diff --git a/changelog.d/fixes/6792-compression-put-all-catalog-engines.md b/changelog.d/fixes/6792-compression-put-all-catalog-engines.md
new file mode 100644
index 0000000000..813620e098
--- /dev/null
+++ b/changelog.d/fixes/6792-compression-put-all-catalog-engines.md
@@ -0,0 +1 @@
+- **fix(api):** the compression config `PUT` schema (`stackedPipelineStepSchema`) now accepts every `ENGINE_CATALOG` id — the structural engines `session-dedup`/`ccr`/`headroom`/`relevance`/`llmlingua`/`omniglyph` and the `aggressive` `ultra` intensity — so a `GET`→`PUT` round-trip of a stacked pipeline no longer 400s on a valid engine the discriminated union had omitted (#6747 — thanks @Pitchfork-and-Torch).
diff --git a/open-sse/services/compression/types.ts b/open-sse/services/compression/types.ts
index fa973fd44e..9bba7973b2 100644
--- a/open-sse/services/compression/types.ts
+++ b/open-sse/services/compression/types.ts
@@ -39,6 +39,7 @@ export type CompressionEngineId =
   | "headroom"
   | "ccr"
   | "llmlingua"
+  | "relevance"
   | "omniglyph";
 
 export interface CavemanRule {
diff --git a/src/lib/db/compression.ts b/src/lib/db/compression.ts
index e5c8372992..c1e92101fe 100644
--- a/src/lib/db/compression.ts
+++ b/src/lib/db/compression.ts
@@ -260,8 +260,9 @@ function normalizeContextEditingConfig(value: unknown): ContextEditingConfig {
 }
 
 // Engines allowed in the global stackedPipeline setting. MUST stay in sync with the
-// compression-combo KNOWN_ENGINE_IDS (src/lib/db/compressionCombos.ts) — otherwise the
-// global setting silently strips engines the combo path accepts (B-PIPELINE-DIVERGENCE).
+// compression-combo KNOWN_ENGINE_IDS (src/lib/db/compressionCombos.ts) and with
+// stackedPipelineStepSchema / ENGINE_CATALOG — otherwise the global setting silently
+// strips engines the combo path accepts (B-PIPELINE-DIVERGENCE / #6747).
 const STACKED_PIPELINE_ENGINE_IDS = new Set([
   "lite",
   "caveman",
@@ -272,6 +273,7 @@ const STACKED_PIPELINE_ENGINE_IDS = new Set([
   "session-dedup",
   "ccr",
   "llmlingua",
+  "relevance",
   "omniglyph",
 ]);
 
@@ -657,8 +659,7 @@ export async function getCompressionSettings(): Promise<CompressionConfig> {
         storedEngines = parseStoredEnginesMap(parsed);
         break;
       case "activeComboId":
-        config.activeComboId =
-          typeof parsed === "string" && parsed.trim() ? parsed.trim() : null;
+        config.activeComboId = typeof parsed === "string" && parsed.trim() ? parsed.trim() : null;
         break;
       case "ultraEngine":
         // Phase 4 (B): SLM tier selector. Only the two known values; anything else
diff --git a/src/lib/db/compressionCombos.ts b/src/lib/db/compressionCombos.ts
index e7f3dcb888..e4333509bd 100644
--- a/src/lib/db/compressionCombos.ts
+++ b/src/lib/db/compressionCombos.ts
@@ -56,6 +56,7 @@ function parseJsonArray<T>(value: unknown, fallback: T[]): T[] {
   }
 }
 
+// Keep in sync with stackedPipelineStepSchema + ENGINE_CATALOG (#6747).
 const KNOWN_ENGINE_IDS = [
   "lite",
   "caveman",
@@ -66,6 +67,7 @@ const KNOWN_ENGINE_IDS = [
   "session-dedup",
   "ccr",
   "llmlingua",
+  "relevance",
 ];
 
 function normalizePipeline(value: unknown): CompressionPipelineStep[] {
@@ -92,8 +94,7 @@ function upgradeLegacySeededDefaultCompressionCombo(): void {
   const row = db
     .prepare("SELECT name, description, pipeline FROM compression_combos WHERE id = ?")
     .get(DEFAULT_COMPRESSION_COMBO_ID) as
-    | { name?: string; description?: string; pipeline?: string }
-    | undefined;
+    { name?: string; description?: string; pipeline?: string } | undefined;
 
   if (!row) return;
 
diff --git a/src/shared/validation/compressionConfigSchemas.ts b/src/shared/validation/compressionConfigSchemas.ts
index bdebd8e63a..c57be7dd7d 100644
--- a/src/shared/validation/compressionConfigSchemas.ts
+++ b/src/shared/validation/compressionConfigSchemas.ts
@@ -141,6 +141,19 @@ export const ultraConfigSchema = z
 
 const noConfigSchema = z.object({}).strict();
 
+// Structural engines (session-dedup / ccr / headroom / relevance / llmlingua) do not
+// expose a fixed intensity enum in ENGINE_CATALOG — accept optional free-form intensity
+// and a loose config bag so GET→PUT round-trips of stackedPipeline succeed (#6747).
+const structuralStepConfigSchema = z.record(z.string(), z.unknown()).optional();
+
+/**
+ * Writable stacked-pipeline step shape for PUT /api/settings/compression and
+ * PUT /api/context/combos/[id]. MUST accept every engine id in ENGINE_CATALOG /
+ * GET /api/compression/engines and every engine the DB normalizer keeps
+ * (src/lib/db/compression.ts STACKED_PIPELINE_ENGINE_IDS). Issue #6747: a 5-engine
+ * discriminator rejected session-dedup/ccr/headroom/relevance/llmlingua on write even
+ * though GET returned them.
+ */
 export const stackedPipelineStepSchema = z.discriminatedUnion("engine", [
   z
     .object({
@@ -159,7 +172,8 @@ export const stackedPipelineStepSchema = z.discriminatedUnion("engine", [
   z
     .object({
       engine: z.literal("aggressive"),
-      intensity: z.literal("standard").optional(),
+      // #6747: previously only "standard"; GET / engines.level may echo "ultra"
+      intensity: z.enum(["standard", "ultra"]).optional(),
       config: aggressiveConfigSchema.optional(),
     })
     .strict(),
@@ -177,6 +191,48 @@ export const stackedPipelineStepSchema = z.discriminatedUnion("engine", [
       config: rtkConfigSchema.optional(),
     })
     .strict(),
+  z
+    .object({
+      engine: z.literal("session-dedup"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
+  z
+    .object({
+      engine: z.literal("ccr"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
+  z
+    .object({
+      engine: z.literal("headroom"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
+  z
+    .object({
+      engine: z.literal("relevance"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
+  z
+    .object({
+      engine: z.literal("llmlingua"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
+  z
+    .object({
+      engine: z.literal("omniglyph"),
+      intensity: z.string().optional(),
+      config: structuralStepConfigSchema,
+    })
+    .strict(),
 ]);
 
 /**
@@ -185,17 +241,23 @@ export const stackedPipelineStepSchema = z.discriminatedUnion("engine", [
  * dropdowns and `stackedPipelineStepSchema`: every engine/intensity offered here is,
  * by construction, accepted by the API update schema.
  *
- * Do NOT add an engine here that is not a branch of `stackedPipelineStepSchema` — the
- * `PUT /api/context/combos/[id]` route validates against that discriminated union and
- * would reject the payload with HTTP 400 (#4955: the UI previously offered `headroom`,
- * `session-dedup`, `ccr`, `llmlingua`, none of which the union accepts, so selecting
- * one silently failed the save). The parity is guarded by a unit test.
+ * Every ENGINE_CATALOG id must appear here (empty intensity list = no level selector).
+ * Parity with `stackedPipelineStepSchema` is guarded by unit tests (#4955 / #6747).
+ * #4955 fixed a UI/schema drift by shrinking the UI; #6747 expands the schema so the
+ * full catalog (and GET stackedPipeline) can round-trip on PUT.
  */
 export const STACKED_PIPELINE_ENGINE_INTENSITIES: Record<string, readonly string[]> = {
-  rtk: ["minimal", "standard", "aggressive"],
-  caveman: ["lite", "full", "ultra"],
+  // Order matches ENGINE_CATALOG stackPriority for readability
+  "session-dedup": [],
+  ccr: [],
   lite: ["lite"],
-  aggressive: ["standard"],
+  rtk: ["minimal", "standard", "aggressive"],
+  headroom: [],
+  relevance: [],
+  caveman: ["lite", "full", "ultra"],
+  aggressive: ["standard", "ultra"],
+  llmlingua: [],
+  omniglyph: [],
   ultra: ["ultra"],
 };
 
diff --git a/tests/unit/compression/combos-engine-ui-schema-parity-4955.test.ts b/tests/unit/compression/combos-engine-ui-schema-parity-4955.test.ts
index 2162dd765d..4855faa60b 100644
--- a/tests/unit/compression/combos-engine-ui-schema-parity-4955.test.ts
+++ b/tests/unit/compression/combos-engine-ui-schema-parity-4955.test.ts
@@ -3,15 +3,17 @@ import assert from "node:assert/strict";
 
 import {
   STACKED_PIPELINE_ENGINE_INTENSITIES,
+  compressionSettingsUpdateSchema,
   stackedPipelineStepSchema,
 } from "../../../src/shared/validation/compressionConfigSchemas.ts";
+import { ENGINE_IDS } from "../../../open-sse/services/compression/engineCatalog.ts";
 
-// Regression guard for #4955: the Engine Combos pipeline editor used to offer engines
-// (headroom, session-dedup, ccr, llmlingua) that `stackedPipelineStepSchema` rejects, so
-// selecting one made `PUT /api/context/combos/[id]` fail with HTTP 400 and the UI swallowed
-// it. The fix routes the dropdown through STACKED_PIPELINE_ENGINE_INTENSITIES, which MUST stay
-// in lockstep with the discriminated union below.
-describe("Engine Combos UI ↔ stackedPipelineStepSchema parity (#4955)", () => {
+// Regression guard for #4955 / #6747:
+// - #4955: UI and API schema must stay in lockstep (no engines the UI offers that PUT rejects).
+// - #6747: PUT must accept every ENGINE_CATALOG / GET stackedPipeline engine so GET→PUT
+//   round-trips of compression settings succeed (session-dedup, ccr, headroom, relevance,
+//   llmlingua were previously rejected by a 5-engine discriminator).
+describe("Engine Combos UI ↔ stackedPipelineStepSchema parity (#4955 / #6747)", () => {
   const unionEngines = stackedPipelineStepSchema.options
     .map((option: { shape: { engine: { value: string } } }) => option.shape.engine.value)
     .sort();
@@ -21,8 +23,18 @@ describe("Engine Combos UI ↔ stackedPipelineStepSchema parity (#4955)", () =>
     assert.deepEqual(uiEngines, unionEngines);
   });
 
+  it("covers every ENGINE_CATALOG id (GET /api/compression/engines parity)", () => {
+    assert.deepEqual([...ENGINE_IDS].sort(), unionEngines);
+  });
+
   it("every (engine, intensity) the UI can emit is accepted by the schema", () => {
     for (const [engine, intensities] of Object.entries(STACKED_PIPELINE_ENGINE_INTENSITIES)) {
+      // Engines with no level selector still need bare { engine } accepted
+      assert.equal(
+        stackedPipelineStepSchema.safeParse({ engine }).success,
+        true,
+        `expected bare { engine: "${engine}" } to be accepted`
+      );
       for (const intensity of intensities) {
         const result = stackedPipelineStepSchema.safeParse({ engine, intensity });
         assert.equal(
@@ -34,18 +46,37 @@ describe("Engine Combos UI ↔ stackedPipelineStepSchema parity (#4955)", () =>
     }
   });
 
-  it("the engines removed from the UI in #4955 are indeed rejected by the schema", () => {
-    for (const engine of ["headroom", "session-dedup", "ccr", "llmlingua"]) {
+  it("accepts structural catalog engines that #4955 had temporarily dropped from the UI (#6747)", () => {
+    for (const engine of ["headroom", "session-dedup", "ccr", "llmlingua", "relevance"]) {
       assert.equal(
-        stackedPipelineStepSchema.safeParse({ engine, intensity: "standard" }).success,
-        false,
-        `engine "${engine}" must not be a valid stacked-pipeline step`
+        stackedPipelineStepSchema.safeParse({ engine }).success,
+        true,
+        `engine "${engine}" must be a valid stacked-pipeline step`
       );
-      assert.equal(
-        STACKED_PIPELINE_ENGINE_INTENSITIES[engine],
-        undefined,
-        `engine "${engine}" must not be offered by the combos UI`
+      assert.ok(
+        Object.prototype.hasOwnProperty.call(STACKED_PIPELINE_ENGINE_INTENSITIES, engine),
+        `engine "${engine}" must be offered by the combos UI`
       );
     }
   });
+
+  it("accepts a full GET-shaped stackedPipeline on settings update (#6747)", () => {
+    const result = compressionSettingsUpdateSchema.safeParse({
+      stackedPipeline: [
+        { engine: "session-dedup" },
+        { engine: "ccr" },
+        { engine: "lite", intensity: "lite" },
+        { engine: "rtk", intensity: "standard" },
+        { engine: "headroom" },
+        { engine: "relevance" },
+        { engine: "caveman", intensity: "full" },
+        { engine: "aggressive", intensity: "ultra" },
+        { engine: "llmlingua" },
+        { engine: "ultra", intensity: "ultra" },
+      ],
+    });
+    assert.equal(result.success, true, () =>
+      result.success ? "" : JSON.stringify(result.error.issues)
+    );
+  });
 });