Software composition analysis (SCA) tools report far more vulnerable dependencies than teams fix: in Snyk Open Source production data, 94% of vulnerability findings have a known fixed version, yet only about 10% are resolved. Agentic coding tools could close this gap, but no benchmark evaluates them on vulnerability-driven upgrades in real, runnable projects across ecosystems, verified independently of the agent. We present VulnBump: 307 upgrade tasks covering 2,066 fixable vulnerabilities in 43 repositories across six package managers, matched to Snyk Open Source production vulnerability distributions. An upgrade passes only if, in a fresh container, dependencies resolve, a rescan shows the targeted vulnerabilities fixed, the project builds, and every previously passing test still passes. A non-agentic baseline that only applies Snyk's recommended version passes 46.9% of upgrades; with the same model (Claude Sonnet 5.5), Snyk's Evo Remediation Agent passes 90.2%, Claude Code + Snyk scan 77.5% and Claude Code unaided 59.6%. The agents fail in opposite ways: Claude Code mostly leaves the vulnerability in place, whereas most of Evo Remediation Agent's failures break the build or tests. An SCA scanner is a key tool for agents: Snyk's guidance raises Claude Code's PASS rate by 17.9 percentage points; without it, Claude Code often settles on a still-vulnerable version. Yet knowing the target is not enough: given the same Snyk findings, Evo Remediation Agent's purpose-built harness passes 12.7 points more than Claude Code + Snyk scan. Major framework migrations remain hard for all harnesses. Because the agents keep projects working at similar rates, evaluating remediation quality in dependency upgrades requires verifying the fix, the build and the tests together.

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