diff --git a/actions/setup/js/model_aliases.cjs b/actions/setup/js/model_aliases.cjs
index 6e4ade7743c..2c80eb7e8bf 100644
--- a/actions/setup/js/model_aliases.cjs
+++ b/actions/setup/js/model_aliases.cjs
@@ -16,6 +16,15 @@ function reduceModelNameToIdentifier(modelName) {
     return normalized;
   }
 
+  // Exact-name shortcuts for opus/sonnet/haiku are handled above. This guard
+  // returns any remaining short (< 6 chars) pure-alphanumeric name verbatim so
+  // that tokens like "o1", "auto", or "gpt" are not padded with spurious zeros.
+  // Names containing non-alphanumeric characters (hyphens, pipes, etc.) fall
+  // through to the family shortcuts or the fallback sanitizer below.
+  if (normalized.length < 6 && /^[a-z0-9]+$/.test(normalized)) {
+    return normalized;
+  }
+
   const VERSION_SUFFIX_PATTERN = "[-_\\s]*([0-9]+)(?:[._-]+([0-9]+))?";
   const FALLBACK_LETTER_LENGTH = 3;
   const FALLBACK_DIGIT_LENGTH = 2;
diff --git a/actions/setup/js/model_aliases.test.cjs b/actions/setup/js/model_aliases.test.cjs
new file mode 100644
index 00000000000..6b468227b2d
--- /dev/null
+++ b/actions/setup/js/model_aliases.test.cjs
@@ -0,0 +1,78 @@
+// @ts-check
+
+import { describe, expect, it } from "vitest";
+import { createRequire } from "module";
+
+const require = createRequire(import.meta.url);
+const { reduceModelNameToIdentifier, formatModelEmojiAlias, formatModelEmojiAliasLegend } = require("./model_aliases.cjs");
+
+describe("reduceModelNameToIdentifier", () => {
+  it('returns "auto" unchanged (short name, 4 chars)', () => {
+    expect(reduceModelNameToIdentifier("auto")).toBe("auto");
+  });
+
+  it("returns safe short model names (< 6 alphanumeric chars) unchanged", () => {
+    expect(reduceModelNameToIdentifier("o1")).toBe("o1");
+    expect(reduceModelNameToIdentifier("o3")).toBe("o3");
+    expect(reduceModelNameToIdentifier("gpt")).toBe("gpt");
+    expect(reduceModelNameToIdentifier("mini")).toBe("mini");
+    expect(reduceModelNameToIdentifier("haiku")).toBe("haiku");
+  });
+
+  it("sanitizes short model names that contain non-alphanumeric characters", () => {
+    // "a|b" has a pipe character that would split a Markdown table row — must be compacted
+    expect(reduceModelNameToIdentifier("a|b")).toBe("abx00");
+    // "gpt-5" has a hyphen, so it falls through to the GPT family shortcut
+    expect(reduceModelNameToIdentifier("gpt-5")).toBe("gpt50");
+  });
+
+  it("processes names of exactly 6 characters through normalization (boundary)", () => {
+    // 6 chars is not < 6, so the short-name guard does not apply; normalization runs
+    expect(reduceModelNameToIdentifier("gpt-4o")).toBe("gpt40");
+  });
+
+  it("returns empty string for empty/null/undefined input", () => {
+    expect(reduceModelNameToIdentifier("")).toBe("");
+    expect(reduceModelNameToIdentifier(null)).toBe("");
+    expect(reduceModelNameToIdentifier(undefined)).toBe("");
+  });
+
+  it("handles known Claude model families", () => {
+    expect(reduceModelNameToIdentifier("claude-sonnet-4.5")).toBe("sonnet45");
+    expect(reduceModelNameToIdentifier("claude-opus-5")).toBe("opus50");
+    expect(reduceModelNameToIdentifier("claude-haiku-4.5")).toBe("haiku45");
+  });
+
+  it("handles GPT model families", () => {
+    expect(reduceModelNameToIdentifier("gpt-5")).toBe("gpt50");
+    expect(reduceModelNameToIdentifier("gpt-4o")).toBe("gpt40");
+  });
+
+  it("handles Gemini model families", () => {
+    expect(reduceModelNameToIdentifier("gemini-3.1-pro")).toBe("gem31pro");
+  });
+
+  it("normalizes to lowercase before processing", () => {
+    expect(reduceModelNameToIdentifier("AUTO")).toBe("auto");
+    expect(reduceModelNameToIdentifier("Auto")).toBe("auto");
+  });
+
+  it("uses fallback identifier for unrecognized longer model names", () => {
+    // "unknown-model" -> compact "unknownmodel" (12 chars) -> letterPart "unk", digitPart "00" -> "unk00"
+    expect(reduceModelNameToIdentifier("unknown-model")).toBe("unk00");
+  });
+});
+
+describe("formatModelEmojiAlias", () => {
+  it('returns "auto" unchanged', () => {
+    expect(formatModelEmojiAlias("auto")).toBe("auto");
+  });
+});
+
+describe("formatModelEmojiAliasLegend", () => {
+  it("produces legend entries for a list of models", () => {
+    const result = formatModelEmojiAliasLegend(["auto", "claude-sonnet-4.5"]);
+    expect(result).toContain("auto=auto");
+    expect(result).toContain("sonnet45=claude-sonnet-4.5");
+  });
+});