diff --git a/services/providers/adapters.py b/services/providers/adapters.py
new file mode 100644
index 0000000..8fb13ec
--- /dev/null
+++ b/services/providers/adapters.py
@@ -0,0 +1,314 @@
+"""
+Track C1.1 — Provider interface adapters.
+
+Two adapter classes that bridge the legacy `TranslationProvider`
+interface (translate(text, target, source) -> str) and the new
+TranslationProvider interface (translate_text(request) -> Response).
+
+Why?
+  The codebase has TWO provider interfaces:
+    1. New: services/providers/base.py
+       - translate_text(TranslationRequest) -> TranslationResponse
+       - translate_batch(List[TranslationRequest]) -> List[TranslationResponse]
+    2. Legacy: services/translation_service.py
+       - translate(text, target, source) -> str
+       - translate_batch(texts, target, source) -> List[str]
+
+  Both are used in production. Translators sometimes receive the new
+  interface, sometimes the legacy one. Previously the translators
+  duck-typed the provider to figure out which interface it supported
+  (see e.g. pptx_translator._translate_with_provider). That was hacky.
+
+  These adapters give a single, explicit way to bridge the two
+  interfaces, so translators can always treat the provider as
+  whichever interface they prefer.
+
+Usage:
+    # Wrap a new-style provider as legacy
+    legacy = NewProviderAsLegacyAdapter(new_provider)
+    legacy.translate("Hello", "fr", "auto")  # returns str
+
+    # Wrap a legacy-style provider as new
+    new = LegacyProviderAsNewAdapter(legacy_provider)
+    new.translate_text(TranslationRequest(...))  # returns TranslationResponse
+"""
+
+from __future__ import annotations
+
+from typing import List, Optional
+
+from core.logging import get_logger
+
+from .base import TranslationProvider as NewTranslationProvider
+from .schemas import TranslationRequest, TranslationResponse, ProviderHealthStatus
+
+logger = get_logger(__name__)
+
+
+# =============================================================================
+# New-style → Legacy
+# =============================================================================
+
+class NewProviderAsLegacyAdapter:
+    """Adapt a new-style provider (translate_text) to the legacy interface.
+
+    The legacy interface is what most translators and the legacy
+    `translation_service` expect:
+        translate(text, target_language, source_language="auto") -> str
+        translate_batch(texts, target, source) -> List[str]
+        get_name() -> str
+        is_available() -> bool
+
+    This adapter is for INTERNAL use — it adapts at the boundary so
+    that a single provider instance can be passed to any code path.
+    """
+
+    def __init__(self, new_provider: NewTranslationProvider):
+        self._new = new_provider
+
+    # ---- Core legacy methods ----
+
+    def translate(
+        self, text: str, target_language: str, source_language: str = "auto"
+    ) -> str:
+        """Translate a single text — legacy signature."""
+        # Lazy import to avoid circular import
+        try:
+            from .schemas import TranslationRequest
+            request = TranslationRequest(
+                text=text,
+                target_language=target_language,
+                source_language=source_language,
+            )
+            response = self._new.translate_text(request)
+            if response.error:
+                raise Exception(
+                    f"[{response.error_code or 'UNKNOWN'}] {response.error}"
+                )
+            return response.translated_text
+        except Exception as e:
+            # If the new provider raised, re-raise so the caller
+            # can decide how to handle it (legacy code expects exceptions).
+            logger.debug(
+                "new_provider_as_legacy_translate_error",
+                error=str(e)[:200],
+            )
+            raise
+
+    def translate_batch(
+        self,
+        texts: List[str],
+        target_language: str,
+        source_language: str = "auto",
+    ) -> List[str]:
+        """Translate multiple texts — legacy signature."""
+        if not texts:
+            return []
+
+        try:
+            from .schemas import TranslationRequest
+            requests = [
+                TranslationRequest(
+                    text=t,
+                    target_language=target_language,
+                    source_language=source_language,
+                )
+                for t in texts
+            ]
+            responses = self._new.translate_batch(requests)
+            return [r.translated_text for r in responses]
+        except Exception as e:
+            logger.debug(
+                "new_provider_as_legacy_batch_error",
+                error=str(e)[:200],
+                count=len(texts),
+            )
+            raise
+
+    # ---- Identity methods (pass-through) ----
+
+    def get_name(self) -> str:
+        return self._new.get_name()
+
+    def is_available(self) -> bool:
+        try:
+            return bool(self._new.is_available())
+        except Exception:
+            return False
+
+    def health_check(self) -> dict:
+        """Best-effort health check, legacy-shaped (plain dict)."""
+        try:
+            status = self._new.health_check()
+            return {
+                "name": status.name,
+                "available": status.available,
+                "latency_ms": status.latency_ms,
+                "error": status.error,
+            }
+        except Exception as e:
+            return {
+                "name": self.get_name(),
+                "available": False,
+                "error": str(e)[:200],
+            }
+
+
+# =============================================================================
+# Legacy-style → New
+# =============================================================================
+
+class LegacyProviderAsNewAdapter(NewTranslationProvider):
+    """Adapt a legacy-style provider (translate(text, ...)) to the new interface.
+
+    Most new code expects the new interface:
+        translate_text(TranslationRequest) -> TranslationResponse
+        translate_batch(List[TranslationRequest]) -> List[TranslationResponse]
+
+    A "legacy" provider is anything that has:
+        translate(text, target, source) -> str
+        translate_batch(texts, target, source) -> List[str]
+        get_name() -> str
+        is_available() -> bool
+    """
+
+    def __init__(self, legacy_provider, provider_name: str = None):
+        self._legacy = legacy_provider
+        # If a custom name is provided, use it; otherwise inherit from the
+        # legacy provider.
+        if provider_name is not None:
+            self._name = provider_name
+        else:
+            try:
+                self._name = legacy_provider.get_name()
+            except Exception:
+                self._name = "legacy"
+
+    # ---- Implement the abstract methods of NewTranslationProvider ----
+
+    def translate_text(self, request: TranslationRequest) -> TranslationResponse:
+        """Translate a single text — new interface."""
+        start = time.time()
+        try:
+            translated = self._legacy.translate(
+                text=request.text,
+                target_language=request.target_language,
+                source_language=request.source_language or "auto",
+            )
+            elapsed_ms = round((time.time() - start) * 1000, 2)
+            return TranslationResponse(
+                translated_text=translated or "",
+                provider_name=self._name,
+                source_language=request.source_language or "auto",
+                target_language=request.target_language,
+                elapsed_ms=elapsed_ms,
+            )
+        except Exception as e:
+            elapsed_ms = round((time.time() - start) * 1000, 2)
+            return TranslationResponse(
+                translated_text="",
+                provider_name=self._name,
+                source_language=request.source_language or "auto",
+                target_language=request.target_language,
+                elapsed_ms=elapsed_ms,
+                error=str(e)[:200],
+                error_code="LEGACY_PROVIDER_ERROR",
+            )
+
+    def translate_batch(
+        self, requests: List[TranslationRequest]
+    ) -> List[TranslationResponse]:
+        """Translate a batch of texts — new interface."""
+        if not requests:
+            return []
+
+        # Group by language pair so we can call the legacy
+        # translate_batch efficiently (legacy signature is per-language-pair).
+        from collections import defaultdict
+        groups: dict = defaultdict(list)
+        for i, r in enumerate(requests):
+            key = (r.target_language, r.source_language or "auto")
+            groups[key].append((i, r))
+
+        results: List[TranslationResponse] = [None] * len(requests)  # type: ignore
+
+        for (target_lang, source_lang), items in groups.items():
+            texts = [r.text for _, r in items]
+            try:
+                translated = self._legacy.translate_batch(
+                    texts=texts,
+                    target_language=target_lang,
+                    source_language=source_lang,
+                )
+            except Exception as e:
+                # If legacy batch fails, fall back to per-text
+                logger.debug(
+                    "legacy_batch_fallback_to_individual",
+                    error=str(e)[:200],
+                )
+                translated = []
+                for t in texts:
+                    try:
+                        translated.append(
+                            self._legacy.translate(t, target_lang, source_lang)
+                        )
+                    except Exception as individual_err:
+                        translated.append("")  # legacy code expects str
+
+            for (orig_idx, r), t in zip(items, translated):
+                results[orig_idx] = TranslationResponse(
+                    translated_text=t or "",
+                    provider_name=self._name,
+                    source_language=source_lang,
+                    target_language=target_lang,
+                )
+
+        return results
+
+    def get_name(self) -> str:
+        # Always prefer the explicit name set at construction. Only fall
+        # back to the legacy provider's get_name if no name was set.
+        if hasattr(self, "_name") and self._name:
+            return self._name
+        try:
+            return self._legacy.get_name()
+        except Exception:
+            return "legacy"
+
+    def is_available(self) -> bool:
+        try:
+            return bool(self._legacy.is_available())
+        except Exception:
+            return False
+
+
+# =============================================================================
+# Auto-detect helper
+# =============================================================================
+
+import time  # noqa: E402
+
+
+def coerce_to_legacy(provider) -> NewProviderAsLegacyAdapter:
+    """Coerce any provider to the legacy interface.
+
+    If the provider is already a legacy provider, return it as-is.
+    If it's a new-style provider, wrap it in NewProviderAsLegacyAdapter.
+    """
+    if isinstance(provider, NewProviderAsLegacyAdapter):
+        return provider
+    if isinstance(provider, NewTranslationProvider):
+        return NewProviderAsLegacyAdapter(provider)
+    # Already legacy-shaped — pass through
+    return provider
+
+
+def coerce_to_new(provider, name: str = None) -> NewTranslationProvider:
+    """Coerce any provider to the new interface.
+
+    If the provider is already a new-style provider, return it as-is.
+    If it's a legacy provider, wrap it in LegacyProviderAsNewAdapter.
+    """
+    if isinstance(provider, NewTranslationProvider):
+        return provider
+    return LegacyProviderAsNewAdapter(provider, provider_name=name)
diff --git a/tests/test_providers_adapter.py b/tests/test_providers_adapter.py
new file mode 100644
index 0000000..d079ad6
--- /dev/null
+++ b/tests/test_providers_adapter.py
@@ -0,0 +1,332 @@
+"""
+Tests for Track C1.1 — NewProviderAsLegacyAdapter and LegacyProviderAsNewAdapter.
+
+Covers:
+  - NewProviderAsLegacyAdapter:
+      * translate() delegates to translate_text()
+      * translate_batch() delegates to translate_batch()
+      * get_name() and is_available() pass through
+      * errors propagate as exceptions (legacy code expects this)
+  - LegacyProviderAsNewAdapter:
+      * translate_text() wraps legacy.translate()
+      * translate_batch() groups by language pair
+      * get_name() and is_available() pass through
+  - coerce_to_legacy / coerce_to_new:
+      * no-op when already in target shape
+      * wraps when in the other shape
+"""
+import pytest
+from typing import List
+from unittest.mock import MagicMock
+
+from services.providers.base import TranslationProvider as NewTranslationProvider
+from services.providers.adapters import (
+    NewProviderAsLegacyAdapter,
+    LegacyProviderAsNewAdapter,
+    coerce_to_legacy,
+    coerce_to_new,
+)
+from services.providers.schemas import (
+    TranslationRequest,
+    TranslationResponse,
+)
+
+
+# ============================================================================
+# Test helpers — new-style mock
+# ============================================================================
+
+class MockNewProvider(NewTranslationProvider):
+    """Minimal new-style provider for testing the adapter."""
+
+    def __init__(self, translations: dict = None, name: str = "mock-new"):
+        self._t = translations or {}
+        self._name = name
+        self.translate_text_calls = []
+        self.translate_batch_calls = []
+
+    def translate_text(self, request: TranslationRequest) -> TranslationResponse:
+        self.translate_text_calls.append(request)
+        translated = self._t.get(request.text, f"NEW_{request.text}")
+        return TranslationResponse(
+            translated_text=translated,
+            provider_name=self._name,
+            source_language=request.source_language or "auto",
+            target_language=request.target_language,
+        )
+
+    def translate_batch(
+        self, requests: List[TranslationRequest]
+    ) -> List[TranslationResponse]:
+        """Override the default (which calls translate_text N times) so we
+        can verify the adapter calls translate_batch directly."""
+        self.translate_batch_calls.append(requests)
+        return [self.translate_text(req) for req in requests]
+
+    def get_name(self) -> str:
+        return self._name
+
+    def is_available(self) -> bool:
+        return True
+
+
+# ============================================================================
+# Test helpers — legacy mock
+# ============================================================================
+
+class MockLegacyProvider:
+    """Minimal legacy provider for testing the adapter."""
+
+    def __init__(self, translations: dict = None, name: str = "mock-legacy"):
+        self._t = translations or {}
+        self._name = name
+        self.calls = []
+
+    def translate(self, text, target_language, source_language="auto"):
+        self.calls.append((text, target_language, source_language))
+        return self._t.get(text, f"LEG_{text}")
+
+    def translate_batch(self, texts, target_language, source_language="auto"):
+        return [self.translate(t, target_language, source_language) for t in texts]
+
+    def get_name(self):
+        return self._name
+
+    def is_available(self):
+        return True
+
+
+# ============================================================================
+# NewProviderAsLegacyAdapter
+# ============================================================================
+
+class TestNewProviderAsLegacyAdapter:
+    def test_translate_delegates(self):
+        new = MockNewProvider({"Hello": "Bonjour"})
+        legacy = NewProviderAsLegacyAdapter(new)
+
+        result = legacy.translate("Hello", "fr", "en")
+        assert result == "Bonjour"
+        assert len(new.translate_text_calls) == 1
+        assert new.translate_text_calls[0].text == "Hello"
+        assert new.translate_text_calls[0].target_language == "fr"
+
+    def test_translate_default_source(self):
+        new = MockNewProvider()
+        legacy = NewProviderAsLegacyAdapter(new)
+        legacy.translate("Hello", "fr")
+        assert new.translate_text_calls[0].source_language == "auto"
+
+    def test_translate_batch_delegates(self):
+        new = MockNewProvider({"A": "TR_A", "B": "TR_B"})
+        legacy = NewProviderAsLegacyAdapter(new)
+        result = legacy.translate_batch(["A", "B"], "fr")
+        assert result == ["TR_A", "TR_B"]
+        assert len(new.translate_batch_calls) == 1
+
+    def test_translate_batch_empty(self):
+        new = MockNewProvider()
+        legacy = NewProviderAsLegacyAdapter(new)
+        result = legacy.translate_batch([], "fr")
+        assert result == []
+
+    def test_get_name_passthrough(self):
+        new = MockNewProvider(name="my-new")
+        legacy = NewProviderAsLegacyAdapter(new)
+        assert legacy.get_name() == "my-new"
+
+    def test_is_available_passthrough(self):
+        new = MockNewProvider()
+        legacy = NewProviderAsLegacyAdapter(new)
+        assert legacy.is_available() is True
+
+    def test_translate_error_raises(self):
+        """Legacy code expects exceptions, not Response objects with errors."""
+
+        class FailingProvider(MockNewProvider):
+            def translate_text(self, request):
+                return TranslationResponse(
+                    translated_text="",
+                    provider_name="failing",
+                    source_language="auto",
+                    target_language="fr",
+                    error="upstream timeout",
+                    error_code="TIMEOUT",
+                )
+
+        legacy = NewProviderAsLegacyAdapter(FailingProvider())
+        with pytest.raises(Exception, match="TIMEOUT"):
+            legacy.translate("Hello", "fr")
+
+    def test_health_check(self):
+        new = MockNewProvider()
+        legacy = NewProviderAsLegacyAdapter(new)
+        status = legacy.health_check()
+        assert status["name"] == "mock-new"
+        assert status["available"] is True
+
+
+# ============================================================================
+# LegacyProviderAsNewAdapter
+# ============================================================================
+
+class TestLegacyProviderAsNewAdapter:
+    def test_translate_text_wraps(self):
+        legacy = MockLegacyProvider({"Hello": "Bonjour"})
+        adapter = LegacyProviderAsNewAdapter(legacy)
+
+        result = adapter.translate_text(TranslationRequest(
+            text="Hello", target_language="fr", source_language="en",
+        ))
+        assert result.translated_text == "Bonjour"
+        assert result.provider_name == "mock-legacy"
+        assert not result.error
+        assert len(legacy.calls) == 1
+
+    def test_translate_text_error_returns_response(self):
+        """New interface wraps errors in the Response, doesn't raise."""
+        class FailingLegacy:
+            def translate(self, *args, **kwargs):
+                raise RuntimeError("kaboom")
+            def get_name(self): return "failing"
+            def is_available(self): return True
+
+        adapter = LegacyProviderAsNewAdapter(FailingLegacy())
+        result = adapter.translate_text(TranslationRequest(
+            text="Hello", target_language="fr", source_language="en",
+        ))
+        assert result.translated_text == ""
+        assert "kaboom" in result.error
+        assert result.error_code == "LEGACY_PROVIDER_ERROR"
+
+    def test_translate_batch_single_group(self):
+        legacy = MockLegacyProvider({"A": "TR_A", "B": "TR_B"})
+        adapter = LegacyProviderAsNewAdapter(legacy)
+
+        results = adapter.translate_batch([
+            TranslationRequest(text="A", target_language="fr", source_language="en"),
+            TranslationRequest(text="B", target_language="fr", source_language="en"),
+        ])
+        assert [r.translated_text for r in results] == ["TR_A", "TR_B"]
+
+    def test_translate_batch_groups_by_language_pair(self):
+        """Requests with different (target, source) should NOT be batched together."""
+        legacy = MockLegacyProvider()
+        adapter = LegacyProviderAsNewAdapter(legacy)
+
+        adapter.translate_batch([
+            TranslationRequest(text="A", target_language="fr", source_language="en"),
+            TranslationRequest(text="B", target_language="es", source_language="en"),
+            TranslationRequest(text="C", target_language="fr", source_language="en"),
+        ])
+
+        # Three calls: A→fr, B→es, C→fr (legacy translate_batch is one
+        # call per language pair)
+        # Two distinct (target, source) pairs → at most 2 batch calls
+        assert len(legacy.calls) >= 2
+
+    def test_translate_batch_empty(self):
+        legacy = MockLegacyProvider()
+        adapter = LegacyProviderAsNewAdapter(legacy)
+        results = adapter.translate_batch([])
+        assert results == []
+
+    def test_translate_batch_order_preserved(self):
+        """The order of results must match the order of requests."""
+        legacy = MockLegacyProvider({"X": "TR_X", "Y": "TR_Y", "Z": "TR_Z"})
+        adapter = LegacyProviderAsNewAdapter(legacy)
+
+        results = adapter.translate_batch([
+            TranslationRequest(text="Z", target_language="fr", source_language="en"),
+            TranslationRequest(text="X", target_language="fr", source_language="en"),
+            TranslationRequest(text="Y", target_language="fr", source_language="en"),
+        ])
+        assert [r.translated_text for r in results] == ["TR_Z", "TR_X", "TR_Y"]
+
+    def test_get_name_passthrough(self):
+        legacy = MockLegacyProvider(name="my-legacy")
+        adapter = LegacyProviderAsNewAdapter(legacy)
+        assert adapter.get_name() == "my-legacy"
+
+    def test_is_available_passthrough(self):
+        legacy = MockLegacyProvider()
+        legacy.is_available = lambda: False
+        adapter = LegacyProviderAsNewAdapter(legacy)
+        assert adapter.is_available() is False
+
+    def test_custom_provider_name_overrides(self):
+        legacy = MockLegacyProvider(name="original-name")
+        adapter = LegacyProviderAsNewAdapter(legacy, provider_name="override-name")
+        assert adapter.get_name() == "override-name"
+
+
+# ============================================================================
+# Coercion helpers
+# ============================================================================
+
+class TestCoercion:
+    def test_new_already_legacy_noop(self):
+        new = MockNewProvider()
+        adapter = NewProviderAsLegacyAdapter(new)
+        result = coerce_to_legacy(adapter)
+        # Should return the existing adapter, not wrap it again
+        assert result is adapter
+
+    def test_new_provider_to_legacy_wraps(self):
+        new = MockNewProvider()
+        result = coerce_to_legacy(new)
+        assert isinstance(result, NewProviderAsLegacyAdapter)
+        assert result._new is new
+
+    def test_legacy_provider_unchanged(self):
+        legacy = MockLegacyProvider()
+        result = coerce_to_legacy(legacy)
+        assert result is legacy
+
+    def test_legacy_already_new_noop(self):
+        legacy = MockLegacyProvider()
+        adapter = LegacyProviderAsNewAdapter(legacy)
+        result = coerce_to_new(adapter)
+        assert result is adapter
+
+    def test_legacy_provider_to_new_wraps(self):
+        legacy = MockLegacyProvider()
+        result = coerce_to_new(legacy)
+        assert isinstance(result, NewTranslationProvider)
+        assert isinstance(result, LegacyProviderAsNewAdapter)
+
+    def test_new_provider_unchanged(self):
+        new = MockNewProvider()
+        result = coerce_to_new(new)
+        assert result is new
+
+
+# ============================================================================
+# Round-trip
+# ============================================================================
+
+class TestRoundTrip:
+    """An adapter should be reversible: a string that comes out of one
+    side should match what the other side would have returned."""
+
+    def test_new_legacy_round_trip(self):
+        new = MockNewProvider({"Hello": "Bonjour", "World": "Monde"})
+        legacy = NewProviderAsLegacyAdapter(new)
+
+        # Round-trip: legacy.translate → new.translate_text
+        via_legacy = legacy.translate("Hello", "fr", "en")
+        via_new = new.translate_text(TranslationRequest(
+            text="Hello", target_language="fr", source_language="en",
+        )).translated_text
+        assert via_legacy == via_new
+
+    def test_legacy_new_round_trip(self):
+        legacy = MockLegacyProvider({"Hello": "Bonjour"})
+        adapter = LegacyProviderAsNewAdapter(legacy)
+
+        # Round-trip: new.translate_text → legacy.translate
+        via_new = adapter.translate_text(TranslationRequest(
+            text="Hello", target_language="fr", source_language="en",
+        )).translated_text
+        via_legacy = legacy.translate("Hello", "fr", "en")
+        assert via_new == via_legacy