diff --git a/c/cagent.c b/c/cagent.c
index c5cc86c..bc1b141 100644
--- a/c/cagent.c
+++ b/c/cagent.c
@@ -58,7 +58,7 @@ static void image_paste_wrapper(char **out, size_t *outlen) {
 int main(int argc, char *argv[]) {
     srand((unsigned int)time(NULL) ^ (unsigned int)getpid());
     /* 默认值 */
-    const char *provider = util_env("BASH_AGENT_PROVIDER", "claude");
+    const char *provider = "claude";
     const char *model = NULL;
     const char *api_key = NULL;
     const char *base_url = NULL;
@@ -169,44 +169,42 @@ int main(int argc, char *argv[]) {
     char *effective_model = NULL;
     char *effective_base_url = NULL;
 
+    /* 先解析当前 provider 的显式参数和环境变量；DeepSeek 回退后才设置模型与传输配置。 */
     if (strcmp(provider, "claude") == 0) {
-        effective_api_key = util_strdup(api_key ? api_key : util_env("ANTHROPIC_API_KEY", ""));
-        effective_base_url = util_strdup(base_url ? base_url : util_env("ANTHROPIC_BASE_URL", ""));
-        effective_model = util_strdup(model ? model : (getenv("MODEL") && getenv("MODEL")[0] ? getenv("MODEL") : "claude-sonnet-4-20250514"));
-
-        /* DeepSeek 自动检测 */
-        if (!effective_api_key[0] && !effective_base_url[0]) {
-            const char *ds_key = getenv("DEEPSEEK_API_KEY");
-            if (ds_key && ds_key[0]) {
-                free(effective_api_key);
-                free(effective_base_url);
-                effective_api_key = util_strdup(ds_key);
-                effective_base_url = util_strdup("https://api.deepseek.com/anthropic");
-                if (!model) {
-                    free(effective_model);
-                    effective_model = util_strdup("deepseek-v4-flash");
-                }
-            }
-        }
-
-        if (!effective_api_key[0] && !effective_base_url[0]) {
-            fprintf(stderr, "Error: no API key. Set ANTHROPIC_API_KEY or use --api-key\n");
-            return 1;
-        }
+        effective_api_key = util_strdup(api_key && api_key[0] ? api_key : util_env("ANTHROPIC_API_KEY", ""));
+        effective_base_url = util_strdup(base_url && base_url[0] ? base_url : util_env("ANTHROPIC_BASE_URL", ""));
     } else if (strcmp(provider, "openai") == 0) {
-        effective_api_key = util_strdup(api_key ? api_key : util_env("OPENAI_API_KEY", ""));
-        effective_base_url = util_strdup(base_url ? base_url : util_env("OPENAI_BASE_URL", ""));
-        effective_model = util_strdup(model ? model : (getenv("MODEL") && getenv("MODEL")[0] ? getenv("MODEL") : "gpt-4o"));
-
-        if (!effective_api_key[0] && !effective_base_url[0]) {
-            fprintf(stderr, "Error: no API key. Set OPENAI_API_KEY or use --api-key\n");
-            return 1;
-        }
+        effective_api_key = util_strdup(api_key && api_key[0] ? api_key : util_env("OPENAI_API_KEY", ""));
+        effective_base_url = util_strdup(base_url && base_url[0] ? base_url : util_env("OPENAI_BASE_URL", ""));
     } else {
         fprintf(stderr, "Error: unknown provider '%s' (use claude|openai)\n", provider);
         return 1;
     }
 
+    /* 仅当前 provider 没有显式配置时，自动回退至 DeepSeek。 */
+    if (!effective_api_key[0] && !effective_base_url[0]) {
+        const char *ds_key = getenv("DEEPSEEK_API_KEY");
+        if (ds_key && ds_key[0]) {
+            provider = "claude";
+            free(effective_api_key);
+            free(effective_base_url);
+            effective_api_key = util_strdup(ds_key);
+            effective_base_url = util_strdup("https://api.deepseek.com/anthropic");
+            if (!model || !model[0]) effective_model = util_strdup("deepseek-v4-flash");
+        }
+    }
+
+    if (!effective_api_key[0] && !effective_base_url[0]) {
+        fprintf(stderr, "Error: no API key. Set %s_API_KEY or use --api-key\n",
+                strcmp(provider, "openai") == 0 ? "OPENAI" : "ANTHROPIC");
+        return 1;
+    }
+
+    if (!effective_model) {
+        effective_model = util_strdup(model && model[0] ? model :
+            (strcmp(provider, "openai") == 0 ? "gpt-4o" : "claude-sonnet-4-20250514"));
+    }
+
     /* 继续 session */
     char *effective_session = util_strdup(session_id);
     if ((do_continue || do_fork) && (!effective_session || !effective_session[0])) {
diff --git a/go/cmd/goagent/main.go b/go/cmd/goagent/main.go
index e492e15..a8e3329 100644
--- a/go/cmd/goagent/main.go
+++ b/go/cmd/goagent/main.go
@@ -127,29 +127,15 @@ Examples:
 		cfg.Model = model
 	}
 
-	// Provider 默认值
+	// 先解析当前 provider 的显式参数和环境变量；DeepSeek 回退后才设置模型与传输配置。
 	switch cfg.Provider {
 	case "claude":
 		if cfg.APIKey == "" {
 			cfg.APIKey = os.Getenv("ANTHROPIC_API_KEY")
 		}
-		// DeepSeek auto-detection: DEEPSEEK_API_KEY 且未显式配置其他 base URL
-		if cfg.APIKey == "" && cfg.BaseURL == "" &&
-			os.Getenv("DEEPSEEK_API_KEY") != "" &&
-			os.Getenv("ANTHROPIC_BASE_URL") == "" &&
-			os.Getenv("OPENAI_BASE_URL") == "" {
-			cfg.APIKey = os.Getenv("DEEPSEEK_API_KEY")
-			cfg.BaseURL = "https://api.deepseek.com/anthropic"
-			if cfg.Model == "" {
-				cfg.Model = "deepseek-v4-flash"
-			}
-		}
 		if cfg.BaseURL == "" {
 			cfg.BaseURL = os.Getenv("ANTHROPIC_BASE_URL")
 		}
-		if cfg.Model == "" {
-			cfg.Model = "claude-sonnet-4-20250514"
-		}
 	case "openai":
 		if cfg.APIKey == "" {
 			cfg.APIKey = os.Getenv("OPENAI_API_KEY")
@@ -157,8 +143,26 @@ Examples:
 		if cfg.BaseURL == "" {
 			cfg.BaseURL = os.Getenv("OPENAI_BASE_URL")
 		}
+	default:
+		fmt.Fprintf(os.Stderr, "Unknown provider: %s (use claude|openai)\n", cfg.Provider)
+		os.Exit(1)
+	}
+
+	// 仅当前 provider 没有显式配置时，自动回退至 DeepSeek。
+	if cfg.APIKey == "" && cfg.BaseURL == "" && os.Getenv("DEEPSEEK_API_KEY") != "" {
+		cfg.Provider = "claude"
+		cfg.APIKey = os.Getenv("DEEPSEEK_API_KEY")
+		cfg.BaseURL = "https://api.deepseek.com/anthropic"
 		if cfg.Model == "" {
+			cfg.Model = "deepseek-v4-flash"
+		}
+	}
+
+	if cfg.Model == "" {
+		if cfg.Provider == "openai" {
 			cfg.Model = "gpt-4o"
+		} else {
+			cfg.Model = "claude-sonnet-4-20250514"
 		}
 	}
 
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index b75435c..657547f 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -198,35 +198,15 @@ pub mod config {
             }
         }
 
+        // 先解析当前 provider 的显式参数和环境变量；DeepSeek 回退后才设置模型与传输配置。
         match cfg.provider.as_str() {
             "claude" => {
                 if cfg.api_key.is_empty() {
                     cfg.api_key = std::env::var("ANTHROPIC_API_KEY").unwrap_or_default();
                 }
-                // DeepSeek auto-detection
-                if cfg.api_key.is_empty()
-                    && cfg.base_url.is_empty()
-                    && std::env::var("DEEPSEEK_API_KEY")
-                        .map(|v| !v.is_empty())
-                        .unwrap_or(false)
-                    && std::env::var("ANTHROPIC_BASE_URL").is_err()
-                    && std::env::var("OPENAI_BASE_URL").is_err()
-                {
-                    cfg.api_key = std::env::var("DEEPSEEK_API_KEY").unwrap();
-                    cfg.base_url = "https://api.deepseek.com/anthropic".to_string();
-                    if cfg.model.is_empty() {
-                        cfg.model = "deepseek-v4-flash".to_string();
-                    }
-                }
                 if cfg.base_url.is_empty() {
                     cfg.base_url = std::env::var("ANTHROPIC_BASE_URL").unwrap_or_default();
                 }
-                if cfg.model.is_empty() {
-                    cfg.model = "claude-sonnet-4-20250514".to_string();
-                }
-                if cfg.api_key.is_empty() && cfg.base_url.is_empty() {
-                    bail!("no API key. Set ANTHROPIC_API_KEY or use --api-key");
-                }
             }
             "openai" => {
                 if cfg.api_key.is_empty() {
@@ -235,15 +215,39 @@ pub mod config {
                 if cfg.base_url.is_empty() {
                     cfg.base_url = std::env::var("OPENAI_BASE_URL").unwrap_or_default();
                 }
-                if cfg.model.is_empty() {
-                    cfg.model = "gpt-4o".to_string();
-                }
-                if cfg.api_key.is_empty() && cfg.base_url.is_empty() {
-                    bail!("no API key. Set OPENAI_API_KEY or use --api-key");
-                }
             }
             _ => bail!("unknown provider: {} (use claude|openai)", cfg.provider),
         }
+
+        // 仅当前 provider 没有显式配置时，自动回退至 DeepSeek。
+        if cfg.api_key.is_empty() && cfg.base_url.is_empty() {
+            if let Ok(ds_key) = std::env::var("DEEPSEEK_API_KEY") {
+                if !ds_key.is_empty() {
+                    cfg.provider = "claude".to_string();
+                    cfg.api_key = ds_key;
+                    cfg.base_url = "https://api.deepseek.com/anthropic".to_string();
+                    if cfg.model.is_empty() {
+                        cfg.model = "deepseek-v4-flash".to_string();
+                    }
+                }
+            }
+        }
+
+        if cfg.api_key.is_empty() && cfg.base_url.is_empty() {
+            match cfg.provider.as_str() {
+                "claude" => bail!("no API key. Set ANTHROPIC_API_KEY or use --api-key"),
+                "openai" => bail!("no API key. Set OPENAI_API_KEY or use --api-key"),
+                _ => unreachable!(),
+            }
+        }
+
+        if cfg.model.is_empty() {
+            cfg.model = match cfg.provider.as_str() {
+                "claude" => "claude-sonnet-4-20250514",
+                "openai" => "gpt-4o",
+                _ => unreachable!(),
+            }.to_string();
+        }
         Ok(())
     }
 
@@ -255,7 +259,7 @@ pub mod config {
                 } else {
                     cfg.base_url.as_str()
                 };
-                format!("{}/messages", base.trim_end_matches('/'))
+                format!("{}/messages", base)
             }
             "openai" => {
                 let base = if cfg.base_url.is_empty() {
@@ -263,7 +267,7 @@ pub mod config {
                 } else {
                     cfg.base_url.as_str()
                 };
-                format!("{}/chat/completions", base.trim_end_matches('/'))
+                format!("{}/chat/completions", base)
             }
             _ => String::new(),
         }
diff --git a/src/agent.sh b/src/agent.sh
index 2f72487..cd11a3e 100755
--- a/src/agent.sh
+++ b/src/agent.sh
@@ -418,12 +418,20 @@ store_session_get_dir() {
 }
 
 store_session_get_latest_dir() {
-    local project_dir latest="" latest_ts=0 dir ts
+    local project_dir latest="" latest_ts=0 dir ts stat_cmd
     project_dir="$(store_session_get_dir)"
     [[ -d "$project_dir" ]] || return 1
+    if stat -c "%Y" "$project_dir" &>/dev/null; then
+        stat_cmd=(stat -c "%Y")
+    else
+        stat_cmd=(stat -f "%m")
+    fi
     for dir in "$project_dir"/*/; do
         [[ -d "$dir" ]] || continue
-        ts=$(stat -f "%m" "$dir/events.jsonl" 2>/dev/null || stat -c "%Y" "$dir/events.jsonl" 2>/dev/null || stat -f "%m" "$dir" 2>/dev/null || stat -c "%Y" "$dir" 2>/dev/null || echo 0)
+        local target="$dir"
+        [[ -f "$dir/events.jsonl" ]] && target="$dir/events.jsonl"
+        ts=$("${stat_cmd[@]}" "$target" 2>/dev/null || echo 0)
+        [[ "$ts" =~ ^[0-9]+$ ]] || ts=0
         (( ts > latest_ts )) && { latest_ts=$ts; latest="$dir"; }
     done
     [[ -n "$latest" ]] && basename "$latest" || return 1
@@ -1626,10 +1634,41 @@ list_sessions() {
 }
 
 validate_config() {
+    # Resolve the selected provider's environment configuration first. Model defaults and
+    # transport settings are applied only after DeepSeek fallback selects the final provider.
     case "$PROVIDER" in
         claude)
-            : "${API_KEY:=$ANTHROPIC_API_KEY}"
+            : "${API_KEY:=${ANTHROPIC_API_KEY:-}}"
             : "${BASE_URL:=${ANTHROPIC_BASE_URL:-}}"
+            ;;
+        openai)
+            : "${API_KEY:=${OPENAI_API_KEY:-}}"
+            : "${BASE_URL:=${OPENAI_BASE_URL:-}}"
+            ;;
+        *)
+            util_die "Unknown provider: $PROVIDER (use claude|openai)"
+            ;;
+    esac
+
+    # Auto-detect DeepSeek only when the selected provider has no explicit configuration.
+    # MODEL is still empty here unless the user supplied -m/--model.
+    if [[ -z "$API_KEY" && -z "$BASE_URL" && -n "${DEEPSEEK_API_KEY:-}" ]]; then
+        PROVIDER="claude"
+        API_KEY="$DEEPSEEK_API_KEY"
+        BASE_URL="https://api.deepseek.com/anthropic"
+        : "${MODEL:=deepseek-v4-flash}"
+    fi
+
+    if [[ -z "$API_KEY" && -z "$BASE_URL" ]]; then
+        case "$PROVIDER" in
+            claude) util_die "No API key. Set ANTHROPIC_API_KEY or use --api-key" ;;
+            openai) util_die "No API key. Set OPENAI_API_KEY or use --api-key" ;;
+        esac
+    fi
+
+    # Build the final transport only after provider fallback has completed.
+    case "$PROVIDER" in
+        claude)
             : "${MODEL:=claude-sonnet-4-20250514}"
             API_URL="${BASE_URL:-https://api.anthropic.com/v1}/messages"
             HEADER_ARGS=(
@@ -1643,8 +1682,6 @@ validate_config() {
             sse_convert()  { cat; }
             ;;
         openai)
-            : "${API_KEY:=$OPENAI_API_KEY}"
-            : "${BASE_URL:=${OPENAI_BASE_URL:-}}"
             : "${MODEL:=gpt-4o}"
             API_URL="${BASE_URL:-https://api.openai.com/v1}/chat/completions"
             HEADER_ARGS=(
@@ -1655,26 +1692,8 @@ validate_config() {
             util_body_convert() { util_awk_run -f "$AWK_DIR/json.awk" -f "$AWK_DIR/transport_openai_body.awk"; }
             sse_convert()  { util_awk_run -f "$AWK_DIR/json.awk" -f "$AWK_DIR/transport_openai_sse.awk"; }
             ;;
-        *)
-            util_die "Unknown provider: $PROVIDER (use claude|openai)"
-            ;;
     esac
 
-    # Auto-detect DeepSeek API key: if DEEPSEEK_API_KEY is set, use DeepSeek's Anthropic endpoint
-    if [[ -z "$API_KEY" && -z "$BASE_URL" && -n "${DEEPSEEK_API_KEY:-}" ]]; then
-        PROVIDER="claude"
-        API_KEY="$DEEPSEEK_API_KEY"
-        BASE_URL="https://api.deepseek.com/anthropic"
-        : "${MODEL:=deepseek-v4-flash}"
-    fi
-
-    if [[ -z "$API_KEY" && -z "$BASE_URL" ]]; then
-        case "$PROVIDER" in
-            claude) util_die "No API key. Set ANTHROPIC_API_KEY or use --api-key" ;;
-            openai) util_die "No API key. Set OPENAI_API_KEY or use --api-key" ;;
-        esac
-    fi
-
     # Unified SSE parser — same for all providers (sse_convert normalizes to Claude format)
     sse_parse() { util_awk_run -v verbose="${VERBOSE:-false}" -f "$AWK_DIR/json.awk" -f "$AWK_DIR/protocol.awk" -f "$AWK_DIR/todo_protocol.awk" -f "$AWK_DIR/claude_sse.awk"; }
 }