diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3b7814aea4..86f027303f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 
 ### 🐛 Bug Fixes
 
+- **fix(dashboard):** passthrough model aliases no longer collide when two namespaced model ids share a last segment. `enx/gpt-5.5` and `enx/codebuddy/gpt-5.5` both auto-generated the alias `gpt-5.5`, so the second model could never be added (the UI just alerted "alias already exists"). Aliases are now disambiguated deterministically — bare last segment when free, then parent-qualified (`codebuddy-gpt-5.5`), then a numeric suffix — while re-adding the exact same model id is still blocked. Regression guard: `tests/unit/passthrough-alias-1850.test.ts`. (thanks @arpicato)
 - **fix(translator):** preserve a Gemini `functionResponse` co-located with other parts (another `functionCall`, or trailing `text`) in the same content when translating **Gemini → OpenAI**. `convertGeminiContent()` early-returned the tool message on the first `functionResponse` part, dropping any co-located parts; such contents are now pre-split (one tool message per `functionResponse`, emitted first, plus one message for the remaining parts). Regression guard: `tests/unit/gemini-to-openai-function-response.test.ts`. (thanks @warelik)
 - **fix(headroom):** detect a python interpreter managed by **mise / pyenv / asdf / conda**. Headroom's python probe (`src/lib/headroom/detect.ts`) searched a hardcoded `PATH`, but version managers expose their interpreters via shim dirs that only join `PATH` through interactive-shell activation — which the non-interactive server never runs, so a managed python (≥3.10) was invisible and Headroom reported it missing. The search path now prepends the well-known shim/bin dirs (`~/.local/share/mise/shims`, `~/.pyenv/shims`, `~/.asdf/shims`, `$CONDA_PREFIX/bin`, `~/.local/bin`, respecting `MISE_DATA_DIR`/`PYENV_ROOT`/`ASDF_DATA_DIR` when set), and a new `HEADROOM_PYTHON` env override lets operators point straight at their interpreter (mirroring `HEADROOM_URL`). Still shell-free (`execFileSync`). Regression guard: `tests/unit/headroom-detect.test.ts` (5). (thanks @loopyd)
 - **fix(executors):** strip the OpenAI-Codex/Claude-CLI `client_metadata` passthrough field for **NVIDIA** requests. NVIDIA's OpenAI-compatible wrapper rejects it with `400 Unsupported parameter`, the same class already handled for `cerebras`/`mistral`; `nvidia` (executor `default`) was missing from the strip allowlist so Codex/Claude-Code passthrough requests 400'd. Regression guard: `tests/unit/executor-default-strip-client-metadata.test.ts` (+nvidia case). (thanks @phidinhmanh)
diff --git a/src/app/(dashboard)/dashboard/providers/[id]/components/PassthroughModelsSection.tsx b/src/app/(dashboard)/dashboard/providers/[id]/components/PassthroughModelsSection.tsx
index fb0d2b6e62..857281e3bc 100644
--- a/src/app/(dashboard)/dashboard/providers/[id]/components/PassthroughModelsSection.tsx
+++ b/src/app/(dashboard)/dashboard/providers/[id]/components/PassthroughModelsSection.tsx
@@ -14,6 +14,7 @@
  */
 import React, { useState, useMemo } from "react";
 import { Button } from "@/shared/components";
+import { generateUniqueModelAlias } from "./passthroughAlias.ts";
 import {
   matchesModelCatalogQuery,
   normalizeModelCatalogSource,
@@ -323,22 +324,18 @@ export default function PassthroughModelsSection({
     : filteredModels;
   const activeCount = allModels.filter((model) => !model.isHidden).length;
 
-  // Generate default alias from modelId (last part after /)
-  const generateDefaultAlias = (modelId: string) => {
-    const parts = modelId.split("/");
-    return parts[parts.length - 1];
-  };
-
   const handleAdd = async () => {
     if (!newModel.trim() || adding) return;
     const modelId = newModel.trim();
-    const defaultAlias = generateDefaultAlias(modelId);
 
-    // Check if alias already exists
-    if (modelAliases[defaultAlias]) {
-      alert(t("aliasExistsAlert", { alias: defaultAlias }));
+    // #1850: block re-adding the SAME model, but disambiguate DISTINCT models
+    // that would otherwise collapse to the same last-segment alias (e.g.
+    // enx/gpt-5.5 vs enx/codebuddy/gpt-5.5 → both "gpt-5.5").
+    if (Object.values(modelAliases).includes(modelId)) {
+      alert(t("aliasExistsAlert", { alias: modelId }));
       return;
     }
+    const defaultAlias = generateUniqueModelAlias(modelId, modelAliases);
 
     setAdding(true);
     try {
diff --git a/src/app/(dashboard)/dashboard/providers/[id]/components/passthroughAlias.ts b/src/app/(dashboard)/dashboard/providers/[id]/components/passthroughAlias.ts
new file mode 100644
index 0000000000..472d9d7907
--- /dev/null
+++ b/src/app/(dashboard)/dashboard/providers/[id]/components/passthroughAlias.ts
@@ -0,0 +1,47 @@
+/**
+ * Generate a unique default alias for a passthrough model id (9router#1850).
+ *
+ * The naive "last path segment" alias collapses distinct namespaced ids to the
+ * same alias — e.g. `enx/codebuddy/gpt-5.5` and `enx/gpt-5.5` both become
+ * `gpt-5.5` — so the second model could never be added (the UI only alerted
+ * "alias already exists"). This disambiguates deterministically:
+ *   1. the bare last segment, if free;
+ *   2. progressively more-qualified names joined with "-" (parent segments
+ *      prepended), if the shorter form is taken;
+ *   3. a numeric suffix on the last segment as a final fallback.
+ *
+ * Pure — no React/DOM deps — so it is unit-testable.
+ */
+export function generateUniqueModelAlias(
+  modelId: string,
+  existingAliases: Record<string, unknown> = {}
+): string {
+  const parts = String(modelId ?? "")
+    .split("/")
+    .filter(Boolean);
+
+  if (parts.length === 0) {
+    // No usable segments (e.g. "" or "///") — fall back to the raw id + numeric.
+    const base = String(modelId ?? "").trim() || "model";
+    return isTaken(base, existingAliases) ? nextNumeric(base, existingAliases) : base;
+  }
+
+  // 1 + 2: try last segment, then last-2 joined, … up to the full path.
+  for (let take = 1; take <= parts.length; take++) {
+    const candidate = parts.slice(parts.length - take).join("-");
+    if (!isTaken(candidate, existingAliases)) return candidate;
+  }
+
+  // 3: every qualified form is taken → numeric suffix on the last segment.
+  return nextNumeric(parts[parts.length - 1], existingAliases);
+}
+
+function isTaken(alias: string, existingAliases: Record<string, unknown>): boolean {
+  return Object.prototype.hasOwnProperty.call(existingAliases, alias);
+}
+
+function nextNumeric(base: string, existingAliases: Record<string, unknown>): string {
+  let i = 2;
+  while (isTaken(`${base}-${i}`, existingAliases)) i++;
+  return `${base}-${i}`;
+}
diff --git a/tests/unit/passthrough-alias-1850.test.ts b/tests/unit/passthrough-alias-1850.test.ts
new file mode 100644
index 0000000000..cc22d84b7d
--- /dev/null
+++ b/tests/unit/passthrough-alias-1850.test.ts
@@ -0,0 +1,48 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { generateUniqueModelAlias } from "../../src/app/(dashboard)/dashboard/providers/[id]/components/passthroughAlias.ts";
+
+/**
+ * Regression guard for upstream 9router#1850.
+ *
+ * The naive last-segment alias collapsed distinct namespaced model ids to the
+ * same alias, blocking the second model from ever being added.
+ */
+
+test("bare last segment when free (preserves the common case)", () => {
+  assert.equal(generateUniqueModelAlias("enx/gpt-5.5", {}), "gpt-5.5");
+  assert.equal(generateUniqueModelAlias("gpt-5.5", {}), "gpt-5.5");
+});
+
+test("#1850: namespaced ids that share a last segment get distinct aliases", () => {
+  const aliases: Record<string, unknown> = {};
+  const a = generateUniqueModelAlias("enx/gpt-5.5", aliases);
+  aliases[a] = { modelId: "enx/gpt-5.5" };
+  const b = generateUniqueModelAlias("enx/codebuddy/gpt-5.5", aliases);
+
+  assert.equal(a, "gpt-5.5");
+  assert.equal(b, "codebuddy-gpt-5.5", "second model must not collide with the first");
+  assert.notEqual(a, b);
+});
+
+test("falls back to a numeric suffix when every qualified form is taken", () => {
+  const aliases: Record<string, unknown> = {
+    "gpt-5.5": {},
+    "codebuddy-gpt-5.5": {},
+    "enx-codebuddy-gpt-5.5": {},
+  };
+  const alias = generateUniqueModelAlias("enx/codebuddy/gpt-5.5", aliases);
+  assert.equal(alias, "gpt-5.5-2");
+});
+
+test("numeric suffix increments past existing numbered aliases", () => {
+  const aliases: Record<string, unknown> = { foo: {}, "foo-2": {}, "foo-3": {} };
+  // single-segment id whose only candidate is taken → numeric fallback skips to -4
+  assert.equal(generateUniqueModelAlias("foo", aliases), "foo-4");
+});
+
+test("degenerate ids do not throw", () => {
+  assert.equal(typeof generateUniqueModelAlias("", {}), "string");
+  assert.equal(typeof generateUniqueModelAlias("///", {}), "string");
+});