diff --git a/CHANGELOG.md b/CHANGELOG.md
index 770be57643..021774944e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,16 @@
 
 _In development — bullets added per PR; finalized at release._
 
+### 🔧 Bug Fixes
+
+- **Engine Combos editor: saving a pipeline no longer fails silently with HTTP 400 (#4955).** The
+  named-combos pipeline dropdown offered four engines (`headroom`, `session-dedup`, `ccr`,
+  `llmlingua`) that the `PUT /api/context/combos/[id]` schema rejects, so selecting one made the
+  save return 400 while the UI swallowed the error. The dropdown is now sourced from a single
+  canonical engine map shared with `stackedPipelineStepSchema` (parity guarded by a unit test), and
+  the editor surfaces save errors and empty-name/empty-pipeline validation instead of failing
+  quietly.
+
 ---
 
 ## [3.8.36] — 2026-06-25
diff --git a/src/app/(dashboard)/dashboard/context/combos/CompressionCombosPageClient.tsx b/src/app/(dashboard)/dashboard/context/combos/CompressionCombosPageClient.tsx
index a9b48c3e47..8bdfc4703d 100644
--- a/src/app/(dashboard)/dashboard/context/combos/CompressionCombosPageClient.tsx
+++ b/src/app/(dashboard)/dashboard/context/combos/CompressionCombosPageClient.tsx
@@ -8,6 +8,7 @@
 // matching `EngineConfigPage` / `CompressionHub`, both of which hydrate cleanly.
 
 import { useEffect, useState } from "react";
+import { STACKED_PIPELINE_ENGINE_INTENSITIES } from "@/shared/validation/compressionConfigSchemas";
 import CompressionHub from "./CompressionHub";
 
 type PipelineStep = { engine: string; intensity?: string };
@@ -29,17 +30,9 @@ const EMPTY_PIPELINE: PipelineStep[] = [
   { engine: "caveman", intensity: "full" },
 ];
 
-const ENGINE_INTENSITIES: Record<string, string[]> = {
-  rtk: ["minimal", "standard", "aggressive"],
-  caveman: ["lite", "full", "ultra"],
-  lite: ["lite"],
-  aggressive: ["standard"],
-  ultra: ["ultra"],
-  headroom: ["standard"],
-  "session-dedup": ["standard"],
-  ccr: ["standard"],
-  llmlingua: ["standard"],
-};
+// Engine list is sourced from the API schema so the dropdown can never offer an engine
+// the `PUT /api/context/combos/[id]` route would reject with HTTP 400 (#4955).
+const ENGINE_INTENSITIES: Record<string, readonly string[]> = STACKED_PIPELINE_ENGINE_INTENSITIES;
 
 function NamedCombosManager() {
   const [combos, setCombos] = useState<CompressionCombo[]>([]);
@@ -55,6 +48,7 @@ function NamedCombosManager() {
   const [assignmentIds, setAssignmentIds] = useState<string[]>([]);
   const [saving, setSaving] = useState(false);
   const [activeComboId, setActiveComboId] = useState<string | null>(null);
+  const [error, setError] = useState<string | null>(null);
 
   const refresh = () => {
     fetch("/api/context/combos")
@@ -88,6 +82,7 @@ function NamedCombosManager() {
     setOutputMode(false);
     setOutputModeIntensity("full");
     setAssignmentIds([]);
+    setError(null);
   };
 
   const loadAssignments = async (id: string) => {
@@ -112,7 +107,15 @@ function NamedCombosManager() {
 
   const saveCombo = async () => {
     const trimmed = name.trim();
-    if (!trimmed) return;
+    if (!trimmed) {
+      setError("Enter a combo name before saving.");
+      return;
+    }
+    if (pipeline.length === 0) {
+      setError("Add at least one pipeline step before saving.");
+      return;
+    }
+    setError(null);
     setSaving(true);
     try {
       const payload = {
@@ -131,7 +134,11 @@ function NamedCombosManager() {
           body: JSON.stringify(payload),
         }
       );
-      if (!res.ok) return;
+      if (!res.ok) {
+        const body = await res.json().catch(() => null);
+        setError(body?.error || `Failed to save combo (HTTP ${res.status}).`);
+        return;
+      }
       const combo = await res.json();
       await fetch(`/api/context/combos/${combo.id}/assignments`, {
         method: "PUT",
@@ -315,6 +322,12 @@ function NamedCombosManager() {
           </div>
         </div>
 
+        {error && (
+          <p className="mt-4 text-sm text-danger" role="alert">
+            {error}
+          </p>
+        )}
+
         <div className="mt-4 flex flex-wrap gap-2">
           <button
             onClick={saveCombo}
diff --git a/src/shared/validation/compressionConfigSchemas.ts b/src/shared/validation/compressionConfigSchemas.ts
index 293c11694f..e5ba212711 100644
--- a/src/shared/validation/compressionConfigSchemas.ts
+++ b/src/shared/validation/compressionConfigSchemas.ts
@@ -177,6 +177,26 @@ export const stackedPipelineStepSchema = z.discriminatedUnion("engine", [
     .strict(),
 ]);
 
+/**
+ * Canonical engine → selectable-intensities map for the named-combos pipeline editor
+ * (Engine Combos UI). This is the SINGLE source of truth shared by the dashboard
+ * 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.
+ */
+export const STACKED_PIPELINE_ENGINE_INTENSITIES: Record<string, readonly string[]> = {
+  rtk: ["minimal", "standard", "aggressive"],
+  caveman: ["lite", "full", "ultra"],
+  lite: ["lite"],
+  aggressive: ["standard"],
+  ultra: ["ultra"],
+};
+
 export const engineToggleSchema = z.object({
   enabled: z.boolean(),
   level: z.string().optional(),
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
new file mode 100644
index 0000000000..2162dd765d
--- /dev/null
+++ b/tests/unit/compression/combos-engine-ui-schema-parity-4955.test.ts
@@ -0,0 +1,51 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  STACKED_PIPELINE_ENGINE_INTENSITIES,
+  stackedPipelineStepSchema,
+} from "../../../src/shared/validation/compressionConfigSchemas.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)", () => {
+  const unionEngines = stackedPipelineStepSchema.options
+    .map((option: { shape: { engine: { value: string } } }) => option.shape.engine.value)
+    .sort();
+
+  it("offers exactly the engines the API update schema accepts (no drift)", () => {
+    const uiEngines = Object.keys(STACKED_PIPELINE_ENGINE_INTENSITIES).sort();
+    assert.deepEqual(uiEngines, 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)) {
+      for (const intensity of intensities) {
+        const result = stackedPipelineStepSchema.safeParse({ engine, intensity });
+        assert.equal(
+          result.success,
+          true,
+          `expected { engine: "${engine}", intensity: "${intensity}" } to be accepted`
+        );
+      }
+    }
+  });
+
+  it("the engines removed from the UI in #4955 are indeed rejected by the schema", () => {
+    for (const engine of ["headroom", "session-dedup", "ccr", "llmlingua"]) {
+      assert.equal(
+        stackedPipelineStepSchema.safeParse({ engine, intensity: "standard" }).success,
+        false,
+        `engine "${engine}" must not be a valid stacked-pipeline step`
+      );
+      assert.equal(
+        STACKED_PIPELINE_ENGINE_INTENSITIES[engine],
+        undefined,
+        `engine "${engine}" must not be offered by the combos UI`
+      );
+    }
+  });
+});