diff --git a/.env.example b/.env.example
index f5e98e7a4..b11153b27 100644
--- a/.env.example
+++ b/.env.example
@@ -25,19 +25,34 @@ OPENAI_ACCESS_TOKEN=
 OPENAI_MODEL=
 OPENAI_URI_BASE=
 
+# Optional: LLM token budget (applies to chat, auto-categorize, merchant detection, PDF processing).
+# Lower these for small-context local models (Ollama, LM Studio, LocalAI).
+# Defaults work for modern cloud OpenAI models without configuration.
+# LLM_CONTEXT_WINDOW=2048
+# LLM_MAX_RESPONSE_TOKENS=512
+# LLM_MAX_HISTORY_TOKENS=
+# LLM_SYSTEM_PROMPT_RESERVE=256
+# LLM_MAX_ITEMS_PER_CALL=25
+
+# Optional: OpenAI-compatible capability flags
+# OPENAI_REQUEST_TIMEOUT=60                  # HTTP timeout in seconds; raise for slow local models
+# OPENAI_SUPPORTS_PDF_PROCESSING=true        # Set to false for endpoints without vision support
+# OPENAI_SUPPORTS_RESPONSES_ENDPOINT=        # Override Responses-API vs chat.completions routing
+# LLM_JSON_MODE=                             # auto | strict | json_object | none
+
 # Optional: External AI Assistant — delegates chat to a remote AI agent
 # instead of calling LLMs directly. The agent calls back to Sure's /mcp endpoint.
 # See docs/hosting/ai.md for full details.
 # ASSISTANT_TYPE=external
 # EXTERNAL_ASSISTANT_URL=https://your-agent-host/v1/chat/completions
-# EXTERNAL_ASSISTANT_TOKEN=your-api-token
+# EXTERNAL_ASSISTANT_TOKEN=your-api-token # pipelock:ignore
 # EXTERNAL_ASSISTANT_AGENT_ID=main
 # EXTERNAL_ASSISTANT_SESSION_KEY=agent:main:main
 # EXTERNAL_ASSISTANT_ALLOWED_EMAILS=user@example.com
 
 # Optional: MCP server endpoint — enables /mcp for external AI assistants.
 # Both values are required. MCP_USER_EMAIL must match an existing user's email.
-# MCP_API_TOKEN=your-random-bearer-token
+# MCP_API_TOKEN=your-random-bearer-token # pipelock:ignore
 # MCP_USER_EMAIL=user@example.com
 
 # Optional: Langfuse config
@@ -82,7 +97,7 @@ EMAIL_SENDER=
 # Database Configuration
 DB_HOST=localhost # May need to be changed to `DB_HOST=db` if using devcontainer
 DB_PORT=5432
-POSTGRES_PASSWORD=postgres
+POSTGRES_PASSWORD=postgres # pipelock:ignore
 POSTGRES_USER=postgres
 
 # Redis configuration
@@ -94,7 +109,7 @@ REDIS_URL=redis://localhost:6379/1
 # REDIS_SENTINEL_HOSTS=sentinel1:26379,sentinel2:26379,sentinel3:26379
 # REDIS_SENTINEL_MASTER=mymaster
 # REDIS_SENTINEL_USERNAME=default
-# REDIS_PASSWORD=your-redis-password
+# REDIS_PASSWORD=your-redis-password # pipelock:ignore
 
 # App Domain
 # This is the domain that your Sure instance will be hosted at. It is used to generate links in emails and other places.
diff --git a/.env.local.example b/.env.local.example
index 75a8de778..1795cc8b9 100644
--- a/.env.local.example
+++ b/.env.local.example
@@ -28,8 +28,22 @@ TWELVE_DATA_API_KEY =
 OPENAI_ACCESS_TOKEN =
 OPENAI_URI_BASE =
 OPENAI_MODEL =
-# OPENAI_REQUEST_TIMEOUT: Request timeout in seconds (default: 60)
-# OPENAI_SUPPORTS_PDF_PROCESSING: Set to false for endpoints without vision support (default: true)
+
+# LLM token budget. Applies to ALL outbound LLM calls: chat history,
+# auto-categorize, merchant detection, provider enhancer, PDF processing.
+# Defaults to Ollama's historical 2048-token baseline so small local models
+# work out of the box — raise explicitly for cloud or larger-context models.
+# LLM_CONTEXT_WINDOW = 2048          # Total tokens the model will accept
+# LLM_MAX_RESPONSE_TOKENS = 512      # Reserved for the model's reply
+# LLM_MAX_HISTORY_TOKENS =           # Derived if unset (context - response - system_reserve)
+# LLM_SYSTEM_PROMPT_RESERVE = 256    # Tokens reserved for the system prompt
+# LLM_MAX_ITEMS_PER_CALL = 25        # Upper bound on auto-categorize / merchant batches
+
+# OpenAI-compatible capability flags (custom/self-hosted providers)
+# OPENAI_REQUEST_TIMEOUT = 60                # HTTP timeout in seconds; raise for slow local models
+# OPENAI_SUPPORTS_PDF_PROCESSING = true      # Set to false for endpoints without vision support
+# OPENAI_SUPPORTS_RESPONSES_ENDPOINT =       # true to force Responses API on custom providers
+# LLM_JSON_MODE =                            # auto | strict | json_object | none
 
 # (example: LM Studio/Docker config) OpenAI-compatible API endpoint config
 # OPENAI_URI_BASE = http://host.docker.internal:1234/
diff --git a/app/controllers/settings/hostings_controller.rb b/app/controllers/settings/hostings_controller.rb
index 59a85939a..063f2a6b0 100644
--- a/app/controllers/settings/hostings_controller.rb
+++ b/app/controllers/settings/hostings_controller.rb
@@ -1,6 +1,15 @@
 class Settings::HostingsController < ApplicationController
   layout "settings"
 
+  # Minimum accepted value for each configurable LLM budget field. Mirrors the
+  # `min:` attribute on the form inputs in `_openai_settings.html.erb` so the
+  # controller rejects what the browser-side validator would reject.
+  LLM_BUDGET_MINIMUMS = {
+    llm_context_window: 256,
+    llm_max_response_tokens: 64,
+    llm_max_items_per_call: 1
+  }.freeze
+
   guard_feature unless: -> { self_hosted? }
 
   before_action :ensure_admin, only: [ :update, :clear_cache, :disconnect_external_assistant ]
@@ -157,6 +166,21 @@ class Settings::HostingsController < ApplicationController
       Setting.openai_json_mode = hosting_params[:openai_json_mode].presence
     end
 
+    LLM_BUDGET_MINIMUMS.each do |key, minimum|
+      next unless hosting_params.key?(key)
+      raw = hosting_params[key].to_s.strip
+      if raw.blank?
+        Setting.public_send("#{key}=", nil)
+        next
+      end
+      parsed = Integer(raw, 10) rescue nil
+      if parsed.nil? || parsed < minimum
+        label = t("settings.hostings.openai_settings.#{key}_label")
+        raise Setting::ValidationError, t(".invalid_llm_budget", field: label, minimum: minimum)
+      end
+      Setting.public_send("#{key}=", parsed)
+    end
+
     if hosting_params.key?(:external_assistant_url)
       Setting.external_assistant_url = hosting_params[:external_assistant_url]
     end
@@ -199,7 +223,7 @@ class Settings::HostingsController < ApplicationController
   private
     def hosting_params
       return ActionController::Parameters.new unless params.key?(:setting)
-      params.require(:setting).permit(:onboarding_state, :require_email_confirmation, :invite_only_default_family_id, :brand_fetch_client_id, :brand_fetch_high_res_logos, :twelve_data_api_key, :tiingo_api_key, :eodhd_api_key, :alpha_vantage_api_key, :openai_access_token, :openai_uri_base, :openai_model, :openai_json_mode, :exchange_rate_provider, :securities_provider, :syncs_include_pending, :auto_sync_enabled, :auto_sync_time, :external_assistant_url, :external_assistant_token, :external_assistant_agent_id, securities_providers: [])
+      params.require(:setting).permit(:onboarding_state, :require_email_confirmation, :invite_only_default_family_id, :brand_fetch_client_id, :brand_fetch_high_res_logos, :twelve_data_api_key, :tiingo_api_key, :eodhd_api_key, :alpha_vantage_api_key, :openai_access_token, :openai_uri_base, :openai_model, :openai_json_mode, :llm_context_window, :llm_max_response_tokens, :llm_max_items_per_call, :exchange_rate_provider, :securities_provider, :syncs_include_pending, :auto_sync_enabled, :auto_sync_time, :external_assistant_url, :external_assistant_token, :external_assistant_agent_id, securities_providers: [])
     end
 
     def update_assistant_type
diff --git a/app/models/assistant/builtin.rb b/app/models/assistant/builtin.rb
index 1d615eb5a..14bc9c05d 100644
--- a/app/models/assistant/builtin.rb
+++ b/app/models/assistant/builtin.rb
@@ -63,6 +63,13 @@ class Assistant::Builtin < Assistant::Base
     responder.respond(previous_response_id: latest_response_id)
   rescue => e
     stop_thinking
+    # If we streamed any partial content before the error, the message was
+    # persisted with the default `complete` status. Demote it to `failed` so
+    # `Assistant::Responder#conversation_history` won't feed a broken turn
+    # back into future prompts.
+    if assistant_message&.persisted?
+      assistant_message.update_columns(status: "failed")
+    end
     chat.add_error(e)
   end
 
diff --git a/app/models/assistant/history_trimmer.rb b/app/models/assistant/history_trimmer.rb
new file mode 100644
index 000000000..02e64f8b9
--- /dev/null
+++ b/app/models/assistant/history_trimmer.rb
@@ -0,0 +1,53 @@
+class Assistant::HistoryTrimmer
+  def initialize(messages, max_tokens:)
+    @messages = messages || []
+    @max_tokens = max_tokens.to_i
+  end
+
+  def call
+    return [] if @messages.empty? || @max_tokens <= 0
+
+    kept = []
+    tokens = 0
+
+    group_tool_pairs(@messages).reverse_each do |group|
+      group_tokens = Assistant::TokenEstimator.estimate(group)
+      break if tokens + group_tokens > @max_tokens
+
+      kept.unshift(*group)
+      tokens += group_tokens
+    end
+
+    kept
+  end
+
+  private
+
+    # Bundles each assistant message that has `tool_calls` with the
+    # consecutive `role: "tool"` results that follow it, so the trimmer
+    # never splits a call/result pair when dropping from the oldest end.
+    def group_tool_pairs(messages)
+      groups = []
+      current_group = nil
+
+      messages.each do |msg|
+        if assistant_with_tool_calls?(msg)
+          groups << current_group if current_group
+          current_group = [ msg ]
+        elsif msg[:role].to_s == "tool" && current_group
+          current_group << msg
+        else
+          groups << current_group if current_group
+          current_group = nil
+          groups << [ msg ]
+        end
+      end
+
+      groups << current_group if current_group
+      groups
+    end
+
+    def assistant_with_tool_calls?(msg)
+      msg[:role].to_s == "assistant" && msg[:tool_calls].is_a?(Array) && msg[:tool_calls].any?
+    end
+end
diff --git a/app/models/assistant/responder.rb b/app/models/assistant/responder.rb
index f2b6d121a..480c69c22 100644
--- a/app/models/assistant/responder.rb
+++ b/app/models/assistant/responder.rb
@@ -79,6 +79,7 @@ class Assistant::Responder
         instructions: instructions,
         functions: function_tool_caller.function_definitions,
         function_results: function_results,
+        messages: conversation_history,
         streamer: streamer,
         previous_response_id: previous_response_id,
         session_id: chat_session_id,
@@ -114,4 +115,46 @@ class Assistant::Responder
     def chat
       @chat ||= message.chat
     end
+
+    def conversation_history
+      messages = []
+      return messages unless chat&.messages
+
+      chat.messages
+          .where(type: [ "UserMessage", "AssistantMessage" ], status: "complete")
+          .includes(:tool_calls)
+          .ordered
+          .each do |chat_message|
+        if chat_message.tool_calls.any?
+          messages << {
+            role: chat_message.role,
+            content: chat_message.content || "",
+            tool_calls: chat_message.tool_calls.map(&:to_tool_call)
+          }
+
+          chat_message.tool_calls.map(&:to_result).each do |fn_result|
+            # Handle nil explicitly to avoid serializing to "null"
+            output = fn_result[:output]
+            content = if output.nil?
+              ""
+            elsif output.is_a?(String)
+              output
+            else
+              output.to_json
+            end
+
+            messages << {
+              role: "tool",
+              tool_call_id: fn_result[:call_id],
+              name: fn_result[:name],
+              content: content
+            }
+          end
+
+        elsif !chat_message.content.blank?
+          messages << { role: chat_message.role, content: chat_message.content || "" }
+        end
+      end
+      messages
+    end
 end
diff --git a/app/models/assistant/token_estimator.rb b/app/models/assistant/token_estimator.rb
new file mode 100644
index 000000000..4f5e3fd60
--- /dev/null
+++ b/app/models/assistant/token_estimator.rb
@@ -0,0 +1,19 @@
+module Assistant::TokenEstimator
+  CHARS_PER_TOKEN = 4
+  SAFETY_FACTOR = 1.25
+
+  def self.estimate(value)
+    chars = char_length(value)
+    ((chars / CHARS_PER_TOKEN.to_f) * SAFETY_FACTOR).ceil
+  end
+
+  def self.char_length(value)
+    case value
+    when nil    then 0
+    when String then value.length
+    when Array  then value.sum { |v| char_length(v) }
+    when Hash   then value.to_json.length
+    else value.to_s.length
+    end
+  end
+end
diff --git a/app/models/chat.rb b/app/models/chat.rb
index d47dcccac..f97fca68b 100644
--- a/app/models/chat.rb
+++ b/app/models/chat.rb
@@ -26,9 +26,9 @@ class Chat < ApplicationRecord
     end
 
     # Returns the default AI model to use for chats
-    # Priority: ENV variable > Setting > OpenAI default
+    # Priority: AI Config > Setting
     def default_model
-      ENV["OPENAI_MODEL"].presence || Setting.openai_model.presence || Provider::Openai::DEFAULT_MODEL
+      Provider::Openai.effective_model.presence || Setting.openai_model
     end
   end
 
diff --git a/app/models/provider/llm_concept.rb b/app/models/provider/llm_concept.rb
index 46e6dcd84..52550111f 100644
--- a/app/models/provider/llm_concept.rb
+++ b/app/models/provider/llm_concept.rb
@@ -40,6 +40,7 @@ module Provider::LlmConcept
     instructions: nil,
     functions: [],
     function_results: [],
+    messages: nil,
     streamer: nil,
     previous_response_id: nil,
     session_id: nil,
diff --git a/app/models/provider/openai.rb b/app/models/provider/openai.rb
index 60c103a9e..052829cc1 100644
--- a/app/models/provider/openai.rb
+++ b/app/models/provider/openai.rb
@@ -4,31 +4,29 @@ class Provider::Openai < Provider
   # Subclass so errors caught in this provider are raised as Provider::Openai::Error
   Error = Class.new(Provider::Error)
 
-  # Supported OpenAI model prefixes (e.g., "gpt-4" matches "gpt-4", "gpt-4.1", "gpt-4-turbo", etc.)
-  DEFAULT_OPENAI_MODEL_PREFIXES = %w[gpt-4 gpt-5 o1 o3]
-  DEFAULT_MODEL = "gpt-4.1"
-
-  # Models that support PDF/vision input (not all OpenAI models have vision capabilities)
+  DEFAULT_MODEL = "gpt-4.1".freeze
+  SUPPORTED_MODELS = %w[gpt-4 gpt-5 o1 o3].freeze
   VISION_CAPABLE_MODEL_PREFIXES = %w[gpt-4o gpt-4-turbo gpt-4.1 gpt-5 o1 o3].freeze
 
-  # Returns the effective model that would be used by the provider
-  # Uses the same logic as Provider::Registry and the initializer
+  # Returns the effective model that would be used by the provider.
+  # Priority: explicit ENV > Setting > DEFAULT_MODEL.
   def self.effective_model
-    configured_model = ENV.fetch("OPENAI_MODEL", Setting.openai_model)
-    configured_model.presence || DEFAULT_MODEL
+    ENV.fetch("OPENAI_MODEL") { Setting.openai_model }.presence || DEFAULT_MODEL
   end
 
   def initialize(access_token, uri_base: nil, model: nil)
     client_options = { access_token: access_token }
-    client_options[:uri_base] = uri_base if uri_base.present?
+    llm_uri_base = uri_base.presence
+    llm_model = model.presence
+    client_options[:uri_base] = llm_uri_base if llm_uri_base.present?
     client_options[:request_timeout] = ENV.fetch("OPENAI_REQUEST_TIMEOUT", 60).to_i
 
     @client = ::OpenAI::Client.new(**client_options)
-    @uri_base = uri_base
-    if custom_provider? && model.blank?
+    @uri_base = llm_uri_base
+    if custom_provider? && llm_model.blank?
       raise Error, "Model is required when using a custom OpenAI‑compatible provider"
     end
-    @default_model = model.presence || DEFAULT_MODEL
+    @default_model = llm_model.presence || self.class.effective_model
   end
 
   def supports_model?(model)
@@ -36,7 +34,18 @@ class Provider::Openai < Provider
     return true if custom_provider?
 
     # Otherwise, check if model starts with any supported OpenAI prefix
-    DEFAULT_OPENAI_MODEL_PREFIXES.any? { |prefix| model.start_with?(prefix) }
+    SUPPORTED_MODELS.any? { |prefix| model.start_with?(prefix) }
+  end
+
+  def supports_responses_endpoint?
+    return @supports_responses_endpoint if defined?(@supports_responses_endpoint)
+
+    env_override = ENV["OPENAI_SUPPORTS_RESPONSES_ENDPOINT"]
+    if env_override.to_s.present?
+      return @supports_responses_endpoint = ActiveModel::Type::Boolean.new.cast(env_override)
+    end
+
+    @supports_responses_endpoint = !custom_provider?
   end
 
   def provider_name
@@ -47,7 +56,7 @@ class Provider::Openai < Provider
     if custom_provider?
       @default_model.present? ? "configured model: #{@default_model}" : "any model"
     else
-      "models starting with: #{DEFAULT_OPENAI_MODEL_PREFIXES.join(', ')}"
+      "models starting with: #{SUPPORTED_MODELS.join(", ")}"
     end
   end
 
@@ -55,9 +64,43 @@ class Provider::Openai < Provider
     @uri_base.present?
   end
 
+  # Token-budget knobs. Precedence: ENV > Setting > default. Defaults match
+  # Ollama's historical 2048-token baseline so local small-context models work
+  # out of the box. Users on larger-context cloud models can raise via ENV or
+  # via the Self-Hosting settings page.
+  def context_window
+    positive_budget(ENV["LLM_CONTEXT_WINDOW"], Setting.llm_context_window, 2048)
+  end
+
+  def max_response_tokens
+    positive_budget(ENV["LLM_MAX_RESPONSE_TOKENS"], Setting.llm_max_response_tokens, 512)
+  end
+
+  def system_prompt_reserve
+    positive_budget(ENV["LLM_SYSTEM_PROMPT_RESERVE"], nil, 256)
+  end
+
+  def max_history_tokens
+    explicit = ENV["LLM_MAX_HISTORY_TOKENS"].presence&.to_i
+    return explicit if explicit&.positive?
+    [ context_window - max_response_tokens - system_prompt_reserve, 256 ].max
+  end
+
+  # Budget available for a one-shot (non-chat) request's full input,
+  # excluding reserved response tokens AND the system/instructions prompt.
+  # Drives the batch slicer for the auto_categorize / auto_detect_merchants /
+  # enhance_provider_merchants calls — each ships ~200–400 tokens of
+  # instructions + JSON schema that aren't counted in `fixed_tokens`.
+  def max_input_tokens
+    [ context_window - max_response_tokens - system_prompt_reserve, 256 ].max
+  end
+
+  def max_items_per_call
+    positive_budget(ENV["LLM_MAX_ITEMS_PER_CALL"], Setting.llm_max_items_per_call, 25)
+  end
+
   def auto_categorize(transactions: [], user_categories: [], model: "", family: nil, json_mode: nil)
     with_provider_response do
-      raise Error, "Too many transactions to auto-categorize. Max is 25 per request." if transactions.size > 25
       if user_categories.blank?
         family_id = family&.id || "unknown"
         Rails.logger.error("Cannot auto-categorize transactions for family #{family_id}: no categories available")
@@ -71,16 +114,20 @@ class Provider::Openai < Provider
         input: { transactions: transactions, user_categories: user_categories }
       )
 
-      result = AutoCategorizer.new(
-        client,
-        model: effective_model,
-        transactions: transactions,
-        user_categories: user_categories,
-        custom_provider: custom_provider?,
-        langfuse_trace: trace,
-        family: family,
-        json_mode: json_mode
-      ).auto_categorize
+      batches = slice_for_context(transactions, fixed: user_categories)
+
+      result = batches.flat_map do |batch|
+        AutoCategorizer.new(
+          client,
+          model: effective_model,
+          transactions: batch,
+          user_categories: user_categories,
+          custom_provider: custom_provider?,
+          langfuse_trace: trace,
+          family: family,
+          json_mode: json_mode
+        ).auto_categorize
+      end
 
       upsert_langfuse_trace(trace: trace, output: result.map(&:to_h))
 
@@ -90,8 +137,6 @@ class Provider::Openai < Provider
 
   def auto_detect_merchants(transactions: [], user_merchants: [], model: "", family: nil, json_mode: nil)
     with_provider_response do
-      raise Error, "Too many transactions to auto-detect merchants. Max is 25 per request." if transactions.size > 25
-
       effective_model = model.presence || @default_model
 
       trace = create_langfuse_trace(
@@ -99,16 +144,20 @@ class Provider::Openai < Provider
         input: { transactions: transactions, user_merchants: user_merchants }
       )
 
-      result = AutoMerchantDetector.new(
-        client,
-        model: effective_model,
-        transactions: transactions,
-        user_merchants: user_merchants,
-        custom_provider: custom_provider?,
-        langfuse_trace: trace,
-        family: family,
-        json_mode: json_mode
-      ).auto_detect_merchants
+      batches = slice_for_context(transactions, fixed: user_merchants)
+
+      result = batches.flat_map do |batch|
+        AutoMerchantDetector.new(
+          client,
+          model: effective_model,
+          transactions: batch,
+          user_merchants: user_merchants,
+          custom_provider: custom_provider?,
+          langfuse_trace: trace,
+          family: family,
+          json_mode: json_mode
+        ).auto_detect_merchants
+      end
 
       upsert_langfuse_trace(trace: trace, output: result.map(&:to_h))
 
@@ -118,8 +167,6 @@ class Provider::Openai < Provider
 
   def enhance_provider_merchants(merchants: [], model: "", family: nil, json_mode: nil)
     with_provider_response do
-      raise Error, "Too many merchants to enhance. Max is 25 per request." if merchants.size > 25
-
       effective_model = model.presence || @default_model
 
       trace = create_langfuse_trace(
@@ -127,15 +174,19 @@ class Provider::Openai < Provider
         input: { merchants: merchants }
       )
 
-      result = ProviderMerchantEnhancer.new(
-        client,
-        model: effective_model,
-        merchants: merchants,
-        custom_provider: custom_provider?,
-        langfuse_trace: trace,
-        family: family,
-        json_mode: json_mode
-      ).enhance_merchants
+      batches = slice_for_context(merchants)
+
+      result = batches.flat_map do |batch|
+        ProviderMerchantEnhancer.new(
+          client,
+          model: effective_model,
+          merchants: batch,
+          custom_provider: custom_provider?,
+          langfuse_trace: trace,
+          family: family,
+          json_mode: json_mode
+        ).enhance_merchants
+      end
 
       upsert_langfuse_trace(trace: trace, output: result.map(&:to_h))
 
@@ -171,7 +222,8 @@ class Provider::Openai < Provider
         pdf_content: pdf_content,
         custom_provider: custom_provider?,
         langfuse_trace: trace,
-        family: family
+        family: family,
+        max_response_tokens: max_response_tokens
       ).process
 
       upsert_langfuse_trace(trace: trace, output: result.to_h)
@@ -207,25 +259,17 @@ class Provider::Openai < Provider
     instructions: nil,
     functions: [],
     function_results: [],
+    messages: nil,
     streamer: nil,
     previous_response_id: nil,
     session_id: nil,
     user_identifier: nil,
     family: nil
   )
-    if custom_provider?
-      generic_chat_response(
-        prompt: prompt,
-        model: model,
-        instructions: instructions,
-        functions: functions,
-        function_results: function_results,
-        streamer: streamer,
-        session_id: session_id,
-        user_identifier: user_identifier,
-        family: family
-      )
-    else
+    if supports_responses_endpoint?
+      # Native path uses the Responses API which chains history via
+      # `previous_response_id`; it does NOT need (and must not receive)
+      # inline message history in the input payload.
       native_chat_response(
         prompt: prompt,
         model: model,
@@ -238,12 +282,48 @@ class Provider::Openai < Provider
         user_identifier: user_identifier,
         family: family
       )
+    else
+      generic_chat_response(
+        prompt: prompt,
+        model: model,
+        instructions: instructions,
+        functions: functions,
+        function_results: function_results,
+        messages: messages,
+        streamer: streamer,
+        session_id: session_id,
+        user_identifier: user_identifier,
+        family: family
+      )
     end
   end
 
   private
     attr_reader :client
 
+    # Returns the first positive integer among env, setting, default. Treats
+    # zero or negative values as "unset" and falls through — a 0-token budget
+    # is never what the user meant.
+    def positive_budget(env_value, setting_value, default)
+      from_env = env_value.to_s.strip.to_i
+      return from_env if from_env.positive?
+      return setting_value.to_i if setting_value.to_i.positive?
+      default
+    end
+
+    # Routes one-shot (non-chat) inputs through the BatchSlicer so large
+    # caller batches are split to fit the model's context window. `fixed` is
+    # the portion of the prompt that stays constant across every sub-batch
+    # (e.g. user_categories, user_merchants), used for fixed-tokens accounting.
+    def slice_for_context(items, fixed: nil)
+      BatchSlicer.call(
+        Array(items),
+        max_items: max_items_per_call,
+        max_tokens: max_input_tokens,
+        fixed_tokens: fixed ? Assistant::TokenEstimator.estimate(fixed) : 0
+      )
+    end
+
     def native_chat_response(
       prompt:,
       model:,
@@ -278,7 +358,7 @@ class Provider::Openai < Provider
           nil
         end
 
-        input_payload = chat_config.build_input(prompt)
+        input_payload = chat_config.build_input(prompt: prompt)
 
         begin
           raw_response = client.responses.create(parameters: {
@@ -344,6 +424,7 @@ class Provider::Openai < Provider
       instructions: nil,
       functions: [],
       function_results: [],
+      messages: nil,
       streamer: nil,
       session_id: nil,
       user_identifier: nil,
@@ -353,7 +434,8 @@ class Provider::Openai < Provider
         messages = build_generic_messages(
           prompt: prompt,
           instructions: instructions,
-          function_results: function_results
+          function_results: function_results,
+          messages: messages
         )
 
         tools = build_generic_tools(functions)
@@ -412,16 +494,24 @@ class Provider::Openai < Provider
       end
     end
 
-    def build_generic_messages(prompt:, instructions: nil, function_results: [])
-      messages = []
+    def build_generic_messages(prompt:, instructions: nil, function_results: [], messages: nil)
+      payload = []
 
       # Add system message if instructions present
       if instructions.present?
-        messages << { role: "system", content: instructions }
+        payload << { role: "system", content: instructions }
       end
 
-      # Add user prompt
-      messages << { role: "user", content: prompt }
+      # Add conversation history or user prompt. History is trimmed to fit the
+      # configured token budget so small-context local models (Ollama, LM Studio,
+      # LocalAI) don't silently truncate. tool_call/tool_result pairs are
+      # preserved atomically by HistoryTrimmer.
+      if messages.present?
+        trimmed = Assistant::HistoryTrimmer.new(messages, max_tokens: max_history_tokens).call
+        payload.concat(trimmed)
+      elsif prompt.present?
+        payload << { role: "user", content: prompt }
+      end
 
       # If there are function results, we need to add the assistant message that made the tool calls
       # followed by the tool messages with the results
@@ -442,7 +532,7 @@ class Provider::Openai < Provider
           }
         end
 
-        messages << {
+        payload << {
           role: "assistant",
           content: "",  # Some OpenAI-compatible APIs require string, not null
           tool_calls: tool_calls
@@ -462,7 +552,7 @@ class Provider::Openai < Provider
             output.to_json
           end
 
-          messages << {
+          payload << {
             role: "tool",
             tool_call_id: fn_result[:call_id],
             name: fn_result[:name],
@@ -471,7 +561,7 @@ class Provider::Openai < Provider
         end
       end
 
-      messages
+      payload
     end
 
     def build_generic_tools(functions)
diff --git a/app/models/provider/openai/batch_slicer.rb b/app/models/provider/openai/batch_slicer.rb
new file mode 100644
index 000000000..577ddba5d
--- /dev/null
+++ b/app/models/provider/openai/batch_slicer.rb
@@ -0,0 +1,45 @@
+class Provider::Openai::BatchSlicer
+  class ContextOverflowError < StandardError; end
+
+  # Splits `items` into sub-batches that respect both a hard item cap and a
+  # token-budget cap. Used by auto_categorize / auto_detect_merchants /
+  # enhance_provider_merchants so callers can pass larger batches and have the
+  # provider fan them out to fit small-context models.
+  def self.call(items, max_items:, max_tokens:, fixed_tokens: 0)
+    items = Array(items)
+    return [] if items.empty?
+
+    available = max_tokens.to_i - fixed_tokens.to_i
+    if available <= 0
+      raise ContextOverflowError,
+            "Fixed prompt tokens (#{fixed_tokens}) exceed context budget (#{max_tokens})"
+    end
+
+    batches = []
+    current = []
+    current_tokens = 0
+
+    items.each do |item|
+      item_tokens = Assistant::TokenEstimator.estimate(item)
+      if item_tokens > available
+        raise ContextOverflowError,
+              "Single item requires ~#{item_tokens} tokens, which exceeds available budget (#{available})"
+      end
+
+      would_exceed_items  = current.size >= max_items.to_i
+      would_exceed_tokens = current_tokens + item_tokens > available
+
+      if would_exceed_items || would_exceed_tokens
+        batches << current unless current.empty?
+        current = []
+        current_tokens = 0
+      end
+
+      current << item
+      current_tokens += item_tokens
+    end
+
+    batches << current unless current.empty?
+    batches
+  end
+end
diff --git a/app/models/provider/openai/chat_config.rb b/app/models/provider/openai/chat_config.rb
index 5e98a66ce..65e683de4 100644
--- a/app/models/provider/openai/chat_config.rb
+++ b/app/models/provider/openai/chat_config.rb
@@ -16,7 +16,9 @@ class Provider::Openai::ChatConfig
     end
   end
 
-  def build_input(prompt)
+  def build_input(prompt: nil)
+    input_messages = prompt.present? ? [ { role: "user", content: prompt } ] : []
+
     results = function_results.map do |fn_result|
       # Handle nil explicitly to avoid serializing to "null"
       output = fn_result[:output]
@@ -36,7 +38,7 @@ class Provider::Openai::ChatConfig
     end
 
     [
-      { role: "user", content: prompt },
+      *input_messages,
       *results
     ]
   end
diff --git a/app/models/provider/openai/pdf_processor.rb b/app/models/provider/openai/pdf_processor.rb
index f65510e87..1cb63e773 100644
--- a/app/models/provider/openai/pdf_processor.rb
+++ b/app/models/provider/openai/pdf_processor.rb
@@ -1,15 +1,16 @@
 class Provider::Openai::PdfProcessor
   include Provider::Openai::Concerns::UsageRecorder
 
-  attr_reader :client, :model, :pdf_content, :custom_provider, :langfuse_trace, :family
+  attr_reader :client, :model, :pdf_content, :custom_provider, :langfuse_trace, :family, :max_response_tokens
 
-  def initialize(client, model: "", pdf_content: nil, custom_provider: false, langfuse_trace: nil, family: nil)
+  def initialize(client, model: "", pdf_content: nil, custom_provider: false, langfuse_trace: nil, family: nil, max_response_tokens:)
     @client = client
     @model = model
     @pdf_content = pdf_content
     @custom_provider = custom_provider
     @langfuse_trace = langfuse_trace
     @family = family
+    @max_response_tokens = max_response_tokens
   end
 
   def process
@@ -175,7 +176,7 @@ class Provider::Openai::PdfProcessor
           { role: "system", content: instructions + "\n\nIMPORTANT: Respond with valid JSON only, no markdown or other formatting." },
           { role: "user", content: content }
         ],
-        max_tokens: 4096
+        max_tokens: max_response_tokens
       }
 
       response = client.chat(parameters: params)
diff --git a/app/models/setting.rb b/app/models/setting.rb
index f16a66d6a..c5aa08d7e 100644
--- a/app/models/setting.rb
+++ b/app/models/setting.rb
@@ -10,6 +10,14 @@ class Setting < RailsSettings::Base
   field :openai_uri_base, type: :string, default: ENV["OPENAI_URI_BASE"]
   field :openai_model, type: :string, default: ENV["OPENAI_MODEL"]
   field :openai_json_mode, type: :string, default: ENV["LLM_JSON_MODE"]
+
+  # LLM token budget (applies to every outbound LLM call: chat, auto-categorize,
+  # merchant detection, enhance-merchants, PDF processing). Defaults track
+  # Ollama's historical 2048-token baseline so local small-context models work
+  # out of the box. ENV overrides Setting at read time in Provider::Openai.
+  field :llm_context_window, type: :integer, default: ENV["LLM_CONTEXT_WINDOW"]&.to_i
+  field :llm_max_response_tokens, type: :integer, default: ENV["LLM_MAX_RESPONSE_TOKENS"]&.to_i
+  field :llm_max_items_per_call, type: :integer, default: ENV["LLM_MAX_ITEMS_PER_CALL"]&.to_i
   field :external_assistant_url, type: :string
   field :external_assistant_token, type: :string
   field :external_assistant_agent_id, type: :string
diff --git a/app/views/settings/hostings/_openai_settings.html.erb b/app/views/settings/hostings/_openai_settings.html.erb
index 6691d9556..b54dd7671 100644
--- a/app/views/settings/hostings/_openai_settings.html.erb
+++ b/app/views/settings/hostings/_openai_settings.html.erb
@@ -63,5 +63,37 @@
                   { disabled: ENV["LLM_JSON_MODE"].present?,
                     data: { "auto-submit-form-target": "auto" } } %>
   <p class="text-xs text-secondary mt-1"><%= t(".json_mode_help") %></p>
+
+  <div class="pt-4 border-t border-secondary">
+    <h3 class="font-medium mb-1"><%= t(".budget_heading") %></h3>
+    <p class="text-xs text-secondary mb-3"><%= t(".budget_description") %></p>
+
+    <%= form.number_field :llm_context_window,
+                          label: t(".context_window_label"),
+                          placeholder: "2048",
+                          value: Setting.llm_context_window,
+                          min: 256,
+                          disabled: ENV["LLM_CONTEXT_WINDOW"].present?,
+                          data: { "auto-submit-form-target": "auto" } %>
+    <p class="text-xs text-secondary mt-1 mb-3"><%= t(".context_window_help") %></p>
+
+    <%= form.number_field :llm_max_response_tokens,
+                          label: t(".max_response_tokens_label"),
+                          placeholder: "512",
+                          value: Setting.llm_max_response_tokens,
+                          min: 64,
+                          disabled: ENV["LLM_MAX_RESPONSE_TOKENS"].present?,
+                          data: { "auto-submit-form-target": "auto" } %>
+    <p class="text-xs text-secondary mt-1 mb-3"><%= t(".max_response_tokens_help") %></p>
+
+    <%= form.number_field :llm_max_items_per_call,
+                          label: t(".max_items_per_call_label"),
+                          placeholder: "25",
+                          value: Setting.llm_max_items_per_call,
+                          min: 1,
+                          disabled: ENV["LLM_MAX_ITEMS_PER_CALL"].present?,
+                          data: { "auto-submit-form-target": "auto" } %>
+    <p class="text-xs text-secondary mt-1"><%= t(".max_items_per_call_help") %></p>
+  </div>
   <% end %>
 </div>
diff --git a/compose.example.ai.yml b/compose.example.ai.yml
index 1265bb23d..a1de3b6e8 100644
--- a/compose.example.ai.yml
+++ b/compose.example.ai.yml
@@ -187,7 +187,7 @@ services:
       - WEBUI_AUTH=False
       - WEBUI_NAME=AI
       - WEBUI_URL=http://localhost:8080
-      - WEBUI_SECRET_KEY=t0p-s3cr3t
+      - WEBUI_SECRET_KEY=t0p-s3cr3t # pipelock:ignore
       - NO_PROXY=host.docker.internal
     extra_hosts:
       - host.docker.internal:host-gateway
@@ -325,7 +325,7 @@ services:
       - POSTGRES_HOST=db
       - POSTGRES_DB=${POSTGRES_DB:-sure_production}
       - POSTGRES_USER=${POSTGRES_USER:-sure_user}
-      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-sure_password}
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-sure_password} # pipelock:ignore
       - SCHEDULE=@daily # Runs once a day at midnight
       - BACKUP_KEEP_DAYS=7 # Keeps the last 7 days of backups
       - BACKUP_KEEP_WEEKS=4 # Keeps 4 weekly backups
diff --git a/compose.example.yml b/compose.example.yml
index 487234038..108fd6b9a 100644
--- a/compose.example.yml
+++ b/compose.example.yml
@@ -130,7 +130,7 @@ services:
       - POSTGRES_HOST=db
       - POSTGRES_DB=${POSTGRES_DB:-sure_production}
       - POSTGRES_USER=${POSTGRES_USER:-sure_user}
-      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-sure_password}
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-sure_password} # pipelock:ignore
       - SCHEDULE=@daily # Runs once a day at midnight
       - BACKUP_KEEP_DAYS=7 # Keeps the last 7 days of backups
       - BACKUP_KEEP_WEEKS=4 # Keeps 4 weekly backups
diff --git a/config/locales/views/settings/hostings/en.yml b/config/locales/views/settings/hostings/en.yml
index 22d1f1d31..0c3f8f7ab 100644
--- a/config/locales/views/settings/hostings/en.yml
+++ b/config/locales/views/settings/hostings/en.yml
@@ -100,6 +100,14 @@ en:
         json_mode_none: None (best for standard models)
         json_mode_json_object: JSON Object
         json_mode_help: "Strict mode works best with thinking models (qwen-thinking, deepseek-reasoner). None mode works best with standard models (llama, mistral, gpt-oss)."
+        budget_heading: Token Budget
+        budget_description: Applies to every LLM call — chat history, auto-categorization, merchant detection, and PDF processing. Defaults are conservative for small-context local models. Raise for cloud models with larger context windows.
+        context_window_label: Context Window (Optional)
+        context_window_help: "Total tokens the model will accept. Default: 2048 — raise to 8192+ for cloud OpenAI or large-context local models."
+        max_response_tokens_label: Max Response Tokens (Optional)
+        max_response_tokens_help: "Tokens reserved for the model's reply. Default: 512. Lower to free up room for longer history."
+        max_items_per_call_label: Max Items Per Batch (Optional)
+        max_items_per_call_help: "Upper bound for auto-categorize / merchant detection batches. Default: 25. Larger batches are auto-sliced to fit the context window."
         title: OpenAI
       yahoo_finance_settings:
         title: Yahoo Finance
@@ -161,6 +169,7 @@ en:
         success: Settings updated
         invalid_onboarding_state: Invalid onboarding state
         invalid_sync_time: Invalid sync time format. Please use HH:MM format (e.g., 02:30).
+        invalid_llm_budget: "%{field} must be a whole number ≥ %{minimum}."
         scheduler_sync_failed: Settings saved, but failed to update the sync schedule. Please try again or check the server logs.
       disconnect_external_assistant:
         external_assistant_disconnected: External assistant disconnected
diff --git a/test/controllers/settings/hostings_controller_test.rb b/test/controllers/settings/hostings_controller_test.rb
index c38585bc3..4ebe20f87 100644
--- a/test/controllers/settings/hostings_controller_test.rb
+++ b/test/controllers/settings/hostings_controller_test.rb
@@ -235,6 +235,61 @@ class Settings::HostingsControllerTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "accepts valid llm budget overrides and blanks clear them" do
+    with_self_hosting do
+      patch settings_hosting_url, params: { setting: {
+        llm_context_window: "4096",
+        llm_max_response_tokens: "1024",
+        llm_max_items_per_call: "40"
+      } }
+
+      assert_redirected_to settings_hosting_url
+      assert_equal 4096, Setting.llm_context_window
+      assert_equal 1024, Setting.llm_max_response_tokens
+      assert_equal 40, Setting.llm_max_items_per_call
+
+      patch settings_hosting_url, params: { setting: {
+        llm_context_window: "",
+        llm_max_response_tokens: "",
+        llm_max_items_per_call: ""
+      } }
+
+      assert_nil Setting.llm_context_window
+      assert_nil Setting.llm_max_response_tokens
+      assert_nil Setting.llm_max_items_per_call
+    end
+  ensure
+    Setting.llm_context_window = nil
+    Setting.llm_max_response_tokens = nil
+    Setting.llm_max_items_per_call = nil
+  end
+
+  test "rejects llm budget below field minimum" do
+    with_self_hosting do
+      patch settings_hosting_url, params: { setting: { llm_context_window: "0" } }
+
+      assert_response :unprocessable_entity
+      assert_match(/must be a whole number/, flash[:alert])
+      assert_nil Setting.llm_context_window
+
+      patch settings_hosting_url, params: { setting: { llm_max_response_tokens: "-5" } }
+
+      assert_response :unprocessable_entity
+      assert_match(/must be a whole number/, flash[:alert])
+      assert_nil Setting.llm_max_response_tokens
+
+      patch settings_hosting_url, params: { setting: { llm_max_items_per_call: "not-a-number" } }
+
+      assert_response :unprocessable_entity
+      assert_match(/must be a whole number/, flash[:alert])
+      assert_nil Setting.llm_max_items_per_call
+    end
+  ensure
+    Setting.llm_context_window = nil
+    Setting.llm_max_response_tokens = nil
+    Setting.llm_max_items_per_call = nil
+  end
+
   test "can clear data only when admin" do
     with_self_hosting do
       sign_in users(:family_member)
diff --git a/test/models/assistant/history_trimmer_test.rb b/test/models/assistant/history_trimmer_test.rb
new file mode 100644
index 000000000..abd94324e
--- /dev/null
+++ b/test/models/assistant/history_trimmer_test.rb
@@ -0,0 +1,93 @@
+require "test_helper"
+
+class Assistant::HistoryTrimmerTest < ActiveSupport::TestCase
+  test "returns empty array for empty input" do
+    assert_equal [], Assistant::HistoryTrimmer.new([], max_tokens: 1000).call
+    assert_equal [], Assistant::HistoryTrimmer.new(nil, max_tokens: 1000).call
+  end
+
+  test "returns empty array when max_tokens is zero or negative" do
+    messages = [ { role: "user", content: "hi" } ]
+    assert_equal [], Assistant::HistoryTrimmer.new(messages, max_tokens: 0).call
+    assert_equal [], Assistant::HistoryTrimmer.new(messages, max_tokens: -10).call
+  end
+
+  test "keeps full history when budget is generous" do
+    messages = [
+      { role: "user", content: "a" * 40 },
+      { role: "assistant", content: "b" * 40 },
+      { role: "user", content: "c" * 40 }
+    ]
+
+    result = Assistant::HistoryTrimmer.new(messages, max_tokens: 10_000).call
+
+    assert_equal messages, result
+  end
+
+  test "drops oldest messages when budget is tight" do
+    messages = [
+      { role: "user", content: "a" * 100 },
+      { role: "assistant", content: "b" * 100 },
+      { role: "user", content: "c" * 100 }
+    ]
+
+    # ~40 tokens per message; budget of 60 should keep only the last one
+    result = Assistant::HistoryTrimmer.new(messages, max_tokens: 60).call
+
+    assert_equal 1, result.size
+    assert_equal "c" * 100, result.first[:content]
+  end
+
+  test "keeps tool_call and tool_result as an atomic pair" do
+    messages = [
+      { role: "user", content: "a" * 40 },
+      { role: "assistant", content: "", tool_calls: [ { id: "c1", type: "function", function: { name: "f", arguments: "{}" } } ] },
+      { role: "tool", tool_call_id: "c1", name: "f", content: "result" },
+      { role: "assistant", content: "follow up" }
+    ]
+
+    # Small budget: should keep the newest assistant follow-up. Pair is either
+    # kept together or dropped together — never split.
+    result = Assistant::HistoryTrimmer.new(messages, max_tokens: 20).call
+
+    tool_call_idx = result.index { |m| m[:role] == "assistant" && m[:tool_calls] }
+    tool_result_idx = result.index { |m| m[:role] == "tool" }
+
+    if tool_call_idx
+      assert_not_nil tool_result_idx, "tool_call without paired tool_result"
+      assert tool_result_idx > tool_call_idx, "tool_result must follow its tool_call"
+    else
+      assert_nil tool_result_idx, "tool_result without paired tool_call leaked through"
+    end
+  end
+
+  test "never splits tool_call + tool_result pair when dropping" do
+    messages = [
+      { role: "assistant", content: "", tool_calls: [ { id: "c1", type: "function", function: { name: "f", arguments: "{}" } } ] },
+      { role: "tool", tool_call_id: "c1", name: "f", content: "x" * 200 },
+      { role: "user", content: "y" * 20 }
+    ]
+
+    # Budget big enough for the user turn, not for the pair
+    result = Assistant::HistoryTrimmer.new(messages, max_tokens: 15).call
+
+    assert_equal 1, result.size
+    assert_equal "user", result.first[:role]
+  end
+
+  test "handles multiple tool results following a single assistant tool_calls message" do
+    messages = [
+      { role: "user", content: "hi" },
+      { role: "assistant", content: "", tool_calls: [
+        { id: "c1", type: "function", function: { name: "f1", arguments: "{}" } },
+        { id: "c2", type: "function", function: { name: "f2", arguments: "{}" } }
+      ] },
+      { role: "tool", tool_call_id: "c1", name: "f1", content: "r1" },
+      { role: "tool", tool_call_id: "c2", name: "f2", content: "r2" }
+    ]
+
+    result = Assistant::HistoryTrimmer.new(messages, max_tokens: 10_000).call
+
+    assert_equal messages, result
+  end
+end
diff --git a/test/models/assistant/token_estimator_test.rb b/test/models/assistant/token_estimator_test.rb
new file mode 100644
index 000000000..a22313c9f
--- /dev/null
+++ b/test/models/assistant/token_estimator_test.rb
@@ -0,0 +1,40 @@
+require "test_helper"
+
+class Assistant::TokenEstimatorTest < ActiveSupport::TestCase
+  test "estimate is zero for nil or empty" do
+    assert_equal 0, Assistant::TokenEstimator.estimate(nil)
+    assert_equal 0, Assistant::TokenEstimator.estimate("")
+    assert_equal 0, Assistant::TokenEstimator.estimate([])
+  end
+
+  test "estimate applies chars/4 with safety factor for strings" do
+    # 100 chars / 4 = 25 tokens, × 1.25 safety factor, ceil = 32
+    assert_equal 32, Assistant::TokenEstimator.estimate("a" * 100)
+  end
+
+  test "estimate sums across arrays" do
+    # Use lengths that avoid ceil rounding drift: 80 chars → 20 tokens → 25.0 ceil → 25.
+    string_estimate = Assistant::TokenEstimator.estimate("a" * 80)
+    array_estimate  = Assistant::TokenEstimator.estimate([ "a" * 80, "a" * 80 ])
+
+    assert_equal string_estimate * 2, array_estimate
+  end
+
+  test "estimate serializes hashes via JSON" do
+    hash = { role: "assistant", content: "hi" }
+    # {"role":"assistant","content":"hi"} => 35 chars / 4 × 1.25 ceil = 11
+    assert_equal 11, Assistant::TokenEstimator.estimate(hash)
+  end
+
+  test "estimate handles nested structures" do
+    nested = [ { role: "user", content: "hello" }, { role: "assistant", content: "world" } ]
+    # Each hash gets JSON-serialized and summed
+    expected = nested.sum { |h| Assistant::TokenEstimator.estimate(h) }
+    assert_equal expected, Assistant::TokenEstimator.estimate(nested)
+  end
+
+  test "estimate coerces unknown types via to_s" do
+    assert Assistant::TokenEstimator.estimate(12345) > 0
+    assert Assistant::TokenEstimator.estimate(:symbol) > 0
+  end
+end
diff --git a/test/models/assistant_test.rb b/test/models/assistant_test.rb
index b396cf7ed..6efc2e782 100644
--- a/test/models/assistant_test.rb
+++ b/test/models/assistant_test.rb
@@ -14,6 +14,10 @@ class AssistantTest < ActiveSupport::TestCase
     @provider = mock
     @expected_session_id = @chat.id.to_s
     @expected_user_identifier = ::Digest::SHA256.hexdigest(@chat.user_id.to_s)
+    @expected_conversation_history = [
+      { role: "user", content: "Can you help me understand my spending habits?" },
+      { role: "user", content: "What is my net worth?" }
+    ]
   end
 
   test "errors get added to chat" do
@@ -100,6 +104,7 @@ class AssistantTest < ActiveSupport::TestCase
     @provider.expects(:chat_response).with do |message, **options|
       assert_equal @expected_session_id, options[:session_id]
       assert_equal @expected_user_identifier, options[:user_identifier]
+      assert_equal @expected_conversation_history, options[:messages]
       text_chunks.each do |text_chunk|
         options[:streamer].call(text_chunk)
       end
@@ -154,6 +159,7 @@ class AssistantTest < ActiveSupport::TestCase
     @provider.expects(:chat_response).with do |message, **options|
       assert_equal @expected_session_id, options[:session_id]
       assert_equal @expected_user_identifier, options[:user_identifier]
+      assert_equal @expected_conversation_history, options[:messages]
       call2_text_chunks.each do |text_chunk|
         options[:streamer].call(text_chunk)
       end
@@ -165,6 +171,7 @@ class AssistantTest < ActiveSupport::TestCase
     @provider.expects(:chat_response).with do |message, **options|
       assert_equal @expected_session_id, options[:session_id]
       assert_equal @expected_user_identifier, options[:user_identifier]
+      assert_equal @expected_conversation_history, options[:messages]
       options[:streamer].call(call1_response_chunk)
       true
     end.returns(call1_response).once.in_sequence(sequence)
@@ -418,6 +425,95 @@ class AssistantTest < ActiveSupport::TestCase
     assert_raises(Assistant::Error) { Assistant.for_chat(nil) }
   end
 
+  test "builtin demotes a partially-streamed assistant message to failed on error" do
+    @assistant.expects(:get_model_provider).with("gpt-4.1").returns(@provider)
+
+    boom = StandardError.new("boom mid-stream")
+
+    @provider.expects(:chat_response).with do |_prompt, **options|
+      # Simulate a partial text chunk landing before the error propagates.
+      options[:streamer].call(provider_text_chunk("partial tokens "))
+      true
+    end.returns(provider_error_response(boom))
+
+    @assistant.respond_to(@message)
+
+    partial = @chat.messages.where(type: "AssistantMessage").order(:created_at).last
+    assert partial.present?, "partial assistant message should be persisted"
+    assert_equal "failed", partial.status
+    assert_equal "partial tokens ", partial.content
+  end
+
+  test "conversation_history excludes failed and pending messages" do
+    # Add a failed assistant turn; it must NOT leak into history.
+    AssistantMessage.create!(
+      chat: @chat,
+      content: "partial error response",
+      ai_model: "gpt-4.1",
+      status: "failed"
+    )
+
+    @assistant.expects(:get_model_provider).with("gpt-4.1").returns(@provider)
+
+    captured_history = nil
+    @provider.expects(:chat_response).with do |_prompt, **options|
+      captured_history = options[:messages]
+      options[:streamer].call(
+        provider_response_chunk(id: "1", model: "gpt-4.1", messages: [ provider_message(id: "1", text: "ok") ], function_requests: [])
+      )
+      true
+    end.returns(provider_success_response(
+      provider_response_chunk(id: "1", model: "gpt-4.1", messages: [ provider_message(id: "1", text: "ok") ], function_requests: []).data
+    ))
+
+    @assistant.respond_to(@message)
+
+    contents = captured_history.map { |m| m[:content] }
+    assert_not_includes contents, "partial error response"
+  end
+
+  test "conversation_history serializes assistant tool_calls with paired tool result" do
+    assistant_msg = AssistantMessage.create!(
+      chat: @chat,
+      content: "Looking that up",
+      ai_model: "gpt-4.1",
+      status: "complete"
+    )
+
+    ToolCall::Function.create!(
+      message: assistant_msg,
+      provider_id: "call_abc",
+      provider_call_id: "call_abc",
+      function_name: "get_net_worth",
+      function_arguments: { foo: "bar" },
+      function_result: { amount: 1000, currency: "USD" }
+    )
+
+    @assistant.expects(:get_model_provider).with("gpt-4.1").returns(@provider)
+
+    captured_history = nil
+    @provider.expects(:chat_response).with do |_prompt, **options|
+      captured_history = options[:messages]
+      options[:streamer].call(
+        provider_response_chunk(id: "1", model: "gpt-4.1", messages: [ provider_message(id: "1", text: "ok") ], function_requests: [])
+      )
+      true
+    end.returns(provider_success_response(
+      provider_response_chunk(id: "1", model: "gpt-4.1", messages: [ provider_message(id: "1", text: "ok") ], function_requests: []).data
+    ))
+
+    @assistant.respond_to(@message)
+
+    tool_call_entry = captured_history.find { |m| m[:role] == "assistant" && m[:tool_calls].present? }
+    tool_result_entry = captured_history.find { |m| m[:role] == "tool" }
+
+    assert_not_nil tool_call_entry, "tool_call message missing from history"
+    assert_not_nil tool_result_entry, "tool_result message missing from history"
+    assert_equal "call_abc", tool_call_entry[:tool_calls].first[:id]
+    assert_equal "call_abc", tool_result_entry[:tool_call_id]
+    assert_equal "get_net_worth", tool_result_entry[:name]
+  end
+
   private
 
     def mock_external_sse_response(sse_body)
diff --git a/test/models/provider/openai/batch_slicer_test.rb b/test/models/provider/openai/batch_slicer_test.rb
new file mode 100644
index 000000000..c56133991
--- /dev/null
+++ b/test/models/provider/openai/batch_slicer_test.rb
@@ -0,0 +1,74 @@
+require "test_helper"
+
+class Provider::Openai::BatchSlicerTest < ActiveSupport::TestCase
+  test "returns empty array for empty input" do
+    assert_equal [], Provider::Openai::BatchSlicer.call([], max_items: 25, max_tokens: 2000)
+  end
+
+  test "single batch when within item cap and token budget" do
+    items = Array.new(10) { |i| { id: i, name: "Item#{i}" } }
+
+    batches = Provider::Openai::BatchSlicer.call(items, max_items: 25, max_tokens: 10_000)
+
+    assert_equal 1, batches.size
+    assert_equal items, batches.first
+  end
+
+  test "splits on item cap" do
+    items = Array.new(30) { |i| { id: i } }
+
+    batches = Provider::Openai::BatchSlicer.call(items, max_items: 10, max_tokens: 100_000)
+
+    assert_equal 3, batches.size
+    assert_equal 10, batches.first.size
+    assert_equal 10, batches.last.size
+    assert_equal items, batches.flatten
+  end
+
+  test "splits on token budget" do
+    big_item = { payload: "x" * 200 }  # ~63 tokens each
+    items = Array.new(10) { big_item }
+
+    # ~125-token budget fits 2 items per batch
+    batches = Provider::Openai::BatchSlicer.call(items, max_items: 100, max_tokens: 125)
+
+    assert batches.size > 1
+    assert_equal items, batches.flatten
+    batches.each do |batch|
+      tokens = Assistant::TokenEstimator.estimate(batch)
+      assert tokens <= 125, "Batch exceeded token budget: #{tokens}"
+    end
+  end
+
+  test "raises ContextOverflowError when single item exceeds budget" do
+    huge = { payload: "x" * 10_000 }
+
+    assert_raises Provider::Openai::BatchSlicer::ContextOverflowError do
+      Provider::Openai::BatchSlicer.call([ huge ], max_items: 25, max_tokens: 500)
+    end
+  end
+
+  test "raises ContextOverflowError when fixed_tokens exceed budget" do
+    assert_raises Provider::Openai::BatchSlicer::ContextOverflowError do
+      Provider::Openai::BatchSlicer.call([ { a: 1 } ], max_items: 25, max_tokens: 500, fixed_tokens: 600)
+    end
+  end
+
+  test "respects fixed_tokens when computing available budget" do
+    items = Array.new(5) { { payload: "x" * 80 } }  # ~25 tokens each
+
+    # Budget of 200 minus fixed 100 = 100 available; fits ~4 items per batch
+    batches = Provider::Openai::BatchSlicer.call(items, max_items: 25, max_tokens: 200, fixed_tokens: 100)
+
+    assert batches.size >= 2
+    assert_equal items, batches.flatten
+  end
+
+  test "never produces empty batches" do
+    items = Array.new(7) { { id: SecureRandom.hex(4) } }
+
+    batches = Provider::Openai::BatchSlicer.call(items, max_items: 2, max_tokens: 10_000)
+
+    batches.each { |b| assert b.any?, "Empty batch produced" }
+  end
+end
diff --git a/test/models/provider/openai_test.rb b/test/models/provider/openai_test.rb
index 879fe20e7..5d3e5e960 100644
--- a/test/models/provider/openai_test.rb
+++ b/test/models/provider/openai_test.rb
@@ -330,4 +330,130 @@ class Provider::OpenaiTest < ActiveSupport::TestCase
 
     @subject.send(:create_langfuse_trace, name: "openai.test", input: { foo: "bar" })
   end
+
+  test "SUPPORTED_MODELS and VISION_CAPABLE_MODEL_PREFIXES are Ruby constants, not YAML-derived" do
+    assert_kind_of Array, Provider::Openai::SUPPORTED_MODELS
+    assert Provider::Openai::SUPPORTED_MODELS.all? { |s| s.is_a?(String) }
+    assert Provider::Openai::SUPPORTED_MODELS.frozen?
+
+    assert_kind_of Array, Provider::Openai::VISION_CAPABLE_MODEL_PREFIXES
+    assert Provider::Openai::VISION_CAPABLE_MODEL_PREFIXES.frozen?
+    assert_equal "gpt-4.1", Provider::Openai::DEFAULT_MODEL
+  end
+
+  test "budget readers default to conservative values" do
+    with_env_overrides(
+      "LLM_CONTEXT_WINDOW" => nil,
+      "LLM_MAX_RESPONSE_TOKENS" => nil,
+      "LLM_SYSTEM_PROMPT_RESERVE" => nil,
+      "LLM_MAX_HISTORY_TOKENS" => nil,
+      "LLM_MAX_ITEMS_PER_CALL" => nil
+    ) do
+      subject = Provider::Openai.new("test-token")
+      assert_equal 2048, subject.context_window
+      assert_equal 512, subject.max_response_tokens
+      assert_equal 256, subject.system_prompt_reserve
+      assert_equal 2048 - 512 - 256, subject.max_history_tokens
+      assert_equal 2048 - 512 - 256, subject.max_input_tokens
+      assert_equal 25, subject.max_items_per_call
+    end
+  end
+
+  test "budget readers respect explicit env overrides" do
+    with_env_overrides(
+      "LLM_CONTEXT_WINDOW" => "8192",
+      "LLM_MAX_RESPONSE_TOKENS" => "1024",
+      "LLM_SYSTEM_PROMPT_RESERVE" => "512",
+      "LLM_MAX_HISTORY_TOKENS" => "4096",
+      "LLM_MAX_ITEMS_PER_CALL" => "50"
+    ) do
+      subject = Provider::Openai.new("test-token")
+      assert_equal 8192, subject.context_window
+      assert_equal 1024, subject.max_response_tokens
+      assert_equal 512, subject.system_prompt_reserve
+      assert_equal 4096, subject.max_history_tokens  # explicit overrides derived
+      assert_equal 8192 - 1024 - 512, subject.max_input_tokens
+      assert_equal 50, subject.max_items_per_call
+    end
+  end
+
+  test "budget readers fall back to Setting when ENV unset" do
+    with_env_overrides(
+      "LLM_CONTEXT_WINDOW" => nil,
+      "LLM_MAX_RESPONSE_TOKENS" => nil,
+      "LLM_MAX_ITEMS_PER_CALL" => nil
+    ) do
+      Setting.llm_context_window = 8192
+      Setting.llm_max_response_tokens = 1024
+      Setting.llm_max_items_per_call = 40
+
+      subject = Provider::Openai.new("test-token")
+      assert_equal 8192, subject.context_window
+      assert_equal 1024, subject.max_response_tokens
+      assert_equal 40, subject.max_items_per_call
+    end
+  ensure
+    Setting.llm_context_window = nil
+    Setting.llm_max_response_tokens = nil
+    Setting.llm_max_items_per_call = nil
+  end
+
+  test "budget readers: ENV beats Setting when both present" do
+    with_env_overrides("LLM_CONTEXT_WINDOW" => "16384") do
+      Setting.llm_context_window = 4096
+      subject = Provider::Openai.new("test-token")
+      assert_equal 16384, subject.context_window
+    end
+  ensure
+    Setting.llm_context_window = nil
+  end
+
+  test "budget readers: zero or negative values fall through to default" do
+    with_env_overrides(
+      "LLM_CONTEXT_WINDOW" => "0",
+      "LLM_MAX_RESPONSE_TOKENS" => nil,
+      "LLM_MAX_ITEMS_PER_CALL" => nil
+    ) do
+      Setting.llm_context_window = 0
+      subject = Provider::Openai.new("test-token")
+      assert_equal 2048, subject.context_window
+    end
+  ensure
+    Setting.llm_context_window = nil
+  end
+
+  test "auto_categorize fans out oversized batches into sequential sub-calls" do
+    with_env_overrides("LLM_MAX_ITEMS_PER_CALL" => "10") do
+      subject = Provider::Openai.new("test-token")
+      transactions = Array.new(25) { |i| { id: i.to_s, name: "txn#{i}", amount: 10, classification: "expense" } }
+      user_categories = [ { id: "cat1", name: "Groceries", is_subcategory: false, parent_id: nil, classification: "expense" } ]
+
+      # Capture the batch size passed to each AutoCategorizer. `.new` is called
+      # once per sub-batch; we record each invocation's transactions count.
+      seen_sizes = []
+      fake_instance = mock
+      fake_instance.stubs(:auto_categorize).returns([])
+      Provider::Openai::AutoCategorizer.stubs(:new).with do |*_args, **kwargs|
+        seen_sizes << kwargs[:transactions].size
+        true
+      end.returns(fake_instance)
+
+      response = subject.auto_categorize(transactions: transactions, user_categories: user_categories)
+
+      assert response.success?
+      assert_equal [ 10, 10, 5 ], seen_sizes
+    end
+  end
+
+  test "build_input no longer accepts inline messages history" do
+    config = Provider::Openai::ChatConfig.new(functions: [], function_results: [])
+    # Positive control: prompt works
+    result = config.build_input(prompt: "hi")
+    assert_equal [ { role: "user", content: "hi" } ], result
+
+    # `messages:` kwarg is no longer part of the signature — calling with it must raise
+    assert_raises(ArgumentError) do
+      config.build_input(prompt: "hi", messages: [ { role: "user", content: "old" } ])
+    end
+  end
 end