diff --git a/.changeset/detect-python-dev-dependencies.md b/.changeset/detect-python-dev-dependencies.md
new file mode 100644
index 00000000..74e4a35b
--- /dev/null
+++ b/.changeset/detect-python-dev-dependencies.md
@@ -0,0 +1,5 @@
+---
+"@taskless/cli": patch
+---
+
+`taskless detect` now finds a Python linter installed as a dev dependency in `pyproject.toml`, so a repository that runs ruff on its defaults, with no `[tool.ruff]` table, no longer reads as having no linter. It reads PEP 735 `[dependency-groups]` (what `uv add --dev` writes), `[tool.uv] dev-dependencies`, `[tool.pdm.dev-dependencies]`, Hatch environment dependencies, and Poetry dependency groups. Poetry groups were previously read by group name rather than by the dependencies inside them.
diff --git a/packages/cli/src/detect/scan.ts b/packages/cli/src/detect/scan.ts
index 1bf8e077..100da90e 100644
--- a/packages/cli/src/detect/scan.ts
+++ b/packages/cli/src/detect/scan.ts
@@ -383,7 +383,35 @@ function requirementName(requirement: string): string {
     .toLowerCase();
 }
 
-/** Python dependency names declared in a parsed `pyproject.toml` (PEP 621 + Poetry). */
+/** Package names from an array of PEP 508 requirement strings. Non-string
+ * entries (a PEP 735 `{ include-group = "..." }`) and non-arrays contribute
+ * nothing. */
+function requirementNames(value: unknown): string[] {
+  if (!Array.isArray(value)) return [];
+  return value
+    .filter((entry): entry is string => typeof entry === "string")
+    .map((entry) => requirementName(entry));
+}
+
+/** Package names from a table of named groups, each an array of PEP 508
+ * requirement strings (`optional-dependencies`, PEP 735 `dependency-groups`,
+ * PDM's `dev-dependencies`). */
+function groupedRequirementNames(value: unknown): string[] {
+  return Object.values(asRecord(value) ?? {}).flatMap((group) =>
+    requirementNames(group)
+  );
+}
+
+/**
+ * Python dependency names declared in a parsed `pyproject.toml`.
+ *
+ * A dev tool like ruff is rarely a runtime dependency, so the dev-dependency
+ * locations matter as much as `[project]`: PEP 735 `[dependency-groups]` (what
+ * `uv add --dev` writes), uv's legacy `[tool.uv] dev-dependencies`, PDM's and
+ * Hatch's own tables, and Poetry's groups. A repository that installs ruff
+ * through any of these and runs it on its defaults has no `[tool.ruff]` table,
+ * so this is the only evidence it lints.
+ */
 function pythonDepsFromPyproject(
   pyproject: Record<string, unknown> | undefined
 ): string[] {
@@ -391,26 +419,41 @@ function pythonDepsFromPyproject(
   const names: string[] = [];
 
   const project = asRecord(pyproject.project);
-  const projectDeps = project?.dependencies;
-  if (Array.isArray(projectDeps)) {
-    for (const entry of projectDeps) {
-      if (typeof entry === "string") names.push(requirementName(entry));
-    }
-  }
-  const optional = asRecord(project?.["optional-dependencies"]);
-  for (const group of Object.values(optional ?? {})) {
-    if (!Array.isArray(group)) continue;
-    for (const entry of group) {
-      if (typeof entry === "string") names.push(requirementName(entry));
-    }
+  names.push(
+    ...requirementNames(project?.dependencies),
+    ...groupedRequirementNames(project?.["optional-dependencies"]),
+    ...groupedRequirementNames(pyproject["dependency-groups"])
+  );
+
+  const tool = asRecord(pyproject.tool);
+  names.push(
+    ...requirementNames(asRecord(tool?.uv)?.["dev-dependencies"]),
+    ...groupedRequirementNames(asRecord(tool?.pdm)?.["dev-dependencies"])
+  );
+  for (const environment of Object.values(
+    asRecord(asRecord(tool?.hatch)?.envs) ?? {}
+  )) {
+    const table = asRecord(environment);
+    names.push(
+      ...requirementNames(table?.dependencies),
+      ...requirementNames(table?.["extra-dependencies"])
+    );
   }
 
-  const poetry = asRecord(asRecord(pyproject.tool)?.poetry);
-  for (const key of ["dependencies", "dev-dependencies", "group"]) {
-    const table = asRecord(poetry?.[key]);
-    if (table) {
-      names.push(...Object.keys(table).map((name) => name.toLowerCase()));
-    }
+  // Poetry keys dependencies by name. Groups nest one level deeper, under
+  // `[tool.poetry.group.<name>.dependencies]`.
+  const poetry = asRecord(tool?.poetry);
+  const poetryTables = [
+    poetry?.dependencies,
+    poetry?.["dev-dependencies"],
+    ...Object.values(asRecord(poetry?.group) ?? {}).map(
+      (group) => asRecord(group)?.dependencies
+    ),
+  ];
+  for (const table of poetryTables) {
+    names.push(
+      ...Object.keys(asRecord(table) ?? {}).map((name) => name.toLowerCase())
+    );
   }
 
   return names;
diff --git a/packages/cli/test/detect.test.ts b/packages/cli/test/detect.test.ts
index d39e4530..c74a5e5f 100644
--- a/packages/cli/test/detect.test.ts
+++ b/packages/cli/test/detect.test.ts
@@ -204,6 +204,48 @@ describe("taskless detect", () => {
     expect(result.languages).toContain("Python");
   });
 
+  it.each([
+    [
+      "PEP 735 [dependency-groups]",
+      '[dependency-groups]\ndev = [{ include-group = "test" }, "ruff>=0.6"]\ntest = ["pytest"]\n',
+    ],
+    ["[tool.uv] dev-dependencies", '[tool.uv]\ndev-dependencies = ["ruff"]\n'],
+    [
+      "[tool.pdm.dev-dependencies]",
+      '[tool.pdm.dev-dependencies]\nlint = ["ruff==0.6.0"]\n',
+    ],
+    [
+      "a Hatch environment",
+      '[tool.hatch.envs.lint]\ndependencies = ["ruff"]\n',
+    ],
+    [
+      "a Poetry dependency group",
+      '[tool.poetry.group.dev.dependencies]\nruff = "^0.6"\n',
+    ],
+  ])(
+    "detects ruff installed only as a dev dependency in %s",
+    async (_location, toml) => {
+      // No [tool.ruff] table: ruff running on its defaults is still a linter.
+      await writeFile(
+        join(cwd, "pyproject.toml"),
+        `[project]\nname = "x"\n\n${toml}`,
+        "utf8"
+      );
+      const result = await detect(cwd);
+      expect(linterNames(result)).toContain("ruff");
+    }
+  );
+
+  it("does not read a Poetry group's name as a dependency", async () => {
+    await writeFile(
+      join(cwd, "pyproject.toml"),
+      '[tool.poetry.group.ruff.dependencies]\npytest = "^8"\n',
+      "utf8"
+    );
+    const result = await detect(cwd);
+    expect(linterNames(result)).not.toContain("ruff");
+  });
+
   it("detects flake8 from a requirements.txt entry", async () => {
     await writeFile(
       join(cwd, "requirements.txt"),