diff --git a/src/plyngent/config/__init__.py b/src/plyngent/config/__init__.py
index e0ea3ee..e05614f 100644
--- a/src/plyngent/config/__init__.py
+++ b/src/plyngent/config/__init__.py
@@ -10,6 +10,7 @@ from .models import DEFAULT_TOOL_DIRECTIVES as DEFAULT_TOOL_DIRECTIVES
 from .models import AgentConfig as AgentConfig
 from .models import AnthropicProvider as AnthropicProvider
 from .models import DatabaseConfig as DatabaseConfig
+from .models import DeepSeekConvention as DeepSeekConvention
 from .models import DeepseekProvider as DeepseekProvider
 from .models import HttpTimeoutConfig as HttpTimeoutConfig
 from .models import ModelConfig as ModelConfig
diff --git a/src/plyngent/config/models.py b/src/plyngent/config/models.py
index edd1546..1703d22 100644
--- a/src/plyngent/config/models.py
+++ b/src/plyngent/config/models.py
@@ -1,7 +1,14 @@
-from typing import Any
+from typing import Any, Literal
 
 from msgspec import Struct, field
 
+# DeepSeek API surface convention (``DeepseekProvider.convention`` and
+# per-model ``ModelConfig.convention`` override).
+# - "" (default) / "openai" → OpenAI-compatible chat completions
+# - "responses"           → OpenAI Responses API (``POST /responses``)
+# - "anthropic"           → legacy value, ignored (still chat completions)
+type DeepSeekConvention = Literal["", "openai", "anthropic", "responses"]
+
 # Built-in persona when ``[agent].system_prompt`` is omitted.
 # Set ``system_prompt = ""`` to omit the persona block only.
 # Override with a multi-line TOML literal (prefer ''' so nested " is fine).
@@ -162,6 +169,8 @@ class ModelConfig(Struct, omit_defaults=True):
     cost_factor: float = 1.0
     preset: str = ""
     url: str = ""
+    # DeepSeek API surface override ("" = inherit provider convention).
+    convention: DeepSeekConvention = ""
 
 
 class HttpTimeoutConfig(Struct, omit_defaults=True):
@@ -244,9 +253,19 @@ def _default_deepseek_models() -> dict[str, ModelConfig]:
 
 
 class DeepseekProvider(ProviderConfig, tag="deepseek"):
-    """Deepseek API provider with optional extras (e.g. convention)."""
+    """DeepSeek API provider.
+
+    ``convention`` selects the API surface on ``https://api.deepseek.com``:
+    - ``""`` / ``"openai"`` → OpenAI-compatible chat completions (default)
+    - ``"responses"`` → OpenAI Responses API (``POST /responses``)
+    - ``"anthropic"`` → legacy value, ignored (still chat completions)
+
+    ``extras`` keeps arbitrary provider keys; the legacy ``extras.convention``
+    key is parsed but ignored (prefer the typed ``convention`` field).
+    """
 
     models: dict[str, ModelConfig] = field(default_factory=_default_deepseek_models)
+    convention: DeepSeekConvention = ""
     extras: dict[str, str] = field(default_factory=dict)
 
 
diff --git a/src/plyngent/config/routing.py b/src/plyngent/config/routing.py
index 4be7124..46a9144 100644
--- a/src/plyngent/config/routing.py
+++ b/src/plyngent/config/routing.py
@@ -5,6 +5,7 @@ from typing import TYPE_CHECKING, Literal, cast
 
 from .models import (
     AnthropicProvider,
+    DeepseekProvider,
     HttpTimeoutConfig,
     ModelConfig,
     OpenAICompatibleProvider,
@@ -30,6 +31,9 @@ class EffectiveProvider:
     timeout: float | HttpTimeoutConfig | None
     provider_tools: list[dict[str, object]]
     source: Provider
+    # DeepSeek API surface (see DeepSeekConvention): "openai" / "anthropic" /
+    # "responses" for deepseek presets; "" (unused) for other presets.
+    convention: str = ""
 
 
 def provider_preset(provider: Provider) -> ProviderPreset:
@@ -73,6 +77,9 @@ def resolve_effective_provider(provider: Provider, *, model: str | None = None)
 
     ``preset`` always decides API conventions: ``openai`` means Responses,
     ``openai-compatible`` means Chat Completions, ``anthropic`` means Messages.
+    Exception: a ``deepseek`` preset may pick the Responses surface via the
+    ``convention`` field (``"responses"``); model-level ``convention`` wins
+    over the provider-level one.
     """
     parent_preset = provider_preset(provider)
     model_cfg = model_config_for(provider, model)
@@ -88,6 +95,13 @@ def resolve_effective_provider(provider: Provider, *, model: str | None = None)
     if preset == "openai" and isinstance(provider, OpenAIProvider) and parent_preset == "openai":
         provider_tools = [dict(item) for item in provider.provider_tools]
 
+    # DeepSeek API surface: model override > provider convention > chat default.
+    convention = ""
+    if preset == "deepseek" and isinstance(provider, DeepseekProvider):
+        convention = provider.convention or "openai"
+        if model_cfg is not None and model_cfg.convention:
+            convention = model_cfg.convention
+
     return EffectiveProvider(
         preset=preset,
         access_key_or_token=provider.access_key_or_token,
@@ -96,6 +110,7 @@ def resolve_effective_provider(provider: Provider, *, model: str | None = None)
         timeout=provider.timeout,
         provider_tools=provider_tools,
         source=provider,
+        convention=convention,
     )
 
 
@@ -108,4 +123,5 @@ def effective_provider_to_config_dict(effective: EffectiveProvider) -> Mapping[s
         "model": effective.model or "",
         "timeout": effective.timeout,
         "provider_tools": effective.provider_tools,
+        "convention": effective.convention,
     }
diff --git a/tests/test_config/plyngent-bad.toml b/tests/test_config/plyngent-bad.toml
index bbeb840..b0197ea 100644
--- a/tests/test_config/plyngent-bad.toml
+++ b/tests/test_config/plyngent-bad.toml
@@ -12,4 +12,4 @@ access_key_or_token = "sk-1145141919810"
 [providers.foo1]
 preset = "deepseek"
 access_key_or_token = "sk-1145141919810"
-convention = "anthropic"
+unknown_field = "should-be-rejected"
diff --git a/tests/test_config/test_config.py b/tests/test_config/test_config.py
index 777baa0..b487500 100644
--- a/tests/test_config/test_config.py
+++ b/tests/test_config/test_config.py
@@ -137,6 +137,116 @@ def test_deepseek_explicit_models_override_defaults() -> None:
     assert set(provider.models) == {"custom-only"}
 
 
+def test_deepseek_convention_from_toml(tmp_path: Path) -> None:
+    path = tmp_path / "deepseek-convention.toml"
+    _ = path.write_text(
+        """
+[providers.ds]
+preset = "deepseek"
+access_key_or_token = "sk-test"
+convention = "responses"
+""",
+        encoding="utf-8",
+    )
+    config = plyngent.config.load(path)
+    provider = config.providers["ds"]
+    assert isinstance(provider, DeepseekProvider)
+    assert provider.convention == "responses"
+
+
+def test_deepseek_convention_invalid_is_bad_provider(tmp_path: Path) -> None:
+    path = tmp_path / "deepseek-bad-convention.toml"
+    _ = path.write_text(
+        """
+[providers.ds]
+preset = "deepseek"
+access_key_or_token = "sk-test"
+convention = "bogus"
+""",
+        encoding="utf-8",
+    )
+    config = plyngent.config.load(path)
+    assert "ds" not in config.providers
+    assert "ds" in config.bad_providers
+
+
+def test_deepseek_model_convention_override_parses(tmp_path: Path) -> None:
+    path = tmp_path / "deepseek-model-convention.toml"
+    _ = path.write_text(
+        """
+[providers.ds]
+preset = "deepseek"
+access_key_or_token = "sk-test"
+
+[providers.ds.models]
+"deepseek-v4-flash" = { text = true, convention = "responses" }
+"deepseek-v4-pro" = { text = true }
+""",
+        encoding="utf-8",
+    )
+    config = plyngent.config.load(path)
+    provider = config.providers["ds"]
+    assert isinstance(provider, DeepseekProvider)
+    assert provider.models["deepseek-v4-flash"].convention == "responses"
+    assert provider.models["deepseek-v4-pro"].convention == ""
+
+
+def test_deepseek_convention_write_back(tmp_path: Path) -> None:
+    path = tmp_path / "deepseek-convention-write.toml"
+    config = ConfigStore(path=path, document=tomlkit.document())
+    config.providers = {
+        "ds": DeepseekProvider(access_key_or_token="sk-test", convention="responses"),
+    }
+    config.write()
+    text = path.read_text(encoding="utf-8")
+    assert 'convention = "responses"' in text
+    again = plyngent.config.load(path)
+    assert again.providers["ds"].convention == "responses"
+
+
+def test_deepseek_convention_default_not_written(tmp_path: Path) -> None:
+    from plyngent.config import ModelConfig
+
+    path = tmp_path / "deepseek-convention-default.toml"
+    config = ConfigStore(path=path, document=tomlkit.document())
+    config.providers = {
+        "ds": DeepseekProvider(
+            access_key_or_token="sk-test",
+            models={"m": ModelConfig(text=True)},
+        ),
+    }
+    config.write()
+    text = path.read_text(encoding="utf-8")
+    assert "convention" not in text
+
+
+def test_deepseek_effective_convention_precedence() -> None:
+    from plyngent.config import ModelConfig, resolve_effective_provider
+
+    provider = DeepseekProvider(
+        access_key_or_token="sk-test",
+        models={
+            "flash": ModelConfig(convention="responses"),
+            "pro": ModelConfig(),
+        },
+    )
+    # Provider default "" → chat completions convention.
+    effective = resolve_effective_provider(provider)
+    assert effective.convention == "openai"
+    # Model override wins.
+    effective = resolve_effective_provider(provider, model="flash")
+    assert effective.convention == "responses"
+    effective = resolve_effective_provider(provider, model="pro")
+    assert effective.convention == "openai"
+    # Provider-level convention is the fallback.
+    provider = DeepseekProvider(access_key_or_token="sk-test", convention="responses")
+    assert resolve_effective_provider(provider).convention == "responses"
+    # Non-deepseek presets leave convention empty.
+    from plyngent.config import OpenAIProvider
+
+    assert resolve_effective_provider(OpenAIProvider(access_key_or_token="sk")).convention == ""
+
+
 def test_read_empty_config() -> None:
     config = plyngent.config.load(Path(__file__).parent / "plyngent-empty.toml")
     assert isinstance(config.providers, Mapping)