From 31e64822076038047f23241982079348d94b0f48 Mon Sep 17 00:00:00 2001
From: SaladDay <1203511142@qq.com>
Date: Wed, 7 Oct 2026 17:20:03 +0000
Subject: [PATCH] Require the harness for a non-empty harness_config

Native parameters belong to one adapter, so validating them against
whichever registered adapter accepts them was an implicit fallback. Delete
the any-adapter loop: an empty kind with parameters now returns
harnessconfig.ErrHarnessRequired.

The resolved Agent decides: a saved Agent after the write (an update keeps
the saved harness) and, for a Session, the inline agent's harness or the
saved Agent's. Both reject with 400 invalid_request_error and a param that
names the missing field.
---
 contracts/agents-api/model-execution.md       |  2 +-
 contracts/agents-api/v1/core_extension.go     | 10 +++++-
 .../agents-api/v1/model_configuration.go      | 10 ++++--
 .../agents-api/v1/saved_core_extension.go     |  4 ++-
 contracts/agents-api/zh/model-execution.md    |  4 +--
 docs/api/public-agent-api.md                  |  2 +-
 docs/zh/api/public-agent-api.md               |  4 +--
 internal/harnessconfig/builtin/native_test.go |  9 +++--
 internal/harnessconfig/native.go              | 33 +++++++++----------
 .../core/internal/agents/configuration.go     | 11 +++++--
 services/core/internal/api/errors_agents.go   |  7 ++++
 .../api/session_model_configuration.go        | 26 +++++++++++----
 .../api/session_model_configuration_test.go   |  6 ++--
 .../unified_model_configuration_http_test.go  | 21 ++++++++++++
 14 files changed, 105 insertions(+), 44 deletions(-)

diff --git a/contracts/agents-api/model-execution.md b/contracts/agents-api/model-execution.md
index ca27f0193..056bc9394 100644
--- a/contracts/agents-api/model-execution.md
+++ b/contracts/agents-api/model-execution.md
@@ -100,7 +100,7 @@ The Harness reaches its frozen upstream through a Session-local credential gatew
 
 ## Native model parameters
 
-`harness_config` holds the selected Harness's native model parameters. Saved Agents accept it in `x_agents_core`, Sessions in the inline `agent.x_agents_core` and in the top-level `x_agents_core`; the top-level value wins.
+`harness_config` holds the selected Harness's native model parameters. Saved Agents accept it in `x_agents_core`, Sessions in the inline `agent.x_agents_core` and in the top-level `x_agents_core`; the top-level value wins. A non-empty object needs an explicitly selected Harness: the Agent's `x_agents_core.harness`, which an update keeps, or for a Session the inline `agent.x_agents_core.harness` or the saved Agent's. The deployment default Harness does not count. Without one, the request fails with 400 `invalid_request_error` and `param` `x_agents_core.harness`, or `agent.x_agents_core.harness` on Session creation.
 
 | Harness | Accepted fields | Applied as |
 | --- | --- | --- |
diff --git a/contracts/agents-api/v1/core_extension.go b/contracts/agents-api/v1/core_extension.go
index 156761009..fe84766c4 100644
--- a/contracts/agents-api/v1/core_extension.go
+++ b/contracts/agents-api/v1/core_extension.go
@@ -2,7 +2,10 @@ package v1
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+
+	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig"
 	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig/builtin"
 )
 
@@ -22,5 +25,10 @@ func (x *AgentsCore) Validate() error {
 	if x.Harness != "" && !builtin.Contains(x.Harness) {
 		return fmt.Errorf("x_agents_core.harness must select a registered harness")
 	}
-	return ValidateHarnessConfig(x.Harness, x.HarnessConfig)
+	// Session admission requires the harness on the resolved Agent: an inline
+	// extension without one keeps the saved Agent's.
+	if err := ValidateHarnessConfig(x.Harness, x.HarnessConfig); err != nil && !errors.Is(err, harnessconfig.ErrHarnessRequired) {
+		return err
+	}
+	return nil
 }
diff --git a/contracts/agents-api/v1/model_configuration.go b/contracts/agents-api/v1/model_configuration.go
index 001285756..729667f5f 100644
--- a/contracts/agents-api/v1/model_configuration.go
+++ b/contracts/agents-api/v1/model_configuration.go
@@ -2,6 +2,7 @@ package v1
 
 import (
 	"encoding/json"
+	"errors"
 
 	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig"
 	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig/builtin"
@@ -28,11 +29,14 @@ func (c ModelConfigurationInput) ValidateHarness(harness string) error {
 	return ValidateNativeModelConfiguration(harness, c.Model, c.HarnessConfig)
 }
 
+// ValidateHarnessConfig returns harnessconfig.ErrHarnessRequired unchanged:
+// the caller names the missing harness field of its own resource.
 func ValidateHarnessConfig(harness string, raw json.RawMessage) error {
-	if err := builtin.Registry().ValidateHarnessConfig(harness, raw); err != nil {
-		return &ModelProviderError{Code: "harness_config_invalid", Param: "harness_config", message: "harness_config contains unsupported or invalid native model parameters"}
+	err := builtin.Registry().ValidateHarnessConfig(harness, raw)
+	if err == nil || errors.Is(err, harnessconfig.ErrHarnessRequired) {
+		return err
 	}
-	return nil
+	return &ModelProviderError{Code: "harness_config_invalid", Param: "harness_config", message: "harness_config contains unsupported or invalid native model parameters"}
 }
 
 func (c ModelConfigurationInput) SafeView() ModelConfigurationView {
diff --git a/contracts/agents-api/v1/saved_core_extension.go b/contracts/agents-api/v1/saved_core_extension.go
index 1b1353a3d..735295d6d 100644
--- a/contracts/agents-api/v1/saved_core_extension.go
+++ b/contracts/agents-api/v1/saved_core_extension.go
@@ -40,7 +40,9 @@ func (x *SavedAgentCoreInput) Validate() error {
 			return err
 		}
 	}
-	if err := ValidateHarnessConfig(x.Harness, x.HarnessConfig); err != nil {
+	// The saved Agent after the write must have the harness: an update keeps
+	// the saved one.
+	if err := ValidateHarnessConfig(x.Harness, x.HarnessConfig); err != nil && !errors.Is(err, harnessconfig.ErrHarnessRequired) {
 		return err
 	}
 	if x.ModelProvider == nil {
diff --git a/contracts/agents-api/zh/model-execution.md b/contracts/agents-api/zh/model-execution.md
index 3961e53e4..bfde567c6 100644
--- a/contracts/agents-api/zh/model-execution.md
+++ b/contracts/agents-api/zh/model-execution.md
@@ -1,7 +1,7 @@
 ---
 title: "模型执行"
 source: contracts/agents-api/model-execution.md
-source_hash: 36c013fb36432a792ee693a7cee46d739710d6e3faba7f8448d418226dae38b4
+source_hash: 2a80d9958e34d744793307b46354dfc887becc632f70d1e5193ee4b7c27d2172
 ---
 
 每个 Session 都运行一个 Harness，并使用一个模型提供商。Core 通过三个固定版本上游协议未定义的 Core 扩展来选择它们：`x_agents_core.harness` 选择 Harness，`x_agents_core.model_provider` 提供端点和密钥，`x_agents_core.harness_config` 携带原生模型参数。Core 没有提供商目录、模型别名解析或产品权限模型；除 Session 和已保存 Agent 配置包外，唯一存储的配置包是每个 Harness 的一个 [deployment default](#deployment-defaults)。本文档定义 Harness—模型提供商协议：[`internal/modelprovider/config.go`](https://github.com/MiniMax-AI/OpenAgentCore/blob/main/internal/modelprovider/config.go) 负责验证冻结的提供商连接并声明[凭据网关](#credential-gateway)转发的内容，每个 Harness 则通过 [`internal/harnessconfig/harness.go`](https://github.com/MiniMax-AI/OpenAgentCore/blob/main/internal/harnessconfig/harness.go) 声明其协议和原生参数。
@@ -102,7 +102,7 @@ Harness 通过 agent host 上 Session 本地的凭据网关访问其冻结的上
 
 ## 原生模型参数 {#native-model-parameters}
 
-`harness_config` 保存所选 Harness 的原生模型参数。已保存 Agent 在 `x_agents_core` 中接受它，Session 则在内联的 `agent.x_agents_core` 和顶层 `x_agents_core` 中接受它；顶层值优先。
+`harness_config` 保存所选 Harness 的原生模型参数。已保存 Agent 在 `x_agents_core` 中接受它，Session 则在内联的 `agent.x_agents_core` 和顶层 `x_agents_core` 中接受它；顶层值优先。非空对象需要显式选择的 Harness：Agent 的 `x_agents_core.harness`（更新时保留已保存的值），或者对 Session 而言，内联的 `agent.x_agents_core.harness` 或已保存 Agent 的 Harness。部署默认 Harness 不算在内。缺少 Harness 时，请求以 400 `invalid_request_error` 失败，`param` 为 `x_agents_core.harness`；创建 Session 时为 `agent.x_agents_core.harness`。
 
 | Harness | 接受的字段 | 应用方式 |
 | --- | --- | --- |
diff --git a/docs/api/public-agent-api.md b/docs/api/public-agent-api.md
index 27200285e..869b0a4c4 100644
--- a/docs/api/public-agent-api.md
+++ b/docs/api/public-agent-api.md
@@ -152,7 +152,7 @@ Any other member is rejected with 400. `api_key` is write-only: reads return `ap
 
 ## Choose a harness and a model
 
-The harness is the agent program that runs a Session: Codex (`codex`), Claude Code (`claude_sdk`) or MiniMax Code (`mcode`). Set `x_agents_core.harness` on the Agent or the inline `agent`; without it, the installation's default harness applies ([`core.default_harness`](../configuration.md#settings), Codex unless the operator changed it).
+The harness is the agent program that runs a Session: Codex (`codex`), Claude Code (`claude_sdk`) or MiniMax Code (`mcode`). Set `x_agents_core.harness` on the Agent or the inline `agent`; without it, the installation's default harness applies ([`core.default_harness`](../configuration.md#settings), Codex unless the operator changed it) and `harness_config` must be empty.
 
 - **Model.** `model` is the provider's exact model ID. An inline Agent may omit it to use the default model configuration of its harness. A saved Agent always needs one.
 - **Provider.** The harness calls your provider with one of the harness's native protocols, through a [credential gateway](../../contracts/agents-api/model-execution.md#credential-gateway) that keeps your key out of the harness; there is no conversion, and a mismatch is rejected when the Session is created. [Model execution](../../contracts/agents-api/model-execution.md#saved-defaults-and-precedence) lists each harness's protocols and which provider a Session uses on each Environment type. A Session freezes its provider at creation.
diff --git a/docs/zh/api/public-agent-api.md b/docs/zh/api/public-agent-api.md
index 354d7ce40..121358a74 100644
--- a/docs/zh/api/public-agent-api.md
+++ b/docs/zh/api/public-agent-api.md
@@ -1,7 +1,7 @@
 ---
 title: "Agents API 指南"
 source: docs/api/public-agent-api.md
-source_hash: f7999939977b8cf4a516c2bea09cd50c8078d6fb05dffeb137dfba45c47839d1
+source_hash: 07bcf81c1f4d131d55e38bf1158805a24e53bae13c79205857dc8fbe25001123
 ---
 
 Core 在 `/v1` 提供 [OpenAI Agents API](https://platform.openai.com/docs/api-reference)。可以使用官方 OpenAI SDK 或普通 HTTP。本指南针对每项常见操作同时展示这两种方式，并说明 Core 与 OpenAI 存在差异的地方。
@@ -154,7 +154,7 @@ Core 可以运行多种 harness，并允许接入你自己的模型访问方式
 
 ## 选择 harness 和模型 {#choose-a-harness-and-a-model}
 
-harness 是运行 Session 的 Agent 程序：Codex（`codex`）、Claude Code（`claude_sdk`）或 MiniMax Code（`mcode`）。请在 Agent 或内联 `agent` 上设置 `x_agents_core.harness`；如果未设置，则使用安装的默认 harness（[`core.default_harness`](../configuration.md#settings)，除非操作员另行更改，否则为 Codex）。
+harness 是运行 Session 的 Agent 程序：Codex（`codex`）、Claude Code（`claude_sdk`）或 MiniMax Code（`mcode`）。请在 Agent 或内联 `agent` 上设置 `x_agents_core.harness`；如果未设置，则使用安装的默认 harness（[`core.default_harness`](../configuration.md#settings)，除非操作员另行更改，否则为 Codex），且 `harness_config` 必须为空。
 
 - **模型。** `model` 是提供商给出的准确模型 ID。内联 Agent 可以省略此字段，以使用其 harness 的默认模型配置。保存的 Agent 始终必须指定模型。
 - **提供商。** harness 会使用其原生协议之一，经由一个让你的密钥不进入 harness 的[凭据网关](../../../contracts/agents-api/zh/model-execution.md#credential-gateway)调用你的提供商；系统不会进行转换，不匹配时会在创建 Session 阶段拒绝请求。[Model execution](../../../contracts/agents-api/zh/model-execution.md#saved-defaults-and-precedence) 列出了每个 harness 的协议，以及各类 Environment 上 Session 使用的提供商。Session 会在创建时冻结其提供商。
diff --git a/internal/harnessconfig/builtin/native_test.go b/internal/harnessconfig/builtin/native_test.go
index e449d1d4f..5560d74b5 100644
--- a/internal/harnessconfig/builtin/native_test.go
+++ b/internal/harnessconfig/builtin/native_test.go
@@ -32,9 +32,8 @@ func TestNativeModelParameters(t *testing.T) {
 		{"claude_sdk", `{"maxThinkingTokens":1024}`, false},
 		{"mcode", `{}`, true},
 		{"mcode", `{"effort":"high"}`, false},
-		{"", `{"effort":"high"}`, true},
-		{"", `{"model_reasoning_effort":"high"}`, true},
-		{"", `{"secret":"private-value"}`, false},
+		{"", `{}`, true},
+		{"", `{"effort":"high"}`, false},
 		{"unknown", `{}`, true},
 		{"unknown", `{"effort":"high"}`, false},
 	} {
@@ -42,14 +41,14 @@ func TestNativeModelParameters(t *testing.T) {
 		if (err == nil) != tc.valid {
 			t.Fatalf("%s %s: %v", tc.kind, tc.raw, err)
 		}
-		if err != nil && err != harnessconfig.ErrHarnessConfig {
+		if err != nil && err != harnessconfig.ErrHarnessConfig && err != harnessconfig.ErrHarnessRequired {
 			t.Fatalf("unsafe error: %v", err)
 		}
 	}
 }
 
 func TestNativeConfigurationBoundary(t *testing.T) {
-	for _, kind := range []string{"codex", "claude_sdk", "mcode", ""} {
+	for _, kind := range []string{"codex", "claude_sdk", "mcode"} {
 		for _, raw := range []string{"null", "[]", "1", `"value"`, `{"unknown":"` + strings.Repeat("x", harnessconfig.MaxHarnessConfigBytes) + `"}`} {
 			if err := Registry().ValidateHarnessConfig(kind, json.RawMessage(raw)); err != harnessconfig.ErrHarnessConfig {
 				t.Fatalf("%s accepted invalid shape: %v", kind, err)
diff --git a/internal/harnessconfig/native.go b/internal/harnessconfig/native.go
index 89cc9a69a..c98038d10 100644
--- a/internal/harnessconfig/native.go
+++ b/internal/harnessconfig/native.go
@@ -9,8 +9,13 @@ import (
 	"github.com/MiniMax-AI/OpenAgentCore/internal/agentdaemon/proto"
 )
 
-// ErrHarnessConfig deliberately excludes caller-controlled keys and values.
-var ErrHarnessConfig = errors.New("invalid or unsupported harness_config")
+var (
+	// ErrHarnessConfig deliberately excludes caller-controlled keys and values.
+	ErrHarnessConfig = errors.New("invalid or unsupported harness_config")
+	// ErrHarnessRequired rejects native parameters without the Harness whose
+	// adapter defines them.
+	ErrHarnessRequired = errors.New("harness_config parameters require a selected harness")
+)
 
 const MaxHarnessConfigBytes = 16 * 1024
 
@@ -44,9 +49,8 @@ func (c Configuration) ParseHarnessConfig(raw proto.HarnessConfig) (map[string]a
 	return result, nil
 }
 
-// ValidateHarnessConfig validates a selected adapter. An empty kind is for saved
-// Agents without a selected Harness: at least one registered adapter must accept
-// the object. Session admission always validates its resolved kind again.
+// ValidateHarnessConfig validates native parameters against the selected
+// adapter. Only an empty object is valid without one.
 func (r Registry) ValidateHarnessConfig(kind string, raw json.RawMessage) error {
 	config, err := decodeHarnessConfig(raw)
 	if err != nil {
@@ -57,20 +61,15 @@ func (r Registry) ValidateHarnessConfig(kind string, raw json.RawMessage) error
 	if len(config) == 0 {
 		return nil
 	}
-	if kind != "" {
-		c, ok := r.Lookup(kind)
-		if !ok {
-			return ErrHarnessConfig
-		}
-		_, err := c.ParseHarnessConfig(raw)
-		return err
+	if kind == "" {
+		return ErrHarnessRequired
 	}
-	for _, c := range r.configurations {
-		if _, err := c.ParseHarnessConfig(raw); err == nil {
-			return nil
-		}
+	c, ok := r.Lookup(kind)
+	if !ok {
+		return ErrHarnessConfig
 	}
-	return ErrHarnessConfig
+	_, err = c.ParseHarnessConfig(raw)
+	return err
 }
 
 // Reject repeated members before storage so values discarded by encoding/json
diff --git a/services/core/internal/agents/configuration.go b/services/core/internal/agents/configuration.go
index 209e98998..d5479a86e 100644
--- a/services/core/internal/agents/configuration.go
+++ b/services/core/internal/agents/configuration.go
@@ -2,9 +2,11 @@ package agents
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 
 	v1 "github.com/MiniMax-AI/OpenAgentCore/contracts/agents-api/v1"
+	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig"
 	"github.com/MiniMax-AI/OpenAgentCore/services/core/internal/jsonobject"
 	"github.com/MiniMax-AI/OpenAgentCore/services/core/internal/metadata"
 )
@@ -55,7 +57,8 @@ func normalizeConfiguration(raw json.RawMessage) (json.RawMessage, error) {
 
 // validateModelExecution checks the saved Harness configuration and that the
 // model provider bundle suits the saved Harness. provider is the bundle being
-// saved, if any.
+// saved, if any. Native parameters without a saved Harness return
+// harnessconfig.ErrHarnessRequired.
 func validateModelExecution(configuration json.RawMessage, provider *v1.ModelProviderInput) error {
 	if provider != nil {
 		if err := provider.Validate(); err != nil {
@@ -69,7 +72,11 @@ func validateModelExecution(configuration json.RawMessage, provider *v1.ModelPro
 		return fmt.Errorf("%w: x_agents_core: %v", ErrInvalidInput, err)
 	}
 	if config.Core != nil {
-		if err := v1.ValidateHarnessConfig(config.Core.Harness, config.Core.HarnessConfig); err != nil {
+		err := v1.ValidateHarnessConfig(config.Core.Harness, config.Core.HarnessConfig)
+		if errors.Is(err, harnessconfig.ErrHarnessRequired) {
+			return err
+		}
+		if err != nil {
 			return fmt.Errorf("%w: %s", ErrInvalidInput, err)
 		}
 	}
diff --git a/services/core/internal/api/errors_agents.go b/services/core/internal/api/errors_agents.go
index 211e6c3fe..6c4607784 100644
--- a/services/core/internal/api/errors_agents.go
+++ b/services/core/internal/api/errors_agents.go
@@ -5,10 +5,15 @@ import (
 	"net/http"
 
 	v1 "github.com/MiniMax-AI/OpenAgentCore/contracts/agents-api/v1"
+	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig"
 	"github.com/MiniMax-AI/OpenAgentCore/internal/obs/log"
 	"github.com/MiniMax-AI/OpenAgentCore/services/core/internal/agents"
 )
 
+// harnessRequiredMessage reports native parameters on an Agent or Session
+// without a selected Harness.
+const harnessRequiredMessage = "harness_config parameters require an explicit x_agents_core.harness."
+
 // writeAgentsError reports an error of the Agent operations.
 func writeAgentsError(w http.ResponseWriter, r *http.Request, err error) {
 	if writeTextValueError(w, r, err) || writeAuditSourceError(w, r, err) || writeCredentialUnavailableError(w, r, err) {
@@ -16,6 +21,8 @@ func writeAgentsError(w http.ResponseWriter, r *http.Request, err error) {
 	}
 	var provider *v1.ModelProviderError
 	switch {
+	case errors.Is(err, harnessconfig.ErrHarnessRequired):
+		writeError(w, http.StatusBadRequest, "invalid_request_error", harnessRequiredMessage, "x_agents_core.harness")
 	case errors.As(err, &provider):
 		writeError(w, http.StatusBadRequest, "unsupported_or_invalid_configuration", provider.Error())
 	case errors.Is(err, agents.ErrNotFound):
diff --git a/services/core/internal/api/session_model_configuration.go b/services/core/internal/api/session_model_configuration.go
index 86304ea40..29006f915 100644
--- a/services/core/internal/api/session_model_configuration.go
+++ b/services/core/internal/api/session_model_configuration.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 
 	v1 "github.com/MiniMax-AI/OpenAgentCore/contracts/agents-api/v1"
+	"github.com/MiniMax-AI/OpenAgentCore/internal/harnessconfig"
 )
 
 // Resolve mutable defaults once, before constructing the immutable Agent. Model
@@ -26,6 +27,22 @@ func (h *Handler) prepareSessionModelConfiguration(ctx context.Context, input *s
 			}
 		}
 	}
+	var inline, session json.RawMessage
+	if input.Agent != nil && input.Agent.XAgentsCore != nil {
+		inline = input.Agent.XAgentsCore.HarnessConfig
+	}
+	if input.XAgentsCore != nil {
+		session = input.XAgentsCore.HarnessConfig
+	}
+	harness := ""
+	if extension != nil {
+		harness = extension.Harness
+	}
+	for _, native := range []json.RawMessage{inline, session} {
+		if errors.Is(v1.ValidateHarnessConfig(harness, native), harnessconfig.ErrHarnessRequired) {
+			return &fieldError{param: "agent.x_agents_core.harness", message: harnessRequiredMessage}
+		}
+	}
 	selected, _ := json.Marshal(configuration{Agent: v1.Agent{XAgentsCore: extension}})
 	engine, err := h.sessionHarness(selected)
 	if err != nil {
@@ -61,12 +78,9 @@ func (h *Handler) prepareSessionModelConfiguration(ctx context.Context, input *s
 	}
 	raw := json.RawMessage(`{}`)
 	source := "unknown"
-	var supplied json.RawMessage
-	if input.Agent != nil && input.Agent.XAgentsCore != nil {
-		supplied = input.Agent.XAgentsCore.HarnessConfig
-	}
-	if input.XAgentsCore != nil && len(input.XAgentsCore.HarnessConfig) > 0 {
-		supplied = input.XAgentsCore.HarnessConfig
+	supplied := inline
+	if len(session) > 0 {
+		supplied = session
 	}
 	if len(supplied) > 0 {
 		raw, source = supplied, "session"
diff --git a/services/core/internal/api/session_model_configuration_test.go b/services/core/internal/api/session_model_configuration_test.go
index 7a7fbc5d7..7d7558886 100644
--- a/services/core/internal/api/session_model_configuration_test.go
+++ b/services/core/internal/api/session_model_configuration_test.go
@@ -24,10 +24,10 @@ func TestSessionNativeConfigurationSources(t *testing.T) {
 		{"model override discards", `{"agent_id":"a","agent":{"model":"other"},"environment":{"type":"openai_hosted"}}`, saved, "other", `{}`, "session", false},
 		{"explicit model discards deployment", `{"agent":{"model":"other"},"environment":{"type":"openai_hosted"}}`, nil, "other", `{}`, "session", false},
 		{"explicit clear", `{"agent_id":"a","environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":{}}}`, saved, "saved-model", `{}`, "session", false},
-		{"inline native", `{"agent":{"model":"other","x_agents_core":{"harness_config":{"model_reasoning_effort":"medium"}}},"environment":{"type":"openai_hosted"}}`, nil, "other", `{"model_reasoning_effort":"medium"}`, "session", false},
-		{"session beats inline", `{"agent":{"model":"other","x_agents_core":{"harness_config":{"model_reasoning_effort":"medium"}}},"environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":{}}}`, nil, "other", `{}`, "session", false},
+		{"inline native", `{"agent":{"model":"other","x_agents_core":{"harness":"codex","harness_config":{"model_reasoning_effort":"medium"}}},"environment":{"type":"openai_hosted"}}`, nil, "other", `{"model_reasoning_effort":"medium"}`, "session", false},
+		{"session beats inline", `{"agent":{"model":"other","x_agents_core":{"harness":"codex","harness_config":{"model_reasoning_effort":"medium"}}},"environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":{}}}`, nil, "other", `{}`, "session", false},
 		{"null rejects", `{"agent":{"model":"other"},"environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":null}}`, nil, "", "", "", true},
-		{"reserved rejects", `{"agent":{"model":"other"},"environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":{"api_key":"secret"}}}`, nil, "", "", "", true},
+		{"reserved rejects", `{"agent":{"model":"other","x_agents_core":{"harness":"codex"}},"environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":{"api_key":"secret"}}}`, nil, "", "", "", true},
 		{"self_hosted deployment", `{"agent":{},"environment":{"type":"self_hosted","workspace_directory":"/tmp/work"}}`, nil, "deployment-model", `{"model_reasoning_effort":"high"}`, "deployment", false},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
diff --git a/services/core/tests/integration/unified_model_configuration_http_test.go b/services/core/tests/integration/unified_model_configuration_http_test.go
index 413b71806..69237eaed 100644
--- a/services/core/tests/integration/unified_model_configuration_http_test.go
+++ b/services/core/tests/integration/unified_model_configuration_http_test.go
@@ -179,6 +179,27 @@ func TestUnifiedModelConfigurationHTTP(t *testing.T) {
 			}
 		})
 	}
+	// Native parameters need the explicit Harness that defines them.
+	t.Run("harness_config needs its harness", func(t *testing.T) {
+		agentError := `{"error":{"message":"harness_config parameters require an explicit x_agents_core.harness.","type":"invalid_request_error","code":"invalid_request_error","param":"x_agents_core.harness"}}`
+		sessionError := strings.Replace(agentError, `"param":"x_agents_core.harness"`, `"param":"agent.x_agents_core.harness"`, 1)
+		harnessless := text(object(call("POST", "/v1/agents", token, `{"model":"saved-model"}`, "", 201))["id"])
+		codex := text(object(call("POST", "/v1/agents", token, `{"model":"saved-model","x_agents_core":{"harness":"codex"}}`, "", 201))["id"])
+		for _, tc := range []struct{ path, body, want string }{
+			{"/v1/agents", `{"model":"saved-model","x_agents_core":{"harness_config":` + high + `}}`, agentError},
+			{"/v1/agents/" + harnessless, `{"x_agents_core":{"harness_config":` + high + `}}`, agentError},
+			{"/v1/agents/sessions", `{"agent":{"x_agents_core":{"harness_config":` + high + `}},"environment":{"type":"openai_hosted"}}`, sessionError},
+			{"/v1/agents/sessions", `{"agent_id":"` + harnessless + `","environment":{"type":"openai_hosted"},"x_agents_core":{"harness_config":` + high + `}}`, sessionError},
+		} {
+			if got := strings.TrimSpace(string(call("POST", tc.path, token, tc.body, uuid.NewString(), 400))); got != tc.want {
+				t.Fatalf("%s: %s", tc.path, got)
+			}
+		}
+		// Native parameters set alone keep the saved Agent's Harness.
+		call("POST", "/v1/agents/"+codex, token, `{"x_agents_core":{"harness_config":`+high+`}}`, "", 200)
+		session := create(`{"agent_id":"`+codex+`","agent":{"x_agents_core":{"harness_config":`+low+`}},"environment":{"type":"openai_hosted"}}`, uuid.NewString())
+		assertSession(session, "saved-model", low, "agent", "session", "deployment", "deployment-canary")
+	})
 	// Disabling tools cannot enable a non-native model protocol.
 	for _, protocol := range []string{"anthropic", "chat_completions"} {
 		t.Run("non-native protocol "+protocol, func(t *testing.T) {