diff --git a/app/server/runtime.cpp b/app/server/runtime.cpp
index e04b2b0c..276c65d5 100644
--- a/app/server/runtime.cpp
+++ b/app/server/runtime.cpp
@@ -11,6 +11,7 @@
 #include "engine/framework/debug/trace.h"
 #include "engine/framework/io/json.h"
 #include "engine/framework/model_spec/metadata.h"
+#include "engine/framework/runtime/errors.h"
 #include "engine/framework/runtime/registry.h"
 
 #include <algorithm>
@@ -1001,6 +1002,12 @@ HttpResponse ServerState::handle(const HttpRequest & request) {
     else {
         response = error_response(404, "unknown endpoint: " + request.path, "not_found");
     }
+  } catch (const engine::runtime::CapacityError & ex) {
+    // The request is too big for the device, which is the caller's to fix --
+    // reporting it as 500 sends them looking for a server fault that is not
+    // there. Checked before ServerBusyError only because both are
+    // runtime_error; the two conditions are disjoint.
+    response = error_response(400, ex.what(), "invalid_request_error");
   } catch (const ServerBusyError & ex) {
     // Non-streaming requests surface the busy state as 503 before any response is
     // sent. (Streaming requests acquire the lock inside the stream body, after
diff --git a/include/engine/framework/runtime/errors.h b/include/engine/framework/runtime/errors.h
new file mode 100644
index 00000000..fe1c3c12
--- /dev/null
+++ b/include/engine/framework/runtime/errors.h
@@ -0,0 +1,17 @@
+#pragma once
+
+#include <stdexcept>
+#include <string>
+
+namespace engine::runtime {
+
+// A request the device cannot serve AT THIS SIZE -- e.g. a transcription
+// prompt plus audio whose prefill graph does not fit in VRAM. Distinct from a
+// genuine internal fault: the caller can fix it by sending less, so servers
+// should surface it as a client error rather than an opaque 500.
+class CapacityError : public std::runtime_error {
+public:
+    explicit CapacityError(const std::string & message) : std::runtime_error(message) {}
+};
+
+}  // namespace engine::runtime
diff --git a/src/models/qwen3_asr/thinker.cpp b/src/models/qwen3_asr/thinker.cpp
index ac710686..e039cd4a 100644
--- a/src/models/qwen3_asr/thinker.cpp
+++ b/src/models/qwen3_asr/thinker.cpp
@@ -12,6 +12,7 @@
 #include "engine/framework/modules/positional_modules.h"
 #include "engine/framework/modules/primitive_modules.h"
 #include "engine/framework/modules/structural_modules.h"
+#include "engine/framework/runtime/errors.h"
 #include "engine/framework/runtime/kv_cache.h"
 #include "engine/framework/sampling/decode_modules.h"
 
@@ -334,7 +335,14 @@ class PrefillGraph {
         ggml_build_forward_expand(graph_, logits_);
         buffer_ = ggml_backend_alloc_ctx_tensors(ctx_.get(), runtime_->backend());
         if (buffer_ == nullptr) {
-            throw std::runtime_error("failed to allocate Qwen3 ASR thinker prefill graph");
+            // Size, not a fault: the graph scales with prompt_steps_, which the
+            // caller controls through the transcription prompt and the length of
+            // the audio. Say which, and by how much, so the remedy is obvious.
+            throw engine::runtime::CapacityError(
+                "Qwen3 ASR prefill graph does not fit in device memory at this size ("
+                + std::to_string(prompt_steps_) + " prompt steps, of which "
+                + std::to_string(audio_tokens_) + " are audio tokens); "
+                "shorten the transcription prompt or the audio");
         }
         const auto pos = modules::qwen_position_ids(prompt_steps_);
         ggml_backend_tensor_set(positions_, pos.data(), 0, pos.size() * sizeof(int32_t));
diff --git a/src/models/voxcpm2/generator.cpp b/src/models/voxcpm2/generator.cpp
index aeffe256..3186a142 100644
--- a/src/models/voxcpm2/generator.cpp
+++ b/src/models/voxcpm2/generator.cpp
@@ -11,6 +11,7 @@
 #include "engine/framework/modules/structural_modules.h"
 #include "engine/framework/modules/weight_binding.h"
 #include "engine/framework/runtime/cache_slots.h"
+#include "engine/framework/runtime/errors.h"
 #include "engine/framework/sampling/torch_random.h"
 #include "engine/models/voxcpm2/assets.h"
 #include "engine/models/voxcpm2/minicpm.h"
@@ -1373,7 +1374,12 @@ class VoxCPM2FeatureGeneratorRuntime::Impl {
 
     if (sequence.rows.empty() ||
         static_cast<int64_t>(sequence.rows.size()) >= config.max_length) {
-      throw std::runtime_error("VoxCPM2 prompt exceeds model cache length");
+      // Caller-controlled: the prompt audio/text decides how many rows this
+      // is. Report the numbers so the remedy is arithmetic, not guesswork.
+      throw engine::runtime::CapacityError(
+          "VoxCPM2 prompt exceeds the model cache length ("
+          + std::to_string(sequence.rows.size()) + " rows, limit "
+          + std::to_string(config.max_length) + "); shorten the prompt");
     }
     return sequence;
   }
@@ -1487,7 +1493,13 @@ class VoxCPM2FeatureGeneratorRuntime::Impl {
     const int64_t patch_elems = config.patch_size * config.feat_dim;
     if (static_cast<int64_t>(prefill.rows.size()) + max_tokens >
         config.max_length) {
-      throw std::runtime_error("VoxCPM2 generation exceeds model cache length");
+      // Same: prefill rows come from the input text, max_tokens from the
+      // request. Both are the caller's to reduce.
+      throw engine::runtime::CapacityError(
+          "VoxCPM2 generation exceeds the model cache length ("
+          + std::to_string(prefill.rows.size()) + " prefill rows + "
+          + std::to_string(max_tokens) + " requested tokens, limit "
+          + std::to_string(config.max_length) + "); shorten the input text");
     }
     base_lm_.reset();
     residual_lm_.reset();