fix(deepseek): reject unsupported multimodal prompts

## About

Fail fast when DeepSeek chat receives unsupported prompt objects instead of
sending invalid request payloads through the OpenAI-compatible transport.
This makes DeepSeek support more predictable by treating non-text prompt
objects as prompt errors unless the provider actually supports them.

## Changes

- Adapt DeepSeek text content as text blocks in the request adapter.
- Raise PromptError for image_url, local_file, and remote_file.
- Fix DeepSeek prompt errors to report the actual unsupported object kind.
- Update DeepSeek adapter specs for unsupported multimodal prompt objects.
- Remove the misleading local-file example from the DeepSeek provider docs.

Author: 0x1eef

CHANGELOG.md

@@ -17,6 +17,11 @@ Changes since `v5.1.0`.
 
 ### Fix
 
+* **Reject unsupported DeepSeek multimodal prompt objects early** <br>
+  Raise `LLM::PromptError` for `image_url`, `local_file`, and
+  `remote_file` in DeepSeek chat requests instead of sending invalid
+  OpenAI-compatible payloads that the provider rejects at runtime.
+
 * **Preserve DeepSeek reasoning content across tool turns** <br>
   Replay `reasoning_content` when serializing prior assistant messages for
   DeepSeek chat completions, so thinking-mode tool calls can continue into

lib/llm/providers/deepseek.rb

@@ -15,7 +15,7 @@ module LLM
   #
   #   llm = LLM.deepseek(key: ENV["KEY"])
   #   ctx = LLM::Context.new(llm)
-  #   ctx.talk ["Tell me about this photo", ctx.local_file("/images/photo.png")]
+  #   ctx.talk "Hello"
   #   ctx.messages.select(&:assistant?).each { print "[#{_1.role}]", _1.content, "\n" }
   class DeepSeek < OpenAI
     require_relative "deepseek/request_adapter"

lib/llm/providers/deepseek/request_adapter/completion.rb

@@ -30,19 +30,28 @@ def adapt
 
     def adapt_content(content)
       case content
+      when LLM::Object
+        adapt_object(content)
       when String
-        content.to_s
+        [{type: :text, text: content.to_s}]
       when LLM::Message
         adapt_content(content.content)
       when LLM::Function::Return
         throw(:abort, {role: "tool", tool_call_id: content.id, content: LLM.json.dump(content.value)})
-      when LLM::Object
-        prompt_error!(content)
       else
         prompt_error!(content)
       end
     end
 
+    def adapt_object(object)
+      case object.kind
+      when :image_url, :local_file, :remote_file
+        prompt_error!(object)
+      else
+        prompt_error!(object)
+      end
+    end
+
     def adapt_message
       case content
       when Array
@@ -64,7 +73,7 @@ def adapt_array
 
     def prompt_error!(object)
       if LLM::Object === object
-        raise LLM::PromptError, "The given LLM::Object with kind '#{content.kind}' is not " \
+        raise LLM::PromptError, "The given LLM::Object with kind '#{object.kind}' is not " \
                                 "supported by the DeepSeek API"
       else
         raise LLM::PromptError, "The given object (an instance of #{object.class}) " \

spec/deepseek/request_adapter_spec.rb

@@ -1,12 +1,15 @@
 # frozen_string_literal: true
 
 require "setup"
+require "tempfile"
 require "llm/providers/deepseek"
 
 RSpec.describe "LLM::DeepSeek::RequestAdapter::Completion" do
   describe "#adapt" do
     subject(:payload) { LLM::DeepSeek::RequestAdapter::Completion.new(message).adapt }
 
+    let(:provider) { LLM.deepseek(key: "test") }
+
     context "with assistant content" do
       let(:message) do
         LLM::Message.new("assistant", "answer", reasoning_content: "thought")
@@ -15,12 +18,43 @@
       it "preserves reasoning content" do
         expect(payload).to eq(
           role: "assistant",
-          content: "answer",
+          content: [{type: :text, text: "answer"}],
           reasoning_content: "thought"
         )
       end
     end
 
+    context "with image content" do
+      let(:message) do
+        LLM::Message.new("user", [ctx.image_url("https://example.com/cat.png")])
+      end
+
+      let(:ctx) { LLM::Context.new(provider) }
+
+      it "raises a prompt error" do
+        expect { payload }.to raise_error(LLM::PromptError, /image_url/)
+      end
+    end
+
+    context "with local file content" do
+      let(:tempfile) do
+        Tempfile.create(["example", ".pdf"]).tap {
+          _1.write("%PDF-1.4\n")
+          _1.rewind
+        }
+      end
+
+      let(:message) do
+        LLM::Message.new("user", [ctx.local_file(tempfile.path)])
+      end
+
+      let(:ctx) { LLM::Context.new(provider) }
+
+      it "raises a prompt error" do
+        expect { payload }.to raise_error(LLM::PromptError, /local_file/)
+      end
+    end
+
     context "with assistant tool calls" do
       let(:message) do
         LLM::Message.new("assistant", nil, {