Fail loudly on invalid config files (#9041)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - The server config loader reads `.paperclip/config.json` and feeds it
into the shared Paperclip config schema.
> - When a config file exists but cannot be parsed or fails schema
validation, Paperclip should not silently ignore it.
> - The current `readConfigFile()` catch block treats invalid files the
same as missing files, so startup falls back to defaults while the
banner can still point at the ignored config path.
> - This pull request keeps the missing-file fallback, but makes present
invalid config files fail with a path-specific error.
> - The benefit is safer startup behavior and a clear diagnostic that
points at the invalid config field.

## Linked Issues or Issue Description

Fixes #8908

## What Changed

- Changed `readConfigFile()` to return `null` only when the config file
is absent.
- Added explicit errors for unreadable/invalid JSON config files.
- Added explicit Zod validation errors that include the config path and
invalid field path without printing config contents.
- Added server tests for missing config, invalid JSON, schema validation
failure, and valid config parsing.

## Verification

- `pnpm exec vitest run server/src/__tests__/config-file.test.ts`
- `pnpm --filter @paperclipai/server typecheck`

## Risks

Low risk for valid configs and missing configs. This intentionally
changes behavior for present invalid config files from silent fallback
to startup failure, which is the issue being fixed.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

OpenAI Codex based on GPT-5, with repository file inspection, GitHub
CLI, and local command execution.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

server/src/__tests__/config-file.test.ts

@@ -0,0 +1,92 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { readConfigFile } from "../config-file.js";
+
+const ORIGINAL_PAPERCLIP_CONFIG = process.env.PAPERCLIP_CONFIG;
+
+function writeConfig(configPath: string, value: unknown): void {
+  fs.writeFileSync(configPath, `${JSON.stringify(value, null, 2)}\n`);
+}
+
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+function minimalConfig(): unknown {
+  return {
+    $meta: {
+      version: 1,
+      updatedAt: "2026-07-05T00:00:00.000Z",
+      source: "configure",
+    },
+    database: {
+      mode: "embedded-postgres",
+    },
+    logging: {
+      mode: "file",
+    },
+    server: {},
+  };
+}
+
+describe("readConfigFile", () => {
+  let tempDir: string;
+  let configPath: string;
+
+  beforeEach(() => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "paperclip-config-file-test-"));
+    configPath = path.join(tempDir, "config.json");
+    process.env.PAPERCLIP_CONFIG = configPath;
+  });
+
+  afterEach(() => {
+    if (ORIGINAL_PAPERCLIP_CONFIG === undefined) {
+      delete process.env.PAPERCLIP_CONFIG;
+    } else {
+      process.env.PAPERCLIP_CONFIG = ORIGINAL_PAPERCLIP_CONFIG;
+    }
+
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  it("returns null when the config file does not exist", () => {
+    expect(readConfigFile()).toBeNull();
+  });
+
+  it("throws a path-specific error when the config file is invalid JSON", () => {
+    fs.writeFileSync(configPath, "{");
+
+    expect(() => readConfigFile()).toThrow(
+      new RegExp(`Invalid Paperclip config at ${escapeRegExp(configPath)}: failed to read or parse JSON`),
+    );
+  });
+
+  it("throws a field-specific error when the config file fails schema validation", () => {
+    const config = minimalConfig();
+    if (typeof config === "object" && config !== null) {
+      (config as { $meta: { source: string } }).$meta.source = "edited-by-hand";
+    }
+
+    writeConfig(configPath, config);
+
+    expect(() => readConfigFile()).toThrow(/Invalid Paperclip config .* \$meta\.source:/);
+  });
+
+  it("parses a valid config file", () => {
+    writeConfig(configPath, minimalConfig());
+
+    expect(readConfigFile()).toMatchObject({
+      $meta: {
+        source: "configure",
+      },
+      database: {
+        mode: "embedded-postgres",
+      },
+      logging: {
+        mode: "file",
+      },
+    });
+  });
+});

server/src/config-file.ts

@@ -1,16 +1,37 @@
 import fs from "node:fs";
 import { paperclipConfigSchema, type PaperclipConfig } from "@paperclipai/shared";
+import { ZodError } from "zod";
 import { resolvePaperclipConfigPath } from "./paths.js";
 
+function formatConfigValidationError(error: ZodError): string {
+  return error.issues
+    .map((issue) => {
+      const issuePath = issue.path.length > 0 ? issue.path.join(".") : "<root>";
+      return `${issuePath}: ${issue.message}`;
+    })
+    .join("; ");
+}
+
 export function readConfigFile(): PaperclipConfig | null {
   const configPath = resolvePaperclipConfigPath();
 
   if (!fs.existsSync(configPath)) return null;
 
+  let raw: unknown;
+  try {
+    raw = JSON.parse(fs.readFileSync(configPath, "utf-8"));
+  } catch (error) {
+    const reason = error instanceof Error ? error.message : String(error);
+    throw new Error(`Invalid Paperclip config at ${configPath}: failed to read or parse JSON: ${reason}`);
+  }
+
   try {
-    const raw = JSON.parse(fs.readFileSync(configPath, "utf-8"));
     return paperclipConfigSchema.parse(raw);
-  } catch {
-    return null;
+  } catch (error) {
+    if (error instanceof ZodError) {
+      throw new Error(`Invalid Paperclip config at ${configPath}: ${formatConfigValidationError(error)}`);
+    }
+
+    throw error;
   }
 }