diff --git a/providers/byteplus-coding.toml b/providers/byteplus-coding.toml
index 1ed0cc1..b4c001d 100644
--- a/providers/byteplus-coding.toml
+++ b/providers/byteplus-coding.toml
@@ -95,6 +95,7 @@ input_cost_per_m = 0.44
 output_cost_per_m = 2.0
 supports_tools = true
 supports_streaming = true
+reasoning_echo_policy = "empty_string"
 aliases = []
 
 [[models]]
diff --git a/providers/deepseek.toml b/providers/deepseek.toml
index 4573f6f..a8f0fe7 100644
--- a/providers/deepseek.toml
+++ b/providers/deepseek.toml
@@ -34,6 +34,10 @@ supports_tools = true
 supports_vision = false
 supports_streaming = true
 supports_thinking = true
+# Thinking mode is on by default and the API rejects multi-turn requests
+# where assistant turns containing tool_calls don't echo back the original
+# reasoning_content. See librefang/librefang#4842.
+reasoning_echo_policy = "echo"
 aliases = ["deepseek-flash"]
 
 [[models]]
@@ -61,4 +65,7 @@ supports_tools = false
 supports_vision = false
 supports_streaming = true
 supports_thinking = true
+# R1 returns reasoning_content in responses but the API rejects multi-turn
+# requests that carry it on previous assistant messages — drivers must strip.
+reasoning_echo_policy = "strip"
 aliases = ["deepseek-r1"]
diff --git a/providers/kimi-coding.toml b/providers/kimi-coding.toml
index ff16666..c127bb1 100644
--- a/providers/kimi-coding.toml
+++ b/providers/kimi-coding.toml
@@ -21,4 +21,5 @@ output_cost_per_m = 0.0
 supports_tools = true
 supports_vision = true
 supports_streaming = true
+reasoning_echo_policy = "empty_string"
 aliases = []
diff --git a/providers/moonshot.toml b/providers/moonshot.toml
index 49fdef4..9c0434a 100644
--- a/providers/moonshot.toml
+++ b/providers/moonshot.toml
@@ -20,6 +20,10 @@ supports_tools = true
 supports_vision = true
 supports_streaming = true
 supports_thinking = true
+# Kimi requires reasoning_content present (empty string) on assistant
+# turns with tool_calls, with thinking disabled wire-side for multi-turn
+# compatibility. See librefang/librefang openai driver.
+reasoning_echo_policy = "empty_string"
 aliases = ["kimi", "kimi-k2.6-0420"]
 
 [[models]]
@@ -34,6 +38,7 @@ supports_tools = true
 supports_vision = true
 supports_streaming = true
 supports_thinking = true
+reasoning_echo_policy = "empty_string"
 aliases = ["kimi-k2.5-0711"]
 
 [[models]]
@@ -47,4 +52,5 @@ output_cost_per_m = 2.3
 supports_tools = true
 supports_vision = true
 supports_streaming = true
+reasoning_echo_policy = "empty_string"
 aliases = []
diff --git a/providers/novita.toml b/providers/novita.toml
index 63a50a9..455f8a7 100644
--- a/providers/novita.toml
+++ b/providers/novita.toml
@@ -36,6 +36,7 @@ supports_tools = true
 supports_vision = false
 supports_streaming = true
 supports_thinking = true
+reasoning_echo_policy = "empty_string"
 aliases = []
 
 [[models]]
diff --git a/schema.toml b/schema.toml
index 14199fd..accce8b 100644
--- a/schema.toml
+++ b/schema.toml
@@ -152,6 +152,13 @@ required = false
 description = "Extended thinking / reasoning support"
 default = false
 
+[provider.sections.models.fields.reasoning_echo_policy]
+type = "string"
+required = false
+description = "How the OpenAI-compatible driver must handle the reasoning_content field on historical assistant turns when this model is used. 'none' (default) omits the field entirely. 'strip' is required by DeepSeek-R1 / deepseek-reasoner — the API rejects multi-turn requests that carry reasoning_content from previous turns. 'echo' is required by DeepSeek V4 Flash and other thinking-mode-on models — the original thinking text MUST be round-tripped on assistant turns that contain tool_calls, otherwise the API returns 400. 'empty_string' is required by Moonshot / Kimi K2 — the field must be present (empty string) on tool_calls turns, with thinking disabled wire-side."
+options = ["none", "strip", "echo", "empty_string"]
+default = "none"
+
 [provider.sections.models.fields.aliases]
 type = "array"
 required = false
diff --git a/scripts/validate.py b/scripts/validate.py
index f892b00..b7b05d3 100755
--- a/scripts/validate.py
+++ b/scripts/validate.py
@@ -35,6 +35,7 @@
 
 VALID_TIERS = {"frontier", "smart", "balanced", "fast", "local"}
 VALID_MODALITIES = {"text", "image", "audio", "video", "music"}
+VALID_REASONING_ECHO_POLICIES = {"none", "strip", "echo", "empty_string"}
 VALID_HAND_CATEGORIES = {
     "communication", "content", "data", "development",
     "devops", "finance", "productivity", "research", "social",
@@ -102,6 +103,13 @@ def validate_provider_file(filepath: Path) -> list[str]:
         if tier is not None and tier not in VALID_TIERS:
             errors.append(f"{filepath.name}: Model '{label}' invalid tier '{tier}'")
 
+        policy = model.get("reasoning_echo_policy")
+        if policy is not None and policy not in VALID_REASONING_ECHO_POLICIES:
+            errors.append(
+                f"{filepath.name}: Model '{label}' invalid reasoning_echo_policy "
+                f"'{policy}' (valid: {', '.join(sorted(VALID_REASONING_ECHO_POLICIES))})"
+            )
+
         for cost_field in (
             "input_cost_per_m",
             "output_cost_per_m",