diff --git a/pkg-r/NEWS.md b/pkg-r/NEWS.md
index 65f63c06..f024a52b 100644
--- a/pkg-r/NEWS.md
+++ b/pkg-r/NEWS.md
@@ -1,5 +1,7 @@
 # shinychat (development version)
 
+* Added support for displaying thinking/reasoning content from models that support extended thinking. Thinking content appears in a collapsible block during streaming and on reload (@simonpcouch, #167).
+
 * Fixed an issue where user chat messages would display the default assistant icon. (#162)
 
 # shinychat 0.3.0
diff --git a/pkg-r/R/chat.R b/pkg-r/R/chat.R
index c73a5f8c..23f6932f 100644
--- a/pkg-r/R/chat.R
+++ b/pkg-r/R/chat.R
@@ -12,11 +12,13 @@ chat_deps <- function() {
     src = "lib/shiny",
     script = list(
       list(src = "chat/chat.js", type = "module"),
-      list(src = "markdown-stream/markdown-stream.js", type = "module")
+      list(src = "markdown-stream/markdown-stream.js", type = "module"),
+      list(src = "thinking/thinking.js")
     ),
     stylesheet = c(
       "chat/chat.css",
-      "markdown-stream/markdown-stream.css"
+      "markdown-stream/markdown-stream.css",
+      "thinking/thinking.css"
     )
   )
 }
@@ -504,6 +506,14 @@ rlang::on_load(
     chat_append_("", chunk = "start", icon = icon)
 
     res <- fastmap::fastqueue(200)
+    thinking_id <- NULL
+
+    send_thinking <- function(type, content = "") {
+      session$sendCustomMessage(
+        "shinychat-thinking",
+        list(id = thinking_id, type = type, content = content)
+      )
+    }
 
     for (msg in stream) {
       if (promises::is.promising(msg)) {
@@ -521,6 +531,21 @@ rlang::on_load(
         }
       }
 
+      if (S7::S7_inherits(msg, ellmer::ContentThinking)) {
+        if (is.null(thinking_id)) {
+          thinking_id <- paste0(
+            "think-",
+            format(Sys.time(), "%H%M%S"),
+            "-",
+            sample.int(1e6, 1)
+          )
+          send_thinking("start", msg@thinking)
+        } else {
+          send_thinking("update", msg@thinking)
+        }
+        next
+      }
+
       if (S7::S7_inherits(msg, ellmer::Content)) {
         msg <- contents_shinychat(msg)
       }
@@ -528,6 +553,10 @@ rlang::on_load(
       chat_append_(msg)
     }
 
+    if (!is.null(thinking_id)) {
+      send_thinking("done")
+    }
+
     chat_append_("", chunk = "end")
 
     res <- res$as_list()
diff --git a/pkg-r/R/contents_shinychat.R b/pkg-r/R/contents_shinychat.R
index 02e2b260..9bd4153c 100644
--- a/pkg-r/R/contents_shinychat.R
+++ b/pkg-r/R/contents_shinychat.R
@@ -136,6 +136,26 @@ S7::method(contents_shinychat, ellmer::ContentText) <- function(content) {
   content@text
 }
 
+S7::method(contents_shinychat, ellmer::ContentThinking) <- function(content) {
+  text <- content@thinking
+  preview <- strsplit(trimws(text), "\n")[[1]][1]
+  if (nchar(preview) > 60) {
+    preview <- paste0(substr(preview, 1, 60), "\u2026")
+  }
+
+  htmltools::div(
+    class = "shinychat-thinking",
+    `data-open` = "false",
+    htmltools::tags$button(
+      type = "button",
+      class = "shinychat-thinking-toggle",
+      `aria-expanded` = "false",
+      htmltools::span(class = "shinychat-thinking-preview", preview)
+    ),
+    htmltools::div(class = "shinychat-thinking-content", text)
+  )
+}
+
 new_tool_card <- function(type, request_id, tool_name, ...) {
   type <- arg_match(type, c("request", "result"))
 
@@ -356,10 +376,34 @@ tool_string <- function(x) {
   }
 }
 
+flush_thinking_buffer <- function(thinking_buffer) {
+  if (length(thinking_buffer) == 0) {
+    return(list())
+  }
+  combined <- ellmer::ContentThinking(paste(thinking_buffer, collapse = "\n\n"))
+  list(combined)
+}
+
+consolidate_thinking <- function(contents) {
+  consolidated <- list()
+  thinking_buffer <- character()
+
+  for (item in contents) {
+    if (S7::S7_inherits(item, ellmer::ContentThinking)) {
+      thinking_buffer <- c(thinking_buffer, item@thinking)
+    } else {
+      consolidated <- c(consolidated, flush_thinking_buffer(thinking_buffer))
+      thinking_buffer <- character()
+      consolidated <- c(consolidated, list(item))
+    }
+  }
+
+  c(consolidated, flush_thinking_buffer(thinking_buffer))
+}
 
 S7::method(contents_shinychat, ellmer::Turn) <- function(content) {
-  # Process all contents in the turn, filtering out empty results
-  compact(map(content@contents, contents_shinychat))
+  consolidated <- consolidate_thinking(content@contents)
+  compact(map(consolidated, contents_shinychat))
 }
 
 S7::method(contents_shinychat, S7::new_S3_class(c("Chat", "R6"))) <- function(
diff --git a/pkg-r/inst/lib/shiny/thinking/thinking.css b/pkg-r/inst/lib/shiny/thinking/thinking.css
new file mode 100644
index 00000000..fbc11673
--- /dev/null
+++ b/pkg-r/inst/lib/shiny/thinking/thinking.css
@@ -0,0 +1,48 @@
+.shinychat-thinking {
+  margin-bottom: 0.75rem;
+  border-left: 2px solid var(--bs-border-color, #dee2e6);
+  padding-left: 0.75rem;
+  font-size: 0.9em;
+  font-style: italic;
+  color: var(--bs-secondary-color, #6c757d);
+  cursor: pointer;
+}
+
+.shinychat-thinking:hover {
+  color: var(--bs-body-color, #212529);
+}
+
+.shinychat-thinking-toggle {
+  background: none;
+  border: none;
+  padding: 0;
+  display: flex;
+  align-items: center;
+  color: inherit;
+  font-size: inherit;
+  font-style: inherit;
+  text-align: left;
+  width: 100%;
+}
+
+.shinychat-thinking-preview {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  flex: 1;
+  min-width: 0;
+}
+
+.shinychat-thinking-content {
+  display: none;
+  margin-top: 0.5rem;
+  white-space: pre-wrap;
+}
+
+.shinychat-thinking[data-open="true"] .shinychat-thinking-preview {
+  display: none;
+}
+
+.shinychat-thinking[data-open="true"] .shinychat-thinking-content {
+  display: block;
+}
diff --git a/pkg-r/inst/lib/shiny/thinking/thinking.js b/pkg-r/inst/lib/shiny/thinking/thinking.js
new file mode 100644
index 00000000..e3bc8fe1
--- /dev/null
+++ b/pkg-r/inst/lib/shiny/thinking/thinking.js
@@ -0,0 +1,106 @@
+(function() {
+  const thinkingBlocks = new Map();
+
+  function findStreamingMessage() {
+    const messages = document.querySelector('shiny-chat-messages');
+    if (!messages) return null;
+    return messages.querySelector('shiny-chat-message[streaming]');
+  }
+
+  function createThinkingContainer(id) {
+    const wrapper = document.createElement('div');
+    wrapper.className = 'shinychat-thinking';
+    wrapper.dataset.id = id;
+    wrapper.dataset.open = 'false';
+
+    const button = document.createElement('button');
+    button.type = 'button';
+    button.className = 'shinychat-thinking-toggle';
+    button.setAttribute('aria-expanded', 'false');
+
+    const preview = document.createElement('span');
+    preview.className = 'shinychat-thinking-preview';
+    preview.textContent = '…';
+
+    button.appendChild(preview);
+
+    const content = document.createElement('div');
+    content.className = 'shinychat-thinking-content';
+
+    wrapper.appendChild(button);
+    wrapper.appendChild(content);
+
+    return { wrapper, preview, content, fullText: '' };
+  }
+
+  function handleThinkingMessage(payload) {
+    const { id, type, content: text } = payload;
+    const retries = payload._retries || 0;
+
+    if (type === 'start') {
+      const message = findStreamingMessage();
+      if (!message) {
+        if (retries < 20) {
+          setTimeout(() => handleThinkingMessage({ ...payload, _retries: retries + 1 }), 50);
+        }
+        return;
+      }
+
+      const parts = createThinkingContainer(id);
+      thinkingBlocks.set(id, parts);
+
+      const target = message.querySelector('shiny-markdown-stream') || message;
+      target.insertAdjacentElement('afterbegin', parts.wrapper);
+    }
+
+    if (type === 'update' || type === 'start') {
+      const parts = thinkingBlocks.get(id);
+      if (!parts) return;
+
+      if (text) {
+        parts.fullText += text;
+        parts.content.textContent = parts.fullText;
+
+        const previewText = parts.fullText.trim().split('\n')[0];
+        parts.preview.textContent = previewText.length > 60
+          ? previewText.slice(0, 60) + '…'
+          : previewText || '…';
+      }
+    }
+
+    if (type === 'done') {
+      const parts = thinkingBlocks.get(id);
+      if (!parts) return;
+
+      if (!parts.fullText.trim()) {
+        parts.wrapper.remove();
+      }
+      thinkingBlocks.delete(id);
+    }
+  }
+
+  function handleThinkingClick(event) {
+    const wrapper = event.target.closest('.shinychat-thinking');
+    if (!wrapper) return;
+
+    const button = wrapper.querySelector('button');
+    const isOpen = wrapper.dataset.open === 'true';
+    wrapper.dataset.open = (!isOpen).toString();
+    if (button) {
+      button.setAttribute('aria-expanded', (!isOpen).toString());
+    }
+  }
+
+  function register() {
+    if (!window.Shiny) return;
+    Shiny.addCustomMessageHandler('shinychat-thinking', handleThinkingMessage);
+  }
+
+  document.addEventListener('click', handleThinkingClick);
+
+  if (window.Shiny) {
+    register();
+  } else {
+    document.addEventListener('shiny:connected', register);
+  }
+})();
diff --git a/pkg-r/tests/testthat/_snaps/chat.md b/pkg-r/tests/testthat/_snaps/chat.md
index 8b0eefaf..f451e1e1 100644
--- a/pkg-r/tests/testthat/_snaps/chat.md
+++ b/pkg-r/tests/testthat/_snaps/chat.md
@@ -55,7 +55,7 @@
         "1.0.0", "")), span("world"))))
     Output
       $deps
-      [{"name":"foo","all_files":true},{"name":"shinychat","script":[{"src":"chat/chat.js","type":"module"},{"src":"markdown-stream/markdown-stream.js","type":"module"}],"stylesheet":["chat/chat.css","markdown-stream/markdown-stream.css"],"all_files":true},{"name":"bslib-tag-require","script":"tag-require.js","all_files":true},{"name":"htmltools-fill","stylesheet":"fill.css","all_files":true}] 
+      [{"name":"foo","all_files":true},{"name":"shinychat","script":[{"src":"chat/chat.js","type":"module"},{"src":"markdown-stream/markdown-stream.js","type":"module"},{"src":"thinking/thinking.js"}],"stylesheet":["chat/chat.css","markdown-stream/markdown-stream.css","thinking/thinking.css"],"all_files":true},{"name":"bslib-tag-require","script":"tag-require.js","all_files":true},{"name":"htmltools-fill","stylesheet":"fill.css","all_files":true}] 
       
       $html
       <shiny-chat-container class="html-fill-item html-fill-container" data-require-bs-caller="chat_ui" data-require-bs-version="5" fill id="chat" placeholder="Enter a message..." style="width:min(680px, 100%);height:auto;">
diff --git a/pkg-r/tests/testthat/test-contents_shinychat.R b/pkg-r/tests/testthat/test-contents_shinychat.R
index 47b38147..33170af9 100644
--- a/pkg-r/tests/testthat/test-contents_shinychat.R
+++ b/pkg-r/tests/testthat/test-contents_shinychat.R
@@ -37,6 +37,17 @@ test_that("opt_shinychat_tool_display handles options and environment variables"
   })
 })
 
+test_that("ContentThinking renders as collapsible HTML", {
+  thinking <- ellmer::ContentThinking("Let me think about this...")
+  result <- contents_shinychat(thinking)
+
+  expect_s3_class(result, "shiny.tag")
+  html <- as.character(result)
+  expect_match(html, "shinychat-thinking")
+  expect_match(html, "shinychat-thinking-content")
+  expect_match(html, "Let me think about this")
+})
+
 test_that("basic Content handling works", {
   ContentHTML <- S7::new_class(
     "ContentHTML",