diff --git a/packages/unplugin/src/core.ts b/packages/unplugin/src/core.ts
index 2b6592f48..30273a25b 100644
--- a/packages/unplugin/src/core.ts
+++ b/packages/unplugin/src/core.ts
@@ -313,6 +313,9 @@ interface WebpackLikeCompiler {
   hooks: {
     done: { tap: (name: string, fn: () => void) => void };
     failed?: { tap: (name: string, fn: () => void) => void };
+    /** `close()` of a watch, which webpack fires in place of `done` when it
+     *  closes mid-compile. */
+    watchClose?: { tap: (name: string, fn: () => void) => void };
     normalModuleFactory: {
       tap: (
         name: string,
@@ -403,6 +406,18 @@ export const unpluginFactory: UnpluginFactory<
 
   async function startPipeline(): Promise<void> {
     signalPipelineStarted();
+    // A webpack compile that ended without done or failed (it restarts one
+    // an edit invalidated, and skips both) left its Session joined and its
+    // claim held: retire it before this run claims the build again. A
+    // webpack compiler runs one compile at a time; the Rollup family can
+    // start environments in parallel, so they keep theirs.
+    if (
+      activeSession !== null &&
+      (meta.framework === 'webpack' || meta.framework === 'rspack')
+    ) {
+      disposeSessionDir(state);
+      releaseClaim();
+    }
     try {
       await runClaimedPipeline();
     } catch (error) {
@@ -933,5 +948,10 @@ export const unpluginFactory: UnpluginFactory<
       disposeSessionDir(state);
       releaseClaim();
     });
+    // A watch closed mid-compile fires neither: the build retires here.
+    compiler.hooks.watchClose?.tap(PLUGIN_NAME, () => {
+      disposeSessionDir(state);
+      releaseClaim();
+    });
   }
 };
diff --git a/packages/unplugin/tests/failed-load-watch.test.ts b/packages/unplugin/tests/failed-load-watch.test.ts
index 92aaf5815..50c0b1e32 100644
--- a/packages/unplugin/tests/failed-load-watch.test.ts
+++ b/packages/unplugin/tests/failed-load-watch.test.ts
@@ -1,12 +1,17 @@
+import { existsSync, readdirSync } from 'node:fs';
 import { join } from 'node:path';
 import { afterEach, describe, expect, test, vi } from 'vitest';
 
 import { setEngineApiOverride } from '../../extract/session/singleton';
 import {
+  buildManifest,
   createProject,
   disposeTempRoots,
+  PLAN_A,
   resetAnimusGlobals,
+  SYSTEM_CONFIG,
 } from '../../extract/tests/session/session-fixtures';
+import { parserStoppingAt } from '../../extract/tests/source-ingestion-fixtures';
 import { unpluginFactory } from '../src/core';
 
 let restoreGlobals: (() => void) | null = null;
@@ -60,3 +65,99 @@ describe("a failed load's observations are watched", () => {
     expect(watched).toEqual(expect.arrayContaining([palette, absent]));
   });
 });
+
+/** A webpack compiler's hooks, recorded so a row can fire them. */
+function fakeWebpackCompiler() {
+  const done: Array<() => void> = [];
+  const failed: Array<() => void> = [];
+  const watchClose: Array<() => void> = [];
+  const taps = { done, failed, watchClose };
+  const hook = (name: keyof typeof taps) => ({
+    tap: (_plugin: string, fn: () => void) => {
+      taps[name].push(fn);
+    },
+  });
+  const compiler = {
+    options: { mode: 'development', resolve: {} },
+    watchMode: true,
+    webpack: {
+      version: '5.105.4',
+      DefinePlugin: class {
+        apply() {}
+      },
+    },
+    hooks: {
+      done: hook('done'),
+      failed: hook('failed'),
+      watchClose: hook('watchClose'),
+      normalModuleFactory: { tap: () => {} },
+    },
+  };
+  return {
+    compiler,
+    fire: (name: keyof typeof taps) => taps[name].forEach((fn) => fn()),
+  };
+}
+
+/** A webpack plugin instance over a project whose system loads, wired to a
+ *  fake compiler; `make` runs its buildStart, which webpack maps to make. */
+function webpackHost(root: string) {
+  const { compiler, fire } = fakeWebpackCompiler();
+  const plugin = unpluginFactory(
+    { system: './src/system.ts', root },
+    // SAFETY: the fake models every compiler member the plugin's webpack
+    // wiring reads (options, watchMode, webpack.DefinePlugin, hooks).
+    {
+      framework: 'webpack',
+      versions: {},
+      webpack: { compiler: compiler as never },
+    }
+  );
+  if (Array.isArray(plugin) || !plugin.webpack || !plugin.buildStart) {
+    throw new Error('expected one plugin with webpack and buildStart hooks');
+  }
+  // SAFETY: as above.
+  plugin.webpack(compiler as never);
+  const buildStart = plugin.buildStart;
+  // SAFETY: the hook reads nothing from its context under webpack.
+  const make = () => buildStart.call({} as never);
+  return { make, fire };
+}
+
+describe('a webpack compile that ends without done or failed', () => {
+  function loadableSystem(): string {
+    restoreGlobals = resetAnimusGlobals();
+    const root = createProject('animus-unplugin-webpack-');
+    setEngineApiOverride(() => ({
+      extractFacts: parserStoppingAt('(('),
+      loadSystemModule: () => ({ ...SYSTEM_CONFIG, dependencies: [] }),
+      analyzeProject: () => buildManifest(PLAN_A),
+      transformFile: (source: string) => ({
+        code: source,
+        hasComponents: false,
+      }),
+      clearAnalysisCache: () => {},
+    }));
+    return root;
+  }
+
+  test('an edit that restarts the compile never strands its Session', async () => {
+    const { make } = webpackHost(loadableSystem());
+
+    await make();
+    // webpack restarted the invalidated compile: no done, no failed.
+    await expect(make()).resolves.toBeUndefined();
+  });
+
+  test('a watch closed mid-compile retires its build', async () => {
+    const root = loadableSystem();
+    const sessions = join(root, '.animus', 'sessions');
+    const { make, fire } = webpackHost(root);
+    await make();
+    expect(readdirSync(sessions)).toHaveLength(1);
+
+    // webpack closed the watch mid-compile: watchClose, no done.
+    fire('watchClose');
+    expect(existsSync(sessions) ? readdirSync(sessions) : []).toEqual([]);
+  });
+});