diff --git a/changelog.d/fixes/8134-github-t5-fallback-filter.md b/changelog.d/fixes/8134-github-t5-fallback-filter.md
new file mode 100644
index 0000000000..cba43ee2a8
--- /dev/null
+++ b/changelog.d/fixes/8134-github-t5-fallback-filter.md
@@ -0,0 +1 @@
+- fix(providers): filter unsupported family-fallback candidates against the provider catalog (#8134)
diff --git a/open-sse/services/modelFamilyFallback.ts b/open-sse/services/modelFamilyFallback.ts
index 81f569764e..3ebfb92bfe 100644
--- a/open-sse/services/modelFamilyFallback.ts
+++ b/open-sse/services/modelFamilyFallback.ts
@@ -140,6 +140,38 @@ export function isContextOverflowError(status: number, errorMessage: string): bo
 
 // ── Fallback Resolution ──────────────────────────────────────────────────────
 
+/**
+ * All notation forms a family-fallback candidate might be registered under
+ * in a provider's catalog: the literal hyphen form, dot-notation variants
+ * (`claude-opus-4-8` -> `claude-opus-4.8`), and — for dated snapshot ids
+ * like `claude-opus-4-5-20251101` — the same variants with the trailing
+ * `-YYYYMMDD` snapshot suffix stripped, so a dated candidate can still
+ * resolve to a provider's undated catalog entry (`claude-opus-4.5`).
+ */
+function candidateNotationVariants(candidate: string): string[] {
+  const variants = [
+    candidate,
+    candidate.replace(/-(\d+)-(\d+)$/, "-$1.$2"),
+    candidate.replace(/-(\d+)-(\d+)-/, "-$1.$2-"),
+  ];
+
+  const dateStripped = candidate.replace(/-\d{8}$/, "");
+  if (dateStripped !== candidate) {
+    variants.push(dateStripped, dateStripped.replace(/-(\d+)-(\d+)$/, "-$1.$2"));
+  }
+
+  return variants;
+}
+
+/**
+ * Resolve a family candidate against the provider's supported model ids.
+ * Returns the matching notation (preferring the first variant found) or
+ * `null` if the candidate is absent from the catalog under every notation.
+ */
+function resolveCandidateNotation(candidate: string, supportedIds: Set<string>): string | null {
+  return candidateNotationVariants(candidate).find((variant) => supportedIds.has(variant)) ?? null;
+}
+
 /**
  * Get the next fallback model from the same family.
  *
@@ -170,11 +202,12 @@ export function getNextFamilyFallback(
   for (const candidate of family) {
     let resolvedCandidate = candidate;
     if (supportedIds && !supportedIds.has(candidate)) {
-      // Try dot-notation variants: claude-opus-4-8 → claude-opus-4.8
-      const dotVariant = candidate.replace(/-(\d+)-(\d+)$/, "-$1.$2");
-      const dotVariant2 = candidate.replace(/-(\d+)-(\d+)-/, "-$1.$2-");
-      if (supportedIds.has(dotVariant)) resolvedCandidate = dotVariant;
-      else if (supportedIds.has(dotVariant2)) resolvedCandidate = dotVariant2;
+      const match = resolveCandidateNotation(candidate, supportedIds);
+      // Provider catalog is known but this candidate has no match under any
+      // notation — it is provably unsupported, so skip it instead of
+      // returning an id the provider will just 400 on again.
+      if (!match) continue;
+      resolvedCandidate = match;
     }
     const fullCandidate = `${prefix}${resolvedCandidate}`;
     if (!triedModels.has(fullCandidate)) {
diff --git a/tests/unit/8134-github-t5-fallback-filter.test.ts b/tests/unit/8134-github-t5-fallback-filter.test.ts
new file mode 100644
index 0000000000..6de8f13416
--- /dev/null
+++ b/tests/unit/8134-github-t5-fallback-filter.test.ts
@@ -0,0 +1,70 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { getRegistryEntry } from "../../open-sse/config/providerRegistry.ts";
+
+const { getNextFamilyFallback } = await import("../../open-sse/services/modelFamilyFallback.ts");
+
+// Regression for #8134 — GitHub Copilot ("github", alias "gh") T5 family fallback
+// returned "claude-opus-4-6" verbatim even though the github registry catalog
+// (Opus 4.8 / 4.8-fast / 4.7 / 4.5) has NO 4.6 tier under any dot/hyphen
+// notation. getNextFamilyFallback() resolved `supportedIds` from the provider's
+// registry but only used it to try notation variants of a candidate, never to
+// filter out a candidate that is provably absent from the catalog — so the
+// unsupported id fell through and was returned anyway, costing a 3rd wasted
+// upstream round-trip before the family was exhausted.
+//
+// Fix: when the provider registry is resolved, getNextFamilyFallback() now
+// skips (continue) any family candidate that has no match in supportedIds
+// under ANY notation (hyphen, dot, or a dated-snapshot id with the date
+// suffix stripped) instead of returning it unfiltered.
+
+test("#8134: github claude-opus-4.8 fallback chain never returns an unsupported tier (claude-opus-4-6)", () => {
+  const github = getRegistryEntry("github");
+  assert.ok(github, "expected the github registry entry to resolve");
+  const githubIds = new Set(github.models.map((m) => m.id));
+  assert.ok(
+    !githubIds.has("claude-opus-4-6") && !githubIds.has("claude-opus-4.6"),
+    "fixture assumption broken: github registry now has a 4.6 tier"
+  );
+
+  const tried = new Set(["github/claude-opus-4.8"]);
+  const first = getNextFamilyFallback("github/claude-opus-4.8", tried);
+  assert.ok(first, "expected a first fallback candidate");
+  const firstBareId = first.replace(/^github\//, "");
+  assert.ok(
+    githubIds.has(firstBareId),
+    `first fallback "${first}" is not in github's registered model catalog: ${[...githubIds].join(", ")}`
+  );
+
+  tried.add(first);
+  const second = getNextFamilyFallback(first, tried);
+  assert.ok(second, "expected a second fallback candidate (family must not be silently exhausted)");
+  const secondBareId = second.replace(/^github\//, "");
+  assert.ok(
+    githubIds.has(secondBareId),
+    `second fallback "${second}" is not in github's registered model catalog: ${[...githubIds].join(", ")}`
+  );
+  assert.notEqual(secondBareId, "claude-opus-4-6");
+  assert.notEqual(secondBareId, "claude-opus-4.6");
+});
+
+test("#8134: getNextFamilyFallback never returns a candidate absent from the resolved provider's catalog", () => {
+  const github = getRegistryEntry("github");
+  assert.ok(github);
+  const githubIds = new Set(github.models.map((m) => m.id));
+
+  let current = "github/claude-opus-4.8";
+  const tried = new Set([current]);
+  for (let hop = 0; hop < 5; hop++) {
+    const next = getNextFamilyFallback(current, tried);
+    if (!next) break;
+    const bareId = next.replace(/^github\//, "");
+    assert.ok(
+      githubIds.has(bareId),
+      `hop ${hop + 1}: "${next}" is not in github's registered model catalog`
+    );
+    tried.add(next);
+    current = next;
+  }
+});