diff --git a/packages/prompt-variance/src/index.ts b/packages/prompt-variance/src/index.ts
index c535243e2..53064fb82 100644
--- a/packages/prompt-variance/src/index.ts
+++ b/packages/prompt-variance/src/index.ts
@@ -17,9 +17,16 @@ export {
 // padding whitespace), so no exact tool fingerprint repeats 3x and the shared
 // threshold guard stays silent. This forbids that specific variation
 // (muse-rule precedent, CL-7869); no signature normalization ships in
-// first-party code. Named export rather than a family row: it travels the
+// first-party code. The re-read bullet is narrowed, not absolute (CL-9921):
+// it discourages redundant unchanged re-read loops while permitting
+// necessary re-reads (unread pages, changed content, targeted uncertainty
+// resolution, compaction-lost evidence) with a stated reason and bounded
+// scope. Named export rather than a family row: it travels the
 // ModelFamilyPolicy.promptResidual seam, composed over the gpt narrate note.
 export const astraResidual: string =
   "Tool discipline (gpt-6-astra worker):\n" +
   "- Never re-issue a tool call that repeats a prior call with only trivial argument changes (path prefix, offset, limit, whitespace).\n" +
-  "- Never re-read a file you have already read this session.";
+  "- Avoid re-reading content you already have; do not loop full-file re-reads that add no new information.\n" +
+  "- A re-read is permitted when it adds information: unread pages (a new offset/limit range), content that changed since the last read, a targeted re-read to resolve a specific uncertainty, or evidence lost to compaction.\n" +
+  "- A necessary re-read states its reason and bounded scope first (path, range, and the question it answers) and does not restart a broad investigation.\n" +
+  "- Boundary: the same path with default offset/limit after a full read is redundant; the same path with a new page range or after an edit is necessary.";
diff --git a/src/agent/astra-reread-guidance.test.ts b/src/agent/astra-reread-guidance.test.ts
new file mode 100644
index 000000000..d7f0d37d7
--- /dev/null
+++ b/src/agent/astra-reread-guidance.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, test } from "bun:test";
+import { ASTRA_PROMPT_RESIDUAL } from "./model-family-policy.js";
+
+// CL-9921: the astra residual permits necessary re-reads while still
+// discouraging redundant unchanged re-read loops. Each test names the
+// scripted scenario the wording must allow (or, for the negative
+// control, keep forbidding).
+describe("astra necessary-reread guidance (CL-9921)", () => {
+  test("changed-file scenario: re-reading content that changed since the last read is permitted", () => {
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("changed since");
+  });
+
+  test("paginated-report scenario: reading unread pages is permitted", () => {
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("unread pages");
+  });
+
+  test("compacted-evidence scenario: targeted re-reads that resolve uncertainty or recover compaction-lost evidence are permitted, with a stated reason and bounded scope", () => {
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("uncertainty");
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("compaction");
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("reason and bounded scope");
+  });
+
+  test("unchanged-repetition negative control: unchanged full-file re-read loops stay discouraged, the absolute ban is gone, and the boundary is stated", () => {
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("full-file re-read");
+    expect(ASTRA_PROMPT_RESIDUAL).toContain("Boundary");
+    expect(ASTRA_PROMPT_RESIDUAL).not.toContain(
+      "Never re-read a file you have already read",
+    );
+  });
+});