diff --git a/src/application/ProviderOperationExecution.test.ts b/src/application/ProviderOperationExecution.test.ts
index 8da9d83cf2..7d456e9d19 100644
--- a/src/application/ProviderOperationExecution.test.ts
+++ b/src/application/ProviderOperationExecution.test.ts
@@ -136,3 +136,25 @@ it("clears a non-JSON partial from the cleanup primary projection returns after
     "partial_observation",
   );
 });
+
+it("freezes a retained partial so a later mutation cannot break projection", () => {
+  const partial = { steps: [] as unknown[] };
+  const primary = new AnalysisCancelledError("capture_browser_scenario", {
+    cause: { requested: "stop during capture" },
+    partialObservation: partial as never,
+  });
+  keepIncompleteObservation(
+    primary,
+    "capture_browser_scenario",
+    "unused because the partial is JSON",
+  );
+  partial.steps.push(undefined);
+
+  expect(primary.partialObservation).toEqual({ steps: [] });
+  expect(Object.isFrozen(primary.partialObservation)).toBe(true);
+  expect(primary.secondaryFailures).toHaveLength(0);
+  expect(projectAnalysisError(primary)).toMatchObject({
+    code: "cancelled",
+    details: { partial_observation: { steps: [] } },
+  });
+});
diff --git a/src/application/ProviderOperationExecution.ts b/src/application/ProviderOperationExecution.ts
index 60e196a3a6..924f33a9ff 100644
--- a/src/application/ProviderOperationExecution.ts
+++ b/src/application/ProviderOperationExecution.ts
@@ -9,7 +9,14 @@ import {
   AnalysisOutputError,
 } from "../domain/analysisErrorCore.js";
 import type { Evidence } from "../domain/evidence.js";
-import { jsonValueValidationIssue } from "../domain/jsonValue.js";
+import {
+  freezeJsonSnapshot,
+  isImmutableJsonSnapshot,
+} from "../domain/immutableJson.js";
+import {
+  jsonValueSchema,
+  jsonValueValidationIssue,
+} from "../domain/jsonValue.js";
 import { projectInputIssues } from "../domain/inputIssueProjection.js";
 import { err, ok, type Result } from "../domain/result.js";
 import {
@@ -97,6 +104,8 @@ export const completeProviderEvidence = (
 
 /**
  * Retain an incomplete observation when projection can serialize it.
+ * A JSON partial is replaced by one frozen snapshot, so a later mutation
+ * cannot make projection throw and projection does not copy it again.
  * A partial that is not JSON is cleared from the returned error and from
  * every cleanup primary projection can select later. Each cleared error
  * records an output failure, leaving cancellation or timeout primary.
@@ -111,15 +120,18 @@ export const keepIncompleteObservation = (
   while (current !== undefined && !seen.has(current)) {
     seen.add(current);
     const partial = current.partialObservation;
-    // Validate without cloning. Projection serializes a kept partial later.
-    if (
-      partial !== undefined &&
-      jsonValueValidationIssue(partial) !== undefined
-    ) {
-      current.partialObservation = undefined;
-      current.retainSecondaryFailure(
-        new AnalysisOutputError(operation, reason),
-      );
+    if (partial !== undefined && !isImmutableJsonSnapshot(partial)) {
+      // Validate without cloning. Copy only a partial that projection keeps.
+      if (jsonValueValidationIssue(partial) === undefined) {
+        current.partialObservation = freezeJsonSnapshot(
+          jsonValueSchema.parse(partial),
+        ) as typeof partial;
+      } else {
+        current.partialObservation = undefined;
+        current.retainSecondaryFailure(
+          new AnalysisOutputError(operation, reason),
+        );
+      }
     }
     current = current.cleanupPrimary;
   }
diff --git a/src/application/ownerObservationExpectation.fixture.ts b/src/application/ownerObservationExpectation.fixture.ts
index d466c57812..f71a73e49c 100644
--- a/src/application/ownerObservationExpectation.fixture.ts
+++ b/src/application/ownerObservationExpectation.fixture.ts
@@ -22,7 +22,7 @@ export const expectKeptPartial = (
   partial: unknown,
   code: "cancelled" | "provider_timeout" = "cancelled",
 ): void => {
-  expect(error.partialObservation).toBe(partial);
+  expect(error.partialObservation).toEqual(partial);
   expect(error.secondaryFailures).toHaveLength(0);
   expect(projectAnalysisError(error)).toMatchObject({
     code,