diff --git a/common/chat-parser-xml-toolcall.cpp b/common/chat-parser-xml-toolcall.cpp
index d7e54cabfa..e4bbfaddd5 100644
--- a/common/chat-parser-xml-toolcall.cpp
+++ b/common/chat-parser-xml-toolcall.cpp
@@ -498,7 +498,9 @@ inline bool parse_xml_tool_calls(common_chat_msg_parser & builder, const struct
             }
             auto val_start = builder.pos();
 
-            // vLLM-style: only try to parse value when there is content; empty/whitespace or non-JSON start = incomplete (avoids "parse empty input" log)
+            // vLLM-style: only try to parse value when there is content; empty/whitespace = incomplete (avoids "parse empty input" log).
+            // When remainder does not look like JSON start, skip try_consume_json and fall through to plain-text path (e.g. "explore").
+            bool looks_like_json = true;
             {
                 const auto & inp = builder.input();
                 const size_t rem_len = (val_start < inp.size()) ? (inp.size() - val_start) : 0;
@@ -511,38 +513,27 @@ inline bool parse_xml_tool_calls(common_chat_msg_parser & builder, const struct
                         (form.key_val_sep2 ? " " + gbnf_format_literal(*form.key_val_sep2) : "")
                     );
                 }
-                // Only call try_consume_json when remainder looks like start of a JSON value (avoids SAX error-at-position-0 → "empty input" log)
+                // Only call try_consume_json when remainder looks like start of a JSON value (avoids SAX error-at-position-0 → "empty input" log).
+                // Otherwise fall through to plain-text path (e.g. subagent_type=explore).
                 size_t pos = 0;
                 while (pos < rest_sv.size() && std::isspace(static_cast<unsigned char>(rest_sv[pos]))) { ++pos; }
                 if (pos >= rest_sv.size()) {
-                    gen_partial_args([&](auto & rest, auto & needle) { arguments[key] = (form.trim_raw_argval ? string_strip(rest) : rest) + needle; });
-                    throw common_chat_msg_partial_exception(
-                        "Expected " + gbnf_format_literal(form.val_end) +
-                        " after " + gbnf_format_literal(form.key_val_sep) +
-                        (form.key_val_sep2 ? " " + gbnf_format_literal(*form.key_val_sep2) : "")
-                    );
-                }
-                std::string_view rest_trim = rest_sv.substr(pos);
-                char c = rest_trim[0];
-                bool looks_like_json = (c == '"' || c == '{' || c == '[' || (c >= '0' && c <= '9') || c == '-');
-                if (!looks_like_json) {
-                    if (c == 't') looks_like_json = (rest_trim.size() <= 4 && std::string_view("true").substr(0, rest_trim.size()) == rest_trim);
-                    else if (c == 'f') looks_like_json = (rest_trim.size() <= 5 && std::string_view("false").substr(0, rest_trim.size()) == rest_trim);
-                    else if (c == 'n') looks_like_json = (rest_trim.size() <= 4 && std::string_view("null").substr(0, rest_trim.size()) == rest_trim);
-                }
-                if (!looks_like_json) {
-                    gen_partial_args([&](auto & rest, auto & needle) { arguments[key] = (form.trim_raw_argval ? string_strip(rest) : rest) + needle; });
-                    throw common_chat_msg_partial_exception(
-                        "Expected " + gbnf_format_literal(form.val_end) +
-                        " after " + gbnf_format_literal(form.key_val_sep) +
-                        (form.key_val_sep2 ? " " + gbnf_format_literal(*form.key_val_sep2) : "")
-                    );
+                    looks_like_json = false;
+                } else {
+                    std::string_view rest_trim = rest_sv.substr(pos);
+                    char c = rest_trim[0];
+                    looks_like_json = (c == '"' || c == '{' || c == '[' || (c >= '0' && c <= '9') || c == '-');
+                    if (!looks_like_json) {
+                        if (c == 't') looks_like_json = (rest_trim.size() <= 4 && std::string_view("true").substr(0, rest_trim.size()) == rest_trim);
+                        else if (c == 'f') looks_like_json = (rest_trim.size() <= 5 && std::string_view("false").substr(0, rest_trim.size()) == rest_trim);
+                        else if (c == 'n') looks_like_json = (rest_trim.size() <= 4 && std::string_view("null").substr(0, rest_trim.size()) == rest_trim);
+                    }
                 }
             }
 
-            // Test if arg_val is a partial JSON
+            // Test if arg_val is a partial JSON (only when remainder looks like JSON; else plain-text path below)
             std::optional<common_json> value_json = std::nullopt;
-            if (!form.raw_argval || !*form.raw_argval) {
+            if ((!form.raw_argval || !*form.raw_argval) && looks_like_json) {
                 try { value_json = builder.try_consume_json(); }
                 catch (const std::runtime_error&) { builder.move_to(val_start); }
                 // TODO: Delete this when json_partial adds top-level support for null/true/false
diff --git a/tools/server/server-context.cpp b/tools/server/server-context.cpp
index ceafcac179..a555a76442 100644
--- a/tools/server/server-context.cpp
+++ b/tools/server/server-context.cpp
@@ -2747,7 +2747,17 @@ private:
 
                 slot.i_batch = -1;
 
-                common_sampler_accept(slot.smpl.get(), id, true);
+                try {
+                    common_sampler_accept(slot.smpl.get(), id, true);
+                } catch (const std::runtime_error & e) {
+                    // Grammar constraint violation (e.g. "Unexpected empty grammar stack") - return 500 instead of aborting
+                    SRV_ERR("slot %d: grammar error, releasing slot: %s\n", slot.id, e.what());
+                    send_error(slot, std::string("Grammar constraint violation: ") + e.what(), ERROR_TYPE_SERVER);
+                    slot.print_timings();
+                    metrics.on_prediction(slot);
+                    slot.release();
+                    continue;
+                }
 
                 // here we have synchronized the llama_context (due to the sampling above), so we can do time measurement
                 const int64_t t_current = ggml_time_us();
@@ -2791,7 +2801,17 @@ private:
                 const size_t n_draft = slot.drafted.size();
 
                 // the accepted tokens from the speculation
-                const auto ids = common_sampler_sample_and_accept_n(slot.smpl.get(), ctx, slot.i_batch_dft, slot.drafted);
+                std::vector<llama_token> ids;
+                try {
+                    ids = common_sampler_sample_and_accept_n(slot.smpl.get(), ctx, slot.i_batch_dft, slot.drafted);
+                } catch (const std::runtime_error & e) {
+                    SRV_ERR("slot %d: grammar error during speculative decoding, releasing slot: %s\n", slot.id, e.what());
+                    send_error(slot, std::string("Grammar constraint violation: ") + e.what(), ERROR_TYPE_SERVER);
+                    slot.print_timings();
+                    metrics.on_prediction(slot);
+                    slot.release();
+                    continue;
+                }
                 slot.i_batch_dft.clear();
                 slot.drafted.clear();