diff --git a/hermes_cli/model_normalize.py b/hermes_cli/model_normalize.py
index 22ab0fa3f..76dace065 100644
--- a/hermes_cli/model_normalize.py
+++ b/hermes_cli/model_normalize.py
@@ -374,7 +374,26 @@ def normalize_model_for_provider(model_input: str, target_provider: str) -> str:
             return bare
         return _dots_to_hyphens(bare)
 
-    # --- Copilot: strip matching provider prefix, keep dots ---
+    # --- Copilot / Copilot ACP: delegate to the Copilot-specific
+    #     normalizer.  It knows about the alias table (vendor-prefix
+    #     stripping for Anthropic/OpenAI, dash-to-dot repair for Claude)
+    #     and live-catalog lookups.  Without this, vendor-prefixed or
+    #     dash-notation Claude IDs survive to the Copilot API and hit
+    #     HTTP 400 "model_not_supported".  See issue #6879.
+    if provider in {"copilot", "copilot-acp"}:
+        try:
+            from hermes_cli.models import normalize_copilot_model_id
+
+            normalized = normalize_copilot_model_id(name)
+            if normalized:
+                return normalized
+        except Exception:
+            # Fall through to the generic strip-vendor behaviour below
+            # if the Copilot-specific path is unavailable for any reason.
+            pass
+
+    # --- Copilot / Copilot ACP / openai-codex fallback:
+    #     strip matching provider prefix, keep dots ---
     if provider in _STRIP_VENDOR_ONLY_PROVIDERS:
         stripped = _strip_matching_provider_prefix(name, provider)
         if stripped == name and name.startswith("openai/"):
diff --git a/hermes_cli/models.py b/hermes_cli/models.py
index 8f23980ac..0208bc9fe 100644
--- a/hermes_cli/models.py
+++ b/hermes_cli/models.py
@@ -1488,6 +1488,19 @@ _COPILOT_MODEL_ALIASES = {
     "anthropic/claude-sonnet-4.6": "claude-sonnet-4.6",
     "anthropic/claude-sonnet-4.5": "claude-sonnet-4.5",
     "anthropic/claude-haiku-4.5": "claude-haiku-4.5",
+    # Dash-notation fallbacks: Hermes' default Claude IDs elsewhere use
+    # hyphens (anthropic native format), but Copilot's API only accepts
+    # dot-notation.  Accept both so users who configure copilot + a
+    # default hyphenated Claude model don't hit HTTP 400
+    # "model_not_supported".  See issue #6879.
+    "claude-opus-4-6": "claude-opus-4.6",
+    "claude-sonnet-4-6": "claude-sonnet-4.6",
+    "claude-sonnet-4-5": "claude-sonnet-4.5",
+    "claude-haiku-4-5": "claude-haiku-4.5",
+    "anthropic/claude-opus-4-6": "claude-opus-4.6",
+    "anthropic/claude-sonnet-4-6": "claude-sonnet-4.6",
+    "anthropic/claude-sonnet-4-5": "claude-sonnet-4.5",
+    "anthropic/claude-haiku-4-5": "claude-haiku-4.5",
 }
 
 
diff --git a/tests/hermes_cli/test_model_normalize.py b/tests/hermes_cli/test_model_normalize.py
index 14861c37a..6de69ab30 100644
--- a/tests/hermes_cli/test_model_normalize.py
+++ b/tests/hermes_cli/test_model_normalize.py
@@ -93,6 +93,59 @@ class TestCopilotDotPreservation:
         assert result == expected
 
 
+# ── Copilot model-name normalization (issue #6879 regression) ──────────
+
+class TestCopilotModelNormalization:
+    """Copilot requires bare dot-notation model IDs.
+
+    Regression coverage for issue #6879 and the broken Copilot branch
+    that previously left vendor-prefixed Anthropic IDs (e.g.
+    ``anthropic/claude-sonnet-4.6``) and dash-notation Claude IDs (e.g.
+    ``claude-sonnet-4-6``) unchanged, causing the Copilot API to reject
+    the request with HTTP 400 "model_not_supported".
+    """
+
+    @pytest.mark.parametrize("model,expected", [
+        # Vendor-prefixed Anthropic IDs — prefix must be stripped.
+        ("anthropic/claude-opus-4.6",   "claude-opus-4.6"),
+        ("anthropic/claude-sonnet-4.6", "claude-sonnet-4.6"),
+        ("anthropic/claude-sonnet-4.5", "claude-sonnet-4.5"),
+        ("anthropic/claude-haiku-4.5",  "claude-haiku-4.5"),
+        # Vendor-prefixed OpenAI IDs — prefix must be stripped.
+        ("openai/gpt-5.4",              "gpt-5.4"),
+        ("openai/gpt-4o",               "gpt-4o"),
+        ("openai/gpt-4o-mini",          "gpt-4o-mini"),
+        # Dash-notation Claude IDs — must be converted to dot-notation.
+        ("claude-opus-4-6",             "claude-opus-4.6"),
+        ("claude-sonnet-4-6",           "claude-sonnet-4.6"),
+        ("claude-sonnet-4-5",           "claude-sonnet-4.5"),
+        ("claude-haiku-4-5",            "claude-haiku-4.5"),
+        # Combined: vendor-prefixed + dash-notation.
+        ("anthropic/claude-opus-4-6",   "claude-opus-4.6"),
+        ("anthropic/claude-sonnet-4-6", "claude-sonnet-4.6"),
+        # Already-canonical inputs pass through unchanged.
+        ("claude-sonnet-4.6",           "claude-sonnet-4.6"),
+        ("gpt-5.4",                     "gpt-5.4"),
+        ("gpt-5-mini",                  "gpt-5-mini"),
+    ])
+    def test_copilot_normalization(self, model, expected):
+        assert normalize_model_for_provider(model, "copilot") == expected
+
+    @pytest.mark.parametrize("model,expected", [
+        ("anthropic/claude-sonnet-4.6", "claude-sonnet-4.6"),
+        ("claude-sonnet-4-6",           "claude-sonnet-4.6"),
+        ("claude-opus-4-6",             "claude-opus-4.6"),
+        ("openai/gpt-5.4",              "gpt-5.4"),
+    ])
+    def test_copilot_acp_normalization(self, model, expected):
+        """Copilot ACP shares the same API expectations as HTTP Copilot."""
+        assert normalize_model_for_provider(model, "copilot-acp") == expected
+
+    def test_openai_codex_still_strips_openai_prefix(self):
+        """Regression: openai-codex must still strip the openai/ prefix."""
+        assert normalize_model_for_provider("openai/gpt-5.4", "openai-codex") == "gpt-5.4"
+
+
 # ── Aggregator providers (regression) ──────────────────────────────────
 
 class TestAggregatorProviders: