diff --git a/.changeset/cuddly-mice-appear.md b/.changeset/cuddly-mice-appear.md
new file mode 100644
index 0000000..60e205a
--- /dev/null
+++ b/.changeset/cuddly-mice-appear.md
@@ -0,0 +1,6 @@
+---
+"@confts/bootstrap": patch
+"confts": patch
+---
+
+Better error handling with diagnostics
diff --git a/packages/bootstrap/src/index.ts b/packages/bootstrap/src/index.ts
index 79610ec..e258bb3 100644
--- a/packages/bootstrap/src/index.ts
+++ b/packages/bootstrap/src/index.ts
@@ -47,6 +47,7 @@ export type AutorunOptions<T> =
 
 export interface StartupOptions<T = unknown> extends ResolveParams {
   autorun?: AutorunOptions<T>;
+  onError?: (error: Error) => void;
 }
 
 // Overload: bootstrap(schema, factory)
@@ -94,11 +95,18 @@ export function bootstrap<
 
   const service: Service<S, T> = {
     async create(createOptions?: ResolveParams): Promise<{ server: T, config: ResolvedConfig<S> }> {
-      const config = resolveConfig(createOptions);
-      return {
-        server: await factory(config),
-        config,
-      };
+      try {
+        const config = resolveConfig(createOptions);
+        return {
+          server: await factory(config),
+          config,
+        };
+      } catch (error) {
+        if (options.onError && error instanceof Error) {
+          options.onError(error);
+        }
+        throw error;
+      }
     },
 
     async run(runOptions?: RunOptions): Promise<void> {
diff --git a/packages/bootstrap/test/bootstrap.test.ts b/packages/bootstrap/test/bootstrap.test.ts
index 1f7f178..88da408 100644
--- a/packages/bootstrap/test/bootstrap.test.ts
+++ b/packages/bootstrap/test/bootstrap.test.ts
@@ -6,7 +6,7 @@ import express from "express";
 import { writeFileSync, mkdtempSync, rmSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
-import { schema, field } from "confts";
+import { schema, field, ConfigError } from "confts";
 import { bootstrap } from "../src";
 
 // Mock server implementation (callback-based)
@@ -349,4 +349,37 @@ describe("bootstrap()", () => {
       await runPromise;
     });
   });
+
+  describe("onError", () => {
+    it("receives ConfigError with diagnostics on config failure", async () => {
+      const requiredSchema = schema({
+        port: field({ type: z.number(), default: 3000 }),
+        apiKey: field({ type: z.string() }), // required, no default
+      });
+      const mockServer = createMockServer();
+      const onError = vi.fn();
+
+      const service = bootstrap(requiredSchema, { onError, env: {} }, () => mockServer);
+
+      await expect(service.create()).rejects.toThrow(ConfigError);
+      expect(onError).toHaveBeenCalledOnce();
+      const err = onError.mock.calls[0][0] as ConfigError;
+      expect(err).toBeInstanceOf(ConfigError);
+      expect(err.diagnostics).toBeDefined();
+      expect(err.diagnostics!.some((d) => d.type === "sourceDecision" && d.key === "port")).toBe(true);
+    });
+
+    it("receives factory errors too", async () => {
+      const onError = vi.fn();
+      const factoryError = new Error("factory failed");
+
+      const service = bootstrap(configSchema, { onError }, () => {
+        throw factoryError;
+      });
+
+      await expect(service.create()).rejects.toThrow("factory failed");
+      expect(onError).toHaveBeenCalledOnce();
+      expect(onError).toHaveBeenCalledWith(factoryError);
+    });
+  });
 });
diff --git a/packages/confts/src/errors.ts b/packages/confts/src/errors.ts
index 2c50d92..40ade33 100644
--- a/packages/confts/src/errors.ts
+++ b/packages/confts/src/errors.ts
@@ -1,10 +1,13 @@
+import type { DiagnosticEvent } from "./types";
+
 export class ConfigError extends Error {
   name = "ConfigError";
 
   constructor(
     message: string,
     public readonly path: string,
-    public readonly sensitive: boolean
+    public readonly sensitive: boolean,
+    public readonly diagnostics?: DiagnosticEvent[]
   ) {
     super(message);
   }
diff --git a/packages/confts/src/resolve.ts b/packages/confts/src/resolve.ts
index 582f4eb..b8b723e 100644
--- a/packages/confts/src/resolve.ts
+++ b/packages/confts/src/resolve.ts
@@ -42,7 +42,8 @@ export function resolve<S extends ConftsSchema<Record<string, unknown>>>(
       throw new ConfigError(
         `Unsupported config file extension: ${ext}. Supported: ${supported || "none"}. Install @confts/yaml-loader for YAML support.`,
         configPath,
-        false
+        false,
+        collector.getEvents()
       );
     }
 
@@ -54,7 +55,8 @@ export function resolve<S extends ConftsSchema<Record<string, unknown>>>(
       throw new ConfigError(
         `Config file not found: ${configPath}`,
         configPath,
-        false
+        false,
+        collector.getEvents()
       );
     }
   } else {
diff --git a/packages/confts/src/values.ts b/packages/confts/src/values.ts
index 8e7862d..5c5e5af 100644
--- a/packages/confts/src/values.ts
+++ b/packages/confts/src/values.ts
@@ -198,7 +198,8 @@ function resolveValue(
     throw new ConfigError(
       `Missing required config at '${pathStr}' (value: ${formatValue(value, sensitive)})`,
       pathStr,
-      sensitive
+      sensitive,
+      collector.getEvents()
     );
   }
 
@@ -208,7 +209,8 @@ function resolveValue(
     throw new ConfigError(
       `Invalid config at '${pathStr}': ${result.error.issues[0]?.message} (value: ${formatValue(value, sensitive)})`,
       pathStr,
-      sensitive
+      sensitive,
+      collector.getEvents()
     );
   }
 
diff --git a/packages/confts/test/errors.test.ts b/packages/confts/test/errors.test.ts
index 9ffa87c..fd416c2 100644
--- a/packages/confts/test/errors.test.ts
+++ b/packages/confts/test/errors.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, expect } from "vitest";
 import { ConfigError, formatValue } from "../src/errors";
+import type { DiagnosticEvent } from "../src/types";
 
 describe("ConfigError", () => {
   it("stores path and sensitive flag", () => {
@@ -14,6 +15,19 @@ describe("ConfigError", () => {
     const err = new ConfigError("msg", "key", false);
     expect(err).toBeInstanceOf(Error);
   });
+
+  it("stores diagnostics when provided", () => {
+    const diagnostics: DiagnosticEvent[] = [
+      { type: "sourceDecision", key: "db.host", picked: "env:DB_HOST", tried: ["env:DB_HOST"] },
+    ];
+    const err = new ConfigError("missing value", "db.password", true, diagnostics);
+    expect(err.diagnostics).toEqual(diagnostics);
+  });
+
+  it("has undefined diagnostics when not provided", () => {
+    const err = new ConfigError("msg", "key", false);
+    expect(err.diagnostics).toBeUndefined();
+  });
 });
 
 describe("formatValue()", () => {
diff --git a/packages/confts/test/resolve.test.ts b/packages/confts/test/resolve.test.ts
index a4ba88b..1bd557b 100644
--- a/packages/confts/test/resolve.test.ts
+++ b/packages/confts/test/resolve.test.ts
@@ -32,6 +32,19 @@ describe("resolve()", () => {
       const s = schema({ key: field({ type: z.string() }) });
       expect(() => resolve(s, { configPath: "/nonexistent/config.json", env: {} })).toThrow(ConfigError);
     });
+
+    it("attaches diagnostics to ConfigError on missing file", () => {
+      const s = schema({ key: field({ type: z.string() }) });
+      try {
+        resolve(s, { configPath: "/nonexistent/config.json", env: {} });
+        expect.fail("should throw");
+      } catch (e) {
+        expect(e).toBeInstanceOf(ConfigError);
+        const err = e as ConfigError;
+        expect(err.diagnostics).toBeDefined();
+        expect(err.diagnostics!.some((d) => d.type === "loader" && d.used)).toBe(true);
+      }
+    });
   });
 
   describe("unsupported formats without loader", () => {
@@ -57,6 +70,21 @@ describe("resolve()", () => {
       expect(() => resolve(s, { configPath, env: {} })).toThrow(ConfigError);
       expect(() => resolve(s, { configPath, env: {} })).toThrow(/unsupported/i);
     });
+
+    it("attaches diagnostics to ConfigError on unsupported extension", () => {
+      const configPath = join(tempDir, "config.toml");
+      writeFileSync(configPath, "key = value");
+      const s = schema({ key: field({ type: z.string() }) });
+      try {
+        resolve(s, { configPath, env: {} });
+        expect.fail("should throw");
+      } catch (e) {
+        expect(e).toBeInstanceOf(ConfigError);
+        const err = e as ConfigError;
+        expect(err.diagnostics).toBeDefined();
+        expect(err.diagnostics!.some((d) => d.type === "loader" && !d.used)).toBe(true);
+      }
+    });
   });
 
   describe("CONFIG_PATH env var", () => {
diff --git a/packages/confts/test/values.test.ts b/packages/confts/test/values.test.ts
index 08360c2..68f3f52 100644
--- a/packages/confts/test/values.test.ts
+++ b/packages/confts/test/values.test.ts
@@ -197,6 +197,40 @@ describe("resolveValues()", () => {
       const s = schema({ port: field({ type: z.number(), env: "PORT" }) });
       expect(() => resolveValues(s, { env: { PORT: "not-a-number" } })).toThrow(ConfigError);
     });
+
+    it("attaches diagnostics to ConfigError on missing required", () => {
+      const s = schema({
+        port: field({ type: z.number(), default: 3000 }),
+        host: field({ type: z.string(), env: "HOST" }),
+      });
+      try {
+        resolveValues(s, { env: {} });
+        expect.fail("should throw");
+      } catch (e) {
+        expect(e).toBeInstanceOf(ConfigError);
+        const err = e as ConfigError;
+        expect(err.diagnostics).toBeDefined();
+        // port resolved before host failed
+        expect(err.diagnostics!.some((d) => d.type === "sourceDecision" && d.key === "port")).toBe(true);
+      }
+    });
+
+    it("attaches diagnostics to ConfigError on validation fail", () => {
+      const s = schema({
+        host: field({ type: z.string(), default: "localhost" }),
+        port: field({ type: z.number(), env: "PORT" }),
+      });
+      try {
+        resolveValues(s, { env: { PORT: "not-a-number" } });
+        expect.fail("should throw");
+      } catch (e) {
+        expect(e).toBeInstanceOf(ConfigError);
+        const err = e as ConfigError;
+        expect(err.diagnostics).toBeDefined();
+        // host resolved before port failed validation
+        expect(err.diagnostics!.some((d) => d.type === "sourceDecision" && d.key === "host")).toBe(true);
+      }
+    });
   });
 
   describe("secretsPath", () => {