diff --git a/open-sse/services/combo/comboStructure.ts b/open-sse/services/combo/comboStructure.ts
index 137561fbaa..9065e123b2 100644
--- a/open-sse/services/combo/comboStructure.ts
+++ b/open-sse/services/combo/comboStructure.ts
@@ -807,16 +807,48 @@ export function filterTargetsByRequestCompatibility(
     return [];
   }
 
+  // #12273: a sole survivor whose catalog window is known-too-small is a
+  // guaranteed context_length_exceeded. Restore the remaining pool so combo.ts
+  // can still try larger-context targets. Unknown context (`null`) is advisory
+  // and must not resurrect hard-rejected targets (vision / output / tools).
+  if (
+    compatible.length === 1 &&
+    (targetReasons.get(compatible[0]) || []).includes("context_window")
+  ) {
+    // #8332: never restore a confirmed-non-vision target onto an image request.
+    const restored = requirements.requiresVision
+      ? targets.filter((target) => !isVisionIncompatibleTarget(target, requirements))
+      : targets;
+    if (restored.length > compatible.length) {
+      log.warn(
+        "COMBO",
+        `${label}: single compatible target ${compatible[0].modelStr} has known context too small for ${requirements.requiredContextTokens} token request; falling back to full pool (#12273)`
+      );
+      return restored;
+    }
+  }
+
   log.info(
     "COMBO",
     `${label}: kept ${compatible.length}/${targets.length} targets for request requirements`
   );
-  log.debug?.(
-    "COMBO",
-    `${label}: rejected targets ${rejected
-      .map((entry) => `${entry.target.modelStr}(${entry.reasons.join("+")})`)
-      .join(", ")}`
-  );
+  // #12273: When pool collapses significantly, log rejection reasons at info
+  // level so the cause is diagnosable without enabling debug logging.
+  if (compatible.length <= 2 && targets.length > 4) {
+    log.info(
+      "COMBO",
+      `${label}: rejected targets ${rejected
+        .map((entry) => `${entry.target.modelStr}(${entry.reasons.join("+")})`)
+        .join(", ")}`
+    );
+  } else {
+    log.debug?.(
+      "COMBO",
+      `${label}: rejected targets ${rejected
+        .map((entry) => `${entry.target.modelStr}(${entry.reasons.join("+")})`)
+        .join(", ")}`
+    );
+  }
   return compatible;
 }
 
diff --git a/tests/unit/combo-context-window-filter.test.ts b/tests/unit/combo-context-window-filter.test.ts
index a78c3d5ba5..74d2646f3e 100644
--- a/tests/unit/combo-context-window-filter.test.ts
+++ b/tests/unit/combo-context-window-filter.test.ts
@@ -450,3 +450,95 @@ test("without an override the small-catalog target is ordered last for the large
     ["unit-override/big", "unit-override/capped"]
   );
 });
+
+// #12273: real Claude Code requests always carry `tools`, and the auto/coding
+// pool mixes coding-capable providers with providers whose catalog marks
+// toolCalling=false. Those non-coding targets are HARD-rejected (tools), so the
+// compat filter can collapse the whole pool to a single too-small-context
+// coding model (e.g. mimo-v2.5-free at 200k) for a much larger request — the
+// larger-context model was never assembled into the candidate pool. Routing to
+// that sole survivor is a guaranteed context_length_exceeded, so the filter
+// must fall back to the full pool instead of silently pinning the request.
+test("#12273 single known-too-small survivor falls back to the full pool", () => {
+  saveModelsDevCapabilities({
+    "unit-collapse": {
+      small: capabilityEntry(200_000),
+    },
+    "unit-noncoding": {
+      nocoder: { ...capabilityEntry(1_000_000), tool_call: false },
+    },
+  });
+  const body = {
+    ...bigContextBody(300_000),
+    tools: [{ type: "function" }], // Claude Code always sends tools
+  };
+
+  const out = filterTargetsByRequestCompatibility(
+    [target("unit-collapse/small"), target("unit-noncoding/nocoder")],
+    body,
+    noopLog
+  );
+
+  // nocoder is hard-rejected (toolCalling=false); small (200k) is the only
+  // compatible survivor but is known to be too small for a 300k request, so the
+  // filter returns the full pool rather than dispatch to a guaranteed failure.
+  assert.deepEqual(
+    out.map((entry) => entry.modelStr),
+    ["unit-collapse/small", "unit-noncoding/nocoder"]
+  );
+});
+
+// Guard against regression: when the single survivor's window DOES fit the
+// request, the filter still collapses (existing behavior preserved).
+test("#12273 single compatible target that fits is still collapsed", () => {
+  saveModelsDevCapabilities({
+    "unit-collapse": {
+      big: capabilityEntry(1_000_000),
+    },
+    "unit-noncoding": {
+      nocoder: { ...capabilityEntry(1_000_000), tool_call: false },
+    },
+  });
+  const body = {
+    ...bigContextBody(300_000),
+    tools: [{ type: "function" }],
+  };
+
+  const out = filterTargetsByRequestCompatibility(
+    [target("unit-collapse/big"), target("unit-noncoding/nocoder")],
+    body,
+    noopLog
+  );
+
+  // big (1M) fits the 300k request, so the collapse is legitimate.
+  assert.deepEqual(
+    out.map((entry) => entry.modelStr),
+    ["unit-collapse/big"]
+  );
+});
+
+// #12278: unknown context is advisory, not "known too small". Collapsing to a
+// single survivor whose context limit is unknown must NOT restore hard-rejected
+// targets (output_tokens here; vision is covered by combo-vision-aware-routing).
+test("#12273 unknown-context sole survivor does not restore hard-rejected targets", () => {
+  saveModelsDevCapabilities({
+    "unit-output": {
+      tiny: capabilityEntryWithLimits(128_000, 128_000, 4096),
+    },
+  });
+  const body = {
+    messages: [{ role: "user", content: "hello" }],
+    max_tokens: 32_000,
+  };
+
+  const out = filterTargetsByRequestCompatibility(
+    [target("unit-unknown/mystery"), target("unit-output/tiny")],
+    body,
+    noopLog
+  );
+
+  assert.deepEqual(
+    out.map((entry) => entry.modelStr),
+    ["unit-unknown/mystery"]
+  );
+});