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◈ Agents & Manager/2026-08-07Advanced

I Asked the Agent to Pick Tests From My Diff and It Said "None" Nine Times Out of Ten

Change-based test selection returned zero tests for most edits. Measuring how far an import graph actually reaches across three repositories, and rebuilding the selection contract around it.

Agent Operations8Testing StrategyStatic AnalysisAntigravity411
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I was trying to shorten the self-verification loop.

Running the whole suite after every agent edit is slow, and when it fails the signal is buried. Working as an indie developer, that wait comes straight out of my own day. Walking backwards from the changed files to the tests that actually depend on them should have been the obvious fix.

I wired it up, ran a few dozen edits through it, and got the same answer nearly every time: zero tests selected.

The whole measurement finished in 0.4 seconds, so my first suspicion was that the file reads were failing. I counted the reverse edges instead — 3,177 of them. The graph was fine. The zero was the honest answer.

Here is what I measured that afternoon.

What was measured

Three public repositories, chosen because their directory habits differ.

RepositoryFiles analyzedRelative import edgesGraph build time
vite1,5401,409303 ms
hono388914240 ms
nest1,7273,177339 ms

One question only. If you change a single source file, what share of the test suite is reachable by walking imports in reverse?

That calculation is exactly what happens inside an agent when you tell it to "run the tests related to this change." A smaller share means a faster loop — and a larger blind spot.

Building the graph

Only relative imports are resolved. Workspace references like @nestjs/common and path aliases are deliberately excluded, because I wanted something an agent could rebuild in a few hundred milliseconds.

import os, re, subprocess
from collections import defaultdict, deque
 
EXTS = [".ts", ".tsx", ".mts", ".js", ".mjs", ".jsx"]
 
# catches both `from "..."` forms and bare side-effect imports
IMP = re.compile(
    r'''(?:^|\n)\s*(?:import|export)\b[^;\n]*?from\s*['"]([^'"]+)['"]'''
    r'''|(?:^|\n)\s*import\s*['"]([^'"]+)['"]'''
)
# an index file made almost entirely of re-exports is treated as a barrel
REEXP = re.compile(r'''(?:^|\n)\s*export\s+(?:\*|\{)[^;]*?from\s*['"]''')
 
 
def resolve(base_dir, spec, fileset):
    """Resolve a relative specifier to a real file, or None."""
    if not spec.startswith("."):
        return None
    raw = os.path.normpath(os.path.join(base_dir, spec))
    cands = [raw] + [raw + e for e in EXTS]
    cands += [os.path.join(raw, "index" + e) for e in EXTS]
    # ESM specifiers often write .js while pointing at .ts
    if raw.endswith(".js"):
        cands += [raw[:-3] + e for e in (".ts", ".tsx", ".mts")]
    for c in cands:
        if c in fileset:
            return c
    return None
 
 
def build(repo):
    out = subprocess.run(["git", "-C", repo, "ls-files"],
                         capture_output=True, text=True).stdout
    paths = [p for p in out.split("\n")
             if p.strip() and os.path.splitext(p)[1] in EXTS]
    fileset = set(paths)
    fwd, barrel = defaultdict(set), set()
 
    for p in paths:
        try:
            src = open(os.path.join(repo, p), encoding="utf-8",
                       errors="ignore").read()
        except OSError:
            continue  # submodule placeholders and similar gaps
        if os.path.basename(p).startswith("index.") and len(REEXP.findall(src)) >= 3:
            barrel.add(p)
        d = os.path.dirname(p)
        for m in IMP.finditer(src):
            t = resolve(d, m.group(1) or m.group(2), fileset)
            if t and t != p:
                fwd[p].add(t)
 
    rev = defaultdict(set)
    for a, bs in fwd.items():
        for b in bs:
            rev[b].add(a)
    return paths, rev, barrel

The errors="ignore" matters more than it looks. git ls-files lists paths whose contents may not be materialized, and an exception there halts the walk and hands you a sparse graph that looks perfectly valid. That was the first thing I checked when the zeros appeared.

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WHAT YOU'LL LEARN
✦Across vite, hono and nest, the tests reachable from a single changed file sit at a median of 0.0-0.7% but a p90 of 0.3-50.4% — designing against the mean will mislead you
✦In vite, 1,029 of 1,148 source files reach no test at all through the import graph. The dangerous failure is not over-selection, it is a zero that says nothing
✦Dropping re-export barrels from the traversal moves nest from 4.0% to 1.4% mean reach and 14.5% to 2.5% at p90 — barrels were inflating perceived impact roughly threefold
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