diff --git a/lua/ai_commit_msg.lua b/lua/ai_commit_msg.lua
index 893623f..8921817 100644
--- a/lua/ai_commit_msg.lua
+++ b/lua/ai_commit_msg.lua
@@ -64,6 +64,20 @@ function M.format_cost(cost_info, format)
   end
 end
 
+-- Small model patterns: auto-select SMALL_MODEL_SYSTEM_PROMPT when the user
+-- hasn't explicitly overridden system_prompt and the model name matches.
+local SMALL_MODEL_PATTERNS = { "haiku", "nano", "lite" }
+
+local function is_small_model(model)
+  local model_lower = model:lower()
+  for _, pattern in ipairs(SMALL_MODEL_PATTERNS) do
+    if model_lower:find(pattern, 1, true) then
+      return true
+    end
+  end
+  return false
+end
+
 -- Get the active provider configuration
 function M.get_active_provider_config()
   local provider_name = M.config.provider
@@ -73,10 +87,17 @@ function M.get_active_provider_config()
     error("No configuration found for provider: " .. tostring(provider_name))
   end
 
-  -- Return a merged config with provider-specific settings
   local active_config = vim.tbl_deep_extend("force", {}, provider_config)
   active_config.provider = provider_name
 
+  -- Auto-select small model prompt if the user hasn't overridden system_prompt
+  local default_provider = config_mod.default.providers[provider_name]
+  local uses_default_prompt = default_provider
+    and active_config.system_prompt == default_provider.system_prompt
+  if uses_default_prompt and active_config.model and is_small_model(active_config.model) then
+    active_config.system_prompt = require("ai_commit_msg.prompts").SMALL_MODEL_SYSTEM_PROMPT
+  end
+
   return active_config
 end
 
diff --git a/lua/ai_commit_msg/config.lua b/lua/ai_commit_msg/config.lua
index 7cb59f7..dadfca9 100644
--- a/lua/ai_commit_msg/config.lua
+++ b/lua/ai_commit_msg/config.lua
@@ -21,7 +21,7 @@ local DEFAULT_SYSTEM_PROMPT = require("ai_commit_msg.prompts").DEFAULT_SYSTEM_PR
 
 ---@class AiCommitMsgConfig
 ---@field enabled boolean Whether to enable the plugin
----@field provider string AI provider to use ("openai", "anthropic", or "gemini")
+---@field provider string AI provider to use ("openai", "anthropic", "gemini", "copilot", or "claude_code")
 ---@field providers table<string, ProviderConfig> Provider-specific configurations
 ---@field auto_push_prompt boolean Whether to prompt for push after commit
 ---@field pull_before_push { enabled: boolean, args: string[] } Whether and how to run `git pull` before pushing
@@ -113,6 +113,12 @@ M.default = {
       system_prompt = DEFAULT_SYSTEM_PROMPT,
       pricing = {},
     },
+    claude_code = {
+      model = "sonnet",
+      prompt = DEFAULT_PROMPT,
+      system_prompt = DEFAULT_SYSTEM_PROMPT,
+      pricing = {},
+    },
   },
 }
 
diff --git a/lua/ai_commit_msg/prompts.lua b/lua/ai_commit_msg/prompts.lua
index 93db4d7..34a33c3 100644
--- a/lua/ai_commit_msg/prompts.lua
+++ b/lua/ai_commit_msg/prompts.lua
@@ -211,4 +211,43 @@ Examples:
 - refactor(router): inline trivial helper
 ]]
 
+-- Optimized for small/fast models (Haiku, GPT-4.1-nano, Flash-Lite).
+-- Shorter, flatter structure. Explicit body rules to avoid terse one-liners.
+M.SMALL_MODEL_SYSTEM_PROMPT = [[
+Generate ONE Conventional Commit message from the git diff below.
+Output plain text only. No markdown, no quotes, no commentary.
+
+FORMAT:
+<type>(scope): description
+
+- bullet explaining what and why
+- bullet for behavior change or secondary impact
+
+TYPES: feat fix docs style refactor perf test build ci chore revert
+
+RULES:
+1. Header <= 72 chars. Imperative mood ("add" not "added"). No period.
+2. Scope: lowercase, parenthesized, pick the primary area changed.
+3. Include 2-4 body bullets for any non-trivial change. Each bullet starts with "- ".
+4. Bullets explain WHY, not just WHAT. Group related changes.
+5. Skip body ONLY for single-line trivial changes (typo, formatting).
+6. Only describe lines that actually changed (+/- in the diff).
+7. Do NOT list files. Do NOT repeat the header in the body.
+8. For breaking changes: add ! after scope and a BREAKING CHANGE: footer.
+9. One commit only. No extras.
+
+EXAMPLES:
+
+feat(auth): add JWT refresh token rotation
+- prevent token reuse after refresh to mitigate replay attacks
+- store token family ID to detect stolen refresh tokens
+- update middleware to validate token lineage
+
+fix(ui): correct dropdown z-index in modal overlay
+- modal content was rendered behind the navigation bar
+- set explicit stacking context on the modal container
+
+chore(deps): bump eslint from 8.56 to 9.0
+]]
+
 return M
diff --git a/lua/ai_commit_msg/providers/claude_code.lua b/lua/ai_commit_msg/providers/claude_code.lua
new file mode 100644
index 0000000..7d94089
--- /dev/null
+++ b/lua/ai_commit_msg/providers/claude_code.lua
@@ -0,0 +1,80 @@
+local M = {}
+
+function M.call_api(config, diff, callback)
+  if vim.fn.executable("claude") ~= 1 then
+    callback(false, "claude CLI not found. Install Claude Code: https://docs.anthropic.com/en/docs/claude-code")
+    return
+  end
+
+  local prompt
+  if config.prompt:find("{diff}", 1, true) then
+    local before, after = config.prompt:match("^(.*)%{diff%}(.*)$")
+    if before and after then
+      prompt = before .. diff .. after
+    else
+      prompt = config.prompt .. "\n\n" .. diff
+    end
+  else
+    prompt = config.prompt .. "\n\n" .. diff
+  end
+
+  vim.schedule(function()
+    require("ai_commit_msg.config").notify(
+      "ai-commit-msg.nvim: Prompt length: " .. #prompt .. " chars",
+      vim.log.levels.DEBUG
+    )
+  end)
+
+  local cmd = {
+    "claude",
+    "-p",
+    "--output-format",
+    "json",
+    "--max-turns",
+    "1",
+    "--system-prompt",
+    config.system_prompt,
+  }
+
+  if config.model then
+    table.insert(cmd, "--model")
+    table.insert(cmd, config.model)
+  end
+
+  vim.system(cmd, { stdin = prompt }, function(res)
+    if res.code ~= 0 then
+      callback(false, "claude CLI failed: " .. (res.stderr or "Unknown error"))
+      return
+    end
+
+    local ok, response = pcall(vim.json.decode, res.stdout)
+    if not ok then
+      callback(false, "Failed to parse claude CLI response: " .. tostring(response))
+      return
+    end
+
+    if response.is_error then
+      callback(false, "Claude Code error: " .. (response.result or "Unknown error"))
+      return
+    end
+
+    vim.schedule(function()
+      require("ai_commit_msg.config").notify(
+        "ai-commit-msg.nvim: Full CLI response: " .. vim.inspect(response),
+        vim.log.levels.DEBUG
+      )
+    end)
+
+    if response.result then
+      local commit_msg = response.result
+      commit_msg = commit_msg:gsub("^```%w*\n", ""):gsub("\n```$", ""):gsub("^`", ""):gsub("`$", "")
+      commit_msg = vim.trim(commit_msg)
+
+      callback(true, commit_msg, nil)
+    else
+      callback(false, "Unexpected claude CLI response format")
+    end
+  end)
+end
+
+return M
diff --git a/lua/ai_commit_msg/providers/init.lua b/lua/ai_commit_msg/providers/init.lua
index 25611f1..24c3f5e 100644
--- a/lua/ai_commit_msg/providers/init.lua
+++ b/lua/ai_commit_msg/providers/init.lua
@@ -1,18 +1,19 @@
 local M = {}
 
+local PROVIDERS = {
+  openai = "ai_commit_msg.providers.openai",
+  anthropic = "ai_commit_msg.providers.anthropic",
+  gemini = "ai_commit_msg.providers.gemini",
+  copilot = "ai_commit_msg.providers.copilot",
+  claude_code = "ai_commit_msg.providers.claude_code",
+}
+
 function M.get_provider(config)
-  local provider_name = config.provider
-  if provider_name == "openai" then
-    return require("ai_commit_msg.providers.openai")
-  elseif provider_name == "anthropic" then
-    return require("ai_commit_msg.providers.anthropic")
-  elseif provider_name == "gemini" then
-    return require("ai_commit_msg.providers.gemini")
-  elseif provider_name == "copilot" then
-    return require("ai_commit_msg.providers.copilot")
-  else
-    error("Unsupported provider: " .. tostring(provider_name))
+  local module = PROVIDERS[config.provider]
+  if not module then
+    error("Unsupported provider: " .. tostring(config.provider))
   end
+  return require(module)
 end
 
 function M.call_api(config, diff, callback)
diff --git a/spec/claude_code_provider_spec.lua b/spec/claude_code_provider_spec.lua
new file mode 100644
index 0000000..90ac6eb
--- /dev/null
+++ b/spec/claude_code_provider_spec.lua
@@ -0,0 +1,157 @@
+-- Minimal vim mock for running outside nvim
+if not rawget(_G, "vim") then
+  _G.vim = {
+    fn = { executable = function() return 1 end },
+    system = function() end,
+    schedule = function(fn) fn() end,
+    json = {
+      encode = function(t) return require("dkjson").encode(t) end,
+      decode = function(s) return require("dkjson").decode(s) end,
+    },
+    trim = function(s) return s:match("^%s*(.-)%s*$") end,
+    inspect = function(t) return tostring(t) end,
+    log = { levels = { DEBUG = 1, INFO = 2, WARN = 3, ERROR = 4 } },
+  }
+end
+
+describe("claude_code provider", function()
+  local claude_code_provider
+  local original_executable = vim.fn.executable
+  local original_vim_system = vim.system
+
+  local default_config = {
+    model = "sonnet",
+    prompt = "{diff}",
+    system_prompt = "Generate a commit message",
+  }
+
+  local function mock_cli_response(response)
+    vim.system = function(cmd, opts, callback)
+      callback({
+        code = 0,
+        stdout = type(response) == "string" and response or vim.json.encode(response),
+      })
+      return cmd
+    end
+  end
+
+  before_each(function()
+    package.loaded["ai_commit_msg.providers.claude_code"] = nil
+    package.loaded["ai_commit_msg.config"] = nil
+    package.preload["ai_commit_msg.config"] = function()
+      return { notify = function() end }
+    end
+    claude_code_provider = require("ai_commit_msg.providers.claude_code")
+    vim.fn.executable = function() return 1 end
+  end)
+
+  after_each(function()
+    vim.fn.executable = original_executable
+    vim.system = original_vim_system
+  end)
+
+  describe("call_api", function()
+    it("calls callback with error when claude CLI not found", function()
+      vim.fn.executable = function() return 0 end
+
+      local result_success, result_message
+      claude_code_provider.call_api({}, "test diff", function(success, message)
+        result_success = success
+        result_message = message
+      end)
+
+      assert.is_false(result_success)
+      assert.truthy(result_message:find("claude CLI not found"))
+    end)
+
+    it("passes model flag from config", function()
+      local captured_cmd
+      vim.system = function(cmd, opts, callback)
+        captured_cmd = cmd
+        callback({
+          code = 0,
+          stdout = '{"type":"result","is_error":false,"result":"feat: test commit","usage":{"input_tokens":10,"output_tokens":5}}',
+        })
+      end
+
+      claude_code_provider.call_api(default_config, "test diff", function() end)
+
+      local found_model = false
+      for i, arg in ipairs(captured_cmd) do
+        if arg == "--model" and captured_cmd[i + 1] == "sonnet" then
+          found_model = true
+          break
+        end
+      end
+      assert.is_true(found_model)
+    end)
+
+    it("pipes prompt via stdin and system prompt via flag", function()
+      local captured_cmd, captured_opts
+      vim.system = function(cmd, opts, callback)
+        captured_cmd = cmd
+        captured_opts = opts
+        callback({
+          code = 0,
+          stdout = '{"type":"result","is_error":false,"result":"feat: test commit","usage":{"input_tokens":10,"output_tokens":5}}',
+        })
+      end
+
+      local config = { model = "haiku", prompt = "{diff}", system_prompt = "Generate a commit message" }
+      claude_code_provider.call_api(config, "my test diff", function() end)
+
+      assert.truthy(captured_opts.stdin)
+      assert.truthy(captured_opts.stdin:find("my test diff"))
+
+      local found_system_prompt = false
+      for i, arg in ipairs(captured_cmd) do
+        if arg == "--system-prompt" and captured_cmd[i + 1] == "Generate a commit message" then
+          found_system_prompt = true
+          break
+        end
+      end
+      assert.is_true(found_system_prompt)
+    end)
+
+    it("parses successful response and extracts usage", function()
+      mock_cli_response({
+        type = "result",
+        is_error = false,
+        result = "feat: add new feature",
+        usage = {
+          input_tokens = 100,
+          cache_creation_input_tokens = 50,
+          cache_read_input_tokens = 200,
+          output_tokens = 20,
+        },
+      })
+
+      local result_success, result_message, result_usage
+      local config = { model = "haiku", prompt = "{diff}", system_prompt = "test" }
+      claude_code_provider.call_api(config, "test diff", function(success, message, usage)
+        result_success = success
+        result_message = message
+        result_usage = usage
+      end)
+
+      assert.is_true(result_success)
+      assert.equals("feat: add new feature", result_message)
+      assert.equals(350, result_usage.input_tokens) -- 100 + 50 + 200
+      assert.equals(20, result_usage.output_tokens)
+    end)
+
+    it("handles error response from claude CLI", function()
+      mock_cli_response('{"type":"result","is_error":true,"result":"Something went wrong"}')
+
+      local result_success, result_message
+      local config = { model = "haiku", prompt = "{diff}", system_prompt = "test" }
+      claude_code_provider.call_api(config, "test diff", function(success, message)
+        result_success = success
+        result_message = message
+      end)
+
+      assert.is_false(result_success)
+      assert.truthy(result_message:find("Something went wrong"))
+    end)
+  end)
+end)