fix(server): a reply gets the context its prompt leaves instead of a 400

server/src/server/admission.h

@@ -2,6 +2,17 @@
 // Admission gate: reject oversized requests with HTTP 400.
 // When compression is enabled, lets oversized requests through — post-compress check is the real gate.
 
+#include <algorithm>
+
+// The reply a request can get: what it asked for, trimmed to the context its
+// prompt leaves (0 when the prompt fills it). Clients ask for their usual cap
+// whatever the prompt, Claude Code 32000 tokens, so a long session would
+// otherwise be refused once prompt + cap passed the context.
+inline int fit_reply_to_context(int prompt_tokens, int max_output, int max_ctx)
+{
+    return (std::max)(0, (std::min)(max_output, max_ctx - prompt_tokens));
+}
+
 inline bool should_reject_oversized(int prompt_tokens, int max_output,
                                     int max_ctx, bool compression_enabled)
 {

server/src/server/http_server.cpp

@@ -2750,18 +2750,29 @@ bool HttpServer::render_and_tokenize_request(
 // Context-length gate. Oversized prompts pass through when compression
 // (pflash) will run — the post-compress check is the real gate.
 bool HttpServer::validate_request_context(
-        SocketHandle fd, const ParsedRequest & req, bool send_failure) {
+        SocketHandle fd, ParsedRequest & req, bool routed) {
     const int prompt_tokens = (int) req.prompt_tokens.size();
     const bool pflash_will_run =
         !req.images && config_.pflash_mode != ServerConfig::PflashMode::OFF &&
         drafter_tokenizer_ != nullptr &&
         (config_.pflash_mode == ServerConfig::PflashMode::ALWAYS ||
          prompt_tokens >= config_.pflash_threshold);
+    if (!routed && !pflash_will_run) {
+        const int fitted =
+            fit_reply_to_context(prompt_tokens, req.max_output, config_.max_ctx);
+        if (fitted > 0 && fitted < req.max_output) {
+            std::fprintf(stderr,
+                "[server] max_tokens=%d trimmed to %d: the prompt takes %d of "
+                "the %d-token context\n",
+                req.max_output, fitted, prompt_tokens, config_.max_ctx);
+            req.max_output = fitted;
+        }
+    }
     if (!should_reject_oversized(
             prompt_tokens, req.max_output, config_.max_ctx, pflash_will_run)) {
         return true;
     }
-    if (send_failure) send_error(fd, 400,
+    if (!routed) send_error(fd, 400,
                context_overflow_message(
                    config_.max_ctx, prompt_tokens, req.max_output));
     return false;
@@ -2943,7 +2954,7 @@ bool HttpServer::handle_model_request(SocketHandle fd, ParsedRequest & req,
         return true;
     }
 
-    if (!validate_request_context(fd, req, admission == nullptr)) {
+    if (!validate_request_context(fd, req, admission != nullptr)) {
         if (admission) *admission = RoutingAdmission::unfit;
         return true;
     }

server/src/server/http_server.h

@@ -667,8 +667,11 @@ class HttpServer {
         const std::vector<ChatMessage> & chat_messages,
         const ParsedRequest & req, bool add_generation_prompt,
         std::string & rendered, std::string & error);
-    bool validate_request_context(SocketHandle fd, const ParsedRequest & req,
-                                  bool send_failure = true);
+    // A direct request's max_tokens is trimmed to the context its prompt
+    // leaves; a routed one (another model may hold it whole) is refused
+    // without a response, so the router can try the next model.
+    bool validate_request_context(SocketHandle fd, ParsedRequest & req,
+                                  bool routed = false);
     // Constrain the reply to the request's structured output format and
     // tool_choice (common/output_constraint.h). False after sending a 400
     // for a format it cannot serve.

server/test/unit/test_admission.cpp

@@ -1,4 +1,4 @@
-// Unit tests for should_reject_oversized — pure, GPU-free.
+// Unit tests for should_reject_oversized and fit_reply_to_context — pure, GPU-free.
 
 #include "CppUnitTestFramework.hpp"
 #include "server/admission.h"
@@ -55,3 +55,17 @@ TEST_CASE(AdmissionFixture, test_effective_overflows_zero_length_hit_is_a_hit) {
 TEST_CASE(AdmissionFixture, test_effective_overflows_full_cache_hit_still_too_large) {
     CHECK(effective_prompt_overflows(200000, 60000, 10000, 65536));
 }
+
+TEST_CASE(AdmissionFixture, test_reply_fits_what_the_prompt_leaves) {
+    // Claude Code asks for 32000 whatever the prompt.
+    CHECK(fit_reply_to_context(110000, 32000, 131072) == 21072);
+    CHECK(fit_reply_to_context(1000, 32000, 131072) == 32000);
+    CHECK(fit_reply_to_context(131072, 32000, 131072) == 0);
+    CHECK(fit_reply_to_context(140000, 32000, 131072) == 0);
+    // Trimmed, the request is no longer oversized; a prompt that fills the
+    // context still is.
+    CHECK(!should_reject_oversized(110000, fit_reply_to_context(110000, 32000, 131072),
+                                   131072, false));
+    CHECK(should_reject_oversized(140000, fit_reply_to_context(140000, 32000, 131072),
+                                  131072, false));
+}