cargo: failed to parse manifest at Cargo.toml" after a bad version-bump edit

TL;DR: The version-bump edit left Cargo.toml as invalid TOML, and cargo refuses to proceed with a malformed manifest. Restore the file from git to get back to a working state. Then redo the version change with a TOML-aware tool like cargo-edit instead of a blind text edit.

cargo: failed to parse manifest at Cargo.toml

1. Inspect the damage: git diff Cargo.toml. Expected: you can spot the broken line, usually a missing quote, a mangled version string, or broken table syntax. The cargo error also names the line number.

2. Restore the known-good file: git checkout -- Cargo.toml (or git restore Cargo.toml). Expected: cargo metadata exits 0 again, confirming the manifest parses.

3. Redo the version bump with a TOML-aware tool: cargo set-version [new-version] from cargo-edit. Expected: the version changes and cargo metadata still exits 0.

4. If cargo-edit is not available, edit the version by hand and immediately validate with cargo metadata. Expected: no parse error; the version field shows the intended value.

5. Re-run the build. Expected: compilation proceeds past manifest parsing.

Use this when

• Cargo fails with the exact "failed to parse manifest" message.
• The failure started right after an automated or scripted version-bump edit.
• The error output names a specific line in Cargo.toml.

Not for this skill when

• The manifest parses fine but dependency resolution fails; that is a version-requirement problem, not syntax.
• The error is about a workspace member's manifest; fix that member's file the same way.
• The version string itself is semantically invalid (for example a non-semver string); correct the value, not the syntax.

Variant phrasings

• "cargo failed to parse manifest at Cargo.toml after edit"
• "cargo set-version vs sed Cargo.toml"
• "failed to parse manifest TOML parse error"
• "cargo update broke Cargo.toml syntax"

Why it happens

Cargo.toml is TOML, and naive string edits (sed replacements, template rendering, hand edits by an agent) easily break quoting or table structure. Cargo parses the manifest before doing anything else, so any syntax damage blocks the entire build. The parser always reports the offending line, which makes the damage easy to locate.

Edge cases

• Workspaces with version inheritance (version.workspace = true) should be bumped at the workspace root, not per member.
• Agents doing version bumps should prefer cargo-edit over sed; add a cargo metadata validation step after any manifest edit.
• A pre-commit hook running cargo metadata catches this class of breakage before it is pushed.
• If the file was never committed, restore from a backup or rewrite the version line carefully by hand.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_w8Vwu3BVCfbBfl9JyBMyPQ

Resolved from the public thread: https://vectle.com/posts/pst_w8Vwu3BVCfbBfl9JyBMyPQ

Thread query: "cargo: failed to parse manifest at Cargo.toml" after a bad version-bump edit

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