diff --git a/docs/semantics.md b/docs/semantics.md
index 58cb803..ac80ca1 100644
--- a/docs/semantics.md
+++ b/docs/semantics.md
@@ -82,12 +82,26 @@ manifest needs to change.
 ```yaml
 core:
   source: { repo: acme/widget, branch: main }
-  build: { pack: dockercompose, base_directory: /, compose_file: docker-compose.yaml }
+  build: { pack: dockercompose, base_directory: /, compose_file: /docker-compose.yaml }
   service_domains:
     api: ["https://api.widget.example.com"]
   env_template: core.prod.env.template
 ```
 
+**Checkout paths are absolute.** `build.compose_file`,
+`build.base_directory` and `build.publish_directory` are all paths *inside the
+repo checkout*, and all three must begin with `/` — the manifest schema refuses
+them otherwise. This is Coolify's own rule, not cast's taste: on create it
+validates `docker_compose_location` against
+`ValidationPatterns::FILE_PATH_PATTERN` and `base_directory`/`publish_directory`
+against `DIRECTORY_PATH_PATTERN` (both anchored on a leading slash; only the
+directory pattern also admits the bare `/` root), and a `docker-compose.yaml`
+with no slash comes back as a bare 422 — *after* `apply` has already created
+the project and the environment. cast refuses at parse time instead, on every
+verb, at zero API cost. It refuses rather than normalizes: the manifest is the
+artifact under review, so the value is fixed in the file, once, not repaired in
+memory on every run.
+
 Internally cast keeps the map vocabulary (`docker_compose_domains:
 {service: string[]}`) all the way through `resolve.ts`/`apply.ts`/diffing;
 only `cli.ts`'s wire-translation layer (`applicationApiFields`) flattens it to
diff --git a/src/manifest.ts b/src/manifest.ts
index 1e4df6b..06e295a 100644
--- a/src/manifest.ts
+++ b/src/manifest.ts
@@ -2,15 +2,47 @@ import { readFileSync } from "node:fs";
 import { parse } from "yaml";
 import { z } from "zod";
 
+// Coolify validates every checkout-relative path on create and 422s anything
+// that is not absolute. Transcribed from coolify v4.1.2:
+// `bootstrap/helpers/api.php::sharedDataApplications()` binds
+// `base_directory`/`publish_directory` to `ValidationPatterns::directoryPathRules()`
+// and `docker_compose_location` to `ValidationPatterns::filePathRules()`, and
+// `app/Support/ValidationPatterns.php` defines the two patterns below. The only
+// difference between them: a file path needs at least one character after the
+// slash, a directory path may be the bare `/` (the checkout root).
+const COOLIFY_FILE_PATH = /^\/[a-zA-Z0-9._/~@+-]+$/;
+const COOLIFY_DIRECTORY_PATH = /^\/[a-zA-Z0-9._/~@+-]*$/;
+
+// These are refinements, not normalizations, and must stay that way: cast does
+// not quietly rewrite what the manifest says. A value that would 422 gets fixed
+// in the file, in a commit, once — not repaired in memory on every run. And the
+// check belongs here, at parse time, because it is a property of the manifest
+// and of nothing else: by the time a create returns its bare 422, `apply` has
+// already made the project and the environment, and the run is half-applied.
+const composeFilePath = z
+  .string()
+  .regex(
+    COOLIFY_FILE_PATH,
+    "compose_file must be an absolute path inside the repo checkout (Coolify 4.1.2 rejects the create otherwise) — write /docker-compose.yaml, not docker-compose.yaml",
+  );
+
+const repoDirectoryPath = (field: string) =>
+  z
+    .string()
+    .regex(
+      COOLIFY_DIRECTORY_PATH,
+      `${field} must be an absolute path inside the repo checkout (Coolify 4.1.2 rejects the create otherwise) — write /apps/core, not apps/core; the checkout root is /`,
+    );
+
 const AppSpecSchema = z
   .object({
     source: z.object({ repo: z.string(), branch: z.string() }).strict(),
     build: z
       .object({
         pack: z.enum(["nixpacks", "static", "dockerfile", "dockercompose"]),
-        base_directory: z.string(),
-        publish_directory: z.string().optional(),
-        compose_file: z.string().optional(),
+        base_directory: repoDirectoryPath("base_directory"),
+        publish_directory: repoDirectoryPath("publish_directory").optional(),
+        compose_file: composeFilePath.optional(),
       })
       .strict(),
     port: z.number().int().optional(),
diff --git a/test/capture.test.ts b/test/capture.test.ts
index 8118428..5d86fe9 100644
--- a/test/capture.test.ts
+++ b/test/capture.test.ts
@@ -251,7 +251,7 @@ environments:
     applications:
       core:
         source: { repo: heavy-duty/incubator, branch: main }
-        build: { pack: dockercompose, base_directory: /, compose_file: docker-compose.yaml }
+        build: { pack: dockercompose, base_directory: /, compose_file: /docker-compose.yaml }
         service_domains:
           api: ["https://api.example.com"]
         env_template: core.prod.env.template
diff --git a/test/manifest.test.ts b/test/manifest.test.ts
index 04d974f..d9f88ab 100644
--- a/test/manifest.test.ts
+++ b/test/manifest.test.ts
@@ -55,7 +55,7 @@ environments:
     applications:
       core:
         source: { repo: acme/widget, branch: main }
-        build: { pack: dockercompose, base_directory: /, compose_file: docker-compose.yaml }
+        build: { pack: dockercompose, base_directory: /, compose_file: /docker-compose.yaml }
         service_domains:
           api: ["https://api.example.com"]
         env_template: core.prod.env.template
@@ -63,7 +63,7 @@ environments:
     });
     const app = m.environments.prod.applications.core;
     expect(app.build.pack).toBe("dockercompose");
-    expect(app.build.compose_file).toBe("docker-compose.yaml");
+    expect(app.build.compose_file).toBe("/docker-compose.yaml");
     expect(app.service_domains).toEqual({ api: ["https://api.example.com"] });
     expect(app.port).toBeUndefined();
     expect(app.healthcheck).toBeUndefined();
@@ -86,7 +86,11 @@ environments:
       }),
     ).toThrow(/compose_file/);
   });
-  it("rejects a dockercompose app with top-level domains", () => {
+  // Coolify 4.1.2 validates docker_compose_location against
+  // ValidationPatterns::FILE_PATH_PATTERN on create and 422s a path with no
+  // leading slash — after `apply` has already made the project and the
+  // environment. The manifest knows this before any API call, so it refuses.
+  it("rejects a compose_file with no leading slash, and names the fix", () => {
     expect(() =>
       loadManifest(`${FIX}manifest.yaml`, {
         overrideText: `
@@ -99,6 +103,92 @@ environments:
         build: { pack: dockercompose, base_directory: /, compose_file: docker-compose.yaml }
         service_domains:
           api: ["https://api.example.com"]
+`,
+      }),
+    ).toThrow(/compose_file must be an absolute path.*\/docker-compose\.yaml/s);
+  });
+  it("rejects a compose_file that is the bare root (a directory, not a file)", () => {
+    expect(() =>
+      loadManifest(`${FIX}manifest.yaml`, {
+        overrideText: `
+project: widget
+environments:
+  prod:
+    applications:
+      core:
+        source: { repo: acme/widget, branch: main }
+        build: { pack: dockercompose, base_directory: /, compose_file: / }
+        service_domains:
+          api: ["https://api.example.com"]
+`,
+      }),
+    ).toThrow(/compose_file must be an absolute path/);
+  });
+  it("rejects a base_directory with no leading slash", () => {
+    expect(() =>
+      loadManifest(`${FIX}manifest.yaml`, {
+        overrideText: `
+project: widget
+environments:
+  prod:
+    applications:
+      core:
+        source: { repo: acme/widget, branch: main }
+        build: { pack: nixpacks, base_directory: apps/core }
+        domains: ["https://api.example.com"]
+`,
+      }),
+    ).toThrow(/base_directory must be an absolute path/);
+  });
+  it("rejects a publish_directory with no leading slash", () => {
+    expect(() =>
+      loadManifest(`${FIX}manifest.yaml`, {
+        overrideText: `
+project: widget
+environments:
+  prod:
+    applications:
+      core:
+        source: { repo: acme/widget, branch: main }
+        build: { pack: static, base_directory: /, publish_directory: dist }
+        domains: ["https://api.example.com"]
+`,
+      }),
+    ).toThrow(/publish_directory must be an absolute path/);
+  });
+  // Coolify's DIRECTORY_PATH_PATTERN admits the bare "/" where FILE_PATH_PATTERN
+  // does not — every manifest in the wild says `base_directory: /`, so a shared
+  // "absolute path" rule that rejected it would refuse every manifest cast has.
+  it("accepts / as base_directory and a nested absolute publish_directory", () => {
+    const m = loadManifest(`${FIX}manifest.yaml`, {
+      overrideText: `
+project: widget
+environments:
+  prod:
+    applications:
+      core:
+        source: { repo: acme/widget, branch: main }
+        build: { pack: static, base_directory: /, publish_directory: /apps/web/dist }
+        domains: ["https://api.example.com"]
+`,
+    });
+    const app = m.environments.prod.applications.core;
+    expect(app.build.base_directory).toBe("/");
+    expect(app.build.publish_directory).toBe("/apps/web/dist");
+  });
+  it("rejects a dockercompose app with top-level domains", () => {
+    expect(() =>
+      loadManifest(`${FIX}manifest.yaml`, {
+        overrideText: `
+project: widget
+environments:
+  prod:
+    applications:
+      core:
+        source: { repo: acme/widget, branch: main }
+        build: { pack: dockercompose, base_directory: /, compose_file: /docker-compose.yaml }
+        service_domains:
+          api: ["https://api.example.com"]
         domains: ["https://api.example.com"]
 `,
       }),
diff --git a/test/resolve.test.ts b/test/resolve.test.ts
index 1cb9de5..0600702 100644
--- a/test/resolve.test.ts
+++ b/test/resolve.test.ts
@@ -295,7 +295,7 @@ environments:
     applications:
       core:
         source: { repo: acme/widget, branch: main }
-        build: { pack: dockercompose, base_directory: /, compose_file: docker-compose.yaml }
+        build: { pack: dockercompose, base_directory: /, compose_file: /docker-compose.yaml }
         service_domains:
           api: ["https://api.widget.example.com"]
         env_template: core.prod.env.template
@@ -315,7 +315,7 @@ environments:
         git_branch: "main",
         build_pack: "dockercompose",
         base_directory: "/",
-        docker_compose_location: "docker-compose.yaml",
+        docker_compose_location: "/docker-compose.yaml",
         docker_compose_domains: {
           api: ["https://api.widget.example.com"],
         },