diff --git a/src/application/desktop/DesktopApplicationState.test.ts b/src/application/desktop/DesktopApplicationState.test.ts
index 8ccc3d3b0f..2be605bd5e 100644
--- a/src/application/desktop/DesktopApplicationState.test.ts
+++ b/src/application/desktop/DesktopApplicationState.test.ts
@@ -910,6 +910,7 @@ describe("inspectDesktopApplicationState database sidecars", () => {
 
   it("reports cancellation during the last database read instead of Evidence", async () => {
     const controller = new AbortController();
+    const reason = { stage: "last database read" };
     let reads = 0;
     const folder = await writeCaptureFolder(
       extended({
@@ -925,7 +926,7 @@ describe("inspectDesktopApplicationState database sidecars", () => {
         ...readers,
         sqlite: async (database, signal) => {
           reads += 1;
-          if (reads === 3) controller.abort(new Error("stopped"));
+          if (reads === 3) controller.abort(reason);
           return readers.sqlite(database, signal);
         },
       },
@@ -934,6 +935,9 @@ describe("inspectDesktopApplicationState database sidecars", () => {
 
     expect(reads).toBe(3);
     expect(result).toBeInstanceOf(AnalysisCancelledError);
+    expect(result instanceof AnalysisCancelledError && result.cause).toBe(
+      reason,
+    );
   });
 });
 
diff --git a/src/application/desktop/DesktopBuildSolves.test.ts b/src/application/desktop/DesktopBuildSolves.test.ts
index 0cbcf77fc9..9648775030 100644
--- a/src/application/desktop/DesktopBuildSolves.test.ts
+++ b/src/application/desktop/DesktopBuildSolves.test.ts
@@ -321,6 +321,7 @@ it.each([
       expect(result.error.cleanupResources).toContain(join(bundle, suffix));
       if (cancel) {
         expect(result.error._tag).toBe("AnalysisCancelledError");
+        expect(result.error.cause).toBe(reason);
         expect(result.error.partialObservation).toMatchObject({
           kind: "partial-task-measurement",
           solves: [{}, {}, {}],
diff --git a/src/application/desktop/DesktopBuildTurns.test.ts b/src/application/desktop/DesktopBuildTurns.test.ts
index 33705bfd9a..b74464ce81 100644
--- a/src/application/desktop/DesktopBuildTurns.test.ts
+++ b/src/application/desktop/DesktopBuildTurns.test.ts
@@ -57,11 +57,14 @@ it("keeps the place of a call cancelled while it waits, so the one after it stil
     return ok("last");
   });
   await settled();
-  caller.abort(new Error("the client went away"));
-  expect(await cancelled).toMatchObject({
+  const reason = { caller: "the client went away" };
+  caller.abort(reason);
+  const cancellation = await cancelled;
+  expect(cancellation).toMatchObject({
     ok: false,
     error: { _tag: "AnalysisCancelledError", operation: "cancelled" },
   });
+  expect(!cancellation.ok && cancellation.error.cause).toBe(reason);
   await settled();
   expect(log).toEqual(["running"]);
   running.end();
diff --git a/src/application/desktop/DesktopRemoteTaskReaders.test.ts b/src/application/desktop/DesktopRemoteTaskReaders.test.ts
index f2aac76297..e8eb37d366 100644
--- a/src/application/desktop/DesktopRemoteTaskReaders.test.ts
+++ b/src/application/desktop/DesktopRemoteTaskReaders.test.ts
@@ -81,9 +81,9 @@ it("waits for an interrupted reader before removing its folder", async () => {
 
 it("preserves a completed view when cancellation arrives during successful cleanup", async () => {
   const controller = new AbortController();
+  const reason = { stage: "reader final cleanup" };
   const owner = new DesktopRemoteTaskReaders(async (argv) => {
-    if (argv[3] === PYTHON_READER_CLEANUP)
-      controller.abort(new Error("cancelled"));
+    if (argv[3] === PYTHON_READER_CLEANUP) controller.abort(reason);
     return success;
   });
   const failure = await owner
@@ -91,6 +91,7 @@ it("preserves a completed view when cancellation arrives during successful clean
     .catch((error: unknown) => error);
   if (!(failure instanceof AnalysisCancelledError))
     throw new Error("expected cancellation");
+  expect(failure.cause).toBe(reason);
   expect(failure.diagnostics).toMatchObject({
     completed_reader_views: { note: { view: { value: 7 } } },
   });
@@ -99,12 +100,13 @@ it("preserves a completed view when cancellation arrives during successful clean
 
 it("keeps cancellation and a completed view when final cleanup also fails", async () => {
   const controller = new AbortController();
+  const reason = { stage: "reader final cleanup" };
   let fail = true;
   const owner = new DesktopRemoteTaskReaders(async (argv) => {
     if (argv[3] !== PYTHON_READER_CLEANUP) return success;
     if (fail) {
       fail = false;
-      controller.abort(new Error("cancelled"));
+      controller.abort(reason);
       return { ...success, exit_code: 1, stderr: "denied" };
     }
     return success;
@@ -115,6 +117,7 @@ it("keeps cancellation and a completed view when final cleanup also fails", asyn
   if (!(failure instanceof ProviderCleanupError))
     throw new Error("expected cleanup failure");
   expect(failure.cleanupPrimary).toBeInstanceOf(AnalysisCancelledError);
+  expect(failure.cleanupPrimary?.cause).toBe(reason);
   expect(failure.diagnostics).toMatchObject({
     completed_reader_views: { note: { view: { value: 7 } } },
   });
diff --git a/src/browser/CdpCaptureValues.test.ts b/src/browser/CdpCaptureValues.test.ts
index 83d5b5adf4..ed95cb551b 100644
--- a/src/browser/CdpCaptureValues.test.ts
+++ b/src/browser/CdpCaptureValues.test.ts
@@ -37,7 +37,9 @@ describe("cancellable browser observation delays", () => {
     ).catch((error: unknown) => error);
 
     expect(beforeStart).toBeInstanceOf(AnalysisCancelledError);
-    expect(beforeStart).toMatchObject({ cause: preAbortedReason });
+    expect(
+      beforeStart instanceof AnalysisCancelledError && beforeStart.cause,
+    ).toBe(preAbortedReason);
 
     vi.useFakeTimers();
     const reason = new Error("capture wait cancelled");
@@ -51,7 +53,9 @@ describe("cancellable browser observation delays", () => {
     const duringWait = await waiting;
 
     expect(duringWait).toBeInstanceOf(AnalysisCancelledError);
-    expect(duringWait).toMatchObject({ cause: reason });
+    expect(
+      duringWait instanceof AnalysisCancelledError && duringWait.cause,
+    ).toBe(reason);
     expect(vi.getTimerCount()).toBe(0);
   });
 });
diff --git a/src/browser/CdpEndpoint.test.ts b/src/browser/CdpEndpoint.test.ts
index 9daa4e4a4f..5f98fe9970 100644
--- a/src/browser/CdpEndpoint.test.ts
+++ b/src/browser/CdpEndpoint.test.ts
@@ -136,7 +136,9 @@ describe("CDP discovery cancellation ownership", () => {
     ).catch((error: unknown) => error);
 
     expect(failure).toBeInstanceOf(AnalysisCancelledError);
-    expect(failure).toMatchObject({ cause: reason });
+    expect(failure instanceof AnalysisCancelledError && failure.cause).toBe(
+      reason,
+    );
     expect(requests).toBe(0);
   });
 
@@ -164,7 +166,9 @@ describe("CDP discovery cancellation ownership", () => {
 
     expect(requestedPath).toBe("/json/version");
     expect(failure).toBeInstanceOf(AnalysisCancelledError);
-    expect(failure).toMatchObject({ cause: reason });
+    expect(failure instanceof AnalysisCancelledError && failure.cause).toBe(
+      reason,
+    );
   });
 
   it("does not replace a primary discovery failure when cancellation arrives later", async () => {
@@ -259,7 +263,9 @@ describe("CDP discovery cancellation ownership", () => {
     ).catch((error: unknown) => error);
 
     expect(failure).toBeInstanceOf(AnalysisCancelledError);
-    expect(failure).toMatchObject({ cause: reason });
+    expect(failure instanceof AnalysisCancelledError && failure.cause).toBe(
+      reason,
+    );
   });
 
   it("keeps a deadline rejection when the caller aborts only after timeout", async () => {
diff --git a/src/browser/CdpPageCaptureScripts.test.ts b/src/browser/CdpPageCaptureScripts.test.ts
index 19b7bfccc0..93fd023d1f 100644
--- a/src/browser/CdpPageCaptureScripts.test.ts
+++ b/src/browser/CdpPageCaptureScripts.test.ts
@@ -112,15 +112,17 @@ it("preserves the caller cancellation reason when a source read is pending", asy
     signal: controller.signal,
   } as unknown as CaptureContext;
 
-  await expect(
-    captureScripts({
-      context,
-      events,
-      rawResources: [],
-      resources: [],
-      frameIds: new Set(),
-    }),
-  ).rejects.toMatchObject({ _tag: "AnalysisCancelledError", cause: reason });
+  const failure = await captureScripts({
+    context,
+    events,
+    rawResources: [],
+    resources: [],
+    frameIds: new Set(),
+  }).catch((error: unknown) => error);
+  expect(failure).toBeInstanceOf(AnalysisCancelledError);
+  expect(failure instanceof AnalysisCancelledError && failure.cause).toBe(
+    reason,
+  );
 });
 
 it.each([