Select from the merge base, not the base branch tip

.github/workflows/reusable.yml

@@ -8,7 +8,7 @@ name: reusable
 #
 #   jobs:
 #     evals:
-#       uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.1
+#       uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.2
 #       secrets:
 #         api_key: ${{ secrets.ANTHROPIC_API_KEY }}
 #       with:
@@ -63,7 +63,7 @@ name: reusable
 # let a reusable workflow interpolate that pin into `uses: amd/skillscope@...`,
 # so each job checks out this repository at `job.workflow_sha` (the commit the
 # caller referenced) and runs the composite action from that tree. Pinning
-# `@v0.1.1` grades with v0.1.1, `@main` grades with main, and a branch grades
+# `@v0.1.2` grades with v0.1.2, `@main` grades with main, and a branch grades
 # with that branch -- so there is no release ritual and nothing to keep in step.
 
 on:

.github/workflows/skill-evals.yml

@@ -16,7 +16,7 @@ name: skill-evals
 #
 #   jobs:
 #     skill-evals:
-#       uses: amd/skillscope/.github/workflows/skill-evals.yml@v0.1.1
+#       uses: amd/skillscope/.github/workflows/skill-evals.yml@v0.1.2
 #       secrets: inherit
 #       with:
 #         skill_globs: skills/*
@@ -62,7 +62,7 @@ name: skill-evals
 # let a reusable workflow interpolate that pin into `uses: amd/skillscope@...`,
 # so each job checks out this repository at `job.workflow_sha` (the commit the
 # caller referenced) and runs the composite action from that tree. Pinning
-# `@v0.1.1` grades with v0.1.1, `@main` grades with main, and a branch grades
+# `@v0.1.2` grades with v0.1.2, `@main` grades with main, and a branch grades
 # with that branch -- so there is no release ritual and nothing to keep in step.
 
 on:
@@ -273,7 +273,8 @@ jobs:
       - name: Check out repository
         uses: actions/checkout@v4
         with:
-          # Need the merge base so `git diff` can see what the pull request changed.
+          # Both commits' full history, so selection can find their merge base
+          # and diff what the pull request changed from there.
           fetch-depth: 0
 
       - name: Check out skillscope
@@ -302,6 +303,10 @@ jobs:
       # it was asked for. Written in Python so the arguments are quoted rather
       # than pasted -- a label like `don't merge` would otherwise arrive at the
       # CLI in pieces.
+      #
+      # Which commits changed what is left to `select --since`, along with
+      # every other decision: it diffs from the merge base, so a branch whose
+      # base has moved on is still planned for what the branch did.
       - name: Work out how to select
         id: how
         shell: python
@@ -323,22 +328,18 @@ jobs:
         run: |
           import os
           import shlex
-          import subprocess
 
           event = os.environ["EVENT"]
           skills = os.environ.get("SKILLS", "").strip()
           base, head = os.environ.get("BASE", ""), os.environ.get("HEAD", "")
           extended = os.environ["EXTENDED"]
 
-          changed = ""
           if event == "pull_request" and base:
-              changed = subprocess.run(
-                  ["git", "diff", "--name-only", base, head],
-                  check=True,
-                  capture_output=True,
-                  text=True,
-              ).stdout
-              args = ["--changed", "--labels", os.environ.get("LABELS", ""), extended]
+              args = [
+                  "--since", base, head,
+                  "--labels", os.environ.get("LABELS", ""),
+                  extended,
+              ]
               if os.environ.get("IGNORE_GATES"):
                   args.append("--ignore-gates")
           else:
@@ -363,10 +364,6 @@ jobs:
               if value:
                   args += [flag, value]
 
-          with open("changed-files.txt", "w", encoding="utf-8") as handle:
-              handle.write(changed)
-          print("Changed files:\n" + (changed or "(none)"))
-
           # Quoted rather than pasted: a JSON label array and a pull-request
           # label like `don't merge` both have to survive the trip intact.
           line = "args=" + " ".join(shlex.quote(a) for a in args)
@@ -381,7 +378,6 @@ jobs:
           command: select
           args: ${{ steps.how.outputs.args }}
           skills: ${{ inputs.skill_globs }}
-          stdin: changed-files.txt
           python-version: ${{ inputs.python_version }}
 
       - name: Emit the plan

README.md

@@ -48,7 +48,7 @@ Simply add an `evals.json` file to your skill and a workflow that points to our
 ```yaml
 jobs:
   evals:
-    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.1
+    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.2
     secrets:
       api_key: ${{ secrets.ANTHROPIC_API_KEY }}
     with:

action.yml

@@ -9,7 +9,7 @@ description: >-
 
 # Pin the tag you want to run, the same way you pin the reusable workflow:
 #
-#   - uses: amd/skillscope@v0.1.1
+#   - uses: amd/skillscope@v0.1.2
 #
 # The Python in that checkout is the harness. There is no second version to
 # resolve, and nothing here fetches a different ref.

bootstrap/launch.py

@@ -5,7 +5,7 @@
 """Run one skillscope command from the composite action's own checkout.
 
 Callers pin a tag on the action or on a reusable workflow in this repo
-(``amd/skillscope@v0.1.1``, ``.../reusable.yml@v0.1.1``). This script installs
+(``amd/skillscope@v0.1.2``, ``.../reusable.yml@v0.1.2``). This script installs
 *that* checkout with ``uvx`` and execs the command. It does not fetch some
 other ref: the ``uses:`` pin is the harness.
 
@@ -91,7 +91,7 @@ def main() -> int:
     if not (source / "pyproject.toml").is_file():
         raise SystemExit(
             f"error: {source} has no pyproject.toml. The action must run from "
-            "a skillscope checkout (for example amd/skillscope@v0.1.1)."
+            "a skillscope checkout (for example amd/skillscope@v0.1.2)."
         )
     version = packaged_version(source) or "unknown"
 

docs/usage.md

@@ -216,7 +216,7 @@ one runner per skill:
 ```yaml
 jobs:
   evals:
-    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.1
+    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.2
     secrets:
       api_key: ${{ secrets.ANTHROPIC_API_KEY }}
     with:
@@ -280,7 +280,7 @@ pays for.
 ```yaml
 jobs:
   skill-evals:
-    uses: amd/skillscope/.github/workflows/skill-evals.yml@v0.1.1
+    uses: amd/skillscope/.github/workflows/skill-evals.yml@v0.1.2
     secrets: inherit
     with:
       routing_room: my-skill,its-neighbour
@@ -295,7 +295,7 @@ workflow file documents every one.
 To run a single command instead of a pipeline, use the action directly:
 
 ```yaml
-- uses: amd/skillscope@v0.1.1
+- uses: amd/skillscope@v0.1.2
   with:
     command: structural
 ```
@@ -304,7 +304,20 @@ Deciding what to run by hand is also possible: `select` emits the plan for a
 change as JSON.
 
 ```bash
-git diff --name-only main HEAD | skillscope select --changed
+skillscope select --since main HEAD
+```
+
+`--since` takes the two commits a pull request names and works the changed
+paths out from their merge base, so a branch is planned for what it changed
+rather than for how it differs from a base that has moved on without it.
+Without that, everything merged into the base since the branch left it comes
+back in the diff, and a base commit touching an [infra
+path](#configuring-the-repo-under-test) re-runs the whole catalog.
+
+Pass a list of paths instead when it was worked out some other way:
+
+```bash
+git diff --name-only main...HEAD | skillscope select --changed
 ```
 
 ## Versions
@@ -315,7 +328,7 @@ the tag you want to run:
 ```yaml
 jobs:
   evals:
-    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.1
+    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.2
 ```
 
 That tag's checkout is what grades your skills. Bump the ref in that one line

examples/amd-skills-checks.yml

@@ -21,7 +21,7 @@ permissions:
 jobs:
   evals:
     name: AMD Skills Checks
-    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.1
+    uses: amd/skillscope/.github/workflows/reusable.yml@v0.1.2
     secrets:
       api_key: ${{ secrets.ANTHROPIC_API_KEY }}
     with:

pyproject.toml

@@ -8,7 +8,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "skillscope"
-version = "0.1.1"
+version = "0.1.2"
 description = "Routing and behavioral test harness for agent skills."
 readme = "README.md"
 requires-python = ">=3.10"

skillscope/__init__.py

@@ -15,4 +15,4 @@
 
 __all__ = ["__version__"]
 
-__version__ = "0.1.1"
+__version__ = "0.1.2"

skillscope/cli.py

@@ -53,7 +53,10 @@
     skillscope routing --only qwen-on-mi300x --keep-logs eval-logs
 
     # what CI should run for a change
-    git diff --name-only BASE HEAD | skillscope select --changed
+    skillscope select --since BASE HEAD
+
+    # the same, from a list of paths worked out some other way
+    git diff --name-only BASE...HEAD | skillscope select --changed
 
 Reports go to stdout as markdown, to ``$GITHUB_STEP_SUMMARY`` under Actions,
 and to a JSON artifact under ``.skillscope/runs/`` in the repo under test.
@@ -228,6 +231,20 @@ def cmd_template(args: argparse.Namespace) -> int:
     return 0
 
 
+def _changed_for_select(args: argparse.Namespace) -> set[str]:
+    """The changed paths to plan from, however the caller chose to say them."""
+    if args.since:
+        lines = select_module.changed_paths(*args.since)
+        # The plan is the only thing on stdout, so what it was decided from
+        # goes to stderr, where a CI log still shows it.
+        print("Changed files:", file=sys.stderr)
+        for path in lines or ["(none)"]:
+            print(f"  {path}", file=sys.stderr)
+    else:
+        lines = sys.stdin.read().splitlines()
+    return {line.strip().replace("\\", "/") for line in lines if line.strip()}
+
+
 def cmd_select(args: argparse.Namespace) -> int:
     available = datasets.skills_with_datasets()
     if args.all:
@@ -240,11 +257,7 @@ def cmd_select(args: argparse.Namespace) -> int:
             return 1
         skills, needs_routing = requested, True
     else:
-        changed = {
-            line.strip().replace("\\", "/")
-            for line in sys.stdin.read().splitlines()
-            if line.strip()
-        }
+        changed = _changed_for_select(args)
         skills = select_module.select_from_changes(changed)
         needs_routing = select_module.routing_needed(changed, args.extended)
 
@@ -747,6 +760,17 @@ def build_parser() -> argparse.ArgumentParser:
     mode = select_parser.add_mutually_exclusive_group(required=True)
     mode.add_argument("--all", action="store_true", help="Every skill with a dataset.")
     mode.add_argument("--changed", action="store_true", help="Read changed paths from stdin.")
+    mode.add_argument(
+        "--since",
+        nargs=2,
+        metavar=("BASE", "HEAD"),
+        help=(
+            "The two commits a pull request names. Changed paths are worked "
+            "out from their merge base, so a branch is planned for what it "
+            "changed rather than for how it differs from a base that has moved "
+            "on. Both commits' history has to be in the checkout."
+        ),
+    )
     mode.add_argument(
         "--names", metavar="A,B,C", help="An explicit comma-separated skill list."
     )

skillscope/select.py

@@ -54,6 +54,8 @@
 from __future__ import annotations
 
 import json
+import subprocess
+import sys
 from pathlib import Path
 
 from . import config, datasets
@@ -65,6 +67,57 @@
 EXTENDED_SUFFIX = "/" + datasets.EXTENDED_DATASET_RELPATH.as_posix()
 
 
+def changed_paths(base: str, head: str) -> list[str]:
+    """What a branch changed, given the two commits a pull request names.
+
+    ``git diff base head`` answers a different question -- how those two trees
+    differ -- and the difference matters as soon as the base branch moves on
+    without the branch. Everything merged into the base since the branch left
+    it comes back in that diff, in reverse, as though this branch had touched
+    it. One skill's pull request then re-runs its neighbours, and a base commit
+    that happened to touch an infra path re-runs the entire catalog.
+
+    So diff from the merge base, which is the only commit both sides agree on
+    and so the only one that makes the answer "what did this branch do".
+
+    Falls back to the plain diff when there is no common ancestor to be found
+    -- a clone shallow enough not to contain one, or histories that really are
+    unrelated -- because selecting too much costs a slow run, and selecting too
+    little ships an untested change.
+    """
+    root = config.active().root
+
+    def git(*args: str) -> subprocess.CompletedProcess:
+        return subprocess.run(
+            ["git", "-C", str(root), *args],
+            capture_output=True,
+            text=True,
+            encoding="utf-8",
+            check=False,
+        )
+
+    fork_point = git("merge-base", base, head)
+    if fork_point.returncode == 0 and fork_point.stdout.strip():
+        base = fork_point.stdout.strip()
+    else:
+        print(
+            f"warning: no merge base for {base} and {head}, so selection is "
+            "falling back to the plain diff between them. It may name files "
+            "this branch never touched.\n"
+            f"{fork_point.stderr.strip()}",
+            file=sys.stderr,
+        )
+
+    diff = git("diff", "--name-only", base, head)
+    if diff.returncode != 0:
+        raise SystemExit(
+            f"error: could not diff {base}..{head} in {root}. CI needs the "
+            "history of both commits to work out what changed, so check out "
+            "with fetch-depth: 0.\n" + diff.stderr.strip()
+        )
+    return [line.strip() for line in diff.stdout.splitlines() if line.strip()]
+
+
 def infra_paths() -> set[str]:
     """Paths that change the shared engine rather than one skill.