diff --git a/internal/runtime/executor/claude_device_profile.go b/internal/runtime/executor/claude_device_profile.go
index aa81eb94..374720b8 100644
--- a/internal/runtime/executor/claude_device_profile.go
+++ b/internal/runtime/executor/claude_device_profile.go
@@ -70,6 +70,7 @@ type claudeDeviceProfile struct {
 	OS             string
 	Arch           string
 	Version        claudeCLIVersion
+	HasVersion     bool
 }
 
 type claudeDeviceProfileCacheEntry struct {
@@ -106,6 +107,7 @@ func defaultClaudeDeviceProfile(cfg *config.Config) claudeDeviceProfile {
 	}
 	if version, ok := parseClaudeCLIVersion(profile.UserAgent); ok {
 		profile.Version = version
+		profile.HasVersion = true
 	}
 	return profile
 }
@@ -161,15 +163,12 @@ func parseClaudeCLIVersion(userAgent string) (claudeCLIVersion, bool) {
 }
 
 func shouldUpgradeClaudeDeviceProfile(candidate, current claudeDeviceProfile) bool {
-	if candidate.UserAgent == "" {
+	if candidate.UserAgent == "" || !candidate.HasVersion {
 		return false
 	}
-	if current.UserAgent == "" {
+	if current.UserAgent == "" || !current.HasVersion {
 		return true
 	}
-	if current.Version == (claudeCLIVersion{}) {
-		return false
-	}
 	return candidate.Version.Compare(current.Version) > 0
 }
 
@@ -183,11 +182,12 @@ func pinClaudeDeviceProfilePlatform(profile, baseline claudeDeviceProfile) claud
 // baseline platform and enforces the baseline software fingerprint as a floor.
 func normalizeClaudeDeviceProfile(profile, baseline claudeDeviceProfile) claudeDeviceProfile {
 	profile = pinClaudeDeviceProfilePlatform(profile, baseline)
-	if profile.UserAgent == "" || profile.Version == (claudeCLIVersion{}) || shouldUpgradeClaudeDeviceProfile(baseline, profile) {
+	if profile.UserAgent == "" || !profile.HasVersion || shouldUpgradeClaudeDeviceProfile(baseline, profile) {
 		profile.UserAgent = baseline.UserAgent
 		profile.PackageVersion = baseline.PackageVersion
 		profile.RuntimeVersion = baseline.RuntimeVersion
 		profile.Version = baseline.Version
+		profile.HasVersion = baseline.HasVersion
 	}
 	return profile
 }
@@ -211,6 +211,7 @@ func extractClaudeDeviceProfile(headers http.Header, cfg *config.Config) (claude
 		OS:             firstNonEmptyHeader(headers, "X-Stainless-Os", baseline.OS),
 		Arch:           firstNonEmptyHeader(headers, "X-Stainless-Arch", baseline.Arch),
 		Version:        version,
+		HasVersion:     true,
 	}
 	return profile, true
 }
diff --git a/internal/runtime/executor/claude_executor_test.go b/internal/runtime/executor/claude_executor_test.go
index 91242802..c163d7ea 100644
--- a/internal/runtime/executor/claude_executor_test.go
+++ b/internal/runtime/executor/claude_executor_test.go
@@ -260,6 +260,58 @@ func TestApplyClaudeHeaders_UpgradesCachedSoftwareFingerprintWhenBaselineAdvance
 	assertClaudeFingerprint(t, thirdPartyReq.Header, "claude-cli/2.1.77 (external, cli)", "0.87.0", "v24.8.0", "MacOS", "arm64")
 }
 
+func TestApplyClaudeHeaders_LearnsOfficialFingerprintAfterCustomBaselineFallback(t *testing.T) {
+	resetClaudeDeviceProfileCache()
+	stabilize := true
+
+	cfg := &config.Config{
+		ClaudeHeaderDefaults: config.ClaudeHeaderDefaults{
+			UserAgent:              "my-gateway/1.0",
+			PackageVersion:         "custom-pkg",
+			RuntimeVersion:         "custom-runtime",
+			OS:                     "MacOS",
+			Arch:                   "arm64",
+			StabilizeDeviceProfile: &stabilize,
+		},
+	}
+	auth := &cliproxyauth.Auth{
+		ID: "auth-custom-baseline-learning",
+		Attributes: map[string]string{
+			"api_key": "key-custom-baseline-learning",
+		},
+	}
+
+	thirdPartyReq := newClaudeHeaderTestRequest(t, http.Header{
+		"User-Agent":                  []string{"curl/8.7.1"},
+		"X-Stainless-Package-Version": []string{"0.10.0"},
+		"X-Stainless-Runtime-Version": []string{"v18.0.0"},
+		"X-Stainless-Os":              []string{"Linux"},
+		"X-Stainless-Arch":            []string{"x64"},
+	})
+	applyClaudeHeaders(thirdPartyReq, auth, "key-custom-baseline-learning", false, nil, cfg)
+	assertClaudeFingerprint(t, thirdPartyReq.Header, "my-gateway/1.0", "custom-pkg", "custom-runtime", "MacOS", "arm64")
+
+	officialReq := newClaudeHeaderTestRequest(t, http.Header{
+		"User-Agent":                  []string{"claude-cli/2.1.77 (external, cli)"},
+		"X-Stainless-Package-Version": []string{"0.87.0"},
+		"X-Stainless-Runtime-Version": []string{"v24.8.0"},
+		"X-Stainless-Os":              []string{"Linux"},
+		"X-Stainless-Arch":            []string{"x64"},
+	})
+	applyClaudeHeaders(officialReq, auth, "key-custom-baseline-learning", false, nil, cfg)
+	assertClaudeFingerprint(t, officialReq.Header, "claude-cli/2.1.77 (external, cli)", "0.87.0", "v24.8.0", "MacOS", "arm64")
+
+	postLearningThirdPartyReq := newClaudeHeaderTestRequest(t, http.Header{
+		"User-Agent":                  []string{"curl/8.7.1"},
+		"X-Stainless-Package-Version": []string{"0.10.0"},
+		"X-Stainless-Runtime-Version": []string{"v18.0.0"},
+		"X-Stainless-Os":              []string{"Linux"},
+		"X-Stainless-Arch":            []string{"x64"},
+	})
+	applyClaudeHeaders(postLearningThirdPartyReq, auth, "key-custom-baseline-learning", false, nil, cfg)
+	assertClaudeFingerprint(t, postLearningThirdPartyReq.Header, "claude-cli/2.1.77 (external, cli)", "0.87.0", "v24.8.0", "MacOS", "arm64")
+}
+
 func TestResolveClaudeDeviceProfile_RechecksCacheBeforeStoringCandidate(t *testing.T) {
 	resetClaudeDeviceProfileCache()
 	stabilize := true