diff --git a/changelog.d/fixes/11912-roundrobin-opencode-zen-collision.md b/changelog.d/fixes/11912-roundrobin-opencode-zen-collision.md
new file mode 100644
index 0000000000..beb24e4a89
--- /dev/null
+++ b/changelog.d/fixes/11912-roundrobin-opencode-zen-collision.md
@@ -0,0 +1 @@
+- fix(routing): stop a round-robin combo's "opencode" targets from collapsing onto the opencode-zen connection (#11912)
diff --git a/open-sse/services/combo/comboStructure.ts b/open-sse/services/combo/comboStructure.ts
index 7d92d7fb45..a12aeb1528 100644
--- a/open-sse/services/combo/comboStructure.ts
+++ b/open-sse/services/combo/comboStructure.ts
@@ -26,6 +26,7 @@ import { containsMediaKind } from "../../utils/mediaParts.ts";
 import { getResolvedModelCapabilities } from "../modelCapabilities.ts";
 import { parseModel, stripContextWindowSuffix } from "../model.ts";
 import { dedupeTargetsByExecutionKey, isRecord } from "./comboData.ts";
+import { resolveComboTargetModelStr } from "./opencodeTargetAlias.ts";
 import { isComboModelVisible } from "./comboVisibility.ts";
 import { getTargetProvider, MAX_COMBO_DEPTH } from "./comboPredicates.ts";
 import { evaluateContextLimit } from "./contextOverrideGate.ts";
@@ -122,8 +123,13 @@ function normalizeRuntimeStep(
     };
   }
 
-  const modelStr = getComboModelString(step);
-  if (!modelStr) return null;
+  const declaredModelStr = getComboModelString(step);
+  if (!declaredModelStr) return null;
+  // #11912: rewrite an ambiguous "opencode/<model>" target to the "oc/" alias
+  // so it stays distinct from an explicit "opencode-zen/<model>" sibling
+  // instead of both collapsing onto the same provider — see
+  // opencodeTargetAlias.ts for the full rationale.
+  const modelStr = resolveComboTargetModelStr(declaredModelStr);
 
   const connectionId = toTrimmedString(step.connectionId);
   const allowedConnectionIds = implicitPinAllowlist(connectionId, step.allowedConnectionIds);
diff --git a/open-sse/services/combo/opencodeTargetAlias.ts b/open-sse/services/combo/opencodeTargetAlias.ts
new file mode 100644
index 0000000000..b3cfa5e26e
--- /dev/null
+++ b/open-sse/services/combo/opencodeTargetAlias.ts
@@ -0,0 +1,43 @@
+/**
+ * Issue #11912 — a combo step declared with the raw "opencode/<model>" prefix
+ * is ambiguous: open-sse/services/model.ts's manual ALIAS_TO_PROVIDER_ID
+ * override canonicalizes ANY "opencode/<model>" string to provider
+ * "opencode-zen" (the api-key gateway) before dispatch. A round-robin combo
+ * mixing declared "opencode/<model>" targets (intended as the free/dynamic
+ * no-auth pool) with an explicit "opencode-zen/<model>" target therefore
+ * collapses every rotation slot onto the SAME provider + connection identity
+ * — every request executes against the single opencode-zen connection
+ * instead of rotating across the free pool, and the account eventually
+ * 429s.
+ *
+ * The combo BUILDER already avoids this for freshly-generated model strings
+ * by emitting the "oc/" alias for the no-auth provider (#2901,
+ * src/lib/combos/builderOptions.ts's rewriteQualifiedModelPrefix). This
+ * mirrors that same substitution at combo TARGET RESOLUTION time so a step
+ * saved — or hand-typed — with the raw "opencode/" prefix still reaches the
+ * true no-auth provider and stays a distinct rotation identity from an
+ * explicit "opencode-zen/<model>" target.
+ *
+ * Deliberately scoped to combo target resolution only — this never touches
+ * open-sse/services/model.ts's general alias-resolution path, so a raw
+ * client request to "opencode/<model>" outside a combo keeps routing to
+ * opencode-zen unchanged (#2798/#3870), and the #7993 sibling credential
+ * lookup (tests/unit/opencode-autocombo-search-pair.test.ts) is unaffected.
+ */
+
+const AMBIGUOUS_OPENCODE_PREFIX = "opencode";
+const OPENCODE_NOAUTH_ALIAS = "oc";
+
+/**
+ * Rewrite a combo-declared model string's "opencode/" prefix to the "oc/"
+ * no-auth alias. Every other prefix (including "opencode-zen/" and
+ * "opencode-go/") passes through untouched.
+ */
+export function resolveComboTargetModelStr(modelStr: string): string {
+  if (typeof modelStr !== "string" || modelStr.length === 0) return modelStr;
+  const slashIndex = modelStr.indexOf("/");
+  if (slashIndex <= 0) return modelStr;
+  const prefix = modelStr.slice(0, slashIndex);
+  if (prefix !== AMBIGUOUS_OPENCODE_PREFIX) return modelStr;
+  return `${OPENCODE_NOAUTH_ALIAS}${modelStr.slice(slashIndex)}`;
+}
diff --git a/src/lib/combos/controlCenter.ts b/src/lib/combos/controlCenter.ts
index d606d2ec93..92a87b2738 100644
--- a/src/lib/combos/controlCenter.ts
+++ b/src/lib/combos/controlCenter.ts
@@ -1,4 +1,6 @@
 import { normalizeComboModels, type ComboStep } from "./steps";
+import { resolveComboTargetModelStr } from "../../../open-sse/services/combo/opencodeTargetAlias.ts";
+import { resolveProviderAlias } from "../../../open-sse/services/model.ts";
 
 type JsonRecord = Record<string, unknown>;
 
@@ -108,9 +110,15 @@ function toString(value: unknown): string | null {
 
 function providerFromModel(model: string | null | undefined): string | null {
   if (!model) return null;
-  const slashIndex = model.indexOf("/");
+  // #11912: resolve through the same "opencode" -> "oc" combo-target alias
+  // treatment (and then the general alias table) that target resolution
+  // applies before dispatch, so this label matches what actually executed
+  // upstream instead of a raw, un-aliased prefix slice.
+  const normalized = resolveComboTargetModelStr(model);
+  const slashIndex = normalized.indexOf("/");
   if (slashIndex <= 0) return null;
-  return model.slice(0, slashIndex);
+  const prefix = normalized.slice(0, slashIndex);
+  return resolveProviderAlias(prefix) || prefix;
 }
 
 function normalizeSuccessRate(value: unknown): number {
diff --git a/tests/unit/issue-11912-opencode-roundrobin-collapse.test.ts b/tests/unit/issue-11912-opencode-roundrobin-collapse.test.ts
new file mode 100644
index 0000000000..7522e9d9a6
--- /dev/null
+++ b/tests/unit/issue-11912-opencode-roundrobin-collapse.test.ts
@@ -0,0 +1,70 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { resolveComboTargets } from "../../open-sse/services/combo/comboStructure.ts";
+import { resolveComboTargetModelStr } from "../../open-sse/services/combo/opencodeTargetAlias.ts";
+import { parseModel } from "../../open-sse/services/model.ts";
+
+// Issue #11912: a round-robin combo built from several "opencode" (free /
+// dynamic no-auth) targets plus one "opencode-zen" (authenticated api-key)
+// target routed 100% of upstream traffic to the opencode-zen connection.
+//
+// Root cause: open-sse/services/model.ts's manual ALIAS_TO_PROVIDER_ID
+// override canonicalizes ANY "opencode/<model>" string to provider
+// "opencode-zen" before dispatch, so every declared "opencode/<model>"
+// combo target and the explicit "opencode-zen/<model>" target resolved to
+// the identical provider identity — round-robin's "7 targets" were never 7
+// distinct upstream accounts.
+//
+// Fix: combo target resolution (comboStructure.ts's normalizeRuntimeStep)
+// now rewrites an ambiguous "opencode/<model>" combo target to the "oc/"
+// no-auth alias, mirroring the combo builder's existing #2901 guard, before
+// the model string reaches dispatch — so it resolves to the true no-auth
+// "opencode" provider and stays distinct from an "opencode-zen/<model>"
+// sibling target.
+
+test("issue #11912: round-robin combo keeps opencode and opencode-zen targets on distinct providers", () => {
+  const targets = resolveComboTargets(
+    {
+      name: "opencode-round-robin",
+      strategy: "round-robin",
+      models: [
+        { kind: "model", model: "opencode/mimo-v2.5-free" },
+        { kind: "model", model: "opencode/mimo-v2.5-free" },
+        { kind: "model", model: "opencode-zen/mimo-v2.5-free" },
+      ],
+    },
+    null
+  );
+
+  assert.equal(targets.length, 3);
+  const [dynamicA, dynamicB, authenticated] = targets;
+
+  assert.notEqual(
+    dynamicA.provider,
+    authenticated.provider,
+    `combo target "opencode/<model>" resolved to provider "${dynamicA.provider}" — it collapsed ` +
+      `onto the same identity as the explicit "opencode-zen/<model>" target instead of routing ` +
+      `to the free/dynamic no-auth pool`
+  );
+  assert.equal(dynamicA.provider, dynamicB.provider);
+  assert.equal(authenticated.provider, "opencode-zen");
+
+  // The rewritten model string must still resolve to the genuine no-auth
+  // provider identity when it later reaches dispatch (parseModel is exactly
+  // what open-sse/services/combo/roundRobinCombo.ts and
+  // resolveModelOrError() call on the resolved target's modelStr).
+  assert.equal(parseModel(dynamicA.modelStr).provider, "opencode");
+  assert.equal(parseModel(authenticated.modelStr).provider, "opencode-zen");
+});
+
+test("resolveComboTargetModelStr rewrites the ambiguous opencode/ prefix to oc/", () => {
+  assert.equal(resolveComboTargetModelStr("opencode/mimo-v2.5-free"), "oc/mimo-v2.5-free");
+  // Siblings and the explicit api-key gateway must pass through untouched.
+  assert.equal(resolveComboTargetModelStr("opencode-zen/mimo-v2.5-free"), "opencode-zen/mimo-v2.5-free");
+  assert.equal(resolveComboTargetModelStr("opencode-go/mimo-v2.5-free"), "opencode-go/mimo-v2.5-free");
+  assert.equal(resolveComboTargetModelStr("oc/mimo-v2.5-free"), "oc/mimo-v2.5-free");
+  // Non-slashed / non-opencode strings are untouched.
+  assert.equal(resolveComboTargetModelStr("bare-model"), "bare-model");
+  assert.equal(resolveComboTargetModelStr("anthropic/claude"), "anthropic/claude");
+});