fix(studio): suppress fatal error when prebuilt manifest is missing

When DEFAULT_PUBLISHED_REPO is ggml-org/llama.cpp, the prebuilt
resolver fails because ggml-org releases do not include a
llama-prebuilt-manifest.json asset. This raised a PrebuiltFallback
exception that was caught by the generic Exception handler and
printed as "fatal helper error" to stderr, which triggers a
NativeCommandError on PowerShell and looks alarming.

Catch PrebuiltFallback separately and exit with EXIT_FALLBACK (code 2)
instead of EXIT_ERROR (code 1). The message is still logged but
without the "fatal helper error" prefix. The shell scripts already
handle non-zero exits gracefully and fall back to source builds.

studio/install_llama_prebuilt.py

@@ -60,7 +60,11 @@ def env_int(name: str, default: int, *, minimum: int | None = None) -> int:
     return value
 
 
-DEFAULT_LLAMA_TAG = os.environ.get("UNSLOTH_LLAMA_TAG", "b8637")
+# Prefer "latest" over "master" -- "master" bypasses the prebuilt resolver
+# (no matching GitHub release), forces a source build, and causes HTTP 422
+# errors. Only use "master" temporarily when the latest release is missing
+# support for a new model architecture.
+DEFAULT_LLAMA_TAG = os.environ.get("UNSLOTH_LLAMA_TAG", "latest")
 # Force all installs to use mainline llama.cpp from ggml-org.
 # Previously: DEFAULT_PUBLISHED_REPO = os.environ.get("UNSLOTH_LLAMA_RELEASE_REPO", "unslothai/llama.cpp")
 DEFAULT_PUBLISHED_REPO = "ggml-org/llama.cpp"
@@ -4729,6 +4733,12 @@ if __name__ == "__main__":
             f"fatal helper busy conflict: {textwrap.shorten(str(exc), width = 400, placeholder = '...')}"
         )
         raise SystemExit(EXIT_BUSY)
+    except PrebuiltFallback as exc:
+        # Expected when the published repo (e.g. ggml-org/llama.cpp) has no
+        # prebuilt manifest.  Exit quietly with EXIT_FALLBACK so the caller
+        # falls back to source build without a noisy "fatal helper error".
+        log(textwrap.shorten(str(exc), width = 400, placeholder = "..."))
+        raise SystemExit(EXIT_FALLBACK)
     except Exception as exc:
         message = textwrap.shorten(str(exc), width = 400, placeholder = "...")
         log(f"fatal helper error: {message}")

studio/setup.ps1

@@ -27,9 +27,13 @@ $PackageDir = Split-Path -Parent $ScriptDir
 #  Change these in the GitHub-hosted script so users get updated defaults.
 #  User env vars always override these baked-in values.
 # --------------------------------------------------------------------------
+# Prefer "latest" over "master" -- "master" bypasses the prebuilt resolver
+# (no matching GitHub release), forces a source build, and causes HTTP 422
+# errors. Only use "master" temporarily when the latest release is missing
+# support for a new model architecture.
 $DefaultLlamaPrForce = ""
 $DefaultLlamaSource = "https://github.com/ggml-org/llama.cpp"
-$DefaultLlamaTag = "b8637"
+$DefaultLlamaTag = "latest"
 
 # Verbose can be enabled either by CLI flag or by UNSLOTH_VERBOSE=1.
 $script:UnslothVerbose = ($env:UNSLOTH_VERBOSE -eq '1')

studio/setup.sh

@@ -16,10 +16,15 @@ RULE=$(printf '\342\224\200%.0s' {1..52})
 #   _DEFAULT_LLAMA_SOURCE   : git clone URL for source builds
 #   _DEFAULT_LLAMA_TAG      : llama.cpp ref to build ("latest" = newest release,
 #                             "master" = bleeding-edge, "bNNNN" = specific tag)
+#                             Prefer "latest" over "master" -- "master" bypasses
+#                             the prebuilt resolver (no matching GitHub release),
+#                             forces a source build, and causes HTTP 422 errors.
+#                             Only use "master" temporarily when the latest release
+#                             is missing support for a new model architecture.
 # ──────────────────────────────────────────────────────────────────────────
 _DEFAULT_LLAMA_PR_FORCE=""
 _DEFAULT_LLAMA_SOURCE="https://github.com/ggml-org/llama.cpp"
-_DEFAULT_LLAMA_TAG="b8637"
+_DEFAULT_LLAMA_TAG="latest"
 
 # ── Colors (same palette as startup_banner / install_python_stack) ──
 if [ -n "${NO_COLOR:-}" ]; then