bundled rust-lld (the self-contained linker, shipped in
+      # the sysroot since 1.90). -fuse-ld=lld would need a system lld and a
+      # different linker, and nightly-only options (-Zthreads, cranelift) are
+      # out: this job pins stable.
     steps:
       - uses: actions/checkout@fbc6f3992d24b796d5a048ff273f7fcc4a7b6c09 # v5.0.1
         with:
@@ -108,18 +122,58 @@ jobs:
             exit 0
           fi
           all=false
+          lock_only=false
           for f in "${files[@]}"; do
             case "$f" in
-              codex-rs/Cargo.toml|codex-rs/Cargo.lock) all=true ;;
+              codex-rs/Cargo.toml) all=true ;;
+              codex-rs/Cargo.lock) lock_only=true ;;
             esac
           done
+          # A workspace manifest change rebuilds every package. A Cargo.lock
+          # change does not: only members whose resolved dependency set
+          # actually moved are selected, by diffing `cargo metadata` (the full
+          # resolve, not --no-deps) on the base against HEAD. Any other path
+          # still selects its owning package, and a member manifest also
+          # selects its workspace reverse-dependencies.
+          others=()
+          for f in "${files[@]}"; do
+            [[ "$f" == "codex-rs/Cargo.lock" ]] && continue
+            others+=("$f")
+          done
           if $all; then
             mapfile -t packages < <(cargo metadata --no-deps --format-version 1 \
               | jq -r '.packages[].name' | sort -u)
           else
             mapfile -t packages < <(cargo metadata --no-deps --format-version 1 \
-              | python3 "${GITHUB_WORKSPACE}/.github/scripts/fork-changed-packages.py" "${files[@]}")
+              | python3 "${GITHUB_WORKSPACE}/.github/scripts/fork-changed-packages.py" "${others[@]}")
+            if $lock_only; then
+              base_meta="$(mktemp)"
+              head_meta="$(mktemp)"
+              cargo metadata --format-version 1 --locked > "$head_meta"
+              # metadata reads the lock next to the workspace manifest. Swap the
+              # base lock in, read it, and put HEAD's lock back. The manifest
+              # stays HEAD's: a manifest change is not lock-only.
+              cp Cargo.lock /tmp/Cargo.lock.head
+              # A failed metadata read must not leave the base lock checked out:
+              # every later cargo invocation would compile against it.
+              trap 'mv /tmp/Cargo.lock.head Cargo.lock' EXIT
+              git show "$base:codex-rs/Cargo.lock" > Cargo.lock
+              cargo metadata --format-version 1 --locked > "$base_meta"
+              mv /tmp/Cargo.lock.head Cargo.lock
+              trap - EXIT
+              mapfile -t locked < <(python3 "${GITHUB_WORKSPACE}/.github/scripts/fork-lock-affected.py" "$base_meta" "$head_meta")
+              if (( ${#locked[@]} == 0 )); then
+                # A lock that "affects nothing" is a bug in the diff far more
+                # often than a fact, and skipping every check would fail open.
+                echo "::warning::Cargo.lock changed but the resolve diff selected nothing; selecting every member"
+                mapfile -t locked < <(jq -r --argjson members "$(jq -c '.workspace_members' "$head_meta")" \
+                  '.packages[] | select(.id as $id | $members | index($id)) | .name' "$head_meta" | sort -u)
+              fi
+              packages+=("${locked[@]}")
+            fi
           fi
+          # Unique, stable order.
+          mapfile -t packages < <(printf '%s\n' "${packages[@]}" | awk 'NF && !seen[$0]++')
           echo "changed: ${packages[*]:-none}"
           {
             echo "list<<EOF"
@@ -150,10 +204,11 @@ jobs:
         shell: bash
         run: |
           set -euo pipefail
-          while IFS= read -r crate; do
-            [[ -z "$crate" ]] && continue
-            cargo fmt -p "$crate" -- --check
-          done <<< "${{ steps.crates.outputs.list }}"
+          mapfile -t crates < <(printf '%s\n' "${{ steps.crates.outputs.list }}" | sed '/^$/d')
+          (( ${#crates[@]} )) || exit 0
+          args=()
+          for crate in "${crates[@]}"; do args+=(-p "$crate"); done
+          cargo fmt "${args[@]}" -- --check
 
       # -D warnings would fail the PR on upstream lint debt in files it never
       # touched, and a fork patch must not have to fix that. Clippy runs without
@@ -165,37 +220,43 @@ jobs:
         run: |
           set -euo pipefail
           printf '%s\n' "${{ steps.crates.outputs.files }}" | sed 's|^codex-rs/||' > /tmp/changed-files
-          while IFS= read -r crate; do
-            [[ -z "$crate" ]] && continue
-            cargo clippy -p "$crate" --all-targets --message-format=json -- -W clippy::all \
-              > "/tmp/clippy-${crate}.json" || true
-            # --slurpfile yields an array of strings. Iterating each element
-            # again (`[$changed[][]]`) is a jq runtime error, and jq exits 0 on
-            # those, so the gate would pass every PR. `any` compares strings
-            # directly. stderr goes to a file synchronously: a process
-            # substitution can still be writing when the check runs, which
-            # would let a jq error pass again.
-            rm -f /tmp/jq-err
-            offenders="$(jq -r --slurpfile changed <(jq -R . /tmp/changed-files) '
-              select(.reason == "compiler-message")
-              | .message
-              | select(.level == "warning" or .level == "error")
-              | . as $m
-              | ($m.spans[] | select(.is_primary)) as $s
-              | select(any($changed[]; . == $s.file_name))
-              | "\($s.file_name):\($s.line_start):\($s.column_start): \($m.level): \($m.message)"
-            ' "/tmp/clippy-${crate}.json" 2>/tmp/jq-err)"
-            if [[ -s /tmp/jq-err ]]; then
-              cat /tmp/jq-err >&2
-              echo "jq failed to read the clippy output" >&2
-              exit 1
-            fi
-            if [[ -n "$offenders" ]]; then
-              echo "$offenders"
-              echo "clippy findings in files this PR changes" >&2
-              exit 1
-            fi
-          done <<< "${{ steps.crates.outputs.list }}"
+          mapfile -t crates < <(printf '%s\n' "${{ steps.crates.outputs.list }}" | sed '/^$/d')
+          (( ${#crates[@]} )) || exit 0
+          args=()
+          for crate in "${crates[@]}"; do args+=(-p "$crate"); done
+          # One invocation for every selected package. Per-crate loops re-resolve
+          # features and rebuild shared dependencies once per crate; cargo shares
+          # that work when the packages are passed together. --timings writes
+          # cargo-timing.html, which the next cargo step overwrites, so copy it
+          # aside immediately.
+          mkdir -p /tmp/cargo-timings
+          cargo clippy "${args[@]}" --all-targets --timings --message-format=json -- -W clippy::all \
+            > /tmp/clippy.json || true
+          [[ -f cargo-timing.html ]] && cp cargo-timing.html /tmp/cargo-timings/clippy-timing.html
+          # The gate is by changed file path, so it reads the whole stream once.
+          # Splitting per crate first drops every lint from a crate whose
+          # package id omits the name (cargo does that when the name equals the
+          # directory's last segment: codex-home, codex-api, ...).
+          rm -f /tmp/jq-err
+          offenders="$(jq -r --slurpfile changed <(jq -R . /tmp/changed-files) '
+            select(.reason == "compiler-message")
+            | .message
+            | select(.level == "warning" or .level == "error")
+            | . as $m
+            | ($m.spans[] | select(.is_primary)) as $s
+            | select(any($changed[]; . == $s.file_name))
+            | "\($s.file_name):\($s.line_start):\($s.column_start): \($m.level): \($m.message)"
+          ' /tmp/clippy.json 2>/tmp/jq-err)"
+          if [[ -s /tmp/jq-err ]]; then
+            cat /tmp/jq-err >&2
+            echo "jq failed to read the clippy output" >&2
+            exit 1
+          fi
+          if [[ -n "$offenders" ]]; then
+            echo "$offenders"
+            echo "clippy findings in files this PR changes" >&2
+            exit 1
+          fi
 
       - name: cargo test
         shell: bash
@@ -235,8 +296,23 @@ jobs:
           # tests at its first failure, so --no-fail-fast does the same within
           # a crate.
           status=0
-          while IFS= read -r crate; do
-            [[ -z "$crate" ]] && continue
-            cargo nextest run -p "$crate" --no-fail-fast "${filter[@]}" || status=1
-          done <<< "${{ steps.crates.outputs.list }}"
+          mapfile -t crates < <(printf '%s\n' "${{ steps.crates.outputs.list }}" | sed '/^$/d')
+          if (( ${#crates[@]} )); then
+            args=()
+            for crate in "${crates[@]}"; do args+=(-p "$crate"); done
+            # One invocation: per-crate loops re-resolved features and rebuilt
+            # shared dependencies once per crate. --timings records the
+            # critical path next to clippy's.
+            cargo nextest run "${args[@]}" --no-fail-fast --timings "${filter[@]}" || status=1
+            [[ -f cargo-timing.html ]] && cp cargo-timing.html /tmp/cargo-timings/nextest-timing.html
+          fi
           exit "$status"
+
+      # Uploaded even when the job failed: the report is most useful then.
+      - name: Upload cargo timings
+        if: always()
+        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
+        with:
+          name: cargo-timings
+          path: /tmp/cargo-timings/
+          if-no-files-found: ignore