diff --git a/shared/api/minja.hpp b/shared/api/minja.hpp
index 7bf110942..5426ce2f7 100644
--- a/shared/api/minja.hpp
+++ b/shared/api/minja.hpp
@@ -2212,6 +2212,61 @@ namespace minja
           }
           return res;
         }
+        else if (method->get_name() == "upper")
+        {
+          vargs.expectArgs("upper method", {0, 0}, {0, 0});
+          // ASCII-only case mapping for deterministic, locale-independent
+          // behavior. Matches upstream minja in practice for ASCII inputs;
+          // non-ASCII bytes are passed through unchanged. Full Unicode
+          // case folding is out of scope for the parser.
+          auto res = str;
+          for (char& c : res) {
+            if (c >= 'a' && c <= 'z') c = static_cast<char>(c - ('a' - 'A'));
+          }
+          return Value(res);
+        }
+        else if (method->get_name() == "lower")
+        {
+          vargs.expectArgs("lower method", {0, 0}, {0, 0});
+          // ASCII-only case mapping; see .upper() comment above.
+          auto res = str;
+          for (char& c : res) {
+            if (c >= 'A' && c <= 'Z') c = static_cast<char>(c + ('a' - 'A'));
+          }
+          return Value(res);
+        }
+        else if (method->get_name() == "replace")
+        {
+          vargs.expectArgs("replace method", {2, 3}, {0, 0});
+          auto before = vargs.args[0].get<std::string>();
+          auto after = vargs.args[1].get<std::string>();
+          auto res = str;
+          if (before.empty())
+          {
+            return Value(res);
+          }
+          // Python str.replace semantics: count < 0 means "replace all";
+          // count >= 0 limits the number of replacements; omitted argument
+          // also means "replace all".
+          int64_t count = (std::numeric_limits<int64_t>::max)();
+          if (vargs.args.size() == 3)
+          {
+            auto requested = vargs.args[2].get<int64_t>();
+            if (requested >= 0)
+            {
+              count = requested;
+            }
+          }
+          size_t start_pos = 0;
+          while (count > 0 &&
+                 (start_pos = res.find(before, start_pos)) != std::string::npos)
+          {
+            res.replace(start_pos, before.length(), after);
+            start_pos += after.length();
+            --count;
+          }
+          return Value(res);
+        }
       }
       throw std::runtime_error("Unknown method: " + method->get_name());
     }
diff --git a/test/pp_api_test/test_tokenizer_chat.cc b/test/pp_api_test/test_tokenizer_chat.cc
index 90e5f3f03..c1e7abe19 100644
--- a/test/pp_api_test/test_tokenizer_chat.cc
+++ b/test/pp_api_test/test_tokenizer_chat.cc
@@ -2255,4 +2255,92 @@ TEST(OrtxTokenizerTest, ChatTemplateDivisionByZero) {
         messages_json.c_str(), nullptr, result.ToBeAssigned(), false, false);
     EXPECT_NE(err, kOrtxOK) << "Expected modulo by zero to return an error.";
   }
+}
+
+// Regression tests for missing string methods (.replace, .upper, .lower) in
+// the vendored minja parser. Chat templates from popular HF models (e.g.
+// smollm3-3b) call these on string values; previously they failed with
+// "Unknown method: <name>". See:
+// - microsoft/onnxruntime-extensions#1081
+// - microsoft/Foundry-Local#800
+namespace {
+
+void ExpectTemplateRenders(OrtxTokenizer* tokenizer,
+                           const std::string& tmpl,
+                           const std::string& expected) {
+  std::string messages_json = R"([{"role":"user","content":"hi"}])";
+  OrtxObjectPtr<OrtxTensorResult> result;
+  auto err = OrtxApplyChatTemplate(
+      tokenizer, tmpl.c_str(), messages_json.c_str(), nullptr,
+      result.ToBeAssigned(), false, false);
+  ASSERT_EQ(err, kOrtxOK) << "template: " << tmpl
+                          << " err: " << OrtxGetLastErrorMessage();
+
+  OrtxObjectPtr<OrtxTensor> tensor;
+  err = OrtxTensorResultGetAt(result.get(), 0, tensor.ToBeAssigned());
+  ASSERT_EQ(err, kOrtxOK) << "OrtxTensorResultGetAt failed for template: " << tmpl;
+  ASSERT_EQ(tensor.Code(), kOrtxOK);
+  const char* text = nullptr;
+  err = OrtxGetTensorData(tensor.get(), reinterpret_cast<const void**>(&text),
+                          nullptr, nullptr);
+  ASSERT_EQ(err, kOrtxOK) << "OrtxGetTensorData failed for template: " << tmpl;
+  ASSERT_NE(text, nullptr) << "OrtxGetTensorData returned null data for template: " << tmpl;
+  EXPECT_STREQ(text, expected.c_str()) << "template: " << tmpl;
+}
+
+}  // namespace
+
+TEST(OrtxTokenizerTest, MinjaStringReplace) {
+  OrtxObjectPtr<OrtxTokenizer> tokenizer(OrtxCreateTokenizer, "data/phi-4-base");
+  ASSERT_EQ(tokenizer.Code(), kOrtxOK);
+
+  // Single replacement.
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'foo and foo'.replace('foo', 'bar') }}",
+                        "bar and bar");
+
+  // Chained replace + rstrip, exactly the pattern smollm3-3b uses.
+  ExpectTemplateRenders(
+      tokenizer.get(),
+      "{{ 'be helpful /no_think and /think  '"
+      ".replace('/no_think', '').replace('/think', '').rstrip() }}",
+      "be helpful  and");
+
+  // Replace with optional count argument (Python str.replace semantics).
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'aaaa'.replace('a', 'X', 2) }}",
+                        "XXaa");
+
+  // Python str.replace semantics: count < 0 means "replace all".
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'aaaa'.replace('a', 'X', -1) }}",
+                        "XXXX");
+
+  // Python str.replace semantics: count == 0 means no replacements.
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'aaaa'.replace('a', 'X', 0) }}",
+                        "aaaa");
+
+  // Replace with empty `before` is a no-op (matches upstream minja behavior).
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'abc'.replace('', 'X') }}",
+                        "abc");
+
+  // No match: original string returned unchanged.
+  ExpectTemplateRenders(tokenizer.get(),
+                        "{{ 'abc'.replace('z', 'X') }}",
+                        "abc");
+}
+
+TEST(OrtxTokenizerTest, MinjaStringUpperLower) {
+  OrtxObjectPtr<OrtxTokenizer> tokenizer(OrtxCreateTokenizer, "data/phi-4-base");
+  ASSERT_EQ(tokenizer.Code(), kOrtxOK);
+
+  ExpectTemplateRenders(tokenizer.get(), "{{ 'Hello, World!'.upper() }}",
+                        "HELLO, WORLD!");
+  ExpectTemplateRenders(tokenizer.get(), "{{ 'Hello, World!'.lower() }}",
+                        "hello, world!");
+  // Idempotent on already-cased strings.
+  ExpectTemplateRenders(tokenizer.get(), "{{ 'ABC'.upper() }}", "ABC");
+  ExpectTemplateRenders(tokenizer.get(), "{{ 'abc'.lower() }}", "abc");
 }
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