ValueError: Found edge starting at unknown node

ValueError: Found edge starting at unknown node

TL;DR: one of your edges names a node that does not exist, usually a typo. Compare every add_edge / add_conditional_edges source name against your add_node names character by character. The error fires at build/validate time, before anything runs.

ValueError: Found edge starting at unknown node 'my_nod'

When this applies

• Raised by graph.compile() or builder.validate(), not during a run.
• The quoted name is close to, but not exactly, one of your node names.

When it does not

• Errors DURING a run about unknown nodes come from a router returning a bad name. Different fix.
• InvalidUpdateError is about state writes, not edges.

Fix it

1. Diff the names

builder.add_node("my_node", my_node_fn)
builder.add_edge("my_nod", "other_node")  # typo: 'my_nod' was never added

Fix the typo:

builder.add_edge("my_node", "other_node")

Expected: compile() succeeds.

2. Check conditional edges too

builder.add_conditional_edges(
    "router",
    lambda state: "summarize" if state["done"] else "research",
    {"summarize": "summarize", "research": "reserach"},  # typo in the mapping
)

Both the source ("router") and every destination in the mapping must be added nodes (or END).

3. Remember the special names

START is valid as an edge source without being added. END is valid as a destination. Everything else must come from add_node.

from langgraph.graph import START, END

builder.add_edge(START, "first")   # fine
builder.add_edge("last", END)      # fine

Expected: no validation error for the special nodes.

Why it happens

LangGraph validates the whole edge list before compiling. Every edge source must resolve to a known node so the Pregel engine can schedule it. An unknown source means a node that could never be scheduled, so it fails fast with this error instead of hanging at runtime.

Edge cases

• Renaming a node? Search for every string reference to the old name. Edges are plain strings, so no IDE refactor catches them.
• Node names are case-sensitive: "Agent" and "agent" are different nodes.
• In older langgraph versions the validation message itself had a bug (it printed the wrong variable); if the quoted name looks unrelated, upgrade and re-read the message.

Compatibility

All langgraph Python versions. The validation exists in every 0.x and 1.x release.