diff --git a/src/resources/extensions/gsd/model-router.ts b/src/resources/extensions/gsd/model-router.ts
index fd76d53ca..fe8bdf0a5 100644
--- a/src/resources/extensions/gsd/model-router.ts
+++ b/src/resources/extensions/gsd/model-router.ts
@@ -114,6 +114,21 @@ export function resolveModelForComplexity(
   const configuredTier = getModelTier(configuredPrimary);
   const requestedTier = classification.tier;
 
+  // If the configured model is unknown (not in MODEL_CAPABILITY_TIER),
+  // honor the user's explicit choice — don't downgrade based on a guess.
+  // Unknown models default to "heavy" in getModelTier, which makes every
+  // standard/light unit get downgraded to tier_models, silently ignoring
+  // the user's configuration. (#2192)
+  if (!isKnownModel(configuredPrimary)) {
+    return {
+      modelId: configuredPrimary,
+      fallbacks: phaseConfig.fallbacks,
+      tier: requestedTier,
+      wasDowngraded: false,
+      reason: `configured model "${configuredPrimary}" is not in the known tier map — honoring explicit config`,
+    };
+  }
+
   // Downgrade-only: if requested tier >= configured tier, no change
   if (tierOrdinal(requestedTier) >= tierOrdinal(configuredTier)) {
     return {
@@ -202,6 +217,16 @@ function getModelTier(modelId: string): ComplexityTier {
   return "heavy";
 }
 
+/** Check if a model ID has a known capability tier mapping. (#2192) */
+function isKnownModel(modelId: string): boolean {
+  const bareId = modelId.includes("/") ? modelId.split("/").pop()! : modelId;
+  if (MODEL_CAPABILITY_TIER[bareId]) return true;
+  for (const knownId of Object.keys(MODEL_CAPABILITY_TIER)) {
+    if (bareId.includes(knownId) || knownId.includes(bareId)) return true;
+  }
+  return false;
+}
+
 function findModelForTier(
   tier: ComplexityTier,
   config: DynamicRoutingConfig,
diff --git a/src/resources/extensions/gsd/tests/model-router.test.ts b/src/resources/extensions/gsd/tests/model-router.test.ts
index c7af7fcca..b22fce7fd 100644
--- a/src/resources/extensions/gsd/tests/model-router.test.ts
+++ b/src/resources/extensions/gsd/tests/model-router.test.ts
@@ -165,3 +165,43 @@ test("falls back to configured model when no light-tier model available", () =>
   assert.equal(result.modelId, "claude-opus-4-6");
   assert.equal(result.wasDowngraded, false);
 });
+
+// ─── #2192: Unknown models honor explicit config ─────────────────────────────
+
+test("#2192: unknown model is not downgraded — respects user config", () => {
+  const config = { ...defaultRoutingConfig(), enabled: true };
+  const result = resolveModelForComplexity(
+    makeClassification("light"),
+    { primary: "gpt-5.4", fallbacks: [] },
+    config,
+    ["gpt-5.4", ...AVAILABLE_MODELS],
+  );
+  assert.equal(result.modelId, "gpt-5.4", "unknown model should be used as-is");
+  assert.equal(result.wasDowngraded, false, "should not be downgraded");
+  assert.ok(result.reason.includes("not in the known tier map"), "reason should explain why");
+});
+
+test("#2192: unknown model with provider prefix is not downgraded", () => {
+  const config = { ...defaultRoutingConfig(), enabled: true };
+  const result = resolveModelForComplexity(
+    makeClassification("standard"),
+    { primary: "custom-provider/my-model-v3", fallbacks: [] },
+    config,
+    ["custom-provider/my-model-v3", ...AVAILABLE_MODELS],
+  );
+  assert.equal(result.modelId, "custom-provider/my-model-v3");
+  assert.equal(result.wasDowngraded, false);
+});
+
+test("#2192: known model is still downgraded normally", () => {
+  const config = { ...defaultRoutingConfig(), enabled: true };
+  // claude-opus-4-6 is known as "heavy" — a light request should downgrade
+  const result = resolveModelForComplexity(
+    makeClassification("light"),
+    { primary: "claude-opus-4-6", fallbacks: [] },
+    config,
+    AVAILABLE_MODELS,
+  );
+  assert.equal(result.wasDowngraded, true, "known heavy model should still be downgraded for light tasks");
+  assert.notEqual(result.modelId, "claude-opus-4-6");
+});