diff --git a/.env.example b/.env.example
index 047ff4b1..4cdbb08a 100644
--- a/.env.example
+++ b/.env.example
@@ -208,6 +208,20 @@ HF_TOKEN=your_huggingface_token_here
 # Optional — defaults to 12
 # GRAPH_MAX_RELATIONSHIPS=12
 
+# ── Vision / Image Captioning (VLM Providers) ──────────────
+# Set VISION_PROVIDER to one of: openai | anthropic | gemini | ollama
+# Leave unset to use OCR / placeholder only.
+# VISION_PROVIDER=openai
+
+# VISION_MODEL=gpt-4o-mini          # openai default
+# VISION_MODEL=claude-3-haiku-20240307  # anthropic default
+# VISION_MODEL=gemini-1.5-flash     # gemini default
+# VISION_MODEL=llava                 # ollama default
+
+# OPENAI_API_KEY=sk-...
+# ANTHROPIC_API_KEY=sk-ant-...
+# GOOGLE_API_KEY=AIza...
+# OLLAMA_BASE_URL=http://localhost:11434
 
 # ── ChromaDB (Vector Store) ─────────────────────────────────
 
diff --git a/backend/app/config.py b/backend/app/config.py
index c8763b39..b8da2e88 100644
--- a/backend/app/config.py
+++ b/backend/app/config.py
@@ -139,9 +139,15 @@ class Settings(BaseSettings):
     # ── Reranker ─────────────────────────────────────────
     RERANKER_MODEL: str = "BAAI/bge-reranker-v2-m3" # Lightweight 384-dim model fine-tuned for relevance ranking
     # ── Vision / Image captioning ─────────────────────
-    VISION_PROVIDER: str | None = None  # e.g. 'openai'
-    VISION_MODEL: str | None = None
+    # Set to: openai | anthropic | gemini | ollama (or leave None)
+    VISION_PROVIDER: str | None = None
+    VISION_MODEL: str | None = None  # overrides provider default model
+
+    # Provider API keys — only the active provider's key is required
     OPENAI_API_KEY: str = ""
+    ANTHROPIC_API_KEY: str = ""
+    GOOGLE_API_KEY: str = ""
+    OLLAMA_BASE_URL: str = "http://localhost:11434"
 
     # ── Workspace Invitation ─────────────────────────
     APP_URL: str = "http://localhost:3000"
diff --git a/backend/app/rag/vision.py b/backend/app/rag/vision.py
index 8699e0eb..c207b6ee 100644
--- a/backend/app/rag/vision.py
+++ b/backend/app/rag/vision.py
@@ -11,6 +11,8 @@
 """
 import base64
 import logging
+import app.vision.providers  # noqa: F401 — triggers self-registration
+from app.vision.registry import get_vision_provider
 from io import BytesIO
 from typing import Any, Dict, List, Optional
 
@@ -187,24 +189,45 @@ def _openai_caption(image_bytes: bytes) -> str:
 
 # ── Public API ───────────────────────────────────────────────────────────────
 
-def caption_image(image_bytes: bytes, page: Optional[int] = None) -> str:
-    """Generate a caption for a single image (bytes).
-
-    Resolution order: OpenAI (if configured) → OCR → placeholder.
-    """
-def caption_image(image_bytes: bytes | List[bytes], page: int | List[int] | None = None) -> str | List[str]:
+def caption_image(
+    image_bytes: "bytes | List[bytes]",
+    page: "int | List[int] | None" = None,
+) -> "str | List[str]":
     """Generate a caption for a single image or a batch of images.
 
-    Order of operations:
-    - If a list of image bytes is passed, returns a list of captions.
-    - If an external VLM provider is configured, attempt to call it.
-    - Fall back to local OCR (pytesseract) if available.
-    - Otherwise return a simple placeholder caption including the page number.
+    Resolution order:
+      1. Configured VLM provider (set VISION_PROVIDER in .env)
+      2. Local OCR via pytesseract
+      3. Placeholder string with page number and dimensions
     """
     if isinstance(image_bytes, list):
-        pages = page if isinstance(page, list) else ([page] * len(image_bytes) if page is not None else [None] * len(image_bytes))
+        pages = (
+            page if isinstance(page, list)
+            else ([page] * len(image_bytes) if page is not None else [None] * len(image_bytes))
+        )
         return [caption_image(img, pg) for img, pg in zip(image_bytes, pages)]
 
+    # Strategy: try the configured VLM provider
+    provider = get_vision_provider(getattr(settings, "VISION_PROVIDER", None))
+    if provider is not None:
+        result = provider.caption(image_bytes)
+        if result:
+            return result
+
+    # Fallback 1: local OCR
+    ocr = _ocr_caption(image_bytes)
+    if ocr:
+        return ocr
+
+    # Fallback 2: placeholder
+    try:
+        pix = fitz.Pixmap(image_bytes)
+        dims = f"{pix.width}x{pix.height} px"
+    except Exception:
+        dims = "unknown size"
+
+    return f"Figure on page {page} ({dims})." if page else f"Figure ({dims})."
+    
     # Placeholder for provider-based captioning (e.g., OpenAI / LLaVA hooks)
     provider = getattr(settings, "VISION_PROVIDER", None)
 
diff --git a/backend/tests/test_vision_providers.py b/backend/tests/test_vision_providers.py
new file mode 100644
index 00000000..9ca33c6f
--- /dev/null
+++ b/backend/tests/test_vision_providers.py
@@ -0,0 +1,93 @@
+"""Tests for the VLM provider Strategy Pattern (issue #592)."""
+from unittest.mock import MagicMock, patch
+import pytest
+
+from app.vision.base import BaseVisionProvider
+from app.vision.registry import _REGISTRY, get_vision_provider, register_provider
+
+
+class TestBaseVisionProvider:
+    def test_cannot_instantiate_abstract_class(self):
+        with pytest.raises(TypeError):
+            BaseVisionProvider()
+
+    def test_concrete_subclass_works(self):
+        class Dummy(BaseVisionProvider):
+            def caption(self, image_bytes: bytes) -> str:
+                return "dummy"
+        assert Dummy().caption(b"x") == "dummy"
+
+
+class TestRegistry:
+    def setup_method(self):
+        self._original = dict(_REGISTRY)
+
+    def teardown_method(self):
+        _REGISTRY.clear()
+        _REGISTRY.update(self._original)
+
+    def test_register_and_retrieve(self):
+        class FakeProvider(BaseVisionProvider):
+            def caption(self, image_bytes: bytes) -> str:
+                return "fake"
+        register_provider("fake", FakeProvider)
+        assert get_vision_provider("fake") is not None
+
+    def test_case_insensitive(self):
+        class P(BaseVisionProvider):
+            def caption(self, image_bytes: bytes) -> str:
+                return ""
+        register_provider("UPPER", P)
+        assert get_vision_provider("upper") is not None
+
+    def test_unknown_returns_none(self):
+        assert get_vision_provider("doesnotexist") is None
+
+    def test_none_returns_none(self):
+        assert get_vision_provider(None) is None
+
+    def test_broken_init_returns_none(self):
+        class Broken(BaseVisionProvider):
+            def __init__(self): raise RuntimeError("fail")
+            def caption(self, image_bytes: bytes) -> str: return ""
+        register_provider("broken", Broken)
+        assert get_vision_provider("broken") is None
+
+
+class TestCaptionImage:
+    def test_uses_provider_when_configured(self):
+        from app.rag.vision import caption_image
+
+        class StubProvider(BaseVisionProvider):
+            def caption(self, image_bytes: bytes) -> str:
+                return "stub caption"
+
+        with patch("app.rag.vision.get_vision_provider", return_value=StubProvider()):
+            assert caption_image(b"img", page=1) == "stub caption"
+
+    def test_falls_back_to_ocr(self):
+        from app.rag.vision import caption_image
+
+        class EmptyProvider(BaseVisionProvider):
+            def caption(self, image_bytes: bytes) -> str:
+                return ""
+
+        with patch("app.rag.vision.get_vision_provider", return_value=EmptyProvider()):
+            with patch("app.rag.vision._ocr_caption", return_value="ocr text"):
+                assert caption_image(b"img", page=1) == "ocr text"
+
+    def test_falls_back_to_placeholder(self):
+        from app.rag.vision import caption_image
+
+        with patch("app.rag.vision.get_vision_provider", return_value=None):
+            with patch("app.rag.vision._ocr_caption", return_value=""):
+                result = caption_image(b"img", page=3)
+        assert "page 3" in result
+
+    def test_batch_mode(self):
+        from app.rag.vision import caption_image
+
+        with patch("app.rag.vision.get_vision_provider", return_value=None):
+            with patch("app.rag.vision._ocr_caption", return_value=""):
+                results = caption_image([b"img1", b"img2"], page=[1, 2])
+        assert isinstance(results, list) and len(results) == 2
\ No newline at end of file
diff --git a/backend/vision/__init__.py b/backend/vision/__init__.py
new file mode 100644
index 00000000..7a1eac67
--- /dev/null
+++ b/backend/vision/__init__.py
@@ -0,0 +1,5 @@
+"""Vision package: pluggable VLM provider strategy for image captioning."""
+from app.vision.registry import get_vision_provider, register_provider
+from app.vision.base import BaseVisionProvider
+
+__all__ = ["BaseVisionProvider", "get_vision_provider", "register_provider"]
\ No newline at end of file
diff --git a/backend/vision/base.py b/backend/vision/base.py
new file mode 100644
index 00000000..6e0462cf
--- /dev/null
+++ b/backend/vision/base.py
@@ -0,0 +1,13 @@
+"""Abstract base class that every VLM provider must implement."""
+from abc import ABC, abstractmethod
+
+
+class BaseVisionProvider(ABC):
+    """Strategy interface for Vision-Language Model providers."""
+
+    @abstractmethod
+    def caption(self, image_bytes: bytes) -> str:
+        """Generate a one-sentence caption for the given image.
+
+        Returns a non-empty string, or empty string on failure (so caller can fall back).
+        """
\ No newline at end of file
diff --git a/backend/vision/providers/__init__.py b/backend/vision/providers/__init__.py
new file mode 100644
index 00000000..4cbbdabf
--- /dev/null
+++ b/backend/vision/providers/__init__.py
@@ -0,0 +1,7 @@
+"""Auto-registers all built-in providers on import."""
+from app.vision.providers import (  # noqa: F401
+    openai_provider,
+    anthropic_provider,
+    gemini_provider,
+    ollama_provider,
+)
\ No newline at end of file
diff --git a/backend/vision/providers/anthropic_provider.py b/backend/vision/providers/anthropic_provider.py
new file mode 100644
index 00000000..07e49dcb
--- /dev/null
+++ b/backend/vision/providers/anthropic_provider.py
@@ -0,0 +1,61 @@
+"""Anthropic Claude Vision provider.
+Activated when VISION_PROVIDER=anthropic and ANTHROPIC_API_KEY is set.
+"""
+import base64
+import logging
+
+from app.config import get_settings
+from app.vision.base import BaseVisionProvider
+from app.vision.registry import register_provider
+
+logger = logging.getLogger(__name__)
+settings = get_settings()
+
+_CAPTION_PROMPT = (
+    "Describe this figure or diagram in one concise sentence "
+    "suitable for use as a search index caption."
+)
+
+
+class AnthropicVisionProvider(BaseVisionProvider):
+
+    def __init__(self) -> None:
+        self._api_key: str = getattr(settings, "ANTHROPIC_API_KEY", "")
+        self._model: str = getattr(settings, "VISION_MODEL", None) or "claude-3-haiku-20240307"
+        if not self._api_key:
+            raise ValueError(
+                "ANTHROPIC_API_KEY must be set when VISION_PROVIDER=anthropic."
+            )
+
+    def caption(self, image_bytes: bytes) -> str:
+        try:
+            import anthropic
+        except ImportError:
+            logger.error("Run: pip install anthropic")
+            return ""
+
+        try:
+            client = anthropic.Anthropic(api_key=self._api_key)
+            b64 = base64.b64encode(image_bytes).decode("utf-8")
+            message = client.messages.create(
+                model=self._model,
+                max_tokens=120,
+                messages=[{
+                    "role": "user",
+                    "content": [
+                        {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": b64}},
+                        {"type": "text", "text": _CAPTION_PROMPT},
+                    ],
+                }],
+            )
+            content = message.content
+            if not content:
+                return ""
+            text_block = next((b for b in content if getattr(b, "type", None) == "text"), None)
+            return text_block.text.strip() if text_block else ""
+        except Exception as exc:
+            logger.debug("Anthropic vision caption failed: %s", exc)
+            return ""
+
+
+register_provider("anthropic", AnthropicVisionProvider)
\ No newline at end of file
diff --git a/backend/vision/providers/gemini_provider.py b/backend/vision/providers/gemini_provider.py
new file mode 100644
index 00000000..7d7a0805
--- /dev/null
+++ b/backend/vision/providers/gemini_provider.py
@@ -0,0 +1,50 @@
+"""Google Gemini Vision provider.
+Activated when VISION_PROVIDER=gemini and GOOGLE_API_KEY is set.
+"""
+import logging
+
+from app.config import get_settings
+from app.vision.base import BaseVisionProvider
+from app.vision.registry import register_provider
+
+logger = logging.getLogger(__name__)
+settings = get_settings()
+
+_CAPTION_PROMPT = (
+    "Describe this figure or diagram in one concise sentence "
+    "suitable for use as a search index caption."
+)
+
+
+class GeminiVisionProvider(BaseVisionProvider):
+
+    def __init__(self) -> None:
+        self._api_key: str = getattr(settings, "GOOGLE_API_KEY", "")
+        self._model: str = getattr(settings, "VISION_MODEL", None) or "gemini-1.5-flash"
+        if not self._api_key:
+            raise ValueError(
+                "GOOGLE_API_KEY must be set when VISION_PROVIDER=gemini."
+            )
+
+    def caption(self, image_bytes: bytes) -> str:
+        try:
+            import google.generativeai as genai
+        except ImportError:
+            logger.error("Run: pip install google-generativeai")
+            return ""
+
+        try:
+            from io import BytesIO
+            import PIL.Image
+            genai.configure(api_key=self._api_key)
+            model = genai.GenerativeModel(self._model)
+            image = PIL.Image.open(BytesIO(image_bytes))
+            response = model.generate_content([_CAPTION_PROMPT, image])
+            text = getattr(response, "text", None)
+            return text.strip() if text else ""
+        except Exception as exc:
+            logger.debug("Gemini vision caption failed: %s", exc)
+            return ""
+
+
+register_provider("gemini", GeminiVisionProvider)
\ No newline at end of file
diff --git a/backend/vision/providers/ollama_provider.py b/backend/vision/providers/ollama_provider.py
new file mode 100644
index 00000000..e95a1f1d
--- /dev/null
+++ b/backend/vision/providers/ollama_provider.py
@@ -0,0 +1,51 @@
+"""Ollama / LLaVA local Vision provider.
+Activated when VISION_PROVIDER=ollama. No API key needed.
+Make sure the model is pulled first: ollama pull llava
+"""
+import base64
+import logging
+
+from app.config import get_settings
+from app.vision.base import BaseVisionProvider
+from app.vision.registry import register_provider
+
+logger = logging.getLogger(__name__)
+settings = get_settings()
+
+_CAPTION_PROMPT = (
+    "Describe this figure or diagram in one concise sentence "
+    "suitable for use as a search index caption."
+)
+
+
+class OllamaVisionProvider(BaseVisionProvider):
+
+    def __init__(self) -> None:
+        self._base_url: str = (
+            getattr(settings, "OLLAMA_BASE_URL", None) or "http://localhost:11434"
+        ).rstrip("/")
+        self._model: str = getattr(settings, "VISION_MODEL", None) or "llava"
+
+    def caption(self, image_bytes: bytes) -> str:
+        try:
+            import httpx
+        except ImportError:
+            logger.error("Run: pip install httpx")
+            return ""
+
+        try:
+            b64 = base64.b64encode(image_bytes).decode("utf-8")
+            response = httpx.post(
+                f"{self._base_url}/api/generate",
+                json={"model": self._model, "prompt": _CAPTION_PROMPT, "images": [b64], "stream": False},
+                timeout=60.0,
+            )
+            response.raise_for_status()
+            text = response.json().get("response", "")
+            return text.strip() if text else ""
+        except Exception as exc:
+            logger.debug("Ollama vision caption failed: %s", exc)
+            return ""
+
+
+register_provider("ollama", OllamaVisionProvider)
\ No newline at end of file
diff --git a/backend/vision/providers/openai_provider.py b/backend/vision/providers/openai_provider.py
new file mode 100644
index 00000000..70a9dade
--- /dev/null
+++ b/backend/vision/providers/openai_provider.py
@@ -0,0 +1,61 @@
+"""OpenAI GPT-4o-mini Vision provider.
+Activated when VISION_PROVIDER=openai and OPENAI_API_KEY is set.
+"""
+import base64
+import logging
+
+from app.config import get_settings
+from app.vision.base import BaseVisionProvider
+from app.vision.registry import register_provider
+
+logger = logging.getLogger(__name__)
+settings = get_settings()
+
+_CAPTION_PROMPT = (
+    "Describe this figure or diagram in one concise sentence "
+    "suitable for use as a search index caption."
+)
+
+
+class OpenAIVisionProvider(BaseVisionProvider):
+
+    def __init__(self) -> None:
+        self._api_key: str = getattr(settings, "OPENAI_API_KEY", "")
+        self._model: str = getattr(settings, "VISION_MODEL", None) or "gpt-4o-mini"
+        if not self._api_key:
+            raise ValueError(
+                "OPENAI_API_KEY must be set when VISION_PROVIDER=openai."
+            )
+
+    def caption(self, image_bytes: bytes) -> str:
+        try:
+            from openai import OpenAI
+        except ImportError:
+            logger.error("Run: pip install openai")
+            return ""
+
+        try:
+            client = OpenAI(api_key=self._api_key)
+            b64 = base64.b64encode(image_bytes).decode("utf-8")
+            response = client.chat.completions.create(
+                model=self._model,
+                max_tokens=120,
+                messages=[{
+                    "role": "user",
+                    "content": [
+                        {"type": "image_url", "image_url": {"url": f"data:image/png;base64,{b64}", "detail": "low"}},
+                        {"type": "text", "text": _CAPTION_PROMPT},
+                    ],
+                }],
+            )
+            choices = response.choices
+            if not choices:
+                return ""
+            content = choices[0].message.content
+            return content.strip() if content else ""
+        except Exception as exc:
+            logger.debug("OpenAI vision caption failed: %s", exc)
+            return ""
+
+
+register_provider("openai", OpenAIVisionProvider)
\ No newline at end of file
diff --git a/backend/vision/registry.py b/backend/vision/registry.py
new file mode 100644
index 00000000..6be5a783
--- /dev/null
+++ b/backend/vision/registry.py
@@ -0,0 +1,34 @@
+"""Provider registry for VLM strategy lookup."""
+import logging
+from typing import Dict, Optional, Type
+
+from app.vision.base import BaseVisionProvider
+
+logger = logging.getLogger(__name__)
+
+_REGISTRY: Dict[str, Type[BaseVisionProvider]] = {}
+
+
+def register_provider(name: str, cls: Type[BaseVisionProvider]) -> None:
+    _REGISTRY[name.lower()] = cls
+    logger.debug("Registered VLM provider: %s → %s", name, cls.__name__)
+
+
+def get_vision_provider(name: Optional[str]) -> Optional[BaseVisionProvider]:
+    if not name:
+        return None
+
+    cls = _REGISTRY.get(name.lower())
+    if cls is None:
+        logger.warning(
+            "Unknown VISION_PROVIDER=%r. Available: %s",
+            name,
+            list(_REGISTRY.keys()) or ["(none)"],
+        )
+        return None
+
+    try:
+        return cls()
+    except Exception as exc:
+        logger.error("Failed to instantiate VLM provider %r: %s", name, exc)
+        return None
\ No newline at end of file