diff --git a/apps/api/src/api/config_profiles.py b/apps/api/src/api/config_profiles.py
index 4fe2c5e..e047c73 100644
--- a/apps/api/src/api/config_profiles.py
+++ b/apps/api/src/api/config_profiles.py
@@ -1,6 +1,8 @@
 """Config profile API endpoints."""
 
 import logging
+import os
+import subprocess
 import uuid
 from typing import Any
 
@@ -21,6 +23,7 @@ from src.services.config_profile_resolver import (
     resolve_profile,
     resolved_profile_to_dict,
 )
+from src.utils.git_url_parser import parse_git_url
 
 logger = logging.getLogger(__name__)
 
@@ -835,3 +838,162 @@ async def resolve_default_profile(
     # Fall back to first created compatible profile
     first = profiles[0]
     return {"profile_id": str(first.id), "profile_name": first.name}
+
+
+class ValidateGitUrlRequest(BaseModel):
+    url: str = Field(description="Git remote URL to validate")
+    ssh_key_id: str | None = Field(default=None, description="Optional SSH key ID for private repos")
+
+
+class ValidateGitUrlResponse(BaseModel):
+    valid: bool
+    suggested_url: str | None = None
+    branches: list[str] | None = None
+    default_branch: str | None = None
+    error: str | None = None
+    error_code: str | None = None
+
+
+@router.post("/validate-git-url", response_model=ValidateGitUrlResponse)
+async def validate_git_url(
+    data: ValidateGitUrlRequest,
+    current_user_id: uuid.UUID = Depends(get_current_user_id),
+    session: AsyncSession = Depends(get_db_session),
+) -> ValidateGitUrlResponse:
+    """Validate a git remote URL and list available branches.
+
+    Parses the URL, suggests corrections for browser URLs, and runs
+    git ls-remote to verify reachability and enumerate branches.
+    """
+    parse_result = parse_git_url(data.url)
+    original_url = data.url.strip()
+    url_to_check = parse_result.get("base_url") or original_url
+
+    if not url_to_check:
+        return ValidateGitUrlResponse(
+            valid=False,
+            error=parse_result.get("message", "Invalid URL"),
+            error_code=parse_result.get("error_code", "INVALID_URL"),
+        )
+
+    # If the URL needed parsing, return suggestion without checking remote
+    if parse_result.get("needs_parsing") and url_to_check != original_url:
+        return ValidateGitUrlResponse(
+            valid=False,
+            suggested_url=url_to_check,
+            error=parse_result.get("message"),
+            error_code=parse_result.get("error_code", "URL_NEEDS_PARSING"),
+        )
+
+    # Optional SSH key for private repos
+    env = None
+    key_path = None
+    if data.ssh_key_id:
+        from src.models.ssh_key import SSHKey
+        from src.services.ssh_keys import _get_fernet
+
+        try:
+            ssh_key_uuid = uuid.UUID(data.ssh_key_id)
+        except ValueError:
+            return ValidateGitUrlResponse(
+                valid=False,
+                error="Invalid SSH key ID format",
+                error_code="INVALID_SSH_KEY",
+            )
+
+        ssh_key = await session.get(SSHKey, ssh_key_uuid)
+        if ssh_key is None or ssh_key.user_id != current_user_id:
+            return ValidateGitUrlResponse(
+                valid=False,
+                error="SSH key not found or not authorized",
+                error_code="SSH_KEY_NOT_FOUND",
+            )
+
+        import tempfile
+
+        fernet = _get_fernet()
+        private_key = fernet.decrypt(ssh_key.private_key_encrypted.encode()).decode()
+        fd, key_path = tempfile.mkstemp(prefix="ssh_key_")
+        try:
+            os.write(fd, private_key.encode())
+        finally:
+            os.close(fd)
+        os.chmod(key_path, 0o600)
+        env = {
+            "GIT_SSH_COMMAND": f"ssh -i {key_path} -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null"
+        }
+
+    try:
+        result = subprocess.run(
+            ["git", "ls-remote", "--heads", url_to_check],
+            capture_output=True,
+            text=True,
+            timeout=30,
+            env={**os.environ, **env} if env else None,
+        )
+    except subprocess.TimeoutExpired:
+        if key_path and os.path.exists(key_path):
+            os.unlink(key_path)
+        return ValidateGitUrlResponse(
+            valid=False,
+            error="Remote repository check timed out",
+            error_code="TIMEOUT",
+        )
+    except FileNotFoundError:
+        if key_path and os.path.exists(key_path):
+            os.unlink(key_path)
+        return ValidateGitUrlResponse(
+            valid=False,
+            error="git command not found on server",
+            error_code="GIT_NOT_FOUND",
+        )
+    finally:
+        if key_path and os.path.exists(key_path):
+            os.unlink(key_path)
+
+    if result.returncode != 0:
+        stderr = result.stderr.strip()
+        if "could not resolve" in stderr.lower() or "unable to access" in stderr.lower():
+            error_msg = "Could not reach repository. Check the URL and network access."
+            error_code = "UNREACHABLE"
+        elif "authentication" in stderr.lower() or "permission denied" in stderr.lower():
+            error_msg = "Authentication failed. Provide an SSH key for private repositories."
+            error_code = "AUTH_FAILED"
+        else:
+            error_msg = f"Repository not accessible: {stderr[:200]}"
+            error_code = "REMOTE_ERROR"
+        return ValidateGitUrlResponse(
+            valid=False,
+            error=error_msg,
+            error_code=error_code,
+        )
+
+    # Parse branches from ls-remote output
+    branches: list[str] = []
+    default_branch = "main"
+    for line in result.stdout.strip().split("\n"):
+        if not line.strip():
+            continue
+        parts = line.split()
+        if len(parts) == 2:
+            ref = parts[1]
+            # refs/heads/branch-name
+            if ref.startswith("refs/heads/"):
+                branch_name = ref[len("refs/heads/"):]
+                branches.append(branch_name)
+                if branch_name in ("main", "master"):
+                    default_branch = branch_name
+
+    if not branches:
+        return ValidateGitUrlResponse(
+            valid=False,
+            error="No branches found in remote repository",
+            error_code="NO_BRANCHES",
+        )
+
+    return ValidateGitUrlResponse(
+        valid=True,
+        suggested_url=url_to_check if url_to_check != original_url else None,
+        branches=branches,
+        default_branch=default_branch,
+    )
diff --git a/apps/api/tests/unit/test_config_profile_resolver.py b/apps/api/tests/unit/test_config_profile_resolver.py
index 1c6c11e..8c5a0d8 100644
--- a/apps/api/tests/unit/test_config_profile_resolver.py
+++ b/apps/api/tests/unit/test_config_profile_resolver.py
@@ -494,7 +494,10 @@ class TestApplyResolvedProfile:
                 "/app": ResolvedMount(
                     target="/app",
                     mode="rw",
-                    files={"config.json": '{"key": "value"}', "nested/file.txt": "hello"},
+                    files={
+                        "config.json": '{"key": "value"}',
+                        "nested/file.txt": "hello",
+                    },
                 )
             },
         )
@@ -531,9 +534,7 @@ class TestApplyResolvedProfile:
         resolved = ResolvedProfile(
             profile_id=uuid.uuid4(),
             profile_name="test",
-            mounts={
-                "/app": ResolvedMount(target="/app", mode="rw", files={})
-            },
+            mounts={"/app": ResolvedMount(target="/app", mode="rw", files={})},
         )
         env, files, volumes, hints = apply_resolved_profile(str(tmp_path), resolved)
         assert volumes == []
diff --git a/apps/web/src/api/config_profiles.ts b/apps/web/src/api/config_profiles.ts
index 048d70c..c8959b2 100644
--- a/apps/web/src/api/config_profiles.ts
+++ b/apps/web/src/api/config_profiles.ts
@@ -167,3 +167,23 @@ export const resolveDefaultProfile = async (
 	});
 	return response.data;
 };
+
+export interface ValidateGitUrlResponse {
+	valid: boolean;
+	suggested_url?: string;
+	branches?: string[];
+	default_branch?: string;
+	error?: string;
+	error_code?: string;
+}
+
+export const validateGitUrl = async (
+	url: string,
+	sshKeyId?: string,
+): Promise<ValidateGitUrlResponse> => {
+	const response = await apiClient.post<ValidateGitUrlResponse>(
+		"/config-profiles/validate-git-url",
+		{ url, ssh_key_id: sshKeyId },
+	);
+	return response.data;
+};
diff --git a/apps/web/src/components/git-mount-editor.tsx b/apps/web/src/components/git-mount-editor.tsx
index 573b5db..85d67ff 100644
--- a/apps/web/src/components/git-mount-editor.tsx
+++ b/apps/web/src/components/git-mount-editor.tsx
@@ -1,5 +1,6 @@
 import { useState, useEffect } from "react";
 import { Icon } from "./icon";
+import { validateGitUrl } from "../api/config_profiles";
 import type { GitMount, GitMountMapping } from "../api/config_profiles";
 
 interface GitMountEditorProps {
@@ -204,6 +205,13 @@ interface GitMountFormProps {
 	onCancel: () => void;
 }
 
+type ValidationState =
+	| { status: "idle" }
+	| { status: "loading" }
+	| { status: "valid"; branches: string[]; defaultBranch: string }
+	| { status: "suggestion"; suggestedUrl: string; message: string }
+	| { status: "invalid"; message: string };
+
 const GitMountForm = ({ mount, onSave, onCancel }: GitMountFormProps) => {
 	const [remoteUrl, setRemoteUrl] = useState(mount.remote_url);
 	const [branch, setBranch] = useState(mount.branch || "");
@@ -213,6 +221,63 @@ const GitMountForm = ({ mount, onSave, onCancel }: GitMountFormProps) => {
 			: [{ source_path: ".", target_path: "" }],
 	);
 	const [errors, setErrors] = useState<Record<string, string>>({});
+	const [validation, setValidation] = useState<ValidationState>({ status: "idle" });
+
+	const isUrlValidated =
+		validation.status === "valid" ||
+		(validation.status === "idle" && mount.remote_url.length > 0);
+
+	const handleCheckUrl = async () => {
+		if (!remoteUrl.trim()) {
+			setErrors((prev) => ({ ...prev, remote_url: "Git URL is required" }));
+			return;
+		}
+		setValidation({ status: "loading" });
+		setErrors((prev) => {
+			const next = { ...prev };
+			delete next.remote_url;
+			return next;
+		});
+		try {
+			const result = await validateGitUrl(remoteUrl.trim());
+			if (result.valid && result.branches) {
+				setValidation({
+					status: "valid",
+					branches: result.branches,
+					defaultBranch: result.default_branch || "main",
+				});
+				if (!branch) {
+					setBranch(result.default_branch || "main");
+				}
+				if (result.suggested_url && result.suggested_url !== remoteUrl.trim()) {
+					setRemoteUrl(result.suggested_url);
+				}
+			} else if (result.suggested_url) {
+				setValidation({
+					status: "suggestion",
+					suggestedUrl: result.suggested_url,
+					message: result.error || "URL needs correction",
+				});
+			} else {
+				setValidation({
+					status: "invalid",
+					message: result.error || "Invalid repository URL",
+				});
+			}
+		} catch {
+			setValidation({
+				status: "invalid",
+				message: "Failed to validate URL. Please try again.",
+			});
+		}
+	};
+
+	const applySuggestion = () => {
+		if (validation.status === "suggestion") {
+			setRemoteUrl(validation.suggestedUrl);
+			setValidation({ status: "idle" });
+		}
+	};
 
 	const validate = (): boolean => {
 		const newErrors: Record<string, string> = {};
@@ -286,46 +351,106 @@ const GitMountForm = ({ mount, onSave, onCancel }: GitMountFormProps) => {
 
 	return (
 		<div style={{ display: "flex", flexDirection: "column", gap: "0.75rem" }}>
-			<div className="form-row" style={{ gap: "0.5rem" }}>
+			<div className="form-row" style={{ gap: "0.5rem", alignItems: "flex-start" }}>
 				<div style={{ flex: 2 }}>
 					<label style={{ fontSize: "0.875rem", fontWeight: 500 }}>
 						Repository URL
 					</label>
-					<input
-						type="text"
-						value={remoteUrl}
-						onChange={(e) => {
-							setRemoteUrl(e.target.value);
-							if (errors.remote_url) {
-								setErrors((prev) => {
-									const next = { ...prev };
-									delete next.remote_url;
-									return next;
-								});
-							}
-						}}
-						placeholder="https://github.com/user/repo.git"
-						className={`form-input ${errors.remote_url ? "error" : ""}`}
-					/>
+					<div style={{ display: "flex", gap: "0.5rem" }}>
+						<input
+							type="text"
+							value={remoteUrl}
+							onChange={(e) => {
+								setRemoteUrl(e.target.value);
+								setValidation({ status: "idle" });
+								if (errors.remote_url) {
+									setErrors((prev) => {
+										const next = { ...prev };
+										delete next.remote_url;
+										return next;
+									});
+								}
+							}}
+							placeholder="https://github.com/user/repo.git"
+							className={`form-input ${errors.remote_url ? "error" : ""}`}
+							style={{ flex: 1 }}
+						/>
+						<button
+							type="button"
+							className="secondary-button small"
+							onClick={handleCheckUrl}
+							disabled={validation.status === "loading"}
+						>
+							{validation.status === "loading" ? (
+								<Icon name="loading" size="sm" />
+							) : (
+								"Check"
+							)}
+						</button>
+					</div>
 					{errors.remote_url && (
 						<span className="error-text">{errors.remote_url}</span>
 					)}
+					{validation.status === "valid" && (
+						<span className="validation-status valid">
+							Repository is accessible ({(validation as Extract<ValidationState, { status: "valid" }>).branches.length} branches)
+						</span>
+					)}
+					{validation.status === "suggestion" && (
+						<div className="url-suggestion">
+							<span>{validation.message}</span>
+							<div className="suggestion-actions">
+								<code className="suggested-url">
+									{validation.suggestedUrl}
+								</code>
+								<button
+									type="button"
+									className="secondary-button small"
+									onClick={applySuggestion}
+								>
+									Use this
+								</button>
+							</div>
+						</div>
+					)}
+					{validation.status === "invalid" && (
+						<span className="validation-status invalid">
+							{validation.message}
+						</span>
+					)}
 				</div>
 				<div style={{ flex: 1 }}>
 					<label style={{ fontSize: "0.875rem", fontWeight: 500 }}>
-						Branch (optional)
+						Branch
 					</label>
-					<input
-						type="text"
-						value={branch}
-						onChange={(e) => setBranch(e.target.value)}
-						placeholder="main"
-						className="form-input"
-					/>
+					{validation.status === "valid" ? (
+						<select
+							value={branch}
+							onChange={(e) => setBranch(e.target.value)}
+							className="form-input"
+						>
+							{(validation as Extract<ValidationState, { status: "valid" }>).branches.map(
+								(b) => (
+									<option key={b} value={b}>
+										{b}
+									</option>
+								),
+							)}
+						</select>
+					) : (
+						<input
+							type="text"
+							value={branch}
+							onChange={(e) => setBranch(e.target.value)}
+							placeholder="main"
+							className="form-input"
+							disabled={!isUrlValidated}
+						/>
+					)}
 				</div>
 			</div>
 
-			<div>
+			<div style={{ opacity: isUrlValidated ? 1 : 0.5, pointerEvents: isUrlValidated ? "auto" : "none" }}>
 				<label style={{ fontSize: "0.875rem", fontWeight: 500 }}>
 					Mappings
 				</label>
@@ -334,6 +459,11 @@ const GitMountForm = ({ mount, onSave, onCancel }: GitMountFormProps) => {
 					style={{ margin: "0 0 0.5rem 0", fontSize: "0.8125rem" }}
 				>
 					Source paths within the repo and where to mount them in the container.
+					{!isUrlValidated && (
+						<span style={{ color: "var(--warning)" }}>
+							{" "}Validate the URL first.
+						</span>
+					)}
 				</p>
 				<div
 					style={{ display: "flex", flexDirection: "column", gap: "0.5rem" }}
diff --git a/openspec/changes/git-mount-url-validation/.openspec.yaml b/openspec/changes/git-mount-url-validation/.openspec.yaml
new file mode 100644
index 0000000..abc0af6
--- /dev/null
+++ b/openspec/changes/git-mount-url-validation/.openspec.yaml
@@ -0,0 +1,4 @@
+name: git-mount-url-validation
+status: completed
+type: feat
+priority: high